An Intruiguing Relationship Between Ants and Cacti

The extrafloral nectaries of  Pachycereus gatesii  appear as tiny red bumps just below the areole.

The extrafloral nectaries of Pachycereus gatesii appear as tiny red bumps just below the areole.

It’s hard to think of a group of plants that are better defended than cacti. Frequently and often elaborately adorned with vicious spines, these succulents make any animal think twice about trying to take a bite. And yet, for some cacti, spines don’t seem to cut it. A surprising amount of species appear to have taken their defense system to a whole new level by recruiting nature’s most tenacious bodyguards, ants.

Plants frequently have a friend in ants. Spend some time observing ants at work and it’s east to see why. These social insects have numbers and strength on their side. Give ants a reason to be invested in your survival and they will certainly see to it that nothing threatens this partnership. For cacti, this involves the secretion of nectar from specialized tissues called extrafloral nectaries.

Extrafloral nectaries are not unique to cacti. A multitude of plant species produce them, often for similar reasons. Ants love a sugary food source and the more reliable that source becomes, the more adamant an ant colony will be at defending it. The odd thing about cacti is that they don’t seem to have settled on a single type of extrafloral nectary to do the trick. In fact, as many as four different types of extrafloral nectaries have been described in the cactus family.

Ants visiting the extrafloral nectaries covering the developing flowers of  Pilosocereus gounellei .

Ants visiting the extrafloral nectaries covering the developing flowers of Pilosocereus gounellei.

Some cacti secrete nectar from highly modified spines. A great example of this can be seen in genera such as Coryphantha, Cylindropuntia, Echinocactus, Ferocactus, Opuntia, Sclerocactus, and Thelocactus. Such spines are usually short and blunt, hardly resembling spines at all. Other cacti secrete nectar from regular looking spines. This adaptation is odd as there does not seem to be anything special about the anatomy of such spines. Examples of this can be seen in genera such as Brasiliopuntia, Calymmanthium, Harrisia, Opuntia, Pereskiopsis, and Quiabentia. Still others secrete nectar from highly reduced leaves that are found at the base of where the spines originate (the areole). Such leaves have been described in Acanthocereus, Leptocereus, Myrtillocactus, Pachycereus, and Stenocereus. They aren’t easy to recognize as leaves either. Most look like tiny scales. Finally, the fourth type of extrafloral nectary comes in the form of specialized regions of the stem tissue. This has been described in genera such as Armatocereus, Leptocereus, and Pachycereus.

Highly modified spines functioning as extrafloral nectaries in  Ferocactus emoryi.

Highly modified spines functioning as extrafloral nectaries in Ferocactus emoryi.

Seemingly normal spines of  Harrisia pomanensis  secreting nectar.

Seemingly normal spines of Harrisia pomanensis secreting nectar.

Regardless of where they form, their function remains much the same. They secrete a form of nectar which ants find irresistible. The more reliable this food source becomes, the more aggressive ant colonies will be in defending it. This is an especially useful form of defense when it comes to small insect herbivores. Whereas spines deter larger herbivores, they don’t do much to deter organisms that can just slip right through them unharmed. Ants also clean the cacti, potentially removing harmful microbes like fungi and bacteria. Though we are only just beginning to understand the depths of this cactus/ant mutualism, what we have discovered already suggests that the relationship between these types of organisms is far more complex than what I have just outlined above.

For instance, it may not just be sugar that the ants are looking for. In arid desert habitats, water may be the most limiting resource for an ant colony and large, succulent cacti are essentially giant water reservoirs. The key is getting to that water. One study that looked at a species of barrel cactus growing in Arizona called Ferocactus acanthodes found that as spring gives way to summer, the concentration of sugars secreted by the extrafloral nectaries decreases. As a result, the nectar becomes far more watery. Amazingly, ant densities on any given barrel cactus actually increased throughout the summer, despite the fact that the nectar was being watered down. Ants are notoriously prone to desiccation so it stands to reason that water, rather than sugar, is the real prize for colonies hanging out on cacti in such hot desert environments.

The incredible floral display of  Ferocactus wislizeni , a species whose reproductive efforts are affected by the types of ants they attract.

The incredible floral display of Ferocactus wislizeni, a species whose reproductive efforts are affected by the types of ants they attract.

Another interesting observation about the cactus/ant mutualism is that it appears that the identity of the ants truly matters. Though defense is the main benefit to the cactus, research suggests that there is a tipping point in how much such defenses benefit cacti. It has been found that although cacti initially benefit from anti-herbivore and cleaning services, extra aggressive ant species can actually drive off potential pollinators. At least one study has shown that when less aggressive ant species tend cacti, they produce more fruits and those fruits contain significantly more seeds than cacti that have been tended by extremely aggressive ant species. This is especially concerning when we think about the growing issue of invasive ants. As more and more non-native ant species displace native ants, this could really tip the balance for some cactus species.

Despite all of the interesting things we have learned about extrafloral nectaries in the family Cactaceae, there are so many questions yet to be answered. For starters, we still do not know how many different taxa produce them in one form or another. It is likely that closer inspection, especially of rare or poorly understood groups, will reveal that far more cacti produce some type of extrafloral nectary. Also, we know next to nothing about the anatomy of the different types of nectaries. How do they differ from one another and how do some, especially those derived from ordinary spines, actually function? Finally, do these nectaries function year round or is there some sort of seasonal pattern to their development and utility. How does this affect the types of ants they attract and how does that in turn affect the survival and reproduction of these cacti? For such a charismatic group of plants as cacti, we still have to much to learn.

Photo Credits: Thanks to Dr. Jim Mauseth and Dr. John Rebman and Dr. Silvia Rodriguez Machado for use of their photos [1] [2] [3]

Further Reading: [1] [2] [3] [4] [5] [6]

Raphides: A Gnarly Form of Plant Defense

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Take a bite out of a dumbcane (Dieffenbachia spp.) or a pothos (Philodendron spp.) and it won’t be long before your mouth and throat start to burn (please don’t actually do that). Eat enough of it and your symptoms may also include intense numbing, oral irritation, excessive drooling, localized swelling, and possibly even kidney and liver failure (again, please don’t). What you are experiencing is a brutal form of plant defense caused by tiny crystals called raphides.

Raphides are tiny, needle-shaped crystals made up of calcium oxalate. A lot of plants accumulate calcium oxalate. Research has shown that in doing so, plants are able to sequester excess calcium in their cells. Many plant lineages then use that calcium oxalate to make raphides. Not all raphides come in the form of needle-like crystals. Often they are ‘H’ shaped or even twinned. Others are blunt, kind of like tiny crystalline cigars.

Cigar-shaped raphides found in the tissues of the polka dot plant ( Hypoestes phyllostachya ).

Cigar-shaped raphides found in the tissues of the polka dot plant (Hypoestes phyllostachya).

How raphides form within the plant is rather fascinating. As far as we can discern, raphide crystals form in vacuoles of specialized cells called “idioblasts.” It is thought that an exquisitely controlled scaffolding or matrix shapes the biomineralization process. To the best of my knowledge, no one has been able to reproduce this process in a laboratory setting. For now, plants are the undeniable masters of raphide manufacturing.

Within the cells, raphides are often associated with acrid and toxic proteins. Together, they comprise one hell of a defense against herbivory. Raphides are only the first part of the defensive equation. When plant tissues containing raphides are damaged, usually by chewing, the raphides shoot out of the idioblasts and into the oral cavity of the herbivore. This is where their needle shape comes in.

Needle-like raphides extracted from the leaves of an  Epipremnum  species.

Needle-like raphides extracted from the leaves of an Epipremnum species.

Raphides wreak havoc on sensitive tissues. They literally act like tiny needles, cutting into and tearing the lining of the mouth, esophagus, and gut. This is only half of the story though. As mentioned, raphides are often packed in with acrid and toxic proteins. The laceration caused by the raphides allows these compounds to enter into the wounds. This is where things can get especially nasty. If the proteins are toxic enough, the herbivore now has far more to worry about than simply the burning sensation.

Raphides are not produced in equal amounts in all tissues. Stems tend to have more than leaves, but raphide content in leaves has also shown to be a function of leaf size. Raphides also differ from species to species. Not all plants that produce raphides produce them in the same shape and quantity. Still, more than 200 plant families contain species that have evolved this form of defense and many of our most prized houseplants fall into this category. However, this should not scare you away from these plants. Provided you or your loved ones don’t go nibbling on the leaves or stems, all will be fine. If anything, this remarkable form of plant defense should earn these plants even more respect than they already get.

Photo Credits: [1] [2] [3]

Further Reading: [1] [2] [3]

Why Are Some Plants Overcompensating?

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Gardeners are all too familiar with herbivory. Countless times I have been awaiting a bloom to burst only to have the buds nipped off the night before they opened. While this can be devastating for many plant species (not to mention my sanity), for certain plant species, an encounter with a hungry herbivore may actually lead to an increase in reproductive fitness.

Overcompensation theory is the idea that, under certain conditions, plants can respond to herbivore damage by producing more shoots, flowers, and seeds. It goes without saying that when this idea was originally proposed in the late 80's, it was met with its fair share of skepticism. Why would a plant capable of producing more shoots and flowers wait to be damaged to do so? The answer may lie in in the realm of biological trade-offs.

Overcompensation may evolve in lineages that tend to grow in habitats where there is a "predictable" amount of herbivory in any given growing season, perhaps a region where large herbivores migrate through annually. Plants in these habitats may conserve dormant growing tips and valuable resources to be used once herbivory has occurred. Perhaps this also serves as a cue to upregulate antiherbivory compounds in new tissues. The trade-off is that the plants incur a cost in the form of fewer flowers and thus reduced reproduction when herbivory is low or absent.

Scarlet gilia ( Ipomopsis aggregata )

Scarlet gilia (Ipomopsis aggregata)

It could also be that plants are exhibiting two different strategies - one to deal with competition and one to deal with herbivory. If herbivory is low, plants may become more competitive, thus favoring rapid vertical growth of one or a couple shoots. When herbivory is high, rapid vertical growth becomes disadventageous and overcompensatory branching and flowering can provide the higher fitness benefits.

These possibilities are not mutually exclusive. In fact, since the late 80's, experts now believe that overcompensation is not an "either/or" phenomenon but rather a spectrum of possibilities that are dictated by the conditions in which the plants are growing. Certainly overcompensation exists but which conditions favor it and which do not?

Research on scarlet gilia (Ipomopsis aggregata), a biennial native to western North America, suggests that overcompensation comes into play only when environmental conditions are most favorable. Soil nutrients seem to play a role in how well a plant can bounce back following herbivore damage. When resources are high, the results can be quite astounding. Early work on this species showed that under proper conditions, plants that were browsed by upwards of 95% produced 2.4 times as much seed as uneaten control plants. What's more, the resulting seedlings were twice as likely to survive than their uneaten counterparts.

Things change for scarlet gilia growing in poor conditions. Low resource availability appears to place limits on how much any given plant may respond to browsing. Also, herbivory can really hamper flowering time. Because scarlet gilia is pollen limited, anything that can cause a disruption in pollinator visits can have serious consequences for seed set. In at least one study, browsed plants flowered later and received fewer pollinator visits as a result.

More recent work has been able to add more nuance to the overcompensation story. For instance, experiments done on two subspecies of field gentian (Gentianella campestris), add further support to the idea that overcompensation is a matter of trade-offs. They showed that, whereas competition with neighboring plants alone could not explain the benefits of overcompensation, browsing certainly can.

Field gentian ( Gentianella campestris )

Field gentian (Gentianella campestris)

Plants growing in environments where herbivory was higher overcompensated by producing more branching, more flowers, and thus more seed, all despite soil nutrients. It appears that herbivory is the strongest predictor of overcompensation for this gentian. What's more, when these data were fed into population models, only the plants that responded to herbivory by overcompensation were predicted to show any sort of population growth in the long term.

Despite all of the interest overcompensation has recieved in the botanical literature, we are only just beginning to understand the biological mechanisms that make it possible. For starters, we know that when a dominant shoot or stem gets damaged or removed, it causes a reduction in the amount of the plant hormone auxin being produced. When auxin is removed, tiny auxiliary buds at the base of the plant are able to break dormancy and begin growing.

Removal of the dominant shoot or stem can also have major impact on the number of chromosomes present in regrowing tissues. Work on Arabidopsis thaliana revealed that when the apical meristem (main growing tip of a vertical stem) was removed, the plant underwent a process called "endoreduplication" in which the cells of the growing tissues actually duplicate their entire genome without undergoing mitosis.

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Endoreduplication is a complex process with lots of biological significance but in plants it is often associated with stress responses. By duplicating the genomes of these new cells, the plants may be able to adjust more rapidly to their environment. This often manifests in changes to leaf size and shape and an uptick in plant defenses. Thus, plants may be able to fine tune the development of new tissues to overcompensate for browsing. Certainly far more work is needed to understand these mechanisms and their functions in more detail.

Overcompensation is not universal. Nonetheless, it is expected to occur in certain plants, especially those with short life cycles, and under certain environmental conditions, mainly when herbivore pressure and nutrient availability are relatively high. That being said, we still have plenty more to learn about this spectrum of strategies. When does it occur and when does it not? How common is it? What are the biological underpinnings of plants capable of overcompensation? Are some lineages more prone to overcompensation than others? Only more research can say for sure!

Photo Credits: [1] [2] [3] [4] [5]

Further Reading: [1] [2] [3] [4]

 

 

Life With Endophytic Fungi

Endophytic fungi living in the cells of a grass leaf.

Endophytic fungi living in the cells of a grass leaf.

Talk about plants long enough and fungi eventually make their way into the conversation. These two walks of life are inextricably linked and probably have been since the earliest days. At this point we are well aware of beneficial fungal partners like mycorrhizae or pathogens like the cedar apple rust. Another type of relationship we are only starting to fully appreciate is that of plants and endophytic fungi living in their above ground tissues. 

Endophytic fungi have been discovered in many different types of plants, however, it is best studied in grasses. The closer we look at these symbiotic relationships, the more complex the picture becomes. There are many ways in which plants can benefit from the presence of these fungi in their tissues and it appears that some plants even stock their seeds with fungi, which appears to give their offspring a better chance at establishment. 

To start, the benefits to the fungi are rather straight forward. They get a relatively safe place to live within the tissues of a plant. They also gain access to all of the carbohydrates the plants produce via photosynthesis. This is not unlike what we see with mycorrhizae. But what about the plants? What could they gain from letting fungi live in and around their cells?

One amazing benefit endophytic fungi offer plants is protection. Fungi are famous for the chemical cocktails they produce and many of these can harm animals. Such benefits vary from plant to plant and fungi to fungi, however, the overall effect is largely the same. Herbivores feeding on plants like grasses that have been infected with endophytic fungi are deterred from doing so either because the fungi make the plant distasteful or downright toxic. It isn't just big herbivores that are deterred either. Evidence has shown that insects are also affected.

There is even some evidence to suggest that these anti-herbivore compounds might have influences farther up the food chain. It usually takes a lot of toxins to bring down a large herbivore, however, some of these toxins have the potential to build up in the tissues of some herbivores and therefore may influence their appeal to predators. Some have hypothesized that the endophytic fungal toxins may make herbivores more susceptible to predators. Perhaps the toxins make the herbivores less cautious or slow them down, making them more likely targets. Certainly more work is needed before anyone can say for sure.

Italian ryegrass ( Lolium multiflorum ) is one of the most studied grasses that host endophytic fungi.

Italian ryegrass (Lolium multiflorum) is one of the most studied grasses that host endophytic fungi.

Another amazing example deals with parasitoids like wasps that lay their eggs in other insects. Researchers found that female parasitoid wasps can discriminate between aphids that have been feeding on plants with endophytic fungi and those without endophytic fungi. Wasp larvae developed more slowly and had a shorter lifespan when raised in aphids that have fed on endophytic fungi plants. As such, the distribution of plants with endophytic symbionts may have serious ramifications for parasitoid abundance in any given habitat.

Another benefit these endophytic fungi offer plants is increased photosynthesis. Amazingly, some grasses appear to photosynthesize better with endophytic fungi living in their tissues than plants without fungi. There are many mechanisms by which this may work but to simplify the matter, it appears that by producing defense compounds, endophytic fungi allow the plant to redistribute their metabolic processes towards photosynthesis and growth. In return, the plants produce more carbohydrates that then feed the fungi living in their tissues. 

One of the most remarkable aspects about the relationship between endophytic fungi and plants is that the plants can pass these fungi on to their offspring. Fungi are able to infect the tissues of the host plants' seeds and therefore can be carried with the seeds wherever they go. As the seedlings grow, so do the fungi. Some evidence suggests this gives infected seedlings a leg up on the competition. Other studies have not found such pronounced effects.

Still other studies have shown that it may not be fungi in the seeds that make a big difference but rather the fungi present in the decaying tissues of plants growing around them. Endophytic fungi have been shown to produce allelopathic compounds that poison neighboring plants. Areas receiving lots of plant litter containing endophytic fungi produced fewer plants but these plants grew larger than areas without endophytic fungi litter. Perhaps this reduces competition in favor of plant species than can host said endophytes. Again, this has potentially huge ramifications for the diversity and abundance of plant species living in a given area.

We are only beginning to understand the role of endophytic fungi in the lives of plants and the communities they make up. To date, it would appear that endophytic fungi are potentially having huge impacts on ecosystems around the globe. It goes without saying that more research is needed.

Photo Credits: [1] [2]

Further Reading: [1] [2] [3] [4] [5] [6]

                                                        

Toxic Nectar

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I was introduced to the concept of toxic nectar thanks to a species of shrub quite familiar to anyone who has spent time in the Appalachian Mountains. Locals will tell you to never place honeybee hives near a patch of rosebay (Rhododendron maximum) for fear of so-called "mad honey." Needless to say, the concept intrigued me.

A quick internet search revealed that this is not a new phenomenon either. Humans have known about toxic nectar for thousands of years. In fact, honey made from feeding bees on species like Rhododendron luteum and R. ponticum has been used more than once during times of war. Hives containing toxic honey would be placed along known routs of Roman soldiers and, after consuming the seemingly innocuous treat, the soldiers would collapse into a stupor only to be slaughtered by armies lying in wait.

Rhododendron luteum

Rhododendron luteum

The presence of toxic nectar seems quite confusing. The primary function of nectar is to serve as a reward for pollinators after all. Why on Earth would a plant pump potentially harmful substances into its flowers?

It is worth mentioning at this point that the Rhododendrons aren't alone. A multitude of plant species produce toxic nectar. The chemicals that make them toxic, though poorly understood, vary almost as much as the plants that make them. Although there have been repeated investigations into this phenomenon, the exact reason(s) remain elusive to this day. Still, research has drummed up some interesting data and many great hypotheses aimed at explaining the patterns.

Catalpa nectar has been shown to deter some ants and butterflies but not large bees.

Catalpa nectar has been shown to deter some ants and butterflies but not large bees.

The earliest investigations into toxic nectar gave birth to the pollinator fidelity hypothesis. Researchers realized that meany bees appear to be less sensitive to alkaloids in nectar than are some Lepidopterans. This led to speculation that perhaps some plants pump toxic compounds into their nectar to deter inefficient pollinators, leading to more specialization among pollinating insects that can handle the toxins.

Another hypothesis is the nectar robber hypothesis. This hypothesis is quite similar to the pollinator fidelity hypothesis except that it extends to all organisms that could potentially rob nectar from a flower without providing any pollination services. As such, it is a matter of plant defense.

The nectar of  Cyrilla racemiflora  is thought to be toxic to some bees.

The nectar of Cyrilla racemiflora is thought to be toxic to some bees.

Others feel that toxic nectar may be less about pollinators or nectar robbers and more about microbial activity. Sugary nectar can be a breeding ground for microbes and it is possible that plants pump toxic compounds into their nectar to keep it "fresh." If this is the case, the antimicrobial benefits could outweigh the cost to pollinators that may be harmed or even deterred by the toxic compounds.

Finally, it could be that toxic nectar may have no benefit to the plant whatsoever. Perhaps toxic nectar is simply the result of selection for defense compounds elsewhere in the plant and therefore is expressed in the nectar as a result of pleiotropy. If this is the case then toxic nectar might not be under as strong selection pressures as is overall defense against herbivores. If so, the plants may not be able to control which compounds eventually end up in their nectar. Provided defense against herbivores outweighs any costs imposed by toxic nectar then plants may not have the ability to evolve away from such traits.

Where Spathodea campanulata is invasive, its nectar causes increased mortality in native bee hives.

Where Spathodea campanulata is invasive, its nectar causes increased mortality in native bee hives.

So, where does the science land us with these hypotheses? Do the data support any of these theories? This is where things get cloudy. Despite plenty of interest, evidence in support of the various hypotheses is scant. Some experiments have shown that indeed, when given a choice, some bees prefer non-toxic to toxic nectar. Also, toxic nectar appears to dissuade some ants from visiting flowers, however, just as many experiments have demonstrated no discernible effect on bees or ants. What's more, at least one investigation found that the amount of toxic compounds within the nectar of certain species varies significantly from population to population. What this means for pollination is anyone's' guess.

It is worth noting that most of the pollination-related hypotheses about toxic nectar have been tested using honeybees. Because they are generalist pollinators, there could be something to be said about toxic nectar deterring generalist pollinators in favor of specialist pollinators. Still, these experiments have largely been done in regions where honeybees are not native and therefore do not represent natural conditions.

Simply put, it is still too early to say whether toxic nectar is adaptive or not. It could very well be that it does not impose enough of a negative effect on plant fitness to evolve away from. More work is certainly needed. So, if you are someone looking for an excellent thesis project, here is a great opportunity. In the mean time, do yourself a favor and don't eat any mad honey.

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4] [5] [6]

 

 

How Trees Fight Disease

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Plants do not have immune systems like animals. Instead, they have evolved an entirely different way of dealing with infections. In trees, this process is known as the "compartmentalization of decay in trees" or "CODIT." CODIT is a fascinating process and many of us will recognize its physical manifestations.

In order to understand CODIT, one must know a little something about how trees grow. Trees have an amazing ability to generate new cells. However, they do not have the ability to repair damage. Instead, trees respond to disease and injury  by walling it off from their living tissues. This involves three distinct processes. The first of these has to do with minimizing the spread of damage. Trees accomplish this by strengthening the walls between cells. Essentially this begins the process of isolating whatever may be harming the living tissues.

This is done via chemical means. In the living sapwood, it is the result of changes in chemical environment within each cell. In heartwood, enzymatic changes work on the structure of the already deceased cells. Though the process is still poorly understood, these chemical changes are surprisingly similar to the process of tanning leather. Compounds like tannic and gallic acids are created, which protect tissues from further decay. They also result in a discoloration of the surrounding wood. 

The second step in the CODIT process involves the construction of new walls around the damaged area. This is where the real compartmentalization process begins. The cambium layer changes the types of cells it produces around the area so that it blocks that compartment off from the surrounding vascular tissues. These new cells also exhibit highly altered metabolisms so that they begin to produce even more compounds that help resist and hopefully stave off the spread of whatever microbes may be causing the injury. Many of the defects we see in wood products are the result of these changes.

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The third response the tree undergoes is to keep growing. New tissues grow around the infected compartment and, if the tree is healthy enough, will outpace further infection. You see, whether its bacteria, fungi, or a virus, microbes need living tissues to survive. By walling off the affected area and pumping it full of compounds that kill living tissues, the tree essentially cuts off the food supply to the disease-causing organism. Only if the tree is weakened will the infection outpace its ability to cope.

Of course, CODIT is not 100% effective. Many a tree falls victim to disease. If a tree is not killed outright, it can face years or even decades of repeated infection. This is why we see wounds on trees like perennial cankers. Even if the tree is able to successfully fight these repeat infections over a series of years, the buildup of scar tissues can effectively girdle the tree if they are severe enough.

CODIT is a well appreciated phenomenon. It has set the foundation for better tree management, especially as it relates to pruning. It is even helping us develop better controls against deadly invasive pathogens. Still, many of the underlying processes involved in this response are poorly understood. This is an area begging for deeper understanding.

Photo Credits: kaydubsthehikingscientist & Alex Shigo

Further Reading: [1]

Delayed Greening

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It goes without saying that leaves are vital to the existence of any photosynthetic plant. They are, after all, the food making organs. This is why plants go to great lengths to protect them. Losing leaves can be extremely costly. One of the most intriguing methods of anti-herbivory in plants is known as delayed greening. Flushes of new growth bathed in reds, whites, and light greens can color forests from top to bottom. 

Delayed greening is a matter of resource conservation and herbivore protection. The cellular machinery that makes photosynthesis possible is costly to produce. It requires large amounts of nutrients, such as nitrogen and phosphorus, that are often in short supply. If a plant can help it, its best to avoid losing a leaf chock full of these precious materials. Delayed greening does just that. 

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Essentially, the process proceeds exactly as it sounds. Young shoots and leaves gradually expand over time, becoming more green as they grow tougher and better defended. When a plant packs its leaves full of photosynthetic machinery right out of the gates, when leaves are small and tender, it runs the risk of loosing all of its investment to a hungry herbivore. In contrast, non-photosynthetic leaves are thought to be less palatable to herbivores because they simply do not have the nutritional content of photosynthetic leaves.

By delaying the development of chlorophyll until the leaf is fully expanded and a bit tougher, some plants are maximizing the chances of successfully increasing their photosynthetic capacity over time. Research has shown that plants that exhibit the delayed greening strategy experience significant reductions in the amount of herbivory over time. What they lose with the lack of photosynthesis early on they make up for in the fact that such leaves last longer.  

Photo Credits: [1] [2]

Further Reading: [1] [2] [3]

 

The Stinging Nettles

We've all been there at some point. It's summer, it's a beautiful day, and you find yourself strolling along a trail. You are walking along, enjoying the sights, sounds, and smells of your environment when you harmlessly brush by a patch of waist-high plants. You don't think anything of it. They are herbaceous and don't readily catch the eye. A few steps later and the burning starts. It is mild at first but wherever your skin met the tissues of those plants an itchy, burning sensation starts to amplify. You have likely just encountered a species of stinging nettle. 

Nettles hail from a handful of genera. There are many different species of nettle but you are most likely to encounter either stinging nettle (Urtica dioica) or the wood nettle (Laportea canadensis), all of which belong to the nettle family (Urticaceae). A closer inspection of the plant reveals that the stems as well as the underside of the leaves are covered in tiny hairs. These hairs are called trichomes. A subset of these trichomes are what caused your discomfort. 

Anatomy of a stinging trichome

Anatomy of a stinging trichome

These trichomes have been honed by natural selection into a very effective defense. They are an elongated cell that sits atop of a multicellular pedestal. They are quite brittle and any contact with them causes their tips to break. They are also hollow and once they are broken, they essentially function like mini hypodermic needles. They penetrate the skin of any animal unlucky enough to brush up against them and inject an irritating fluid into the tissues of their "attacker." The fluid itself is quite interesting. Chemical analyses have revealed that it consists of a complex mixture of histamines, acetylcholine, serotonin, and even formic acid. Chemists are still working out the exact makeup of this chemical weapon and how much variation there is between different stinging species. 

As you might have deduced by this point, these stinging hairs are a defense mechanism. They protect the plant from herbivores. However, not all herbivores are deterred by this defense. It was found that invertebrates don't seem to have any issue navigating the stinging hairs. Instead, it is thought that the stinging nature of these plants evolved in response to large mammalian herbivores. This makes some sense as larger herbivores pose more of a threat to the entire plant than do invertebrates.

Stinging nettle ( Urtica dioica ) 

Stinging nettle (Urtica dioica

Even more interesting is the response of some nettles to varying levels of herbivory. It has been found that heavily damaged plants will regrow leaves and stems with higher densities of stinging hairs than those of plants that have experienced lower rates of herbivory. This too makes a lot of sense. Stinging hairs require resources to produce so plants that have not experienced high rates of herbivory do not bother allocating precious resources to their production.

Even more interesting is the fact that for stinging nettle (U. dioica), male and female plants tend to have differing densities of stinging hairs. Female plants produce more stinging hairs than males. It is thought that since females must invest more resources into producing seeds than males do into producing pollen, they must also invest in more protection for these valuable reproductive assets. 

These nettles are not alone in producing such stinging trichomes. Many other plant species have converged on this defensive strategy. If you have ever experienced this for yourself, you can really understand just how effective it can be. 

Wood nettle ( Laportea canadensis )

Wood nettle (Laportea canadensis)

Photo Credits: [1] [2] [3] [4]

Further Reading: [1] [2] [3] [4]

Got Herbivores? Turn Them Into Cannibals!

Plants have to deal with quite a lot in their day to day lives. They can't get up and move like animals can. Due to their sessile nature, plants rely on a suite of physical and chemical traits for defense. The world of plant chemistry is quite amazing and thanks to new research published in Nature Ecology & Evolution, it has gotten even more interesting. Under attack by herbivorous insects, some plant species are able to turn their vegetarian predators into cannibals. 

Cannibalism in insects is not unheard of, even among the herbivorous species. When the going gets tough, why not eat your sibling or your neighbor? Well, research using tomatoes and the army beetworm (Spodoptera exigua) suggests that plants might be able to induce this behavior in caterpillars long before it would happen naturally. It makes sense too. Plants that are able to induce cannibalistic behavior via chemical means not only reduce grazing pressures on their own tissues, they also reduce the number of herbivores in the system.

The chemical in question here is called methyl jasmonate. It is a volatile organic compound produced by a plethora of plant species and is thought to play in role in a diverse array of biological functions such as germination, root growth, fruit ripening, and defense. It is often released when a plant becomes damaged. Neighboring plants are able to pick up on this compound and will begin to beef up their own defenses in response. After all, if your neighbor is being attacked, there is a decent chance you will be too. 

Researchers investigating the effects of this chemical on the beetworm (a common aggricultural pest) found that plants that were treated with methyl jasmonate induced beetworms to turn on one another through cannibalism. Caterpillars hanging out on plants that were not treated with methyl jasmonate only turned to cannibalism after they had consumed all of the leaves available, if at all. 

The researchers are now gearing up to figure out whether inducing cannibalism also helps to spread disease among caterpillars. This exciting new form of plant defenses opens up doors to many new questions and potentially safer forms of pest control. Considering the near ubiquity of methyl jasmonate in the botanical world, it begs the question as to how common this form of defense really is. 

Photo Credits: [1] [2] 

Further Reading: [1] [2]

Convergent Carnivores

A carnivorous lifestyle has evolved independently in numerous plant lineages. Despite the similarities between genera like Nepenthes, Sarracenia, and Cepholotus they are not closely related. Researchers have wondered how the highly modified leaves of various carnivorous plant species evolved into the insect trapping and digesting organs that we see today. Thanks to a recent article published in Nature, it has been revealed that the mechanisms responsible for carnivory in plants are a case of convergent evolution.

This research all started with the Australian pitcher plant Cepholotus follicularis. More closely related to wood sorrels (Oxalis spp.) than either of the other two pitcher plant families, this species offers a unique window into the genetic controls on pitcher development. Cepholotus produces two different kinds of leaves - normal, photosynthetic leaves and the deadly pitcher leaves that have made it famous the world over.

By observing which genes are activated during the development of these different types of leaves, the research team was able to identify which alleles have been modified. In doing so, they were able to identify genes involved in producing the nectar that attracts their insect prey as well as the genes involved in producing the slippery waxy coating that keeps trapped insects from escaping. But they also found something even more interesting.

Next, the team took a closer look at the digestive fluids produced by Cepholotus as well as many other unrelated carnivorous plant species from around the world. In doing so, the team made a startling discovery. They found that the genes involved in synthesizing the deadly digestive cocktails among these disparate lineages have a similar evolutionary origin.

Although they are unrelated, the ability to digest insects seems to have its origins in defending plants against fungi. You have probably heard someone say that fungi are more similar to animals than they are plants. Well, the polymer that makes up the cell walls of fungi is the same polymer that makes up the exoskeleton of insects - chitin. By comparing the carnivorous plant genes to those of the model plant Arabidopsis, the team found that similar genes became active when plants were exposed to fungal pathogens.

It appears that carnivorous plants around the world have all converged on a system in which genes used to defend themselves against fungal infection have been co-opted to digest insect bodies. Taken together, these results show that the path to carnivory in plants is surprisingly narrow. Evolution doesn't always require the appearance of new alleles but rather a retooling of genes that are already in place. 

Photo Credits: [1] [2]

Further Reading: [1]

 

 

The Gas Plant

Meet the gas plant, Dictamnus albus. This lovely herbaceous species is native to southern Europe, north Africa, and Asia. The gas plant is a member of the citrus family, Rutaceae, and like many members of this group, it has very showy blossoms. Its affiliation with the citrus fruits on your counter isn't the only interesting thing about this species. As the common name might suggest, this plant does something quite strange. 

During the heat of summer, parts of the gas plant exude an oily substance that smells much like the fruits of its cousin, the lemon. These oils have been known to cause contact dermatitis not unlike the sap of giant hogweed. However, this is not the strangest aspect of the gas plants oily nature. One of the properties of these oils is that they are highly volatile. So volatile in fact that they can ignite. 

Another common name for this species is burning bush (though not the one of biblical lore). If air temperatures get high enough or if someone takes a match to this plant on a hot day, the oils covering its tissues will ignite in a flash. The oils burn off so quickly that it is of no consequence to the plant. It goes on growing like nothing ever happened. If you're like me then you have one burning question after reading this - why?!

Despite how incredible this phenomenon may seem, it doesn't appear that too many people have looked into its function. Research has identified a highly flammable organic compound within the oils called isoprene. In plants, isoprenes are thought to protect against heat stress. This is bolstered by the fact that the gas plant produces these compounds during the heat of summer. 

Another possibility is that spontaneous ignition of these compounds could create small wildfires that clear the surrounding area of competition. I have not seen any evidence suggesting this. Yet another possibility is that this is simply an unrelated side effect of oil production. Since the plant is not hurt by the quick burst of flames, it simply hasn't had any reason to evolve a less flammable alternative. Evolution is funny like that. 

Still don't believe what you are reading? Check out this video:

Photo Credit: Jörg Hempel (Wikimedia Commons)

Further Reading: [1]

 

Trees Know Who Nibbles

Not being able to escape form your predators makes for an interesting challenge. Plants, being the static organisms that they are, have risen to this challenge in some amazing ways. From thorns, to hairs, and even chemical warfare, there is no end to the strategies plants have evolved to discourage herbivores.

These adaptations come at a cost. Whether its physical or chemical, defenses require ample resources to produce. That is why so many plant species have evolved induced defense mechanisms. They don't bother producing their chemical cocktails until they sense that herbivores are munching on their tissues. Whereas the literature is rife with examples of insect-induced defenses in plants, few studies have ever investigated whether plants can sense mammalian herbivory. 

A 2016 paper published in Functional Ecology suggests that some trees certainly can. By looking at the responses of two different tree species, sycamore maple (Acer pseudoplantanus) and European beech (Fagus sylvatica), the research team found that trees seem to be able to distinguish between being clipped and being browsed, in this case, by deer. Deer populations are at historically high densities. This has had severe ramifications on forest health and composition. Tree saplings are especially vulnerable, so much so that we are witnessing an alarming decrease in tree replacement over time. 

The research team tested if trees could sense herbivory in a pretty ingenious way. They set out into the forest with pruners. Saplings were subjected to two different treatments - simple pruning and pruning followed by the addition of deer saliva. The team then took a look at how each tree responded on a molecular level. What they found was quite startling. Trees that were subjected to pruning alone began producing a class of hormones called jasmonates. This was not surprising as jasmonates are involved in some generic plant defenses. The most interesting results came from the treatments in which deer saliva was added.

With the addition of deer saliva to pruned beech twigs, it was discovered that the trees increased their production of metabolites related to growth of buds and leaves. They also found that the addition of deer saliva caused an increase in the production of defense compounds, specifically tannins. Tannins bind to proteins in animal guts, making them harder to digest. In maples specifically, they also found an increase in certain types of flavanols, which have shown to have anti-herbivory properties in insects and humans, however, more work is needed to see if they do in fact deter other mammalian herbivores.

Although we still don't know what exactly the trees are responding to in deer saliva, these results nonetheless offer the first evidence of trees not only being able to perceive mammalian herbivores but also responding with an increase in defense compounds. Although they only looked at two tree species, it stands to reason that such responses are wide spread throughout many plant lineages. It also calls into question previous research that used simple pruning as a proxy for herbivory. Taken together, the picture of plants being unresponsive backdrops to more charismatic fauna is entirely erroneous. Plants are proving to be quite "aware" of their environment.

Photo Credit: [1]

Further Reading: [1]

 

 

Grass Defenses

Cut grass, sword grass, ripgut? All of these names have been applied to grasses. It may seem strange to attach such sharp adjectives to grasses but run through a prairie full of Leersia oryzoides or Spartina pectinata with shorts on and you will quickly learn why. As innocuous as they may look, grasses are well defended.

What looks like the mouth of a shark is actually a blade of grass. As you can see, it is covered in microscopic, razor sharp daggers. The daggers themselves are specialized structures called "phytoliths." Grasses manufacture phytoliths from silica that they absorb from the soil. Not all species produce these daggers. Some distribute phytoliths throughout their leaves, essentially packing themselves with tiny granules of glass. Their presence is an adaptation against herbivory. 

It's not hard to imagine how effective silica daggers can be. Run your finger along the stem or leaves of one of these grasses and you are likely to draw blood. Early settlers coined the term "ripgut grass" because the bellies of horses and other livestock would get seriously lacerated from running through it. Whereas this defense is rather straight forward, the other types of phytoliths are a little more subtle in their effectiveness. 

Silica is tough and chewing on leaves chock full of it can do a real number on your teeth. That is the main reason why the teeth of many grass grazers alive today grow continuously. If their teeth were like ours, the phytoliths within the blades of grass would wear them down to useless nubs. In fact, the evolution of phytoliths in grass is thought to have ushered in a new age of grazing mammals via the extinction of those that could not cope with these microscopic defenses. 

It's not just about teeth either. Insects feeding on blades of grass may be able to get past the phytoliths without an issue but the story changes once it makes it to the gut. Silica particles have been shown to interfere with digestion. Caterpillars feeding on grasses containing high amounts of silica in their leaves had decreased levels of digestion efficiency, which resulted in reduced growth rates. Unlike other plants, grasses can handle certain amounts of grazing because their growth tips are located beneath the soil rather than near the tips. As such, they can afford to gradually weaken the effectiveness of their predators. 

I have gained a new appreciation for grasses since moving to the prairies. This diverse order of plants has shaped the world we see today in a very big way. Because they don't have showy flowers, grasses are often overlooked as nothing more than turf. In reality, they are fascinating organisms supremely adapted to what the environment throws at them. One could only wish to be as hardy as a grass. 

Photo Credits: [1] [2]

Further Reading: [1] [2] 

Sand Armor

Plants go through a lot to protect themselves from the hungry jaws of herbivores. They have evolved a multitude of ways in which to do this - toxins, stinging hairs, thorns, and even camouflage. And now, thanks to research by a team from UC Davis, we can add sand to this list. 

At this point you may be asking "sand?!" Stick with me here. Undoubtedly you have noticed that sticky plants often have bits of whatever substrate they are growing in stuck to their stems and leaves. You wouldn't be the first to notice this. Back in 1996 a term was coined for this very phenomenon. It has been called “psammophory,” which translates to "sand-carrying."

Over 200 species of plants hailing from 88 genera in 34 families have been identified as psammorphorous. The nature of this habit has been an object of inquiry for at least a handful of researchers over the last few decades. Hypotheses have ranged from protection from physical abrasion, reduction of water loss, reduced surface temperature, reduced solar radiation, and protection from herbivory. 

It was this last hypothesis that seemed to stick. Indeed, many plants produce crystalline structures in their tissues (phytoliths, raphides, etc., which are often silica or calcium based) to deter herbivores. Sand, being silica based, is known to cause tooth wear in humans, ungulates, and rodents. Perhaps a coating of sand is enough to drive away insects and other hungry critters looking to snack on a plant. 

By controlling the amount and color of the sand stuck to plants, the researchers were able to demonstrate that plants covered in sand were less palatable to both mammalian and insect herbivores. In total, sand-covered individuals received significantly less damage to their leaves than individuals that had their sand coat removed. By altering the color of the sand, the researchers were able to demonstrate that this was not a function of camouflage. In total, the presence of sand led to an overall increase in fitness due to a decrease in damage over time. These results are the first conclusive evidence in support of psammophory as yet another fantastic plant defense mechanism. 

Photo Credit: Franco Folini (bit.ly/1RApG1R) and Wolfram Burner (
http://bit.ly/1RMNR9V)

Further Reading:
http://onlinelibrary.wiley.com/wol1/doi/10.1890/15-1696/abstract

The Truth About Coffee

Mmm mmm coffee. This wonderful elixir has taken over the world. Though individual tastes and preferences vary, there is no denying that most folks who turn to coffee enjoy its effects as a stimulant. Many an In Defense of Plants post has been written in a coffee-fueled frenzy. Even as I write this piece, I am taking breaks to sip on a warm mug of the stuff. Coffee has plenty of proponents as well as its fair share of nay sayers but the health effects don't really concern me much. Today I would rather talk with you about the shrubs that are behind all of this. 

The coffee we drink comes from a handful of shrubs in the genus Coffea. Native to parts of Africa, these shrubs are distant relatives of plants like buttonbush (Cephalanthus occidentalis) and the bedstraws (Galium sp.). The "beans" that we brew coffee from are not beans at all but rather a type of pit or stone found in the center of a bright red berry. Before they are roasted, the "beans" are actually green. Plants in this genus produce an alkaloid compound known as caffeine. Though it may seem strange, the purpose of caffeine is not to stimulate the human nervous system (though it is a wonderful side effect) but rather it is produced as a defense mechanism for the plant. Making this compound is a complex process that involves many metabolic steps within the tissues of the plant. There are certain factions out there who would like to argue that this is proof against evolution but, as always, evidence seems to be the downfall of their argument. 

Creationists will tell you that the adaptations we see throughout the living world are too complex to have happened by accident. In reality, there is a vast amount of evidence that disputes this. Caffeine is one such example. It has evolved independently multiple times in many different plant lineages. Looking at the genome of coffee, researchers at the University at Buffalo (my alma mater) found that the genes involved in the synthesis of caffeine did not arise all at once. Instead, the genes duplicated multiple times throughout the history of this genus with each duplication coding for another step in the process of producing the caffeine molecule. The interesting part is that each step of this evolutionary process produced a chemical that was itself useful to the plant. The precursor compounds are bitter and toxic to the kinds of animals that like to nibble on the plant. 

As it turns out, the benefits that the plants get from caffeine aren't restricted to defense either. Coffee, as well as other flowering plants such as citrus, produce small amounts of caffeine in their nectar. Researchers at Arizona State University found that bees were 3 times more likely to remember a flowers scent when there was caffeine in the nectar than if there wasn't. This serves a great benefit to the plant producing it because it means that its flowers are much more likely to get pollinated. As it turns out, humans aren't the only species that enjoys a good buzz from caffeine.

Before we get too excited over coffee, we must remember that is definitely has its downside. Worldwide, we humans drink roughly 2.25 billion cups of the stuff every day. In order to produce that much coffee, humans have turned somewhere around 11 million hectares of land into coffee plantations. This has come at an extreme cost to the environment. Also, being a tropical species, the types of habitat used to grow coffee were once lush, tropical rain forests. A majority of coffee consumed around the world is produced in monocultures. Where there once stood towering trees and a lush understory is now an open, chemically-laden field of coffee shrubs. There is hope, however, and it is rising in popularity. 

If you enjoy coffee as much as I do, you should certainly consider switching over to shade grown coffee. I have attached a fair amount of literature at the bottom of this post but the long story short of it is that growing coffee is much less harmful to the environment when it is grown in a forest rather than open plantations. The structural complexity of shade grown coffee farms allows a greater diversity of plant and animal species to coexist with one another. Species diversity and richness are significantly higher on shade grown farms than on open field plantations. 

So, there you have it. Coffee is as complex as it is interesting. We humans are simply lucky to have stumbled across a plant that interacts with our brain chemistry in wonderful ways. Certainly coffee has benefitted in the long run. 

Photo Credit: Ria Tan (http://bit.ly/1pFQD1J)

Further Reading:
http://www.sciencemag.org/content/345/6201/1181.full

https://asunews.asu.edu/20130307_beesandcaffeine


http://s.si.edu/1o6wOaj

http://www.sciencedaily.com/releases/2012/08/120807101357.htm

http://bit.ly/1S6dLVV

Shady Spines

Tephrocactus articulatus

Tephrocactus articulatus

Fondling cacti with your bare hands is often ill-advised. These spiny plants are icons of plant defense mechanisms. Cactus spines are actually modified leaves/bud scales. They develop from a bundle of cells called "primordia" that are nearly indistinguishable from leaf primordia. Unlike leaves, however, cactus spines are not made up of living tissue. The genes for leaf development are shut off in these cells and instead, genes for wood fibers are ramped up, creating the stiff structures many of us have had to pry out of our skin.

It is easy to assume that spines are simply there for defense. For a lot species they certainly do the trick. However, for many other species, spines serve another important purpose - they provide shade. This is exemplified by the fact that cacti growing in rainforests and cloudy highlands often have reduced or no spines at all.

For cacti living in the sun-baked regions of the world, sunburn is a serious issue to contend with. Full sunlight can damage sensitive photosynthetic machinery and while intense UV rays wreak havoc on the genome. As such, any adaptation that can shelter these sensitive tissues to some degree is advantageous.

Cephalocereus senilis

Cephalocereus senilis

Spines also buffer the cactus from huge temperature swings. Think of fuzzy or papery spines as a sort of blanket covering the cactus. These spines create a boundary between air immediately surrounding the cactus and the cold nighttime air of these arid climates. This insulation can come in handy as desert temperatures can drop quite low when the sun goes down.

Another benefit spines have is to catch and direct water to the base of the plant. Rain is often scarce in these habitats so when it does occur, a cactus needs to be ready. Water collects on the spines and then runs down to the base. They also act as dew catchers, causing water vapor to condense on their surfaces. In this way, cacti are able to take advantage of every last drop available.

Though they certainly offer some protection, many of these shade spines are too thin and flexible to deter a hungry herbivore. That is where secondary compounds come into play. It is no wonder why some cacti are extremely toxic to herbivores. Whether they are for shade, protection, or water harvesting, cacti spines have managed to capture our imagination and knowing a bit more about their function makes these plants even more impressive.

Photo Credit: [1] [2]

Further Reading: [1] [2] [3]