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]

                                                        

How a Giant Parasitic Orchid Makes a Living

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Imagine a giant vine with no leaves and no chlorophyll scrambling over decaying wood and branches of a warm tropical forest. As remarkable as that may seem, that is exactly what Erythrorchis altissima is. With stems that can grow to upwards of 10 meters in length, this bizarre orchid from tropical Asia is the largest mycoheterotrophic plant known to science.

Mycoheterotrophs are plants that obtain all of their energy needs by parasitizing fungi. As you can probably imagine, this is an extremely indirect way for a plant to make a living. In most instances, this means the parasitic plants are stealing nutrients from the fungi that were obtained via a partnership with photosynthetic plants in the area. In other words, mycoheterotrophic plants are indirectly stealing from photosynthetic plants.

In the case of E. altissima, this begs the question of where does all of the carbon needed to build a surprising amount of plant come from? Is it parasitizing the mycorrhizal network associated with its photosynthetic neighbors or is it up to something else? These are exactly the sorts of questions a team from Saga University in Japan wanted to answer.

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All orchids require fungal partners for germination and survival. That is one of the main reasons why orchids can be so finicky about where they will grow. Without the fungi, especially in the early years of growth, you simply don't have orchids. The first step in figuring out how this massive parasitic orchid makes its living was to identify what types of fungi it partners with. To do this, the team took root samples and isolated the fungi living within.

By looking at their DNA, the team was able to identify 37 unique fungal taxa associated with this species. Most surprising was that a majority of those fungi were not considered mycorrhizal (though at least one mycorrhizal species was identified). Instead, the vast majority of the fungi associated with with this orchid are involved in wood decay.

Stems climbing on fallen dead wood (a) or on standing living trees (b). A thick and densely branched root clump (c) and thin and elongate roots (d) [Source]

Stems climbing on fallen dead wood (a) or on standing living trees (b). A thick and densely branched root clump (c) and thin and elongate roots (d) [Source]

To ensure that these wood decay fungi weren't simply partnering with adult plants, the team decided to test whether or not the wood decay fungi were able to induce germination of E. altissima seeds. In vitro germination trials revealed that not only do these fungi induce seed germination in this orchid, they also fuel the early growth stages of the plant. Further tests also revealed that all of the carbon and nitrogen needs of E. altissima are met by these wood decay fungi.

These results are amazing. It shows that the largest mycoheterotrophic plant we know of lives entirely off of a generalized group of fungi responsible for the breakdown of wood. By parasitizing these fungi, the orchid has gained access to one of the largest pools of carbon (and other nutrients) without having to give anything back in return. It is no wonder then that this orchid is able to reach such epic proportions without having to do any photosynthesizing of its own. What an incredible world we live in!

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Photo Credits: [1] [2]

Further Reading: [1]

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]

The Extraordinary Catasetum Orchids

Male  Catasetum osculatum

Male Catasetum osculatum

Orchids, in general, have perfect flowers in that they contain both male and female organs. However, in a family this large, exceptions to the rules are always around the corner. Take, for instance, orchids in the genus Catasetum. With something like 166 described species, this genus is interesting in that individual plants produce either male or female flowers. What's more, the floral morphology of the individual sexes are so distinctly different from one another that some were originally described as distinct species. 

Female  Catasetum osculatum

Female Catasetum osculatum

In fact, it was Charles Darwin himself that first worked out that plants of the different sexes were indeed the same species. The genus Catasetum enthralled Darwin and he was able to procure many specimens from his friends for study. Resolving the distinct floral morphology wasn't his only contribution to our understanding of these orchids, he also described their unique pollination mechanism. The details of this process are so bizarre that Darwin was actually ridiculed by some scientists of the time. Yet again, Darwin was right. 

Catasetum longifolium

Catasetum longifolium

If having individual male and female plants wasn't strange enough for these orchids, the mechanism by which pollination is achieved is quite explosive... literally. 

Catasetum orchids are pollinated by large Euglossine bees. Attracted to the male flowers by their alluring scent, the bees land on the lip and begin to probe the flower. Above the lip sits two hair-like structures. When a bee contacts these hairs, a structure containing sacs of pollen called a pollinia is launched downwards towards the bee. A sticky pad at the base ensures that once it hits the bee, it sticks tight. 

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Male Catasetum flower in action. Taken from BBC's Kingdom of Plants.

Bees soon learn that the male flowers are rather unpleasant places to visit so they set off in search of a meal that doesn't pummel them. This is quite possibly why the flowers of the individual sexes look so different from one another. As the bees visit the female flowers, the pollen sacs on their back slip into a perfect groove and thus pollination is achieved. 

Eulaema polychroma  visiting  Catasetum integerrimum

Eulaema polychroma visiting Catasetum integerrimum

The uniqueness of this reproductive strategy has earned the Catasetum orchids a place in the spotlight among botanists and horticulturists alike. It begs the question, how is sex determined in these orchids? Is it genetic or are there certain environmental factors that push the plant in either direction? As it turns out, light availability may be one of the most important cues for sex determination in Catasetum

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A paper published back in 1991 found that there were interesting patterns of sex ratios for at least one species of Catasetum. Female plants were found more often in younger forests whereas the ratios approached an even 1:1 in older forests. What the researchers found was that plants are more likely to produce female flowers under open canopies and male flowers under closed canopies. In this instance, younger forests are more open than older, more mature forests, which may explain the patterns they found in the wild. It is possible that, because seed production is such a costly endeavor for plants, individuals with access to more light are better suited for female status. 

Catasetum macrocarpum

Catasetum macrocarpum

Aside from their odd reproductive habits, the ecology of these plants is also quite fascinating. Found throughout the New World tropics, Catasetum orchids live as epiphytes on the limbs and trunks of trees. Living in the canopy like this can be stressful and these orchids have evolved accordingly. For starters, they are deciduous. Most of the habitats in which they occur experience a dry season. As the rains fade, the plants will drop their leaves, leaving behind a dense cluster of green pseudobulbs. These bulbous structures serve as energy and water stores that will fuel growth as soon as the rains return. 

Catasetum silvestre in situ

Catasetum silvestre in situ

The canopy can also be low in vital nutrients like nitrogen and phosphorus. As is true for all orchids, Catasetum rely on an intimate partnership with special mychorrizal fungi to supplement these ingredients. Such partnerships are vital for germination and growth. However, the fungi that they partner with feed on dead wood, which is low in nitrogen. This has led to yet another intricate and highly specialized relationship for at least some members of this orchid genus. 

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Mature Catasetum are often found growing right out of arboreal ant nests. Those that aren't will often house entire ant colonies inside their hollowed out pseudobulbs. This will sometimes even happen in a greenhouse setting, much to the chagrin of many orchid growers. The partnership with ants is twofold. In setting up shop within the orchid or around its roots, the ants provide the plant with a vital source of nitrogen in the form of feces and other waste products. At the same time, the ants will viciously attack anything that may threaten their nest. In doing so, they keep many potential herbivores at bay.  

Female  Catasetum planiceps

Female Catasetum planiceps

To look upon a flowering Catasetum is quite remarkable. They truly are marvels of evolution and living proof that there seems to be no end to what orchids have done in the name of survival. Luckily for most of us, one doesn't have to travel to the jungles and scale a tree just to see one of these orchids up close. Their success in the horticultural trade means that most botanical gardens house at least a species or two. If and when you do encounter a Catasetum, do yourself a favor and take time to admire it in all of its glory. You will be happy that you did. 

Photo Credits: [1] [2] [3] [4] [5] [6] [7] [8] [9] 

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

Cedar-Apple Rust

I have had my eye on these strange brown golf ball shaped growths growing on the twigs of a cedar in my neighborhood for about a year now. I first took notice of them late last spring. They looked pretty nasty but I knew they had to be something interesting. Indeed, interesting doesn't even come close to the reality. 

These odd little growths are actually a single stage in the complex life cycle of a group of fungi in the genus Gymnosporangium. Collectively they are referred to as cedar-apple galls. Its a group of fungi whose hosts include junipers and relatives of the apple. Wherever these two lineages coexist you are bound to find this fungus. 

Gymnosporangium have a rather interesting life cycle that includes multiple hosts. The golf ball shaped galls will appear on the twigs of a juniper nearly a year after being infected with spores. They grow in size until they reach a point in which they will barely fit in the palm of your hand. The gall itself is covered in a series of depressions, making it look quite out of place in a natural setting. After a year on the tree, the galls enter into their next stage of development. 

Usually triggered by the first warm rains of spring, strange gelatinous protrusions start to poke out of each depression on the gall's surface. These protrusions continue to swell until the entire gall is covered in bright orange finger-like masses. These are where the spores are produced. These spores, however, cannot infect another juniper. Instead, they need to land on the next host to complete their life cycle. 

If the spores land on a member of the family Rosaceae (though usually apples - genus Malus - are preferred), then the second stage of the life cycle begins. Spores can germinate on both the leaves and the fruit but instead of turning into a large brown gall, they take on a different appearance. This is what makes this fungus readily apparent as a type of rust. A patch of orange will begin to grow. Upon closer inspection one can see that the orange patch is actually a series of small cup-like structures full of spores. 

Come fall, the spores are ready to be dispersed by wind. With any luck, these spores will land back on a juniper tree and the cycle will start anew. Because of its propensity for apple crops, cedar-apple rust fungi are considered to be quite a pest. In a more natural setting, however, it is one of the most unique and interesting fungi you can find. It looks truly alien if you aren't already aware of its existence. 

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

Further Reading: [1] [2]

On Fungi and Forest Diversity

One simply can't talk about plants without eventually talking about fungi. The fact of the matter is the vast majority of plant species rely on fungal interactions for survival. This mutualistic relationship is referred to as mycorrhizal. Fungi in the soil colonize the root system of plants and assist in the acquisition of nutrients such as nitrogen and phosphorus. In return, most photosynthetic plants pay their mycorrhizal symbionts with carbohydrates. 

There are two major categories of mycorrhizal fungi - ectomycorrhizae (EMF) and arbuscular mycorrhizae (AMF). Though there are a variety of different species of fungi that fall into either of these groups, their strategies are pretty much the same. EMF make up roughly 10% of all the known mycorrhizal symbionts. The prefix "ecto" hints at the fact that these fungi form on the outside of root cells. They form a sort of sheath that encases the outside of the root as well as a "hartig net" around the outside of individual cells within the root cortex. AMF, on the other hand, literally penetrate the root cells and form two different kinds of structures once inside. One of these structures looks like the crown of a tree, hence the term "arbuscular." What's more, they are considered the oldest mycorrhizal group to have evolved. 

The type of mycorrhizal fungi a plant partners with has greater implications that simple nutrient uptake. Evidence is now showing that the dominant fungi of a region can actually influence the overall health and diversity forest ecosystems. The mechanism behind this has a lot to do with the two different categories discussed above. 

Researchers have discovered that trees partnering with AMF experience negative feedbacks in biomass whereas those that partner with EMF experience positive feedbacks in biomass. When grown in soils that previously harbored similar tree species, trees that partnered with AMF grew poorly whereas trees that partnered with EMF grew much better. Additionally, by repeating the experiments with seedlings, researchers found that seedling survival was reduced for AMF trees whereas seedling survival increased in EMF trees. 

What is going on here? If mycorrhizae are symbionts, why would there be any detrimental effects? The answer to this may have something to do with soil pathogens. Thinking back to the major differences between EMF and AMF, you will remember that it comes down to the way in which they form their root associations. EMF form a protective sheath around the roots whereas AMF penetrate the cells.  As it turns out, this has major implications for pathogen resistance. Because they form a sheath around the entire root, EMF perform much better at keeping pathogens away from sensitive root tissues. The same can't be said for AMF. Researchers found that AMF trees experienced significantly more root damage when grown in soils that previously contained AMF trees. 

The differences in the type of feedback experienced by EMF and AMF trees can have serious consequences for tree diversity. Because EMF trees are healthier and experience increased seedling establishment in soils containing other EMF species, it stands to reason that this would lead to a dominance of EMF species, thus reducing the variety of species capable of establishing in that area. Conversely, areas dominated by AMF trees may actually be more diverse due to the reduction in fitness that would arise if AMF trees started to dominate. Though they are detrimental, the negative feedbacks experienced by AMF trees may lead to healthier and more diverse forests in the grand scheme of things. 

Infographic by [1]

Further Reading: [1]