Early Land Plants Made The World Muddy

Cooksonia  is one of the earliest land plants to have evolved.

Cooksonia is one of the earliest land plants to have evolved.

Try to picture the world before life moved onto land. It would have been a vastly different landscape than anything we know today. For one, there would have been no soil. Before life moved onto land, there was nothing organic around to facilitate soil formation. This would have changed as terrestrial habitats were slowly colonized by microbes and eventually plants. A recent paper published in Science is one of the first to demonstrate that the rise in certain sediments on land, specifically mud-forming clays, coincided with the rise in deep-rooted land plants.

This was no small task. The research duo had to look at thousands of reports spanning from the Archean eon, some 3.5 billion years ago, to the Carboniferous period, some 358 million years ago. By looking for the relative amounts of a sedimentary rock called mudrock in terrestrial habitats, they were able to see how the geology of terrestrial habitats was changing through time. What they found was that the presence of mudrock increased by orders of magnitude around the same time as early land plants were beginning to colonize land. Before plants made it onto land, mudrocks comprised a mere 1% of terrestrial sediments. By the end of the Carboniferous period, mudrocks had risen to 26%.

This begs the question, why are mudrocks so significant? What do they tell us about what was going on in terrestrial environments? A key to these questions lies in the composition of mudrocks themselves. Mudrock is made up of fine grained sediments like clay. There are many mechanisms by which clay can be produced and certainly this was going on well before plants made it onto the scene. The difference here is in the quantity of clay-like minerals in these sediments. Whereas bacteria and fungi do facilitate the formation of clay minerals, they do so in small quantities.

A little bit of moss goes a long way for erosion control!

A little bit of moss goes a long way for erosion control!

The real change came when plants began rooting themselves into the soil. In pushing their roots down into sediments, plants act as conduits for increased weathering of said minerals. Roots not only increase the connectivity between subsurface geology and the atmosphere, they also secrete substances like organic acids and form symbiotic relationships with cyanobacteria and fungi that accelerate the weather process. No purely tectonic or chemical processes can explain the rate of weathering that must have taken place to see such an increase in these fine grained minerals.

What's more, the presence of rooted plants on land would have ensured that these newly formed muds would have stuck around on the landscape much longer. Whereas in the absence of plants, these sediments would have been washed away into the oceans, plants were suddenly holding onto them. Plant roots act as binders, holding onto soil particles and preventing erosion. Aside from their roots, the rest of these early land plants would have also held onto sediments via a process known as the baffling effect. As water and wind pick up and move sediments, they inevitably become trapped in and around the stems and leaves of plants. Even tiny colonies of liverworts and moss are capable of doing this and entire mats of these would have contributed greatly to not only the formation of these sediments, but their retention as well.

The movement of plants onto land changed the course of history. It was the beginning of massive changes to come and much of that started with the gradual formation of soils. We owe everything to these early botanical pioneers.

Photo Credit: [1]

Further Reading: [1]

The Nitrogen-Fixing Abilities of Cycads

Encephalartos_turneri_-_Koko_Crater_Botanical_Garden_-_IMG_2328.JPG

Long before the first legumes came onto the scene, the early ancestors of Cycads were hard at work fixing atmospheric nitrogen. However, they don't do this on their own. Despite being plentiful in Earth's atmosphere, gaseous nitrogen is not readily available to most forms of life. Only a special subset of organisms are capable of turning gaseous nitrogen into forms usable for life. Some of the first organisms to do this were the cyanobacteria, which has led them down the path towards symbioses with various plants on many occasions. 

Cycads are but one branch of the gymnosperm tree. Their lineage arose at some point between the Carboniferous and Permian eras. Throughout their history it would seem that Cycads have done quite well in poor soils. They owe this success to a partnership they struck up with cyanobacteria. Although it is impossible to say when exactly this happened, all extant cycads we know of today maintain this symbiotic relationship with these tiny prokaryotic organisms. 

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

Cross section of a coralloid cycad root showing the green cyanobacteria inside.

The relationship takes place in Cycad roots. Cycads don't germinate with cyanobacteria in tow. They must acquire them from their immediate environment. To do so, they begin forming specialized structures called precoralloid roots. Unlike other roots that generally grow downwards, these roots grow upwards. They must situate themselves in the upper layer of soil where enough light penetrates for cyanobacteria to photosynthesize.

The cyanobacteria enter into the precoralloid roots through tiny cracks and take up residence. This causes a change in root development. The Cycad then initiates their development into true coralloid roots, which will house the cyanobacteria from that point on. Cycads appear to be in full control of the relationship, dolling out carbohydrates in return for nitrogen depending on the demands of their environment. Coralloid roots can shed and reform throughout the lifetime of the plant. It is quite remarkable to think about how nitrogen-fixing symbiotic relationships between plants and microbes have evolved independently throughout the history of life on this planet.

Photo Credits: [1] [2]

Further Reading: [1] [2]

 

Can Plants "Hear" Running Water?

A recently published study suggests that some plants are capable of using sound to locate water. That's right, sound. Although claims of plants liking or disliking certain types of music still belong in the realm of pseudoscience, this research does suggest that plants may be capable of detecting vibrations in an interpretable way. 

To test this idea, Dr. Monica Gagliano of the University of Western Australia germinated pea seeds in specially designed mazes. Each maze looked like an upside-down Y. At the end of each arm, she devised a series of treatments that would force the pea seedlings to "choose" their desired rooting direction. In some treatments she used standing water coupled with water running through a tube. In others she simply played the sounds of running water against the sound of white noise.

The peas were allowed to grow for five days and afterwards were checked to see which direction their roots were growing. Amazingly, the peas seemed to be able to distinguish the sound of actual running water even when there was no moisture gradient present. Peas given the option of sitting or running water in a tube grew their roots towards the tube a majority of the time. Again, this was in the absence of any sort of water gradient in order to eliminate the chances that the peas were simply honing in on humidity.

Interestingly, plants that were played the sound of running water and white noise through speakers seemed to do what they could to avoid the noise. Although the research did not investigate why the peas had an aversion to the recordings, Dr. Gagliano suspects that it might have something to do with the low frequency magnetic currents emitted by the speakers. Previous research has shown that even weak localized magnetic fields are enough to disrupt the structure of developing root cells. 

All of this taken together paints a fascinating picture of plant sensory capabilities. One should take note, however, that the sample sizes used in this experiment were quite small. More, larger experiments will be needed to fully understand these patterns as well as the mechanisms behind them. Still, these findings shed light on cases in which tree roots seem to be so adept at finding sewer pipes, even in the absence of leaks. It also lends to the findings that the roots of trees such as scrub oaks and box elders will often opt for more stable and reliable sources of ground water over the fluctuating uncertainty of nearby stream sources. Finally, there is something to be said that we share as many as 10 of the 50 genes involved in human hearing with plants.

Our understanding of plant sensory capabilities is really starting to blossom (pun intended). Plants aren't the static, sessile organisms so many make them out to be. They are living, breathing organisms fighting for survival. I, for one, am excited for what new discoveries await. 

Photo Credits: [1] [2]

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]

 

 

Plants May Be Piping Light to Their Roots

Plants just might be piping more than just carbohydrates down to their roots. A study published in Science Signalling offers the first evidence that plants may actually be piping light down underground. No this isn't a metaphor either.

The presence of photoreceptors in the roots has been a bit of a puzzle ever since they were identified. A handful of hypotheses have been put forth in attempt to explain their function. It has been suggested that these photoreceptors are able to sense minuscule amounts of light penetrating through the soil. However, this research suggests there is another mechanism.

A team of researchers based out of South Korea found that certain stem tissues efficiently conducted wavelengths of red light down to the roots. Now before we get too ahead of ourselves, it should be noted that these are minuscule amounts of light. It certainly isn't enough for photosynthesis. However, it is light. Detectors placed under the soil at the ends of roots confirmed that light was indeed being transmitted.

Light is conducted through the tissues in much the same way as fiber optic cables. It is likely that the affinity for red wavelengths in particular has to do with the fact that it can travel farther than other, more intense wavelengths.

By experimenting with gene expression and light exposure, the team was able to demonstrate that light being piped to the roots activates a transcription factor involved in root growth and response to gravity. When the researchers blocked the ability to transmit light they found that root growth was severely stunted. Taken together, these results suggest that not only do roots receive information regarding light conditions above ground, they also directly perceive it.

It should be noted that all of this research was done on a single species, Arabidopsis thaliana. The question remains how common this phenomenon is throughout the plant kingdom. Most plants have photoreceptors in their roots, suggesting this light-piping ability is widespread.

Photo Credit: Dr John Runions/Science Photo Library

Further Reading: [1]

Kin Selection in Plants

Apparently some plants can recognize their relatives...

The plant world is highly competitive. Since they can't move around, plant have gotten quite creative in terms of defense and competition. From brute force to chemical warfare, plants are not the static entities that most write them off as. And, while most of what we see is going on above ground, underground, things get even more crazy.

Recent evidence shows that the sea rocket (Cakile edentula) seems to be able to distinguish between plants that it shares DNA with and plants that it doesn't. According to a study done by researchers at McMaster University in Hamilton, Canada, plants that grow around genetic relatives allocated less to root growth than those around non-relatives. Basically, when planted near a non-relative, the sea rocket will expand its root system to try and get the most out of its surroundings. When planted near a relative, the plant limits this expansion. So what does this mean? Well, they believe that the plants are recognizing their relation to other plants and attempting to limit the amount of competition for nutrients and water to genetically related individuals.

So, is this altruism? Not exactly. According to evolutionary geneticist John Kelly, its more along the lines of reduced antagonism. Sea rockets tend to grow in high disturbance beach habitats and because of their short lifespan they frequently self-pollinate. Their seed capsules also tend to stay on the mother plant and because of this, groups of clones tend to be found within close proximity to each other.

If they were to be as aggressive to their relatives as they would be with non-relatives, then they would be essentially competing with copies of their own DNA. From an evolutionary standpoint, preserving copies of your DNA, even in individuals other than yourself, is a boost to overall fitness. The researchers make it a point to note that, in this study, they were not looking at overall lifetime fitness of the plants in question. They do not know if reduced root mass, in this situation, incurs any positive or negative fitness to individuals overall. It should be noted that studies have shown that, at least in some plant species, reduced root mass seems to incur greater reproductive efforts. It is possible that sea rocket, in the presence of related individuals, can produce more seed.

How do the plants recognize their relation to their neighbors? The mechanism is not known at this point. My guess is that there is some form of chemical signature that the plants can recognize. How this information is processed is another story entirely. More and more we are discovering how complex the botanical world really is. According to the researchers, they feel that this type of relationship is not unique to this species alone. Research like this is opening new doors into uncharted and exciting territory.

Further Reading:
http://plants.usda.gov/java/profile?symbol=caed

http://rsbl.royalsocietypublishing.org/content/3/4/435.full