The first footholds for land plants may have been much smaller than imagined

A polysaccharide found in the cell walls of land plants has now been found outside the cell. This chemical, Xyloglucan, could be one of clues as to how plants moved onto land.

There are plenty of advantages to life in the water. One is that it’s a lot easier to support a large body in the water than on land. This is as true for plants as it is animals, so how did plants get the strength to thrive on land? Biologist have been looking to a chemical called Xyloglucan for the answer. While cellulose gives the cell walls strength, it’s thought that xyloglucan is a glue that helps organise the cellulose. Luiz-Eduardo Del-Bem of the Universidade Federal de Minas Gerais (UFMG), said: “A vast number of people are interested in xyloglucan. From plant biologists trying to understand what it actually does in plant cell walls and why it evolved, to biotechnologists interested in developing methods to disassemble its monosaccharides to make it fermentable to increase bioethanol production from plant biomass, to the cosmetic industry that uses its sticky properties in antiwrinkle and moisturizing lotions.”

Algae under the microscope
Algae, a microscopic view, by Elif Bayraktar / Getty Images.

So creating this versatile substance could have been the step that plants needed to stand up on land. It’s a great idea, but there’s one problem. Del-Bem explained: “I have found the whole genetic repertoire of enzymes needed to synthesize and degrade xyloglucan in a group of green algae.” If you’re thinking of complex algae flopping by rock pools are the algae he’s talking about, then think again. These algae could be small Del-Bem said: “We have evidence that a particular group of green algae, named charophytes, have all the genes needed to produce xyloglucan. Most of these algae are unicellular or have very simple structure made up of few cells (members of the Class Charophyceae are the only exception), which leads to the conclusion that it probably didn’t evolve as a molecule associated with the strength needed to the upright growth of land plants. There must be something else.”

The reason it doesn’t seem to have a role in upright growth is partly that these charophytes and land plants have a common ancestor, which is likely to be the first land plant. This first land plant isn’t around, but Del-Bem can explain how we can know some of the traits it would have had. “The first lineages of charophytes that eventually gave rise to what we call land plants or embryophytes are not around anymore. We can, however, reconstruct the gene content of land plants first and last common ancestors by comparing the genomes of extant plants and algae. We know since the 1970s that the charophytes are most likely the ancestor of all land plants. So the availability of genomic information of diverse groups of land plants and charophytes makes it possible to perform comparative genomic analyses that give us clues about the nature of the ancestral charophytes that first colonized land environments and most likely gave rise to the embryophytes directly on land.”

So why do plants have xyloglucan? Earlier this year, Galloway and colleagues published an interesting finding. They had studied xyloglucan, but not in the cell wall. They had found that plants were releasing it from their roots, and it was helping the roots aggregate soil particles by sticking them together. Del-Bem, working on charophytes realised this could explain why the algae he studied had the ability to make xyloglucan. “A simple and elegant explanation for the case of xyloglucan in charophytes is that these molecules evolved on terrestrial algae that colonized land environments before the emergence of land plants and the selective pressures are probably related with the soil modification properties of xyloglucan. The evolution of xyloglucan is possibly linked to the adaptation of these algae to survive on land, in direct contact with the substrates, making them able to aggregate soil particles around the cells creating a more favorable microenvironment.”

Once you can aggregate soil, you can start shaping the ground to your advantage. This might have been useful for algae. Del-Bem notes: “One possible explanation is that xyloglucan could have helped in the formation of biological soil crusts where terrestrial algae grow. Later, in land plants, it is probably linked to the creation of spaces around the roots and rhizoids that can help plants establishing its root system and increase the water flux around it.”

If Del-Bem is correct then this has a major impact on how we think of life colonising the land. For a start, both the plants and locations of the colonisation might be a lot different to how biologists imagined. Del-Bem concludes: “I think this research helps us understand the ecological circumstances in which the emergence of the first embryophytes took place. It is likely that the processes happened on land environments rather than in fresh water, as first thought. It also tells us that the first ancestors of the land plants colonized terrestrial environments long before the first true land plants. I think the comparative genomics of land plants and charophytes will give us good evidence of how this lineage was able to overcome the challenges of living on land and helps us understand how terrestrial photosynthetic organisms paved the way for the evolution of complex terrestrial life as we see today. This is an essential part of the history of life on Earth and helps us understand why we humans are here today. Without terrestrial photosynthetic organisms, there would be no primary production on land, and the terrestrial ecosystems that we know today would not exist. This is an essential part of the story of how life conquered the land.”

Reference List

Del-Bem, L-E., & Vincentz, M. G. (2010). Evolution of xyloglucan-related genes in green plants. BMC Evolutionary Biology, 10(1), 341. https://doi.org/10.1186/1471-2148-10-341

Galloway, A. F., Pedersen, M. J., Merry, B., Marcus, S. E., Blacker, J., Benning, L. G., … Knox, J. P. (2017). Xyloglucan is released by plants and promotes soil particle aggregation. New Phytologist, 217(3), 1128–1136. https://doi.org/10.1111/nph.14897

Del-Bem, L.-E. (2018). Xyloglucan evolution and the terrestrialization of green plants. New Phytologist, 219(4), 1150–1153. https://doi.org/10.1111/nph.15191