The importance of being sticky

The composition of the cell walls of land plants allows them to grow upright and gives them a sturdy structure that is essential for living out of water. This is possible thanks to a complex matrix made of cellulose fibrils, proteins and polysaccharides. One of these polysaccharides is called Xyloglucan, and it sticks cellulose fibrils together in a dynamic way.

For a while scientists thought Xyloglucan was only present in land plant cell walls, and that it was needed to make them stiffer than the algal ones. These two characteristics together would make it part of that mysterious ‘land plant starter pack’ that somehow allowed a group of freshwater green algae (charophyte) to start one of the most important events in the history of life on earth: the colonisation of land. Evolutionary biologists are still working hard on that somehow, and they are probably going to be busy for a while longer.

However, 10 years ago it was discovered that Xyloglucan is actually present in some charophyte algae, the same group from which land plants evolved. This implied some other function for Xyloglucan, not necessarily related to the cell wall strength needed by plants to stand up straight out of water. This is confirmed by the appearance of the seaweed used for this study, which is called Water Silk and floats softly in ponds around the world, making fun of confused evolutionary biologists.

 

The freshwater algae Water Silk, also known as Spirogyra. (Credit: Carnat Joel. CC BY 2.0)

In the same year, it turned out that an Arabidopsis plant (plant scientists’ favourite weed) lacking Xyloglucan was doing fine: smugly healthy and not looking at all like a floppy seaweed on the shore. It seemed that, after all, the presence of Xyloglucan in cell walls was not that essential for land colonisation, and was mostly linked to cell expansion.

In a new Viewpoint paper published in New Phytologist, Luiz-Eduardo Del-Bem builds up on the research published earlier this year by Galloway et al., who found out that Xyloglucan has a role outside the cell wall. They observed that all major land plant groups in fact release it from roots or rhyzoids into the soil, where its stickiness makes it a potent aggregator of soil particles, “suggesting that it is involved in soil formation and its properties”.

 

Detection of xyloglucan (XG) secretion from plant surfaces with XG MAb LM25. (a) Bright field image of Arabidopsis seedling grown on plant agar solid media for 14 d paired with nitrocellulose print of solid media surface (after removal of the seedling) which was then probed with LM25. Bars, 10 mm. (b) Secretion by day‐30 Marchantia polymorpha gemma. Bright field image on agar and immunoprint of agar surface after gemma removal. Arrows indicate corresponding rhizoid tips. T, thallus outlined in print by dashed line. Bars, 1 mm. (c) Whole mount immunolabelling of day‐14 M. polymorpha thallus/rhizoid in situ on agar block with LM25. Blue represents Calcofluor White labelling of cell walls and green represents LM25‐FITC. T, thallus. Bars, 1 mm. (Credit: Galloway et al. 2018 )

Del-Bem tried to put this new finding in the context of land plant evolution. The most widely spread idea is that plants evolved from freshwater seaweeds present in frequently drying pools. Another view states that the ancestors of land plants were already on land, in the form of microscopic algae living in damp soils. According to this theory, modern plants evolved in an environment similar to biological soil crusts, which are communities of living organisms that grow on soil surfaces across the world. Del-Bem argues that you can still find many charophyte algae “growing in soil and other moist substrates, fulfilling numerous important ecological functions as components of biological soil crust”.

In a primordial soil crust, it would have been an advantage for those algae to produce and release a sticky molecule like Xyloglucan, as it would have improved the surroundings by aggregating soil particles around them. Thus Xyloglucan may have paved the way for the evolution of all land plants by modifying soils and creating a favourable environment for colonisation, rather than simply  supporting the plant’s body against gravity.

In summary, a tacky molecule could be challenging crucial concepts in plant evolution. Hearing this kind of news, the evolutionary biologist that is inside me shifts from being confused to being thrilled!

Zoe Nemec Venza

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References:

Del‐Bem, L. (2018), Xyloglucan evolution and the terrestrialization of green plants. New Phytologist. doi: 10.1111/nph.15191


Zoe Nemec Venza

Zoe Nemec Venza is a Sainsbury PhD student at the University of Bristol, in Dr Jill Harrison’s lab, funded by the Gatsby Charitable Foundation. She graduated from her Masters degree at the Universita’ di Pisa, with Prof. Francesco Licausi. Her research interests are mostly located in the broad field of plant development. She is particularly fascinated by how cell identity is established and by how developmental pathways changed during evolution. Zoe is doing an internship at the New Phytologist Trust during summer 2018.