Land Plants Get By With a Little Help from Their Friends

Written by: El Hebert '24
Edited by: Megan List '24

All over the Earth, root systems like this one (belonging to a black locust) intertwine with helpful fungi. Ninjatacoshell, Wikimedia Commons.

In the pitch-black world beneath the soil, a hidden alliance is at work: nearly 80% of all land plants mingle their root systems with fungi, sharing resources. This intimate partnership equips the plants with the tools of survival, and likely even helped them make the first leap from sea to land.

The associated fungi are known as mycorrhizae, literally meaning “fungus-roots,” since they fan out underground like an extension of the plant. Their own bodies are pale and threadlike. Some species cling to the tips of roots, but the most common family - the arbuscular mycorrhizae - actually penetrate among the root cells [1]. 

The image of a fungus colonizing one’s tissues may evoke thoughts of a zombie-apocalypse scenario for us humans, but plants derive substantial benefits from their relationship with fungi. They supply the fungi with fatty acids and the sugary products of photosynthesis, and in exchange, the fungi support the soil, seek out new nutrients, and soften the stressors of the environment.

For example, all plants need phosphorus to grow, but they tend to exhaust local supplies quickly. Far-reaching mycorrhizae slurp up phosphorus and pass it back to the host plant. Together, the pair can survive in much more hostile soils [2].

Mycorrhizae protect their hosts from excessive salinity [3], heavy metal poisoning [4], and parasitic worms [5]. Their networks often link multiple plants together, allowing them to pipe nutrients and signal chemicals back and forth. Through this “wood-wide web,” plants redistribute resources and even communicate. Studies on beans, tomatoes, and Douglas firs have all demonstrated how an individual warns its fungus-linked neighbors of a threatening pest, causing them to mount chemical defences - even when the above-ground portions of the plants were sealed in plastic bags [6]. In dense ecosystems, mycorrhizae may act like the cables of the Internet, alive with the signs of competition and collaboration.

The close tie between land plants and their fungi is an ancient one. Some mycorrhizae emerged more recently, jumping on board with plants as they began to define ecosystems. But the arbuscular family evolved its unique lifestyle nearly 400 million years ago, and stuck to it ever since [1]. Plants themselves only colonized the land 450 million years ago. In fact, the oldest recognizable plant fossils sport root structures that look a lot like fungal joining points.

Earlier this year, researchers from the CNRS and the Université de Toulouse III found compelling evidence that plants partnered with fungi almost as soon as they left the water. The researchers monitored protein manufacturing in liverworts - a primitive, moss-like plant - as they encountered and joined an arbuscular fungus. Just as observed in flowering plants, these liverwort roots lit up with activity, producing new proteins essential to plant-fungus relations. One in particular, MpaWRI, helps deliver fatty acids to the fungus. In fact, mutant liverworts that lack the corresponding gene can’t link with their fungal partners.

Closely related genes are present in flowering plants too, but not in algae. The researchers concluded that MpaWRI must have evolved in the last common ancestor between liverworts and those distant cousins: the earliest land plant. Each of that family’s descendants - every tree, flower and food crop - comes with a program for the mycorrhizal relationship. Perhaps they first reached for help from fungi in response to the new demands of the dry seashore [7].

And, of course, who followed them but our very own fishy ancestors? Searching for food and shelter, they climbed into a world already teeming with life, linked together beneath their newly-evolved feet.

References

[1] Egerton-Warburton LM, Querejeta JI, Finkelman SL, Allen MF. Mycorrhizal fungi. In: Hillel D, editor. Encyclopedia of Soils in the Environment. Elsevier; 2005 [cited 2021 Jul 9]. p. 533-542. DOI: 10.1016/B0-12-348530-4/00455-0.

[2] Ceasar SA. Regulation of low phosphate stress in plants. In: Durgesh KT, Vijay PS, Devendra KC, et al, editors. Plant Life Under Changing Environment. Academic Press; 2020 [cited 2021 Jul 9]. p. 123-156. DOI: 10.1016/B978-0-12-818204-8.00007-2.

[3] Tofighi C, Khavari-Nejad RA, Najafi F, Razavi K, Rejali F. Brassinosteroid (BR) and arbuscular mycorrhizal (AM) fungi alleviate salinity in wheat. Journal of Plant Nutrition. 2017 May 9 [cited 2021 Jul 9];40(8):1091-8. DOI: 10.1080/01904167.2016.1263332.

[4] Yasmeen T, Airong L, Shahid I, et al. Biotechnological Tools in the Remediation of Cadmium Toxicity. In: Hasanuzzaman M, Prasad MNV, Nahar K, et al, editors. Cadmium Tolerance in Plants. Academic Press; 2019 [cited 2021 Jul 9]. p. 497-520. DOI: 10.1016/B978-0-12-815794-7.00018-7.

[5] Solanki K, Kashyap PL, Kumari B, et al. Mycorrhizal fungi and its importance in plant health amelioration. In: Solanki MK, Kashyap PL, Ansari RA, Kumari B, editors. Microbiomes and Plant Health. Academic Press; 2021 [cited 2021 Jul 9]; p. 205-223. DOI: 10.1016/B978-0-12-819715-8.00006-9.

[6] Gilbert L, Johnson D. Plant–Plant Communication Through Common Mycorrhizal Networks. In: Becard G, editor. Advances in Botanical Research. Academic Press; 2017 [cited 2021 Jul 9];82. p. 83-97. DOI: 10.1016/bs.abr.2016.09.001.

[7] Rich MK, Vigneron N, Libourel C, Keller J, Xue L, Hajheidari M, Radhakrishnan GV, Le Ru A, Diop SI, Potente G, Conti E. Lipid exchanges drove the evolution of mutualism during plant terrestrialization. Science. 2021 May 21 [cited 2021 Jul 9];372(6544):864-8. DOI: 10.1126/science.abg0929.