Mycorrhizae is the union of plant root and fungal mycelia. Joined together, they continually bargain resources as plants offer sugar from photosynthesis, and the fungi trade scant soil nutrients like nitrogen and phosphorus. The mycelial surface area with its adept extracellular digestion enhance its ability to find and mobilize soil nutrients that many plants otherwise could not access. Now there are plenty of specialized plants that have evolved adaptations to tap into their environment’s resource pool without the need of mycorrhizae, but the majority of plants today do partake in this mutualism. As soon as I learned that 90% of all plants form mycorrhizal relationships, I knew I was in the right field. 90% sounds crazy though. Was this mutualism a newer adaptation that spread quickly throughout the plant kingdom because it greatly enhanced their fitness? Or is it an ancient, conserved symbiosis that the first land plants engaged in when they started colonizing terrestrial ecosystems 460 million years ago? Because so many plant species form mycorrhizae, I hypothesize the latter; an ancient mutualism.
I recently found out that the oldest root like structures evolved to access nutrients and water, but not directly. Since fungi were already proficient in acquiring nutrients it makes sense that given enough time, they paired up with an autotroph that needed some help. Photosynthesis is quite the chemical and physical feat as organisms on Earth harness the energy from our nearest star. When these two paired together, they greatly enhanced their fitness and ultimately changed the ecological trajectory of life on Earth.
Plant roots have come a long way from the prehistoric non-vascular subterranean stems called rhizomes. In this review paper, botanists and mycologists alike theorize that roots evolved for two reason. Thick rhizomes structurally supported aboveground mass, and thin rhizomes housed mycorrhizal fungi. Evolutionarily speaking, plants could not make the jump from a single cell living in solution, to a multicellular, resource acquiring machine overnight. They depended on the glomeromycota to forage for soil resources early on, which is why nearly all land plants have conserved this symbiosis.
From Brundrett 2002
The first and most prehistoric mycorrhizal fungus are the glomeromycota. These fungi don’t form spore bearing mushrooms like we’re used to seeing. Instead, they reproduce asexually to form long lasting soil born glomerospores. This division of fungi form the most intimate relationships with plants called endomycorrhizae. Endomycorrhizae exchange materials through arbuscules; branch-like structures that form within the cells of the outer root. Recently, a fossil of a glomerospore was found from a region of rock formed during the Ordovician. It was this exact time 460 million years ago that land plant appeared on Earth. Coincidence? I think not. Plant roots became elongated, highly branched plant appendages that ultimately increased the surface area where arbuscular mycorrhizal fungi (AMF) could engage in and trade resources.
Pictures of AMF structures I took
AMF morphology- Bonfante & Genre 2010
Soon enough, plant roots that followed this trajectory became so highly branched, they acquired the ability to inhabit the forest floor alone without paying a sugar reward to a fungal counterpart. The majority of non-mycorrhizal plants today are of recently evolved lineages. Many non-mycorrhizal plants create fungal inhibitory zones around their roots by releasing noxious chemicals into the soil that reduce spore germination. Though, it’s not all black or white. Facultative plants can engage in mycorrhizae, but do not require them. Whether or not plants facilitate mycorrhizae depends on habitat type. Nutrient poor substrate increases the chance of finding a facultative plant housing AMF.
It would be simple enough to say that land plants evolutionarily traversed from requiring AMF, to being facultative, eventually to become independent from its ancient symbiote, but you know it’s not that simple. The orchids are new to the face of Earth, but their relationship with mycorrhizae is completely obligatory. Mycorrhizae of orchids nearly nurse seeds into germination, and support the plant for months without any sugar rewards. It’s almost like the fungus is investing resources to get paid with sugar at a later date; a relationship that I find hard to evolve. Maybe orchids trick the fungus early on. Starting off parasitic until its photosynthetic machinery starts efficiently generating sugars? I truly don’t know, and look forward to writing a piece on orchid mycorrhizae once I learn more.
Encyclia cordigera, an obligate fungal mutualist found in Costa Rica.
Today, we see that the modern forest floor was set in motion by the earliest land plants and AMF. It’s neat to find evolutionary and ecological trajectories, but they are never a straight line with an end goal. Species compete, coalesce, and parasitize their way to extend their existence. The vast majority go extinct. What we see in nature are the descendants of organisms that made it. We also get to see the plasticity of their ancestor’s genes, in a cutthroat concerto called life on Earth.