Non-vascular lower plants like mosses, liverworts and hornworts represent the primitive traits ancient plants had when they were just getting a foothold on land nearly 500 million years ago. A common misconception is to call these lower plants ‘less evolved’. Every living species today, every individual, represents millions of years of compatible genes recombining. All extant species are equally just as evolved. For simplicities sake, I sometimes refer to lower plants as more ancient lineages, but I am referring to the ancestral traits they’re comprised of. Again, evolution doesn’t fix what isn’t broken, so these non-vascular plants, the descendants of some of the first terrestrial pioneers still function in ecosystems today.
These lower plants have the ability to form a symbiosis with arbuscular mycorrhizal fungi (AMF). While aquatic plants flourished in oceans and lakes before they colonized land, they were without fungal symbiotes. Suspended nutrients in the water column become absorbed directly through the plants tissues, so in these watery habitats, roots where irrelevant for nutrient uptake. A filamentous network on a terrestrial habitat is pretty imperative for plants to succeed on land, but true roots and vascular tissue didn’t evolve for about another 60 million years. The early plants that paired with nutrient and water scavenging AMF did better than the others without fungal symbiotes. This ancient interaction shifted the planet’s evolutionary trajectory towards a mycorrhizal planet.
The genes required to allow mycorrhizal interactions are functional in more than 90% of all living plants and are conserved across all terrestrial plant lineages. The 10% of plants that don’t utilize these ‘SYM genes’ live in nutrient rich habitats and/or have very complex root systems and usually are exemplified by more recently evolved plant genera. As plant roots became more complex, some higher fungi transitioned into this symbiotic niche, becoming specialized symbiotes also known as ectomycorrhizal fungi (ECM). Many tree species today form mutualisms with ECM beneath the forest floor, as these fungi provide a series of ecological services. Some of these ECM fungi and higher plants have become even more specialized, forming a type of ectomycorrhizal relationship called ericoid mycorrhizae.
Some new research coming to light shows the adaptability of lower plants and higher fungi. ECM initially evolved to combine with higher plants with complex root systems. This 2018 paper by Jill Kowal and her teamreveals that a higher, ascomycete fungus that specializes in taking part in a highly specialized ericoid mycorrhizal relationship can actually form a long lasting, symbiotic relationship with liverworts and their primitive root-like structures called rhizoids. The fungus Pezoloma ericae and the liverwort Cephalozia bicuspidata were the two species of interest in the study.
These researchers of the forest floor measured the flow of phosphorus (33P) from the fungus to the plant and the carbon (14CO2) from the plant to the fungus to better understand their relationship. Although these simple plants don’t have a large photosynthetic output, this study shows that fungal derived nutrients flow into the liverwort in return for a sugar reward. This is the first instance recorded for an ericoid specialized fungus to form a nutrient symbiosis with a non-vascular plant. Although it is predicted that 20% of liverworts (roughly 1000 species) have associations with ascomycete fungi, this study is the first to show the flow of resources.
Pezoloma ericae forms a nutritional symbiosis with plants within the Ericaceae. Plants within this family are known to inhabit nutrient poor habitats and have had a large area of niche overlap with these lower, leafy liverworts. It is no surprise that a mutualism evolved between the two, since they’ve cohabited the same ecosystems for millions of years. Additionally, the small size of Pezoloma ericae makes it a perfect fit to associate with these less productive lower plants because they don’t require much carbon. Although this fungus has a close evolutionary relationship with these higher vascular plants, it also takes advantage of plants with more ancestral traits.
This liverwort-ascomycete complex enhances not only the fitness of both parties involved, but the function of their present ecosystems as they act as a fungal inoculum for their neighboring ericaceous plants. The nutrient scavenging abilities and low carbon demand for this tiny fungus makes it exceedingly suitable for engaging in this unique liverwort mutualism. This study highlights the adaptability of both organisms, further revealing the importance of taking advantage of services your neighbor can provide. Evolution doesn’t fix what isn’t broken, and when you have an efficient fungal symbiote, the need for more complex roots becomes irrelevant.