Mycorrhiza-like Associations In Early Land Plants

Life is moving a little too fast for me right now, so I’ve been a little slow at posting here.  At least I feel like I have been spending my time in the laboratory, which is probably where my focus should lie anyway.  That being said, I did get caught up on reading some newer papers this weekend.This paper by Humphreys et alMutualistic mycorrhiza-like symbiosis in the most ancient group of land plants” is an article in Nature’s fairly new Nature Communications electronic journal and was published online on November 2nd of this year.  This paper is significant because it attempts to experimentally address hypotheses of ecological selection for symbiotic associations of early land plants and the fungi these plants harbor on their roots.  Through both fossil evidence and genetic clock estimations it is believed that these symbiotic associations were present 400 million years ago when the first plants made it onto land.  These hypotheses of early plant and fungal symbiosis were introduced more than 35 years ago in a classic paper by Pirozynski & Malloch in the journal Biosystems (“The origin of land plants: a matter of mycotropism”).  The authors address these hypotheses by using a system of studying a complex thalloid liverwort, Marchantia paleacea (image link), believed to be a member of an ancient extant basal clade of plants, and members of the fungal phyla Glomeromycota (Glomus group Ab).  Because these small liverworts have limited root mass, the use of mycorrhizal fungi to acquire nitrogen and phosphorus could be extremely beneficial in the early colonization of land.

Using a comparison of mycorrhizal and non-mycorrhizal liverwort plants at ambient and elevated CO2, the research group showed that mycorrhizal liverworts exhibited an increase in host plant photosynthesis, growth, and fitness (in the case of liverworts, more asexual propagules).  This increase could be attributable to an increase in nitrogen, and most notably, phosphorus.  These findings have been shown in both arbuscular and ectomycorrhizal plants in previous research, but this is the first example of finding this phenomenon in “early” plants, as well as elevated levels of CO2 (1500 ppb) believed to represent the atmosphere of the Paleozoic.  The researchers propose that the elevated atmospheric CO2 during the Paleozoic actually amplified the benefits from mycorrhizal associations, notably the acquisition of phosphorus, at a time when land was colonized by early plants.