I am fortunate that among the first impressions of my day I take in this view of land plants. Given my proclivity for microbiology my mind inevitably wonders to what I cannot see, the underground network of filamentous fungi connecting the roots of those plants, the mycorrhizae. Whether you are new to mycorrhizae or a longtime aficionado, I heartily recommend you read (or re-read) Elio's marvelous mycorrhiza primer. In short, nearly all terrestrial plants develop close associations between their roots and mycorrhizal fungi. In so doing the plants are aided in their acquisition of various nutrients, including phosphate, sulfur, nitrogen, and water. In exchange, the fungi obtain organic compounds for their growth. The most abundant of these fungi are the endomycorrhizal arbuscular fungi, so named because they enter the plant roots and therein form highly membranous structures that appear as miniature trees (arbuscles).
It is difficult to imagine that terrestrial landscapes on Earth were completely devoid of land plants until some 450 million years ago, when plant "terrestrialization" occurred. The prevailing hypothesis on this evolutionary milestone is that mycorrhizae were there all along, helping plants adapt to desiccation, temperature fluctuations, etc. Much of the evidence supporting this idea came from the fossil record. So, the question remained as to whether arbuscular mycorrhiza co-evolved with land plants all along. A paper published in 2021 provided excellent evidence that this was indeed the case.
Before summarizing the results, a quick reminder of land plant evolution. All land plants evolved from algae. The earliest land plants had no vasculature, and their present-day descendants, the bryophytes (mosses, liverworts, and hornworts) remain so. Vascular plants split from bryophytes and evolved (sequentially) into ferns, gymnosperms (conifers) and angiosperms (flowering plants).
Prior to this most recent work, investigators identified numerous genes in flowering plants required for the arbuscular mycorrhizal symbioses. One key transcriptional regulator is WRINKLED. In the new study, the authors identified the WRINKLED homolog in the model bryophyte, Marchantia paleacea. They studied the genes controlled by WRINKLED in the bryophyte and found much similarity with this regulon in flowering plants. In addition, because the mycorrhizal fungus they studied (Rhizophagus irregularis) is a fatty acid auxotroph, they could show that fatty acid synthesized by the plant are provided to the fungus. This is in fact, a trait shared by all land plants. The authors conclude: "The fact that the same trait is regulated by orthologous genetic pathways in Bryophytes and in angiosperms indicates its conservation across land plants, covering 450 million years of diversification and dispersion. These results validate the long-standing proposed hypothesis that the mutualistic arbuscular mycorrhizal symbiosis was one of the traits that evolved in the first land plants, enabling terrestrialization."
Next time you come across a land plant be mindful of its connectedness to mycorrhizae and of the significance of this long and intimate relationship. It's no wonder Elio closed his post with these wise words about mycorrhizae: "I honestly believe that all literate people should be mindful of them, and that this knowledge will help safeguard them from threats due to the despoiling of our environment."
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