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Description
Soluble nitrogen is necessary for the growth of all oxygenic photoautotrophs (cyanobacteria, algae, plants), and today Nitrogen gas (N2) is fixed from the atmosphere by prokaryotes into the soil, which is piled upon the tallest sheaths of the Earth’s rocky crust below. Life had colonized the land by approximately 500 million years ago and it is likely that plants were among the first terrestrial taxa. Despite evolving traits for land-living such as heat tolerance and desiccation resistance, plants never evolved to withstand nitrogen limitation. Early Earth soils were not saturated with soluble nitrogen, such as ammonium, nor were they held to dry land by any root-bearing plants which had yet to evolve. Today, nitrogen-fixing cyanobacteria of the order Nostocales (Nostoc), such as Nostoc punctiforme, is found in mutual symbioses among the most ancient lineages of plants, including the non-vascular Bryophyta. Here I propose the hypothesis that the ancestors of the nitrogen-fixing cyanobacteria, Nostocales, likely played a vital role in the ability of plants (Embryophyta) to colonize land. Methods for building evidence in support of this hypothesis are explored, and initial data supporting said hypothesis are generated. The bryophytes – Anthoceros punctatus, Blasia pusilla, and Physcomitrium patens, as well as the plant relative Chara braunii, were grown on nitrogen-starved defined media. After nitrogen starvation, half were inoculated with Nostoc punctiforme via controlled colonizations which were confirmed using epifluorescent microscopy. Bryophyte health between nitrogen-starved and Nostoc-inoculated groups, and the symbiotic competency of several strains of Nostocales with the common plant symbiont Anthoceros were accessed using chlorophyll fluorescence. Plant health significantly improved across all inoculated groups. Transcriptomes of Physcomitrium, an unknown Nostoc symbiont, in the presence and absence of Nostoc were generated. Differential gene expression patterns among Physcomitrium treatment groups mirror previous nitrogen-starved bryophyte plus & minus Nostoc transcriptome studies.
