mRNA and smallRNA sequencing of Azolla, its organelles and symbiotic partners during transition to sexual reproduction. mRNA and sRNA of the Azolla symbiosis sexual transition

Azolla ferns and the filamentous cyanobacteria Nostoc azollae constitute a model symbiosis that enabled colonization of the water surface with traits highly desirable for development of more sustainable crops: their floating mats capture CO2 and fixate N2 at high rates using light energy. Their mode of sexual reproduction is heterosporous. Regulation of the transition from vegetative to spore-forming phases in ferns is largely unknown, yet a pre-requisite for Azolla domestication, and of particular interest since ferns represent the sister lineage of seed plants. A. filiculoides fern sporocarp formation required far-red light (FR), was increased by fern density, and inhibited by nitrogen in the medium. Sporocarps induced with FR could be crossed to verify species attribution of strains from the Netherlands but not from the Iranian Anzali lagoon; the latter strain was assigned to a novel species cluster from South America. Red-dominated light suppresses the formation of dissemination stages in both gametophyte and sporophyte-dominated lineages of plants, theresponse likely is a convergent ecological strategy to open fields. FR-responsive transcripts included MIKCC homologues of CMADS1 and miR319-controlled GAMYB transcription factors in the fern, transporters in N. azollae, and ycf2 in chloroplasts. Loci of conserved miRNA in the fern lineage included miR172, yet FR only induced miR529 and miR535, and reduced miR319 and miR159. Phylogenomic analyses of MIKCC transcription factors suggested that control of flowering and flower organ specification may have originated from the diploid to haploid phase transition in the homosporous common ancestor of ferns and seed plants.