ABSTRACT
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 phototrophically. 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.
Far-red light (FR) induced sporocarp formation in A. filiculoides. Sporocarps obtained, when crossed, verified species attribution of Netherlands strains but not Iran’s Anzali lagoon. FR-responsive transcripts included CMADS1 MIKCC-homologues and miRNA-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.
Suppression of sexual reproduction in both gametophyte and sporophyte-dominated plant lineages by red light is likely a convergent ecological strategy in open fields as the active control networks in the different lineages differ. MIKCC transcription factor control of flowering and flower organ specification, however, likely originated from the diploid to haploid phase transition in the homosporous common ancestor of ferns and seed plants.
Competing Interest Statement
The authors have declared no competing interest.
Footnotes
Material distribution footnote: The author responsible for distribution of materials integral to the findings presented in this article in accordance with the policy described in the Instructions for Authors (www.plantcell.org) is: Henriette Schluepmann (h.schlupmann{at}uu.nl).
Abstract and Title: were re-written to emphasize the potential relevance of the work to researchers of sexual reproduction in seed plants. Introduction: The section introducing the symbiosis as a model for high CO2 and N2 fixation has been re-written. CSLM-images of the shoot apical meristem with the N. azollae apical colony are added to emphasize the potential role of the symbiont in controlling meristem identity in the sporophyte shoot apex. None of the figures were altered.