Abstract

At the genetic level, the evolutionary history of land plants (embryophytes) is frequently the story of redeployment of existing genes for new purposes. A good example of this is the LEAFY transcription factor (LFY), best known from the flowering plant (angiosperm) clade as a master regulator of flowering and floral development. However, angiosperms are only the most recent land plant group to evolve, and LFY predates the evolution of flowering, with homologues present in all land plant clades and in algal relatives. Our understanding of LFY function outside the flowering plants has remained extremely limited due a scarcity of tractable non-angiosperm model species. Most data has been obtained from studies in the model moss, Physcomitrella patens. As a representative of the most basal lineages of non-vascular land plants (the bryophytes), we thus have information regarding the developmental functions of LFY at the two extreme ends of the land plant phylogeny, but not in intermediate lineages (i.e. within the vascular plants). With the enormous evolutionary divergence that this comparison encompasses (approximately 500 million years), our ability to infer the intervening evolutionary changes in LFY function from these two extremes has been limited.

Recent technical advances have now made functional genetic analysis possible in a model species from one of these intermediate lineages- a non-seed vascular plant, the fern Ceratopteris richardii. Expression and functional analysis of endogenous LFY homologues in C. richardii suggest that in ancestral vascular plants LFY functioned outside of the reproductive phase to promote development of apical meristems, in both the sporophyte and gametophyte phases of the land plant life-cycle. This result clarifies our understanding of LFY functions previously reported in P. patens, and of the evolutionary changes in shoot genetic networks that led to the emergence of specialised reproductive apices within the seed-bearing plants (angiosperms and gymnosperms).