ABSTRACT
During land plant evolution, determinate spore-bearing axes (retained in extant bryophytes such as mosses) were progressively transformed into indeterminate branching shoots with specialized reproductive axes that form flowers. The LEAFY transcription factor, which is required for the first zygotic cell division in mosses and primarily for floral meristem identity in flowering plants, may have facilitated developmental innovations during these transitions. Mapping the LEAFY evolutionary trajectory has been challenging, however, because there is no functional overlap between mosses and flowering plants, and no functional data from intervening lineages. Here, we report a transgenic analysis in the fern Ceratopteris richardii that reveals a role for LEAFY in maintaining cell divisions in the apical stem cells of both haploid and diploid phases of the lifecycle. These results support an evolutionary trajectory in which an ancestral LEAFY module that promotes cell proliferation was progressively co-opted, adapted and specialized as novel shoot developmental contexts emerged.
AUTHOR CONTRIBUTIONS
VSD cloned the CrLFY coding sequences and made the RNAi constructs during a sabbatical visit to the University of Oxford; ARP and EHR performed transformations in Ceratopteris richardii and EHR maintained T0 transgenic plants; ARP cloned the CrLFY1 promoter, made GUS reporter constructs, validated transgenic reporter lines, conducted GUS staining, and performed gel blot analysis of CrLFY copy number; SJC and KDHH screened, validated and characterized the RNAi lines; KDHH performed ontogenetic gene expression analysis; VSD performed statistical analyses; SJC conducted in situ localization experiments; JAL performed the phylogenetic analysis; JAL & ARP wrote the first draft of the paper, all authors contributed to the final version.
ACKNOWLEDGEMENTS
Work in JAL’s lab was funded by an ERC Advanced Investigator Grant (EDIP) and by the Gatsby Charitable Foundation. VSD’s visit to JAL’s lab was funded in part by NSF/EDEN IOS # 0955517. Work in VSD’s lab was funded by the Royalty Research Fund and Bridge Funding Program, University of Washington. We thank Brittany Dean for her contribution to transgenic line screening and validation.
Footnotes
COMPETING INTERESTS The authors declare that they have no competing interests
