(CS-29-2) The dual roles of a fern EXCESS MICROSPOROCYTES1: suppression of sporangium formation during sporophyte development and promotion of antheridium in the gametophyte
The timing of reproductive transition is crucial for species survival in land plants. At the cell level, the leucine-rich repeat receptor-like kinase EXCESS MICROSPOROCYTES1 (EXS/EMS1) functions to define the reproductive and somatic cell boundary. Loss of EMS1 function in Arabidopsis, or its homologs in maize and rice, results in supernumerary microsporocytes (male) and defective tapetum which renders pollen inviable. In addition, this receptor kinase also affects macrosporogenesis (female) in rice. The role of EMS1 in gametogenesis is not possible to observe in Arabidopsis due to pollen lethality. Unlike flowering plants whose sporogenesis and gametogenesis are coupled, the ferns partition the two processes in two free-living generations, the sporophyte and the gametophyte. Ferns do not flower; spores are produced on the reproductive leaves. The biology of ferns offers an excellent system to dissect the function of EMS1 in the reproduction transition in both sporogenesis and gametogenesis, which informs our understanding of vascular plant reproductive transition. Like the majority of ferns, the model fern Ceratopteris is homosporous (produces one kind of spore). Once germinated, a Ceratopteris spore can develop into a hermaphrodite or male. Here, we report the identification of EMS1 homologs (CrEMS1) with concentration on one member of the gene family for expression and functional analyses. We used RT-qPCR, in situ hybridization, and created RNAi suppression lines to discover this CrEMS1 plays a key role in the transition from vegetative growth to reproductive grow in the sporophyte, a role not observed in the flowering plants. In the gametophytes, CrEMS1 regulates the antheridium (male) but not the archegonium (female) development, a role resembling that seen in Arabidopsis microsporogenesis. Analyses of other three homologues in Ceratopteris will shed light on the relationship between the EMS1 gene family evolution and reproductive transition in vascular plants.