The transcription factor PpRKD evokes female developmental fate in the sexual reproductive organs of Physcomitrium patens.

The sexual reproductive organs of bryophytes - in which gametes necessary for fertilization are produced, namely, male antheridia and female archegonia - are formed from vegetative haploid gametophytes. In dioicous bryophytes such as Marchantia polymorpha, the genes within the sex-determining regions in distinct sexual strains have been identified. However, in monoicous bryophytes such as Physcomitrium patens, how the two sex fates are specified on the same gametophyte remained unknown. Here, we identified an RWP-RK domain-containing transcription factor in P. patens, PpRKD, as a factor required for the development of female organs, based on the absence of archegonia in loss-of-function Pprkd mutants and the specific expression of PpRKD in archegonia. When ectopically induced, the expression of PpRKD resulted in the repression of antheridial development and the emergence of archegonium-like organs. Furthermore, the young primordia inside the antheridial bundle displayed typical archegonial division patterns, suggesting that PpRKD confer female fate to antheridium primordia. This study represents the first instance where the function of sex determination has been identified among RKD orthologs in land plants. This finding should provide a new framework for the molecular evolutionary context of the genes in the RKD family, considering the recent elucidation of their roles in algae.