Mutant phenotypes and comprehensive expression analyses reveal roles for CLAVATA in moss vegetative and reproductive development and fertility

The CLAVATA pathway regulates meristem size in angiosperms, but bryophytes have distinct meristematic activities to vascular plants, and gametophytic CLAVATA functions are divergent between species. Here we analysed the promoter activities of all CLAVATA peptide and receptor-encoding genes in the moss Physcomitrium patens, and using mutants, identified requirements for PpCLV1 and PpRPK2 receptors in male and female reproductive development and fertility. In gametophytes, all 12 CLAVATA genes were expressed in foraging filaments (caulonemata) and leaves (phyllids), but most tissues showed highly specific patterns of promoter activity. PpCLE3 expression specifically marked gametophyte shoot (gametophore) apical cells and PpCLV1b and PpRPK2 expression marked overlapping apical domains. Expression in male (antheridia) and female (archegonia and eggs) reproductive tissues led us to use mutants to identify roles for PpCLV1a, PpCLV1b and PpRPK2 in fertility and reproductive development. In sporophytes, the foot was a common site of PpCLE expression, and all genes were expressed in stomata. No PpCLE activity specifically marked the embryonic apical cells, and embryonic PpCLV1b and PpRPK2 expression marked distinct apical and basal domains. Thus, P. patens stem cell activity is likely regulated by different genes in gametophytes and sporophytes, and promoter evolution was a likely driver of diversification of CLAVATA function. Plain language summaryWhilst gene gain and duplication contributed to the origin of land plants and diversification of seed plants, significant gene loss is associated with morphological adaptation in bryophytes. In the moss, Physcomitrium patens, CLAVATA genes expanded from low ancestral numbers, showing exquisite cell type specificity in expression. Our results suggest co-option of CLAVATA into many different developmental processes during moss evolution.