Characterisation of rapid alkalinisation factors in Physcomitrium patens reveals functional conservation in tip growth.

Small signalling peptides are key molecules for cell-to-cell communications in plants. The cysteine-rich signalling peptide, rapid alkalinisation factors (RALFs) family are involved in diverse developmental and stress responses and have expanded considerably during land plant evolution, implying neofunctionalisations in the RALF family. However, the ancestral roles of RALFs when land plant first acquired them remain unknown. Here, we functionally characterised two of the three RALFs in bryophyte Physcomitrium patens using loss-of-function mutants, overexpressors, as well as fluorescent proteins tagged reporter lines. We showed that PpRALF1 and PpRALF2 have overlapping functions in promoting protonema tip growth and elongation, showing a homologous function as the Arabidopsis RALF1 in promoting root hair tip growth. Although both PpRALFs are secreted to the plasma membrane on which PpRALF1 symmetrically localised, PpRALF2 showed a polarised localisation at the growing tip. Notably, proteolytic cleavage of PpRALF1 is necessary for its function. Our data reveal a possible evolutionary origin of the RALF functions and suggest that functional divergence of RALFs is essential to drive complex morphogenesis and to facilitate other novel processes in land plants.

A PSTAIRE-type cyclin-dependent kinase controls light responses in land plants.

Light is a critical signal perceived by plants to adapt their growth rate and direction. Although many signaling components have been studied, how plants respond to constantly fluctuating light remains underexplored. Here, we showed that in the moss Physcomitrium (Physcomitrella) patens, the PSTAIRE-type cyclin-dependent kinase PpCDKA is dispensable for growth. Instead, PpCDKA and its homolog in Arabidopsis thaliana control light-induced tropisms and chloroplast movements by probably influencing the cytoskeleton organization independently of the cell cycle. In addition, lower PpCDKA kinase activity was required to elicit light responses relative to cell cycle regulation. Thus, our study suggests that plant CDKAs may have been co-opted to control multiple light responses, and owing to the bistable switch properties of PSTAIRE-type CDKs, the noncanonical functions are widely conserved for eukaryotic environmental adaptation.