Chloroplast Hibernation-Promoting Factor PSRP1 Prevents Ribosome Degradation Under Darkness Independently of 100S Dimer Formation.

Ribosome hibernation is a conserved translational stress response in bacteria, regulated by the hibernation-promoting factor (HPF). Plastid-specific ribosomal protein 1 (PSRP1) is the chloroplast ortholog of bacterial HPF. Although bacterial HPFs have been extensively characterized, both structurally and mechanistically, the physiological roles and mechanisms of PSRP1 in plant chloroplasts remain unclear. Here, we aimed to clarify the role of PSRP1 in chloroplast ribosome hibernation by examining its function under dark-stress conditions in the moss Physcomitrium patens. The PSRP1 knockout mutant exhibited moderate but statistically significant growth defects under both long- and short-day conditions compared to those of the wild-type plants. Moreover, the mutant displayed pronounced growth delay when co-cultured with wild-type plants, indicating a competitive disadvantage. Under dark conditions, wild-type plants exhibit increased PSRP1 protein accumulation, whereas the knockout mutant displayed reduction in chloroplast rRNA content. Notably, although PSRP1 is capable of inducing 100S dimers, we detected no chloroplast 100S dimers either in vivo or in vitro, suggesting a chloroplast-specific ribosome protection mechanism distinct from that of bacteria. These findings reveal PSRP1-mediated chloroplast ribosome protection and could provide new insights into plant stress tolerance.