The conserved plant PM19 protein functions as an osmosensor and regulator of germination

How plants perceive water, especially during the critical stages of seed formation and germination, is key to their survival. During development and germination, seeds undergo large changes in water content, down to around 10% during maturation and up to 90% again within 24 hours of germination. We find the evolutionary conserved Arabidopsis plasma membrane protein PM19L1 to be an osmosensor, regulating dormancy and germination under osmotic stress. The PM19L1 protein structurally resembles the yeast osmosensor Sho1, with four transmembrane domains, and PM19L1 complements the osmosensitive sho1 mutant. Arabidopsis pm19l1 mutants have enhanced dormancy and reduced germination under salt and osmotic stress, and enhanced ABA levels. In a striking parallel to osmosensing in yeast, signalling downstream of PM19L1 involves a MAP kinase signal transduction pathway. PM19L1 is a positive regulator of ABI3, which promotes the late maturation of the seed, and negatively regulates the ABI4 and ABI5 dormancy- regulating transcription factors. These results have implications for the study of dormancy, drought, and salinity tolerance in crops, and may provide an insight into evolutionary adaptation of plants to a terrestrial environment.