The Arabidopsis PM19L1 Protein Functions as a Regulator of Germination Under Osmotic Stress

How plants perceive and respond to water availability, especially during the critical stages of seed formation and germination, is key to their survival. During development, ripening, and germination, seeds undergo large changes in water content, down to around 10% during maturation and up to 90% again within 24 h of germination. However, the mechanisms by which plants perceive and respond to their osmotic environment remain largely unknown. The results presented here indicate that the osmotic environment of the seed is perceived by the PM19L1 protein. We find the Arabidopsis plasma membrane protein PM19L1 is evolutionarily conserved in all land plants, is highly expressed in seeds and seedlings, and regulates germination under osmotic stress, as shown by the reduced germination of the pm19l1 mutant under salt and osmotic stress. The PM19L1 protein structurally resembles the yeast osmosensor Sho1, and expression of PM19L1 in yeast will complement the osmosensitive sho1 mutant, thus PM19L1 can function as an osmosensor. In contrast to the Sho1-mediated mechanisms for osmotic tolerance in yeast, PM19L1 does not control osmolyte levels in plants, but is a regulator of genes governing abscisic acid and gibberellin synthesis, and of transcription factors that mediate the abscisic acid response. In the pm19l1 mutant, expression of genes for ABI3, LEC1, and FUS3, which promote the late maturation of the seed, is downregulated, whereas expression of the ABI4 and ABI5 transcription factors, which confer abscisic acid-dependent inhibition of germination, is upregulated. The role of PM19L1 as an osmosensor in the plant was verified by ectopic expression of PM19L1 which conferred the ability of vegetative plants to respond to imposed osmotic stress by enhanced expression of ABI3, LEC1, and FUS3. This suggests a function for PM19L1 as a factor that integrates information on the osmotic environment to modulate the developmental fate of the seed during development and germination. Analysis of endogenous hormone levels and phenotypes of digenic mutants, for example pm19l1/abi3 and pm19l1/abi4, will help confirm and refine this model. In a further parallel to ShoI osmosensing in yeast, intracellular signaling downstream of PM19L1 in the plant likely involves a MAP kinase signal transduction pathway, as shown by split ubiquitin analysis for protein-protein interactions, and by pull-down assays from plant extracts. The MAP kinase proteins AtMKK2 and AtMKK3 specifically bind to PM19L1, and the atmkk2, and atmkk3 mutants have strikingly similar germination and gene expression phenotypes to pm19l1; however, corroboration of the role of these proteins in the signaling pathway will require further analysis of knockout and gain of function MKK mutants in the pm19l1 background. These results have implications for the study of dormancy, drought, and salinity tolerance in land plants including crops, and may also provide an insight into evolutionary adaptation of plants to a terrestrial environment.