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The Salt Overly Sensitive(SOS)pathway regulates intracellular sodium ion(Na+)homeostasis and salt tolerance in plants.The mechanisms that control inhibition of the SOS pathway when plants grow in the absence of salt stress have remained elusive.In this study,we demonstrate that the 14-3-3 proteins,λ and κ,interact with SOS2 and repress its kinase activity.Growth in the presence of salt decreased the interaction between SOS2 and the 14-3-3 proteins leading to kinase activation in planta.14-3-3 λ interacts with the SOS2 junction domain,which has previously been shown to be important for its kinase activity.A phosphorylation site(Ser294)was identified within this domain by mass spectrometry.Mutation of Ser294 to Alanine(A)or Aspartate(D)did not affect SOS2 kinase activity in the absence of 14-3-3s.However,the S/A mutation decreased the 14-3-3-dependent inhibition of SOS2 activity,while the S/D mutation increased this inhibitory effect.These results identify 14-3-3 proteins as important regulators of salt tolerance.The phosphorylation-dependent inhibition mediated by the binding of 14-3-3 proteins to the SOS2 regulatory domain is a novel mechanism that confers basal repression of the SOS pathway in the absence of salt stress.