The phytohormone auxin plays a central role in shaping plant growth and development.With decades of genetic and biochemical studies,numerous core molecular components and their networks,underlying auxin biosynthesis,transport,and signaling,have been identified.Notably,protein phosphorylation,catalyzed by kinases and oppositely hydrolyzed by phosphatases,has been emerging to be a crucial type of post-translational modification,regulating physiological and developmental auxin output at all levels.In this review,we comprehensively discuss earlier and recent advances in our understanding of genetics,biochemistry,and cell biology of the kinases and phosphatases participating in auxin action.We provide insights into the mechanisms by which reversible protein phosphorylation defines developmental auxin responses,discuss current challenges,and provide our perspectives on future directions involving the integration of the control of protein phosphorylation into the molecular auxin network.