A subset of plasma membrane-localized PP2C.D phosphatases negatively regulate SAUR-mediated cell expansion in Arabidopsis

The plant hormone auxin regulates numerous growth and developmental processes throughout the plant life cycle. One major function of auxin in plant growth and development is the regulation of cell expansion. Our previous studies have shown that SMALL AUXIN UP RNA (SAUR) proteins promote auxin-induced cell expansion via an acid growth mechanism. These proteins inhibit the PP2C.D family phosphatases to activate plasma membrane (PM) H⁺-ATPases and thereby promote cell expansion. However, the functions of individual PP2C.D phosphatases are poorly understood. Here, we investigated PP2C.D-mediated control of cell expansion and other aspects of plant growth and development. The nine PP2C.D family members exhibit distinct subcellular localization patterns. Our genetic findings demonstrate that the three plasma membrane-localized members, PP2C.D2, PP2C.D5, and PP2C.D6, are the major regulators of cell expansion. These phosphatases physically interact with SAUR19 and PM H⁺-ATPases, and inhibit cell expansion by dephosphorylating the penultimate threonine of PM H⁺-ATPases. PP2C.D genes are broadly expressed and are crucial for diverse plant growth and developmental processes, including apical hook development, phototropism, and organ growth. GFP-SAUR19 overexpression suppresses the growth defects conferred by PP2C.D5 overexpression, indicating that SAUR proteins antagonize the growth inhibition conferred by the plasma membrane-localized PP2C.D phosphatases. Auxin and high temperature upregulate the expression of some PP2C.D family members, which may provide an additional layer of regulation to prevent plant overgrowth. Our findings provide novel insights into auxin-induced cell expansion, and provide crucial loss-of-function genetic support for SAUR-PP2C.D regulatory modules controlling key aspects of plant growth.