PtrSAUR32 Interacts with PtrPP2C.Ds to Regulate Root Growth in Citrus

Small auxin-up-regulated RNA (<i>SAUR</i>) genes are involved in the regulation of dynamic and adaptive growth in higher plants. However, their function and mode of action in citrus root growth are still unknown. Here, we demonstrate that in <i>Poncirus trifoliata</i>, <i>PtrSAUR32</i> acted downstream of the auxin response factor PtrARF8 to regulate root growth by interacting with PtrPP2C.Ds, subfamily type 2C protein phosphatases which interacted with H-ATPase and PtrHA. In this study, several members of <i>SAUR</i> family in <i>Poncirus trifoliata</i> are identified to be associated with the growth and development of the roots. Among them, <i>PtrSAUR32</i> was found to be highly expressed in the RT (root tip), and the level of its expression was significantly positively corelated to the length of primary roots (<i>p</i> < 0.01). The overexpression of <i>PtrSAUR32</i> in citrus significantly promoted the growth of primary roots. In <i>PtrSAUR32</i> transgenic citrus plants, the expressions of several auxin biosynthesis and transport genes were altered in accordance with the expression of <i>PtrSAUR32</i>. Y1H and dual-luciferase reporter assays proved that the expression of <i>PtrSUAR32</i> is regulated by PtrARF8. Y2H and BiFC assay results indicated that PtrSAUR32 interacted with PtrPP2C.Ds subfamily members PtrPP2C.D1, PtrPP2C.D6, and PtrPP2C.D7, of which PtrPP2C.D7 could interact with PtrHA in vivo.