ABP1 and ROP Mediated Auxin Signaling in Arabidopsis
Auxin is a multi-functional hormone essential for plant development and pattern formation. A nuclear auxin perception and signaling system controlling auxin-induced gene expression has been established. However this nuclear-based auxin signaling cannot explain all the actions of auxin in plant cells. In this dissertation, we have demonstrated a cytoplasmic auxin signaling mechanism that can regulate cell morphogenesis through ROP GTPase signaling.
In the first charpter, we use leaf pavement cells (PCs) as a model system to investigate how auxin modulate the interdigitation growth between adjacent cells. PCs develop interdigitated lobes and indentations to form a puzzle-piece shape and provide a useful system to investigate the spatial coordination of cell expansion among cells in a two-dimensional plane. PC interdigitation is compromised in leaves deficient in either auxin biosynthesis or its export by the auxin transport facilitator PINFORMED 1 (PIN1) which is localized at the lobe tip. Auxin coordinately activates two Rho GTPases, ROP2 and ROP6, which promotes the formation of complementary lobes and indentations across cells, respectively. Activation of both ROP2 and ROP6 by auxin occurs within 30 seconds and depends on AUXIN-BINDING PROTEIN 1 (ABP1). These findings reveal ROP-based novel auxin signaling mechanisms, which modulate the spatial coordination of cell expansion across a field of cells.
However, the mechanism about how auxin activates ROPs is still unknown since ABP1 localized to the outer surface of plasma membrane and cannot directly bind with ROPs. In the second chapter, we reported a family of receptor like kinases (RLK) called TMK that could bind with ABP1 on plasma membrane and activate ROP GTPase in pavement cells. tmk mutant showed similar phenotype and auxin insensitivity as in abp1 mutant. And both ROP2 signaling and ROP6 signaling are not activated in tmk mutant. ABP1 and TMK1 are confirmed in a same complex on PM by co-immunoprecipitation. In the meanwhile, TMK kinase domain can directly bind with N terminal of SPK1, which is a putative ROPGEF that can activate ROP2. Therefore, we concluded that TMK and ABP1 are forming an auxin receptor complex on PM that mediate auxin activation of ROP signaling pathway.