SnRK1 Is Involved In Leaf Pavement Cell Morphogenesis Through Regulation Of ROP GTPase Activity In Arabidopsis

Cell polarity is fundamental to cell development,growth,and morphogenesis.The formation and maintenance of cell polarity play important roles throughout the life cycle of plants.The Intercalary pattern of leaf pavement cells requires harmonized directional outgrowth and invagination of these cells,and represents signal communication between cells for the cooperated cell polar growth.The plant specific Rho-like small GTPase ROPs are highly conserved molecular switches that typically function as "hubs" in signaling networks to control fundamental cellular processes.Evidence suggests that ROP GTPase is important for cytoskeletal reorganization-mediated pavement cell morphogenesis in plants,through an antagonistic interaction between ROP2-RIC4-Microfilament and ROP6-RIC1-Microtubule signaling pathways.The function of ROPs as a signaling switch depends on their activity status.However,the signals or the mechanisms that regulate ROP activity during pavement cell morphogenesis are poorly understood.In this study,a new ROP signaling component(pleiotropic regulatory locus 1)PRL1 was identified by EMS screening.The pavement cell shape was alteraed in PRL1 loss-of-function mutant prll-9,showing wider neck width and decreased lobe density.This observation indicated that the pavement cell polarity was abnormal.Assays of ROP activity demonstrated that the change of pavement cell morphogenesis in prll-9 mutant was attributed to increased activity of ROP2 and simultaneous reduction of ROP6 activity.It was reported that PRL1 is a substrate receptor of the CUL4-DDB1-PRL1 E3 ligase complex that involved in the degradation of SnRKl.The observations in this study found that a loss-of-function mutant cul4cs had a similar phenotype to prll-9 mutant.In addition,the pavement cell shape of SnRK1.1 OE lines was also identical with prll-9 and cul4cs mutants.Furthermore,the activity of ROP2 increased and ROP6 activity decreased simultaneously as well.SnRK1 is a cytoplasmic kinase,homologous with SNF1 in yeast and AMPK in mammal,which involved in intracellular energy homeostasis and responding to sugar signaling.Treatment of sucrose to activate the activity of SnRKl also changed the pavement cell shape.These investigations suggested that CUL4-DDB1-PRL1 E3 ligase complex mediated SnRKl.1 degradation might be involved in the regulation of ROP activity to control pavement cell morphogenesis.The results of yeast two-hybrid,pull-down assay,and Co-immunoprecipitation(Co-IP)experiments showed that SnRKl interacted with RopGAP1,an upstream regulator of ROP GTPases,RopGAPl interacted with ROP2 specifically.These data suggested that SnRK1 may regulate the activities of ROP2 and ROP6 via their upstream regulators,such as phosphosphorylating RopGAPl directly by SnRK1.In summary,this study proposed a PRL1-dependent of SnRK1 degradation pathway,which is involed in regulating pavement cell morphogenesis via adjusting ROP activities.In addition,this finding also implys that the transformation of cell morphogenesis may be a strategy for plant adaptation to the change of energy status and/or under environmental stress.