Emerging evidences exhibit that mitogen-activated protein kinase (MAPK/MPK) signaling pathways are connected with many aspects of plant development. The complexity of MAPK cascades raises challenges not only to identify the MAPK module in planta but also to define the specific role of an individual module. So far, our knowledge of MAPK signaling has been largely restricted to a small subset of MAPK cascades. Our previous study has characterized an Arabidopsis bushy and dwarf1 (bud1) mutant, in which the MAP Kinase Kinase 7 (MKK7) was constitutively activated, resulting in multiple phenotypic alterations. In this study, we found that MPK3 and MPK6 are the substrates for phosphorylation by MKK7 in planta. Genetic analysis showed that MKK7-MPK6 cascade is specifically responsible for the regulation of shoot branching, hypocotyl gravitropism, filament elongation, and lateral root formation, while MKK7-MPK3 cascade is mainly involved in leaf morphology. We further demonstrated that the MKK7-MPK6 cascade controls shoot branching by phosphorylating Ser 337 on PIN1, which affects the basal localization of PIN1 in xylem parenchyma cells and polar auxin transport in the primary stem. Our results not only specify the functions of the MKK7-MPK6 cascade but also reveal a novel mechanism for PIN1 phosphorylation, establishing a molecular link between the MAPK cascade and auxin-regulated plant development.

Signaling mediated by cell surface receptor kinases is central to the coordination of growth patterns during organogenesis. Receptor kinase signaling is in part controlled through endocytosis and subcellular distribution of the respective receptor kinase. For the majority of plant cell surface receptors, the underlying trafficking mechanisms are not characterized. In Arabidopsis, tissue morphogenesis requires the atypical receptor kinase STRUBBELIG (SUB). Here, we studied the endocytic mechanism of SUB. Our data revealed that a functional SUB–enhanced green fluorescent protein (EGFP) fusion is ubiquitinated in vivo. We further showed that plasma membrane-bound SUB:EGFP becomes internalized in a clathrin-dependent fashion. We also found that SUB:EGFP associates with the trans-Golgi network and accumulates in multivesicular bodies and the vacuole. Co-immunoprecipitation experiments revealed that SUB:EGFP and clathrin are present within the same protein complex. Our genetic analysis showed that SUB and CLATHRIN HEAVY CHAIN (CHC) 2 regulate root hair patterning. By contrast, genetic reduction of CHC activity ameliorates the floral defects of sub mutants. Taken together, the data indicate that SUB undergoes clathrin-mediated endocytosis, that this process does not rely on stimulation of SUB signaling by an exogenous agent, and that SUB genetically interacts with clathrin-dependent pathways in a tissue-specific manner.