Abstract
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.
Highlights
Mitogen-activated protein kinase (MAPK/MPK) cascades play important roles in a broad spectrum of signals, including biotic and abiotic stresses and hormone-mediated development in higher plants [1]
In the MAPK signaling module, MAPK kinases (MKKs) are of particular importance because they serve as the convergence and divergence points in the MAPK signal transduction
Our results demonstrated that MPK6 and MPK3 are two major downstream targets of MAP Kinase Kinase 7 (MKK7)
Summary
Mitogen-activated protein kinase (MAPK/MPK) cascades play important roles in a broad spectrum of signals, including biotic and abiotic stresses and hormone-mediated development in higher plants [1]. The Arabidopsis genome encodes a large number of MAPK cascade components with more than 60 MAPKKKs, 10 MKKs, and 20 MPKs, which participate in regulating many essential biological processes [1,2]. As there are 10 MKKs and 20 MPKs in the Arabidopsis genome, the signaling specificity of the MAPK modules should partially rely on the diversity of the MPKs and their downstream signaling events. The different roles of MPK3 and MPK6 in certain biological events have been recently reported [17,18,19,20,21,22,23], the signaling specificity of the two MPKs in more diverse biological processes remains to be elucidated
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