Nuclear Signaling of Plant MAPKs

Abstract

Mitogen-activated protein kinases (MAPKs) are conserved protein kinases in eukaryotes that establish signaling modules where MAPK kinase kinases (MAPKKKs) activate MAPK kinases (MAPKKs) which in turn activate MAPKs. In plants, they are involved in the signaling of multiple environmental stresses and developmental programs. MAPKs phosphorylate their substrates and this post-translational modification (PTM) contributes to the regulation of proteins. PTMs may indeed modify the activity, subcellular localization, stability or trans-interactions of modified proteins. Plant MAPKs usually localize to the cytosol and/or nucleus, and in some instances they may also translocate from the cytosol to the nucleus. Upon the detection of environmental changes at the cell surface, MAPKs participate in the signal transduction to the nucleus, allowing an adequate transcriptional reprogramming. The identification of plant MAPK substrates largely contributed to a better understanding of the underlying signaling mechanisms. In this review, we highlight the nuclear signaling of plant MAPKs. We discuss the activation, regulation and activity of plant MAPKs, as well as their nuclear re-localization. We also describe and discuss known nuclear substrates of plant MAPKs in the context of biotic stress, abiotic stress and development and consider future research directions in the field of plant MAPKs.

Keywords: abiotic stress; biotic stress; development; mitogen-activated protein kinase; nucleus; phosphorylation; signaling.

Figure 1

Overview of plant MAPK characteristics and signaling. MAPKs constitute the last tier of MAPK modules which are involved in the signaling of multiple environmental stresses, initially perceived by receptors (R), and developmental programs. MAPKs are activated by the dual phosphorylation of their T-E/D-Y motif in their activation loop by MAPKKs. This activation is reversible and MAPK deactivation occurs via their dephosphorylation by protein phosphatases. The kinetics of MAPK activation is variable depending on the stimulus, for instance in a window of several minutes during MTI, or of several hours during ETI. Once activated, MAPKs phosphorylate their substrates (S) on one or several S/T-P motifs, and besides the mandatory presence of these phosphorylation motifs, several mechanisms contribute to the substrate specificity, such as the presence of docking motifs. Substrate phosphorylation by MAPKs may have different consequences, for example their subcellular relocalization. While a few extensively characterized Arabidopsis MAPKs exhibit multiple subcellular localizations, about one third of studied plant MAPKs seem to localize only/predominantly to the nucleus, and two thirds are present in both the cytosol and nucleus. Interestingly, a few cases of MAPK subcellular relocalization were observed upon different stimuli. In almost all instances, the relocalization corresponded to a MAPK nuclear shuttling. The large majority of bona fide MAPK substrates identified so far are transcription factors (TF) (Table 1), phosphorylated either in the cytosol or nucleus. These phosphorylated TF then contribute to the transcriptional reprogramming, allowing the regulation of important processes in which MAPKs are involved.