Abstract
Plants use different signalling pathways to respond to external stimuli. Intracellular signalling via calcium-dependent protein kinases (CDPKs) or mitogen-activated protein kinases (MAPKs) present two major pathways that are widely used to react to a changing environment. Both CDPK and MAPK pathways are known to be involved in the signalling of abiotic and biotic stresses in animal, yeast and plant cells. Here, we show the essential function of the CDPK CPK3 (At4g23650) for salt stress acclimation in Arabidopsis thaliana, and test crosstalk between CPK3 and the major salt-stress activated MAPKs MPK4 and MPK6 in the salt stress response. CPK3 kinase activity was induced by salt and other stresses after transient overexpression in Arabidopsis protoplasts, but endogenous CPK3 appeared to be constitutively active in roots and leaves in a strictly Ca2+-dependent manner. cpk3 mutants show a salt-sensitive phenotype comparable with mutants in MAPK pathways. In contrast to animal cells, where crosstalk between Ca2+ and MAPK signalling is well established, CPK3 seems to act independently of those pathways. Salt-induced transcriptional induction of known salt stress-regulated and MAPK-dependent marker genes was not altered, whereas post-translational protein phosphorylation patterns from roots of wild type and cpk3 plants revealed clear differences. A significant portion of CPK3 was found to be associated with the plasma membrane and the vacuole, both depending on its N-terminal myristoylation. An initial proteomic study led to the identification of 28 potential CPK3 targets, predominantly membrane-associated proteins.
Highlights
Plants as sessile organisms must respond to changes in environmental conditions, such as different forms of stress or different light intensities
We show the essential function of the calcium-dependent protein kinases (CDPKs) CPK3 (At4g23650) for salt stress acclimation in Arabidopsis thaliana, and test crosstalk between CPK3 and the major salt-stress activated mitogen-activated protein kinases (MAPKs) MPK4 and MPK6 in the salt stress response
CPK3 kinase activity was induced by salt and other stresses after transient overexpression in Arabidopsis protoplasts, but endogenous CPK3 appeared to be constitutively active in roots and leaves in a strictly Ca2+dependent manner. cpk3 mutants show a salt-sensitive phenotype comparable with mutants in MAPK pathways
Summary
Plants as sessile organisms must respond to changes in environmental conditions, such as different forms of stress or different light intensities. Many extracellular signals elicit changes in the cellular Ca2+ concentrations in plants (Cheng et al, 2002; Sanders et al, 2002; Ludwig et al, 2004) The decoding of these calcium signals is performed by protein kinases, such as the calcium-dependent protein kinases (CDPKs), that mediate cellular responses either directly by changing enzymatic activities via protein phosphorylation or indirectly by changing gene expression patterns (Sathyanarayanan and Poovaiah, 2004). Whereas crosstalk between Ca2+-dependent signalling and MAPK pathways has been elaborately studied in animal systems (Agell et al, 2002; Rozengurt, 2007), it has hardly been addressed in plant signal transduction These signalling pathways mediate changes in gene expression by modifiying the transcriptional machinery (Xiong et al, 2002), leading to altered gene expression patterns. Evidence for other targets of CDPKs, including different metabolic enzymes such as nitrate reductase (NR), sucrose synthase (SuSy) or sucrose phosphate synthase (SPS), has been summarized by Cheng et al (2002), Harper et al (2004) and Harper and Harmon (2005)
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