| An increase in cytoplasmic calcium is one of the common early physiological events following pathogen detection by plants. However, little is known about the specific functions of calcium in defense signaling or the specific calcium sensors detecting these changes. Increasing evidence has suggested that calcium dependent protein kinases (CDPKs) may act as key translators converting calcium signals to changes in protein phosphorylation and gene expression. In this study, I demonstrated that treatment of Arabidopsis mesophyll protoplasts with an elicitor active peptide (flg22) from the general bacterial elicitor flagellin transiently activates endogenous CDPKs. Transient coexpression in protoplasts of a flg22-inducible reporter construct with constitutively active forms of 33 of the 34 Arabidopsis CDPKs identified five highly homologous CDPKs (AtCPK4, 5, 6, 11, and 26) important for flg22-mediated defense responses. Global gene expression analysis using microarrays showed that, while the genes regulated by these proteins were overlapping with one another and with genes regulated by flg22, these genes were distinct from those regulated by a previously-known transcription factor (WRKY29) acting downstream of flg22. Thus, these CDPKs represent a previously unknown branch of the flg22-signaling pathway. Transient expression in protoplasts or stable constitutive expression in transgenic plants of another Arabidopsis CDPK (AtCPK2) provided evidence that this CDPK may also be important for flg22-regulated gene expression. Global gene expression profiling with this CDPK showed that its downstream targets overlap with those of the other defense-related CDPKs, but not with those of WRKY29. Interestingly, despite the redundancy in many of the genes regulated by AtCPK2 and the other defense-related CDPKs, this protein is unique in that (1) it may induce these changes through a different mechanism, and (2) it has an apparent dual function in plant defense and growth and development. I hypothesize that AtCPK2 activation is responsible in part for the growth inhibiting effects of flg22 treatment. This is the first time in plants that GeneChip experiments have been used to probe the relationship between different signaling components downstream of the same receptor, and suggests that such methods could be useful for the analysis of other pathways. |
