Differential innate immune signalling via Ca2+ sensor protein kinases

Innate immunity represents the first line of inducible defence against microbial infection in plants and animals(1-3). In both kingdoms, recognition of pathogen- or microbe-associated molecular patterns (PAMPs or MAMPs, respectively), such as flagellin, initiates convergent signalling pathways involving mitogen-activated protein kinase ( MAPK) cascades and global transcriptional changes to boost immunity(1-4). Although Ca2+ has long been recognized as an essential and conserved primary mediator in plant defence responses, how Ca2+ signals are sensed and relayed into early MAMP signalling is unknown(5,6). Using a functional genomic screen and genome-wide gene expression profiling, here we show that four calcium-dependent protein kinases (CDPKs) are Ca2+-sensor protein kinases critical for transcriptional reprogramming in plant innate immune signalling. Unexpectedly, CDPKs and MAPK cascades act differentially in four MAMP-mediated regulatory programs to control early genes involved in the synthesis of defence peptides and metabolites, cell wall modifications and redox signalling. Transcriptome profile comparison suggests that CDPKs are the convergence point of signalling triggered by most MAMPs. Double, triple and quadruple cpk mutant plants display progressively diminished oxidative burst and gene activation induced by the 22-amino-acid peptide flg22, as well as compromised pathogen defence. In contrast to negative roles of calmodulin and a calmodulin-activated transcription factor in plant defence(7,8), the present study reveals Ca2+ signalling complexity and demonstrates key positive roles of specific CDPKs in initial MAMP signalling.