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Ca2+, cAMP, and transduction of non-self perception during plant immune responses

Ca2+ influx is an early signal initiating cytosolic immune responses to pathogen perception in plant cells; molecular components linking pathogen recognition to Ca2+ influx are not delineated. Work presented here provides insights into this biological system of non-self recognition and response activation. We have recently identified a cyclic nucleotide-activated ion channel as facilitating the Ca2+ flux that initiates immune signaling in the plant cell cytosol. Work in this report shows that elevation of cAMP is a key player in this signaling cascade. We show that cytosolic Ca2+ elevation, nitric oxide (NO) and reactive oxygen species generation, as well as immune signaling, lead to a hypersensitive response upon application of pathogens and/or conserved molecules that are components of microbes and are all dependent on cAMP generation. Exogenous cAMP leads to Ca2+ channel-dependent cytosolic Ca2+ elevation, NO generation, and defense response gene expression in the absence of the non-self pathogen signal. Inoculation of leaves with a bacterial pathogen leads to cAMP elevation coordinated with Ca2+ rise. cAMP acts as a secondary messenger in plants; however, no specific protein has been heretofore identified as activated by cAMP in a manner associated with a signaling cascade in plants, as we report here. Our linkage of cAMP elevation in pathogen-inoculated plant leaves to Ca2+ channels and immune signaling downstream from cytosolic Ca2+ elevation provides a model for how non-self detection can be transduced to initiate the cascade of events in the cell cytosol that orchestrate pathogen defense responses.