Calcium-dependent Protein Kinases, CDPKs

In nature plants are constantly challenged by simultaneous abiotic and biotic stresses, and under conflicting stress scenarios prioritization of stress responses is required for plant survival. Calcium-dependent protein kinase CPK5 is a central hub in local and distal immune signaling, required upstream of hormone salicylic acid (SA)-dependent systemic acquired resistance (SAR). Here we show that CPK5 signaling-dependent immune responses are effectively blocked and pathogen resistance is reverted either upon treatment of plants with abscisic acid (ABA) or in genetic mutant backgrounds lacking PP2C phosphatase activities including abi1-2. Consistently, enhanced immune responses occur upon co-expression of CPK5 kinase with active variants of ABI1 phosphatase ABI1G180S and ABI1G181A. Biochemical studies and mass spectrometry-based phosphosite analysis reveal a direct ABI1 phosphatase-catalyzed de-phosphorylation of CPK5 at T98, a CPK5 auto-phosphorylation site. CPK5T98A, mimicking continuous de-phosphorylation through ABI1, correlates with an increase in kinase activity and CPK5 function in ROS production. CPK5T98D, mimicking a CPK5 auto-phosphorylated status under ABA-induced phosphatase inhibition, leads to inactivated CPK5 causative to an immediate stop of immune responses.Our work reveals an elegant mechanism for plant stress prioritization, where the ABA-dependent phosphatase ABI1, negative regulator of abiotic responses, functions as positive regulator of biotic stress responses, stabilizing CPK5-dependent immune responses in the absence of ABA. This mechanism allows continuous immune signaling during pathogen survey in environmentally non-challenging conditions. Under severe abiotic stress, immune signaling is discontinued via a direct biochemical intersection through a phosphatase/kinase pair recruiting two key regulatory enzymes of these antagonistic signaling pathways.