Prioritization of abiotic and biotic plant stress responses through ABI1 phosphatase and CPK5 calcium-dependent protein kinase switch
Authors: Seybold Heike , Bortlik Jennifer , Jiang Xiyuan , Liese Anja , Conrads Benjamin , Hoehenwarter Wolfgang , Matschi Susanne , Romeis Tina 
Published: November 2019

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 for hormone salicylic acid (SA)-dependent immunity and pathogen resistance. Here we show that CPK5-dependent immune responses and pathogen resistance are inhibited upon abscisic acid (ABA) treatment or in genetic mutant backgrounds lacking PP2C phosphatase activities includingabi1-2, whereas immune responses are enhanced by co-expression of active ABI1 phosphatase variants. Biochemical studies and mass spectrometry-based phospho-site analysis reveal a direct ABI1 phosphatase-catalyzed de-phosphorylation of CPK5 auto-phosphorylation site T98. Mimicking continuous de-phosphorylation in CPK5T98Aleads to enhanced ROS production and more resistant plants, mimicking the auto-phosphorylated status in CPK5T98D, reduces CPK5-mediated immune responses. Mechanistic insight identifies differential phosphorylation at T98 in the N-terminal domain of CPK5 to control the level of interaction between the kinase and its substrate protein rather than CPK5 catalytic activity. Thus, CPK5-catalyzed immune signaling may become discontinued even at an elevated cytoplasmic calcium concentration.

Our work reveals an elegant mechanism for stress response prioritization in plants: The ABA-dependent phosphatase ABI1, negative regulator of abiotic responses, functions as positive regulator of biotic stress responses, stabilizing CPK5-dependent immune signaling in the absence of ABA. Continuous pathogen survey activates plant immunity in environmentally friendly conditions, whereas under severe abiotic stress the phosphatase/kinase pair prohibits immune signaling through a direct biochemical switch involving two key regulatory enzymes of these antagonistic pathways.

Significance Statement

Plants challenged by simultaneous abiotic and biotic stresses must prioritize in conflicting scenarios to guarantee survival. Pathogen resistance and immune memory depends on the phytohormone salicylic acid (SA). Adaptation to abiotic stress signaling involves the phytohormone abscisic acid (ABA). We identify a direct biochemical switch by which ABA-mediated abiotic signaling prioritizes over SA-dependent immune responses via reversible phosphorylation at a single protein mark involving two key regulatory enzymes of these antagonistic pathways. Phosphatase ABI1 de-phosphorylates calcium-dependent protein kinase CPK5 at an auto-phosphorylation site T98, which effects the interaction efficiency between the kinase and its substrate. Under abiotic stress ABA mediates phosphatase inhibition, which facilitates prolonged auto-phosphorylation of CPK5, preventing CPK5 substrate interaction and ultimately stop CPK5-mediated immune signaling.