GENETIC AND FUNCTIONAL CHARACTERIZATION OF CALCIUM/CALMODULIN/ATSR1-MEDIATED SIGNAL PATHWAY IN PLANT DEFENSE RESPONSE
Calcium (Ca2+) is accepted as a predominant second messenger in plant cells. Cytosolic and nuclear free Ca2+ concentration in plant cells changes rapidly and dynamically in response to various endogenous or environmental stimuli. In turn, alterations in the Ca2+ concentration are sensed by Ca2+ binding proteins such as calmodulin, which decode and relay the information encoded by Ca2+ into specific biochemical, cellular and physiological responses. Elevation in calcium concentration in plant cells is an early event during plant defense response and is caused by Ca2+ influx into the cytosol. Our previous research on AtSR1, a calmodulin-binding transcription factor in Arabidopsis, revealed a novel connection between calcium signaling and salicylic acid (SA)-mediated plant immunity. Interestingly, AtSR1 suppresses immune response through the repression of the transcription of EDS1 which encodes a critical regulator of both SA-mediated defense and R gene-mediated defense. The fact that plants are able to achieve effective defense response following pathogen challenge indicates possible mechanisms to relieve the repression of plant immune response by calcium/calmodulin/AtSR1. Here our data revealed that this calcium/calmodulin/AtSR1-mediated suppression could be relieved by the degradation of AtSR1 protein through the ubiquitin-proteasome system. We identified the AtSR1-interaction protein 1 (SR1IP1) through CytoTrap two-hybrid library screening. Genetic analysis showed that the loss-of-function mutant of SR1IP1 is more susceptible to Pseudomonas syringae pv. tomato DC3000 (Pst. DC3000) and overexpression of SR1IP1 confers elevated resistance to Pst. DC3000 treatment, suggesting that SRIIP1 is a positive regulator of plant defense response. SR1IP1 has a BTB domain in its N-terminus and a NPH3 domain in its C-terminal region, and BTB domain is a typical domain structure of the substrate adaptor in cullin3-based E3 ubiquitin ligases. Our biochemical data confirmed that SR1IP1 interacts with Arabidopsis cullin3A (AtCUL3A), and mediates ubiquitination and degradation of AtSR1, especially when challenged with P. syringae. These results reveal a novel mechanism regulating the actions of Ca2+/calmodulin-mediated signaling to coordinate an effective defense response following pathogen challenge.