Calcium-dependent protein kinase 5 links calcium-signaling with SARD1-dependent immune memory in systemic acquired resistance

Abstract

Summary- Systemic acquired resistance (SAR) prepares infected plants for faster and stronger defense activation upon subsequent attacks. SAR requires an information relay from primary infection to distal tissue and the initiation and maintenance of a self-maintaining phytohormone salicylic acid (SA)-defense loop.- In spatial and temporal resolution we show that calcium-dependent protein kinase CPK5 contributes to immunity and SAR. In local basal resistance CPK5 functions upstream of SA-synthesis, -perception, and -signaling. In systemic tissue, enhanced CPK5 signaling leads to an accumulation of SAR marker genes including transcription factor Systemic Acquired Resistance Deficient 1 (SARD1).- Plants of enhanced CPK5-, but not CPK6-, signaling display a ‘super-priming’ phenotype of enhanced resistance toward a secondary bacterial infection. In sard1 background, CPK5-mediated basal resistance is still mounted but systemic ‘super-priming’ is lost.- The biochemical analysis determines CPK5 half maximal kinase activity for calcium K50 [Ca2+] to ∼100 nM close to the cytoplasmic resting level. This low activation threshold uniquely qualifies CPK5 to decode subtle changes in calcium prerequisite to immune signal relay and to onset and maintenance of priming at later time points in distal tissue. Our data explain why CPK5 functions as a hub in basal and systemic plant immunity.