16-Systemic acquired resistance (SAR) prepares infected plants for faster and stronger 17 defense activation upon subsequent attacks. SAR requires an information relay from primary 18 infection to distal tissue and the initiation and maintenance of a self-maintaining 19 phytohormone salicylic acid (SA)-defense loop. 20 -In spatial and temporal resolution we show that calcium-dependent protein kinase CPK5 21 contributes to immunity and SAR. In local basal resistance CPK5 functions upstream of SA-22 synthesis, -perception, and -signaling. In systemic tissue, enhanced CPK5 signaling leads to 23 an accumulation of SAR marker genes including transcription factor Systemic Acquired 24Resistance Deficient 1 (SARD1). 25 -Plants of enhanced CPK5-, but not CPK6-, signaling display a 'super-priming' phenotype of 26 enhanced resistance toward a secondary bacterial infection. In sard1 background, CPK5-27 mediated basal resistance is still mounted but systemic 'super-priming' is lost. 28 -The biochemical analysis determines CPK5 half maximal kinase activity for calcium K50 29[Ca 2+ ] to ~100 nM close to the cytoplasmic resting level. This low activation threshold 30 uniquely qualifies CPK5 to decode subtle changes in calcium prerequisite to immune signal 31 relay and to onset and maintenance of priming at later time points in distal tissue. Our data 32 explain why CPK5 functions as a hub in basal and systemic plant immunity. 33 34

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, CPK5 signaling leads to accumulation of SAR‐inducing metabolite N‐hydroxy‐L‐pipecolic acid (NHP) and SAR marker genes, including Systemic Acquired Resistance Deficient 1 (SARD1) Plants of increased CPK5, but not CPK6, signaling display an ‘enhanced SAR’ phenotype towards a secondary bacterial infection. In the sard1‐1 background, CPK5‐mediated basal resistance is still mounted, but NHP concentration is reduced and enhanced SAR is lost. The biochemical analysis estimated CPK5 half maximal kinase activity for calcium, K50 [Ca2+], to be c. 100 nM, close to the cytoplasmic resting level. This low threshold uniquely qualifies CPK5 to decode subtle changes in calcium, a prerequisite to signal relay and 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.