Role of AtCPK5 and AtCPK6 in the regulation of the plant immune response triggered by rhamnolipids in Arabidopsis

Rhamnolipids (RLs) are a class of glycolipids naturally synthesized by bacteria potentially used in the biocontrol of plant pathogenic microorganisms in agriculture. While RLs trigger an immune response in plants, little is known about the signaling mechanisms involved after RL perception. Calcium Dependent Protein Kinases (CDPKs/CPKs) are a large family of kinases involved in various functions in plants including the regulation of the resistance response to phytopathogens. Here, we investigated the role of AtCPK5 and AtCPK6 in RL-triggered immunity in Arabidopsis thaliana (hereafter referred to as Arabidopsis). Gene expression analysis showed that RLs up-regulate the expression of both genes in Arabidopsis leaves. Using a functional approach, we demonstrated that signaling events exemplified by reactive oxygen species (ROS) production and defense gene expression including AtWRKY46 , AtFRK1 and AtPR1 are increased in cpk5/6 mutant plants compared to wild-type (WT) plants. Moreover, cpk5 mutants displayed intermediary responses while cpk6 mutation alone did not change the immune profile activated by RLs, except for AtFRK1 . Those results suggest that AtCPK5 and AtCPK6 regulate RL-triggered defenses, with AtCPK5 likely playing a more important role. Yet, the cpk5/6 mutations did not affect RL-triggered electrolyte leakage nor enhanced RL-mediated resistance to the bacterial pathogen Pseudomonas syringae pv tomato DC3000. Our results therefore show that additional signaling components regulate AtCPK5 and AtCPK6 -mediated plant immune responses triggered by RLs.