CARK3 Interacts with OST1 to Regulate Stomatal Immunity in Arabidopsis

The Arabidopsis serine/threonine protein kinase CARK3, also known as PTO-INTERACTING 1-4 (Pti1-4) in Arabidopsis thaliana, is involved in mitogen-activated protein kinase signaling to regulate the immune response. However, the molecular mechanism underlying CARK3's function in these processes remains poorly characterized. Here, we found that CARK3 physically interacted with and phosphorylated OST1 in vitro, and this interaction was dependent on CARK3's kinase activity, but not on OST1's kinase activity. Overexpression of CARK3 in Arabidopsis enhanced resistance to Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) by promoting stomatal closure, increasing reactive oxygen species accumulation, upregulating immunity-related gene expression, and activating phosphorylation of MPK3 and MPK6. In contrast, loss-of-function CARK3 mutants reduced resistance to Pst DC3000 compared with wild-type Col-0 plants. Overexpression of OST1 in cark3-4 mutants partially rescued the susceptibility of cark3-4 to Pst DC3000 infection. Collectively, our results establish critical evidence for the role of the CARK3-OST1 regulatory module in Arabidopsis immune responses.