Pathogen Xanthomonas campestris-induced immune-related genes integrated with phytohormones are involved in stomatal closure by interactively regulating ROS and Ca2+ signaling in the Xcc–Brassica napus pathosystem

This study aimed to characterize Xanthomonas campestris pv. campestris (Xcc)-induced ABA interaction with immune-related genes in stomatal movement. Xcc-responsive alterations in resistance genes, Ca2+, reactive oxidative species, and phytohormones were compared with those of exogenous ABA application (Exo-ABA) as being linked to stomatal movement. Xcc infection up-regulated botrytis-induced kinase 1 (BIK1) with a significant increase in salicylic acid (SA) and jasmonic acid (JA) levels at the early phase of infection (2 days after post-inoculation; DPI), this treatment also increased TIR-NB-LRR-type R-gene (TAO1) and R-gene–mediated signaling gene (SGT1) expression as well as SA and ABA accumulation during the later phase (7–15 DPI). In contrast, the Exo-ABA treatment did not activate these genes, even though the ABA-receptor gene PYR1 was up-regulated. The accumulation of Ca2+ and H2O2 in Xcc-inoculated leaves resulted from the activation of calcium-dependent protein kinase 5 (CDPK5), SA synthesis and signaling genes (ICS1 and NPR1), and ABA-related genes (NCED3 and MYC2), while in Exo-ABA treated leaves via the activation of H2O2-related genes (NADPH oxidase and OXI1) and calmodulin (CaM) in predominant ABA responses, leading to a depression of SA responses. Furthermore, the expression of immune-responsive mitogen-activated protein kinase 3 (MAPK3) was prominent in Xcc-inoculated leaves, while MAPK9 was more prominently expressed in Exo-ABA-treated leaves. Both Xcc infection and Exo-ABA treatments led to stomatal closure, accompanied by enhanced expression of slow anion channel 1 (SLAC1). These results indicate that Xcc-induced BIK1-SA-JA interaction acts as pattern-triggered immunity (PTI) in the early phase, and then after which TAO1-SA-ABA mediates CDPK5-dependent Ca2+ and H2O2 generation, leading to MAPK3-SLAC1-mediated stomatal closure in the later phase, while in Exo-ABA-treated leaves via ABA-mediated CaM-dependent H2O2 accumulation and stomata closure.