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 ExoABA 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.

Automated summary

This study aimed to investigate the interaction between Xanthomonas campestris pv. campestris (Xcc) and the plant hormone abscisic acid (ABA) in stomatal movement. The results showed that Xcc infection induced the expression of several immune-related genes and accumulation of phytohormones, leading to stomatal closure. Exogenous ABA application, on the other hand, induced stomatal closure through different mechanisms. These findings are important for understanding the molecular mechanisms underlying the interaction between plants and pathogenic bacteria.