Inhibition of SIMPK1, SIMPK2, and SIMPK3 Disrupts Defense Signaling Pathways and Enhances Tomato Fruit Susceptibility to Botrytis cinerea

Mitogen-activated protein kinases (MAPKs) are major components of defense signaling pathways that transduce extracellular stimuli into intracellular responses in plants. Our previous study indicated that SIMPK1/2/3 were associated with nitric oxide induced defense response in tomato fruit In this study, we determine whether SIMPK1/2/3 influence the tomato fruit s innate immunity and whether plant hormones and reactive oxygen species (ROS) are involved in SIMPK1/2/3 defense signaling pathways. Treatment with 10 mu M U0126 significantly inhibited the relative expression, of SIMPK1, SIMPK2, and SIMPK3 (P < 0.05). U0126 treated fruit showed higher concentrations of auxin indole acetic acid (IAA), abscisic acid (ABA), and gibberellic acid (GA), but a lower concentration of methyl jasmonate (MeJA). The activities of defense enzymes, including beta-1,3-glucanases (GLU), chitinase (CHI), phenylalanine ammonia lyase (PAL), and polyphenol oxidase (PPO), decreased after U0126 treatment. Meanwhile, H2O2 content increased, and catalase (CAT), ascorbate peroxidase (APX), and peroxidase (POD) activities decreased after U0126 treatment. U0126 treatment enhanced the susceptibility of tomoto fruit to Botrytis cinerea and resulted in more severe gray mold tot. These results demonstrate that inhibition of SIMPK1/2/3 disrupts tomato fruit defense signaling pathways' and enhances the susceptibility to B. cinerea and also that plant hormones and ROS are associated with SIMPK1/2/3 defense signaling pathways.