Regulation of plant immunity systems by phytohormones

Abstract

Various phytohormones play important roles in regulation of induced resistance systems that are important self-protection systems to protect the whole plant body. Systemic acquired resistance (SAR) is one such immune system governed by salicylic acid, which is utilized to control rice blast disease in rice field by exploiting plant activators that stimulate SA-mediated signaling. The action mechanism studies revealed that probenazole (PBZ) stimulates the point upstream of SA biosynthesis, whereas other stimulate the downstream. The effects of plant activators were found to be suppressed by ABA-mediated signaling, which is activated by environmental stresses. Conversely, activation of SA-mediated signaling suppressed ABA signaling. Thus, there is an antagonistic crosstalk between SA-mediated signaling for disease resistance and ABA-mediated environmental stress responses. Whereas the SA-mediated signaling has a growth-inhibiting effect, priming of plant immunity, induced by stimuli of certain chemicals and microbes, has protective effects without growth inhibition. We demonstrated that strigolactone-mediated signaling and interaction with symbionts induced priming in Arabidopsis and tomato, respectively. Further studies from these findings are expected to contribute to the development of effective disease control technology for crops.