Ethylene—A key regulator of submergence responses in rice

Ethylene is a major regulator of submergence responses in rice (Oryza sativa). This gaseous phytohormone rapidly accumulates in tissues of submerged plants due to physical entrapment and active biosynthesis during the stress, triggering a range of acclimation responses including shoot elongation, adventitious root formation and carbohydrate metabolism. In addition, ethylene coordinates the balance of gibberellic acid (GA) and abscisic acid (ABA) contents, which facilitates GA-promoted elongation of shoots during submergence. Besides cell elongation, the interaction of ethylene with GA and ABA also regulates adventitious root formation, but these developmental processes are modulated by distinct regulatory networks of the three hormones. Deepwater and lowland rice can overcome submergence stress through antithetical modes of responses. Deepwater rice promotes internode elongation during submergence to project photosynthetic organs above the air–water interface. This escape response is mediated by a high rate of ethylene production and sensitivity to the hormone. By contrast, submergence-tolerant lowland rice restricts carbohydrate consumption used for cell division and cell elongation that promotes underwater elongation. This tolerance is the result of limited ethylene production and sensitivity. We propose that response strategies have evolved in rice to survive distinct conditions of submergence.