Physiology of submergence tolerance in rice: A functional study of the rice Sub1 gene

Submergence stress limits yield on 22 million hectares of lowland rice in South and Southeast Asia, demonstrating the need for varieties with improved submergence tolerance (ST). ST may also benefit irrigated systems where controlled flooding suppresses weed growth. The response of rice to submergence depends on flooding regime. In regions where flash-flooding occurs and water recedes within two weeks, ST cultivars predominate. In contrast to deepwater rice, which avoids submergence stress by elongating in response to flooding, ST cultivars exhibit little or no elongation, but have enhanced capacity for tolerance of submergence. ST in rice, previously viewed as a quantitative trait, is now associated with the Sub1 gene. This study, which examines the effect of Sub1 on physiology of ST in rice, represents a considerable improvement in methodology in the examination of flooding tolerance. Previous studies utilized flooding intolerant species or made comparisons between species or cultivars. According to the results of this study, Sub1 alters fermentation, carbohydrate metabolism and growth under anoxia and is associated with reduced submergence-induced shoot elongation. Treatments which increase leaf elongation have no effect on carbohydrate reserves but decrease glutathione reductase activity, increase oxidative stress, and decrease survival. These data indicate a relationship between elongation growth and oxidative stress defense responses and the lack of a relationship between carbohydrate reserves and leaf elongation. This finding suggests that some mechanisms controlled by Sub1 may occur independent of the gene's effect on elongation and indicate the possibility of combining ST with elongation. Sub1 engenders a suite of physiological responses including altered fermentation, carbohydrate metabolism, elongation, and oxidative stress tolerance. Sub1 increases coleoptile growth under anoxia but decreases leaf growth under submergence. Such a diversity of effects indicates that Sub1 encodes a transcription factor, or a protein involved in the perception or transduction of a signal generated by submergence stress. Rice varieties transformed with Sub1 will improve rice production and provide an excellent model system to advance basic understanding of the Sub1 gene and of flooding tolerance in plants. This study provides a framework for ultimately establishing the link between Sub1 structure and function.