When floodwaters rise, some rice varieties rapidly grow taller to keep from drowning. A new study identifies the key gene responsible for the deepwater rice adaptation - SD1 (SEMIDWARF1) - a variant of which was responsible for the increased grain yield that launched the rice Green Revolution. The ability of this gene to function in such diverse roles in cultivated rice highlights the "intrinsic complexity and molecular plasticity of plant adaptation strategies," the authors say. Deepwater rice varieties can survive months-long periods of inundation in flood-prone areas by rapidly elongating their stems, to keep the plant's leaves above the water's surface. While the plant hormones ethylene and gibberellin (GA) were thought to trigger the meters-long growth spurt, the genetic mechanics and origin of this adaptation have remained elusive. Takeshi Kuroha et al. performed a genome-wide association study using a selection of Asian rice and deepwater rice varieties to identify the factors regulating this submergence-induced growth. They now reveal a novel molecular mechanism they call the "ethylene-gibberellin relay." When submerged, the gaseous plant hormone ethylene accumulates within the plants, which induces the expression of a transcription factor triggering SD1 to increase the production of the GAs - one of which, GA4, promotes rapid stem growth. An evolutionary analysis of various rice species suggests that this modern-day flood prone variety was selectively bred from the wild ancestor of rice in Bangladesh, where seasonal flooding is a regular occurrence. Kuroha et al.'s study indicates that variants of the SD1 gene have been co-opted by humans for rice cultivation in very different systems, when higher yields were sought as part of the Green Revolution, and under heavy monsoon flooding, as evaluated here. Identifying further genetic variation in wild rice could offer useful adaptive solutions, which could be bred into new modern rice varieties, particularly as climate change triggers radical shifts in weather.
###
Journal
Science