A new molecular module that regulates rice grain size and yield is described in three independent papers published online this week in Nature Plants. The three groups take different approaches to show that growth regulating factors (GRFs) are repressed by the small RNA molecule miR396 and that, together, they control both grain size and number-suggesting that they could be targets for significantly increasing crop yield in the future.
Rice yield can be increased in two ways: by increasing the number of flowers or spikelets on the plant and, hence, its number of grains, or by increasing the size of the individual grains of rice.
In one paper, Shaoqing Li and colleagues investigate why hybrid rice has a larger number of spikelets than its parental lines. They find that the expression of miR396 is inhibited in hybrids, and so miR396-mediated repression of the GRF6gene is released.The authors find the resulting increased expression of GRF6 activates the biosynthesis and signalling of a plant hormone that promotes spikelet development.
In the other two studies, Chengcai Chu, Mingfu Zhao and colleagues and Yunhai Li, Xudong Zhu and colleagues report two different forms of GRF4 that both greatly increase grain weight. They find that these versions of GRF4 contain mutations that make them insensitive to miR396 repression. Chu and colleagues find that increased GRF4 expression activates another type of plant hormone response that further promotes grain development and increases grain size. Yunhai Li and colleagues find that GRF4 also increases grain size and weight by interacting with transcription coactivators (proteins that help activate gene expression).
Taken together, these three studies show that the miR396-GRF module controls grain yield through multiple molecular pathways and that, though both controlled by miR396, GRF6 and GRF4 increase grain yield through different mechanisms. This knowledge may help guide the future breeding of high-yield rice varieties.