Brassinosteroids promote development of rice pollen grains and seeds by triggering expression of Carbon Starved Anther, a MYB domain protein

Summary Transport of photoassimilates from leaf tissues (source regions) to the sink organs is essential for plant development. Here, we show that a phytohormone, the brassinosteroids ( BR s) promotes pollen and seed development in rice by directly promoting expression of Carbon Starved Anther ( CSA ) which encodes a MYB domain protein. Over‐expression of the BR ‐synthesis gene D11 or a BR ‐signaling factor Os BZR 1 results in higher sugar accumulation in developing anthers and seeds, as well as higher grain yield compared with control non‐transgenic plants. Conversely, knockdown of D11 or Os BZR 1 expression causes defective pollen maturation and reduced seed size and weight, with less accumulation of starch in comparison with the control. Mechanically, Os BZR 1 directly promotes CSA expression and CSA directly triggers expression of sugar partitioning and metabolic genes during pollen and seed development. These findings provide insight into how BR s enhance plant reproduction and grain yield in an important agricultural crop.