Grain size is a key component of grain yield and quality in crops. Several core players of auxin signaling have been revealed to modulate grain size; however, to date, few genetically defined pathways have been reported, and whether phosphorylation could boost degradation of Aux/IAA proteins is uncertain. Here, we show that TGW3 (also called OsGSK5) interacts with and phosphorylates OsIAA10. Phosphorylation of OsIAA10 facilitates its interaction with OsTIR1 and subsequent destabilization, but this modification hinders its interaction with OsARF4. Our genetic and molecular evidence identifies an OsTIR1-OsIAA10-OsARF4 axis as key for grain size control. In addition, physiological and molecular studies suggest that TGW3 mediates the brassinosteroid response, the effect of which can be relayed through the regulatory axis. Collectively, these findings define a auxin signaling pathway to regulate grain size, in which phosphorylation of OsIAA10 enhances its proteolysis and potentiates OsIAA10-OsARF4-mediated auxin signaling.
水稻是我國重要的主糧作物。籽粒大小是決定稻米外觀品質(zhì)和產(chǎn)量的重要農(nóng)藝性狀。近年來，水稻籽粒大小的調(diào)控機理研究取得了較大進展，許多重要相關(guān)基因被克隆和分析。然而，目前與之相關(guān)的遺傳調(diào)控網(wǎng)絡(luò)較少被報道，限制了人們對籽粒大小調(diào)控機理的認知，也制約了其在作物高產(chǎn)優(yōu)質(zhì)分子育種實踐中的利用。
此前，中國科學(xué)院植物研究所宋獻軍研究組發(fā)現(xiàn)了一個控制水稻谷粒長度和產(chǎn)量的遺傳位點TGW3，其目標(biāo)基因編碼一個GSK3類型的蛋白激酶。以此為基礎(chǔ)，研究借助酵母雙雜交文庫篩選技術(shù)，鑒定到TGW3的一個新的互作因子OsIAA10。研究發(fā)現(xiàn)，TGW3能夠直接對OsIAA10蛋白的絲氨酸位點S68、S75和S97進行磷酸化修飾，且這些絲氨酸位點的磷酸化，調(diào)控了水稻籽粒大小。進一步，研究發(fā)現(xiàn)，OsIAA10的磷酸化，增加了與OsTIR1之間的互作和蛋白降解，并降低了其與OsARF4之間的蛋白互作。同時，遺傳學(xué)數(shù)據(jù)表明，存在一個OsTIR1-OsIAA10-OsARF4介導(dǎo)的生長素信號通路，調(diào)控水稻籽粒大小。此外，生理學(xué)和分子生物學(xué)數(shù)據(jù)表明，TGW3還介導(dǎo)了水稻植株對油菜素內(nèi)酯的反應(yīng)，且該效應(yīng)能夠通過上述信號通路進行傳遞。該研究揭示了一個調(diào)控水稻籽粒大小的新通路，為進一步改良作物的相關(guān)農(nóng)藝性狀提供重要靶標(biāo)。