The microRNA miR393 has been shown to play a role in plant development and in the stress response by targeting mRNAs that code for the auxin receptors in Arabidopsis. In this study, we verified that two rice auxin receptor gene homologs (OsTIR1 and OsAFB2) could be targeted by OsmiR393 (Os for Oryza sativa). Two new phenotypes (increased tillers and early flowering) and two previously observed phenotypes (reduced tolerance to salt and drought and hyposensitivity to auxin) were observed in the OsmiR393-overexpressing rice plants. The OsmiR393-overexpressing rice demonstrated hyposensitivity to synthetic auxin-analog treatments. These data indicated that the phenotypes of OsmiR393-overexpressing rice may be caused through hyposensitivity to the auxin signal by reduced expression of two auxin receptor genes (OsTIR1 and OsAFB2). The expression of an auxin transporter (OsAUX1) and a tillering inhibitor (OsTB1) were downregulated by overexpression of OsmiR393, which suggested that a gene chain from OsmiR393 to rice tillering may be from OsTIR1 and OsAFB2 to OsAUX1, which affected the transportation of auxin, then to OsTB1, which finally controlled tillering. The positive phenotypes (increased tillers and early flowering) and negative phenotypes (reduced tolerance to salt and hyposensitivity to auxin) of OsmiR393-overexpressing rice present a dilemma for molecular breeding.
擬南芥中，小RNA分子miR393 已被證實通過作用于轉(zhuǎn)錄后的生長素受體基因的mRNAs，參與調(diào)控植物的生長發(fā)育及脅迫應(yīng)答。本文中我們證實了2類水稻生長素受體同源基因OsTIR1 和OsAFB2 都是OsmiR393 的靶標，OsmiR393 過表達水稻植株除了會導(dǎo)致之前報道過的對鹽和干旱的耐性降低和對生長素遲鈍之外還會導(dǎo)致水稻分蘗數(shù)增加和早花，OsmiR393 過表達水稻植株對人工合成的生長素類似物處理也不敏感。以上表明OsmiR393 過表達水稻植株可能通過下調(diào)OsTIR1 和OsAFB2 基因從而降低對生長素信號的敏感性，過表達OsmiR393 也能導(dǎo)致生長素運輸?shù)鞍?EM>OsAUX1 和分蘗抑制基因OsTB1 表達的下調(diào)。這可能表明OsmiR393 對分蘗的控制存在一個基因鏈即通過調(diào)控生長素受體基因OsTIR1，OsAFB2 到影響生長素運輸基因OsAUX1，最后到影響OsTB1 基因的表達，最終影響水稻分蘗。OsmiR393 的過表達一方面能著提高水稻分蘗數(shù)和促進早花，另一方面又降低了對鹽的耐性和對生長素的敏感性，這對分子育種來說是一個進退兩難的問題。