a National Key Laboratory of Crop Genetic Improvement, National Engineering Research Center of Rapeseed , Huazhong Agricultural University , Wuhan , China.
Autophagy. 2018;14(4):702-714. doi: 10.1080/15548627.2017.1407888. Epub 2018 Jan 29.
Sulfonylurea (SU) herbicides inhibit branched-chain amino acid (BCAA) biosynthesis by targeting acetolactate synthase. Plants have evolved target-site resistance and metabolic tolerance to SU herbicides; the GCN2 (general control non-repressible 2) pathway is also involved in SU tolerance. Here, we report a novel SU tolerance mechanism, autophagy, which we call 'homeostatic tolerance,' is involved in amino acid signaling in Arabidopsis. The activation and reversion of autophagy and GCN2 by the SU herbicide tribenuron-methyl (TM) and exogenous BCAA, respectively, confirmed that TM-induced BCAA starvation is responsible for the activation of autophagy and GCN2. Genetic and biochemical analyses revealed a lower proportion of free BCAA and more sensitive phenotypes in atg5, atg7, and gcn2 single mutants than in wild-type seedlings after TM treatment; the lowest proportion of free BCAA and the most sensitive phenotypes were found in atg5 gcn2 and atg7 gcn2 double mutants. Immunoblotting and microscopy revealed that TM-induced activation of autophagy and GCN2 signaling do not depend on the presence of each other, and these 2 pathways may serve as mutually compensatory mechanisms against TM. TM inhibited the TOR (target of rapamycin), and activated autophagy in an estradiol-induced TOR RNAi line, suggesting that TM-induced BCAA starvation activates autophagy, probably via TOR inactivation. Autophagy and GCN2 were also activated, and independently contributed to TM tolerance in plants conferring metabolic tolerance. Together, these data suggest that autophagy is a proteolytic process for amino acid recycling and contributes to GCN2-independent SU tolerance, probably by its ability to replenish fresh BCAA.
磺酰脲（SU）类除草剂通过靶向乙酰乳酸合成酶来抑制支链氨基酸（BCAA）的生物合成。植物已经进化出针对 SU 类除草剂的靶标位点抗性和代谢耐受性；GCN2（一般控制非阻遏 2）途径也参与 SU 耐受性。在这里，我们报告了一种新的 SU 耐受性机制，自噬，我们称之为“稳态耐受性”，它参与了拟南芥中氨基酸信号转导。SU 除草剂三甲嘧磺隆（TM）和外源性 BCAA 分别激活和逆转自噬和 GCN2，证实 TM 诱导的 BCAA 饥饿是自噬和 GCN2 激活的原因。遗传和生化分析表明，与野生型幼苗相比，TM 处理后，atg5、atg7 和 gcn2 单突变体中游离 BCAA 的比例较低，表型更敏感；atg5 gcn2 和 atg7 gcn2 双突变体中游离 BCAA 的比例最低，表型最敏感。免疫印迹和显微镜观察表明，TM 诱导的自噬和 GCN2 信号转导的激活不依赖于彼此的存在，这 2 条途径可能作为相互补偿的机制来对抗 TM。TM 抑制雷帕霉素（TOR），并在雌二醇诱导的 TOR RNAi 系中激活自噬，表明 TM 诱导的 BCAA 饥饿激活自噬，可能通过 TOR 失活。自噬和 GCN2 也被激活，并在赋予代谢耐受性的植物中独立地促进 TM 耐受性。总的来说，这些数据表明自噬是一种氨基酸回收的蛋白水解过程，通过补充新鲜的 BCAA，有助于 GCN2 非依赖性 SU 耐受性。
