Autophagy contributes to sulfonylurea herbicide tolerance via GCN2-independent regulation of amino acid homeostasis

ABSTRACT

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.

KEYWORDS:

• Acetolactate synthase
• amino acid signaling
• Arabidopsis
• autophagy
• GCN2
• herbicide tolerance
• TOR

Acknowledgments

We thank Dr. Xingwang Li and Dr. Changbin Gao for helpful discussions and suggestions; Mr. Shouchuang Wang, Mr. Jianbo Cao, and Mrs. Dongqin Li for technical assistance. This work was financially supported by the National Key Research and Development Program of China (grant number 2016YFD0101300), the National Natural Science Foundation of China (grant number 31271761), and the Program for Modern Agricultural Industrial Technology System (grant number CARD-12).

Disclosure of potential conflicts of interest

There were no potential conflicts of interest to be disclosed.