Chloroplast stress caused by maltose hyperaccumulation activates chlorophagy via the core autophagy machinery

Chlorophagy is an autophagy pathway that delivers chloroplast components into the vacuole for degradation, thus eliminating damaged chloroplasts. Chloroplast degradation is observed in Arabidopsis (Arabidopsis thaliana) mutants of MALTOSE-EXCESS 1 (MEX1), a maltose exporter in the chloroplast inner envelope membrane. However, whether autophagy is involved in the mex1 phenotypes is unknown. To extend our understanding of the signals that emanate from damaged chloroplasts and activate chlorophagy, we investigated how mex1 chloroplasts are degraded. Chlorotic mature leaves caused by maltose hyperaccumulation in mex1 plants contained swollen chloroplasts in the cytoplasm and degrading chloroplasts in the vacuole, together with heightened expression of autophagy-related (ATG) genes. The vacuolar degradation of mex1 chloroplasts required the core ATG proteins ATG7 and ATG10. ATG8-labeled structures accumulated on the surfaces of swollen mex1 chloroplasts. These findings indicate that maltose hyperaccumulation triggers chlorophagy via the core autophagy machinery. Notably, phenotypic analysis of mex1 atg double mutant plants suggested that excess chlorophagy aggravates the chlorosis seen in mex1 leaves. Transcriptome deep sequencing indicated that maltose-excess stress shares a similar transcriptomic response with high-light stress, which also triggers chlorophagy. Therefore, the signals inducing chlorophagy may be highly stimulated in mex1 leaves, making mex1 mutants effective tools for chlorophagy research.