Fig. 6.

COST1 is a negative regulator of autophagy. (A) Confocal microscopy analysis of autophagy in the same autophagosome marker line GFP-ATG8e in the genetic background of WT, cost1, and COST1 overexpression lines #35 and #42. (Scale bar, 50 µm) Ten-day-old seedlings were treated with dehydration for 6 h or starvation for 16 h and representative images are shown. (B) Analysis of GFP-ATG8e cleavage as an indicator of autophagy activity in WT, cost1, and two COST1 overexpression lines with and without 6-h dehydration. Ponceau staining was used as a loading control. (C) Assay for drought tolerance of WT, cost1, and two COST1 overexpression lines. Four-week-old plants of the indicated genotypes were subjected to water withholding for 2 wk and a representative image is shown. (D) A working model of COST1 function in the drought response. Under normal growth conditions, COST1 inhibits stress responses by directly interacting with ATG8, leading to degradation and thus favoring plant growth. In stress conditions, COST1 proteins are degraded by both the 26S proteasome and autophagy, releasing ATG8, and thus the repression of autophagy, and in turn conferring drought tolerance.