In stressful conditions such as drought and high temperature, a plant cell’s chloroplasts can become damaged and produce harmful reactive oxygen species (ROS). Researchers at the Salk Institute have uncovered that plants produce an enzyme to signal cells to degrade ROS-producing chloroplasts before they do too much damage (Science 2015, DOI: 10.1126/science.aac7444). The team led by Jesse D. Woodson and Joanne Chory first created a strain of Arabidopsis thaliana sensitive to photooxidative stress. With transmission electron microscopy the researchers observed cells degrading damaged chloroplasts, which leak out their contents. With these green organelles destroyed, young plants never became green. The researchers then bred a second mutant plant that also underwent photooxidative stress but did turn green, indicating that chloroplasts were damaged but not destroyed. A genetic screen of these plants revealed a mutation disabling an enzyme called plant U-box 4 E3 ubiquitin ligase, suggesting that chloroplast degradation depends on that enzyme acting as a stress signal. Understanding this mechanism may help scientists create better drought- or temperature-resistant plants.