The identification of a protein that selectively clears damaged chloroplasts from plant cells reveals how plants maintain a "clean workshop" during the process of photosynthesis. Chloroplasts play an important role in transforming light into useable energy for plants, but when these energy powerhouses are damaged, they release harmful substances. When the plant detects this damage, signals are sent to genes involved in chloroplast function and stress adaptation.
Some evidence suggests that plastid ferrochelatases 1 and 2 (FC1 and FC2), enzymes that convert protoporphyrin-IX (Proto) to heme, may play a role in the quality control of individual chloroplasts. Proto is a photosensitizing molecule that generates oxygen, which increases oxidative stress within cells. Jesse Woodson and colleagues therefore created two strains of mutant plants each lacking one of these enzymes and subjected the plants to varying doses of light to observe changes in their chloroplasts.
After being in the dark for prolonged periods of time and then suddenly exposed to light, FC1 mutants and controls were able to green, but the FC2 strain did not. A closer look under the microscope revealed this strain had damaged chloroplasts. As well, there was accumulation of Proto and consequently an increased amount of oxygen and expression of oxidative stress responsive genes.
The team then searched for additional mutations within the FC2 strain that could counter this effect, identifying Plant U-Box 4 (PUB4) E3 ubiquitin ligase, a regulatory protein involved in cell death and development.
Tests of FC2 mutants that also had the PUB4 mutation revealed accumulated Proto and oxygen, but no degradation of the plants' chloroplasts, indicating that PUB4 plays a direct role in signaling chloroplast degradation. Looking closer at healthy plants, the researchers found that PUB4 plays a selective role in chloroplast quality control, further highlighting the function of this protein in reducing oxidative stress.