New research at the Salk Institute for Biological Studies reveals the details of how plants manage their energy intake, which could potentially be harnessed to improve yield. Plants are stuck where they are planted, so they use a variety of ways to cope with environmental challenges. Chloroplasts convert sunlight into chemical energy to enable the plant to grow. The command center of the cell, the nucleus, occasionally sends out signals to destroy all of the 50-100 chloroplasts in the cell, such as when leaves drop off. The research team found how the plant nucleus begins to degrade and reuse the materials of select, malfunctioning chloroplasts, a mechanism that had been suspected but never shown until now.

While studying a mutant version of Arabidopsis, the team noticed the plant was making defective chloroplasts that created a reactive, toxic molecule called singlet oxygen that accumulated in the cells. The cells were marking the damaged chloroplasts for degradation with a protein tag called ubiquitin, which is used in organisms from yeast to humans to modify the function of a protein. Under closer investigation, the team observed that a protein called PUB4 was initiating the tagging. While PUB4 had been tied to cell death, the Salk team showed that it initiates the degradation of chloroplasts by placing ubiquitin tags to mark the organelle for cellular recycling.

"We've discovered a new pathway that lets a cell do a quality control check on the chloroplasts," says Jesse Woodson, Salk staff scientist and first author of the paper published in Science. Joanne Chory, senior author of the paper said, "Understanding the basic biology of plants like this selective chloroplast degradation leads us a step closer to learning how to control chloroplasts and design crops that are more resistant to stressors."

For more details, read the news article at the Salk Institute website.

Figure: In plants, chloroplasts can accumulate high levels of toxic singlet oxygen, a reactive oxygen species formed during photosynthesis. In these cells, most of the chloroplasts (green organelles) and mitochondria (red organelles) appear healthy. However, the chloroplast in the top left of the image is being selectively degraded and is interacting with the central vacuole (blue).