As photoautotrophs, plants use chlorophyll to harvest light energy. Thus, high levels of chlorophyll might be beneficial to plant growth. However, abnormal accumulation of chlorophyll precursors beyond the capacity of the POR enzyme could cause photobleaching. To prevent photobleaching due to the abnormal accumulation of chlorophyll precursors, the biosynthesis of chlorophyll precursors must be carefully regulated. To investigate the underlying mechanisms that fine-tune chlorophyll biosynthesis, we looked for mutants that abnormally accumulate photosensitizing chlorophyll precursors. We identified a mutant that undergoes photobleaching due to the imbalanced expression of chlorophyll biosynthetic genes. We are currently investigating which gene is the causal gene for photobleaching.