Decreasing photosystem antenna size by inhibiting chlorophyll synthesis: A double-edged sword for photosynthetic efficiency

Improving photosynthetic efficiency has long been considered as an important strategy to increase crop yield. Optimization of antenna size of photosynthetic systems is one strategy to increase plant photosynthetic efficiency. However, applying this strategy to improve photosynthesis received conflicting results, and the reasons behind these conflicts are unclear. In this study, we constructed transgenic rice with amiRNA targeting to YGL1, which encodes a key enzyme of chlorophyll a/b synthesis in chlorophyll biosynthesis pathway, to generate different lines with different leaf chlorophyll contents and antenna sizes to test under what conditions reduction of antenna size can improve photosynthesis. We found that leaf photosynthesis, canopy photosynthesis (A_c), biomass and grain yield of the heterozygote were not significantly different from those of wild type (WT) while the A_c, biomass and yield of the homozygote were lower than those of WT. Further, when the maximal quantum yield of photosystem II (F_v/F_m) was larger than 0.8, decreasing antenna size by reducing chlorophyll biosynthesis didn't affect leaf photosynthesis. In view of this phenomenon, we proposed that the accumulation of protoporphyrin and the reduced photoprotection capacity might be the cause of the decrease in F_v/F_m and A_c. Therefore, this study shows that reduction of antenna size by inhibiting chlorophyll synthesis can lead to improved light distribution and photosystem efficiency, as long as photodamage and photobleaching can be avoided to maintain the photosystem II efficiency. The double-edged sword effect of inhibiting chlorophyll synthesis on photosynthetic efficiency should be considered when the antenna size is manipulated to gain higher photosynthesis.