Decreasing photosystem antenna size: a double-edged sword for canopy photo-synthetic efficiency

Optimization of antenna size of photosynthetic systems is one strategy to increase plant canopy photosynthesis and crop yield potential. The relationship between antenna size and photosynthesis rate has been extensively studied recently. However, conflicting results have been obtained. Here we show that the extent of decrease in antenna size is a major factor determining the consequences of decreasing antenna on photosynthesis and growth-related parameters. Specifically, we constructed transgenic rice lines with artificial microRNA (amiRNA) targeting to Chlorophyll Synthesis (YGL1) to generate transgene heterozygous and homozygous lines with different leaf chlorophyll contents and antenna sizes. We found that canopy photosynthesis (Ac), biomass and grain yield of the heterozygote were not significantly different from those of WT while the Ac, biomass and grain yield of the homozygote were lower than those of WT. Further, when the maximal quantum yield of photosystem II (Fv/Fm) was larger than 0.8, decreasing antenna size by reducing chlorophyll biosynthesis did not affect leaf photosynthesis; but when Fv/Fm was lower than 0.8, there is a positive relationship between antenna size and leaf photosynthesis. There is large variation in both leaf chlorophyll content and antenna size in elite rice cultivars, suggesting that there is a large scope to decrease leaf chlorophyll content to increase nitrogen use efficiency as long as the quantum yield of PSII is not compromised.