LAUSR

Photosynthesis provides food, fibre and fuel that support our society; understanding the mechanisms controlling dynamic changes in this process helps identify new options to improve photosynthesis. Photosynthesis shows diel changes, which have been largely attributed to external light/dark conditions, as well as internal gene expression and the post-translational modification of critical enzymes. Here we report diel fluctuations of magnesium (Mg) in rice ( Oryza sativa ) chloroplasts, which may function as a rhythm regulator contributing to the post-translational regulation of photosynthetic CO 2 assimilation in rice. We found that a chloroplast-localized Mg 2+ transporter gene, OsMGT3 , which is rhythmically expressed in leaf mesophyll cells, partly modulates Mg fluctuations in rice chloroplasts. Knockout of OsMGT3 substantially reduced Mg 2+ uptake, as well as the amplitude of free Mg 2+ fluctuations in chloroplasts, which was closely associated with a decrease in ribulose 1,5-bisphosphate carboxylase activity in vivo and a consequent decline in the photosynthetic rate. In addition, the mesophyll-specific overexpression of OsMGT3 remarkably improved photosynthetic efficiency and growth performance in rice. Taken together, these observations demonstrate that OsMGT3-dependent diel Mg fluctuations in chloroplasts may contribute to Mg-dependent enzyme activities for photosynthesis over the daily cycle. Enhancing Mg 2+ input to chloroplasts could be a potential approach to improving photosynthetic efficiency in plants. Photosynthesis in plants responds to light fluctuations and displays circadian rhythms in various aspects. Here a new regulator of photosynthetic rhythms is identified in rice and shown to control diurnal magnesium fluctuations in the chloroplast. Further engineering attempts yield enhanced rice growth and photosynthesis.