Up-regulation of bundle sheath electron transport capacity under limiting light in C4Setaria viridis

C 4 photosynthesis is a biochemical pathway that operates across mesophyll and bundle sheath (BS) cells to increase CO 2 concentration at the site of CO 2 fixation. C 4 plants benefit from high irradiance but their efficiency decreases under shade causing a loss of productivity in crop canopies. We investigated shade acclimation responses of a model NADP-ME monocot Setaria viridis focussing on cell-specific electron transport capacity. Plants grown under low light (LL) maintained CO 2 assimilation rates similar to high light plants but had an increased chlorophyll and light-harvesting-protein content, predominantly in BS cells. Photosystem II (PSII) protein abundance, oxygen-evolving activity and the PSII/PSI ratio all increased in LL BS cells indicating a higher capacity for linear electron flow. PSI, ATP synthase, Cytochrome b 6 f and the chloroplastic NAD(P) dehydrogenase complex, which constitute the BS cyclic electron flow machinery, were all upregulated in LL plants. A decline in PEP carboxylase activity in mesophyll cells and a consequent shortage of reducing power in BS chloroplasts was associated with the more oxidised redox state of the plastoquinone pool in LL plants and the formation of PSII - light-harvesting complex II supercomplexes with an increased oxygen evolution rate. Our results provide evidence of a redox regulation of the supramolecular composition of Photosystem II in BS cells in response to shading. This newly identified link contributes to understanding the regulation of PSII activity in C 4 plants and will support strategies for crop improvement including the engineering of C 4 photosynthesis into C 3 plants. Significance statement The efficiency of C 4 photosynthesis decreases under low irradiance causing a loss of productivity in crop canopies. We investigate shade acclimation of a model NADP-ME monocot, analysing cell-specific protein expression and electron transport capacity. We propose a regulatory pathway controlling abundance and activity of Photosystem II in bundle sheath cells in response to irradiance.