Overexpression of Xanthophyll Cycle Genes Leads to Faster NPQ Acclimation in the C4 Monocot Setaria viridis

Slow relaxation and induction of Non-Photochemical Quenching (NPQ) limits plant productivity under fluctuating light in C3 plants but it is unknown if this trait extends to C4 plants. Three genes have been implicated in determining NPQ induction and relaxation rates under intermittent leaf shading; VDE and ZEP interconvert the xanthophyll pigments violaxanthin and zeaxanthin, and PsbS regulates the overall level of NPQ. Here we report the overexpression of the Arabidopsis thaliana VDE, ZEP, and PsbS in the NADP-Malic Enzyme C4 monocot Setaria viridis. We demonstrate these transgenic plants have faster NPQ induction and relaxation under fluctuating light but observe no impact on Photosystem II photochemistry yield. Under illumination, faster NPQ induction resulted in higher transient NPQ levels and a lower overall chlorophyll fluorescence yield. Together this introduces the potential for lower productivity due to over-quenching of chlorophyll during light fluctuation. Although we observed faster NPQ acclimation rates in our transgenic lines, hyperspectral imaging and pigment analysis suggest that there is little photosynthetic advantage to increasing NPQ acclimation over wildtype due to naturally fast C4 acclimation rates. These results indicate that further work is necessary in evaluating the role of NPQ acclimation in C4 growth dynamics towards the goal of improving crop productivity.