Evaluating effects of elevated and accelerated NPQ relaxation on yield, physiology and transcription in soybean

Improving photosynthetic efficiency is proposed as a means of enhancing crop productivity under rising atmospheric CO2 concentrations ([CO2]). To assess whether additive gains can be realized from simultaneous improvements in light use-efficiency and carbon assimilation, we investigated the physiological and molecular responses to elevated CO2 (e[CO2]) of a transgenic soybean line (VPZ-34A) expressing the genes violaxanthin de-epoxidase (VDE), Photosystem II subunit S (PsbS) and zeaxanthin epoxidase (ZEP) from Arabidopsis thaliana (arabidopsis) (AtVPZ). Plants were grown at a free-air CO2 enrichment facility (SoyFACE) in 2021, with physiological and transcriptomic measurements collected during vegetative (V4-5) and reproductive stages (R5-6), then compared to final biomass and seed production parameters. VPZ-34A showed increased maximum quantum photosynthetic efficiency of photosynthesis ({varphi}PSIImax) under fluctuating light during vegetative stages, but no increase in maximum efficiency of carbon assimilation ({varphi}CO2, max), and no significant increase in seed or biomass related traits under ambient or e[CO2]. Transcriptome analysis showed the VPZ construct impacted a small number of genes, among them downregulation of a mitochondrial thioredoxin (Trx o1) and up-regulation of two AP2 transcription factors in VPZ lines may suggest alterations in redox balance and seed development. In conclusion, the improvements in {varphi}PSIImax and NPQ in VPZ-34A were unaffected by e[CO2] but didnt translate to stimulation of yield. Our data highlight the complexity of VPZ manipulation on plant performance and provide avenues for future research to optimize this strategy to improve crop yields.