Intersections: photosynthesis, abiotic stress, and the plant microbiome

Climate change impacts environmental conditions that affect photosynthesis. This review examines the effect of combinations of elevated atmospheric CO₂, long photoperiods, and/or unfavorable nitrogen supply. Under moderate stress, perturbed plant source-sink ratio and redox state can be rebalanced but may result in reduced foliar protein content in C₃ plants and a higher carbon-to-nitrogen ratio of plant biomass. More severe environmental conditions can trigger pronounced photosynthetic downregulation and impair growth. We comprehensively evaluate available evidence that microbial partners may be able to support plant productivity under challenging environmental conditions by providing (1) nutrients, (2) an additional carbohydrate sink, and (3) regulators of plant metabolism, especially plant redox state. In evaluating the latter mechanism, we note parallels to metabolic control in photosymbioses and microbial regulation of human redox biology.