A Deficiency in the Flavoprotein of Arabidopsis Mitochondrial Complex II Results in Elevated Photosynthesis and Better Growth in Nitrogen-Limiting Conditions

Mitochondrial complex II (succinate dehydrogenase [SDH]) plays roles both in the tricarboxylic acid cycle and the respiratory electron transport chain. In Arabidopsis (Arabidopsis thaliana), its flavoprotein subunit is encoded by two nuclear genes, SDH1-1 and SDH1-2. Here, we characterize heterozygous SDH1-1/sdh1-1 mutant plants displaying a 30% reduction in SDH activity as well as partially silenced plants obtained by RNA interference. We found that these plants displayed significantly higher CO₂ assimilation rates and enhanced growth than wild-type plants. There was a strong correlation between CO₂ assimilation and stomatal conductance, and both mutant and silenced plants displayed increased stomatal aperture and density. By contrast, no significant differences were found for dark respiration, chloroplastic electron transport rate, CO₂ uptake at saturating concentrations of CO₂ , or biochemical parameters such as the maximum rates of carboxylation by Rubisco and of photosynthetic electron transport. Thus, photosynthesis is enhanced in SDH-deficient plants by a mechanism involving a specific effect on stomatal function that results in improved CO₂ uptake. Metabolic and transcript profiling revealed that mild deficiency in SDH results in limited effects on metabolism and gene expression, and data suggest that decreases observed in the levels of some amino acids were due to a higher flux to proteins and other nitrogen-containing compounds to support increased growth. Strikingly, SDH1-1/sdh1-1 seedlings grew considerably better in nitrogen-limiting conditions. Thus, a subtle metabolic alteration may lead to changes in important functions such as stomatal function and nitrogen assimilation.