Implications of terminal oxidases in the regulation of soybean photosynthetic performance under different light intensities

Soybean [Glycine max (L.) Merr.] is a widely cultivated crop, whose growth and development are usually influenced by fluctuating light environment in fields. In the present study, two soybean cultivars (Nandou12 and Nan 032-4) were selected to study the regulatory mechanisms of capability to do photosynthetic work under different light intensities. Results showed that the net photosynthetic rate (P n ) of Nandou 12, which has a higher yield in field condition, was significantly higher than that of Nan 032-4 under all the light intensities studied. Measurements on stomatal conductance (G s) and intercellular CO2 concentration (C i) suggested that the difference of P n between these two cultivars was mainly caused by non-stomatal factors. Transcript detection found that the expression of genes encoding plastid terminal oxidase (PTOX) and alternative oxidase (AOX), which represent the terminal oxidases of chlororespiration in chloroplasts and alternative respiration in mitochondria, respectively, was up-regulated in both cultivars when light intensity increased, implying potential roles of these two terminal oxidases in photosynthetic regulation. Further analysis showed that the expression level of PTOX increased more obviously in Nandou 12, together with higher non-photochemical quenching (NPQ) and higher activities of superoxide dismutase (SOD) and ascorbate peroxidase (APX) than that in Nan 032-4, when light intensity increased. However, Nan 032-4 performed a more obvious up-regulation of AOX genes, as well as higher activities of NAD-malate dehydrogenase (MDH) and NADP-MDH than Nandou 12, when light intensity increased. These distinctions were consequently suggested to be potential reasons for the difference of photosynthetic performance between two soybean cultivars.