Diazotrophic Endophytes Alleviate Down-Regulation of Photosynthesis Under Elevated CO2 Concentrations in Rice Leaves.

Most of plants experience acclimation of photosynthesis under long-term exposure to elevated atmospheric CO₂: Down-regulation of CO₂ assimilation by sink limitation of photosynthesis and further stunt of increase of biomass occur under high CO₂ concentration, which have been elucidated through a body of literature. Recent studies showed that diazotrophic endophytes are able to biologically fix atmospheric di-nitrogen and provide host plants with this fundamental nutrient resource in exchange for photosynthates. We hypothesized that these two key features of the N fixing endophytes eliminate down-regulation of CO₂ assimilation in elevated CO₂. To test this hypothesis, we examined biochemical responses of CO₂ assimilation (AC_i curves) of rice leaves with and without the inoculation of the symbiotic bacteria using a commercial gas exchange system. WP5 (Rahnella sp. CDC 2987-79), originally isolated from wild black cottonwood trees (Populus trichocarpa), was inoculated (E+) to M-206 Japonica rice (Oryza sativa) seedlings seven days after germination. They were grown in the ambient (400 ppm, AMB) and the elevated (800 ppm, ELE) CO₂ concentrations with the mock-inoculated control (E-) seedlings. Sixty five days after germination, E- plants grown in ELE showed apparent down-regulation of CO₂ assimilation compared to those grown under AMB. However, E+ plants did not show the down-regulation under ELE compared to those under AMB. This is possibly related to the additional nitrogen supplied by diazotrophic endophytes, allowing the host plants to utilize this resource for building photosynthetic machinery or larger sinks. The endophytes themselves acting as alternative sinks to drain photosynthates in return for the supply of nitrogen can also be a possible explanation of the increase of CO₂ utilization. Our results suggest that nitrogen fixing endophytes can be an effective means for improving plant growth in the future climate by maximizing the growth promotion by elevated CO2 through elimination of photosynthetic down-regulation.