Salicaceae Endophyte Impacts on Physiological Functions of Host Plants: Water Relations, Photosynthesis, and Respiration in Rice ( Oryza sativa

Endophytes are bacteria, fungi, and yeast that live inside plants. Endophytes provide fitness benefits to the host plant while receiving carbohydrates in return. A wide range of bacteria and yeast endophyte strains was isolated from native Salicaceae trees growing in a riparian area. Previous studies on these Salicaceae endophyte isolates have shown their symbiotic traits in various host crops across taxa. Biological di-nitrogen fixation, phytohormone production, and an enhancement of drought stress tolerance in plants were important beneficial functions of these endophyte isolates. However, their impacts on the physiology of the host plant had not been examined in-depth. The focus of the present study was on water relations, photosynthesis, and respiration of the host plant to better understand symbiotic associations in the plant eco-physiological context. Select Salicaceae diazotrophic endophytes were inoculated into rice as a model C3 plant, and the effects on the physiology were assessed in a series of greenhouse experiments. The inoculated plants showed reduced stomatal conductance in the afternoon than control plants. Stomatal density of the inoculated plants was also lower than that of control plants. The accumulation of leaf ABA during the afternoon was facilitated in the inoculated plants. The stomatal responses of the inoculated plants led to decreases in transpiration, and further to increases in water use efficiency of the plants. The endophyte inoculation alleviated down-regulation of the host plant photosynthesis to elevated CO2. Moreover, the inoculated plants showed the improvements in photosynthesis compared to control plants. The improvements featured increases in electron transport rate of the photosynthetic light reactions and increases in internal CO2 conductance of the CO2 diffusion pathways in leaves. The inoculated plants showed increases in respiration rates. In vitro respiration rates of the microbes were positively correlated to the concentrations of carbohydrate supply and the number of the microbes on growing media. The in planta and the in vitro assay results provided an estimation of microbial respiratory CO2 release in the host plant. The estimate was approximately 15% of total assimilated CO2 through photosynthesis. This suggests microbial respiratory CO2 could be a significant amount and possibly reenter the photosynthetic CO2 assimilatory pathways. The stomatal closure, the photosynthetic improvements, and the respiration responses together imply the possibility of the re-assimilation and partially explain the increases in water use efficiency of the plant. Further investigation will be required to confirm the re-assimilation hypothesis with convincing empirical evidence. The key to uncover future significant findings will be an understanding of source-sink relations and carbon-nitrogen relations in plants with endophytes -- the resource exchanges between the host plant and endophytes.