Strain-Specific Effects of Epichloë bromicola Symbionts on Photosynthesis and Chloroplast Ultrastructure in Hordeum bogdanii

Epichloë endophytes can confer diverse benefits to host grasses, but the differences in effects between strains from different populations are poorly understood. In this study, we compared the impacts of two Epichloë bromicola strains isolated from distinct geographic populations of Hordeum bogdanii: GS1 (from Linze County, Gansu Province) and WS1 (from Wensu County, Xinjiang Province). Through controlled inoculation experiments, we established two new symbionts—HE2 (WS1 transferred to endophyte-free GF plants) and HE3 (GS1 transferred to endophyte-free WF plants)—alongside the natural symbionts GI (GS1-harboring) and WI (WS1-harboring) and corresponding endophyte-free controls (GF and WF). Symbiosis was confirmed by microscopic observation of blue-stained hyphae, re-isolation of fungi, and molecular identification using tef and tub gene sequences. Strikingly, the two strains exerted opposite effects on host photosynthesis. GS1-colonized plants (GI and HE3) maintained normal chloroplast ultrastructure, showed increased chlorophyll a, chlorophyll b, and carotenoid contents, and exhibited enhanced net photosynthetic rate, transpiration rate, and stomatal conductance, comparable to or exceeding those of control WF. In contrast, WS1-colonized plants (WI and HE2) had deformed chloroplasts, reduced pigment contents, and depressed gas exchange parameters, similar to control GF. Both newly generated symbionts accumulated more starch grains than their natural counterparts, indicating altered carbon partitioning. Phenotypic patterns were consistent across natural and novel associations, suggesting that fungal genotype drives outcomes. Differing physiological effects caused by strains from the same species and the same host but different populations indicate the importance of strain-level selection in agricultural applications. GS1 shows promise as a growth-promoting bioinoculant to enhance photosynthesis and productivity in forage grasses, particularly under marginal conditions. This study highlights how intraspecific variation and local adaptation shape grass–endophyte interactions and informs targeted use of symbionts in sustainable agriculture.