An Epichloë fungal endophyte regulates grain size in barley.

Grain size represents a key agronomic trait and a major determinant of barley grain yield potential. We observed that the grain size of Epichloë bromicola-inoculated barley plants were significantly larger than those of the uninoculated barley plants. However, the mechanisms and functions by which the endophyte fungus E. bromicola modulates barley grain size have not been investigated. In this study, we compared grain size between uninoculated (KE-) and E. bromicola-inoculated (KE+) barley plants to elucidate the association between E. bromicola colonization and barley grain size. Quantitative grain morphometric analysis further showed that grain width and grain area increased in KE+ by 6.8% and 5.2% compared with KE-, respectively. Structural analyze revealed that E. bromicola increased starch granule density in seeds. Phytohormone analyze showed that IAA enhanced nutrient transport by activating endosperm transfer cells, while ABA increased starch and storage protein synthesis for better grain filling. Integrated transcriptomic, and metabolomic analyses revealed that E. bromicola activated multiple hormone pathways, and enhanced the antioxidant system to regulate grain size. These findings suggest that inoculation with E. bromicola may improve barley grain yield performance. This study provides mechanistic insights into E. bromicola-mediated regulation of major seed agronomic traits and proposes a promising strategy for optimizing plant architecture by E. bromicola inoculation in sustainable agriculture.