Cultivar-Specific Seed Endophytes in Medicago sativa and Their Association with Germination Under Salt Stress

Medicago sativa (alfalfa) establishment is often limited by soil salinity, especially during germination. Seed-associated microorganisms may influence plant performance, but cultivar-associated seed endophytic communities and their relationship with germination under salt stress remain unclear. We evaluated germination of five alfalfa cultivars under 0, 100, and 150 mM NaCl and applied a seed-derived endophytic microbial suspension from Saiwo to Gleat seeds under salt stress. Bacterial and fungal communities were characterized by 16S rRNA and ITS amplicon sequencing, and culturable bacteria were tested for plant growth-promoting traits under non-saline conditions. Saiwo showed higher germination at 100 mM NaCl, and the Saiwo seed-derived suspension significantly improved Gleat germination under salt stress. Pseudomonadota dominated bacterial communities, with Liberibacter as the most abundant genus in Saiwo and Pantoea enriched in Gleat and Vegron. Fungal communities were dominated by Ascomycota, with Fusarium, Alternaria, and Cladosporium being the main genera and Penicillium being enriched in Saiwo. The isolated strains, mainly Bacillus, Erwinia, Kocuria, Leclercia, and Pantoea, exhibited growth on Ashby N-free medium, siderophore production, and phosphate and potassium solubilization, but these traits were not evaluated under saline conditions. These results indicate that cultivar-associated seed endophytic communities are associated with germination under salt stress, and the Saiwo seed-derived suspension has potential for improving alfalfa germination in saline soils.