Abstract:

To explore the diversity of seed-borne bacteria of imported alfalfa （Medicago sativa） and their pathogenicity to animals and plants， 34 samples of lucerne seeds imported from North America and Europe were collected and analyzed. After Petri-dish isolation and culture， 39 seed-borne bacterial isolates were obtained. The taxonomic status of the bacterial isolates was determined through observations of phenotypic characteristics and 16S rDNA amplicon sequencing. Twenty-one representative strains with potential pathogenicity were selected for evaluation in a plant model （alfalfa， M. sativa） and an animal model （mouse， Mus musculus）. Petri-dishes test and pot experiments were performed to study the impact of the bacterial isolates on alfalfa plants. The mouse infection model consisted of intraperitoneal injection of a bacterial suspension （10⁹ CFU·mL^-1） at a dose of 0.1 mL·10 g^-1. We found that the alfalfa seeds harbored a diverse community of bacteria. These bacterial isolates were grouped in 39 morphospecies that corresponded to three phyla and 15 genera. Out of 39 isolates， 24 （61.54%） were Gram positive and 15 （38.46%） Gram negative. Among them， Firmicutes was the predominant phylum observed in all groups and Bacillus was the dominant genus. The results of pathogenicity tests showed that bacterial strain GCKH （Rhodococcus sp.） was only pathogenic to alfalfa while ZSR17 （Acinetobacter sp.）， ZSR25 （Escherichia sp.）， and R1 （Massilia sp.） were pathogenic to mice. Notably， ZF1 （Erwinia sp.）， ZS3 （Erwinia sp.）， CQ10 （Pantoea sp.）， and ZS6 （Pantoea sp.） could cause disease in both alfalfa and mouse hosts， indicating that they are potential co-pathogenic bacteria capable of cross-kingdom infection. The results provide a preliminary overview of the taxonomic status of seed-borne bacteria carried by lucerne seeds imported from Europe and America and the potential harmfulness of these bacterial strains. These findings lay a theoretical foundation for the integrated management and prevention of the invasion and transmission of the plant-pathogenic， animal-pathogenic， or co-pathogenic bacteria on forage seeds that have the potential to cause infections in plants and animals.

Key words: lucerne, seed-borne bacteria, diversity, cross-kingdom co-pathogen of plants and animals, pathogenic bacteria