Plasmids encode and can mobilize onion pathogenicity in Pantoea agglomerans

Area of Science:

• Plant Pathology
• Microbial Genomics
• Bacteriology

Background:

• Pantoea agglomerans is a known cause of onion bulb rot in the USA.
• Not all P. agglomerans strains exhibit pathogenicity towards onions, necessitating characterization of virulent strains.

Purpose of the Study:

• To identify genetic and genomic signatures distinguishing onion-pathogenic P. agglomerans strains.
• To understand the role of plasmids and specific gene clusters in P. agglomerans pathogenicity to onions.

Main Methods:

• Collected over 300 P. agglomerans strains from symptomatic onions across 11 US states.
• Performed whole-genome sequencing and phylogenetic analysis of P. agglomerans strains.
• Investigated plasmid contexts and gene content, including HiVir, alt, and cop genes.

Main Results:

• Identified two distinct P. agglomerans phylogroups; Phylogroup II strains were pathogenic to onion.
• Pathogenic strains encoded the HiVir pantaphos biosynthetic cluster, a key pathogenicity factor.
• The pOnion plasmid (pCB1C) was shown to transfer pathogenicity to non-pathogenic strains, conferring Allium thiosulfinate tolerance and copper resistance.

Conclusions:

• The HiVir cluster and specific plasmid contexts (pOnion) are crucial for P. agglomerans onion pathogenicity.
• Plasmids play a central role in mediating P. agglomerans-onion plant interactions.
• Understanding plasmid ecology is vital for managing bacterial diseases in onions.