PurK, N5-Carboxyaminoimidazole Ribonucleotide Synthetase, an Exocrine Protein Induced by Potato Plants, Influences the Virulence Through Motility Modulation in Pectobacterium brasiliense NJAU180

Bacterial pathogens secrete effector proteins that suppress plant immune responses and facilitate infection. This study focuses on <i>Pectobacterium brasiliense</i> NJAU180, a bacterial pathogen causing severe blackleg disease in potato plants in Inner Mongolia, China. Using exoproteomic analysis, plant-induced extracellular proteins were identified by comparing culture supernatants from <i>P. brasiliense</i> NJAU180 grown in minimal medium (MM) alone and in the presence of aseptically grown potato plantlets at an early growth stage (OD₆₀₀ ≈ 0.5). The results reveal PurK as a novel plant-induced extracellular protein, and deletion of <i>purK</i> markedly reduces virulence. PurK, <i>N</i>⁵-carboxyaminoimidazole ribonucleotide synthetase, is a key enzyme in <i>de novo</i> purine biosynthesis. Its impact on virulence is distinct from the conventional production of plant cell wall-degrading enzymes: PurK promotes motility by modulating transcription of flagellar genes, acting through its three domains as an integrated unit to infect successfully. Extracellularly detected PurK suppresses callose deposition, a PAMP-triggered immunity (PTI)-like defense, while it also triggers a strong hypersensitive response and upregulates expression of PTI marker genes such as <i>PR2</i> and <i>WRKY7</i> when secreted into the host plant. Although PurK interacts specifically with PurE, our data indicate that PurK's pathogenic effects operate independently of purine biosynthesis. This study reveals a reliable experimental model for more accurate assessment of microbe-plant interactions and highlights new functional roles for PurK in <i>P. brasiliense</i> NJAU180 pathogenesis and identifies potential targets for disease control strategies.