The Genetic Requirements for HiVir-Mediated Onion Necrosis by , a Necrotrophic Plant Pathogen.

is an unusual bacterial pathogen that lacks typical virulence determinants yet causes extensive necrosis in onion foliage and bulb tissues. The onion necrosis phenotype is dependent on the expression of the phosphonate toxin, pantaphos, which is synthesized by putative enzymes encoded by the HiVir (high virulence) gene cluster. The genetic contributions of individual genes in HiVir-mediated onion necrosis remain largely unknown, except for the first gene, (phosphoenolpyruvate mutase, ), whose deletion resulted in the loss of onion pathogenicity. In this study, using gene-deletion mutation and complementation, we report that, of the ten remaining genes, to are also strictly required for the HiVir-mediated onion necrosis and in-planta bacterial growth, whereas to partially contributed to these phenotypes. As the HiVir gene cluster is a common genetic feature shared among the onion-pathogenic . strains that could serve as a useful diagnostic marker of onion pathogenicity, we sought to understand the genetic basis of HiVir-positive yet phenotypically deviant (non-pathogenic) strains. We identified and genetically characterized inactivating single nucleotide polymorphisms in the essential genes of six phenotypically deviant . strains. Finally, inoculation of cell-free spent medium of the isopropylthio-β-galactoside (IPTG)-inducible promoter (P)-driven HiVir strain caused . -characteristic red onion scale necrosis as well as cell death symptoms in tobacco. Co-inoculation of the spent medium with essential mutant strains restored in-planta populations of the strains to the wild-type level, suggesting that necrotic tissues are important for the proliferation of . in onion. [Formula: see text] Copyright © 2023 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license.