Gitaitis, R. | bioRxiv Science

bioRxiv2022

A novel biosynthetic gene cluster across the Pantoea species complex is important for pathogenicity in onion

Onion center rot is caused by at least four species of Pantoea (P. ananatis, P. agglomerans, P. allii, and P. stewartii subsp. indologenes). Critical onion pathogenicity determinants for P. ananatis were recently described but whether those determinants are common among other onion-pathogenic Pantoea species remains unknown. In this work, we report onion pathogenicity determinants in P. stewartii subsp. indologenes and P. allii. We identified two distinct secondary metabolite biosynthetic gene clusters in different strains of onion pathogenic P. stewartii subsp. indologenes. One cluster is similar to the previously described HiVir phosphonate biosynthetic cluster of P. ananatis and another is a novel putative phosphonate biosynthetic gene cluster, which we name "Halophos". The Halophos gene cluster was also identified in P. allii strains. Both clusters are predicted to be phosphonate biosynthetic clusters based on the presence of a characteristic phosphenolpyruvate phosphomutase (pepM) gene. The deletion of pepM gene from either the P. stewartii subsp. indologenes HiVir or Halophos clusters caused loss of necrosis on onion leaves and red onion scales, and resulted in significantly lower bacterial populations compared to the corresponding wildtype and complemented strains. Seven (halB-halH) out of eleven genes (halA-halK) in the Halophos gene cluster are required for onion necrosis phenotypes. The onion non-pathogenic strain PNA15-2 gained the capacity to cause foliar necrosis on onion via exogenous expression of a minimal seven gene Halophos cluster (halB -halH). Furthermore, cell-free culture filtrates of PNA14-12 expressing the intact Halophos-gene cluster caused necrosis on onion leaves consistent with the presence of a secreted toxin. Together, these observations indicated that pepM genes in both phosphonate biosynthetic gene clusters (HiVir and Halophos) are important for Pantoea spp. onion pathogenicity and the biosynthetic product of the Halophos cluster causes necrosis on onion leaf tissue. Overall, this is the first report of onion pathogenicity determinants in P. stewartii subsp. indologenes and P. allii. Author summaryOnion center rot is caused by multiple Pantoea species including P. stewartii subsp. indologenes and P. allii. We identified two distinct secondary metabolite biosynthetic clusters associated with onion pathogenic strains, the validated HiVir phosphonate cluster and a putative phosphonate biosynthetic cluster that we named as Halophos based on the associated "halo" phenotype on the red onion scales. We found that pepM genes from each cluster (HiVir and Halophos) are required for onion infection by P. stewartii subsp. indologenes and P. allii but not for millet infection by P. stewartii subsp. indologenes. Conversely, the T3SS was important for millet infection by P. stewartii subsp. indologenes but not onion infection. Induction of the intact Halophos cluster was associated with the accumulation of a necrosis-inducing factor in culture, which suggests it might be a secreted phytotoxin. Seven of the eleven Halophos cluster genes are required for onion necrosis phenotypes and expression of this minimal cluster conferred a limited onion necrosis phenotype to an onion-non-pathogenic Pantoea strain. We provide evidence of a Halophos biosynthetic gene cluster to be associated with onion pathogenicity in strains of P. stewartii subsp. indologenes and P. allii.

microbiology↗

bioRxiv2020

Pan-genome-wide analysis of Pantoea ananatis identified genes linked to pathogenicity in onion

Pantoea ananatis is a member of a Pantoea spp. complex that causes center rot of onion, which significantly affects onion yield and quality. This pathogen does not have typical virulence factors like type II or type III secretion systems but appears to require a biosynthetic gene-cluster, HiVir/PASVIL (located chromosomally), for a phosphonate secondary metabolite, and the onion-virulence regions, OVR (localized on a megaplasmid), for onion pathogenicity and virulence, respectively. We conducted a deep pan-genome-wide association study (pan-GWAS) to predict additional genes associated with pathogenicity in P. ananatis using a panel of diverse strains (n = 81). We utilized a red-onion scale necrosis assay as an indicator of pathogenicity. Based on this assay, we differentiated pathogenic (n = 51)- vs. non-pathogenic (n = 30)-strains phenotypically. Pan-GWAS revealed a large core genome of 3,153 genes and a flexible accessory genome of [≤]5,065 genes. Phylogenomic analysis using pan-GWAS and presence and absence variants (PAVs) distinguished red-scale necrosis inducing (pathogenic) strains from non-scale necrosis inducing (non-pathogenic) strains of P. ananatis. The pan-GWAS also predicted 42 genes, including 14 from the previously identified HiVir/PASVIL cluster associated with pathogenicity, and 28 novel genes that were not previously associated with pathogenicity in onion. Of the 28 novel genes identified, eight have annotated functions of site-specific tyrosine kinase, N-acetylmuramoyl-L-alanine amidase, TraR/DksA family transcriptional regulator, and HTH-type transcriptional regulator. The remaining 20 genes are currently hypothetical. This is the first report of using pan-GWAS on P. ananatis for the prediction of novel genes contributing to pathogenicity in onion, which will be utilized for further functional analyses. Pan-genomic differences (using PAVs) differentiated onion pathogenic from non-pathogenic strains of P. ananatis, which has been difficult to achieve using single or multiple gene-based phylogenetic analyses. The pan-genome analysis also allowed us to evaluate the presence and absence of HiVir/PASVIL genes and 11 megaplasmid-borne OVR-A genes regarded as the alt cluster that aid in P. ananatis colonization in onion bulbs. We concluded that HiVir/PASVIL genes are associated with pathogenic P. ananatis strains and the alt gene cluster alone is not responsible for pathogenicity on onion. The pan-genome also provides clear evidence of constantly evolving accessory genes in P. ananatis that may contribute to host-range expansion and niche-adaptation. Author summaryPantoea ananatis is a major bacterial pathogen that causes center rot of onion and diseases of a number of other plant species. In order to understand the genome architecture and identify genes responsible for pathogenicity in onion, a pan-genome analysis was performed. We used 81 strains of P. ananatis collected over 20 years from different regions of the state of Georgia, USA. The pan-genome study identified a core genome with a conserved set of genes and an accessory genome that displayed variation among strains. We conducted pan-GWAS (pan-genome-wide association study) using presence and absence variants (PAVs) in the genomes and associated onion-pathogenic phenotypes based on a red-onion scale necrosis assay. The study resulted in identification of genes, including a cluster of chromosomal HiVir/PASVIL genes, that are significantly associated with the onion pathogenic phenotype. In addition, we identified 28 novel genes, a majority of which (n = 20) have hypothetical functions. We concluded and further substantiated earlier findings that a cluster of genes is responsible for pathogenicity on onion. The pan-genome analysis also allowed us to evaluate the presence and absence of HiVir/PASVIL genes and 11 megaplasmid-borne OVR-A genes regarded as the alt cluster that aid in bacterial colonization of onion bulbs by P. ananatis strains. We concluded that HiVir/PASVIL genes are associated with onion-pathogenic strains, and the alt gene cluster alone is not responsible for pathogenicity on onion. This study also provides potential evidence of constantly evolving accessory genes in P. ananatis which may help in host range expansion and adaptation to diverse niches.

genomics↗