Comprehensive pangenomic, phylogenetic, and genome comparison analyses demonstrate that the causal agent of castor bean (Ricinus communis L.) bacterial blight is a pathovar of the Xanthomonas euvesicatoria species

Abstract

Xant homonas axonopodis pv. ricini is the major bacterial pathogen of castor bean ( Ricinus communis L.), an important source for oil production in several countries; only castor bean has been reported as its natural host. Recently, its pathotype strain ICMP 5747 was shown to cause disease in eucalypt plants and to group with Xanthomonas euvesicatoria in a multi-locus sequence analysis (MLSA), raising doubts about its taxonomic position. The objective of this study was to assemble the genome sequence of strain ICMP 5747 and employ it in MLSA using Bayesian inference and maximum likelihood methods with four housekeeping genes, phylogenomics based on conserved homologous genes, and genome sequence comparisons against validly published or proposed Xanthomonas species to elucidate its taxonomic position. A 5.1 Mb draft genome sequence with a 65% GC content and 99.7% completeness was assembled. MLSA, phylogenomic reconstruction based on 368 conserved genes, and genome comparisons demonstrated that strain ICMP 5747 is more closely related to X. euvesicatoria strains than to the X. axonopodis type strain. Serendipitously, it was also found that the X. axonopodis pv . commiphoreae pathotype strain LMG 26789 is also more closely related to X. euvesicatoria strains. This study proposes the transfer of X. axonopodis pvs. ricini and commiphoreae to the X. euvesicatoria species, as X. euvesicatoria pv . ricini comb. nov. and X. euvesicatoria pv . commiphoreae comb. nov. to help establish the phylogenetic relationships among Xanthomonas species and determine the etiology of plant diseases.

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