Vascular plant pathogens travel long distances through host veins, leading to life-threatening, systemic infections. In contrast, nonvascular pathogens remain restricted to infection sites, triggering localized symptom development. The contrasting features of vascular and nonvascular diseases suggest distinct etiologies, but the basis for each remains unclear. Here, we show that the hydrolase CbsA acts as a phenotypic switch between vascular and nonvascular plant pathogenesis. cbsA was enriched in genomes of vascular phytopathogenic bacteria in the family Xanthomonadaceae and absent in most nonvascular species. CbsA expression allowed nonvascular Xanthomonas to cause vascular blight, while cbsA mutagenesis resulted in reduction of vascular or enhanced nonvascular symptom development. Phylogenetic hypothesis testing further revealed that cbsA was lost in multiple nonvascular lineages and more recently gained by some vascular subgroups, suggesting that vascular pathogenesis is ancestral. Our results overall demonstrate how the gain and loss of single loci can facilitate the evolution of complex ecological traits.
The plant immune system has evolved to resist attack by pathogens and pests. However, successful phytopathogens deliver effector proteins into plant cells where they hijack the host cellular machinery to suppress the plant immune responses and promote infection. This manipulation of the host cellular pathways is done by the pathogen using various enzymatic activities, protein- DNA or protein- protein interactions. Rice is one the major economically important crops and its yield is affected by several pathogens and pests. In this review, we summarize the various effectors at the plant- pathogen/ pest interface for the major pathogens and pests of rice, specifically, on the mode of action and target genes of the effector proteins. We then compare this across the major rice pathogens and pests in a bid to understand probable conserved pathways which are under attack from pathogens and pests in rice. This analysis highlights conserved patterns of effector action, as well as unique host pathways targeted by the pathogens and pests.
Bacteria respond to environmental cues in different ways. Phase variation is one such adaptation where heritable and reversible changes in DNA aid bacteria to alter the expression of specific genes. The bacterial plant pathogenXanthomonas oryzaepv.oryzae(Xoo) causes the serious bacterial blight disease of rice. The mucoid phenotype of Xoo colonies is attributed to the secreted exopolysaccharide (EPS), xanthan gum. Spontaneous non-mucoid variants of Xoo which are deficient in EPS production and virulence were observed to accumulate in long-term stationary phase cultures. This phenomenon was termed stationary phase variation and variant colonies as stationary phase variants (SPV). Several but not all of these SPVs have been earlier described to carry spontaneous insertions of endogenous insertion sequence elements in the gum operon which encodes genes involved in EPS biosynthesis. In this study, we show that a number of SPVs harbour variations in the lipopolysaccharide (LPS) outer antigen (O-antigen) biosynthetic gene cluster. The data revealed that the vast majority of variations are due to either insertion of endogenous insertion sequence (IS) elements or slipped strand mispairing (SSM). Also, it was observed that many of these SPVs exhibited reversion to wild type mucoid phenotype via restoration of the wild type genotype. The results indicate that the phenomenon of phase variation is occurring in the LPS O-antigen biosynthetic gene cluster of Xoo.
Two yellow pigmented bacterial strains were isolated from healthy rice seeds. The strains designated as PPL560T and PPL568 were identified as members of genus Xanthomonas based on analysis of biochemical and 16S rRNA gene sequence retrieved from whole genome sequence. Isolates formed a distinct monophyletic lineage with X. sontii and X. sacchari as the closest relatives in the phylogenetic tree based on core gene content shared by reported species in genus Xanthomonas. Pairwise ortho Average Nucleotide Identity and digital DNA-DNA hybridisation values calculated against other species of Xanthomonas were below their respective cut-offs. In planta studies revealed that PPL560T and PPL568 are non-pathogenic to rice plants upon leaf clip inoculation. Absence of type III secretion system related genes and effectors further supported their non-pathogenic status. Herein, we propose Xanthomonas indica sp. nov. as novel species of genus Xanthomonas with PPL560T=MTCC13185 as its type strain and PPL568 as another constituent member.
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