Novel insights into the genetic basis and transcriptomic elucidation of virulence attenuation in <i>Pseudomonas syringae</i> pv. <i>actinidiae</i> variants

<i>Pseudomonas syringae</i> pv. <i>actinidiae</i> (Psa) is the causal agent of kiwifruit bacterial canker, but the factors affecting its pathogenicity in natural settings remain poorly explored. In this study, we isolated two Psa strains, G126 and G282, from infected kiwifruit orchards in Guizhou Province of China. Both isolates, categorized as Psa-biovar 3, were confirmed through Psa3-specific primers and phylogenomic analysis. Pathogenicity assays on kiwifruit cultivar "Hongyang" leaves and branches showed significantly reduced numbers of necrotic spots and reduced lesion sizes upon infection with G282 compared to the G1 positive control strain, while G126 showed a nonpathogenic phenotype. Additionally, both strains failed to induce a hypersensitive response in nonhost <i>Nicotiana tabacum</i> plants and exhibited significantly reduced promoter activity of the <i>hrpR/S</i>, <i>hrpL</i>, and <i>hrpA</i> genes, which are crucial for the type III secretion system (T3SS). Genomic sequencing revealed that the T3SS of G126 was defective due to a single-nucleotide polymorphism in the <i>hrpR</i> gene, while G282 was entirely deficient in the type VI secretion system (T6SS), which potentially regulates the expression of T3SS genes. Transcriptomic analysis showed widespread alterations in key aspects of the secretion system, protein transport, and signal transduction, further supporting the phenotype characteristics exhibited by the strains. This study enhances our understanding of the genetic basis of nonpathogenic and partially pathogenic Psa isolates, highlighting the functional interdependencies between T3SS and T6SS.