TALome and phenotypic analysis of Pakistani Xanthomonas oryzae pv. oryzae population revealed novel virulent TALEs contributing to bacterial blight of rice

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo), is an important disease in rice-growing countries, including Pakistan, where it was first reported in the mid-1970s. Transcription activator-like effectors (TALEs) play vital roles in many plant diseases caused by Xanthomonas spp.; however, Pakistani Xoo TALome diversity and their contribution to pathogenicity is largely unknown. In this study, 101 Xoo strains were screened using specific PCR primers. The genomic DNA from these strains underwent BamHI digestion and hybridized with the internal SphI fragment of PthXo1. Southern blot analysis revealed 16 to 20 putative tale fragments among the tested strains. These strains were further classified into 11 genotypes based on the number and size of the hybridizing bands. Genotypes 1, 2, 3, and 4 represented 24, 2, 51, and 17 strains, respectively. Pathogenicity assays on near-isogenic lines (NILs) containing different resistance (R) genes exhibited that CBB23 was incompatible with all tested Pakistani-Xoo genotypes, whereas IRBB5 and IRBB4 showed resistance against specific genotypes. In contrast, paddy trails on NILs containing single, double, and triple mutants of OsSWEET11a, OsSWEET13, and OsSWEET14 in the effector binding elements (EBEs) of cv. Kitaake revealed that KP-22 and LD-5 harbor novel virulent TAL effector/s. Interestingly, the expression analysis of six clade-III OsSWEET genes suggests that novel TALE/s targeting unidentified susceptibility gene/s. Altogether, this study highlights gene-for-gene relationships between tested rice lines and Pakistani-Xoo strains. This is the first report providing the diversity of TALEs and their relationship to R and S (susceptibility) genes. Further identification of novel virulent TALE/s and their cognate target/s is warranted to precisely elucidate their role in BB.