Research

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the important diseases of rice worldwide. Like other phytopathogenic Xanthomonads, Xoo secretes effector proteins directly in to the rice cell through a hrp-encoded specialized tubular structure, referred to as type three secretion systems (T3SS). Bacteria use the T3SS-effector arsenal to suppress the pathogen-associated molecular pattern (PAMP)-triggered immune (PTI) response of rice and induce blight. There are two classes of effectors, namely TAL (Transcription activation like domain), of the avrBs3/pthA family and Xop (Xanthomonas outer protein) effectors. We demonstrated 21 Xop and 15 TALE effectors in one of the virulent Indian strain, X. oryzae pv. oryzae race 4 using gene specific PCR and reverse northern analysis. The full length nucleotide sequences of these effector genes (xopN, xopQ, xopR, xopF) were submitted in the GenBank (JX 485640, KC479821, JX 485643, KF939317). We found that core effectors like XopF, XopR plays crucial role during blight disease development. We demonstrated that both the effectors are required for growth, and for suppression of rice immune responses including the suppression of callose deposition, ROS production and transcript expression of PTI responsive genes by Xoo. Right now our focus is to investigate the cognate interactors in rice for the key Xop effectors with the aim to explore the sensible strategy to develop the host resistance. Besides, our group is now interested to decipher the role TAL effectors (TALE) in blight induction and to identify the major TALEs and its corresponding SWEET genes that are being induced by Xoo TALEs during blight pathogenesis. The ultimate goal of our research team is to have detailed insight into Xoo::rice interaction via T3SS effectors and to contribute with the updated understanding to BB resistance programme.

The phylogeny based on xopN gene indicated that the Indian Xoo strains formed clad with strains from Korea (KACC10331) and Japan (MAFF311018), while Philippines strain PXO99 belonged to a different clad. This suggests that both the Indian strains of Xoo are closely related to Korean and Japanese strains with respect to XopN effector, one of the conserved effectors in Xanthomonads.

We have carried out virulence profiling of 780 isolates of Xoo covering 13 states of North-west and –eastern rice growing states. The isolates are grouped into 6 pathogenic races based on phenotypic reaction on 12 NILs. The distribution of these races was varied across the states. Race 4, 6 were most virulent and predominantly occurred on rice in most of the north-west states including Punjab, Haryana, UP, UK, Himachal. While race 2, 3 were predominant in north-eastern states including Assam, Meghalaya, Manipur. Race 1, and 5 has got sporadic distribution. Among the R-genes Xa4 was very effective followed by Xa11, xa13 and Xa21.

Bacterial blight caused by Xanthomonas axonopodis pv. punicae (Xap) has emerged as a potential threat to pomegranate production in India, the biggest producer of pomegranate in the world. We identified six Xop T3SS effectors (XopC2, XopE1, XopL, XopN, XopQ and XopZ) in Xap. We found that the core Xop effectors like XopC2, XopL, XopN play important role during blight deveopment. We found that these effectors are required for maximal Xap pathogenicity in its natural host pomegranate. Wild type Xap but not the mutants (lacking the effector gene) produced large intense watersoaking lesions on pomegranate leaves. Further, the mutants showed substantial reduction in in planta colonization relative to the wild strain. We examined that the mutants induced more callose deposition in infected pomegranate leaves. Taken together, our study shows that these core effectors favours pathogen for its unrestricted spread and multiplication in the plant via suppressing pomegranate innate immune responses including ROS production, down regulation of immune responsive genes in pomegranate. We further documented that all these effectors are localized to the plasma membrane, possible site of their action. Our understanding on Xop effectors provide detailed insight into the effector mediated pathogenesis. Now we aimed at to identify the cognate receptors in the pomegranate for these effectors with the ultimate aim to develop strategies for broad and durable resistance towards the blight pathogen.

Developed a specific PCR based detection kit for Xathomonas axonopodis pv. punicae (Xap), the incitant of pomegranate bacterial blight. Determined the phylogeny of Xap strains using multiple loci viz. 16S rRNA, gyrB, T3SS-effector genes and established its close linkage with X. citri subsp. malvacearum. Investigated the antibacterial properties of nanocopper against Xap. A patent filed on “Nanocopper”-a novel formulation to combat bacterial blight of pomegranate, rice and bean.