A mutation in an exoglucanase of Xanthomonas oryzae pv. oryzae,which confers an endo mode of activity,affects bacterial virulence,but not the induction of immune responses,in rice

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight, a serious disease of rice. Xoo secretes a repertoire of cell wall‐degrading enzymes, including cellulases, xylanases and pectinases, to degrade various polysaccharide components of the rice cell wall. A secreted Xoo cellulase, CbsA, is not only a key virulence factor of Xoo, but is also a potent inducer of innate immune responses of rice. In this study, we solved the crystal structure of the catalytic domain of the CbsA protein to a resolution of 1.86 Å. The core structure of CbsA shows a central distorted TIM barrel made up of eight β strands with N‐ and C‐terminal loops enclosing the active site, which is a characteristic structural feature of an exoglucanase. The aspartic acid at the 131st position of CbsA was predicted to be important for catalysis and was therefore mutated to alanine to study its role in the catalysis and biological functions of CbsA. Intriguingly, the D131A CbsA mutant protein displayed the enzymatic activity of a typical endoglucanase. D131A CbsA was as proficient as wild‐type (Wt) CbsA in inducing rice immune responses, but was deficient in virulence‐promoting activity. This indicates that the specific exoglucanase activity of the Wt CbsA protein is required for this protein to promote the growth of Xoo in rice.     

Reciprocal adaptation of rice and Xanthomonas oryzae pv. oryzae: cross-species 2D GWAS reveals the underlying genetics

A 1D/2D genome-wide association study strategy was adopted to investigate the genetic systems underlying the reciprocal adaptation of rice (Oryza sativa) and its bacterial pathogen, Xanthomonas oryzae pv. oryzae (Xoo) using the whole-genome sequencing and large-scale phenotyping data of 701 rice accessions and 23 diverse Xoo strains. Forty-seven Xoo virulence-related genes and 318 rice quantitative resistance genes (QR-genes) mainly located in 41 genomic regions, and genome-wide interactions between the detected virulence-related genes and QR genes were identified, including well-known resistance genes/virulence genes plus many previously uncharacterized ones. The relationship between rice and Xoo was characterized by strong differentiation among Xoo races corresponding to the subspecific differentiation of rice, by strong shifts toward increased resistance/virulence of rice/Xoo populations and by rich genetic diversity at the detected rice QR-genes and Xoo virulence genes, and by genome-wide interactions between many rice QR-genes and Xoo virulence genes in a multiple-to-multiple manner, presumably resulting either from direct protein–protein interactions or from genetic epistasis. The observed complex genetic interaction system between rice and Xoo likely exists in other crop–pathogen systems that would maintain high levels of diversity at their QR-loci/virulence-loci, resulting in dynamic coevolutionary consequences during their reciprocal adaptation.

A complex system of genetic interactions leads to reciprocal adaptation between rice and its bacterial pathogen, Xanthomonas oryzae pv. oryzae.     

Genotypic and Pathotypic Diversity of Xanthomonas oryzae pv. oryzae Strains in Taiwan

Rice bacterial leaf blight, caused by Xanthomonas oryzae pv. oryzae [(Ishiyama) Swings et al. 1990] (Xoo), is a major rice disease of the second crop season in Taiwan. A total of 88 Xoo strains collected from 10 major rice cultivating areas in Taiwan from 1986, 1997, 2000, 2004, and 2011 were characterized by repetitive‐element PCR (REP‐PCR) fingerprinting and virulence analyses. Among the five genetic clusters identified by the pJEL1/pJEL2 (IS1112‐based) and REP1R‐Dt/REP2‐D [repetitive extragenic palindromic (REP)‐based] primer sets, clusters A, C and D contained Xoo strains from geographically distant regions, which suggests a high frequency of Xoo dispersal in Taiwan. The 88 Xoo strains were evaluated by inoculations on IRBB near‐isogenic lines and five Taiwan rice cultivars. A subset of 45 moderately or highly virulent strains were classified into 15 pathotypes by their compatible or incompatible reactions on IR24 and 12 IRBB near‐isogenic lines, each containing a single resistance gene. Analysis of molecular haplotypes and pathotypes revealed a partial relationship. IRBB5, IRBB21 and IRBB4 were incompatible with 96%, 96% and 73% of the strains, so xa5, Xa21 and Xa4 can recognize most of the Xoo strains in Taiwan and elicit resistance. In contrast, IRBB3 (Xa3), IRBB8 (xa8), IRBB10 (Xa10), IRBB11 (Xa11), IRBB13 (xa13) and IRBB14 (Xa14) were susceptible to almost all of the 45 Xoo strains. Inoculation trials revealed significant differences in the susceptibility of five Taiwan cultivars to Xoo (from high to low susceptibility: Taichung Sen 10 >  IR24, Taichung Native 1 >  Taichung 192, Taikeng 9, Tainan 11). This study provides useful information for resistance breeding and the development of disease management strategies against bacterial blight disease of rice.     

gltB/D Mutants of Xanthomonas oryzae pv. oryzae are Virulence Deficient

Xanthomonas oryzae pv. oryzae (Xoo) causes bacterial blight, a serious disease of rice. Upon clip inoculation of rice leaves, Xoo causes typical V-shaped lesions whose leading edge moves through the mid-veinal region. We have isolated a virulence deficient mutant of Xoo, referred to as BXO808 that causes limited lesions which primarily extend through the side-veinal regions of rice leaves. Functional complementation studies identified a clone, pSR19, from a cosmid genomic library that restored wild-type virulence and lesion phenotype to BXO808. Transposon mutagenesis of the pSR19 clone, marker exchange experiments, and targeted mutagenesis, revealed that the BXO808 phenotype is due to mutation in the gltB/D genes of Xoo, which encode glutamate synthase subunits α and β, respectively. The gltB/D mutants that were generated in this study also exhibited virulence deficiency, an altered lesion phenotype and growth deficiency on minimal medium with low levels of ammonium as a sole nitrogen source. This is the first report that mutations in the gltB/D genes of Xoo cause virulence deficiency.     

Pathotypic variation of Xanthomonas oryzae pv. oryzae in Bangladesh

Bacterial Blight (BB) caused by Xanthomonas oryzae pv. oryzae (Xoo), a destructive disease of rice. Altogether, 96 isolates of Xoo were collected from 19 rice growing districts of Bangladesh in irrigated and rainfed seasons during 2014 to assess pathotypic variation. Pathotypic analyses on a set of 12 Near Isogenic Lines (NILs) of rice containing resistance genes viz. Xa1, Xa2, Xa3, Xa4, Xa5, Xa7, Xa8, Xa10, Xa11, Xa13, Xa14 and Xa21 and two check varieties IR24 and TN1 by leaf clip-inoculation technique. A total of 24 pathotypes were identified based on their virulence patterns on NILs tested. Among these, pathotypes VII, XII, and XIV considered as major, containing maximum number of isolates, (9.38% each) frequently distributed in North to Mid-Eastern districts of Bangladesh. Most virulent pathotype I recorded from Habiganj and Brahmanbaria. This pathotypic variation explained the pathogenic relatedness of X. oryzae pv. oryzae populations from diverse geographic areas in Bangladesh.     

Dissection of the genetic architecture of rice resistance to Xanthomonas oryzae pv. oryzae using a genomewide association study

Bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), usually causes serious rice yield loss in many countries. Rice breeders have used resistance (R) genes to control the disease but many of the resistant cultivars become susceptible few years after releasing. Identification of new R genes to Xoo is one of the main objectives in rice breeding programs. In this study, we used a genomewide association study (GWAS) to analyse the resistance against the Xoo race C1 using the Rice Diversity Panel 1 (RDP1). Disease evaluation of the RDP1 population to C1 indicated that the AUS subgroup conferred a higher level of resistance to C1 than other subgroups. Genomewide association mapping identified 15 QTLs that are distributed on chromosomes 1, 2, 3, 4, 5, 6, 8, 9, 10 and 12. Some of them are located in the regions without known resistance loci or QTLs. This study demonstrated the effectiveness of GWAS on the genetic dissection of rice resistance to Xoo and provided many Xoo resistance‐associated SNP markers for rice breeding.     

Two thiadiazole compounds promote rice defence against Xanthomonas oryzae pv. oryzae by suppressing the bacterium's production of extracellular polysaccharides

Thiazole, isothiazole, thiadiazole, and their derivatives are used to control various human, animal and plant diseases. In addition to having direct anti‐microbial and anti‐fungal properties, these compounds are thought to induce host defences, but the mechanism of defence induction remains poorly understood. This article reports that the thiadiazoles of zinc thiazole and bismerthiazol induce H₂O₂ accumulation, up‐regulation of defence‐related genes, callose deposition and hypersensitive response‐like cell death in rice leaves infected with Xanthomonas oryaze pv. oryzae (Xoo) strain ZJ173, but not in non‐infected leaves. These defence responses in Xoo‐infected leaves were suppressed by the exogenous application of catalase, which reduces H₂O₂ accumulation. The application of extracellular polysaccharides (EPSs) extracted from strain ZJ173 significantly compromised rice defence against ZJ173 with or without thiadiazole treatment. The EPS‐deficient Xoo mutant ?gumH triggered a stronger defence than its parent strain ZJ173. The thiadiazole treatments reduced EPS production by strain ZJ173, but not by the thiadiazole‐resistant strain 2‐1‐1, which is thiadiazole resistant in vivo, but not in vitro; moreover, enhanced defence was not detected in thiadiazole‐treated rice inoculated with 2‐1‐1. Based on these data, we infer that zinc thiazole and bismerthiazol promote rice defence against Xoo by inhibiting the production of bacterial EPS.