Phylogenetic Characterization and Genome Sequence Analysis of Burkholderia glumae Strains Isolated in Thailand as the Causal Agent of Rice Bacterial Panicle Blight.

Burkholderia glumae is one of the most critical rice-pathogenic bacteria, and it causes bacterial panicle blight (BPB) in rice plants. In 2017, BPB symptoms were observed from rice fields in Chiang Rai, Northern Thailand. Sixty-one isolates obtained from the symptomatic panicles of rice were initially identified as B. glumae by polymerase chain reaction (PCR) using species-specific primers. Among them, six selected strains isolated from the susceptible japonica rice cultivar DOA2 were characterized in terms of morpho-physiology, pathology, phylogenetics, and genomics. Our genome sequence analysis of the six selected strains revealed the presence of multiple prophages, which may reflect the high level of diversity in this bacterial species through dynamic horizontal gene transfer processes, including phage infection. This notion was supported by the results of phylogenetic and phylogenomic analyses, which showed the formation of several subgroups not related to the years of isolation or the geographical origins. This study reports the isolation of B. glumae as the causal pathogen of BPB disease in japonica rice in Thailand and provides genomic resources to better understand the biology and diversity of this plant pathogenic bacterium. Further studies with a vast collection of B. glumae strains from various rice-growing regions around the world are needed to elucidate the evolution, variability, and lifestyle of the pathogen.

Identification of Bacterial Blight Resistance Loci in Rice (Oryza sativa L.) against Diverse Xoo Thai Strains by Genome-Wide Association Study.

Bacterial leaf blight (BLB) is a serious disease affecting global rice agriculture caused by Xanthomonas oryzae pv. oryzae (Xoo). Most resistant rice lines are dependent on single genes that are vulnerable to resistance breakdown caused by pathogen mutation. Here we describe a genome-wide association study of 222 predominantly Thai rice accessions assayed by phenotypic screening against 20 Xoo isolates. Loci corresponding to BLB resistance were detected using >142,000 SNPs. We identified 147 genes according to employed significance thresholds across chromosomes 1-6, 8, 9 and 11. Moreover, 127 of identified genes are located on chromosomal regions outside estimated Linkage Disequilibrium influences of known resistance genes, potentially indicating novel BLB resistance markers. However, significantly associated SNPs only occurred across a maximum of six Xoo isolates indicating that the development of broad-spectrum Xoo strain varieties may prove challenging. Analyses indicated a range of gene functions likely underpinning BLB resistance. In accordance with previous studies of accession panels focusing on indica varieties, our germplasm displays large numbers of SNPs associated with resistance. Despite encouraging data suggesting that many loci contribute to resistance, our findings corroborate previous inferences that multi-strain resistant varieties may not be easily realised in breeding programs without resorting to multi-locus strategies.

Isolation and Characterization of Bacteriophages Infecting Burkholderia glumae, the Major Causal Agent of Bacterial Panicle Blight in Rice.

Bacterial panicle blight (BPB), caused by Burkholderia glumae, is one of the most severe seed-borne bacterial diseases of rice in the world, which can decrease rice production by ≤75%. Nevertheless, there are few effective measures to manage this disease. In an attempt to develop an alternative management tool for BPB, we isolated and characterized phages from soil and water that are effective to lyse several strains of B. glumae. After tests of host ranges, the phages NBP1-1, NBP4-7, and NBP4-8 were selected for further comprehensive characterization, all of which could lyse B. glumae BGLa14-8 (phage sensitive) but not B. glumae 336gr-1 (phage insensitive). This result indicates that the phages killing B. glumae cells have specific host ranges at the strain level within the bacterial species. In the greenhouse condition of this study, foliar application of the phage NBP4-7 reduced the severity of BPB caused by B. glumae BGLa14-8 ≤62% but did not cause any significant effect on the infection by B. glumae 336gr-1. Electron microscopy and whole-genome sequencing were also performed to characterize the three selected phages. Transmission electron microscopy revealed that the selected phages belong to the family Myoviridae. Furthermore, whole-genome sequence analysis indicated that the three phages belong to a same species and are closely related to the Burkholderia phage KL3, a member of the Myoviridae family.

Genome-Wide Association Analysis Identifies Resistance Loci for Bacterial Leaf Streak Resistance in Rice (Oryza sativa L.).

Bacterial leaf streak (BLS) caused by Xanthomonas oryzae pv. oryzicola (Xoc) is one of the most devastating diseases in rice production areas, especially in humid tropical and subtropical zones throughout Asia and worldwide. A genome-wide association study (GWAS) analysis conducted on a collection of 236 diverse rice accessions, mainly indica varieties, identified 12 quantitative trait loci (QTLs) on chromosomes 1, 2, 3, 4, 5, 8, 9 and 11, conferring resistance to five representative isolates of Thai Xoc. Of these, five QTLs conferred resistance to more than one Xoc isolates. Two QTLs, qBLS5.1 and qBLS2.3, were considered promising QTLs for broad-spectrum resistance to BLS. The xa5 gene was proposed as a potential candidate gene for qBLS5.1 and three genes, encoding pectinesterase inhibitor (OsPEI), eukaryotic zinc-binding protein (OsRAR1), and NDP epimerase function, were proposed as candidate genes for qBLS2.3. Results from this study provide an insight into the potential QTLs and candidate genes for BLS resistance in rice. The recessive xa5 gene is suggested as a potential candidate for strong influence on broad-spectrum resistance and as a focal target in rice breeding programs for BLS resistance.

An xa5 Resistance Gene-Breaking Indian Strain of the Rice Bacterial Blight Pathogen Xanthomonas oryzae pv. oryzae Is Nearly Identical to a Thai Strain.

The rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) constrains production in major rice growing countries of Asia. Xoo injects transcription activator-like effectors (TALEs) that bind to and activate host "susceptibility" (S) genes that are important for disease. The bacterial blight resistance gene xa5, which reduces TALE activity generally, has been widely deployed. However, strains defeating xa5 have been reported in India and recently also in Thailand. We completely sequenced and compared the genomes of one such strain from each country and examined the encoded TALEs. The two genomes are nearly identical, including the TALE genes, and belong to a previously identified, highly clonal lineage. Each strain harbors a TALE known to activate the major S gene SWEET11 strongly enough to be effective even when diminished by xa5. The findings suggest international migration of the xa5-compatible pathotype and highlight the utility of whole genome sequencing and TALE analysis for understanding and responding to breakdown of resistance.

Genome-wide association mapping of virulence gene in rice blast fungus Magnaporthe oryzae using a genotyping by sequencing approach.

Magnaporthe oryzae is a fungal pathogen causing blast disease in many plant species. In this study, seventy three isolates of M. oryzae collected from rice (Oryza sativa) in 1996-2014 were genotyped using a genotyping-by-sequencing approach to detect genetic variation. An association study was performed to identify single nucleotide polymorphisms (SNPs) associated with virulence genes using 831 selected SNP and infection phenotypes on local and improved rice varieties. Population structure analysis revealed eight subpopulations. The division into eight groups was not related to the degree of virulence. Association mapping showed five SNPs associated with fungal virulence on chromosome 1, 2, 3, 4 and 7. The SNP on chromosome 1 was associated with virulence against RD6-Pi7 and IRBL7-M which might be linked to the previously reported AvrPi7.

Sampling for disease absence-deriving informed monitoring from epidemic traits.

Monitoring for disease requires subsets of the host population to be sampled and tested for the pathogen. If all the samples return healthy, what are the chances the disease was present but missed? In this paper, we developed a statistical approach to solve this problem considering the fundamental property of infectious diseases: their growing incidence in the host population. The model gives an estimate of the incidence probability density as a function of the sampling effort, and can be reversed to derive adequate monitoring patterns ensuring a given maximum incidence in the population. We then present an approximation of this model, providing a simple rule of thumb for practitioners. The approximation is shown to be accurate for a sample size larger than 20, and we demonstrate its use by applying it to three plant pathogens: citrus canker, bacterial blight and grey mould.

A Strain of an Emerging Indian Xanthomonas oryzae pv. oryzae Pathotype Defeats the Rice Bacterial Blight Resistance Gene xa13 Without Inducing a Clade III SWEET Gene and Is Nearly Identical to a Recent Thai Isolate.

The rice bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) injects transcription activator-like effectors (TALEs) that bind and activate host "susceptibility" (S) genes important for disease. Clade III SWEET genes are major S genes for bacterial blight. The resistance genes xa5, which reduces TALE activity generally, and xa13, a SWEET11 allele not recognized by the cognate TALE, have been effectively deployed. However, strains that defeat both resistance genes individually were recently reported in India and Thailand. To gain insight into the mechanism(s), we completely sequenced the genome of one such strain from each country and examined the encoded TALEs. Strikingly, the two strains are clones, sharing nearly identical TALE repertoires, including a TALE known to activate SWEET11 strongly enough to be effective even when diminished by xa5. We next investigated SWEET gene induction by the Indian strain. The Indian strain induced no clade III SWEET in plants harboring xa13, indicating a pathogen adaptation that relieves dependence on these genes for susceptibility. The findings open a door to mechanistic understanding of the role SWEET genes play in susceptibility and illustrate the importance of complete genome sequence-based monitoring of Xoo populations in developing varieties with effective disease resistance.