Genetic and functional characterization of the rice bacterial blight disease resistance gene xa5

Transcriptome and Metabolome Analysis of Rice Cultivar CBB23 after Inoculation by Xanthomonas oryzae pv. oryzae Strains AH28 and PXO99A

Bacterial leaf blight (BLB), among the most serious diseases in rice production, is caused by Xanthomonas oryzae pv. oryzae (Xoo). Xa23, the broadest resistance gene against BLB in rice, is widely used in rice breeding. In this study, the rice variety CBB23 carrying the Xa23 resistance gene was inoculated with AH28 and PXO99A to identify differentially expressed genes (DEGs) associated with the resistance. Transcriptome sequencing of the infected leaves showed 7997 DEGs between the two strains at different time points, most of which were up-regulated, including cloned rice anti-blight, peroxidase, pathology-related, protein kinase, glucosidase, and other coding genes, as well as genes related to lignin synthesis, salicylic acid, jasmonic acid, and secondary metabolites. Additionally, the DEGs included 40 cloned, five NBS-LRR, nine SWEET family, and seven phenylalanine aminolyase genes, and 431 transcription factors were differentially expressed, the majority of which belonged to the WRKY, NAC, AP2/ERF, bHLH, and MYB families. Metabolomics analysis showed that a large amount of alkaloid and terpenoid metabolite content decreased significantly after inoculation with AH28 compared with inoculation with PXO99A, while the content of amino acids and their derivatives significantly increased. This study is helpful in further discovering the pathogenic mechanism of AH28 and PXO99A in CBB23 rice and provides a theoretical basis for cloning and molecular mechanism research related to BLB resistance in rice.

Rapid generation of fragrant thermo-sensitive genic male sterile rice with enhanced disease resistance via CRISPR/Cas9

MAIN CONCLUSION

The three, by mutagenesis produced genes OsPi21, OsXa5, and OsBADH2, generated novel lines exhibiting desired fragrance and improved resistance. Elite sterile lines are the basis for hybrid rice breeding, and rice quality and disease resistance become the focus of new sterile lines breeding. Since there are few sterile lines with fragrance and high resistance to blast and bacterial blight at the same time in hybrid rice production, we here integrated the simultaneous mutagenesis of three genes, OsPi21, OsXa5, and OsBADH2, into Zhi 5012S, an elite thermo-sensitive genic male sterile (TGMS) variety, using the CRISPR/Cas9 system, thus eventually generated novel sterile lines would exhibit desired popcorn-like fragrance and improved resistance to blast and bacterial blight but without a loss in major agricultural traits such as yield. Collectively, this study develops valuable germplasm resources for the development of two-line hybrid rice with disease resistance, which provides a way to rapid generation of novel TGMS lines with elite traits.

Research Progress on Cloning and Function of Xa Genes Against Rice Bacterial Blight

Bacterial blight (BB) of rice caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most serious bacterial diseases that hinder the normal growth and production of rice, which greatly reduces the quality and yield of rice. The effect of traditional methods such as chemical control is often not ideal. A series of production practices have shown that among the numerous methods for BB controlling, breeding and using resistant varieties are the most economical, effective, and environmentally friendly, and the important basis for BB resistance breeding is the exploration of resistance genes and their functional research. So far, 44 rice BB resistance genes have been identified and confirmed by international registration or reported in journals, of which 15 have been successfully cloned and characterized. In this paper, research progress in recent years is reviewed mainly on the identification, map-based cloning, molecular resistance mechanism, and application in rice breeding of these BB resistance genes, and the future influence and direction of the remained research for rice BB resistance breeding are also prospected.

Engineering broad-spectrum disease-resistant rice by editing multiple susceptibility genes

Rice blast and bacterial blight are important diseases of rice (Oryza sativa) caused by the fungus Magnaporthe oryzae and the bacterium Xanthomonas oryzae pv. oryzae (Xoo), respectively. Breeding rice varieties for broad-spectrum resistance is considered the most effective and sustainable approach to controlling both diseases. Although dominant resistance genes have been extensively used in rice breeding and production, generating disease-resistant varieties by altering susceptibility (S) genes that facilitate pathogen compatibility remains unexplored. Here, using CRISPR/Cas9 technology, we generated loss-of-function mutants of the S genes Pi21 and Bsr-d1 and showed that they had increased resistance to M. oryzae. We also generated a knockout mutant of the S gene Xa5 that showed increased resistance to Xoo. Remarkably, a triple mutant of all three S genes had significantly enhanced resistance to both M. oryzae and Xoo. Moreover, the triple mutant was comparable to the wild type in regard to key agronomic traits, including plant height, effective panicle number per plant, grain number per panicle, seed setting rate, and thousand-grain weight. These results demonstrate that the simultaneous editing of multiple S genes is a powerful strategy for generating new rice varieties with broad-spectrum resistance.

Genome-Wide Association Study Dissects Resistance Loci against Bacterial Blight in a Diverse Rice Panel from the 3000 Rice Genomes Project

BACKGROUND

Bacterial blight (BB), caused by Xanthomonas oryzae pv. oryzae (Xoo) is one of the most devastating bacterial diseases of rice in temperate and tropical regions. Breeding and deployment of resistant cultivars carrying major resistance (R) genes has been the most effective approach for BB management. However, because of specific interaction of each R gene with the product of the corresponding pathogen avirulence or effector gene, new pathogen strains that can overcome the deployed resistance often emerge rapidly. To deal with ever-evolving Xoo, it is necessary to identify novel R genes and resistance quantitative trait loci (QTL).

RESULTS

BB resistance of a diverse panel of 340 accessions from the 3000 Rice Genomes Project (3 K RGP) was evaluated by artificial inoculation with four representative Xoo strains, namely Z173 (C4), GD1358 (C5), V from China and PXO339 (P9a) from Philippines. Using the 3 K RG 4.8mio filtered SNP Dataset, a total of 11 QTL associated with BB resistance on chromosomes 4, 5, 11 and 12 were identified through a genome-wide association study (GWAS). Among them, eight resistance loci, which were narrowed down to relatively small genomic intervals, coincided with previously reported QTL or R genes, e.g. xa5, xa25, xa44(t). The other three QTL were putative novel loci associated with BB resistance. Linear regression analysis showed a dependence of BB lesion length on the number of favorable alleles, suggesting that pyramiding QTL using marker-assisted selection would be an effective approach for improving resistance. In addition, the Hap2 allele of LOC_Os11g46250 underlying qC5-11.1 was validated as positively regulating resistance against strain C5.

CONCLUSIONS

Our findings provide valuable information for the genetic improvement of BB resistance and application of germplasm resources in rice breeding programs.

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.

OsNPR3.3-dependent salicylic acid signaling is involved in recessive gene xa5-mediated immunity to rice bacterial blight

Salicylic acid (SA) is a key natural component that mediates local and systemic resistance to pathogens in many dicotyledonous species. However, its function is controversial in disease resistance in rice plants. Here, we show that the SA signaling is involved in both pathogen-associated-molecular-patterns triggered immunity (PTI) and effector triggered immunity (ETI) to Xanthomonas oryzae pv. Oryzae (Xoo) mediated by the recessive gene xa5, in which OsNPR3.3 plays an important role through interacting with TGAL11. Rice plants containing homozygous xa5 gene respond positively to exogenous SA, and their endogenous SA levels are also especially induced upon infection by the Xoo strain, PXO86. Depletion of endogenous SA can significantly attenuate plant resistance to PXO86, even to 86∆HrpXG (mutant PXO86 with a damaged type III secretion system). These results indicated that SA plays an important role in disease resistance in rice plants, which can be clouded by high levels of endogenous SA and the use of particular rice varieties.

TALEN-based editing of TFIIAy5 changes rice response to Xanthomonas oryzae pv. Oryzae

The xa5 gene encodes a basal transcription factor (TFIIAγ) protein with wide spectrum resistance to bacterial blight caused by Xanthomonas oryzae pv. Oryzae (Xoo) in rice. It was only found in a few rice ecotypes, and the recessive characteristics limited its application in breeding. Here, we employed a TALEN-based technique to edit its dominant allelic TFIIAγ5 and obtained many mutant TFIIAγ5 genes. Most of them reduced rice susceptibility to varying degrees when the plants were challenged with the Xoo. In particular, the knocked-out TFIIAγ5 can reduce the rice susceptibility significantly, although it cannot reach the xa5-mediated resistance level, indicating TFIIAγ5 is a major component involved in disease susceptibility. In addition, the mutant encoding the protein with deletion of the 32nd amino acid or amino acid insertion between 32nd and 33rd site confers rice with the similar resistance to that of the knocked-out TFIIAγ5. Thus, the amino acids around 32nd site are also the important action sites of TFIIAγ5 besides the 39th amino acid previously reported. Moreover, the integration of xa5 into TFIIAγ5-knockout plants conferred them with a similar resistance as IRBB5, the rice variety containing the homozygous xa5 gene. Thus, TFIIAγ5 was not simply regarded as a resistant or a susceptible locus, as the substitution of amino acids might shift its functions.

Deployment of Genetic and Genomic Tools Toward Gaining a Better Understanding of Rice-Xanthomonas oryzae pv. oryzae Interactions for Development of Durable Bacterial Blight Resistant Rice

Rice is the most important food crop worldwide and sustainable rice production is important for ensuring global food security. Biotic stresses limit rice production significantly and among them, bacterial blight (BB) disease caused by Xanthomonas oryzae pv. oryzae (Xoo) is very important. BB reduces rice yields severely in the highly productive irrigated and rainfed lowland ecosystems and in recent years; the disease is spreading fast to other rice growing ecosystems as well. Being a vascular pathogen, Xoo interferes with a range of physiological and biochemical exchange processes in rice. The response of rice to Xoo involves specific interactions between resistance (R) genes of rice and avirulence (Avr) genes of Xoo, covering most of the resistance genes except the recessive ones. The genetic basis of resistance to BB in rice has been studied intensively, and at least 44 genes conferring resistance to BB have been identified, and many resistant rice cultivars and hybrids have been developed and released worldwide. However, the existence and emergence of new virulent isolates of Xoo in the realm of a rapidly changing climate necessitates identification of novel broad-spectrum resistance genes and intensification of gene-deployment strategies. This review discusses about the origin and occurrence of BB in rice, interactions between Xoo and rice, the important roles of resistance genes in plant's defense response, the contribution of rice resistance genes toward development of disease resistance varieties, identification and characterization of novel, and broad-spectrum BB resistance genes from wild species of Oryza and also presents a perspective on potential strategies to achieve the goal of sustainable disease management.

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.

Dominant and Recessive Major R Genes Lead to Different Types of Host Cell Death During Resistance to Xanthomonas oryzae in Rice

The bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo) is the most devastating bacterial disease of rice worldwide. A number of dominant major disease resistance (MR) genes and recessive MR genes against Xoo have been cloned and molecularly characterized in the last two decades. However, how these MR genes mediated-resistances occur at the cytological level is largely unknown. Here, by ultrastructural examination of xylem parenchyma cells, we show that resistances to Xoo conferred by dominant MR genes and recessive MR genes resulted in different types of programmed cell death (PCD). Three dominant MR genes Xa1, Xa4, and Xa21 and two recessive MR genes xa5 and xa13 that encode very different proteins were used in this study. We observed that Xa1-, Xa4-, and Xa21-mediated resistances to Xoo were associated mainly with autophagy-like cell death featured by the formation of autophagosome-like bodies in the xylem parenchyma cells. In contrast, the xa5- and xa13-mediated resistances to Xoo were associated mainly with vacuolar-mediated cell death characterized by tonoplast disruption of the xylem parenchyma cells. Application of autophagy inhibitor 3-methyladenine partially compromised Xa1-, Xa4-, and Xa21-mediated resistances, as did Na₂HPO₄ alkaline solution to xa5- and xa13-mediated resistances. These results suggest that autophagy-like cell death is a feature of the dominant MR gene-mediated resistance to Xoo and vacuolar-mediated cell death is a characteristic of the recessive MR gene-mediated resistance.

Panicle blast 1 (Pb1) resistance is dependent on at least four QTLs in the rice genome

BACKGROUND

Rice blast is the most serious disease afflicting rice and there is an urgent need for the use of disease resistance (R) genes in blast tolerance breeding programs. Pb1 is classified as a quantitative resistance gene and it does not have fungal specificity. Pb1-mediated resistance develops in the latter stages of growth. However, some cultivars, such as Kanto209 (K209), cultivar name Satojiman, despite possessing Pb1, do not exert resistance to rice blast during the reproductive stage.

RESULTS

We found that the expression of WRKY45 gene downstream of Pb1 was weakly induced by rice blast inoculation at the full heading stage in K209. Genetic analysis using the SNP-based Golden Gate assay of K209 crossing with Koshihikari Aichi SBL (KASBL) found at least four regions related to the resistance in the rice genome (Chr8, Chr9, Chr7, Chr11). Mapping of QTL related to Chr7 confirmed the existence of factors that were required for the resistance of Pb1 in the 22 to 23 Mbp region of the rice genome.

CONCLUSION

We clarified how the K209 cultivar is vulnerable to the blast disease despite possessing Pb1 and found the DNA marker responsible for the quantitative resistance of Pb1. We identified the QTL loci required for Pb1-mediated resistance to rice panicle blast. Pb1 was negatively dependent on at least three QTLs, 7, 9 and 11, and positively dependent on one, QTL 8, in the K209 genome. This finding paves the way for creating a line to select optimal QTLs in order to make use of Pb1-mediated resistance more effectively.

Plant innate immunity in rice: a defense against pathogen infection

A large number of pathogenic microorganisms cause rice diseases that lead to enormous yield losses worldwide. Such losses are important because rice is a staple food for more than half of the world's population. Over the past two decades, the extensive study of the molecular interactions between rice and the fungal pathogen Magnaporthe oryzae and between rice and the bacterial pathogen Xanthomonas oryzae pv. oryzae has made rice a model for investigating plant–microbe interactions of monocotyledons. Impressive progress has been recently achieved in understanding the molecular basis of rice pathogen-associated molecular pattern-immunity and effector-triggered immunity. Here, we briefly summarize these recent advances, emphasizing the diverse functions of the structurally conserved fungal effectors, the regulatory mechanisms of the immune receptor complexes, and the novel strategies for breeding disease resistance. We also discuss future research challenges.

Molecular recognition of avirulence protein (avrxa5) by eukaryotic transcription factor xa5 of rice (Oryza sativa L.): insights from molecular dynamics simulations

The avirulence gene avrxa5 of bacterial blight pathogen Xanthomonas oryzae pv. oryzae (Xoo) recognized by the resistant rice lines having corresponding resistance (xa5) gene in a gene-for-gene manner. We used a combinatorial approach involving protein-protein docking, molecular dynamics (MD) simulations and binding free energy calculations to gain novel insights into the gene-for-gene mechanism that governs the direct interaction of R-Avr protein. From the best three binding poses predicted by molecular docking, MD simulations were performed to explore the dynamic binding mechanism of xa5 and avrxa5. Molecular Mechanics/Poisson Boltzmann Surface Area (MM/PBSA) techniques were employed to calculate the binding free energy and to uncover the thriving force behind the molecular recognition of avrxa5 by eukaryotic transcription factor xa5. Binding free energy analysis revealed van der Waals term as the most constructive component that favors the xa5 and avrxa5 interaction. In addition, hydrogen bonds (H-bonds) and essential electrostatic interactions analysis highlighted amino acid residues Lys54/Asp870, Lys56/Ala868, Lys56/Ala866, Lys56/Glu871, Ile59/His862, Gly61/Phe858, His62/Arg841, His62/Leu856, Ser101/Ala872 and Ser105/Asp870 plays pivotal role for the energetically stability of the R-Avr complex. Insights gained from the present study are expected to unveil the molecular mechanisms that define the transcriptional activator mediated transcriptome modification in host plants.

Novel insights into rice innate immunity against bacterial and fungal pathogens

Rice feeds more than half of the world's population. Rice blast, caused by the fungal pathogen Magnaporthe oryzae, and bacterial blight, caused by the bacterial pathogen Xanthomonas oryzae pv. oryzae, are major constraints to rice production worldwide. Genome sequencing and extensive molecular analysis has led to the identification of many new pathogen-associated molecular patterns (PAMPs) and avirulence and virulence effectors in both pathogens, as well as effector targets and receptors in the rice host. Characterization of these effectors, host targets, and resistance genes has provided new insight into innate immunity in plants. Some of the new findings, such as the binding activity of X. oryzae transcriptional activator-like (TAL) effectors to specific rice genomic sequences, are being used for the development of effective disease control methods and genome modification tools. This review summarizes the recent progress toward understanding the recognition and signaling events that govern rice innate immunity.

Molecular phylogeny, homology modeling, and molecular dynamics simulation of race-specific bacterial blight disease resistance protein (xa5) of rice: a comparative agriproteomics approach

Rice (Oryza sativa L.), a model plant belonging to the family Poaceae, is a staple food for a majority of the people worldwide. Grown in the tropical and subtropical regions of the world, this important cereal crop is under constant and serious threat from both biotic and abiotic stresses. Among the biotic threats, Xanthomonas oryzae pv. oryzae, causing the damaging bacterial blight disease in rice, is a prominent pathogen. The xa5 gene in the host plant rice confers race-specific resistance to this pathogen. This recessive gene belongs to the Xa gene family of rice and encodes a gamma subunit of transcription factor IIA (TFIIAγ). In view of the importance of this gene in conferring resistance to the devastating disease, we reconstructed the phylogenetic relationship of this gene, developed a three-dimensional protein model, followed by long-term molecular dynamics simulation studies to gain a better understanding of the evolution, structure, and function of xa5. The modeled structure was found to fit well with the small subunit of TFIIA from human, suggesting that it may also act as a small subunit of TFIIA in rice. The model had a stable conformation in response to the atomic flexibility and interaction, when subjected to MD simulation at 20 nano second in aqueous solution. Further structural analysis of xa5 indicated that the protein retained its basic transcription factor function, suggesting that it might govern a novel pathway responsible for bacterial blight resistance. Future molecular docking studies of xa5 underway with its corresponding avirulence gene is expected to shed more direct light into plant-pathogen interactions at the molecular level and thus pave the way for richer agriproteomic insights.

Rice versus Xanthomonas oryzae pv. oryzae: a unique pathosystem

Bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is a devastating disease of rice worldwide. The qualitative or pathogen race-specific resistance to this pathogen conferred by major disease resistance (MR) genes has been widely used in rice improvement. Accumulating genetic and molecular data have revealed that the molecular mechanisms of rice qualitative resistance to Xoo are largely different from those of qualitative resistance in other plant-pathogen pathosystems. In this review, we focus on the unique features of rice qualitative resistance to Xoo based on MR genes that have been identified and characterized. The distinctiveness of the rice-Xoo interaction provides a unique pathosystem to elucidate the diverse molecular mechanisms in plant qualitative resistance.

Identification of the novel recessive gene pi55(t) conferring resistance to Magnaporthe oryzae

The elite rice cultivar Yuejingsimiao 2 (YJ2) is characterized by a high level of grain quality and yield, and resistance against Magnaporthe oryzae. YJ2 showed 100% resistance to four fungal populations collected from Guangdong, Sichuan, Liaoning, and Heilongjiang Provinces, which is a higher frequency than that shown by the well-known resistance (R) gene donor cultivars such as Sanhuangzhan 2 and 28zhan. Segregation analysis for resistance with F(2) and F(4) populations indicated the resistance of YJ2 was controlled by multiple genes that are dominant or recessive. The putative R genes of YJ2 were roughly tagged by SSR markers, located on chromosomes 2, 6, 8, and 12, in a bulked-segregant analysis using genome-wide selected SSR markers with F(4) lines that segregated into 3 resistant (R):1 susceptible (S) or 1R:3S. The recessive R gene on chromosome 8 was further mapped to an interval ≈1.9 cM/152 kb in length by linkage analysis with genomic position-ready markers in the mapping population derived from an F(4) line that segregated into 1R:3S. Given that no major R gene was mapped to this interval, the novel R gene was designated as pi55(t). Out of 26 candidate genes predicted in the region based on the reference genomic sequence of the cultivar Nipponbare, two genes that encode a leucine-rich repeat-containing protein and heavy-metal-associated domain-containing protein, respectively, were suggested as the most likely candidates for pi55(t).

Colonization of rice leaf blades by an African strain of Xanthomonas oryzae pv. oryzae depends on a new TAL effector that induces the rice nodulin-3 Os11N3 gene

African strains of Xanthomonas oryzae pv. oryzae contain fewer TAL effectors than Asian strains, and their contribution to pathogenicity is unknown. Systematic mutagenesis of tal genes was used to decipher the contribution of each of the eight TAL effector paralogs to pathogenicity of African X. oryzae pv. oryzae BAI3. A strain mutated in talC was severely affected in the production of disease symptoms. Analysis of growth in planta upon leaf-clip inoculation showed that mutant bacteria multiplied only at the site of inoculation at the apex of the leaf, suggesting a requirement for talC during colonization of vascular tissues. Such tissue-specific effect of a tal mutant is a novel phenotype, which has not yet been characterized in other xanthomonads. Microarray experiments comparing the host response of rice leaves challenged with BAI3(R) vs. BAI3(R)ΔtalC were performed to identify genes targeted by TalC. A total of 120 upregulated and 21 downregulated genes were identified, among them Os11N3, which is a member of the MtN3/saliva family. Based on semiquantitative reverse transcription-polymerase chain reaction and β-glucuronidase reporter assays, we show that Os11N3 is directly upregulated by TalC and identify a TalC DNA target box within the Os11N3 upstream sequence.

Xanthomonas AvrBs3 family-type III effectors: discovery and function

Xanthomonads are bacterial plant pathogens that cause diseases on many plant species, including important crops. Key to pathogenicity of most Xanthomonas pathovars is a Hrp-type III secretion (T3S) system that translocates effector proteins into plant cells. Within the eukaryotic cell, the effectors are thought to perform a variety of tasks to support bacterial virulence, proliferation, and dissemination. We are only beginning to understand the host targets of different effectors. The largest effector family found in Xanthomonas spp. is the AvrBs3/PthA or TAL (transcription activator-like) family. TAL effectors act as transcriptional activators in the plant cell nucleus. Specificity of TAL effectors is determined by a novel modular DNA-binding domain. Here, we describe the discovery of TAL effectors and their structure, activity, and host targets.

A benefit of high temperature: increased effectiveness of a rice bacterial blight disease resistance gene

*Continuous planting of crops containing single disease resistance (R) genes imposes a strong selection for virulence in pathogen populations, often rendering the R gene ineffective. Increasing environmental temperatures may complicate R-gene-mediated disease control because high temperatures often promote disease development and reduce R gene effectiveness. Here, performance of one rice bacterial blight disease R gene was assessed in field and growth chamber studies to determine the influence of temperature on R gene effectiveness and durability. *Disease severity and virulence of Xanthomonas oryzae pv. oryzae (Xoo) populations were monitored in field plots planted to rice with and without the bacterial blight R gene Xa7 over 11 yr. The performance of Xa7 was determined in high- and low-temperature regimes in growth chambers. *Rice with Xa7 exhibited less disease than lines without Xa7 over 11 yr, even though virulence of Xoo field populations increased. Xa7 restricted disease more effectively at high than at low temperatures. Other R genes were less effective at high temperatures. *We propose that Xa7 restricts disease and Xoo population size more efficiently in high temperature cropping seasons compared with cool seasons creating fluctuating selection, thereby positively impacting durability of Xa7.

Transcription activator-like type III effector AvrXa27 depends on OsTFIIAgamma5 for the activation of Xa27 transcription in rice that triggers disease resistance to Xanthomonas oryzae pv. oryzae

The transcription activator-like (TAL) type III effector AvrXa27 from Xanthomonas oryzae pv. oryzae (Xoo) strain PXO99(A) activates the transcription of the host resistance gene Xa27, which results in disease resistance to bacterial blight (BB) in rice. In this study, we show that AvrXa27-activated Xa27 transcription requires host general transcription factor OsTFIIAgamma5. The V39E substitution in OsTFIIAgamma5, encoded by the recessive resistance gene xa5 in rice, greatly attenuates this activation in xa5 and Xa27 double homozygotes on inoculation with Xa27-incompatible strains. The xa5 gene also causes attenuation in the induction of Xa27 by AvrXa27 expressed in rice. The xa5-mediated attenuation of Xa27-mediated resistance to PXO99(A) is recessive. Intriguingly, xa5-mediated resistance to xa5-incompatible strains is also down-regulated in the xa5 and Xa27 double homozygotes. In addition, AvrXa27 expressed in planta shows weak virulence activity in the xa5 genetic background and causes enhanced susceptibility of the plants to BB inoculation. The results suggest that TAL effectors target host general transcription factors to directly manipulate the host transcriptional machinery for virulence and/or avirulence. The identification of xa5-mediated attenuation of Xa27-mediated resistance to Xoo provides a guideline for breeding resistance to BB when pyramiding xa5 with other resistance genes.

Fine genetic mapping of xa24, a recessive gene for resistance against Xanthomonas oryzae pv. oryzae in rice

Bacterial blight, caused by Xanthomonas oryzae pv. oryzae (Xoo), is the most devastating plant bacterial disease worldwide. Different bacterial blight resistance (R) genes confer race-specific resistance to different strains of Xoo. We fine mapped a fully recessive gene, xa24, for bacterial blight resistance to a 71-kb DNA fragment in the long arm of rice chromosome 2 using polymerase chain reaction-based molecular markers. The xa24 gene confers disease resistance at the seedling and adult stages. It mediates resistance to at least the Philippine Xoo races 4, 6 and 10 and Chinese Xoo strains Zhe173, JL691 and KS-1-21. Sequence analysis of the DNA fragment harboring the dominant (susceptible) allele of xa24 suggests that this gene should encode a novel protein that is not homologous to any known R proteins. These results will greatly facilitate the isolation and characterization of xa24. The markers will be convenient tools for marker-assisted selection of xa24 in breeding programs.

Manipulation of the eukaryotic transcriptional machinery by bacterial pathogens

Pathogenic bacteria inject a diverse set of effectors into eukaryotic host cells and manipulate the cellular environment to enhance bacterial fitness. There is mounting evidence that the transcription machinery in the nucleus of plant cells is a target of various bacterial effectors. These effectors seem to mimic the actions of host nuclear components. To monitor and counteract such virulence-promoting strategies, plants have acquired unique immune-sensing mechanisms.