Leaf Blight Of Rice Research Articles

The host basal transcription factor IIA subunits coordinate for facilitating infection of TALEs-carrying bacterial pathogens in rice

Many plant-pathogenic Xanthomonas rely largely on secreting virulence transcription activator-like effectors (TALEs) proteins into plant nucleus to activate host susceptibility genes to cause disease, the process is dependent on pathogen TALEs association with host plants basal transcription factor IIA small subunit TFIIAγ. TFIIAγ together with large subunit TFIIAαβ constitute as a key component of RNA polymerase II complex for transcriptome initiation. However, whether TFIIAαβ coordinates or competes with pathogen TALEs for interaction with TFIIAγ to activate transcript of TALEs-targeting genes is unclear. Here, we showed that TALE-carrying bacterial pathogens Xanthomonas oryzae pv. oryzae (Xoo) and Xanthomonas oryzae pv. oryzicola (Xoc), the causal agent for bacterial leaf blight and bacterial leaf streak in rice, using their major virulence TALEs to physically associate with N-terminal of OsTFIIAγ5. OsTFIIAα and OsTFIIAβ which are post-translationally mature proteins of OsTFIIAαβ separately bound to N- and C-terminal of OsTFIIAγ5. OsTFIIAα coordinated with TALEs for binding with OsTFIIAγ5 to upregulate rice susceptibility genes to cause disease. Conversely, suppression of OsTFIIAαβ attenuated TALEs-targeting genes transcription, thus improved broad-spectrum disease resistance of rice to Xoo and Xoc. These results provide an applicable strategy for improving resistance to TALE-carrying pathogens in rice by appropriate suppression of plant basal transcription factors expression.

Bacterial Leaf Blight of Rice: An Overview of Epidemiology and Management with Special Reference to-Indian-Sub-Continent

Bacterial Leaf Blight of Rice: An Overview of Epidemiology and Management with Special Reference to-Indian-Sub-Continent

Herbal Extracts in Combination with Nanosilver Inhibit Blight Disease Caused by Xanthomonas oryzae pv. oryzae in Rice

Bacterial rice leaf blight (BLB), caused by Xanthomonas oryzae pv. oryzae (Xoo), has a massive impact on the quality and productivity of rice. Besides BLB resistant rice cultivars, herbal extracts and nanosilver have increasingly demonstrated their important roles in controlling the disease as alternatives to synthetic chemical pesticides. Therefore, this research aimed to examine the Xoo antibacterial effects of several herbal extracts and nanosilver in vitro and in vivo. In the study, Wedelia chinensis Osbeck Merr., Clerodendrum fragrans Vent., Excoecaria cochinchinensis Lour., Polyathia longifolia var. Pendula, and Caesalpinia sappan L. were extracted by maceration with six types of solvents (distilled water, 70% ethanol, chloroform, n-hexane, and 100% acetonitrile), then used in an agar diffusion test to evaluate their Xoo antibacterial effects. The results showed that 70% ethanol was the best extracting solvent for the targeted plants. C. fragrans, E. Cochinchinensis, and C. sappan showed significant antibacterial effects with inhibition zone diameters of 28.50 cm, 21.00 cm, and 25.70 cm, respectively. Finally, the individual extract from C. fragrans, E. Cochinchinensis, and C. sappan were combined with nanosilver particles and used to access BLB inhibition capacity in vivo, using the rice cultivar IR24 as the target for Xoo infection. Application of the C. fragrans extract resulted in resistance of IR24 rice to BLB. Similar results were also observed in the infected rice when products combining nanosilver and E. cochinchinensis or C. sappan were applied to infected rice leaves.

Evaluation of antibacterial activity of mangosteen (Garcinia mangostana L.) pericarp extract against rice leaf blight bacteria (Xanthomonas oryzae pv. oryzae) at various temperatures and durations of fruit storage

Bioactive contents of mangosteen pericarp can be used as antibacterial agent. This study aimed to examine the antibacterial activity of mangosteen pericarp extract (Garcinia mangostana L.) against rice leaf blight bacteria (Xanthomonas oryzae pv. oryzae) at different temperature and duration of fruit storage, and to identify the antibacterial bioactive metabolites present in the pericarp extract. The antibacterial experiment was arranged with Randomized Completely Block Design (RCBD) with two factors, namely: storage temperature (0, 13.5, and 27 °C) and storage duration (1, 7, and 14 days), with 3 replicates. There was significant interaction between storage temperature and storage duration towards the antibacterial activity of mangosteen pericarp extract. The best was showed by the fruits that was kept in temperature of 13,5 °C for 7 days of storage with inhibition zone of 24.46 mm and the minimum inhibitory concentration (MIC) of 25%. Gas chromatography-mass spectrometry (GC-MS) result showed antibacterial bioactive compounds including (-)-Alpha-Copaene and 2H,6H-Pyran [3,2-B] Xanthen-6-One,5,8-Dihydroxy-9-Methoxy-2,2-Dimethyl-10-(3-Methyl-2-Butenyl)-on the fruits stored in temperature of 0 °C and duration of 1 day, and Copaene on the fruits stored during 7 days on temperature of 13,5 °C. In conclusion, mangosteen pericarp is a promising source of antibacterial bioactive against Xoo pathogen.

Plant growth promotion and suppression of bacterial leaf blight in rice by Paenibacillus polymyxa Sx3.

The effects and mechanisms of Paenibacillus polymyxa Sx3 on growth promotion and the suppression of bacterial leaf blight in rice were evaluated in this study. The results from a plate assay indicated that Sx3 inhibited the growth of 20 strains of Xanthomonas oryzae pv. oryzae (Xoo). Rice seedling experiments indicated that Sx3 promoted plant growth and suppressed bacterial leaf blight. In addition, bacteriologicaltests showed that Sx3 was able to fix nitrogen, solubilize phosphate and produce indole acetic acid, indicating that various mechanisms may be involved in the growth promotion by Sx3. The culture filtrate of P. polymyxa Sx3 reduced bacterial growth, biofilm formation and disrupted the cell morphology of Xoo strain GZ 0005, as indicated by the transmission and scanning electron microscopic observations. In addition, MALDI-TOF MS analysis revealed that Sx3 could biosynthesize two types of secondary metabolites fusaricidins and polymyxin P. In summary, this study clearly indicated that P. polymyxa Sx3 has strong invitro and invivo antagonistic activity against Xoo, which may be at least partially attributed to its production of secondary metabolites. SIGNIFICANCE AND IMPACT OF THE STUDY: Antagonistic bacteria can grow well in their originating environment. However, it is unclear whether antagonistic bacteria were able to survive in different ecological environments. This study revealed that Paenibacillus polymyxa Sx3 isolated from rhizosphere soil of cotton significantly promoted the plant growth and suppressed bacterial leaf blight in rice. Therefore, it could be inferred that P. polymyxa Sx3 has the potential to be used as biocontrol agents in plants grown in different ecological environments.

FOURIER TRANSFORM INFRARED CHARACTERIZATION OF SALICYLIC ACID TREATED RICE PLANTS AGAINST Xanthomonas oryzae pv. oryzae

<p>Salicylic acid (SA) plays an important role in induction of plant defence against a wide range of biotic and abiotic stresses through physiological, biochemical and molecular responses. The study was carried out to evaluate defence reponses of SA- induced rice plants against leaf blight (LB) disease, caused by <em>X</em><em>anthomonas oryzae</em> pv. <em>oryzae</em> (Xoo). In this study, a series of SA solutions at concentrations of 0.25, 0.50, 0.75, 1.0, 2.0 and 4.0 mM were evaluated for their potential to protect rice LB in the net house conditions. Biochemical analyses were characterized by means of a FTIR spectroscope. Our results showed that at inoculated leaves, the elicitor of SA at a concentration of 1.0 mM had highest reduction of disease severity approximately 34 % at 21 days after inoculation, compared to the control. Moreover, SA application and Xoo inoculation were efficient for enhancing of carbohydrates, lipids and proteins modifications at both above and below leaves compared to the inoculated leaf. These results collectively suggest that the elicitor of SA strongly induced the systemic resistance in rice plants.</p>

Bacterial leaf blight resistance in rice: a review of conventional breeding to molecular approach.

Breeding for disease resistant varieties remains very effective and economical in controlling the bacterial leaf blight (BLB) of rice. Breeders have played a major role in developing resistant rice varieties against the BLB infection which has been adjudged to be a major disease causing significant yield reduction in rice. It would be difficult to select rice crops with multiple genes of resistance using the conventional approach alone. This is due to masking effect of genes including epistasis. In addition, conventional breeding takes a lot of time before a gene of interest can be introgressed. Linkage drag is also a major challenge in conventional approach. Molecular breeding involving markers has facilitated the characterization and introgression of BLB disease resistance genes. Biotechnology has brought another innovation in form of genetic engineering (transgenesis) of rice. Although, molecular breeding cannot be taken as a substitute for conventional breeding, molecular approach for combating BLB disease in rice is worthwhile given the demand for increased production of rice in a fast growing population of our society. This present article highlights the recent progress from conventional to molecular approach in breeding for BLB disease resistant rice varieties.

In silico Identification of Resistance and Defense Related Genes for Bacterial Leaf Blight (BLB) in Rice

In silico Identification of Resistance and Defense Related Genes for Bacterial Leaf Blight (BLB) in Rice

English

Bacterial leaf blight (BLB) of rice is one of the most destructive diseases affecting rice fields. Xanthomonas oryzae pv. oryzae (Xoo) is the causal agent of BLB. Two BLB resistance genes, Xa21 and Xa7, were transferred into the susceptible indica cultivar Bacthom7 (BT7) by using marker-assisted selection with markers pTA248 for Xa21 and ID7 for Xa7. Improved BT7 lines carrying the two resistance genes were inoculated with three isolates of the Xoo from Northern Vietnam and evaluated for agronomic traits. Artificial inoculation of 13 lines with three Xoo races identified nine highly resistant lines with wide-spectrum resistance to Xoo, including D1, D2, D3, D6, D7, D8, D9, D10 and D12. These lines were similar to recurrent parent BT7 with regard to external appearance, yield performance and grain quality. On the basis of agronomic traits and the level of resistance to BLB, two promising lines, D6 and D9 were further selected. These two lines could efficiently contribute to rice production for food security and food safety in northern Vietnam.   Key words: Xanthomonas oryzae pv. oryzae, resistance genes, near-isogenic lines, marker-assisted selection (MAS), improved rice lines.&nbsp

Identification of a Novel Semi-Dominant Spotted-Leaf Mutant with Enhanced Resistance to Xanthomonas oryzae pv. oryzae in Rice.

Many spotted-leaf mutants show enhanced disease resistance to multiple pathogen attacks; however, the mechanisms are largely unknown. Here, we reported a novel semi-dominant spotted-leaf mutant 24 (spl24) obtained from an ethyl methane sulfonate (EMS)-induced IR64 mutant bank. spl24 developed tiny brown lesions on the leaf tip and spread down gradually to the leaf base as well as the sheath at the early heading stage. The performances of major agronomic traits such as the plant height, panicle length, number of panicles/plant, and 1000-grain weight were significantly altered in spl24 when compared to the wild-type IR64. Furthermore, spl24 exhibited a premature senescing phenotype with degeneration of nuclear acids, significantly reduced soluble protein content, increased level of malonaldehyde (MDA), and lowered activities of reactive oxygen species (ROS) scavenging enzymes. Disease evaluation indicated that spl24 showed enhanced resistance to multiple races of Xanthomonas oryzae pv. oryzae, the causal pathogen of bacterial leaf blight in rice, with elevated expression of pathogenesis-related genes, salicylic acid (SA) signaling pathway-associated genes revealed by real-time quantitative PCR and high-throughput RNA sequencing analysis. Genetic analysis and gene mapping indicated that the lesion mimic phenotype was controlled by a novel semi-dominant nuclear gene. The mutation, tentatively termed as OsSPL24, was in a 110 kb region flanked by markers Indel-33 and Indel-12 in chromosome 11. Together, our data suggest that spl24 is a novel lesion mimic mutant with enhanced innate immunity and would facilitate the isolation and functional characterization of the target gene.

Genetic variability to improve yield and resistance to bacterial leaf blight in rice

One hundred F2 and F3 single plant progenies along with their parents and the infector rows were evaluated for 13 quantitative traits to assess the genetic variability, heritability, associations and estimation of number of genes for bacterial leaf blight resistance and yield related traits. In both F2 and F3, a wide range of variability was present for all the traits, indicating considerable scope for selection. High heritability accompanied with high genetic advance was observed for plant height, panicle length, panicle weight, number of effective tillers per plant and grain yield per plant. Positive and significant correlation were observed between grain yield per plant with days to 50% flowering, plant height, panicle length, panicle weight, number of effective tillers per plant, test weight and flag leaf length in either one of the generation or in combination of both F2 and F3. Path-coefficients analysis showed that plant height, panicle length and test weight had high direct positive effect in both the F2 and F3. The estimated number of gene(s) in the segregating generations for bacterial leaf blight was 2.14 in the F2 and 0.92 in the F3 revealed that two genes were involved in the inheritance of resistance to bacterial leaf blight in F2, while only one gene in F3 due to dominance effect.

Investigation of bacterial leaf blight of rice through various detection tools and its impact on crop yield in Punjab, Pakistan

Investigation of bacterial leaf blight of rice through various detection tools and its impact on crop yield in Punjab, Pakistan

AGRONOMIC MANAGEMENT OF BACTERIAL LEAF BLIGHT OF RICE (ORYZA SATIVA L.) DISEASE AND YIELD ENHANCEMENT THROUGH DIFFERENT COMBINATIONS OF NITROGEN AND POTASH DOSES

This research trial was conducted at Rice Research Institute, Kala Shah Kaku, during kharif 2010, to assess the influence of nitrogen (N) and potash (K) on the incidence and severity of Bacterial leaf blight of rice (BLB) (Oryzae sativa L.). The experiment was laid out in a Randomized Complete Block Design in factorial arrangement with three replications using net plot size of 2.25 m × 6.0 m. The experiment was comprised of 0, 75, 100, 125 kg N ha-1 and 0, 50, 75, 100 kg K ha-1. Thirty days old nursery of Super Basmati was transplanted manually in puddle field. Data on disease severity and yield parameters of rice were recorded using standard procedures. Yield parameters were affected significantly by various combinations of N & K. In case of BLB, minimum diseased leaf area (15.76 % DLA) was observed where N alone was applied @ 75 kg ha-1 in contrast to 125 kg N + 50 kg K ha-1 which showed maximum DLA (43.17 %). Highest paddy yield of 4.32 t ha-1 was recorded where rice was fertilized @ 75 kg N + 100 kg K ha-1, while the minimum paddy yield of 2.40 t ha-1 was obtained where only 75 kg K ha-1 was applied. Optimum fertilization @ 75 kg N + 100 kg K ha-1 improved the harvest index value up to 20.32 %. Minimum harvest index of 15.61 % was obtained where K alone was applied at the rate of 100 kg ha-1. Maximum gross income (Rs.154652 ha-1), net return (Rs.154652 ha-1), and BCR (1.32) were obtained where rice crop was fertilized @ 75 kg N and 100 kg K ha-1.

Assessing synergism of combined applications of Trichoderma harzianum and Pseudomonas fluorescens to control blast and bacterial leaf blight of rice

Combined application of biocontrol agents (BCAs) have been used to exploit their synergistic potential. But the analysis of additive, synergistic and antagonistic effects of BCAs has rarely been explored for the biocontrol of plant diseases. This study was conducted to evaluate in vitro and in vivo biocontrol potential of the combined application of Trichoderma harzianum strain Th3 and Pseudomonas fluorescens strain RRb11, and the fungicide carbendazim against the rice blast (RB) pathogen, Magnaporthe oryzae, and the bacterial leaf blight (BLB) pathogen, Xanthomonas oryzae pv. oryzae. Combined application of T. harzianum Th3 and P. fluorescens RRb11 synergistically reduced severity of RB by 69.5% in comparison to the untreated control, displaying a synergy factor (SF) of 1.29. P. fluorescens RRb 11 alone and in combination with T. harzianum Th3 and the mixture of T. harzianum Th3 + carbendazim was antagonistic (SF = 0.71 and 0.45, respectively). The combined application of T. harzianum Th3 and P. fluorescens RRb 11 also enhanced several rice plant growth and yield parameters. Additive, synergistic or antagonistic effects among the two BCAs were assessed by applying the numerical hypothesis of ‘Bliss Independence’ which suggested that the combined application of the two BCAs was synergistic in reducing RB but antagonistic with respect to BLB. Out of the six combined application interactions for both the diseases only one interaction (T. harzianum Th3 and P. fluorescens RRb11) was synergistic against RB, which suggested that synergism is a rare event in combined use of BCAs.

Functional analysis of African Xanthomonas oryzae pv. oryzae TALomes reveals a new susceptibility gene in bacterial leaf blight of rice.

Most Xanthomonas species translocate Transcription Activator-Like (TAL) effectors into plant cells where they function like plant transcription factors via a programmable DNA-binding domain. Characterized strains of rice pathogenic X. oryzae pv. oryzae harbor 9–16 different tal effector genes, but the function of only a few of them has been decoded. Using sequencing of entire genomes, we first performed comparative analyses of the complete repertoires of TAL effectors, herein referred to as TALomes, in three Xoo strains forming an African genetic lineage different from Asian Xoo. A phylogenetic analysis of the three TALomes combined with in silico predictions of TAL effector targets showed that African Xoo TALomes are highly conserved, genetically distant from Asian ones, and closely related to TAL effectors from the bacterial leaf streak pathogen Xanthomonas oryzae pv. oryzicola (Xoc). Nine clusters of TAL effectors could be identified among the three TALomes, including three showing higher levels of variation in their repeat variable diresidues (RVDs). Detailed analyses of these groups revealed recombination events as a possible source of variation among TAL effector genes. Next, to address contribution to virulence, nine TAL effector genes from the Malian Xoo strain MAI1 and four allelic variants from the Burkinabe Xoo strain BAI3, thus representing most of the TAL effector diversity in African Xoo strains, were expressed in the TAL effector-deficient X. oryzae strain X11-5A for gain-of-function assays. Inoculation of the susceptible rice variety Azucena lead to the discovery of three TAL effectors promoting virulence, including two TAL effectors previously reported to target the susceptibility (S) gene OsSWEET14 and a novel major virulence contributor, TalB. RNA profiling experiments in rice and in silico prediction of EBEs were carried out to identify candidate targets of TalB, revealing OsTFX1, a bZIP transcription factor previously identified as a bacterial blight S gene, and OsERF#123, which encodes a subgroup IXc AP2/ERF transcription factor. Use of designer TAL effectors demonstrated that induction of either gene resulted in greater susceptibility to strain X11-5A. The induction of OsERF#123 by BAI3Δ1, a talB knockout derivative of BAI3, carrying these designer TAL effectors increased virulence of BAI3Δ1, validating OsERF#123 as a new, bacterial blight S gene.

Lack of a Cytoplasmic RLK, Required for ROS Homeostasis, Induces Strong Resistance to Bacterial Leaf Blight in Rice.

Many scientific findings have been reported on the beneficial function of reactive oxygen species (ROS) in various cellular processes, showing that they are not just toxic byproducts. The double-edged role of ROS shows the importance of the regulation of ROS level. We report a gene, rrsRLK (required for ROS-scavenging receptor-like kinase), that encodes a cytoplasmic RLK belonging to the non-RD kinase family. The gene was identified by screening rice RLK mutant lines infected with Xanthomonas oryzae pv. oryzae (Xoo), an agent of bacterial leaf blight of rice. The mutant (ΔrrsRLK) lacking the Os01g02290 gene was strongly resistant to many Xoo strains, but not to the fungal pathogen Magnaporthe grisea. ΔrrsRLK showed significantly higher expression of OsPR1a, OsPR1b, OsLOX, RBBTI4, and jasmonic acid-related genes than wild type. We showed that rrsRLK protein interacts with OsVOZ1 (vascular one zinc-finger 1) and OsPEX11 (peroxisomal biogenesis factor 11). In the further experiments, abnormal biogenesis of peroxisomes, hydrogen peroxide (H2O2) accumulation, and reduction of activity of ROS-scavenging enzymes were investigated in ΔrrsRLK. These results suggest that the enhanced resistance in ΔrrsRLK is due to H2O2 accumulation caused by irregular ROS-scavenging mechanism, and rrsRLK is most likely a key regulator required for ROS homeostasis in rice.

Niclosamide Blocks Rice Leaf Blight by Inhibiting Biofilm Formation of Xanthomonas oryzae.

Rice (Oryza sativa) is the leading source of nutrition for more than half of the world’s population, and by far it is the most important commercial food crop. But, its growth and production are significantly hampered by the bacterial pathogen Xanthomonas oryzae pv. oryzae (Xoo) which causes leaf blight disease. Earlier studies have reported the antibacterial ability of FDA-approved niclosamide drug against Xoo. However, the underlying mechanism by which niclosamide blocks the growth of Xoo remained elusive. In the present study, by employing the microbiological, microscopical, molecular, bioinformatics and analytical tools we found that niclosamide can directly inhibit the growth of the Xoo by hampering the biofilm formation and the production of xanthomonadin and exopolysaccharide substances (EPS) required for relentless growth and virulence of Xoo. Interestingly, niclosamide was found to specifically suppress the growth of Xoo without affecting other bacteria like Escherichia coli. Our electron microscopic observations disclosed that niclosamide disrupts the membrane permeability of Xoo and causes the release of intracellular components. Similarly, the molecular docking analysis disclosed the molecular interaction of niclosamide with the biofilm, virulence and quorum sensing related proteins, which was further substantiated by relative gene expression analysis where niclosamide was found to significantly downregulate the expression of these key regulatory genes. In addition, considerable changes in chemical structures were detected by Fourier Transform Infrared Spectroscopy (FTIR) in response to niclosamide treatment. Overall, our findings advocate the utilization of niclosamide as a safe and potent alternative antibacterial compound to control bacterial blight disease in rice.

Dissecting the virulence-related functionality and cellular transcription mechanism of a conserved hypothetical protein in Xanthomonas oryzae pv. oryzae.

Hypothetical proteins without defined functions are largely distributed in all sequenced bacterial genomes. Understanding their potent functionalities is a basic demand for bacteriologists. Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial leaf blight of rice, is one of the model systems for the study of molecular plant pathology. One-quarter of proteins in the genome of this bacterium are defined as hypothetical proteins, but their roles in Xoo pathogenicity are unknown. Here, we generated in-frame deletions for six hypothetical proteins selected from strain PXO99A and found that one of them (PXO_03177) is required for the full virulence of this strain. PXO_03177 is conserved in Xanthomonas, and is predicted to contain two domains relating to polysaccharide synthesis. However, we found that mutation of this gene did not affect the production or modification of extracellular polysaccharides (EPSs) and lipopolysaccharides (LPSs), two major polysaccharides produced by Xoo relating to its infection. Interestingly, we found that inactivation of PXO_03177 significantly impaired biofilm formation and tolerance to sodium dodecyl sulfate (SDS), both of which are considered to play key roles during Xoo infection in rice leaves. These findings thus enable us to define a function for PXO_03177 in the virulence of Xoo. Furthermore, we also found that the global regulator Clp controls the transcription of PXO_03177 by direct binding to its promoter region, presenting the first cellular regulatory pathway for the modulation of expression of this hypothetical protein gene. Our results provide reference information for PXO_03177 homologues in Xanthomonas.

Xanthomonas oryzae pv. oryzae type III effector PthXo3JXOV suppresses innate immunity, induces susceptibility and binds to multiple targets in rice.

Transcription activator-like (TAL) effectors encoded by tal genes were recognized as a key virulence strategy used by Xanthomonas oryzae pv. oryzae (Xoo) to cause bacterial leaf blight of rice. TAL effector PthXo3 is a major virulence factor identified in a Philippine Xoo strain PXO61, and it can induce the expression of susceptibility gene OsSWEET14 by binding to the effector-binding element (EBE) in the promoter region. In this study, pthXo3 homologous genes were also identified and isolated from Xoo Chinese strain OS198 and Japanese strain JXOV, which were named as pthXo3OS198 and pthXo3JXOV, respectively. When pthXo3JXOV was delivered into PXO99A, the resulting strain PXO99A/pthXo3JXOV significantly increased virulence in 18 out of 23 rice varieties tested, with the most prominent increase in lesion length and bacteria propagation in rice IRBB13. PthXo3JXOV suppresses the plant's innate immunity by inhibiting hypersensitive response (HR) and callose deposition. The Agrobacterium tumefaciens-mediated transient expression assays showed that, besides OsSWEET14, PthXo3JXOV also interacts with other targets by binding to the EBEs in their promoter regions. Our results suggest that PthXo3JXOV may interact with multiple targets to execute its virulence functions.

Concomitant Regulation by a LacI-Type Transcriptional Repressor XylR on Genes Involved in Xylan and Xylose Metabolism and the Type III Secretion System in Rice Pathogen Xanthomonas oryzae pv. oryzae.

The hypersensitive response and pathogenicity (hrp) genes of Xanthomonas oryzae pv. oryzae, the causal agent of bacterial leaf blight of rice, encode components of the type III secretion system and are essential for virulence. Expression of hrp genes is regulated by two key hrp regulators, HrpG and HrpX; HrpG regulates hrpX and hrpA, and HrpX regulates the other hrp genes on hrpB-hrpF operons. We previously reported the sugar-dependent quantitative regulation of HrpX; the regulator highly accumulates in the presence of xylose, followed by high hrp gene expression. Here, we found that, in a mutant lacking the LacI-type transcriptional regulator XylR, HrpX accumulation and hrp gene expression were high even in the medium without xylose, reaching the similar levels present in the wild type incubated in the xylose-containing medium. XylR also negatively regulated one of two xylose isomerase genes (xylA2 but not xylA1) by binding to the motif sequence in the upstream region of the gene. Xylose isomerase is an essential enzyme in xylose metabolism and interconverts between xylose and xylulose. Our results suggest that, in the presence of xylose, inactivation of XylR leads to greater xylan and xylose utilization and, simultaneously, to higher accumulation of HrpX, followed by higher hrp gene expression in the bacterium.