Rice Blast Pathogen Magnaporthe Oryzae Research Articles

Polymorphic residues in rice NLRs expand binding and response to effectors of the blast pathogen.

Accelerated adaptive evolution is a hallmark of plant-pathogen interactions. Plant intracellular immune receptors (NLRs) often occur as allelic series with differential pathogen specificities. The determinants of this specificity remain largely unknown. Here, we unravelled the biophysical and structural basis of expanded specificity in the allelic rice NLR Pik, which responds to the effector AVR-Pik from the rice blast pathogen Magnaporthe oryzae. Rice plants expressing the Pikm allele resist infection by blast strains expressing any of three AVR-Pik effector variants, whereas those expressing Pikp only respond to one. Unlike Pikp, the integrated heavy metal-associated (HMA) domain of Pikm binds with high affinity to each of the three recognized effector variants, and variation at binding interfaces between effectors and Pikp-HMA or Pikm-HMA domains encodes specificity. By understanding how co-evolution has shaped the response profile of an allelic NLR, we highlight how natural selection drove the emergence of new receptor specificities. This work has implications for the engineering of NLRs with improved utility in agriculture.

Reconstruction of an SSR-based Magnaporthe oryzae physical map to locate avirulence gene AvrPi12

BackgroundPathogen avirulence (Avr) genes can evolve rapidly when challenged by the widespread deployment of host genes for resistance. They can be effectively isolated by positional cloning provided a robust and well-populated genetic map is available.ResultsAn updated, SSR-based physical map of the rice blast pathogen Magnaporthe oryzae (Mo) has been constructed based on 116 of the 120 SSRs used to assemble the last map, along with 18 newly developed ones. A comparison between the two versions of the map has revealed an altered marker content and order within most of the Mo chromosomes. The avirulence gene AvrPi12 was mapped in a population of 219 progeny derived from a cross between the two Mo isolates CHL42 and CHL357. A bulked segregant analysis indicated that the gene was located on chromosome 6, a conclusion borne out by an analysis of the pattern of segregation shown by individual isolates. Six additional PCR-based markers were developed to improve the map resolution in the key region. AvrPi12 was finally located within the sub-telomeric region of chromosome 6, distal to the SSR locus LSM6–5.ConclusionsThe improved SSR-based linkage map should be useful as a platform for gene mapping and isolation in Mo. It was used to establish the location of AvrPi12, thereby providing a starting point for its positional cloning.

Morphological and molecular characterization of Magnaporthe oryzae causing rice blast disease in Odisha

Rice blast disease is one of the most destructive disease causing enormous yield losses to rice in different rice growing regions of the world. The present study aims to identify and characterize the rice blast pathogen Magnaporthe oryzae from different rice growing regions of Odisha. Twenty isolates were collected and categorized into three groups based on colony colour i.e., grayish blackish, grayish and white, and in two group based on the texture of the colony as smooth and rough. Molecular characterization of blast isolates was done with two ITS primers. The sequence analysis of the OD-2 blast isolate showed maximum similarity with Malaysia isolate. In phylogenetic analysis, the blast isolates were grouped into two major clusters. The Odisha isolate (OD-2) along with Chhattisgarh, are in one cluster whereas, blast isolates from China and Japan are in another cluster. Our study showed high degree of morphological and genetic variation among the isolates collected from different locations. The finding of this study would enhance the practical application of research in the field of epidemiology, disease control and developing breeding strategies for blast disease.

Comparative proteomic analysis between nitrogen supplemented and starved conditions in Magnaporthe oryzae

BackgroundFungi are constantly exposed to nitrogen limiting environments, and thus the efficient regulation of nitrogen metabolism is essential for their survival, growth, development and pathogenicity. To understand how the rice blast pathogen Magnaporthe oryzae copes with limited nitrogen availability, a global proteome analysis under nitrogen supplemented and nitrogen starved conditions was completed.MethodsM. oryzae strain 70–15 was cultivated in liquid minimal media and transferred to media with nitrate or without a nitrogen source. Proteins were isolated and subjected to unfractionated gel-free based liquid chromatography-tandem mass spectrometry (LC-MS/MS). The subcellular localization and function of the identified proteins were predicted using bioinformatics tools.ResultsA total of 5498 M. oryzae proteins were identified. Comparative analysis of protein expression showed 363 proteins and 266 proteins significantly induced or uniquely expressed under nitrogen starved or nitrogen supplemented conditions, respectively. A functional analysis of differentially expressed proteins revealed that during nitrogen starvation nitrogen catabolite repression, melanin biosynthesis, protein degradation and protein translation pathways underwent extensive alterations. In addition, nitrogen starvation induced accumulation of various extracellular proteins including small extracellular proteins consistent with observations of a link between nitrogen starvation and the development of pathogenicity in M. oryzae.ConclusionThe results from this study provide a comprehensive understanding of fungal responses to nitrogen availability.

Genetic Structure of the Rice Blast Pathogen (Magnaporthe oryzae) over a Decade in North Central California Rice Fields

Rice blast, caused by the ascomycete Magnaporthe oryzae, is one of the most destructive rice diseases worldwide. Even though the disease has been present in California since 1996, there is no data for the pathogen population biology in the state. Using amplified fragment length polymorphisms and mating-type markers, the M. oryzae population diversity was investigated using isolates collected when the disease was first established in California and isolates collected a decade later. While in the 1990 samples, a single multilocus genotype (MLG) was identified (MLG1), over a decade later, we found 14 additional MLGs in the 2000 isolates. Some of these MLGs were found to infect the only rice blast-resistant cultivar (M-208) available for commercial production in California. The same samples also had a significant decrease of MLG1. MLG1 was found infecting the resistant rice cultivar M-208 on one occasion whereas MLG7 was the most common genotype infecting the M-208. MLG7 was identified in the 2000 samples, and it was not present in the M. oryzae population a decade earlier. Our results demonstrate a significant increase in genotypic diversity over time with no evidence of sexual reproduction and suggest a recent introduction of new virulent race(s) of the pathogen. In addition, our data could provide information regarding the durability of the Pi-z resistance gene of the M-208. This information will be critical to plant breeders in developing strategies for deployment of other rice blast resistance genes/cultivars in the future.

Analysis of the subcellular localization and dynamics of cyclic AMP/PKA signaling proteins in the model Rice Blast pathogen Magnaporthe oryzae reveals a functional dependency for the regulatory (RpkA-GFP) and catalytic (CpkA-mCherry) subunits as they are

Analysis of the subcellular localization and dynamics of cyclic AMP/PKA signaling proteins in the model Rice Blast pathogen Magnaporthe oryzae reveals a functional dependency for the regulatory (RpkA-GFP) and catalytic (CpkA-mCherry) subunits as they are

Subcellular compartmentation, interdependency and dynamics of the cyclic AMP-dependent PKA subunits during pathogenic differentiation in rice blast.

The cAMP-dependent PKA signalling plays a central role in growth, asexual development and pathogenesis in fungal pathogens. Here, we functionally characterised RPKA, the regulatory subunit of cAMP/PKA and studied the dynamics and organisation of the PKA subunits in the rice blast pathogen Magnaporthe oryzae. The RPKA subunit was essential for proper vegetative growth, asexual sporulation and surface hydrophobicity in M. oryzae. A spontaneous suppressor mutation, SMR19, that restored growth and conidiation in the RPKA deletion mutant was isolated and characterised. SMR19 enhanced conidiation and appressorium formation but failed to suppress the pathogenesis defects in rpkAΔ. The PKA activity was undetectable in the mycelial extracts of SMR19, which showed a single mutation (val242leu) in the highly conserved active site of the catalytic subunit (CPKA) of cAMP/PKA. The two subunits of cAMP/PKA showed different subcellular localisation patterns with RpkA being predominantly nucleocytoplasmic in conidia, while CpkA was largely cytosolic and/or vesicular. The CpkA anchored RpkA in cytoplasmic vesicles, and localisation of PKA in the cytoplasm was governed by CpkA in a cAMP-dependant or independent manner. We show that there exists a tight regulation of PKA subunits at the level of transcription, and the cAMP signalling is differentially compartmentalised in a stage-specific manner in rice blast.

Economic and Environmental Impact of Rice Blast Pathogen (Magnaporthe oryzae) Alleviation in the United States.

Rice blast (Magnaporthe oryzae) is a key concern in combating global food insecurity given the disease is responsible for approximately 30% of rice production losses globally—the equivalent of feeding 60 million people. These losses increase the global rice price and reduce consumer welfare and food security. Rice is the staple crop for more than half the world’s population so any reduction in rice blast would have substantial beneficial effects on consumer livelihoods. In 2012, researchers in the US began analyzing the feasibility of creating blast-resistant rice through cisgenic breeding. Correspondingly, our study evaluates the changes in producer, consumer, and environmental welfare, if all the rice produced in the Mid-South of the US were blast resistant through a process like cisgenics, using both international trade and environmental assessment modeling. Our results show that US rice producers would gain 69.34 million dollars annually and increase the rice supply to feed an additional one million consumers globally by eliminating blast from production in the Mid-South. These results suggest that blast alleviation could be even more significant in increasing global food security given that the US is a small rice producer by global standards and likely experiences lower losses from blast than other rice-producing countries because of its ongoing investment in production technology and management. Furthermore, results from our detailed life cycle assessment (LCA) show that producing blast-resistant rice has lower environmental (fossil fuel depletion, ecotoxicity, carcinogenics, eutrophication, acidification, global warming potential, and ozone depletion) impacts per unit of rice than non-blast resistant rice production. Our findings suggest that any reduction in blast via breeding will have significantly positive impacts on reducing global food insecurity through increased supply, as well as decreased price and environmental impacts in production.

Fluorescent reporter analysis revealed the timing and localization of AVR-Pia expression, an avirulence effector of Magnaporthe oryzae.

In order to facilitate infection, the rice blast pathogen Magnaporthe oryzae secretes an abundance of proteins, including avirulence effectors, to diminish its host's defences. Avirulence effectors are recognized by host resistance proteins and trigger the host's hypersensitive response, which is a rapid and effective form of innate plant immunity. An understanding of the underlying molecular mechanisms of such interactions is crucial for the development of strategies to control disease. However, the expression and secretion of certain effector proteins, such as AVR-Pia, have yet to be reported. Reverse transcription-polymerase chain reaction (RT-PCR) revealed that AVR-Pia was only expressed during infection. Fluorescently labelled AVR-Pia indicated that AVR-Pia expression was induced during appressorial differentiation in the cells of both rice and onion, as well as in a penetration-deficient (Δpls1) mutant capable of developing melanized appressoria, but unable to penetrate host cells, suggesting that AVR-Pia expression is independent of fungal penetration. Using live-cell imaging, we also documented the co-localization of green fluorescent protein (GFP)-labelled AVR-Pia and monomeric red fluorescent protein (mRFP)-labelled PWL2, which indicates that AVR-Pia accumulates in biotrophic interfacial complexes before being delivered to the plant cytosol. Together, these results suggest that AVR-Pia is a cytoplasmic effector that is expressed at the onset of appressorial differentiation and is translocated to the biotrophic interfacial complex, and then into the host's cytoplasm.

Killing Two Birds with One Stone: Natural Rice Rhizospheric Microbes Reduce Arsenic Uptake and Blast Infections in Rice.

Our recent work has shown that a rice thizospheric natural isolate, a Pantoea sp (hereafter EA106) attenuates Arsenic (As) uptake in rice. In parallel, yet another natural rice rhizospheric isolate, a Pseudomonas chlororaphis (hereafter EA105), was shown to inhibit rice blast pathogen Magnaporthe oryzae. Considering the above, we envisaged to evaluate the importance of mixed stress regime in rice plants subjected to both As toxicity and blast infections. Plants subjected to As regime showed increased susceptibility to blast infections compared to As-untreated plants. Rice blast pathogen M. oryzae showed significant resistance against As toxicity compared to other non-host fungal pathogens. Interestingly, plants treated with EA106 showed reduced susceptibility against blast infections in plants pre-treated with As. This data also corresponded with lower As uptake in plants primed with EA106. In addition, we also evaluated the expression of defense related genes in host plants subjected to As treatment. The data showed that plants primed with EA106 upregulated defense-related genes with or without As treatment. The data shows the first evidence of how rice plants cope with mixed stress regimes. Our work highlights the importance of natural association of plant microbiome which determines the efficacy of benign microbes to promote the development of beneficial traits in plants.

The role of a cytosolic superoxide dismutase in barley-pathogen interactions.

Reactive oxygen species (ROS), including superoxide ( O2·-/ HO2·) and hydrogen peroxide (H2 O2 ), are differentially produced during resistance responses to biotrophic pathogens and during susceptible responses to necrotrophic and hemi-biotrophic pathogens. Superoxide dismutase (SOD) is responsible for the catalysis of the dismutation of O2·-/ HO2· to H2 O2 , regulating the redox status of plant cells. Increased SOD activity has been correlated previously with resistance in barley to the hemi-biotrophic pathogen Pyrenophora teres f. teres (Ptt, the causal agent of the net form of net blotch disease), but the role of individual isoforms of SOD has not been studied. A cytosolic CuZnSOD, HvCSD1, was isolated from barley and characterized as being expressed in tissue from different developmental stages. HvCSD1 was up-regulated during the interaction with Ptt and to a greater extent during the resistance response. Net blotch disease symptoms and fungal growth were not as pronounced in transgenic HvCSD1 knockdown lines in a susceptible background (cv. Golden Promise), when compared with wild-type plants, suggesting that cytosolic O2·-/ HO2· contributes to the signalling required to induce a defence response to Ptt. There was no effect of HvCSD1 knockdown on infection by the hemi-biotrophic rice blast pathogen Magnaporthe oryzae or the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei, but HvCSD1 also played a role in the regulation of lesion development by methyl viologen. Together, these results suggest that HvCSD1 could be important in the maintenance of the cytosolic redox status and in the differential regulation of responses to pathogens with different lifestyles.

Identification of UV-Induced Diterpenes Including a New Diterpene Phytoalexin, Phytocassane F, from Rice Leaves by Complementary GC/MS and LC/MS Approaches.

Rice phytoalexins are regarded as one of the most important weapons against pathogenic microorganisms. We attempted to identify novel phytoalexins and their derivatives using GC/MS and LC/MS analyses. Diterpene derivatives, 9β-pimara-7,15-diene-3β,6β,19-triol, 1, stemar-13-en-2α-ol, 2, and 1α,2α-dihydroxy-ent-12,15-cassadiene-3,11-dione, 3, were isolated from UV-irradiated rice leaves by chromatographic methods. These structures were confirmed by 1D- and 2D-NMR and MS analyses. Interestingly, all three compounds were accumulated following an infection by the rice blast pathogen Magnaporthe oryzae. Compounds 1 and 2 exhibited weak antifungal activity and may be the biosynthetic intermediates of rice phytoalexins momilactones and oryzalexin S, respectively. Compound 3 exhibited relatively high inhibitory activity against the fungal mycelial growth of M. oryzae to the same extent as the known phytoalexin phytocassane A. We conclude that 3 is a member of the cassane-type phytoalexin family and propose the name phytocassane F.

Development and evaluation of near-isogenic lines for major blast resistance gene(s) in Basmati rice.

A set of NILs carrying major blast resistance genes in a Basmati rice variety has been developed. Also, the efficacy of pyramids over monogenic NILs against rice blast pathogen Magnaporthe oryzae has been demonstrated. Productivity and quality of Basmati rice is severely affected by rice blast disease. Major genes and QTLs conferring resistance to blast have been reported only in non-Basmati rice germplasm. Here, we report incorporation of seven blast resistance genes from the donor lines DHMASQ164-2a (Pi54, Pi1, Pita), IRBLz5-CA (Pi2), IRBLb-B (Pib), IRBL5-M (Pi5) and IRBL9-W (Pi9) into the genetic background of an elite Basmati rice variety Pusa Basmati 1 (PB1). A total of 36 near-isogenic lines (NILs) comprising of 14 monogenic, 16 two-gene pyramids and six three-gene pyramids were developed through marker-assisted backcross breeding (MABB). Foreground, recombinant and background selection was used to identify the plants with target gene(s), minimize the linkage drag and increase the recurrent parent genome (RPG) recovery (93.5-98.6%), respectively, in the NILs. Comparative analysis performed using 50,051 SNPs and 500 SSR markers revealed that the SNPs provided better insight into the RPG recovery. Most of the monogenic NILs showed comparable performance in yield and quality, concomitantly, Pusa1637-18-7-6-20 (Pi9), was significantly superior in yield and stable across four different environments as compared to recurrent parent (RP) PB1. Further, among the pyramids, Pusa1930-12-6 (Pi2+Pi5) showed significantly higher yield and Pusa1633-7-8-53-6-8 (Pi54+Pi1+Pita) was superior in cooking quality as compared to RP PB1. The NILs carrying gene Pi9 were found to be the most effective against the concoction of virulent races predominant in the hotspot locations for blast disease. Conversely, when analyzed under artificial inoculation, three-gene pyramids expressed enhanced resistance as compared to the two-gene and monogenic NILs.

Global genome and transcriptome analyses of Magnaporthe oryzae epidemic isolate 98-06 uncover novel effectors and pathogenicity-related genes, revealing gene gain and lose dynamics in genome evolution.

Genome dynamics of pathogenic organisms are driven by pathogen and host co-evolution, in which pathogen genomes are shaped to overcome stresses imposed by hosts with various genetic backgrounds through generation of a variety of isolates. This same principle applies to the rice blast pathogen Magnaporthe oryzae and the rice host; however, genetic variations among different isolates of M. oryzae remain largely unknown, particularly at genome and transcriptome levels. Here, we applied genomic and transcriptomic analytical tools to investigate M. oryzae isolate 98-06 that is the most aggressive in infection of susceptible rice cultivars. A unique 1.4 Mb of genomic sequences was found in isolate 98-06 in comparison to reference strain 70-15. Genome-wide expression profiling revealed the presence of two critical expression patterns of M. oryzae based on 64 known pathogenicity-related (PaR) genes. In addition, 134 candidate effectors with various segregation patterns were identified. Five tested proteins could suppress BAX-mediated programmed cell death in Nicotiana benthamiana leaves. Characterization of isolate-specific effector candidates Iug6 and Iug9 and PaR candidate Iug18 revealed that they have a role in fungal propagation and pathogenicity. Moreover, Iug6 and Iug9 are located exclusively in the biotrophic interfacial complex (BIC) and their overexpression leads to suppression of defense-related gene expression in rice, suggesting that they might participate in biotrophy by inhibiting the SA and ET pathways within the host. Thus, our studies identify novel effector and PaR proteins involved in pathogenicity of the highly aggressive M. oryzae field isolate 98-06, and reveal molecular and genomic dynamics in the evolution of M. oryzae and rice host interactions.

The RhoGAP SPIN6 associates with SPL11 and OsRac1 and negatively regulates programmed cell death and innate immunity in rice.

The ubiquitin proteasome system in plants plays important roles in plant-microbe interactions and in immune responses to pathogens. We previously demonstrated that the rice U-box E3 ligase SPL11 and its Arabidopsis ortholog PUB13 negatively regulate programmed cell death (PCD) and defense response. However, the components involved in the SPL11/PUB13-mediated PCD and immune signaling pathway remain unknown. In this study, we report that SPL11-interacting Protein 6 (SPIN6) is a Rho GTPase-activating protein (RhoGAP) that interacts with SPL11 in vitro and in vivo. SPL11 ubiquitinates SPIN6 in vitro and degrades SPIN6 in vivo via the 26S proteasome-dependent pathway. Both RNAi silencing in transgenic rice and knockout of Spin6 in a T-DNA insertion mutant lead to PCD and increased resistance to the rice blast pathogen Magnaporthe oryzae and the bacterial blight pathogen Xanthomonas oryzae pv. oryzae. The levels of reactive oxygen species and defense-related gene expression are significantly elevated in both the Spin6 RNAi and mutant plants. Strikingly, SPIN6 interacts with the small GTPase OsRac1, catalyze the GTP-bound OsRac1 into the GDP-bound state in vitro and has GAP activity towards OsRac1 in rice cells. Together, our results demonstrate that the RhoGAP SPIN6 acts as a linkage between a U-box E3 ligase-mediated ubiquitination pathway and a small GTPase-associated defensome system for plant immunity.

Molecular Diversity and Mating Type Distribution of the Rice Blast Pathogen Magnaporthe oryzae in North-East and Eastern India

Rice blast, caused by Magnaporthe oryzae, is the most devastating disease of rice and severely affects crop stability and sustainability worldwide. In this study, a total of 63 single spore isolates, collected during 2010–2013 from different cultivars in the different rice growing regions of North-East and Eastern India were used for molecular diversity and mating type analysis. DNA fingerprinting was used to study the diversity among the collections of 63 isolates by using POT2-TIR rep-PCR and MGR586-TIR. Different lineages were detected for 63 M. oryzae isolates by Pot2-TIR and eight for MGR586-TIR fingerprints at 75 % similarity. Among the lineages detected by Pot2-TIR, lineage A and I represented the maximum number of isolates whereas other lineages represent fewer numbers of isolates. Generally all the lineages contained isolates of mixed geographical origin. Isolates from Jharkhand were distributed in all the seven lineages. The MGR586-TIR DNA fingerprinting detected eight lineages, out of which three (Lineages F, G, H) were site specific but were represented only by single isolate. Lineage C contained isolates of Jharkhand only. The lineage A was the largest represented 46 isolates from all the states except Madhya Pradesh. Optimization of the sampling may result in considerable improvement in the results as clustering of isolates from Jharkhand in a few lineages and detection of different lineages with limited isolates from other states could be ascribed to improper sampling. MGR586-TIR fingerprints appeared to differentiate the isolates more strongly compared to POT2-TIR as is obvious from the distance among isolates of the same lineage (Lineage A) arbitrarily grouped together at 75 % similarity. All the 63 isolates were also investigated for MAT1-1 and MAT1-2 mating-type distribution by PCR based molecular markers. Of the 63 M. oryzae isolates collected, 16 (25 %) of the isolates were the mating type MAT1-1 while 35 (56 %) were mating type MAT1-2. The MAT1-2 isolates predominated in Jharkhand and Assam while MAT1-1 is more predominant in the isolates of Odisha. Both MAT1-1 and MAT1-2 were equally distributed in the isolates of Meghalaya and Tripura. Only single isolate from Jharkhand was positive for both the mating type. The results indicated that sexual recombination might be the one reason for lineage diversity in M. oryzae in fields of large rice-growing regions in North-East and Eastern states of India.

Function and interaction of the coupled genes responsible for Pik-h encoded rice blast resistance.

Pik-h, an allele of Pik, confers resistance against the rice blast pathogen Magnaporthe oryzae. Its positional cloning has shown that it comprises a pair of NBS-LRR genes, Pikh-1 and Pikh-2. While Pikh-1 appears to be constitutively transcribed, the transcript abundance of Pikh-2 responds to pathogen challenge. The Pikh-1 CC (coiled coil) domain interacts directly with both AvrPik-h and Pikh-2. Transient expression assays demonstrated that Pikh-2 mediates the initiation of the host defence response. Nucleocytoplasmic partitioning of both Pikh-1 and Pikh-2 is required for their functionalities. In a proposed mechanistic model of Pik-h resistance, it is suggested that Pikh-1 acts as an adaptor between AvrPik-h and Pikh-2, while Pikh-2 transduces the signal to trigger Pik-h-specific resistance.

Potential for using crude extract of Sarocladium oryzae for suppression of rice blast

Crude extract of an isolate of the fungus Sarocladium oryzae (CNPAF So 20G), containing the antimicrobial cerulenin, was produced and its antagonistic potential on the rice blast pathogen Magnaporthe oryzae was asessed. Cerulenin was detected in crude extract of S. oryzae through thin layer chromatography showing a Rf value of 0.63. The quantity of cerulenin in the crude extract was 237 µg.mL-1. The in vitro inhibition of germination and appressorial formation of M. oryzae was assessed on an artificial hydrophobic surface, using eight different doses of cerulenin ranging from 0.05 to 30.0 µg.mL-1. The LD50 values calculated based on the Probit-log analysis for germination and appressorial formation were 1.298 ± 0.123 µg.mL-1 and 0.0705 ± 0.0062 µg.mL-1 of cerulenin, respectively. The 30.0 µg.mL-1 of cerulenin dose inhibited 98% and 99%, germination and appressorial formation, respectively. The mode of action of cerulenin was studied by staining conidia with Calcofluor White and fluorescent microscopy showing its effect on plasma membrane. Crude extract of S. oryzae suppressed 63% of rice blast disease in greenhouse conditions. The results indicate that a product based on cerulenin and/or S. oryzae has a great potential to be used in biological control of rice blast.

Involvement of mitogen activated protein kinase kinase 6 in UV induced transcripts accumulation of genes in phytoalexin biosynthesis in rice.

BackgroundUltra violet radiation leads to accumulation of phytoalexins (PA) in rice (Oryza sativa) which are typically accumulated when the plants are infected with rice blast pathogen Magnaporthe oryzae. Although extensive works have been done in elucidating phytoalexin biosynthesis, UV stress signal transduction leading to accumulations of rice phytoalexin is largely unknown.ResultsIn the present study, the involvement of mitogen activated protein kinase (MAPK) cascade has been shown in UV induced regulation of genes in phytoalexin biosynthesis in rice. UV induced activation of MAPK and expression of PA biosynthesis genes were shown to be inhibited with staurosporin and MAPK inhibitors. Transcript regulation studies and kinase assays indicated involvement of OsMKK6 in the process. Transgenic rice overexpressing constitutive active OsMKK6EE exhibited higher expression of genes of PA biosynthesis pathway upon UV stress and also upon infection with M. oryzae.ConclusionThese results suggest a key role of OsMKK6 in regulation of UV responsive expression of genes of PA biosynthesis in rice. This study will help to elucidate the intricate signalling components of UV leading to phytoalexins biosynthesis in rice.Electronic supplementary materialThe online version of this article (doi:10.1186/1939-8433-6-35) contains supplementary material, which is available to authorized users.

Global gene expression in rice blast pathogen Magnaporthe oryzae treated with a natural rice soil isolate

The rhizospheric microbiome is comprised of many microbes, some of which reduce the virulence of their phytopathogenic neighbors; however, the mechanisms underlying these interactions are largely unknown. Rice soil isolate Pseudomonas chlororaphis EA105 strongly inhibits Magnaporthe oryzae's in vitro growth by restricting fungal diameter as well as inhibiting the formation of the appressorium, required for penetration. We were interested in elucidating M. oryzae's response to EA105 treatment, and utilized a microarray approach to obtain a global perspective of EA105 elicited changes in this pathogen. Based on this analysis, three genes of interest were knocked out in M. oryzae 70-15, and their sensitivity to EA105 treatment as well as their ability to infect rice was determined. Priming rice plants with EA105 prior to M. oryzae infection decreased lesion size, and the mutants were tested to see if this effect was retained. A null 70-15 mutant in a trichothecene biosynthesis gene showed less susceptibility to bacterial treatment, forming more appressoria than the parental type 70-15. A similar pattern was seen in a null mutant for a stress-inducible protein, MGG_03098. In addition, when this mutant was inoculated onto the leaves of EA105-primed rice plants, lesions were reduced to a greater extent than in 70-15, implicating the lack of this gene with an increased ISR response in rice. Understanding the global effect of biocontrol bacteria on phytopathogens is a key for developing successful and lasting solutions to crop loss caused by plant diseases and has the potential to greatly increase food supply.