Differential responses of rice to inoculation with wild-type and non-pathogenic mutants of Magnaporthe oryzae

An update on redox signals in plant responses to biotic and abiotic stress crosstalk: insights from cadmium and fungal pathogen interactions

Complex signalling pathways are involved in plant protection against single and combined stresses. Plants are able to coordinate genome-wide transcriptional reprogramming and display a unique programme of transcriptional responses to a combination of stresses that differs from the response to single stresses. However, a significant overlap between pathways and some defence genes in the form of shared and general stress-responsive genes appears to be commonly involved in responses to multiple biotic and abiotic stresses. Reactive oxygen and nitrogen species, as well as redox signals, are key molecules involved at the crossroads of the perception of different stress factors and the regulation of both specific and general plant responses to biotic and abiotic stresses. In this review, we focus on crosstalk between plant responses to biotic and abiotic stresses, in addition to possible plant protection against pathogens caused by previous abiotic stress. Bioinformatic analyses of transcriptome data from cadmium- and fungal pathogen-treated plants focusing on redox gene ontology categories were carried out to gain a better understanding of common plant responses to abiotic and biotic stresses. The role of reactive oxygen and nitrogen species in the complex network involved in plant responses to changes in their environment is also discussed.

The elicitor-responsive gene for a GRAS family protein, CIGR2, suppresses cell death in rice inoculated with rice blast fungus via activation of a heat shock transcription factor, OsHsf23

We show that a rice GRAS family protein, CIGR2, is a bonafide transcriptional activator, and through this function, targets the B-type heat shock protein-encoding gene OsHsf23 (Os09g0456800). CIGR2 (Os07g0583600) is an N-acetylchitooligosaccharide elicitor-responsive gene whose activity, through the direct transcriptional control of OsHsf23, is required for mediating hypersensitive cell death activation during pathogen infection. RNAi lines of CIGR2 and OsHsf23 similarly exhibited the higher level of granulation in the epidermal cells of leaf sheath inoculated with an avirulent isolate of rice blast fungus. Interestingly, we did not observe altered levels of resistance, suggesting that CIGR2 suppresses excessive cell death in the incompatible interaction with blast fungus via activation of OsHsf23.

Chestnut Leaf Inoculation Assay as a Rapid Predictor of Blight Susceptibility

American chestnuts (Castanea dentata), effectively eliminated from eastern North America by chestnut blight in the twentieth century, are the subject of multiple restoration efforts. Screening individual trees (or tree types) for blight resistance is a critical step in all of these programs. Traditional screening involves inoculating stems of >3-year-old trees with the blight fungus (Cryphonectria parasitica), then measuring resulting cankers a few months later. A quicker, nondestructive, quantitative assay, usable on younger plants, would enhance restoration efforts by speeding the screening process. The assay presented here meets these requirements by inoculating excised leaves with the blight fungus and measuring resulting necrotic lesions. Leaves can be collected from few-month-old seedlings or fully mature trees, and results are measured after less than a week. Leaves from several lines of both American and Chinese chestnuts were inoculated, as well as the congener Allegheny chinquapin, and experimental leaf assay results correlate well with stem assay results from these species. Inoculations with virulent and hypovirulent blight fungi strains also showed relative patterns similar to traditional inoculations. Given the correlations to established stem assay results, this procedure could be a valuable tool to quickly evaluate blight resistance in American chestnut trees used for restoration.

Simultaneous RNA-seq analysis of a mixed transcriptome of rice and blast fungus interaction

A filamentous fungus, Magnaporthe oryzae, is a causal agent of rice blast disease, which is one of the most serious diseases affecting cultivated rice, Oryza sativa. However, the molecular mechanisms underlying both rice defense and fungal attack are not yet fully understood. Extensive past studies have characterized many infection-responsive genes in the pathogen and host plant, separately. To understand the plant-pathogen interaction comprehensively, it is valuable to monitor the gene expression profiles of both interacting organisms simultaneously in the same infected plant tissue. Although the host-pathogen interaction during the initial infection stage is important for the establishment of infection, the detection of fungal gene expression in infected leaves at the stage has been difficult because very few numbers of fungal cells are present. Using the emerging RNA-Seq technique, which has a wide dynamic range for expression analyses, we analyzed the mixed transcriptome of rice and blast fungus in infected leaves at 24 hours post-inoculation, which is the point when the primary infection hyphae penetrate leaf epidermal cells. We demonstrated that our method detected the gene expression of both the host plant and pathogen simultaneously in the same infected leaf blades in natural infection conditions without any artificial treatments. The upregulation of 240 fungal transcripts encoding putative secreted proteins was observed, suggesting that these candidates of fungal effector genes may play important roles in initial infection processes. The upregulation of transcripts encoding glycosyl hydrolases, cutinases and LysM domain-containing proteins were observed in the blast fungus, whereas pathogenesis-related and phytoalexin biosynthetic genes were upregulated in rice. Furthermore, more drastic changes in expression were observed in the incompatible interactions compared with the compatible ones in both rice and blast fungus at this stage. Our mixed transcriptome analysis is useful for the simultaneous elucidation of the tactics of host plant defense and pathogen attack.

Two homologous putative protein tyrosine phosphatases, OsPFA-DSP2 and AtPFA-DSP4, negatively regulate the pathogen response in transgenic plants

Protein phosphatases, together with protein kinases, regulate protein phosphorylation and dephosphorylation, and play critical roles in plant growth and biotic stress responses. However, little is known about the biological functions of plant protein tyrosine dual-specificity phosphatase (PFA-DSP) in biotic stresses. Here, we found that OsPFA-DSP2 was mainly expressed in calli, seedlings, roots, and young panicles, and localized in cytoplasm and nucleus. Ectopic overexpression of OsPFA-DSP2 in rice increased sensitivity to Magnaporthe grisea (M. grisea Z1 strain), inhibited the accumulation of hydrogen peroxide (H₂O₂) and suppressed the expression of pathogenesis-related (PR) genes after fungal infection. Interestingly, transgenic Arabidopsis plants overexpressing AtPFA-DSP4, which is homologous to OsPFA-DSP2, also exhibited sensitivity to Pseudomonas syringae pv. tomato DC3000 (Pst DC3000), reduced accumulation of H₂O₂ and decreased photosynthesic capacity after infection compared with Col-0. These results indicate that OsPFA-DSP2 and AtPFA-DSP4 act as negative regulators of the pathogen response in transgenic plants.

The single functional blast resistance gene Pi54 activates a complex defence mechanism in rice

The Pi54 gene (Pi-k(h)) confers a high degree of resistance to diverse strains of the fungus Magnaporthe oryzae. In order to understand the genome-wide co-expression of genes in the transgenic rice plant Taipei 309 (TP) containing the Pi54 gene, microarray analysis was performed at 72 h post-inoculation of the M. oryzae strain PLP-1. A total of 1154 differentially expressing genes were identified in TP-Pi54 plants. Of these, 587 were up-regulated, whereas 567 genes were found to be down-regulated. 107 genes were found that were exclusively up-regulated and 58 genes that were down- regulated in the case of TP-Pi54. Various defence response genes, such as callose, laccase, PAL, and peroxidase, and genes related to transcription factors like NAC6, Dof zinc finger, MAD box, bZIP, and WRKY were found to be up-regulated in the transgenic line. The enzymatic activities of six plant defence response enzymes, such as peroxidase, polyphenol oxidase, phenylalanine ammonia lyase, β-glucosidase, β-1,3-glucanase, and chitinase, were found to be significantly high in TP-Pi54 at different stages of inoculation by M. oryzae. The total phenol content also increased significantly in resistant transgenic plants after pathogen inoculation. This study suggests the activation of defence response and transcription factor-related genes and a higher expression of key enzymes involved in the defence response pathway in the rice line TP-Pi54, thus leading to incompatible host-pathogen interaction.

HYR1-mediated detoxification of reactive oxygen species is required for full virulence in the rice blast fungus

During plant-pathogen interactions, the plant may mount several types of defense responses to either block the pathogen completely or ameliorate the amount of disease. Such responses include release of reactive oxygen species (ROS) to attack the pathogen, as well as formation of cell wall appositions (CWAs) to physically block pathogen penetration. A successful pathogen will likely have its own ROS detoxification mechanisms to cope with this inhospitable environment. Here, we report one such candidate mechanism in the rice blast fungus, Magnaporthe oryzae, governed by a gene we refer to as MoHYR1. This gene (MGG_07460) encodes a glutathione peroxidase (GSHPx) domain, and its homologue in yeast was reported to specifically detoxify phospholipid peroxides. To characterize this gene in M. oryzae, we generated a deletion mutantΔhyr1 which showed growth inhibition with increased amounts of hydrogen peroxide (H₂O₂). Moreover, we observed that the fungal mutants had a decreased ability to tolerate ROS generated by a susceptible plant, including ROS found associated with CWAs. Ultimately, this resulted in significantly smaller lesion sizes on both barley and rice. In order to determine how this gene interacts with other (ROS) scavenging-related genes in M. oryzae, we compared expression levels of ten genes in mutant versus wild type with and without H₂O₂. Our results indicated that the HYR1 gene was important for allowing the fungus to tolerate H₂O₂ in vitro and in planta and that this ability was directly related to fungal virulence.

The role of catalase-peroxidase secreted by Magnaporthe oryzae during early infection of rice cells

The biological role of a secretory catalase of the rice blast fungus Magnaporthe oryzae was studied. The internal amino acid sequences of the partially purified catalase in the culture filtrate enabled us to identify its encoding gene as a catalase-peroxidase gene, CPXB, among four putative genes for catalase or catalase-peroxidase in M. oryzae. Knockout of the gene drastically reduced the level of catalase activity in the culture filtrate and supernatant of conidial suspension (SCS), and increased the sensitivity to exogenously added H₂O₂ compared with control strains, suggesting that CPXB is the major gene encoding the secretory catalase and confers resistance to H₂O₂ in hyphae. In the mutant, the rate of appressoria that induced accumulation of H₂O₂ in epidermal cells of the leaf sheath increased and infection at early stages was delayed; however, the formation of lesions in the leaf blade was not affected compared with the control strain. These phenotypes were complimented by reintroducing the putative coding regions of CPXB driven by a constitutive promoter. These results suggest that CPXB plays a role in fungal defense against H₂O₂ accumulated in epidermal cells of rice at the early stage of infection but not in pathogenicity of M. oryzae.

Exploring molecular signaling in plant-fungal symbioses using high throughput RNA sequencing

Plant-fungal symbioses are a common feature in nature. They vary from pathogenic interactions, where fungi subvert plant resources for their own use, to mutualistic associations, where both fungus and host benefit from the interaction. Although the ecological importance of plant-fungal symbioses has long been recognized and the biology of several key associations are now well studied, new technologies have the potential to allow fresh insight into the molecular basis of plant-fungal interactions. One such technique - high throughput RNA sequencing - has recently been used to explore the molecular basis of cross-species communications. Here, we give a brief overview of this emerging technology, and present a general guide for employing the methodology to dissect plant-fungal symbiosis.

Perception of the chitin oligosaccharides contributes to disease resistance to blast fungus Magnaporthe oryzae in rice

Chitin is a component of fungal cell walls, and its fragments act as elicitors in many plants. The plasma membrane glycoprotein CEBiP, which possesses LysM domains, is a receptor for the chitin elicitor (CE) in rice. Here, we report that the perception of CE by CEBiP contributes to disease resistance against the rice blast fungus, Magnaporthe oryzae, and that enhanced responses to CE by engineering CEBiP increase disease tolerance. Knockdown of CEBiP expression allowed increased spread of the infection hyphae. To enhance defense responses to CE, we constructed chimeric genes composed of CEBiP and Xa21, which mediate resistance to rice bacterial leaf blight. The expression of either CRXa1 or CRXa3, each of which contains the whole extracellular portion of CEBiP, the whole intracellular domain of XA21, and the transmembrane domain from either CEBiP or XA21, induced cell death accompanied by an increased production of reactive oxygen and nitrogen species after treatment with CE. Rice plants expressing the chimeric receptor exhibited necrotic lesions in response to CE and became more resistant to M. oryzae. Deletion of the first LysM domain in CRXA1 abolished these cellular responses. These results suggest that CEs are produced and recognized through the LysM domain of CEBiP during the interaction between rice and M. oryzae and imply that engineering pattern recognition receptors represents a new strategy for crop protection against fungal diseases.