Differential requirement of Oryza sativa RAR1 in immune receptor-mediated resistance of rice to Magnaporthe oryzae

Identification of a novel major QTL from Chinese wheat cultivar Ji5265 for Fusarium head blight resistance in greenhouse

A novel major QTL for FHB resistance was mapped to a 6.8 Mb region on chromosome 2D in a Chinese wheat cultivar Ji5265, and diagnostic KASP markers were developed for detecting it in a worldwide wheat collection. Fusarium head blight (FHB) is a serious disease in wheat (Triticum aestivum L.) and causes significant reductions in grain yield and quality worldwide. Breeding for FHB resistance is the most effective strategy to minimize the losses caused by FHB; therefore, identification of major quantitative trait loci (QTLs) conferring FHB resistance and development of diagnostic markers for the QTLs are prerequisites for marker-assisted selection (MAS). Ji5265 is a Chinese wheat cultivar resistant to FHB in multiple environments. An F₆ population of 179 recombinant inbred lines (RILs) was developed from Ji5265 × Wheaton. The population was genotyped by genotyping-by-sequencing (GBS) and phenotyped for FHB Type II resistance in greenhouses. A major QTL, designated as QFhb-2DL, was mapped in a 6.8 Mb region between the markers GBS10238 and GBS12056 on the long arm of chromosome 2D in Ji5265 and explained ~ 30% of the phenotypic variation for FHB resistance. The effect of QFhb-2DL on FHB resistance was validated using near-isogenic lines (NILs) derived from residual heterozygotes from an F₆ RIL of Ji5265 × Wheaton. The two flanking markers were converted into Kompetitive allele-specific PCR (KASP) markers (KASP10238 and KASP12056) and validated to be diagnostic in a collection of 2,065 wheat accessions. These results indicate that QFhb-2DL is a novel major QTL for resistance to FHB spread within a spike (Type II) and the two KASP markers can be used for MAS to improve wheat FHB resistance in wheat breeding programs.

The hybrid lethality of interspecific F1 hybrids of Nicotiana: a clue to understanding hybrid inviability-a major obstacle to wide hybridization and introgression breeding of plants

Reproductive isolation poses a major obstacle to wide hybridization and introgression breeding of plants. Hybrid inviability in the postzygotic isolation barrier inevitably reduces hybrid fitness, consequently causing hindrances in the establishment of novel genotypes from the hybrids among genetically divergent parents. The idea that the plant immune system is involved in the hybrid problem is applicable to the intra- and/or interspecific hybrids of many different taxa. The lethality characteristics and expression profile of genes associated with the hypersensitive response of the hybrids, along with the suppression of causative genes, support the deleterious epistatic interaction of parental NB-LRR protein genes, resulting in aberrant hyper-immunity reactions in the hybrid. Moreover, the cellular, physiological, and biochemical reactions observed in hybrid cells also corroborate this hypothesis. However, the difference in genetic backgrounds of the respective hybrids may contribute to variations in lethality phenotypes among the parental species combinations. The mixed state in parental components of the chaperone complex (HSP90-SGT1-RAR1) in the hybrid may also affect the hybrid inviability. This review article discusses the facts and hypothesis regarding hybrid inviability, alongside the findings of studies on the hybrid lethality of interspecific hybrids of the genus Nicotiana. A possible solution for averting the hybrid problem has also been scrutinized with the aim of improving the wide hybridization and introgression breeding program in plants.

Genome-Wide Association Study of Local Thai Indica Rice Seedlings Exposed to Excessive Iron

Excess soluble iron in acidic soil is an unfavorable environment that can reduce rice production. To better understand the tolerance mechanism and identify genetic loci associated with iron toxicity (FT) tolerance in a highly diverse indica Thai rice population, a genome-wide association study (GWAS) was performed using genotyping by sequencing and six phenotypic data (leaf bronzing score (LBS), chlorophyll content, shoot height, root length, shoot biomass, and root dry weight) under both normal and FT conditions. LBS showed a high negative correlation with the ratio of chlorophyll content and shoot biomass, indicating the FT-tolerant accessions can regulate cellular homeostasis when encountering stress. Sixteen significant single nucleotide polymorphisms (SNPs) were identified by association mapping. Validation of candidate SNP using other FT-tolerant accessions revealed that SNP:2_21262165 might be associated with tolerance to FT; therefore, it could be used for SNP marker development. Among the candidate genes controlling FT tolerance, RAR1 encodes an innate immune responsive protein that links to cellular redox homeostasis via interacting with abiotic stress-responsive Hsp90. Future research may apply the knowledge obtained from this study in the molecular breeding program to develop FT-tolerant rice varieties.

The role of chaperone complex HSP90-SGT1-RAR1 as the associated machinery for hybrid inviability between Nicotiana gossei Domin and N. tabacum L

Two cultured cell lines (GTH4 and GTH4S) of a Nicotiana interspecific F1 hybrid (N. gossei × N. tabacum) were comparatively analyzed to find genetic factors related to hybrid inviability. Both cell lines proliferated at 37 °C, but after shifting to 26 °C, GTH4 started to die similar to the F1 hybrid seedlings, whereas GTH4S survived. As cell death requires de novo expression of genes and proteins, we compared expressed protein profiles between the two cell lines, and found that NgSGT1, a cochaperone of the chaperone complex (HSP90-SGT1-RAR1), was expressed in GTH4 but not in GTH4S. Agrobacterium-mediated transient expression of NgSGT1, but not NtSGT1, induced cell death in leaves of N. tabacum, suggesting its possible role in hybrid inviability. Cell death in N. tabacum was also induced by transient expression of NgRAR1, but not NtRAR1. In contrast, transient expression of any parental combinations of three components revealed that NgRAR1 promoted cell death, whereas NtRAR1 suppressed it in N. tabacum. A specific inhibitor of HSP90, geldanamycin, inhibited the progression of hypersensitive response-like cell death in GTH4 and leaf tissue after agroinfiltration. The present study suggested that components of the chaperone complex are involved in the inviability of Nicotiana interspecific hybrid.

The overexpression of OsACBP5 protects transgenic rice against necrotrophic, hemibiotrophic and biotrophic pathogens

The most devastating diseases in rice (Oryza sativa) are sheath blight caused by the fungal necrotroph Rhizoctonia solani, rice blast by hemibiotrophic fungus Magnaporthe oryzae, and leaf blight by bacterial biotroph Xanthomonas oryzae (Xoo). It has been reported that the Class III acyl-CoA-binding proteins (ACBPs) such as those from dicots (Arabidopsis and grapevine) play a role in defence against biotrophic pathogens. Of the six Arabidopsis (Arabidopsis thaliana) ACBPs, AtACBP3 conferred protection in transgenic Arabidopsis against Pseudomonas syringae, but not the necrotrophic fungus, Botrytis cinerea. Similar to Arabidopsis, rice possesses six ACBPs, designated OsACBPs. The aims of this study were to test whether OsACBP5, the homologue of AtACBP3, can confer resistance against representative necrotrophic, hemibiotrophic and biotrophic phytopathogens and to understand the mechanisms in protection. Herein, when OsACBP5 was overexpressed in rice, the OsACBP5-overexpressing (OsACBP5-OE) lines exhibited enhanced disease resistance against representative necrotrophic (R. solani & Cercospora oryzae), hemibiotrophic (M. oryzae & Fusarium graminearum) and biotrophic (Xoo) phytopathogens. Progeny from a cross between OsACBP5-OE9 and the jasmonate (JA)-signalling deficient mutant were more susceptible than the wild type to infection by the necrotroph R. solani. In contrast, progeny from a cross between OsACBP5-OE9 and the salicylic acid (SA)-signalling deficient mutant was more susceptible to infection by the hemibiotroph M. oryzae and biotroph Xoo. Hence, enhanced resistance of OsACBP5-OEs against representative necrotrophs appears to be JA-dependent whilst that to (hemi)biotrophs is SA-mediated.

Mutation of a Nucleotide-Binding Leucine-Rich Repeat Immune Receptor-Type Protein Disrupts Immunity to Bacterial Blight

Most characterized plant resistance proteins belong to the nucleotide-binding domain and Leu-rich repeat-containing (NLR) family. NLRs are present in an auto-inhibited state in the absence of specific pathogens, while gain-of-function mutations in NLRs usually cause autoimmunity. Here, we show that a gain-of-function mutation, weaker defense (wed), which caused a Phe-to-Leu substitution in the nucleotide-binding domain of a typical NLR in rice (Oryza sativa), led to enhanced susceptibility to Xanthomonas oryzae pv. Oryzae The unexpected accumulation of salicylic acid (SA), along with downregulation of NONEXPRESSOR OF PR1 (NPR1), in wed indicates the potential presence of a feedback regulation loop of SA biosynthesis in rice. Epistasis analyses illustrated that SA accumulation and the NLR-associated components RAR1, OsRac1, and PhyB are dispensable for the wed phenotypes. Intriguingly, besides pattern-triggered immunity, effector-triggered immunity conferred by different resistance proteins, including Xa3/Xa26, Xa4, and Xa21, was also disturbed by wed to a certain extent, indicating the existence of shared regulatory mechanisms for various defense systems. The identification of wed therefore provides a unique system for genetic dissection of shared immune signaling pathways activated by different types of immune receptors.

A Single-Nucleotide Polymorphism in the Promoter of a Hairpin RNA Contributes to Alternaria alternata Leaf Spot Resistance in Apple (Malus × domestica)

Apple leaf spot caused by the Alternaria alternata f. sp mali (ALT1) fungus is one of the most devastating diseases of apple (Malus × domestica). We identified a hairpin RNA (hpRNA) named MdhpRNA277 that produces small RNAs and is induced by ALT1 infection in 'Golden Delicious' apple. MdhpRNA277 produces mdm-siR277-1 and mdm-siR277-2, which target five resistance (R) genes that are expressed at high levels in resistant apple variety 'Hanfu' and at low levels in susceptible variety 'Golden Delicious' following ALT1 infection. MdhpRNA277 was strongly induced in 'Golden Delicious' but not 'Hanfu' following ALT1 inoculation. MdhpRNA277 promoter activity was much stronger in inoculated 'Golden Delicious' versus 'Hanfu'. We identified a single-nucleotide polymorphism (SNP) in the MdhpRNA277 promoter region between 'Golden Delicious' (pMdhpRNA277-GD) and 'Hanfu' (pMdhpRNA277-HF). The transcription factor MdWHy binds to pMdhpRNA277-GD, but not to pMdhpRNA277-HF Transgenic 'GL-3' apple expressing pMdhpRNA277-GD:MdhpRNA277 was more susceptible to ALT1 infection than plants expressing pMdhpRNA277-HF:MdhpRNA277 due to induced mdm-siR277 accumulation and reduced expression of the five target R genes. We confirmed that the SNP in pMdhpRNA277 is associated with A. alternata leaf spot resistance by crossing. This SNP could be used as a marker to distinguish between apple varieties that are resistant or susceptible to A. alternata leaf spot.

Molecular Basis of Citrus sunki Susceptibility and Poncirus trifoliata Resistance Upon Phytophthora parasitica Attack

Coevolution has shaped the molecular basis of an extensive number of defense mechanisms in plant-pathogen interactions. Phytophthora parasitica, a hemibiothrophic oomycete pathogen and the causal agent of citrus root rot and gummosis, interacts differently with Citrus sunki and Poncirus trifoliata, two commonly favored citrus rootstocks that are recognized as susceptible and resistant, respectively, to P. parasitica. The molecular core of these interactions remains elusive. Here, we provide evidence on the defense strategies employed by both susceptible and resistant citrus rootstocks, in parallel with P. parasitica deployment of effectors. Time course expression analysis (quantitative real-time polymerase chain reaction) of several defense-related genes were evaluated during i) plant disease development, ii) necrosis, and iii) pathogen effector gene expression. In C. sunki, P. parasitica deploys effectors, including elicitins, NPP1 (necrosis-inducing Phytophthora protein 1), CBEL (cellulose-binding elicitor and lectin activity), RxLR, and CRN (crinkler), and, consequently, this susceptible plant activates its main defense signaling pathways that result in the hypersensitive response and necrosis. Despite the strong plant-defense response, it fails to withstand P. parasitica invasion, confirming its hemibiothrophic lifestyle. In Poncirus trifoliata, the effectors were strongly expressed, nevertheless failing to induce any immunity manipulation and disease development, suggesting a nonhost resistance type, in which the plant relies on preformed biochemical and anatomical barriers.

Identification of a Polyketide Synthase Gene in the Synthesis of Phleichrome of the Phytopathogenic Fungus Cladosporium phlei

Phleichrome, a pigment produced by the phytopathogenic fungus Cladosporium phlei, is a fungal perylenequinone whose photodynamic activity has been studied intensively. To determine the biological function of phleichrome and to engineer a strain with enhanced production of phleichrome, we identified the gene responsible for the synthesis of phleichrome. Structural comparison of phleichrome with other fungal perylenequinones suggested that phleichrome is synthesized via polyketide pathway. We recently identified four different polyketide synthase (PKS) genes encompassing three major clades of fungal PKSs that differ with respect to reducing conditions for the polyketide product. Based on in silico analysis of cloned genes, we hypothesized that the non-reducing PKS gene, Cppks1, is involved in phleichrome biosynthesis. Increased accumulation of Cppks1 transcript was observed in response to supplementation with the application of synthetic inducer cyclo-(l-Pro-l-Phe). In addition, heterologous expression of the Cppks1 gene in Cryphonectria parasitica resulted in the production of phleichrome. These results provide convincing evidence that the Cppks1 gene is responsible for the biosynthesis of phleichrome.