Rsc3K, a SMV resistance gene localized on chromosome 2 in soybean cv. Kefeng No.1, interacts with virulence determinant P3

Soybean mosaic virus (SMV) is one of the most devastating viral pathogens in Glycine max (L.) Merr (soybean). Twenty-two SMV strains (SC1-SC22) isolated in China were identified based on their responses to ten soybean cultivars. By using the F2-derived F3 (F2:3) and recombinant inbred line (RIL) populations of resistant Soybean cultivar (cv.) Kefeng No.1 × susceptible cv. Nannong 1138-2, we localized the gene mediating resistant to SMV-SC3 strain to a 90 kb interval on chromosome 2 in Kefeng No.1. Bean pod mottle vi-rus (BPMV)-induced gene silencing (VIGS) were used to study the gene function of candidate genes in the mapping interval and revealed that an recombinant gene, later named as Rsc3K, caused by internal deletion of a genomic DNA fragement in Kefeng No.1, is the resistant gene to SMV-SC3. By shuffling genes between avirulent isolate SC3 and avirulent SMV isolate 1129, we found that P3 is the virulence determinant causing resistance on Kefeng No.1. We showed the interaction between Rsc3K and P3 by the yeast two-hybrid (Y2H) and bimolecular fluorescent complementation (BiFC) assays. In conclusion, this study demonstrated that Rsc3K plays a crucial role in resistance of Kefeng No.1 to SMV-SC3 by direct interaction with viral protein P3.

Mlo Resistance to Powdery Mildew (Blumeria graminis f. sp. hordei) in Barley Landraces Collected in Yemen

Barley (Hordeumvulgare L.) is one of the most important cereal crops in the world. Powdery mildew on barley, which is caused by the pathogen Blumeria graminis f. sp. hordei, occurs world-wide and can result in severe yield loss. Thousands of barley accessions are stored in national gene banks, and their characterization for breeding purposes is needed. This study was conducted to determine the resistance to powdery mildew in 33 barley landraces from Yemen, which were obtained from the ICARDA gene bank. Twenty differential isolates of barley powdery mildew were used. Nine single plant lines were selected from five landraces, based on tests that were performed with 30 plants per landrace, after inoculation with the most avirulent isolate of barley powdery mildew available. Two of these landraces originated from the Al Bayda province in Yemen, and three others originated from Dhamar, Sanaa, and Taizz, respectively. Next, single plant lines were tested using a set of 20 differential isolates of powdery mildew. Two lines that were selected from landrace from the Al Bayda province in Yemen, showed disease reaction designated as 0(4), which is specific for the presence of Mlo resistance. The new source of highly effective Mlo powdery mildew resistance that is described in this study could be used in barley breeding programs.

Responses of Tomato Genotypes to Avirulent and Mi-Virulent Meloidogyne javanica Isolates Occurring in Israel

The behavior of naturally virulent Meloidogyne isolates toward the tomato resistance gene Mi in major tomato-growing areas in Israel was studied for the first time. Virulence of seven selected isolates was confirmed over three successive generations on resistant (Mi-carrying) and susceptible (non-Mi-carrying) tomato cultivars. Diagnostic markers verified the predominance of Meloidogyne javanica among virulent isolates selected on resistant tomato cultivars or rootstocks. To better understand the determinants of nematode selection on Mi-carrying plants, reproduction of Mi-avirulent and virulent isolates Mjav1 and Mjv2, respectively, measured as eggs per gram of root, on non-Mi-carrying, heterozygous (Mi/mi) and homozygous (Mi/Mi) genotypes was evaluated. Although no reproduction of Mjav1 was observed on Mi/Mi genotypes, some reproduction was consistently observed on Mi/mi plants; reproduction of Mjv2 on the homozygous and heterozygous genotypes was similar to that on susceptible cultivars, suggesting a limited quantitative effect of the Mi gene. Histological examination of giant cells induced by Mi-virulent versus avirulent isolates confirmed the high virulence of Mjv2 on Mi/mi and Mi/Mi genotypes, allowing the formation of well-developed giant-cell systems despite the Mi gene. Analysis of the plant defense response in tomato Mi/Mi, Mi/mi, and mi/mi genotypes to both avirulent and virulent isolates was investigated by quantitative real-time polymerase chain reaction. Although the jasmonate (JA)-signaling pathway was clearly upregulated by avirulent and virulent isolates on the susceptible (not carrying Mi) and heterozygous (Mi/mi) plants, no change in signaling was observed in the homozygous (Mi/Mi) resistant line following incompatible interaction with the avirulent isolate. Thus, similar to infection promoted by the avirulent isolate on the susceptible genotype, the Mi-virulent isolate induced the JA-dependent pathway, which might promote tomato susceptibility during the compatible interaction with the homozygous (Mi/Mi) resistant line. These results have important consequences for the management of Mi resistance genes for ensuring sustainable tomato farming.

Evidence for a Minor Gene–for–Minor Gene Interaction Explaining Nonhypersensitive Polygenic Partial Disease Resistance

Partial resistance is a quantitative type of resistance that, by definition of Parlevliet, is not based on hypersensitivity. It is largely pathotype nonspecific, although some minor isolate-specific responses have been reported. In order to elucidate the isolate specificity of individual genes for partial resistance, three barley recombinant inbred line mapping populations were analyzed for resistance to the leaf rust fungus Puccinia hordei. The mapping populations were inoculated with one isolate avirulent and two isolates virulent to resistance gene Rph7g. Six significant quantitative trait loci (QTLs) were detected. Of these, two (Rphq3 and Rphq11) were detected with only the avirulent isolate (1.2.1.) and one (Rphq18) only with both virulent isolates (CO-04 and 28.1). The effectiveness of these QTLs was tested with 14 isolates, using a tester set of genotypes containing alleles for resistance or susceptibility for these QTLs. QTL Rphq18 was effective to only two isolates, CO-04 and 28.1, whereas Rphq3 and Rphq11 were ineffective to CO-04 and 28.1 but effective to all other isolates, except one. This resulted in a significant Person's differential interaction, which is a hallmark of a gene–for–gene interaction. The minor gene–for–minor gene interaction is not based on hypersensitivity and there is no evidence that the resistance is based on genes belonging to the nucleotide-binding leucine-rich repeat class.

Identification and Characterization of Tomato Mutants Affected in the Rx-Mediated Resistance to PVX Isolates

Five tomato mutants affected in the Rx-mediated resistance against Potato virus X (PVX) were identified by screening a mutagenized population derived from a transgenic, Rx1-expressing ‘Micro-Tom’ line. Contrary to their parental line, they failed to develop lethal systemic necrosis upon infection with the virulent PVX-KH2 isolate. Sequence analysis and quantitative reverse-transcription polymerase chain reaction experiments indicated that the mutants are not affected in the Rx1 transgene or in the Hsp90, RanGap1 and RanGap2, Rar1 and Sgt1 genes. Inoculation with the PVX-CP4 avirulent isolate demonstrated that the Rx1 resistance was still effective in the mutants. In contrast, the virulent PVX-KH2 isolate accumulation was readily detectable in all mutants, which could further be separated in two groups depending on their ability to restrict the accumulation of PVX-RR, a mutant affected at two key positions for Rx1 elicitor activity. Finally, transient expression of the viral capsid protein elicitor indicated that the various mutants have retained the ability to mount an Rx1-mediated hypersensitive response. Taken together, the results obtained are consistent with a modification of the specificity or intensity of the Rx1-mediated response. The five Micro-Tom mutants should provide very valuable resources for the identification of novel tomato genes affecting the functioning of the Rx gene.

Identification and characterisation of a novel class I endo-β-1,3-glucanase regulated by salicylic acid, ethylene and fungal pathogens in strawberry

The identification of a full length cDNA encoding an endo-β-1,3-glucanase (FaOGBG-5) from strawberry (Fragaria × ananassa Duch) is reported. The analysis of the deduced amino acid sequence of FaOGBG-5 showed that it shares typical structural features and a high degree of identity with other plant β-1,3-glucanases of the class I. The expression of FaOGBG-5 in plants infected with a virulent isolate of Colletotrichum acutatum and an avirulent isolate of Colletotrichum fragariae was examined. Induction of expression was observed with both pathogens but exhibited a delayed high expression with the virulent one. Additionally, the accumulation of FaOGBG-5 transcripts was also observed after treatments with the stress related hormones salicylic acid and ethylene. Results obtained suggest that the β-1,3-glucanase encoded by FaOGBG-5 may be implicated in plant defence against biotic and abiotic stress.

Study of symptoms and gene expression in four Pinus species after pinewood nematode infection

Pine wilt disease, caused by the pinewood nematode Bursaphelenchus xylophilus (Steiner and Buhrer) Nickle, is originating severe infections in pine trees. The disease is detected when external symptoms appear (e.g. needle chlorosis), but trees could remain asymptomatic for long periods and serve as a long-term host. The primary goal of this study was to assess the effect of inoculation with an avirulent isolate of B. xylophilus (C14-5) on different Pinus spp. seedlings (P. sylvestris, P. nigra, P. pinea and P. pinaster). At the same time, seedlings were also inoculated with a virulent strain, HF, in order to compare the phenotypic and genomic results of the two types of inoculations. The effect of inoculation was determined in terms of expression of various Pinus genes potentially involved in the response to the disease.The results suggest that P. pinea and P. nigra are more resistant to infection by the nematode than P. sylvestris and P. pinaster. The phenotypic and genetic differences were more marked among P. pinea and P. pinaster.

One of Two Major Paralogs of AVR-Pita1 Is Functional in Japanese Rice Blast Isolates

We analyzed the avirulence gene AVR-Pita1 in Japanese rice blast isolates to determine how they gain virulence toward rice cultivars containing the Pita resistance gene. An avirulent isolate, OS99-G-7a (G7a), from a Japanese commercial field contained two paralogs of AVR-Pita1, designated as AVR-Pita1JA and AVR-Pita1JB. Analysis of virulent, independent mutants derived from G7a, a single avirulent progenitor strain, indicated that AVR-Pita1JA was functional but AVR-Pita1JB was nonfunctional. The most frequent mutation was loss of AVR-Pita1JA. Analyses of field isolates collected from diverse areas in Japan revealed that most of the AVR-Pita1 genes carried by Japanese isolates were identical to AVR-Pita1JA or AVR-Pita1JB. The relationship between these major paralogs in Japanese isolates and the virulence of the strains carrying them indicate that AVR-Pita1JA is functional but AVR-Pita1JB is not, as is the case in G7a. Isolates that show virulence toward rice cultivars containing the Pita gene are presumed to have evolved virulence from avirulent origins via loss of AVR-Pita1JA, except for one case in which virulence resulted from a base substitution. In this study, we discuss the properties and specificities of Japanese rice blasts that relate to virulence against Pita-containing rice. Furthermore, we present a method to amplify AVR-Pita1JA and AVR-Pita1JB separately and, specifically, to monitor functional AVR-Pita1 in Japan.