Race nonspecific resistance for potato late blight

The late blight fungus (Phytophthora infestans) rots susceptible species of potato plants. None of the major varieties of potato (Solanum tuberosum) grown in the USA is resistant to US-8, the most prevalent genotype of the fungus. Now, Junqi Song, James Bradeen and colleagues have cloned the RB gene from the wild diploid potato species, Solanum bulbocastanum, using a map-based approach in combination with long-range PCR. Transgenic plants containing the gene, normally fully susceptible, displayed broad-spectrum late blight resistance.     

广西玉米种质资源对纹枯病的抗性鉴定

在人工接种条件下,对860份玉米材料进行了纹枯病抗性鉴定和评价,旨在为进一步开展玉米纹枯病抗性育种和分子生物学研究奠定基础。结果显示,在鉴定的860份玉米材料中,没有发现免疫的材料,高抗、抗、中抗、感和高感的比例分别为3．49％、28．60％、26．16％、10．35％和31．40％;在农家种、群体种和杂交种（组合）等杂合体中,抗病材料所占比例较大,而自交系等纯合体中,感病材料所占比例较大,表明玉米杂合体种质资源材料中可能蕴藏着不同的抗纹枯病基因,特别是广西农家种中可能存在对纹枯病具有稳定抗性的材料,值得进一步研究和利用。     

广西玉米种质资源对纹枯病的抗性鉴定

在人工接种条件下,对860份玉米材料进行了纹枯病抗性鉴定和评价,旨在为进一步开展玉米纹枯病抗性育种和分子生物学研究奠定基础。结果显示,在鉴定的860份玉米材料中,没有发现免疫的材料,高抗、抗、中抗、感和高感的比例分别为3.49%、28.60%、26.16%、10.35%和31.40%;在农家种、群体种和杂交种（组合）等杂合体中,抗病材料所占比例较大,而自交系等纯合体中,感病材料所占比例较大,表明玉米杂合体种质资源材料中可能蕴藏着不同的抗纹枯病基因,特别是广西农家种中可能存在对纹枯病具有稳定抗性的材料,值得进一步研究和利用。     

Carbohydrates and resistance to <Emphasis Type="Italic">Phytophthora infestans</Emphasis> in potato plants

Plants generally deal with biotic or abiotic stresses by altering components as for example cell wall constituents and metabolites. Infection by Phytophthora infestans, the causal agent of late blight, constitutes a stress condition for the plants and they react to it with changes arising in their metabolism depending on the resistance level of the plants. The present work compares two potato hybrids differing in their level of horizontal resistance to late blight. Carbohydrate content in stems and leaves of infected and uninfected plants was determined by HPLC. Some carbohydrates accumulated in the stems of the resistant hybrid infected by P. infestans, whereas they remained unchanged in the susceptible hybrid. On the other hand, in the leaves, these carbohydrates accumulated only in the infected susceptible hybrid.     

The Rpi-blb2 gene from Solanum bulbocastanum is an Mi-1 gene homolog conferring broad-spectrum late blight resistance in potato

The necessity to develop potato and tomato crops that possess durable resistance against the oomycete pathogen Phytophthora infestans is increasing as more virulent, crop-specialized and pesticide resistant strains of the pathogen are rapidly emerging. Here, we describe the positional cloning of the Solanum bulbocastanum-derived Rpi-blb2 gene, which even when present in a potato background confers broad-spectrum late blight resistance. The Rpi-blb2 locus was initially mapped in several tetraploid backcross populations, derived from highly resistant complex interspecific hybrids designated ABPT (an acronym of the four Solanum species involved:S. acaule, S. bulbocastanum, S. phureja and S. tuberosum), to the same region on chromosome 6 as the Mi-1 gene from tomato, which confers resistance to nematodes, aphids and white flies. Due to suppression of recombination in the tetraploid material, fine mapping was carried out in a diploid intraspecific S. bulbocastanum F1 population. Bacterial artificial chromosome (BAC) libraries, generated from a diploid ABPT-derived clone and from the resistant S. bulbocastanum parent clone, were screened with markers linked to resistance in order to generate a physical map of the Rpi-blb2 locus. Molecular analyses of both ABPT- and S. bulbocastanum-derived BAC clones spanning the Rpi-blb2 locus showed it to harbor at least 15 Mi-1 gene homologs (MiGHs). Of these, five were genetically determined to be candidates for Rpi-blb2. Complementation analyses showed that one ABPT- and one S. bulbocastanum-derived MiGH were able to complement the susceptible phenotype in both S. tuberosum and tomato. Sequence analyses of both genes showed them to be identical. The Rpi-blb2 protein shares 82% sequence identity to the Mi-1 protein. Significant expansion of the Rpi-blb2 locus compared to the Mi-1 locus indicates that intrachromosomal recombination or unequal crossing over has played an important role in the evolution of the Rpi-blb2 locus. The contrasting evolutionary dynamics of the Rpi-blb2/Mi-1 loci in the two related genomes may reflect the opposite evolutionary potentials of the interacting pathogens.     

马铃薯非共生血红蛋白基因StHb1的克隆及表达

采用RT-PCR技术成功分离了马铃薯StHb1基因序列.经半定量RT-PCR分析表明,StHb1基因的表达在抗性品种(陇薯三号)和感性品种(荷兰十五)块茎中均受致病疫霉的侵染所抑制;StHb1基因在正常生长的马铃薯块茎组织中表达量最高;外源NO和H2O2的作用可明显地抑制StHb1基因的表达,但在抗性品种中该基因受抑制的程度低于感性品种.上述试验结果暗示了StHb1基因与马铃薯对致病疫霉侵染的抗性应答具有一定的相关性.     

An ancient R gene from the wild potato species Solanum bulbocastanum confers broad-spectrum resistance to Phytophthora infestans in cultivated potato and tomato

Late blight, caused by the oomycete pathogen Phytophthora infestans, is the most devastating disease for potato cultivation. Here, we describe the positional cloning of the Rpi-blb1 gene from the wild potato species Solanum bulbocastanum known for its high levels of resistance to late blight. The Rpi-blb1 locus, which confers full resistance to complex isolates of P. infestans and for which race specificity has not yet been demonstrated, was mapped in an intraspecific S. bulbocastanum population on chromosome 8, 0.3 cM from marker CT88. Molecular analysis of a bacterial artificial chromosome (BAC) clone spanning the Rpi-blb1 locus identified a cluster of four candidate resistance gene analogues of the coiled coil, nucleotide-binding site, leucine-rich repeat (CC-NBS-LRR) class of plant resistance (R) genes. One of these candidate genes, designated the Rpi-blb1 gene, was able to complement the susceptible phenotype in a S. tuberosum and tomato background, demonstrating the potential of interspecific transfer of broad-spectrum late blight resistance to cultivated Solanaceae from sexually incompatible host species. Paired comparisons of synonymous and non-synonymous nucleotide substitutions between different regions of Rpi-blb1 paralogues revealed high levels of synonymous divergence, also in the LRR region. Although amino acid diversity between Rpi-blb1 homologues is centred on the putative solvent exposed residues of the LRRs, the majority of nucleotide differences in this region have not resulted in an amino acid change, suggesting conservation of function. These data suggest that Rpi-blb1 is relatively old and may be subject to balancing selection.     

Effector genomics accelerates discovery and functional profiling of potato disease resistance and phytophthora infestans avirulence genes

Potato is the world's fourth largest food crop yet it continues to endure late blight, a devastating disease caused by the Irish famine pathogen Phytophthora infestans. Breeding broad-spectrum disease resistance (R) genes into potato (Solanum tuberosum) is the best strategy for genetically managing late blight but current approaches are slow and inefficient. We used a repertoire of effector genes predicted computationally from the P. infestans genome to accelerate the identification, functional characterization, and cloning of potentially broad-spectrum R genes. An initial set of 54 effectors containing a signal peptide and a RXLR motif was profiled for activation of innate immunity (avirulence or Avr activity) on wild Solanum species and tentative Avr candidates were identified. The RXLR effector family IpiO induced hypersensitive responses (HR) in S. stoloniferum, S. papita and the more distantly related S. bulbocastanum, the source of the R gene Rpi-blb1. Genetic studies with S. stoloniferum showed cosegregation of resistance to P. infestans and response to IpiO. Transient co-expression of IpiO with Rpi-blb1 in a heterologous Nicotiana benthamiana system identified IpiO as Avr-blb1. A candidate gene approach led to the rapid cloning of S. stoloniferum Rpi-sto1 and S. papita Rpi-pta1, which are functionally equivalent to Rpi-blb1. Our findings indicate that effector genomics enables discovery and functional profiling of late blight R genes and Avr genes at an unprecedented rate and promises to accelerate the engineering of late blight resistant potato varieties.     

Tagging QTLs for late blight resistance and plant maturity from diploid wild relatives in a cultivated potato (Solanum tuberosum) background

Phytophthora infestans causes an economically important disease of potato called late blight. The epidemic is controlled chemically but resistant potatoes can become an environment-friendly and financially justified alternative solution. The use of diploid Solanum tuberosum derived from European tetraploid cultivars enabled the introgression of novel genes encoding foliage resistance and tuber resistance from other species into the modern cultivated potato gene pool. This study evaluated the resistance of the obtained hybrids, its quality, expression in leaflets and tubers and its relation to the length of vegetation period. We also identified genetic loci involved in late blight resistance and the length of vegetation period. A family of 156 individuals segregating for resistance to late blight was assessed by three laboratory methods: detached leaflet, tuber slice and whole tuber test, repeatedly over 5 years. Length of vegetation period was estimated by a field test over 2 years. The phenotypic distributions of all traits were close to normal. Using sequence-specific PCR markers of known chromosomal position on the potato genetic map, six quantitative trait loci (QTLs) for resistance and length of vegetation period were identified. The most significant and robust QTL were located on chromosomes III (explaining 17.3% of variance observed in whole tuber tests), IV (15.5% of variance observed in slice tests), X (15.6% of variance observed in leaflet tests) and V (19.9% of variance observed in length of vegetation period). Genetic characterization of these novel resistance sources can be valuable for potato breeders and the knowledge that the most prominent QTLs for resistance and vegetation period length do not overlap in this material is promising with respect to breeding early potatoes resistant to P. infestans. Electronic supplementary material The online version of this article (doi:) contains supplementary material, which is available to authorized users.     

马铃薯种质资源的晚疫病抗性鉴定

采用离体叶片接种法,对43份马铃薯种质资源进行晚疫病抗性鉴定、比较和评价。以接种5 d后的感病品种‘Désirée’和抗病品种‘加湘1号’叶片症状为对照,鉴定出5份表现为高抗的种质资源,其中包含3个Solanum phurejia和2个S. tuberosum ssp. andigena材料。另外,还鉴定出14份中抗材料(No. 7～20)。结果表明,野生种和安第斯山栽培亚种马铃薯资源的抗性材料较为丰富,可作为晚疫病抗病育种的亲本。     

Using half-normal probability plot and regression analysis to differentiate complex traits: differentiating disease response of multigenic resistance and susceptibility in tomatoes to multiple pathogen isolates

The need for a new analytical approach was encountered in the course of characterizing newly developed tomato lines resistant to late blight. Late blight resistant tomato lines were created in independent breeding programs using the accession Solanum pimpinellifolium L. (formerly Lycopersicon pimpinellifolium (L.) Miller) L3708 as the source of the resistance. However, initial field observation suggested that the late blight resistance in the lines produced by two independent breeding programs differed. Possible causes included a partial transfer of the late blight resistance derived from S. pimpinellifolium L3708 or the possibility of race specificity of this resistance. A crucial issue was determining the most appropriate and robust analytical method to use with data from laboratory analyses of the responses of nine tomato lines against five P. infestans isolates. Prior analysis by standard ANOVA revealed significant differences across tomato lines but could not determine whether the disease responses in the CLN-R lines were different from those of the heterozygous F₁ hybrids, created by crossing susceptible tomatoes with the fixed CU-R lines. A different analytical method was needed. Therefore, sporangia numbers/leaflet and diseased area data were analyzed using a half-normal probability plot and regression analysis. The results of this analysis show its utility for genetic or pathology studies. Considering only populations of the uniform tomato lines, this method confirms the results obtained by using a standard ANOVA, but provides a clearer demonstration of the distributions of the individuals within the populations and how this distribution impacts variance and the difference among the populations. This method also allows a joint analysis of the uniform lines with an additional population that is less uniform, because it is segregating. Such an analysis would be invalid using a standard ANOVA. The results of this joint analysis determined that the additional population was divergent from the fixed CU-R lines, and, against some isolates, against the CLN-R lines as well. Half-normal probability plot analysis method would be applicable more broadly beyond analysis of disease resistance data. It could be useful for data from populations that are not normally distributed, for traits which are affected by epistatic gene action, and could be useful for selection of extremes.     

Comparative analysis of Phytophthora infestans induced gene expression in potato cultivars with different levels of resistance

Differential gene expression was analyzed after infection with Phytophthora infestans in six potato cultivars with different levels of resistance to late blight. To verify the infection of the potato leaflets, the amount of phytopathogen mRNA within the plant material was quantified by real-time quantitative PCR. The expression of 182 genes selected from two subtracted cDNA libraries was studied with cDNA array hybridization using RNA from non-infected and infected potato leaflets. Gene up- and down-regulation were clearly detectable in all cultivars 72 h post inoculation. Gene expression patterns in susceptible cultivars differed from those in potato varieties with a higher level of resistance. In general, a stronger gene induction was observed in the susceptible cultivars compared to the moderately to highly resistant potato varieties. Five genes with the highest homology to stress and/or defence-related genes were induced specifically in the susceptible cultivars. Four genes responded to pathogen attack independently of the level of resistance of the cultivar used, and three genes were repressed in infected tissue of most cultivars. Even in the absence of P. infestans infection, six genes showed higher expression levels in the somewhat resistant cultivars Bettina and Matilda. Possible reasons for the different levels of gene expression are discussed.     

Biochemical and molecular determinance of resistance and susceptibility in Solanum tuberosum (potato) plants challenged with Phytopthora infestans

Determinance of resistance was studied in four different varieties of Solanum tuberosum using biochemical and molecular parameters. It was clearly evident that due to infection of P. infestans, the total protein, total phenol and lignin were induced in all four verities; however, the induction was found more in resistant varieties compared to susceptible varieties. Induction of proteins was also determined by SDS-PAGE analysis. Deposition of lignin was showed by histological comparison using Phloroglucinol HCl staining. Higher deposition of lignin in resistant varieties could be considered as reliable characters related to disease resistance and could be used as biochemical markers for late blight resistance. The RAPD profile generated using eight different decameric primers showed both, polymorphic as well as monomorphic bands. There were many unique bands found only in resistant varieties. These polymorphic bands could also be served as molecular markers for screening of LB resistance potato varieties.     

Identification of candidate genes in rice for resistance to sheath blight disease by whole genome sequencing

Recent advances in whole genome sequencing (WGS) have allowed identification of genes for disease susceptibility in humans. The objective of our research was to exploit whole genome sequences of 13 rice (Oryza sativa L.) inbred lines to identify non-synonymous SNPs (nsSNPs) and candidate genes for resistance to sheath blight, a disease of worldwide significance. WGS by the Illumina GA IIx platform produced an average 5× coverage with ~700 K variants detected per line when compared to the Nipponbare reference genome. Two filtering strategies were developed to identify nsSNPs between two groups of known resistant and susceptible lines. A total of 333 nsSNPs detected in the resistant lines were absent in the susceptible group. Selected variants associated with resistance were found in 11 of 12 chromosomes. More than 200 genes with selected nsSNPs were assigned to 42 categories based on gene family/gene ontology. Several candidate genes belonged to families reported in previous studies, and three new regions with novel candidates were also identified. A subset of 24 nsSNPs detected in 23 genes was selected for further study. Individual alleles of the 24 nsSNPs were evaluated by PCR whose presence or absence corresponded to known resistant or susceptible phenotypes of nine additional lines. Sanger sequencing confirmed presence of 12 selected nsSNPs in two lines. “Resistant” nsSNP alleles were detected in two accessions of O. nivara that suggests sources for resistance occur in additional Oryza sp. Results from this study provide a foundation for future basic research and marker-assisted breeding of rice for sheath blight resistance.