Local adaptation to temperature in populations and clonal lineages of the Irish potato famine pathogen Phytophthora infestans

Environmental factors such as temperature strongly impact microbial communities. In the current context of global warming, it is therefore crucial to understand the effects of these factors on human, animal, or plant pathogens. Here, we used a common‐garden experiment to analyze the thermal responses of three life‐history traits (latent period, lesion growth, spore number) in isolates of the potato late blight pathogen Phytophthora infestans from different climatic zones. We also used a fitness index (FI) aggregating these traits into a single parameter. The experiments revealed patterns of local adaptation to temperature for several traits and for the FI, both between populations and within clonal lineages. Local adaptation to temperature could result from selection for increased survival between epidemics, when isolates are exposed to more extreme climatic conditions than during epidemics. We also showed different thermal responses among two clonal lineages sympatric in western Europe, with lower performances of lineage 13_A2 compared to 6_A1, especially at low temperatures. These data therefore stress the importance of thermal adaptation in a widespread, invasive pathogen, where adaptation is usually considered almost exclusively with respect to host plants. This must now be taken into account to explain, and possibly predict, the global distribution of specific lineages and their epidemic potential.     

Tracking historic migrations of the Irish potato famine pathogen,Phytophthora infestans

The plant pathogen Phytophthora infestans causes late blight, a devastating disease on potato that led to the Irish potato famine during 1845-1847. The disease is considered a reemerging problem and still causes major epidemics on both potato and tomato crops worldwide. Theories on the origin of the disease based on an examination of the genetic diversity and structure of P. infestans populations and use of historic specimens to understand modern day epidemics are discussed.     

马铃薯晚疫病生防木霉菌的筛选及鉴定

采用马铃薯活体筛选法从268株木霉菌中筛选获得两株对致病疫霉有较强抑菌活性的木霉菌株R-5和T-15。这两株木霉菌代谢液可抑制病原菌生长及孢子囊萌发。温室防病试验发现,接种两株木霉菌可以减轻晚疫病的发生。田间试验进一步证明,两株木霉菌对晚疫病具有良好的田间防治效果,防效分别达到了72.4%和70.0%。经分子生物学方法鉴定,两株木霉菌分别为拟康氏木霉和棘孢木霉。实验构建的以活体筛选为基础的生防木霉菌筛选方法是一种可行高效的生防木霉菌筛选方式。     

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。     

Differential gene expression in two potato lines differing in their resistance to Phytophthora infestans

Horizontal resistance to late blight in the potato is a primary objective of many breeding programs. Knowledge of the physiological and biochemical mechanisms underlying it, however, is scarce. The purpose of the present study was the identification of these physiological and biochemical factors in plant material obtained by crossing a late blight resistant Solanum phureja clone with a susceptible dihaploid of S. tuberosum subsp. tuberosum. The mRNA RT-PCR differential display method was used to compare the gene expression patterns of a resistant hybrid with that of a susceptible one. By sequence homology, we identified several genes with diverse functions, including genes known to be involved in resistance or stress responses and genes known to be involved in primary or secondary metabolism.     

中国部分地区马铃薯寄主上致病疫霉SSR基因型分析

利用两对SSR引物对两个基因座Pi4B和Pi4G进行了PCR扩增,测定了中国部分地区66个致病疫霉Phyophthora infestans(马铃薯晚疫病菌)菌株和2个参考菌株的SSR基因型,并对菌株的基因型进行了鉴定和命名.在被测定的66个致病疫霉菌株中,共产生了7种SSR基因型D-03,D-05,D-06,G-02,H-01,F-01和F-06,其中F-06为本研究新命名的基因型.F-01基因型菌株53个,占总菌株数目的80.3%,该基因型为中国致病疫霉的优势基因型.在对两个基因座Pi4B和Pi4G产生的等位基因统计分析发现基因座Pi4B产生的多样性比Pi4G高.对SSR数据揭示的河北、黑龙江和云南3个不同省份致病疫霉遗传多样性的比较发现,河北省和黑龙江省致病疫霉遗传多样性几乎相同,然而与云南省致病疫霉有较大的遗传差异.此外,发现致病疫霉SSR基因型与其对甲霜灵抗性无相关性.     

一个马铃薯种质资源圃致病疫霉群体的分析

由致病疫霉Phytophthora infestans引起的晚疫病是马铃薯生产上最严重的病害之一,认识其群体结构特征,可为晚疫病防控策略的制定以及抗病品种的合理布局提供指导。对2009年采自宁夏一个种植有93个品种(品系)的马铃薯种质资源圃的致病疫霉进行了交配型、致病型和线粒体DNA单倍型分析,结果表明,116个致病疫霉菌株中存在A1、A2和自育型3种交配型,发生频率分别为24.1%、57.8%和18.1%,A2交配型为优势类型;对其中43个菌株的致病型进行测试,检测到两种致病类型:1.2.3.4.5.6.7.8.9.10.11和3.4.10,发生频率分别为95.3%和4.7%,可克服所有11个抗病基因的1.2.3.4.5.6.7.8.9.10.11类型占绝对优势;对62个菌株的线粒体DNA单倍型进行分析,检测到Ia和IIa两种类型,发生频率分别为74.2%和25.8%。综合表型和基因型数据分析发现,该马铃薯种质资源圃中致病疫霉群体致病型单一,但致病型毒力因子高度复合;线粒体DNA分析表明,该马铃薯种质资源圃引入了遗传背景较为复杂的致病疫霉"新"群体。     

Evaluation of fungicides to control potato late blight (Phytophthora infestans) in the plains of Nepal

Fungicide application is an effective management option to control late blight of potato (caused by Phytophthora infestans). Field experiments were conducted to evaluate the efficacy of recently introduced and previously used fungicides on late blight management and potato yields in the western plains of Nepal in 2015 and 2016 crop seasons. Fungicides and a non-treated control (NTC) were replicated three times in a randomized block design planted with late blight susceptible cultivar Cardinal. Chlorothalonil, copper oxychloride, dimethomorph, fenamidone + mancozeb, mancozeb and metalaxyl were sprayed in 2015. In 2016, carbendazim was used instead of chlorothalonil. The area under disease progress curve (AUDPC) was consistently reduced in years by dimethomorph (90% and 65% in 2015 and 2016, respectively), fenamidone + mancozeb (68% and 62%) and mancozeb (40% and 47%) compared with the NTC. Similarly, tuber yield was increased with the application of dimethomorph (266% and 146% in 2015 and 2016, respectively), fenamidone + mancozeb (211% and 155%) and mancozeb (136% and 116%) compared with the NTC. Chlorothalonil reduced AUDPC by 43% and increased tuber yield by 170% in 2015. Other fungicides either had inconsistent results or did not reduce late blight severity and consequent effects on potato yield. The overall benefit–cost ratio was highest for dimethomorph in both years. These results show efficacy of dimethomorph, fenamidone + mancozeb and mancozeb in reducing late blight severity and increasing potato tuber yield in the plains of Nepal.     

Trade‐offs and evolution of thermal adaptation in the Irish potato famine pathogen Phytophthora infestans

Temperature is one of the most important environmental parameters with crucial impacts on nearly all biological processes. Due to anthropogenic activity, average air temperatures are expected to increase by a few degrees in coming decades, accompanied by an increased occurrence of extreme temperature events. Such global trends are likely to have various major impacts on human society through their influence on natural ecosystems, food production and biotic interactions, including diseases. In this study, we used a combination of statistical genetics, experimental evolution and common garden experiments to investigate the evolutionary potential for thermal adaptation in the potato late blight pathogen, Phytophthora infestans, and infer its likely response to changing temperatures. We found a trade‐off associated with thermal adaptation to heterogeneous environments in P. infestans, with the degree of the trade‐off peaking approximately at the pathogen's optimum growth temperature. A genetic trade‐off in thermal adaptation was also evidenced by the negative association between a strain's growth rate and its thermal range for growth, and warm climates selecting for a low pathogen growth rate. We also found a mirror effect of phenotypic plasticity and genetic adaptation on growth rate. At below the optimum, phenotypic plasticity enhances pathogen's growth rate but nature selects for slower growing genotypes when temperature increases. At above the optimum, phenotypic plasticity reduces pathogen's growth rate but natural selection favours for faster growing genotypes when temperature increases further. We conclude from these findings that the growth rate of P. infestans will only be marginally affected by global warming.     

Oospore Germination and Formation by the Late Blight Pathogen Phytophthora infestans in vitro and under Field Conditions

The ability of the late blight pathogen Phytophthora infestans to form oospores in leaves of seven potato cultivars was examined at different incubation temperatures under controlled environmental conditions and under field conditions. At 10°C, the oospore formation in three intermediate-resistant cultivars all differed significantly from each other (P < 0.05), with the lowest amount formed in cv. Asterix. This latter cultivar did not form oospores at any other temperature. Under field conditions oospores were formed abundantly in a naturally infected field. A significant date by cultivar interaction showed that P. infestans increased the oospore formation in foliage by time in cvs Columbo, Hertha and Matilda, whereas no significant differences between dates were found for other cultivars. The genetic structure of P. infestans in the naturally infected field plot, where oospores formed abundantly, was studied by using amplified fragment length polymorphism and a high genetic diversity was revealed. Oospore germination from two Scandinavian (A1 and A2) P. infestans isolates was stimulated in visible light and in 1 : 2 and 1 : 10 soil extract. The effect of light and nutrients on oosporogenesis is discussed.     

The plant pathogen Phytophthora andina emerged via hybridization of an unknown Phytophthora species and the Irish potato famine pathogen, P. infestans

Emerging plant pathogens have largely been a consequence of the movement of pathogens to new geographic regions. Another documented mechanism for the emergence of plant pathogens is hybridization between individuals of different species or subspecies, which may allow rapid evolution and adaptation to new hosts or environments. Hybrid plant pathogens have traditionally been difficult to detect or confirm, but the increasing ease of cloning and sequencing PCR products now makes the identification of species that consistently have genes or alleles with phylogenetically divergent origins relatively straightforward. We investigated the genetic origin of Phytophthora andina, an increasingly common pathogen of Andean crops Solanum betaceum, S. muricatum, S. quitoense, and several wild Solanum spp. It has been hypothesized that P. andina is a hybrid between the potato late blight pathogen P. infestans and another Phytophthora species. We tested this hypothesis by cloning four nuclear loci to obtain haplotypes and using these loci to infer the phylogenetic relationships of P. andina to P. infestans and other related species. Sequencing of cloned PCR products in every case revealed two distinct haplotypes for each locus in P. andina, such that each isolate had one allele derived from a P. infestans parent and a second divergent allele derived from an unknown species that is closely related but distinct from P. infestans, P. mirabilis, and P. ipomoeae. To the best of our knowledge, the unknown parent has not yet been collected. We also observed sequence polymorphism among P. andina isolates at three of the four loci, many of which segregate between previously described P. andina clonal lineages. These results provide strong support that P. andina emerged via hybridization between P. infestans and another unknown Phytophthora species also belonging to Phytophthora clade 1c.     

Genotypic Analysis of Russian Isolates of Phytophthora infestans from the Moscow Region, Siberia and Far East

Phytophthora infestans samples were collected during 1997 and 1998 at multiple sites in Russia from Sakhalin Island in the Far East across Siberia (nine sites, 160 isolates) to the Moscow region (four sites, 325 isolates). In addition, 12 isolates that were obtained previously were included. All isolates were analysed for mating type, and sensitivity to metalaxyl. Isolates from within any of the nine sites outside of the Moscow region were monomorphic for mating type and nearly monomorphic for metalaxyl resistance. In contrast, both A1 and A2 isolates were detected in the Moscow region, and these isolates were also polymorphic for metalaxyl resistance. In two sites in Siberia only A2 mating type strains were detected, in the other six sites in Siberia and in Sakahlin Island, only A1 mating types were detected. A subset of isolates ( n =191) was also analysed for pathotype (virulence to 10 R-genes, each in a distinct differential genotype). All isolates were highly complex (mean number of virulences approximately 8.4 of a maximum number of 10). All isolates ( n =43) from Sakahlin Island were virulent to all 10 of the R-genes tested. A further subset of isolates ( n =70, including 12 isolates collected before 1997) was analysed for genotype at the Glucose- 6-phosphate isomerase and Peptidase loci, mtDNA haplotypes, and RFLP pattern using the RG57 probe. The US-1 clonal lineage (previously dominant) was not detected in the 1997–98 sample. The populations of P. infestans near Moscow in 1997 and 1998 was highly diverse with 15 unique genotypes (including both mating types) among a sample of 18 isolates. In contrast, the populations of P. infestans in Siberia had limited diversity, with only three multilocus genotypes detected and most populations were dominated by the SIB-1 clonal lineage. This lineage accounted for 31 of the 39 strains collected in Siberia that were assayed for multilocus genotype.     

The R1 gene for potato resistance to late blight (Phytophthora infestans) belongs to the leucine zipper/NBS/LRR class of plant resistance genes

Late blight caused by the oomycete Phytophthora infestans is the most destructive disease in potato cultivation worldwide. New, more virulent P. infestans strains have evolved which overcome the genetic resistance that has been introgressed by conventional breeding from wild potato species into commercial varieties. R genes (for single-gene resistance) and genes for quantitative resistance to late blight are present in the germplasm of wild and cultivated potato. The molecular basis of single-gene and quantitative resistance to late blight is unknown. We have cloned R1, the first gene for resistance to late blight, by combining positional cloning with a candidate gene approach. The R1 gene is member of a gene family. It encodes a protein of 1293 amino acids with a molecular mass of 149.4 kDa. The R1 gene belongs to the class of plant genes for pathogen resistance that have a leucine zipper motif, a putative nucleotide binding domain and a leucine-rich repeat domain. The most closely related plant resistance gene (36% identity) is the Prf gene for resistance to Pseudomonas syringae of tomato. R1 is located within a hot spot for pathogen resistance on potato chromosome V. In comparison to the susceptibility allele, the resistance allele at the R1 locus represents a large insertion of a functional R gene.     

河北省番茄上致病疫霉Phytophthora infestans群体结构的分析

番茄晚疫病是河北省番茄生产上最具毁灭性的病害之一,对引起该病害的致病疫霉群体结构进行分析有利于病害的防治。利用对峙培养法和菌落直径法对2007-2008年采自河北省保定、沧州和唐山分离自番茄的49个致病疫霉菌株进行了交配型和甲霜灵抗性的表型测定,结果表明该群体所有菌株均为A1交配型,以甲霜灵敏感菌株为主,抗性菌株仅7株。利用聚合酶链式反应-限制性片段长度多态性(PCR-RFLP)、简单序列重复(SSR)和扩增片段长度多态性(AFLP)等分子技术对该群体的基因型进行了分析,结果表明供试菌株线粒体基因型均为Ia型,共鉴定出了Ⅰ、Ⅱ和Ⅲ3种SSR基因型,AFLP聚类分析在相似系数0.87时可以形成α、β和γ等3个不同的分支。河北省所有番茄上致病疫霉菌株均分布在α分支上,该分支又可进一步分为7个亚分支。AFLP亚分支与甲霜灵抗性和地理来源均无明显相关性,但Ⅱ型SSR与甲霜灵抗性和地理来源有明显的相关性。综合表型和基因型数据说明河北省番茄上致病疫霉群体结构比较单一,遗传多样性程度较低。     

A genetic analysis of quantitative resistance to late blight in potato: towards marker-assisted selection

Late blight caused by the oomycete Phytophthora infestans is the most important fungal disease in potato cultivation worldwide. Resistance to late blight is controlled by a few major genes (R genes) which can be easily overcome by new races of P. infestans and/or by an unknown number of genes expressing a quantitative type of resistance which may be more durable. Quantitative resistance of foliage to late blight was evaluated in five F₁ hybrid families originating from crosses among seven different diploid potato clones. Tuber resistance was evaluated in four of the families. Two of the families were scored for both foliage maturity and vigour. The five families were genotyped with DNA-based markers and tested for linkage with the traits analysed. QTL (quantitative trait locus) analysis identified at least twelve segments on ten chromosomes of potato having genes that affect reproducibly foliage resistance. Two of those segments also have major R genes for resistance to late blight. The segments are tagged by 21 markers that can be analyzed based on PCR (polymerase chain reaction) with specific oligonucleotide primers. One QTL was detected for tuber resistance and one for foliage vigour. Two QTLs were mapped for foliage maturity. Major QTL effects on foliage and tuber resistance to late blight and on foliage maturity and vigour were all linked with marker GP179 on linkage group V of potato. Plants having alleles at this QTL, which increased foliage resistance, exhibited decreased tuber resistance, later maturity and more vigour.     

Potato StMPK7 is a downstream component of StMKK1 and promotes resistance to the oomycete pathogen Phytophthora infestans

A cascade formed by phosphorylation events of mitogen-activated protein kinases (MAPKs) takes part in plant stress responses. However, the roles of these MAPKs in resistance of potato (Solanum tuberosum) against Phytophthora pathogens is not well studied. Our previous work showed that a Phytophthora infestans RXLR effector targets and stabilizes the negative regulator of MAPK kinase 1 of potato (StMKK1). Because in Arabidopsis thaliana the AtMPK4 is the downstream phosphorylation target of AtMKK1, we performed a phylogenetic analysis and found that potato StMPK4/6/7 are closely related and are orthologs of AtMPK4/5/11/12. Overexpression of StMPK4/7 enhances plant resistance to P. infestans and P. parasitica. Yeast two-hybrid analysis revealed that StMPK7 interacts with StMKK1, and StMPK7 is phosphorylated on flg22 treatment and by expressing constitutively active StMKK1 (CA-StMKK1), indicating that StMPK7 is a direct downstream signalling partner of StMKK1. Overexpression of StMPK7 in potato enhances potato resistance to P. infestans. Constitutively active StMPK7 (CA-StMPK7; StMPK7^{D198G, E202A}) was found to promote immunity to Phytophthora pathogens and to trigger host cell death when overexpressed in Nicotiana benthamiana leaves. Cell death triggered by CA-StMPK7 is SGT1/RAR1-dependent. Furthermore, cell death triggered by CA-StMPK7 is suppressed on coexpression with the salicylate hydroxylase NahG, and StMPK7 activation promotes salicylic acid (SA)-responsive gene expression. We conclude that potato StMPK7 is a downstream signalling component of the phosphorelay cascade involving StMKK1 and StMPK7 plays a role in immunity to Phytophthora pathogens via an SA-dependent signalling pathway.