Aluminum induces cross-resistance of potato to Phytophthora infestans

The phenomenon of cross-resistance allows plants to acquire resistance to a broad range of stresses after previous exposure to one specific factor. Although this stress–response relationship has been known for decades, the sequence of events that underpin cross-resistance remains unknown. Our experiments revealed that susceptible potato (Solanum tuberosum L. cv. Bintje) undergoing aluminum (Al) stress at the root level showed enhanced defense responses correlated with reduced disease symptoms after leaf inoculation with Phytophthora infestans. The protection capacity of Al to subsequent stress was associated with the local accumulation of H₂O₂ in roots and systemic activation of salicylic acid (SA) and nitric oxide (NO) dependent pathways. The most crucial Al-mediated changes involved coding of NO message in an enhanced S-nitrosothiol formation in leaves tuned with an abundant SNOs accumulation in the main vein of leaves. Al-induced distal NO generation was correlated with the overexpression of PR-2 and PR-3 at both mRNA and protein activity levels. In turn, after contact with a pathogen we observed early up-regulation of SA-mediated defense genes, e.g. PR1, PR-2, PR-3 and PAL, and subsequent disease limitation. Taken together Al exposure induced distal changes in the biochemical stress imprint, facilitating more effective responses to a subsequent pathogen attack.     

Phenotypic markers of Phytophthora infestans

Abstract

Phytophthora infestans is one of the most destructive pathogens of potato and causal agents of notorious disease late blight. Different chemicals are used to control the pathogen of late blight but the most commonly used is metalaxyl; its extensive use of has caused decreased sensitivity in the P. infestans population. The metalaxyl sensitivity of the Pakistani population of P. infestans is investigated in the present study. For this purpose, 178 isolates of P. infestans were obtained from the lesions of diseased potato leaves and stems, and samples were collected from the different potato-growing areas of Pakistan, where late blight is a problem. Sensitivity of the isolates of P. infestans was investigated by metalaxyl sensitivity test and with the help of test isolates were divided into three categories, i.e. sensitive, intermediate and resistant, based on their Co-efficient of mycelial growth inhibition (CMGI) values. During the study, highest percentage (50.17%) of resistant isolates was observed in the population of Punjab (zone 2), whereas the lower percentage (33.33%) was observed in the population of Swat valley (zone 6b). In the present study, it was discovered that P. infestans late blight-causing fungus has adopted more resistance against metalaxyl because of its wide use.     

Susceptibility of potato tubers to infection by Phytophthora infestans

Phytophthora infestans infects King Edward potato tubers more readily through inoculated eyes than through lenticels, but more lenticels than eyes became infected when whole tubers were sprayed with inoculum. The resistance of lenticels but not of eyes increased as tubers aged. The spores did not infect through intact periderm. The likelihood of tubers on plants grown in pots becoming infected by sporangial suspension poured on to the soil increased the nearer the tubers were to the soil surface, the stem, or the side of the pot. Naturally infected tubers, and those sprayed with sporangial suspension, had most eyes infected at the rose end, and most lenticels infected on the middle region of the tuber. Of naturally infected tubers, on which the site of infection could be identified, most were infected through eyes at the rose end.     

Genetic Analyses of Interspecific Hybrids between Phytophthora infestans and Phytophthora mirabilis

Goodwin, S. B., and Fry, W. E. 1994. Genetic analyses of interspecific hybrids between Phytophthora infestans and Phytophthora mirabilis. Experimental Mycology 18, 20-32. Four crosses were made between isolates of two host-specific Phytophthora species. Phytophthora infestans and Phytophthora mirabilis. In the two most successful crosses involving a common P. infestans A2 parent, allozyme analysis confirmed that 79 of 86 progeny were interspecific hybrids, 3 were presumed selfs, and 4 were either selfs or nonrecombinant parental types. Mating type, alleles at the allozyme locus glucose-6-phosphate isomerase, and the + alleles at a number of DNA fingerprinting loci segregated independently according to Mendelian expectation. Three DNA fingerprinting loci were tightly linked in P. mirabilis, but no other linkages were detected among these markers. Mitochondrial DNA was uniparentally inherited, mostly from the P. infestans parent. Growth rate segregated as a quantitative character. None of the 68 progeny tested infected Mirabilis jalapa (the host of P. mirabilis), 3 infected potato, and 4 were weakly pathogenic to tomato. Because most of the F₁ hybrids could not infect any of the hosts infected by the parents, host specialization could provide a postzygotic as well as a prezygotic reproductive isolating mechanism for P. infestans and P. mirabilis in central Mexico. These results indicate that P. mirabilis probably is capable of a regular outcrossing mating system.     

Genetic analysis of Phytophthora infestans populations in the Nordic European countries reveals high genetic variability

Late blight, caused by the oomycete Phytophthora infestans, is the most important disease of potato (Solanum tuberosum). The pathogen is highly adaptable and to get an overview of the genetic variation in the Nordic countries, Denmark, Finland, Norway and Sweden we have analyzed 200 isolates from different fields using nine simple-sequence repeat (SSR) markers. Forty-nine alleles were detected among the nine SSR loci and isolates from all four Nordic countries shared the most common alleles across the loci. In total 169 multilocus genotypes (based on seven loci) were identified among 191 isolates. The genotypic diversities, quantified by a normalized Shannon's diversity index (H(s)), were 0.95 for the four Nordic countries. The low F(ST) value of 0.04 indicates that the majority of variation is found within the four Nordic countries. The large number of genotypes and the frequency distribution of mating types (60% A1) support the hypothesis that sexual reproduction is contributing notably to the genetic variation of P. infestans in the Nordic countries.     

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。     

Exoproteinases of the Oomycete Phytophthora infestans

When grown in a medium containing heat-stable potato tuber proteins, the oomycete Phytophthora infestans (Mont.) de Bary produces a set of exoproteinases active at neutral and mildly basic pH values. These extracellular proteinases have been shown (by SDS-PAGE in the presence of gelatin) to include at least six components differing in molecular weight. Inhibitory analysis and studies of the effects of the enzymes on various synthetic substrates show that the culture liquid of P. infestans contains mainly serine proteinases (specific for trypsin and subtilisin) and metalloproteinases. Their activity is suppressed by proteinase-inhibiting proteins from potato tubers. It is suggested that exoproteinases of P. infestans may be the metabolic target for natural proteinase inhibitors from potato.     

The hypersensitive response is associated with host and nonhost resistance to Phytophthora infestans

The interaction between Phytophthora infestans (Mont.) de Bary and Solanum was examined cytologically using a diverse set of wild Solanum species and potato (S. tuberosum L.) cultivars with various levels of resistance to late blight. In wild Solanum species, in potato cultivars carrying known resistance (R) genes and in nonhosts the major defense reaction appeared to be the hypersensitive response (HR). In fully resistant Solanum species and nonhosts, the HR was fast and occurred within 22 h. This resulted in the death of one to three cells. In partially resistant clones, the HR was induced between 16 and 46 h, and resulted in HR lesions consisting of five or more dead cells, from which hyphae were occasionally able to escape to establish a biotrophic interaction. These results demonstrate the quantitative nature of the resistance to P. infestans. The effectiveness of the HR in restricting growth of the pathogen differed considerably between clones and correlated with resistance levels. Other responses associated with the defense reaction were deposition of callose and extracellular globules containing phenolic compounds. These globules were deposited near cells showing the HR, and may function in cell wall strengthening. Received: 22 April 1999 / Accepted: 4 November 1999     

Differentiation of Phytophthora infestans sporangia from other airborne biological particles by flow cytometry.

The ability of two different flow cytometers, the Microcyte (Optoflow) and the PAS-III (Partec), to differentiate sporangia of the late-blight pathogen Phytophthora infestans from other potential airborne particles was compared. With the PAS-III, light scatter and intrinsic fluorescence parameters could be used to differentiate sporangia from conidia of Alternaria or Botrytis spp., rust urediniospores, and pollen of grasses and plantain. Differentiation between P. infestans sporangia and powdery mildew conidia was not possible by these two methods but, when combined with analytical rules evolved by genetic programming methods, could be achieved after staining with the fluorescent brightener Calcofluor white M2R. The potential application of these techniques to the prediction of late-blight epiphytotics in the field is discussed.     

Effect of immunomodulators on potato resistance and susceptibility to Phytophthora infestans

The mechanisms of induced resistance and susceptibility of potato (Solanum tuberosum L.) tubers to late blight agent (Phytophthora infestans Mont de Bary) were studied using an elicitor chitosan and an immunosuppressor laminarin. It was elucidated that treatment of disks from potato tubers with chitosan resulted in salicyclic acid (SA) accumulation due to activation of benzoate-2-hydroxylase and hydrolysis of SA conjugates. Such SA accumulation in potato tissues inhibited one of the antioxidant enzymes, catalase, inducing an oxidative burst and resistance development. The mechanisms of induced susceptibility to the late blight causal agent were studied using an unspecific immunosuppressor, laminarin, an analogue of natural specific suppressor of potato immune responses, β-1,3,β-1,6-glucan. It was established that the development of immunosuppression in tissues treated with laminarin did not affect the SA level in tissues. However, catalase sensitivity to SA reduced in laminarin-treated tissues, and the enzyme activity increased. In its turn, this might result in the reduced level of hydrogen peroxide in the cells and, as a sequence, in the increased potato susceptibility to late blight.     

Cytology of induced systemic resistance of tomato to Phytophthora infestans

The infection of tomato leaves by Phytophthora infestans was followed using cytological methods. Fungal ingress and plant reactions in untreated and induced resistant plants were studied. Systemic disease resistance was induced by a local pre-infection with the same fungus. Induction retarded fungal progress at the leaf surface, epidermis and in the mesophyll. The reduced numbers of germinated cysts indicate the presence of fungitoxic substances on the leaf surface of induced plants. Frequency of fungal penetration through the outer epidermal cell wall was reduced, but only in plants exhibiting a high level of induced resistance. Autofluor-escent material, indicating the presence of lignin-like substances, accumulated rapidly beneath some of the appressoria, but this plant response was similar in induced and non-induced plants. Staining with aniline blue indicated that callose deposition was not involved in induced resistance. Thus, none of the cytologically investigated plant reactions correlated with the reduced penetration frequency observed. In the mesophyll, however, the cytological picture corresponding to a hypersensitive reaction occurred more often in induced plants. It is concluded that reduction of disease severity by induction is the result of the combined action of several successive defence reactions.Dedicated to the memory of Professor H. Grisebach     

Resistance of tomatoes to late blight (Phytophthora infestans)

Race T₁ of Phytophthora infestans was first isolated in Central Europe. Tomato varieties carrying the dominant gene Ph were easily infected by T₁ but showed a different degree of field resistance. In September 1967 more than 80% of all isolations from the experimental plots at Greifswald were determined as the aggressive race T₁. This race was also isolated in a large screening test over several provinces of the GDR from potato and tomato fields where no plants carrying dominant resistance genes were present in the neighbourhood. The fungus was able to pass from potato leaves or tubers to detached tomato leaves or intact tomato plants (cv. Fanal) without any reduction in sporulation. The field-resistant tomato variety Ru?ový Ker and the highly field-resistant variety Atom were crossed. In F₂ and subsequent generations, young plants were selected following inoculation of 14 mm leaf-discs. Weak sporulations on the leaf-discs were almost invariably correlated with a reduced level of infection of fruit in the field. Data from crosses of Atom with more or less susceptible varieties suggested the presence of incompletely dominant genes. It is concluded that at least two genes in the variety Atom control field resistance and the gene-symbols Phf and Phf-2 are proposed for them.