Genetic diversity among late blight resistant and susceptible potato genotypes

RAPD polymerase chain reaction analysis was used to study the genetic diversity among a wild potato variety Solanum demissum (very resistant to late blight) and six potato cultivars (Hanna, Lady-Olympia, Lady-Rosetta, Spunta, Diamant and Cara) varied in their resistance to Phytophthora infestans. Cluster analysis of six potato genotypes showed that, all tested genotypes were separated into two clusters (1 and 2). Cluster 1, included only the wild potato variety (S. demissum), whereas cluster 2 divided into two groups (G1 and G2). Late blight high resistant cultivars Hanna and Cara were grouped in G1. Group 2 included the moderate resistant cultivar Spunta and the susceptible cultivars Diamant, Lady-Rosetta and Lady-Olympia. The potato cultivars that showed highest genetic similarity to the wild potato variety were the resistant cultivars Hanna and Cara. Lowest genetic similarity was obtained with the susceptible cultivars Lady-Rosetta, Diamant and Lady-Olympia. RAPD primer K17 yielded a band with molecular weight of 936 bp found in all susceptible potato cultivars (Lady-Rosetta, Lady-Olympia and Diamant). On the other hand, band with molecular weight of 765 bp were detected in the wild potato and the resistant cultivars Hanna and Cara. Results of this study suggested that, the RAPD marker technique could be beneficial for revealing the genetic variability of different genotypes of potato varied in their resistibility to late blight.     

Comparative proteomic analysis of bacterial wilt susceptible and resistant tomato cultivars

To investigate the molecular mechanisms of bacterial resistance in susceptible and resistant cultivars of tomato, a proteomic approach was adopted. Four cultivars of tomato were selected on the basis of their response to bacterial (Pseudomonas solanacearum) inoculation wherein cultivar Roma and Riogarande, and cultivar Pusa Ruby and Pant Bahr were considered as resistant and susceptible cultivars, respectively. Proteins were extracted from leaves of 3-week-old seedlings of the four cultivars and separated by 2-DE. A total of nine proteins were found to be differentially expressed between the susceptible and resistant cultivars. Amino acid sequences of these proteins were determined with a protein sequencer. The identified proteins belongs to the categories of energy, protein destination and storage, and defense. Of these proteins, a 60 kDa chaperonin and an apical membrane antigen were significantly upregulated in resistant cultivars compared with susceptible cultivars. Application of jasmonic acid and salicylic acid resulted in significant changes in levels of apical membrane antigen and protein disulfide-isomerase. Taken together, these results suggest that apical membrane antigen might be involved in bacterial resistance process through salicylic acid induced defense mechanism signaling in tomato plants.     

An analysis of responses of resistant and of susceptible tomato plants to Verticillium infection

Comparative studies were made of the responses of resistant and of susceptible Gem tomato plants to infection by Verticillium albo-atrum. When inoculated through roots, there were striking differences in their responses. In susceptible plants, the foliar symptoms and amount of mycelium in the stem increased rapidly for some time. Then the mycelium started to disappear from the stem; this was accompanied by a check to the normal progress of symptoms, and by the formation of tyloses. In resistant plants, a limited invasion of the root and lower stem was accompanied by rapid and extensive tylosis. The mycelium soon disappeared from the stem and the plant then recovered from the initially mild symptoms. There was an inverse relationship between the amount of mycelium and the extent of tylosis in infected plants. The growth of susceptible plants was markedly reduced by infection. Total leaf area was much less because the newly produced leaves did not expand normally. The root system in infected plants was smaller because there were many fewer tertiary roots. In resistant plants infection stimulated growth. Tomato cuttings inoculated with conidia reacted similarly to root-inoculated plants. Hyphae grew well in the vascular system of susceptible cuttings whereas in resistant cuttings the pathogen started to grow but soon disappeared. Detached leaves of susceptible plants, inoculated through cut ends, wilted more than did leaves from resistant plants. It is suggested that resistance is mainly of the active type that develops after infection.     

Comparative transcriptome profiling of resistant and susceptible sugarcane genotypes in response to the airborne pathogen Fusarium verticillioides

Molecular Biology Reports - Fusarium verticillioides is the pathogen associated with pokkah boeng disease (PBD), the most significant airborne disease of sugarcane. The molecular mechanisms that...     

The reactions of susceptible and resistant tomato cultivars to strains of Verticillium albo-atrum

Three strains of Verticillium albo-atrum causing severe wilt of tomato (T), progressive (Hp ) and fluctuating (HF) wilt of hop, were inoculated through the roots of four tomato cultivars at different inoculum concentrations. Symptoms were assessed visually 42 days after inoculation, and quantitatively on the change in total leaf area compared with controls. Distribution of mycelium and tyloses was determined by sections at 2 cm intervals of root, stem and petiole. Cultivars Loran Blood and Moscow showed resistance to disease expression at all levels of inoculum concentration with the T strain. Bonny Best and Potentate were both susceptible to this strain, but whereas in Potentate, disease severity increased from mild to severe with increase in inoculum concentration, Bonny Best was severely diseased at the lowest level of inoculum. All cultivars showed some susceptibility to the HP and HF strains; the ‘resistance’ of Loran Blood and Moscow was no longer apparent and Bonny Best was most severely affected. The relative susceptibilities to the strains were HF Bonney Best > Loran Blood > Potentate > Moscow, HP Bonny Best > Loran Blood, Moscow > Potentate, T Bonny Best > Potentate > Loran Blood, Moscow. In general, vascular colonization was less in the cultivars Loran Blood and Moscow with all three fungal strains at io⁵propagules/ml level of inoculum, but this was not always correlated with an increase in disease severity. With the exception of the host-pathogen combinations Bonny Best/T, Bonny Best/HF, Potentate/T and Moscow/T, increasing the inoculum concentration to 10⁷propagules/ml increased disease severity but had little or no effect of increasing vascular colonization. In Bonny Best/T, Bonny Best/HF and Potentate/T vascular colonization was reduced with the higher level of inoculum. Moscow showed complete resistance to symptom expression and little vascular colonization with the T strain at 10⁵prop./ml. At 10⁷prop./ml resistance was maintained but there was very extensive growth of mycelium in the vessels. Tylosis resulted from an interaction of host, fungal strain and the level of inoculum and was not always correlated with the degree of vascular colonization. Contrary to previous reports the resistant varieties Loran Blood and Moscow developed acute disease symptoms after inoculation with HP and HF and these were associated with a high level of tylosis rather than mycelial growth. Tylosis and disease severity but not mycelial growth increased with higher levels of inoculum. The results suggested that susceptibility to Verticillium wilt was a complex response depending on host cultivar, fungal strain and the initial inoculum concentration. In some cultivar-pathogen combinations susceptibility was directly proportional to the amount of mycelium present in the vessels, while in others a physiological resistance mechanism independent of the degree of colonization appeared to operate. In a third category, increased disease development rather than resistance was associated with high levels of tylosis.     

Comparative proteome analysis of Xanthomonas campestris pv. campestris in the interaction with the susceptible and the resistant cultivars of Brassica oleracea

Black rot of cruciferous plants, caused by Xanthomonas campestris pv. campestris , causes severe losses in agriculture around the world. This disease affects several cultures, including cabbage and broccoli, among others. Proteome studies of this bacterium have been reported; however, most of them were performed using the bacterium grown under culture media conditions. Recently, we have analyzed the proteome of X. campestris pv. campestris during the interaction with the susceptible cultivar of Brassica oleracea and several proteins were identified. The objective of the present study was to analyze the expressed proteins of X. campestris pv. campestris during the interaction with the resistant cultivar of B. oleracea . The bacterium was infiltrated in the leaves of the resistant plant and recovered for protein extraction and two-dimensional electrophoresis. The protein profile was compared with that of the bacterium isolated from the susceptible host and the results obtained revealed a group of proteins exclusive to the resistant interaction. Among the proteins identified in this study were plant and bacterium proteins, some of which were exclusively expressed during the resistant interaction.     

Phenol metabolism in downy mildew resistant and susceptible genotypes of pearl millet

Biochemical characterisation of pearl millet genotypes was carried at pre- (45 DAS) and post-infection (57 DAS i.e. 7 days after infection) stages. Total phenol content at pre-infection stage did not show inherent resistance or susceptibility. While the total phenol content was found to be higher in susceptible genotypes at post-infection stage, qualitative analysis of phenol through high-performance thin layer chromatography showed absence of ferulic acid in resistant genotypes at pre-infection stage. Peroxidase (POX) activity was higher in susceptible genotypes at both the stages of analysis. Constitutive activity of phenylalanine ammonia lyase was higher in resistant genotype whereas induced activity was recorded higher in susceptible genotypes. Native poly-acrylamide gel electrophoresis isozyme banding pattern of POX showed some inducible band(s) due to disease infection in resistant and susceptible genotypes.     

Life-history parameters of western flower thrips on susceptible and resistant cucumber genotypes

The life-history parameters reproduction rate, developmental time and age specific survival of the western flower thrips,Frankliniella occidentalis (Pergande) [Thysanoptera: Thripidae], were determined on susceptible and resistant cucumber (Cucumis sativus L.) genotypes. Both newly emerged andF. occidentalis females of mixed ages showed a substantial reduction (36 to 50%) of the reproduction rate on all resistant genotypes, in particular after the second day. On the resistant genotypes 9127 and 9140,F. occidentalis had a prolonged developmental period. This was primarily due to a prolongation of the second larval stage. On all resistant genotypes,F. occidentalis suffered from high (82 to 97%) preadult mortality, predominantly at the second larval stage. It is conclude that the resistant genotypes do not cause an immediate intoxication of adult nor preadult thrips stages.     

Inoculation of susceptible and resistant potato plants with the late blight pathogen Phytophthora infestans: effects on an aphid and its parasitoid

Plants are exposed to microbial pathogens as well as herbivorous insects and their natural enemies. Here, we examined the effects of inoculation of potato plants, Solanum tuberosum L. (Solanaceae), with the late blight pathogen Phytophthora infestans (Mont.) de Bary (Peronosporales: Pythiaceae) on an aphid species commonly infesting potato crops and one of the aphid's major parasitoids. We observed the peach‐potato aphid, Myzus persicae Sulzer (Hemiptera: Aphididae), and its natural enemy, the biocontrol agent Aphidius colemani Viereck (Hymenoptera: Braconidae), on potato either inoculated with water or P. infestans. Population growth of the aphid, parasitism rate of its natural enemy, and other insect life‐history traits were compared on several potato genotypes, the susceptible cultivar Désirée and genetically modified (GM) isogenic lines carrying genes conferring resistance to P. infestans. Effects of P. infestans inoculation on the intrinsic rate of aphid population increase and the performance of the parasitoid were only found on the susceptible cultivar. Insect traits were similar when comparing inoculated with non‐inoculated resistant GM genotypes. We also tested how GM‐plant characteristics such as location of gene insertion and number of R genes could influence non‐target insects by comparing insect performance among GM events. Different transformation events leading to different positions of R‐gene insertion in the genome influenced aphids either with or without P. infestans infection, whereas effects of position of R‐gene insertion on the parasitoid A. colemani were evident only in the presence of inoculation with P. infestans. We conclude that it is important to study different transformation events before continuing with further stages of risk assessment of this GM crop. This provides important information on the effects of plant resistance to a phytopathogen on non‐target insects at various trophic levels.     

QTL analysis of quantitative resistance to Phytophthora infestans (late blight) in tomato and comparisons with potato.

Quantitative trait loci (QTLs) for resistance to Phytophthora infestans (late blight) were mapped in tomato. Reciprocal backcross populations derived from cultivated Lycopersicon esculentum x wild Lycopersicon hirsutum (BC-E, backcross to L. esculentum; BC-H, backcross to L. hirsutum) were phenotyped in three types of replicated disease assays (detached-leaflet, whole-plant, and field). Linkage maps were constructed for each BC population with RFLPs. Resistance QTLs were identified on all 12 tomato chromosomes using composite interval mapping. Six QTLs in BC-E (lb1a, lb2a, lb3, lb4, lb5b, and lb11b) and two QTLs in BC-H (lb5ab and lb6ab) were most consistently detected in replicated experiments or across assay methods. Lycopersicon hirsutum alleles conferred resistance at all QTLs except lb2a. Resistance QTLs coincided with QTLs for inoculum droplet dispersal on leaves, a trait in L. hirsutum that may contribute to resistance, and dispersal was mainly associated with leaf resistance. Some P. infestans resistance QTLs detected in tomato coincided with chromosomal locations of previously mapped R genes and QTLs for resistance to P. infestans in potato, suggesting functional conservation of resistance within the Solanaceae.     

Influence of bacterial leaf blight on the photosynthetic characteristics of resistant and susceptible rice

The effect of crop disease on photosynthetic characteristics is important for disease control. Two varieties, Shenzhou 98 and Neiwuyou 8015 with resistance and susceptibility to bacterial leaf blight (BLB), respectively, were selected, and the responses of the net photosynthetic rate (P_N) to active photon flux density (PPFD) and intercellular carbon dioxide concentration (C_i), as well as chlorophyll fluorescence, pigments and stomatal resistance (SR), were measured. The results showed that BLB infection greatly decreased the maximum photosynthetic rate (P_max), light saturation point (LSP), carboxylation efficiency (CE), maximal fluorescence (F_m) and actual photochemical efficiency of PSII ( Φ _PSII) but increased the light compensation point (LCP) and dark respiratory rate (R_D), which suggested that the performance of rice photosynthesis was decreased by BLB infection. The BLB infection had a lower effect on resistant rice Shenzhou 98 than on susceptible rice Neiwuyou 8015. The reduction of pigment and increased SR caused by BLB infection may have resulted in the decline in the photosynthetic rate. Significant effects of the BLB infection were observed on chlorophyll fluorescence F_m and Φ_PSII in resistant and susceptible rice. These parameters may be useful for noninvasive monitoring of plant disease considering the negative effect caused by other stresses.     

Comparative response of Metarhizium brunneum to the cuticles of susceptible and resistant hosts

Earlier studies demonstrated that Metarhizium brunneum, usually a broad-host pathogen of arthropods, is unable to complete its pathogenic life cycle when inoculated on the fungus-resistant tick, Hyalomma excavatum engorged females. While the fungus penetrates the cuticle of fungus-susceptible tick, Rhipicephalus annulatus females, it is unable to penetrate the cuticle of fungus-resistant tick, and even perishes on its surface. This is probably due to high concentration of antifungal fatty acids and probably also due to a hypersensitive-like response of the tick. To understand the metabolic pathways occurring in the fungal hyphae upon encountering the cuticles, we compared the response of the fungus to cuticle from susceptible and resistant tick cuticles by 2D-gels. The intracellular proteomes of M. brunneum Mb7 exposed to cuticle of the fungus-susceptible tick, R. annulatus, and to the fungus-resistant tick, H. excavatum engorged females were compared after exposure to either cuticles. By means of liquid chromatography-mass spectrometry/mass spectrometry we identified in both proteomes common proteins involved in biological processes as well as unique proteins identified only in the proteome of fungus exposed to fungus-resistant tick cuticle. These proteins were identified in high probability as heat shock proteins, four key enzymes of the glyoxylate cycle, and proteins associated with hypoxia, and exposure to antifungal drugs. These findings are discussed within the M. brunneum-tick pathosystem in relation to tick resistance and host resistance in general.