Abstract: We report on the growth pattern of the root rot pathogen Phytophthora quercina in roots of oak seedlings and on the localization of its elicitin quercinin. Some hours after infection, the pathogen was mainly found in the intercellular spaces of the cortical parenchyma. Some parenchyma cells were also invaded by P. quercina and the plasmalemma of those cells as well as of neighbouring cells separated from the cell wall. Transmission electron microscopy studies proved that P. quercina penetrated the cell walls forming haustorial-like structures in invaded cells. These structures and cell wall appositions were covered with electron dense material.
Immunofluorescence investigations in combination with a specific anti cryptogein antibody clearly showed that the P. quercina elicitin quercinin was located within the hyphal cell wall and seems to be released into invaded cells. In addition, immunogold labelling showed that quercinin was found in the intercellular spaces as well as in penetrated cells. Finally, we demonstrated with ELISA that quercinin was produced by the pathogen in infected tissue during the whole growth phase.
Abstract: The effect of temperature and photoperiod on the expression of resistance against Phytophthora infestans in five potato cultivars with and without resistance (R) genes was investigated. Four experiments were carried out under controlled conditions in growth chambers. Two cultivars (393295.236 and 391046.22) without known R genes from the International Potato Center (CIP) in Lima, Peru, two Mexican cultivars with major R genes (Tollocan and Malinche), and a susceptible cultivar (Atlantic) were used in this study. Plants were grown for 32 days in growth chambers at two temperatures (16 and 24 C) and two photoperiods (12 and 16 h day length), then inoculated with a compatible isolate of P. infestans and incubated in a mist chamber at 18 C. The inoculation efficiency, the percentage of lesions that did not grow beyond the inoculation spot, the sporangia density, and the AUDPC were recorded. The percentage of arrested lesions decreased with temperature in the two most resistant cultivars (393295.236 and Malinche), and the AUDPC was lower at 16 than at 24 C in four of the five cultivars. The inoculation efficiency and the sporangia density were not affected by change in temperature. Sporangia density increased at 16 h photoperiod; however, the final infected leaf area was not affected. Our results demonstrate that the expression of horizontal and vertical resistance was affected by temperature; however, the relative resistance ranking among cultivars was the same in the four experiments with different temperatures and photoperiods. It is assumed that the resistance in the Mexican cultivars may be conferred by minor resistance genes and by the residual effect of defeated R genes. These results emphasize the difficulty in differentiating between horizontal and vertical resistance.
Abstract: Two related segregating populations of Theobroma cacao L. were analysed for their resistance to Phytophthora palmivora. The first F1 population was obtained by crossing two susceptible cacao clones of Catongo (a highly homozygous genotype) and Pound 12(a highly heterozygous genotype) and the second population was obtained by backcrossing a single F1 tree with Catongo. The genetic maps obtained for each population were compared. The F1 map includes 162 loci and the backcross has 140 loci. The two maps, F1 and BC1, exhibit high co-linear loci organization covering respectively, 772 and 944 cM. Phytophthora resistance was assessed by measuring the size increase of a lesion at five (DL5)and ten days (DL10) after pod inoculation. Six different QTL were detected in the F1 and BC1 populations. One QTL was found in both populations, and appeared to be a major component of disease resistance, and explaining nearly 48% of the phenotypic variance in the F1 population. The absence of some yield QTL detection in the BC1 in comparison with the F1 population is due to the lack of transmission of the favouring alleles for these QTL from the single F1 tree used for the backcross. The phenotypic variance explained by the action of the quantitative trait alleles indicated that genetic factors of both major and minor effects were involved in the control of the character studied. QTL conferring increased resistance to Phytophthora were identified in both susceptible parents, suggesting the presence of transgressive traits and the possibility of selection in cacao. Pleiotropic and epistatic effects for the QTL were also detected. Finally, the use of marker assisted selection (MAS) in cacao breeding programs is discussed.
Abstract: Reciprocal, bi-parental cross progenies of Solanum caripense, a diploid (2n = 2x = 24), self-incompatible, non-tuberizing, wild herb occurring throughout the Andes from Bolivia to Costa Rica, were used for the construction of parental framework linkage maps. Unexpectedly, low levels of polymorphism were encountered. The map of crp-1 comprises in total 287 cM and contains 89 markers in 10 linkage groups, whereas the map of crp-4 spans a genetic distance of 310 cM distributed over 66 markers in 12 l inkage groups. Gene-specific markers detecting R gene analogs, a kinase similar to the tomato Pto gene, and a homolog of SGT1 were also mapped. Two QTLs associated with resistance were
located on the parental linkage maps by composite interval analysis of marker-trait associations. Within the cytoplasmic background of parent crp-4, a portion of the susceptible progenies carried the marker alleles for resistance at the position of the QTLs. This suggested that resistance may have been abrogated by an additional, independent nuclear factor that interacts with the cytoplasm of parent crp-4 to cause a susceptible phenotype. By single-marker analyses of marker-trait association, three additional marker loci independent of the QTLs were detected that significantly contributed to the resistance phenotype. S. caripense, a wild plant that has not been subjected to domestication and breeding should be a valuable source of resistance to late blight.
Abstract: Sequencing and annotation of a contiguous stretch of genomic DNA (112.3 kb) from the oomycete plant pathogen Phytophthora infestans revealed the order, spacing and genomic context of four members of the elicitin (inf) gene family. Analysis of the GC content at the third codon position (GC3) of six genes encoded in the region, and a set of randomly selected coding regions as well as random genomic regions, showed that a high GC3 value is a general feature of Phytophthora genes that can be exploited to optimize gene prediction programs for Phytophthora species. At least one-third of the annotated 112.3-kb P. infestans sequence consisted of transposons or transposon-like elements. The most prominent were four Tc3/gypsy and Tc1/copia type retrotransposons and three DNA transposons that belong to the Tc1/mariner, Pogo and PiggyBac groups, respectively. Comparative analysis of other available genomic sequences suggests that transposable elements are highly heterogeneous and ubiquitous in the P. infestans genome.
Abstract: Late blight, caused by Phytophthora infestans, is the most important disease of potato worldwide and foliar resistance is an important component of managing late blight in the field. The objective of this research was to identify germplasm for use in breeding cultivars with foliar resistance to P. infestans. More than 500 clones were tested from 1997 to 2002 in inoculated (US8 genotype) field experiments conducted at the Michigan State University Muck Soils Research Farm in Bath, Michigan. All of the current commercial cultivars tested were classified as susceptible to P. infestans. The most resistant clones were A90586-11, AWN86514-2, B0718-3, Jacqueline Lee (MSG274-3), MSI152-A, MSJ307-2, MSJ317-1, MSJ453-4Y, MSJ456-2, MSJ456-4, MSJ461-1, MSK101-2, MSK128-1, NY121, LBR8, LBR9, Tollocan, and Torridon. Some of these resistant selections were from crosses with B0718-3, Jacqueline Lee, and Tollocan suggesting that the resistance to P. infestans was transmissible. These resistant clones will provide the opportunity to breed late-blight-resistant cultivars from a diverse pool of cultivated germplasm. Consistent foliar reaction to P. infestans over years suggested that the Michigan State University Muck Soils Research Farm is a valuable location for North American breeders to assess the reaction of potato germplasm to the US8 genotype of late blight.
Abstract: A detailed study was carried out to characterize the Phytophthora species associated with capsule rot and leaf blight diseases in cardamom. Phytophtliora was isolated from rot affected plant parts such as leaf, capsule and pseudostem collected from different cardamom growing tracts of ldukki and Wyanad districts of Kerala during the monsoon and post monsoon periods. Morphological and physiological studies such as colony morphology, growth rate, morphology of sporangia, pedicel length, caducity, chlamydospores, sex organs, compatibility type, growth on synthetic medium and temperature relationship was studied using 25 Phytophthora isolates from cardamom. Two groups of Plrytophthora could be identified from infected capsules, leaves and pseudo stem. The group one having pigmentation, chlamydospore production and growth at >35 °C where as the group two having no pigmentation, no chlamydospore production and no growth at 35 °C. The group one is identified as P. nicotianae var. nicotianae and group two as P. meadii. From the studies it has been confirmed that two species of Phytophthora viz. P. meadii, and P. nicotianae var. nicotianae are involved in initiating capsule rot occurring during the monsoon season of which P. meadii is the predominant one, The leaf blight occurring during the post monsoon season is due to P. nicotianae var. nicotianae. However, there is certain amount of overlapping of these in initiating the disease in cardamom. P. palnaivora could not be isolated from the infected plant parts as reported earlier.
Abstract: We analysed changes in the transcript population produced in habanero pepper (Capsicum chinense) cell suspensions by the addition of whole mycelium homogenates from a pathogenic isolate of Phytophthora capsici, to identify plant cellular processes modified by the oomycete effectors. The elicitation produced several defence-like cellular responses: alkalinisation of the medium, a two-step oxidative burst, induction of β-1,3-glucanases, and activation of mitogen-activated protein kinases. The elicitation modified the accumulation of transcripts representative of diverse metabolic pathways, including ethylene biosynthetic enzymes, MAP kinases and defence-related products, like PR proteins, but did not affect the expression of C. chinense NPR1 and WRKY orthologue genes, which are important modulators of plant defence responses. Interestingly, apart from some defence-related genes, inoculation of six-leaf-stage habanero pepper plantlets with the pathogenic isolate revealed few systemic modifications in the transcript patterns. All plantlets ultimately died, even though the in planta inoculation induced the strong accumulation of two MAPK transcripts. As few resistance-related genes were expressed in susceptible habanero pepper plantlets that died, either the extent or the timing of the defence response could be insufficient to establish a proper response against Phytophthora blight.
Abstract: Main and interaction effects of day-length and pathogen isolate on the reaction and expression of field resistance to Phytophthora infestans were analyzed in a sample of standard clones for partial resistance to potato late blight, and in the BCT mapping population derived from a backcross of Solanum berthaultii to Solanum tuberosum. Detached leaves from plants grown in field plots exposed to short- and long day-length conditions were independently inoculated with two P. infestans isolates and incubated in chambers under short- and long photoperiods, respectively. Lesion growth rate (LGR) was used for resistance assessment. Analysis of variance revealed a significant contribution of genotype × isolate × day-length interaction to variation in LGR indicating that field resistance of genotypes to foliar late blight under a given day-length depended on the infecting isolate. An allele segregating from S. berthaultii with opposite effects on foliar resistance to late blight under long- and short day-lengths, respectively, was identified at a quantitative trait locus (QTL) that mapped on chromosome 1. This allele was associated with positive (decreased resistance) and negative (increased resistance) additive effects on LGR, under short- and long day-length conditions, respectively. Disease progress on whole plants inoculated with the same isolate under field conditions validated the direction of its effect in short day-length regimes. The present study suggests the occurrence of an isolate-specific QTL that displays interaction with isolate behavior under contrasting environments, such as those with different day-lengths. This study highlights the importance of exposing genotypes to a highly variable population of the pathogen under contrasting environments when stability to late blight resistance is to be assessed or marker-assisted selection is attempted for the manipulation of quantitative resistance to late blight.
Abstract: Really interesting new gene (RING) finger proteins function as ubiquitin ligase and play key roles in biotic and abiotic stresses. A new RING-H2 finger protein gene, StRFP1, was cloned from Phytophthora infestans-inoculated leaves of potato (Solanum tuberosum) clone 386209.10, which is free of R1-R11 genes. The deduced amino acid sequence was characterized by an N-terminal transmembrane domain, a GLD region and a RING-H2 finger signature. StRFP1 is homologous to the tobacco NtACRE132 protein and belongs to the ATL family. The DNA gel blot analysis and mapping revealed that StRFP1, an intron-free gene, had one to two copies in the potato genome and was located on chromosome 3. RT-PCR assays showed that StRFP1 was constitutively expressed in potato plants and significantly induced in detached potato leaves by P. infestans and plant defense-related signal molecules, abscisic acid, salicylic acid and methyl jasmonate. Transient expression studies revealed that StRFP1 fused with GFP localized to the plasma membrane or out of that in onion epidermal cells. The function of StRFP1 in potato resistance against late blight was further investigated by constructing overexpression and RNA interference (RNAi) vectors, which were introduced into potato cv. E-potato 3, respectively. By challenging the detached leaves with mixture races of P. infestans, all of the StRFP1-overexpressing plants displayed slower disease development than non-transformed controls in terms of the lesion growth rate (LGR). In contrast, StRFP1-silencing plants through RNAi were more susceptible to pathogen infection. The present results demonstrate that StRFP1 contributes to broad-spectrum resistance against P. infestans in potato.
Abstract