Abstract: The different subspecies of Phytophthora alni, P. alni subsp. alni (Paa), P. alni subsp. uniformis (Pau), and P. alni subsp. multiformis (Pam), are recent and widespread pathogens of alder in Europe. They are believed to be a group of emergent heteroploid hybrids between two phylogenetically close Phytophthora species. Nuclear and mitochondrial DNA analyses were performed, using a broad collection of P. alni and two closely related species, P. cambivora and P. fragariae. Paa possesses three different alleles for each of the nuclear genes we studied, two of which are present in Pam as well, whereas the third matches the single allele present in Pau. Moreover, Paa displays common mtDNA patterns with both Pam and Pau. A combination of the data suggests that Paa may have been generated on several occasions by hybridization between Pam and Pau, or their respective ancestors. Pau might have P. cambivora as a species ancestor, whereas Pam seems to have either been generated itself by an ancient reticulation or by autopolyploidization.
Abstract: The genes of the mitochondrial and cytosolic malate dehydrogenase (mMDH and cMDH) of Phytophthora infestans were cloned and overexpressed in Escherichia coli as active enzymes. The catalytic properties of these proteins were determined: both enzymes have a similar specific activity. In addition, the natural mitochondrial isoenzyme was semi-purified from mycelia and its catalytic properties determined: the recombinant mitochondrial isoform behaved as the natural enzyme. A phylogenetic analysis indicated that mMDH, present in all stramenopiles studied, can be useful to study the relationships between these organisms. MDH with the conserved domain DH_cytoplasmic_cytosolic is absent in some stramenopiles as well as in fungi. This enzyme seems to be less related within the stramenopile group. The Phytophthora cMDHs have an insertion of six amino acids that is also present in the stramenopile cMDHs studied, with the exception of Thalassiosira pseudonana cMDH, and is absent in other known eukaryotic cMDHs.
Abstract: Motile zoospores of species of oomycetes encyst rapidly to form walled cysts that germinate and infect host plants. Differences in the protein composition of the plasma membrane and endomembranes of zoospores and cysts of the oomycete Phytophthora nicotianae have been explored by comparing patterns of polypeptides in one- and two-dimensional gels of microsomal fractions. Against a backdrop of common components, this comparison revealed that at least 53
proteins were specific to, or occurred preferentially in, the microsomal fraction of one spore type or the other. In addition, proteins common to zoospore and cyst plasma membranes were further investigated by immunocytochemical labelling with a panel of 10 monoclonal antibodies raised against P. nicotianae spore components. The results of immunolocalisation
studies and immunoblotting showed that the antibodies reacted with four different sets of membrane proteins, and were grouped accordingly. Group 1 antibodies bound preferentially to the bladder of the water expulsion vacuole or a region of the cell surface associated with it. Group 2 antibodies reacted with a protein of high relative molecular weight (>200 kDa) found in zoospores and cyst plasma membranes and in the cleavage membranes of sporangia. Group 3 antibodies reacted with a set of proteins occurring in the plasma membrane of zoospores and cysts, in the peripheral cisternae, in the spongiome membranes of the water expulsion vacuole, in the cleavage membranes of sporangia, and in the plasma membrane and apical vesicle membranes in hyphae and germinating cysts. Group 4 antibodies bound to a set of proteins present in the zoospore and cyst plasma membrane, in sporangial cleavage membranes and in the spongiome but not present in the peripheral cisternae, hyphae or germinating cysts. The results document the presence of proteins that are common to the plasma membrane of all stages of the asexual life cycle of P. nicotianae, of proteins that occur in the plasma membrane of the asexual spores but not of hyphae, and of proteins that occur in the membranes of either zoospores or cysts. The results also indicate that zoospore and cyst plasma membrane proteins may be present in specific subsets of other membranes within the zoospores.
Abstract: Elicitins, holoproteins which act as inducers of hypersensitivity on tobacco, were considered as a characteristic of Phytophthora. They are also produced, along with glycosylated isoforms, by three species belonging to the related genus Pythium, Py. vexans, Py. oedochilum and Py. marsipium, while other Pythium species do not possess such proteins. Various elicitin-like sequences were determined, bringing novel features to the elicitin family, such as an histidine residue and C-terminal extensions on the deduced peptide sequences. As the unique elicitin content of these species supports a distinct location among Pythiaceae, we suggest the separation of vexans, Py. oedochilum and Py. marsipium from Pythium and consider them as linking species between Phytophthora and Pythium.
Abstract:Phytophthora foot rot of black pepper caused by Phytophthora capsici is a major disease of black pepper (Piper nigrum) throughout Vietnam. To understand the population structure of P. capsici, a large collection of P. capsici isolates from black pepper was studied on the
basis of mating type, random amplified microsatellites (RAMS) and repetitive extragenic palindromic (REP) fingerprinting. Two mating types A1 and A2 were detected in four provinces in two climatic regions, with A1:A2 ratios ranging from 1:3 to 1:5. In several instances A1 and A2 mating types were found to co-exist in the same farm or black pepper pole, suggesting the potential for sexual reproduction of P. capsici in the field in Vietnam although its
contribution to disease epidemics is uncertain. RAMS and REP DNA fingerprinting analysis of 118 isolates of P. capsici from black pepper showed that the population was genetically more diverse where two mating types were found, although the overall genetic diversity was low with most of the isolates belonging to one clonal group. The implication of these findings is discussed. The low diversity among isolates suggests that the P. capsici population may have originated from a single source. There was no genetic differentiation of isolates from different climatic regions. In addition to the large clonal group, several isolates with unique RAMS/REP phenotypes were also detected. Most of these unique phenotypes belonged to the minority A1mating type. This may have significant implications for a gradual increase in overall genetic diversity.
Abstract: Vegetative growth rate, and size of sporangia, chlamydospores and oospores from 94 P. ramorum isolates were measured and the isolates were paired in vitro with four different heterothallic Phytophthora species isolated from infected nursery plants in Germany. P. ramorum isolates originated from different European countries and from Canada and the USA. 66 of the 67 European isolates were determined as mating type A1; only one isolate was of mating type A2. Of the 27 North American isolates tested, seven (all from nurseries) were determined to be the A1 mating type and 17 to be the A2 mating type. Three isolates did not produce gametangia during the incubation period. Discriminant analysis of all data allowed a grouping based on the vegetative growth rate. The two groups corresponded with the mating type no matter whether the isolates originated from Europe or North America. The A1 isolates were much more homogeneous in their morphology than the A2 isolates. They grew faster, had larger chlamydospores and did not produce gametangia with P. cambivora. Within the A2 group, the single European isolate of mating type A2 (BBA 16/02) and three US isolates showed intermediate characters and were classified with the discriminant function into that of the opposite mating type. The morphological characters and the mating behaviour of the isolates will be discussed.
Abstract: A number of fine root pathogens, including Phytophthora cinnamomi, Pythium ultimum var. ultimum, Pythium undulatum, Pythium violae, Fusarium sp., and two incompletely identified Verticillium species, were isolated from soils taken from under Scots pine trees at five sites in north Scotland, including semi-natural forests and plantations. At least two root pathogens were recovered from each forest. Morphological and molecular data supported the identification of Phytophthora cinnamomi from three of the sites investigated. Isolates of Phytophthora cinnamomi, Pythium ultimum var. ultimum and an incompletely identified Fusarium sp. caused growth reductions of Scots pine seedlings, as determined by dry weight; the most virulent species were Phytophthora cinnamomi and Fusarium sp. The most severe disease symptoms were caused by a mixed inoculum containing Phytophthora cinnamomi, Pythium ultimum var. ultimum and Fusarium sp., or by the Fusarium isolate alone. These nonspecific pathogens may persist on the roots of understorey and herbaceous plants in the pine forests.
Abstract: A total of 79 isolates of Fusarium semitectum were characterized by morphological and IGS-RFLP analysis to assess its intraspecific variation. Based on morphological characteristics, the isolates of F. semitectum were classified into 2 distinct groups, morphotypes I and II. Morphotype I was characterized by longer macroconidia (3 - septate: 31.03 ± 2.57 µm; 5 - septate: 40.17 ± 1.85 µm), 0 - 7 septate with 5 - septate was the most common, absence of chlamydospores, presence of sporodochia, abundant-floccose mycelium, peach colony appearance, peach to orange pigmentations and fast growing. While isolates of morphotype II produced shorter macroconidia (3 - septate: 24.98 ± 1.87 µm; 5 - septate: 35.24 ± 2.07 µm), 0 - 5 septate with 3 - septate was the most common, with (56%) or without chlamydospores (44%), without sporodochia, abundant-floccose and abundant-powdery mycelium, beige to brown colonies, brown to dark brown pigmentations and slow growing. Corresponding to the morphological characterization, IGS-RFLP analysis indicated that the 79 isolates could be divided into 2 different clusters assigned as RFLP groups I and II. 49 IGS haplotypes were produced by 8 restriction enzymes (AluI, Bsu15I, BsuRI, Eco881, Hin6I, MspI, PstI and TaqI) which indicated a high level of intraspecific variation and polymorphism among the 79 isolates. This is the first report of F. semitectum associated with H. polyrhizus.
Abstract: Molecular characterization of Phytophthora isolates from tobacco, tomato, carnation, gerbera, crossandra and periwinkle was carried out using internal transcribed spacer(ITS) regions of rDNA gene repeat and amplified fragment length polymorphism (AFLP). All the isolates of Phytophthora were identical in morphology and ITS-RFLP patterns, indicating P. nicotianae and P. parasitica are synonymous. However, based on AFLP, four sub groups were evident within population of P. nicotianae (P. parasitica): Group I, includes isolates from tomato, tobacco and periwinkle. Isolates from carnation represents Group II. Isolates from gerbera falls under group III. The crossandra isolates formed a Group IV. Thus, present study showed the existence of four molecular sub groups within population of P. nicotianae (P. parasitica) in India.
Abstract: Phytophthora, an oömycetous organism with ~70 recognized species, is one of the most destructive of plant pathogens affecting a wide host range consisting of economically important crops having a widespread occurrence in the wet tropical regions; and as a result, almost a third of the Phytophthora species reported worldwide are from India. Identification and taxonomy of Phytophthora is still acknowledged to be 'difficult' primarily due to its morphological plasticity. An attempt was made, therefore, to identify and characterize the Phytophthora population prevalent on some of the common high value economically important crops grown in this region; brinjal [aubergine], betel vine (Piper betle), guava, sesame, roselle, chilli (Capsicum annuum), black pepper (Piper nigrum), pointed gourd [Trichosanthes dioica] and taro [Colocasia esculenta] both through molecular methods such as restriction fragment length polymorphism of the internal transcribed spacer (ITS) region of rDNA, sequencing of the ITS region and also morphological parameters. The Phytophthora species prevalent on these crops, which were identified by these mentioned molecular methods, were P. melonis, P. nicotianae, P. colocasiae and P. capsici. Multiple occurrences of different Phytophthora species on a single crop were also observed. Data on all of the mentioned aspects of the isolates, twenty-six in number, under accession at the World Phytophthora Collection, USA, are discussed.
Abstract