Syoyo Nishimura

Structure of AF-toxin II, one of the host-specific toxins produced by alternaria alternata strawberry pathotype

Abstract AF-toxin II was a mixture of three stereo-isomers, whose structures were determined as 2a , 2b and 2c by means of chemical and spectroscopic methods.

(+)-catechin acts as an infection-inhibiting factor in strawberry leaf.

ABSTRACT An infection-inhibiting factor (IIF) was isolated from strawberry leaves and identified as (+)-catechin. This compound inhibited the formation of infection hyphae from appressoria of Alternaria alternata, but allowed both spore germination and appressorial formation. It is a normal component of strawberry leaves, but further accumulates as the major IIF in response to inoculation with nonpathogenic spores of A. alternata. The accumulation of (+)-catechin on a susceptible host was not induced, however, by inoculation with pathogenic spores of the strawberry pathotype or by inoculation with nonpathogenic spores supplemented with host-specific toxin (AF-toxin I). These results imply that (+)-catechin acts as a protective agent during induced resistance and that AF-toxin I acts as a fungal suppressor of induced resistance.

Efficient integrative transformation of the phytopathogenic fungus Alternaria alternata mediated by the repetitive rDNA sequences.

An attempt was made to transform Alternaria alternata protoplasts using a plasmid vector, pDH25, bearing the Escherichia coli hygromycin B (Hy) phosphotransferase gene (hph) under the control of the Aspergillus nidulans trpC promoter. Transformants arose on a selective medium containing 100 micrograms Hy/ml. There were two types of transformants, forming large and small colonies on the selective medium. Transformation with one microgram of the vector produced an average of 4.5 large colonies and 600 small ones. In large-colony transformants, the vector often integrated into the recipient chromosome in the form of highly rearranged tandem arrays. To increase transformation efficiency, fragments of the highly repetitive ribosomal RNA gene cluster (rDNA) of A. alternata were used to construct four new vectors for homologous recombination system. Use of these vectors gave higher transformation efficiency than the original plasmid. The best vector, pDH25r1a, gave rise to large-colony transformants at a frequency 20 times higher than pDH25. Transformation events in A. alternata with pDH25r1a occurred by homologous recombination as a single crossover between the plasmid-borne rDNA segment and its homologue in the chromosome, often giving rise to tandemly repeated vector DNA.

Host Recognition : Can Accessibility to Fungal Invasion be Induced by Host-Specific Toxins Without Necessitating Necrotic Cell Death?

Although plants in nature are continually exposed to diverse fungal spores as potential parasites, only few of these fungi successfully establish a distinct or specific relationship of parasitism which allows them to invade, grow and reproduce on a given plant species; in order to survive, higher plants have evolved defense mechanisms which operate against all but the few specialized parasites that can cause disease in a given species. Conceptually, a specific host-parasite relationship appears to have originated from a kind of co-evolution between both organisms. A mechanism that determines such a parasitical specificity is comprised of three basic processes (Nishimura and Kohmoto, 1983): a) Spores of a fungal parasite release on germination a host recognition factor, for example, host-specific toxin (HST) in advance of invasion, b) the released signal factor selectively binds to receptor sites in the host cells, and c) the accessible state or susceptibility of host cells to possible hyphal invasion is simply disposed by the signal transduction.

Organization of ribosomal RNA genes in Alternaria alternate Japanese pear pathotype, a host-selective AK-toxin-producing fungus

SummaryDNA encoding ribosomal RNA (rRNA) of Alternaria alternata Japanese pear pathotype has been cloned in λ, replacement vector, λ, Fix. Restriction endonuclease mapping and Southern hybridization with the 18S and 28S rRNAs of Saccharomyces cerevisiae revealed the A. alternata rDNA to be tandemly repeating 8.15-kilobase pair unit. The restriction fragments of the unit were then subcloned in the plasmid vector Bluescribe M13- and partially sequenced. The determined sequences were compared with previously reported sequences of S. cerevisiae rRNAs and their genes. The locations of DNA sequences encoding the 5.8S, 18S, and 28S rRNAs were determined by homology search using reported sequences. The complete DNA sequence for 5.8S rRNA of the fungus was found to be highly conserved at more than 90 % homology in the fungi analyzed. However, sequence diversities were observed in limited regions involved in a helix structure, the helix (e), found at position 116–137.

Biosynthetic origin of (8R,9S)-9,10-epoxy-8-hydroxy-9-methyl-deca-(2E,4Z,6E)-trienoic acid, a precursor of AK-toxins produced by Alternaria alternata

Abstract The biosynthetic origin of (8R,9S)-9,10-epoxy-8-hydroxy-9-methyl-deca-(2E,4Z,6E)-trienoic acid, a biological precursor of host-selective AK-toxins produced by Alternaria alternata Japanese pear pathotype, was shown to be six molecules of acetic acid by means of 13C NMR spectroscopy.

Rice-specific toxins produced by Bipolaris zeicola, race 3; evidence for role as pathogenicity factors for rice and maize plants*

Bipolaris zeicola race 3, originally reported as a pathogen causing a leaf spot disease of maize, is also highly pathogenic to rice plants. Its spore-germination fluids (SGFs) were specifically phytotoxic to rice leaves but induced susceptibility to infection by nonpathogens on leaves of both rice and maize. Effective compounds were partially purified from the SGFs which caused leaf chlorosis only on rice and enabled spores of nonpathogenic Alternaria alternata and B. victoriae to infect leaf tissues of rice and maize, as judged by the formation of disease symptoms on the leaves. The partially purified compounds were further purified by HPLC and found to consist of at least three separate components. The host-specific phytotoxicity to rice and the susceptibility-inducing activity of the components with rice and maize were evident only when combined, but not when used singly. HPLC analysis of extracts from SGFs and culture filtrates of several isolates of Bipolaris and Helminthosporium demonstrated that the components were produced only by B. zeicola race 3. We conclude that the complex of the components may be a pathogenicity factor of B. zeicola race 3 for rice and maize plants.