MOLECULAR PATHOLOGY OF PLANT INFECTION MEDIATED BY HOST-SPECIFIC TOXINS

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An interdisiplinary project for 3 years was conducted to elucidate the molecular basis of plant infection by toxigenic fungal pathogens with host-specific toxins (HST). A proposed scheme for disease establishment that an HST released during spore germination binds a putative toxin-binding site (receptor) in potential host plant, where the HST-signal induces the state of accessibility to the invader, and then the producing fungus is eventually able to colonize the plant, was developed as follows:1) ACT-, BZR-, AT- and Pyricularia oryzae-toxins were isolated from germination fluids of the pathogens, and structures of the former two toxins were elucidated. ACT-toxin shared in structure a decatrtienoic acid with AK- and AF-toxins, while BZR-toxin composed of homologs of cyclic depsipeptides that were bioactive only when they co-exisited at least two members (co-toxin). On the other hand, a structure-activity corelation study revealed that there is coincidence between the host ranges of the … More Japanese pear, strawberry and tangerine pathotypes of Alternaria alternata and the plant ranges sensitive to AK-, AF- and ACT-toxins they produced, suggesting that host-specificity of the fungus is precisely determined at the molecular level.2) Biosynthetic pathways of AK-, AF- and ACT-toxins were proposed, depending on structural analysis of intermediate metabolites and tracer experiments. Enzymatic basis for AM-toxin synthesis also examined.3) Cascade-like signal transduction processes of HSTs actions were dissected by means of physiological, biochemical and ultrastructural approaches.4) A highly efficient integrative transformation system was estalished for Alternaria alternata: the vector pDH25rla was constructed by inserting a highly repetitive ribosomal RNA gene cluster (rDNA) sequence into a unique Xbal site of pDH25. Using this, 3 genes for fungal melanin biosynthesis (ALM, BRM1, BRM2) were isolated as a model for cloning pathogenicity genes.5) For HST-receptor studies, anti-HST anibodies were generated. A possible receptor protein was detected in the plasma membrane fraction of susceptible pear calli but not of resistant cell line derived from the same susceptible calli. Less