Study of the role of phototoxins in toxin-induced symptom development

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A number of phytopathogenic fungi produce variety of phytotoxins. The role of the phytotoxins in plant diseases, however, still needs to be clarified. Construction of genomic DNA library of causal fungus of potato early blight, Alternaria solani, and homology based PCR allowed us to identify polyketide synthase (PKS) responsible for biosynthesis of reduced polyketadie, most likely phytotoxin solanapyrone. Expression of the identifed PKS gene in Asperguillus oryzae made production of a new pyrone whose structure shows striking similarity to solanapyrone.Physiological effects of alternaric acid (AA), a host-specific compound produced by Alternaria solani on a putative plasma membrane kinase of RiCDPK of potato and on hypersensitive cell death of tomato and potato cells were studied. We demonstrated that a fusion protein encoded by Ca^2+-dependent protein kinase from potato RiCDPK2 gene of cv. Rishiri, purified from E. coli host cells, was stimulated by 25 uM AA from A. solani. In this study, the kinase activity of fusion protein RiCDPK2 is differentially affected by the host-specific toxin (HST) AA and inhibited by solanapyrone A (non-HST), both produced by A. solani. Moreover, from the results, Ca^2+ and Mg2+ may play an important role in the interaction between the toxins and RiCDPK2-fusion protein. We suggest that the HST from A. solani may regulate RiCDPK2 kinase activity during the infection process, leading to the inhibition of HR in the compatible interaction between the host plant and A. solani, as does the suppressor of hypersensitivity from Phytophthom infestans in the host cells. The toxic compounds, which are produced by the pathogen, may help the pathogen establish successful infection by suppressing rapid hypersensitive cell death during the host-parasite interaction.