VIEW ARTICLE

Physiology and Biochemistry

Phytotoxins Produced by Germinating Spores of Bipolaris oryzae. J. Z. Xiao, Ph.D. graduate student, Laboratory of Plant Pathology, faculty of agriculture, Nagoya University, Nagoya 464, Japan; M. Tsuda(2), N. Doke(3), and S. Nishimura(4). (2)Associate professor, Pesticide Research Institute, Kyoto University, Kyoto 606, Japan; (3)(4)Professor, and professor, respectively, Laboratory of Plant Pathology, faculty of agriculture, Nagoya University, Nagoya 464, Japan. Phytopathology 81:58-64. Accepted for publication 9 May 1990. Copyright 1991 The American Phytopathological Society. DOI: 10.1094/Phyto-81-58.

Bipolaris oryzae produces ophiobolin A and ophiobolin B during spore germination. Ophiobolin A, 6-epiophiobolin A, anhydroophiobolin A, 6-epianhydroophiobolin A, and ophiobolin I are produced during culture in a nutrient medium. Each compound showed nonselective phytotoxicity to host and nonhost plants in assays for root elongation and induction of leaf chlorosis. Ophiobolin A was most toxic, with an ID₅₀ value of 5 ?g/ml, followed by 6-epiophiobolin A, with an ID₅₀ value of 10 ?g/ml for inhibition of root elongation. Others were toxic only at levels of more than 100 ?g/ml. At a concentration of approximately 3 ?g/ml, ophiobolin A also induced susceptibility of rice leaf tissues to a non-pathogenic isolate of Alternaria alternata. However, ophiobolins are not likely to function as disease determinants of B. oryzae, because their concentrations in spore-germination fluids were only about one one-thousandth of that required for biological activity. The spore-germination fluids nevertheless exhibited extensive susceptibility-inducing activity and phytotoxicity. These activities were host selective for rice and were detectable soon after spore germination, when ophiobolins were not detectable at effective concentrations.