ALTERNATIVE RESPIRATION - A BIOCHEMICAL-MECHANISM OF RESISTANCE TO AZOXYSTROBIN (ICIA-5504) IN SEPTORIA-TRITICI

The mechanism of resistance to ICIA 5504 (azoxystrobin) in a Septoria tritici mutant raised in the laboratory has been investigated. This mu tant was approximately 10 times less sensitive than the wild-type stra in in in-vitro tests towards spore germination or fungal growth. Gluco se oxidation in whole cells was inhibited in the wild type (80% inhibi tion at 0.1 mu g ml(-1)), whereas in the resistant mutant, oxygen upta ke was stimulated (50% stimulation at 1.0 mu g ml(-1)). Respiration of the wild-type strain was inhibited by antimycin A and cyanide but not that of the mutant. These results indicate the existence of an effici ent alternative respiratory pathway in the mutant, which was inhibited by the addition of 2 mM salicylhydroxamate (SHAM). Using mitochondria , antimycin A and ICIA 5504 did not completely inhibit NADH oxidation in either strain. Addition of SHAM inhibited part of the antimycin- an d ICIA 5504-insensitive oxygen uptake only in mutant mitochondria. For complete inhibition of oxygen reduction, SHAM and cyanide need to be present. Thus, three systems of electron transfer from exogenous NADH to oxygen are present in S. tritici mitochondria: the cytochrome pathw ay which is sensitive to ICIA 5504 and antimycin A inhibition in both strains, the system of NADH-cytochrome c reductase which bypasses the methoxyacrylate inhibition at the cytochrome bc(1) complex, and the al ternative oxidase which is inhibited by SHAM, and which is partially f unctioning only in mitochondria isolated from the ICIA 5504-resistant mutant. When the S. tritici isolates were tested for their in-vivo sen sitivity to ICIA 5504 on wheat, the resistant strain was controlled be tter than the wild type. This indicates that the decreased ATP formati on by the alternative pathway of respiration was inadequate for effici ent parasitic growth on the host.