Fungicides and sterol - def icient mutants of Ustiago maydis : plasma membrane physico - chemical characteristics do not explain growth inhibition

Department of Agricultural Sciences, University of Bristol, institute of Arable Crops Research, Long Ashton Research Station, Bristol BS18 9AF, UK Plasma membrane vesicles from erg1 I and erg2 sterol-deficient mutants and from wild-type Ustilago maydis sporidia treated with and without inhibitors of sterol 14a-demethylase or sterol A8-A7 isomerase (triadimenol and fenpropimorph fungicides, respectively) were purified by aqueous two-phase partitioning. Changes in plasma membrane lipid composition were mostly restricted to sterols and complex lipid-bound fatty acids (CLB fatty acids). There was a greater accumulation of abnormal sterols (14a-methylor A8unsaturated sterols) in plasma membranes from sterol-def icient mutants than from those treated with their fungicide counterparts. However, greater growth inhibition was observed on fungicide-treated wild-type than on mutants. Changes in CLB fatty acids were restricted to alterations in the relative proportion of linoleic acid (18:2) with respect to oleic acid (18: 1). The 18:2 to 18:l ratio found in CLB fatty acids in plasma membranes could be correlated to rates of sporidial growth but not to accumulation of a particular abnormal sterol or to the extent of sterol replacement. Plasma membrane permeability to protons was increased moderately in the mutants only. No changes were observed in plasma membrane fluidity. Plasma membrane H+ATPase activity was increased up to twofold in those cases with lower growth rate. It was concluded that fungiciddnduced growth inhibition in U. maydis was not due to accumulation of abnormal sterols in plasma membranes but probably due to intracellular ATP depletion by the H+-ATPase and that changes in 18:2 to 18: 1 ratio in CLB fatty acids were not directly dependent on the plasma membrane physical state or lipid composition but were possibly part of a stress adaptation mechanism.