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Natural production of organochlorines by terrestrial fungi and plants
08/2008 About 300 natural chlorine-containing compounds have been isolated to date from terrestrial fungi and plants ranging from compounds as simple as gaseous chloromethane released by many wood-rotting fungi to the complex chlorinated polyacetylenes and sesquiterpene lactones biosynthesized by plants of the Aster family. Some are surprisingly heavily chlorinated, e.g. 1,2,3,4-tetrachlorobenzene which comprises 1% of the oil of the needlerush from the Mississippi delta. Many chlorinated natural products act as antifeedants protecting the plant against attack by insects and herbivores while some formed by fungi such as Penicillium have antibiotic activity and may give the organism some competitive advantage over rival micro-organisms. Others appear to have important biochemical roles. Thus a 3,5-dichlorohexanophenone produced by Dictyostelium discoideum is a key biological signal molecule triggering the transformation of undifferentiated cells of the slime mould into fruiting bodies. Another fascinating natural product is the 4-chloro analogue of the plant hormone, indoleacetic acid; the methyl ester of this chloroindole occurs in seeds of the common pea at the surprisingly high level of 6 mg/kg fresh weight. Similar levels of the free acid are found in developing seed of the broad bean and as much as 16 mg/kg in the young leaves of the legume. It has been proposed that this very potent auxin is involved in the mobilization of food reserves from leaf to seed during ripening. A most unexpected discovery in wheat grain and potato tuber is the presence, albeit in trace amounts, of a chlorinated benzodiazepine with a structure identical to that of the widely used tranquilizer, diazepam. The role of the compound in plant metabolism is unknown. Substantial amounts of chlorinated aromatic compounds are excreted by fungi during growth in forest environments. Thus chlorinated anisyl metabolites of fungal origin have been observed at concentrations of 15 to 75 mg/kg in wood and leaf litter. Soil from the 'fairy rings' created by the wood blewitt has been shown to contain 40 mg/kg of such compounds which are believed to play an important part in fungal lignin degradation acting as recyclable substrates for key enzymes in the process. Our studies over the last 15 years at The Queen's University of Belfast have focused on chloromethane production by fungi and higher plants and its role in fungal and plant biochemistry. The ability to volatilize chloride ion as chloromethane is widespread amongst the Hymenochaetaceae, a large family of wood-rotting fungi whose bracket-like perennial fruiting bodies are common on temperate and tropical trees. Of 63 species examined from this family over half released chloromethane during growth and, with some, yields were very high. Thus Phellinus pomaceus, a species frequently found on fruit trees such as the plum, can volatilize 80 to 90% of inorganic chloride from its growth substrates. Globally, chloromethane release to the atmosphere by this family of fungi has been conservatively estimated at 160,000 tonnes annually with the bulk originating from tropical forests. Unexpectedly, our investigations have revealed that chloromethane has a vital metabolic function in these fungi acting as a novel type of biological methylating agent in the biosynthesis of various natural products. Indeed the utilization of chloromethane in this role extends to many fungi which never actually release the compound during growth. Thus chloromethane appears to be widely employed by wood-rotting fungi as a methyl donor in the biosynthesis of veratryl alcohol (3,4-dimethoxybenzyl alcohol), a key component of the lignin degrading system. Interestingly, a wide variety of higher plant species also emit chloromethane. The leaves of cabbage and the tubers of some varieties of potato are particular prolific producers. Recent work in Belfast with a potato variety whose tubers release up to 1 mg/kg/day of chloromethane suggest that a significant proportion of the halocarbon biosynthesised by the tuber is utilized biochemically within the tuber although the precise metabolic function has not yet been determined. |
