BIOSYNTHESIS OF HALOMETHANES AND METHANETHIOL BY HIGHER-PLANTS VIA A NOVEL METHYLTRANSFERASE REACTION

Biogenic emissions of halomethanes (CH3CI, CH3Br and CH3I) and methanethiol (CH3SH) are of major significance to atmospheric chemistry, but there is little information on such emissions from higher plants, We present evidence that plants can produce all these gases through an identical methyltransferase reaction, A survey of 118 herbaceous species, based on CH3I production by leaf discs supplied with KI, detected the presence of in vivo halide methyltransferase activity in 87 species. The activities ranged over nearly 4 orders of magnitude, Plants generally considered salt tolerant had relatively low activities, and salinization of three such species did not increase the activity, The highest activities were found in the family Brassicaceae. Leaf extracts of Brassica oleracea catalysed the S-adenosyl-L-methionine-dependent methylation of the halides I-, Br- and CI- to the respective halomethanes. In addition, the extract similarly methylated HS- (bisulphide) to CH3SH. These two types of enzyme activity (halide and bisulphide methyltransferase) were also present in all of the 20 species comprising a subsample that represented the range of C3(H)I emissions observed in the initial survey of in vivo CH3I production ability, and in a marine red alga Endocladia muricata. Moreover, the two activities occurred in approximately the same ratio in all the higher plants tested, These findings highlight the potential of higher plants to contribute to the atmospheric budget of halomethanes and methanethiol, The halide and bisulphide methyltransferase activities may also provide a mechanism for the elimination of halide and HS- ions, both of which are known to be phytotoxic.