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 Kl, 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‐methioninc‐dependenl niethylalion 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 CH3I 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 atmosphericbudget of halomethanes and melhanethiol. 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.

A novel S-adenosyl-L-methionine:halide/bisulfide methyltransferase (EC 2.1.1.-) was purified approximately 1000-fold to apparent homogeneity from leaves of Brassica oleracea. The enzyme catalyzed the S-adenosyl-L-methionine-dependent methylation of the halides iodide, bromide, and chloride to monohalomethanes and of bisulfide to methanethiol. The dual function of the enzyme was demonstrated through co-purification of the halide- and bisulfide-methylating activities in the same ratio and by studies of competition between the alternative substrates iodide and bisulfide. The purification procedure included gel filtration, anion exchange chromatography, and affinity chromatography on adenosine-agarose. Elution of the protein from a chromatofocusing column indicated a pI value of 4.8. The pH optimum of halide methylation (5.5-7.0) was different from that of bisulfide methylation (7.0-8.0). The molecular mass values for the native and denatured protein were 29.5 and 28 kDa, respectively, suggesting that the active enzyme is a monomer. The enzyme had the highest specificity constant for iodide and the next highest for bisulfide. Substrate interaction kinetics and product inhibition patterns were consistent with an Ordered Bi Bi mechanism.

The first large-scale network of 79 tree-ring chronologies in the Eastern Mediterranean and Near East (EMNE; 33°N–42°N, 21°E–43°E) is described and analyzed to identify the seasonal climatic signal in indices of annual ring width. Correlation analysis and cluster analysis are applied to tree-ring data and gridded climate data to assess the climate signal embedded in the network in preparation for climate field reconstructions and formal proxy/model intercomparison experiments. The lengths of the 79 combined chronologies range from 89 to 990 years. The monthly correlations and partial correlations reveal a pervasive positive association with May, June, and sometimes July precipitation, positive correlations with winter and spring (December through April) temperatures, and negative relationships with May through July temperature, although as expected, there are site-to-site exceptions to these general patterns. Cluster analysis suggests three groups of sites based on their association with climate. The chronologies for the EMNE have coherent seasonal precipitation and temperature signals across a fairly broad geographical domain. The predominant signal is a positive growth response to May–June precipitation. Collectively, the findings suggest that the network can be exploited to develop season-specific field reconstructions of precipitation and drought history in the EMNE.

No abstract

The physiological relevance of a novel thiol methyltransferase from cabbage, and its possible role in sulphur metabolism have been investigated. The enzyme was absent from the chloroplast, the site of sulphate reduction, and was localized in the cytosol. Potential substrates were initially screened on the basis of their ability to inhibit the methylation of iodide, a previously known substrate for the enzyme. Thiocyanate, 4,4¢-thiobisbenzenethiol, thiophenol, and thiosalicylic acid were identified as possible substrates. Methylation of these thiols by the purified enzyme using [Methyl-3 H]S-adenosyl-L-methionine confirmed their nature as substrates. The purified enzyme strongly preferred thiocyanate as a methyl acceptor. The enzyme had K m values of 11, 51, 250 and 746 mmol m -3 for thiocyanate, 4,4¢-thiobisbenzenethiol, thiophenol and thiosalicylic acid, respectively. The identity of methylthiocyanate as the product of thiocyanate methylation by the purified enzyme was confirmed by mass spectrometry. The enzyme was strictly associated with glucosinolate-containing plants. Thiol substrates of the enzyme are known products of glucosinolate hydrolysis. Our observations indicate that this enzyme could be involved in the detoxification of reactive thiols produced upon glucosinolate degradation in these plants.