A study of the biosynthesis of the methoxyl groups of lignin in tobacco plants.

Previous studies in our laboratory have been concerned with the pathway of carbon in the formation of methyl groups in photosynthesizing plants. In the biosynthesis of the N-methyl group of nicotine in tobacco plants, it was shown that, of several possible precursors, formaldehyde was incorporated to the greatest extent (1). This finding led to the suggestion that a “l-carbon” unit at the oxidation state of formaldehyde was reduced to methyl groups in various plant products. It was further shown that the a-carbons of glycine’ and glycolate and the P-carbon of serine were also incorporated into the N-methyl group of nicotine to a relatively large extent, although to a lesser extent than was formaldehyde (l-3). These results were interpreted to indicate that, in the formation of the N-methyl group, a 2or 3-carbon compound is decomposed to yield the “l-carbon” unit at the oxidation state of formaldehyde. The biosynthesis of O-methyl groups, which occurs in plants but not in animals, likewise has received attention. The O-methyl group of the alkaloid, ricinine, synthesized by etiolated castor bean seedlings, was shown to arise from the methyl group of methionine. However, formate did not appear to be reduced to methyl groups in these plants (4). In growing, photosynthesizing barley and tobacco, the O-methyl groups of the cell wall constituent lignin were shown to arise either by transmethylation from methionine or by reduction of formate or of the a-carbon of glycolate (3, 5). Formate was incorporated to a relatively small extent in comparison with methionine or glycolate. It was therefore of interest to study some other possible precursors of the O-methyl group which might occur in the metabolic pathway from carbon dioxide.