Studies on the Biosynthesis of Pectinic Acid Methyl Esters

Pectin esterase in relation to leaf abscission in coleus and phaseolus

Pectin esterase (PE) activities in abscission zones, other portions of leaves, and adjacent stem tissues were compared in attached leaves and abscissing petioles (previously debladed) of Coleus blumei Benth. and Phaseolus vulgaris L., cv. Canadian Wonder. Earlier findings of Osborne in bean were confirmed and changes in PE activity in coleus were shown to resemble those in bean in some respects. In both plants PE was lower in the distal portion of abscission zones of abscissing petioles than in that portion of attached leaves but this difference was not as large or as consistently clear-cut in coleus as in bean. The general level of PE activity was an order of magnitude lower and changes associated with abscission were smaller in coleus than in bean. Auxin treatment of debladed petioles of coleus prevented abscission and resulted in small increases in PE activity in abscission zones and most of the other regions sampled. The largest increase was observed in the stem tissue adjacent to the attached leaf opposite the debladed, auxin treated one.The activity of coleus PE was highest in the pH range from 7.3 to 7.6. The pH of distal tissue from abscission zones of abscissing petioles was 5.8. This was 0.7 pH units lower than that of proximal tissue from the same zones.PE from both coleus and bean appears to be denatured by freezing and/or thawing.

Incorporation of methanol into pectic substance

Others have shown that l-methionine is utilized in the biosynthesis of methyl ester groups in pectic substance. Methanol, like l-methionine, is used for methyl ester biosynthesis by detached parsley leaves (Petroselinum crispum). When a combination of methanol-(3)H and methanol-(14)C is given to parsley leaves, methanol recovered from pectic substance by alkaline hydrolysis has a (3)H/(14)C ratio about one-fourth that of the mixture administered. Unlike l-methionine, methanol is oxidized prior to its utilization as a carbon source for methyl ester biosynthesis.Parsley leaves given methyl d-galacturonate-methyl-(14)C also form pectic substance in which methyl groups are labeled but incorporation appears to proceed by way of methanol metabolism after hydrolysis of methyl d-galacturonate.

The utilization of carbon-1 compounds by plants. II. The formation and metabolism of formate by higher plant tissues

Tissue slices of carrot root and sunflower cotyledons readily produce formate-C14when incubated with glyoxylate-1,2-C14and glycine-2-C14solutions. Formate-C14feeding experiments using a wide variety of higher plant tissues have shown that this compound is readily oxidized to carbon dioxide. This oxidation probably involves a formic dehydrogenase system which has been demonstrated to occur in several of the tissues. Large amounts of formate-C14were also incorporated into serine, methionine, and methionine sulfoxide. Degradations of the serine produced from formate-C14showed that the bulk of the C14was present in the 3 position. The incorporation of formate into serine by radish cotyledons was stimulated under anaerobic conditions and by additions of glycine. Synthesis of methionine and methionine sulfoxide from formate-C14was stimulated by additions of homocysteine. The results are interpreted as an indication that formate-C14can be oxidized to carbon dioxide by formic dehydrogenase and can serve as a precursor of carbon-1 units that are to be utilized for the biosynthesis of serine and methionine.