Pectic Metabolism of Growing Cell Walls

Abstract

Avena coleoptile cell walls contained, on a dry weight basis, 5.1% of pectic substances as measured by the carbazole method. Methyl ester determination by alkaline hydrolysis showed that on the average, 40% of the pectic material was esterified. Methyl ester determination by pectinesterase yielded a value approximately 90% of that found by alkaline hydrolysis. The cell walls contained a considerable amount of labile alkyl groups other than methyl ester. The nature of these alkyl groups is not now known. Hydrolysis of cell walls produced small but significant amounts of formaldehyde. Pectinase hydrolysis of cell walls followed by ion exchange separations and paper chromato-graphy were used to identify the component sugars and uronic acids of the cell wall. The sugar fraction contained ribose, galactose, glucose, xylose, and arabinose, with the last 2 sugars predominating, as well as at least 1 ultraviolet absorbing and 1 ultraviolet fluorescing material. The acidic fraction contained 2 ultraviolet absorbing and 2 ultraviolet fluorescing substances in addition to galacturonic acid. The cation exchange capacity of coleoptile cell walls equaled the number of free pectic carboxyl groups. The cation exchange capacity of living Avena coleoptile sections, previously determined, is likewise equal to the number of free pectic carboxyl groups which the tissue contains. Accordingly, it is concluded that the cation exchange capacity of coleoptile sections is determined by the deesterified portion of pectic substances present. Incubation of coleoptile tissue with IAA induces an accelerated formation of the methyl ester of the pectic substances of cell walls, although no analytically detectable net change in cell wall content of pectic substance in methyl ester content, or in degree of pectic esterification was found. This is deduced from experiments in which tissue was incubated for 5 hours in methionine-C14H3 and in the presence or absence of IAA, followed by isolation of the pectic methyl ester groups as methanol. The specific activity of the pectic methyl ester groups of lAA-treated tissue was as much as 1.7 times that of the non-auxin treated tissue. The IAA-induced accelerated formation of methyl ester occurred exclusively in the hot-water-soluble pectic fraction of the cell walls which represents 15% of the total pectic substance of the cell walls. Incubation of coleoptile tissue with IAA caused a greater increase in incorporation of methionine methyl into the methyl ester groups of the cold-water-soluble, 70% alcohol-precipitable pectic fraction than into the pectin of the cell walls themselves. The high specific radioactivity of the methyl ester groups of the cold-water-soluble pectin fraction suggests a relatively direct pathway for the transfer of methionine methyl to the methyl ester of pectic substances. These observations also suggest that the cold-water-soluble pectic material may be a precursor of cell wall pectic substances. Little or no incorporation of the methyl group of methionine into the acetyl (or similar acyl) groups of the cell walls could be found. No net change in the acetyl (or similar acyl) groups resulted from incubation of coleoptile tissue with IAA.