Polysaccharide Synthesis from GDP-Glucose in Pea Epicotyl Slices

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Abstract
Pea epicotyl slices were incubated with GDP-[14C]glucose (1 μM), and the incorporation of radioactivity into products insoluble in hot 0·5 M NaOH was measured. All the radioactivity in the alkali-insoluble product was present as β-(1→4)-linked glucose residues. The following tests suggest that the product was not cellulose: 88% of it was soluble in hot 4·4 M NaOH, it was shown to be of relatively low molecular weight by gel filtration in cadoxen, and its synthesis was not inhibited by inhibitors of cellulose synthesis. When non-radioactive GDP-mannose (10 μM) was included in incubation medium, the incorporation continued for a longer period, but its initial rate was not increased. This suggests that the product was a glucomannan. Incubation of pea epicotyl slices with 50 μM GDP-[14C]glucose however, gave products which appeared to include a significant proportion of cellulose: 25% of the product was insoluble in hot 4·4 M NaOH, the incorporation continued up to at least 90 min of incubation, and the product was of higher molecular weight than that from 1 μM GDP-glucose. The tests described permit positive evidence for cellulose biosynthesis in in vitro systems to be obtained.