Particulate Glucan Synthetase Activity: Dependence on Acceptor, Activator, and Plant Growth Hormone

Abstract

Particulate fractions isolated from the growing region of the epicotyl of Pisum sativum L. var Alaska are capable of transferring glucose from uridine or guanosine diphosphate glucose-14C to buffer-insoluble products which are partly alkali-soluble and partly alkali-insoluble. Cellobiose activates the reaction; carboxymethylcellulose and cellodextrins act as competitive acceptor molecules.When the epicotyl is decapitated, glucan synthetase activity disappears from particulate fractions of the growing region within 3 days unless the hormone indoleacetic acid (IAA) is added to the tissue, in which event activity is retained. Other growth regulators (gibberellic acid, benzyladenine) have no such effect. Loss of activity is not due to a change in solubility of enzyme or product or to the absence of substrate, activator, or acceptor molecules, nor is it accompanied by comparable losses in total protein or cellulase activity from the particulate fraction. It is concluded that IAA is needed for the formation and/or protection of an essential part of the insoluble synthetase complex.

Regulation of β-Glucan Synthetase Activity by Auxin in Pea Stem Tissue

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In studies of hormonal regulation of cell wall polysaccharide synthesis, auxin-treated tissue yields greater activity of UDPglucose: /3-1 ,4-glucan glucosyltransferase (",3-glucan synthetase") than untreated controls (1,6,15,21,27). This effect may be part of the mechanism by which wall synthesis is stimulated by auxin.

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Changes in Molecular Weight of Cellulose in the Pea Epicotyl during Growth

“…Since the latter contain much mo than young tissues (Fig. 1), the absolute z DP component per cell actually increase ( Pea seedlings are probably not unique Insofar as other studies have been made, ce relatively low in young tissues, e.g., growi 19,21) and high in mature tissues, e.g., Moreover, auxin-induced cellulase activ (generated) in growing tissues (7, 15), althoi sis continues in plant cells after growth until senescence begins (16,19,25,26 (Fig. 3) …”

“…Such correlations have been observed in Acetobacter xylinlum cultures (11). In the pea epicotyl, it is established (2,23,26) that enzymes that use activated glucose donors to form glucan which is f8-l ,4-linked and alkali-insoluble are localized in membranes. These may be expected to retain their synthesizing capacities during tissue development, because excised epicotyl segments continue to form cellulose long after they have ceased growing (16).…”

“…These may be expected to retain their synthesizing capacities during tissue development, because excised epicotyl segments continue to form cellulose long after they have ceased growing (16). In contrast, enzymes which hydrolyze cellulose or its derivatives are present both in buffer-soluble and membrane fractions of peas (5,8,26). Almost all such activity is confined to the youngest meristematic regions (4,17).…”

“…Synthetase reaction rate was proportional to the concentration of microsomal protein. It was not catalyzed by soluble fractions, it was activated by cellobiose and able to transfer glucose from either guanosine or uridine sugar nucleotides to exogenously supplied /3-1,4-linked primers (26). The assay is referred to here as a measure of cellulose synthetase activity, with recognition that the alkali-insoluble product may not have been completely cellulose and not all ,B-1 , 4-linked products were necessarily recovered in this fraction.…”