Sucrose synthase is associated with the cell wall of tobacco pollen tubes

Sucrose synthase (EC-Number 2.4.1.13) is a key enzyme of sucrose metabolism in plant cells, providing carbon for respiration and for the synthesis of cell wall polymers and starch. Since sucrose synthase is important for plant cell growth, insights into its structure, localization and features is useful for defining the relationships between nutrients, growth and cell morphogenesis. We used the pollen tube of Nicotiana tabacum L. as a cell model to characterize the main features of sucrose synthase with regards to cell growth and cell wall synthesis. Apart from its role during sexual reproduction, the pollen tube is a typical tip-growing cell and the proper construction of its cell wall is essential for correct shaping and direction of growth. The outer cell wall layer of pollen tubes consists of pectins but the inner layer is composed of cellulose and callose; both polymers require metabolic precursors in the form of UDP-glucose, which is synthesised by sucrose synthase. We identified an 88-kD polypeptide in the soluble, plasma membrane and Golgi fraction of pollen tubes. The protein was also found in association with the cell wall. After purification, the protein showed an enzyme activity similar to that of maize sucrose synthase. Distribution of sucrose synthase was affected by Brefeldin A and depended on the nutrition status of the pollen tube because an absence of metabolic sugars in the growth medium caused sucrose synthase to distribute differently during tube elongation. Analysis by bidimensional electrophoresis indicated that sucrose synthase exists as two isoforms, one of which is phosphorylated and more abundant in the cytoplasm and cell wall, while the other is not phosphorylated and is specific to the plasma membrane. Results indicate that the protein has a role in construction of the extracellular matrix and thus in the morphogenesis of pollen tubes