A membrane-associated form of sucrose synthase and its potential role in synthesis of cellulose and callose in plants.

Abstract

Sucrose synthase (SuSy; EC 2.4.1.13; sucrose + UDP = UDPglucose + fructose) has always been studied as a cytoplasmic enzyme in plant cells where it serves to degrade sucrose and provide carbon for respiration and synthesis of cell wall polysaccharides and starch. We report here that at least half of the total SuSy of developing cotton fibers (Gossypium hirsutum) is tightly associated with the plasma membrane. Therefore, this form of SuSy might serve to channel carbon directly from sucrose to cellulose and/or callose synthases in the plasma membrane. By using detached and permeabilized cotton fibers, we show that carbon from sucrose can be converted at high rates to both cellulose and callose. Synthesis of cellulose or callose is favored by addition of EGTA or calcium and cellobiose, respectively. These findings contrast with the traditional observation that when UDPglucose is used as substrate in vitro, callose is the major product synthesized. Immunolocalization studies show that SuSy can be localized at the fiber surface in patterns consistent with the deposition of cellulose or callose. Thus, these results support a model in which SuSy exists in a complex with the j8-glucan synthases and serves to channel carbon from sucrose to glucan.The well-characterized cellulose synthase from the bacterium Acetobacter xylinum is a plasma-membrane-localized enzyme that clearly uses UDPglucose (UDP-Glc) both in vivo and in vitro as substrate for synthesis of 13-1,4-glucan microfibrils (1). The high levels (2) and turnover rate (3) of UDP-Glc also suggest that it is the substrate for higher plant cellulose synthesis. However, when isolated plasma membranes of higher plants are supplied with UDP-Glc, the major product synthesized is usually not cellulose but callose (j3-1,3-glucan; ref. 4). A recent study with developing cotton fibers (5) has shown that a subfraction of membrane proteins can synthesize a higher ratio of cellulose to callose, but the rate of cellulose synthesis was far below that observed in vivo.In plants, UDP-Glc can potentially be synthesized by two different pathways. One route involves the enzyme UDP-Glc pyrophosphorylase (EC 2.7.7.9). Levels of this enzyme are usually very high in plant cells, but it probably functions primarily in the direction of UDP-Glc degradation, particularly in nonphotosynthetic tissues (6). The second route involves the enzyme sucrose synthase (SuSy; EC 2.4.1.13). Like the phosphorylase reaction, the reaction catalyzed by SuSy is freely reversible, but the high levels of this enzyme and steady-state measurements of levels of its substrates and products in nonphotosynthetic tissues suggest that it functions primarily in the direction of sucrose degradation and UDP-Glc synthesis (7,8). SuSy has formerly been studied as a cytoplasmic enzyme that provides carbon for respiration and cell wall polysaccharide and starch synthesis (9, 10). Evidence for a biosynthetic role of SuSy is provided by substantially reduced starch deposition and extensive cell wall degeneration in mut...

Callose Synthase GSL7 Is Necessary for Normal Phloem Transport and Inflorescence Growth in Arabidopsis

“…It has been suggested that UDPglucose for the synthesis of cell wall components is supplied via a specific association between SUS and cell wall biosynthetic enzymes (Amor et al, 1995;Verma and Hong, 2001;Salnikov et al, 2003;Persia et al, 2008). SUS is frequently associated with plasma membranes, consistent with the idea that it supplies UDPglucose to either cellulose synthase or callose synthase or both.…”

Sucrose synthase expression pattern in the rhythmically growing shoot of common oak (Quercus robur L.)

“…starch [7] and cell wall synthesis [3]. A positive correlation between sucrose synthase activity, sink strength and sucrose import has been demonstrated in many sink organs, suggesting a role for this enzyme in carbohydrate partitioning between sink and source organs [24,28,30].…”

Sugar/osmoticum levels modulate differential abscisic acid-independent expression of two stress-responsive sucrose synthase genes in Arabidopsis

“…such as the synthesis of cell wall [11] and starch [12,13]. In young leaves (sink tissue), Sus has supposedly a role in breaking down sucrose imported from mature leaves (source tissue).…”