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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...

Despite rapidly decreasing costs and innovative technologies, sequencing of angiosperm genomes is not yet undertaken lightly. Generating larger amounts of sequence data more quickly does not address the difficulties of sequencing and assembling complex genomes de novo. The cotton (Gossypium spp.) genomes represent a challenging case. To this end, a coalition of cotton genome scientists has developed a strategy for sequencing the cotton genomes, which will vastly expand opportunities for cotton research and improvement worldwide.

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Cotton fibers are single-celled extensions of the seed epidermis. They can be isolated in pure form as they undergo staged differentiation including primary cell wall synthesis during elongation and nearly pure cellulose synthesis during secondary wall thickening. This combination of features supports clear interpretation of data about cell walls and cellulose synthesis in the context of high throughput modern experimental technologies. Prior contributions of cotton fiber to building fundamental knowledge about cell walls will be summarized and the dynamic changes in cell wall polymers throughout cotton fiber differentiation will be described. Recent successes in using stable cotton transformation to alter cotton fiber cell wall properties as well as cotton fiber quality will be discussed. Futurec prospects to perform experiments more rapidly through altering cotton fiberwall properties via virus-induced gene silencing will be evaluated.