Biosynthesis and degradation of starch.

Sucrose and starch biosynthesis Arrival of sucrose into terminal sink organs is a dynamic process that can take either a symplastic and/or apoplastic route depending on the type of organ and developmental state (Patrick, 1997). In either case, the overall ability of the heterotrophic organ to attract photoassimilates is strongly dependent on the capacity of individual cells to import, metabolize and store sucrose, ultimately determining plant productivity and crop yield (Ho, 1988). In organs where apoplastic sucrose unloading occurs, different routes can be envisaged for the subsequent uptake into storage cells: (a) hydrolysis of sucrose by an apoplastic invertase and subsequent uptake of glucose and fructose, and/or (b) import of sucrose by plasmalemma bound carriers. In addition, sucrose can also be taken up by endocytosis and transported to the central vacuole (Etxeberria et al., 2005) Subsequent conversion of internalized sucrose to starch involves a series of enzymatic reactions wherein sucrose synthase predominantly controls sucrose breakdown and production of a C6 starch precursor molecule entering the amyloplast (Baroja-Fernandez et al., 2003). Results obtained from sycamore cultured cells, potato tuber slices and developing barley endosperms treated with potent endocytic inhibitors suggest the presence of an important pool of apoplastic sucrose produced by endocytosis prior to its subsequent conversion into starch in heterotrophic organsn of both monoand di-cotyldonous plants (Baroja-Fernandez et al., 2006) Pathway of starch biosynthesis Genetic and biochemical data demonstrate that starch biosynthesis is mediated by at least four enzymes: ADPglucose pyrophosphorylase (ADP: α-D-glucose-1-phosphate adenyltransferase, AGPase, EC 2.7.7.27), starch synthase (ADP-glucose:(1,4)-α-D-glucan 4-α-D-glucosyltransferase, SS, EC 2.4. 1.21 and 2.4. 1.242), starch-branching enzyme ((1,4)-αD-glucan: (1 ,4) -α-D-glucan 6-α-D-( (1 ,4) -α-D-glucano) transferase SBE, EC 2.4. 1.18), and starch-debranching enzyme (DBE; EC 3.2.1.68 for isoamylase-type and EC 3.2.1.41 for pullulanase-type). In starch biosynthesis, ADP-glucose is the glucose donor for α-glucan elongation and is formed from ATP and glucose 1-phosphate (G1P) by AGPase. SS catalyzes the transfer of glucose moiety from ADP-glucose to the nonreducing end of an α-glucan and is responsible for the elongation of amylose and amylopectin molecules. SBE catalyzes hydrolysis of an α-1,4-linkage and the subsequent transfer of an α-1,4 glucan to form an α-1,6 branching point. Hence, the catalytic actions of these enzymes synthesize nonbranched and branched glucans; the sequential actions by SS, theoretically synthesize non-branched glucans and a cooperative actions of SS and SBE produces branched glucans. In fact, the analysis of glucans synthesized by E. coli cells co-expressing maize SSs and SBEs suggested that the function of SS and SBE is important for determining the structure of α-glucan, but the in vivo synthesis of amylose and amylopectin is not so simple. Sugary-1 (su1) mutants isolated form maize and rice have the reduced starch content and accumulation of water-soluble and highly branched phytoglycogens. Since the su1 loci encoded the genes for