a mutant lacking starch synthase 4 is impaired in its ability to initiate starch granules, its chloroplasts rarely contain more than one large granule, and the plants have a pale appearance and reduced growth. The chloroplastic alpha-amylase AMY3 abundance is reduced in mutants lacking starch synthase 3 and/or 4
downregulating the expression of isoform SSIIa results in either shrunken or opaque grains with an elevated proportion of A-type starch granules. Shrunken seeds have severely reduced starch content and cannot be maintained in succeeding generations
glycogen synthesis occurs via ADP-glucose in Streptomyces coelicolor, with the pathway being strictly regulated in connection with other routes involved with oligo- and polysaccharides, as well as with antibiotic synthesis in the bacterium, metabolic scenario for glucose 6-phosphate partitioning, overview
granule-bound proteins involved in amylopectin synthesis are partitioned into the starch granule as a result of their association within protein complexes, and starch synthase IIa plays a crucial role in trafficking starch synthase I and starch branching enzyme IIb into the granule matrix. A mutant starch synthase IIa that has lost catalytic activity and is unable to bind to starch additionally leads to greatly reduced activities of starch synthase I and starch branching enzyme IIb
granule bound starch synthase I is the dominant enzyme controlling long amylose chain synthesis. The enzyme plays a key role in determining the amylose content of cereal grains, and it competes with starch branching enzyme I during starch biosynthesis
the enzyme plays a key role in granule initiation, allowing it to proceed in a way that avoids premature degradation of primers by starch hydrolases, such as the chloroplastic alpha-amylase AMY3
the enzyme plays an important role in transient starch synthesis of photosynthetic tissue and storage starch synthase of storage tissue at the total developmental stages
glycogen synthesis occurs via ADP-glucose in Streptomyces coelicolor, with the pathway being strictly regulated in connection with other routes involved with oligo- and polysaccharides, as well as with antibiotic synthesis in the bacterium, metabolic scenario for glucose 6-phosphate partitioning, overview
function of isoform SSIV in the priming process of starch granule formation. SSIV is necessary and sufficient to establish the correct number of starch granules observed in wild-type chloroplasts. The role of SSIV in granule seeding can be replaced, in part, by SSIII. The simultaneous elimination of both proteins prevents Arabidopsis thaliana from synthesizing starch. The soluble starch synthase activity in ssI ssII ssIV mutant plants is 32% of the activity determined in wild-type plants, whereas no soluble SS activity can be detected in ssI ssII ssIII mutant plants
construction of double mutant isoforms ss1- ss2- or ss1- ss3- lines using confirmed null mutations. Double mutant plants develop similarly to the wild type, although they accumulate less leaf starch in both short-day and long-day diurnal cycles. Lines containing only SS2 and SS4, or SS3 and SS4, are able to produce substantial amounts of starch granules. In both double mutants the residual starch is structurally modified including higher ratios of amylose to amylopectin, altered glucan chain length distribution within amylopectin, abnormal granule morphology, and altered placement of alpha(1-6) branch linkages relative to the reducing end of each linear chain. Starch Ssynthase activity affects not only chain elongation but also the net result of branch placement accomplished by the balanced activities of starch branching enzymes and starch debranching enzymes. Isoform SS3 partially overlaps in function with isoform SS1 for the generation of short glucan chains within amylopectin
construction of double mutants defective in isoforms SSI and SSIIIa. In the F2 generation, two opaque seed types were found to have either the ss1ss1/SS3ass3a or the SS1ss1/ss3ass3a genotype. The endosperm of the two types of opaque seeds displays lower starch synthase activity and contains the unique starch with modified fine structure, round-shaped starch granules, high amylose content, and specific physicochemical properties. The seed weight is about 90% of that of the wild type. The amount of granule-bound starch synthase I and the activity of ADP-glucose diphosphorylase are higher than in the wild type and parent mutant lines. The double-recessive homozygous mutant prepared from both ss1 and ss3a null mutants is considered sterile, while the mutant produced by the leaky ss1 mutant with the ss3a null mutant is fertile
double mutant endosperms which are affected in both isoform SSIII and ISA2, encoding a noncatalytic subunit of heteromeric isoamylase-type starch-debranching enzyme, are starch deficient and accumulate phytoglycogen. Despite lack of functional isa2, ISA1 homomeric enzyme complexes assemble in both double mutants and are enzymatically active in vitro. Thus, SSIII is required for normal starch crystallization and the prevention of phytoglycogen accumulation when the only isoamylase-type debranching activity present is ISA1 homomer, but not in the wild-type condition, when both ISA1 homomer and ISA1/ISA2 heteromer are present
grain qualities and physicochemical properties of starch are determined by SSIIIa. Biosynthesis of resistant starch is regulated by SSIIIa. Mutation of SSIIIa gene results in elevation of resistant starch