Phytoglycogen

Phytoglycogen-branching enzyme has been found in du however, no phytoglycogen was isolated by Black et al.23 Preiss and Boyer396 reported that the du mutation lowered starch synthase II activity and also lowered branching enzyme Ha activity. Gao et al.397 used a molecular approach to clone the du gene in maize endosperms and, based on amino acid sequence similarity of the predicted protein product with the soluble starch synthase III of potato,398 concluded that du most likely encodes the 180 000 molecular weight, primer-dependent soluble starch synthase described previously.399,400,401... 

Although low levels of WSP have been reported in ae du kernels (Table 3.5), Black et al.23 concluded that no phytoglycogen accumulates in ae du kernels in a dent background. In contrast, kernels of ae du in a sweet com background produce numerous plastids with one or two starch granules surrounded by a thick layer of non-crystalline phytoglycogen-like polysaccharide.43... 

The mature kernel phenotype of the triple mutant ae su su2 differs from that of any of the component mutants (Table 3.3). Mature kernel dry weight is similar to that of ae su, and sugar and starch concentrations are intermediate between those of su and su su2 and those of ae, su2, ae su and ae su2.212 Mature and immature kernels contain intermediate levels of WSP (Table 3.5). This WSP has not been characterized and may or may not be similar to the phytoglycogen accumulating in su kernels. Starch from ae su su2 kernels has been reported to contain 31-54% apparent amylose (Table 3.6). Starches from ae su su2 have not been separated by SEC, and thus the relative sizes of the polysaccharides and degrees of branching have not been established. 

The mature kernel phenotype of du su wx is similar to that of su (Table 3.3). The quantity of sugars is similar to that in su.210 212 Addition of du to su wx causes an increase in WSP (Table 3.5) and a decrease in starch content.272 The phytoglycogen from du su wx has a (3-amylolysis limit and chain length similar to that of su. The enhanced phytoglycogen accumulation may result from the additive effect of the branching enzymes present in each of the component single mutants.23... 

Endosperm development in ae du su wx is similar to that in du su wx, with the type-II minor gradient observed and the central endosperm cavity being present by 27 days post-pollination.43 Starch granule and phytoglycogen plastid development in ae du su wx is similar to that in su, except that the quadruple mutant has greater apparent phytoglycogen content at 16 days post-pollination than does su or any other mutant combination.43 However, with development, there is increasing deterioration of the plastids and central endosperm cells.43... 

Interaction of these mutants further clarifies the biosynthetic pathway. For example, the wx mutant is epistatic to all other known maize endosperm mutants and no amylose accumulates (Table 3.6). Mutants such as sh2, bt2 and sit cause major reductions in starch accumulation, but when in combination with wx, the starch produced is all amylopectin.271 In the double mutant ae wx, wx prevents the production of amylose and ae reduces the degree of branching, resulting in the accumulation of a loosely-branched polysaccharide.88 The su mutant is epistatic to du, su2 and wx relative to accumulation of phytoglycogen, but ae and sh2 are partially epistatic to su, causing a marked reduction in the su stimulated phytoglycogen accumulation (Table 3.6). The addition of du or wx to ae su partially overcomes the ae inhibitory effect, and phytoglycogen accumulates. 

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