Several laboratories have investigated the structures of starch produced by the mutants that affect starch synthesis (14, 75, 76, 83, 92, 99, 100). It is difficult to compare the reports vigorously because the methods of starch isolation and preparation of amylose and amylopectin fractions, the genetic background in which the mutants are represented, and the chromatographic separations of native or debranched starches may differ from laboratory to laboratory. Further, Asaoka et al (101) have reported that the environmental conditions, under which rice is grown ,can modify starches significantly. It would be useful if the data from these laboratories included the amounts of each fraction per endosperm in addition to the fraction's percentage of the total. Nevertheless, there are general points of agreement. The Wx mutants, which synthesize starch consisting mainly of amylopectin that is the same as that produced by nonmutant endosperms, are epistatic to all other mutants in double mutant combinations. However, in double mutant with wx , all modified amylopectins were synthesized in the ae,dul and sul mutants. The ae wx double mutants produce starches, which have apparent amylose contents of 15-25%, measured by iodine affinity or by blue value. These double mutants do not produce a true amylose fraction. The high iodine affinity is the result of an anomalous amylopectin with longer chain lengths than nonmutant amylopectin (14).
6. IS THERE A ROLE FOR PHOSPHORYLASE IN STARCH SYNTHESIS?
Until the discovery of the glucosyl transferases that transfer glucose from nucleoside diphosphate glucose to the nonreducing ends of growing starch or glycogen molecules, it was assumed that the enzyme responsible for lengthening the chains was starch phosphorylase (E.C.18.104.22.168). Because the action of this enzyme (Glen + Glc-l-P <—> Glcn+1 + Pi) is readily reversible and the amounts of Pi in homogenates of starch synthesizing storage tissues would be inimical to starch synthesis, it was necessary to postulate that much of the Pi was effectively sequestered away from the sites of starch synthesis. Yet there is no evidence to demonstrate conclusively that a-glucan phosphoiylase activity does not make a contribution. Phosphorylase activity in the developing endosperm increases and then decreases over time roughly in parallel with enzymes that are known to participate in starch synthesis (102). Ozbun et al (103) also confirmed the large increase in phosphoiylase activity during endosperm development in starch synthesis. Badenhuizen et al (104) were convinced of a role in starch synthesis for phosphoiylase. The most forceful evidence was that normal starch could be synthesized in sterile potato tubers even when most of SS activity was lost during growth at 30°C, because phosphorylase is much more thermostable (105).
The coordinated biochemical, genetic and molecular genetic analysis of maize mutants conditioning specific phenotypes will be an invaluable research tool for dissecting the primary reaction mechanism of starch synthesis in cereal storage tissues. The potential exists for future investigations to clarify aspects that are still unclear and possibly to uncover unsuspective pathways and reactions. These include two most important problems of starch synthesis such as the primary event of starch synthesis is the formation of primer and the mechanism of the formation of phosphorylated starch in vivo. From physiological principle for consideration, the understanding of the coordinated biochemical reactions of the isoforms of the enzymes involved in the biochemical pathways of starch synthesis is a desirable task. A more intensive search of the numerous mutants induced by mutagen treatments and transposon insertions may identify such mutants. The transposon mutants would have the advantage of providing a route to cloning the gene. With the exponential advance and expansion of molecular genetic knowledge and technology, we also look forward to the future possibility of targeted gene replacement that would allow the generation of mutants at a locus where none now exists. An understanding of starch synthesis would allow alterations of the process by plant breeding and genetic engineering to produce unique and invaluable starches for human need.
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