Adp

* Glc-1,6-Bisphosphate

Fig. 3. A possible " Phosphohexose cycle" pathway involved in starch synthesis in maize

Erlander (91) suggested that phytoglycogen is an intermediate in the pathway to starch, and the phytoglycogen is subsequently debranched with debranched chains forming the amy lose component of starch. This is not the route by which amylose is synthesized (7), and no enzyme present in developing maize endosperm is capable of debranching phytoglycogen (92). Boyer et al (86) also noted that A:B chain ratio of phytoglycogen, which is 1:1, could not produce the 2:1 chain ratio of amylopectin by debranching. Pan and Nelson (8) suggested that the primary defect of the sul mutation was a deficiency of a debranching enzyme, and emphasized that the role of the enzyme in nonmutant endosperms was a dynamic one with both branching and debranching occuring simultaneously. The attention of debranching activity by the sul mutation alleles at the sul locus comes from the observation (72) that sul alleles are epistatic to mutant at almost all other loci affecting starch synthesis (i.e.,the double mutant endosperms produce large quantities of glycogen). One exception is the ae sul double mutant. The ae mutant lacks BEIIb (4, 75) and results in the production of amylopectin with longer and less branched chains (75, 93). The other exception is the sh2 mutant, and the double mutant sh2 sul. They lack the copious production of phytoglycogen (72, 93). The nonmutant alleles at the sh2 locus, in concert with those at the bt2 locus catalyze the synthesis of ADPGlc , the substrate for starch synthetase. This observation raises an interesting question. Is there a pathway, other than through ADPGlc, that leads to the synthesis of unbranched starch? Or is it simply that one or more of the branching enzymes are inhibited by the high concentration of reducing sugars resulting from the presence of the sh2 allele?

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