Sugar metabolism in developing kernels of starch-deficient endosperm mutants of maize

Sugar metabolism in kernels of starch-deficient endosperm mutants of maize (Zea mays L.) was examined to determine how single locus mutations of carbohydrate metabolism affect carbohydrate metabolism as a whole. Activities of 14 enyzmes were measured in extracts from endosperms from isogenic lines of normal, shrunken, shrunken-2, shrunken-4, brittle-1, and brittle-2 maize in an OH43 background. Nearly every enzyme activity examined was affected in some or all of the mutants. Sucrose synthase and aldolase activities were lower in all mutants compared to normal. ADP-Glc pyrophosphorylase activity in immature kernels was much higher in brittle endosperms than in normal, but absent in brittle-2 and shrunken-2 endosperms. The activity in those genotypes exhibiting activity was positively correlated with sucrose concentration in the kernels. Sucrose may be modulating the coarse control of ADP-Glc pyrophosphorylase activity by affecting the genetic transcription of message for this enzyme. Sorbitol dehydrogenase activity was negatively correlated with its substrate, fructose, supporting the hypothesis that sorbitol dehydrogenase converts fructose produced during sucrose degradation into sorbitol. Glucokinase activity was positively correlated with mature kernel dry weight. This supports the hypothesis that glucokinase activity may limit sucrose utilization. Shrunken-extracts 4 had lower activities for a number of enzymes, supporting the view that this mutant may have an impediment to protein synthesis. Elevated sucrose levels were evenly distributed throughout 20-day postpollination shrunken-2 kernels, whereas a sucrose concentration gradient existed in normal kernels between the basal region and the upper endosperm. This gradient is apparently generated by the utilization of sugars and may facilitate the movement of sugars into developing corn kernels.