Alterations in Carbohydrate Intermediates in the Endosperm of Starch-Deficient Maize (Zea mays L.) Genotypes

Abstract

Metabolite levels in kernels of selected starch-deficient mutants of maize (Zea mays L.) were investigated to gain insight into partitioning of carbohydrate metabolism during kernel development. Several free sugars, hexose phosphates, triose phosphates, fructose-2,6-bisphosphate, and pyrophosphate were measured in normal, shrunken, shrunken-2, amylose extender dull waxy, and brittle genotypes, which were in a near-isogenic W64A background. These mutants were selected to include at least one lesion in both the cytosolic (shrunken) and amyloplastic (shrunken-2) compartments. All the starch-deficient genotypes contained elevated levels of fructose-2,6-bisphosphate and triose phosphates but reduced levels of pyrophosphate, indicating an enhanced glycolytic utilization of carbohydrates in response to the reduced utilization of sugars for starch synthesis. The shrunken kernels (sucrose synthase deficient) contained reduced levels of glucose-1-phosphate, glucose-6-phosphate, and fructose-6-phosphate, and this reduction paralleled the reduction in starch accumulation, but levels of triose phosphates were elevated. In shrunken-2 kernels, glucose-1-phosphate, glucose-6-phosphate, and fructose-6-phosphate, dihydroxyacetone phosphate, and glyceraldehyde-3-phosphate were increased, but fructose-1,6-bisphosphate was lower. These findings support the view that hexose phosphate transport across the amyloplast envelope is more important for starch biosynthesis than transport of triose phosphates. The amylose extender dull waxy mutation showed less dramatic effects on hexose phosphates, but the triose phosphates were greatly increased. The brittle mutation, which has an unknown lesion, showed distinctly similar changes in metabolite levels with shrunken-2, suggesting that the lesion may be associated with the amyloplast.

Transcript Profiling of the Anoxic Rice Coleoptile

“…Samples (0.05-0.3 g fresh weight) were rapidly frozen in liquid nitrogen and ground to a powder. Samples were then extracted as described by Tobias et al (1992). After centrifugation, the supernatant was used for the analysis of Suc, Fru, and Glc, whereas the starch-containing pellet was extracted using 10% KOH, centrifuged, and the neutralized supernatant treated with 2.5 units amyloglucosidase (from Rhizopus niger) for 3 h to release Glc.…”

Overexpression of Arabidopsis Hexokinase in Tomato Plants Inhibits Growth, Reduces Photosynthesis, and Induces Rapid Senescence

“…Glucose 6-phosphate and fructose 6-phosphate were extracted and assayed spectrophotometrically on 1 g fresh weight of mature leaves, according to Tobias et al (1992).…”

Brittle-1, an Adenylate Translocator, Facilitates Transfer of Extraplastidial Synthesized ADP-Glucose into Amyloplasts of Maize Endosperms1

“…ADP-Glc is transported into the amyloplast stroma via BT1, which may be the same transporter as the ADP-Glc/AMP adenylate translocator described by Mö hlmann et al (1997). The importance of the BT1 translocator to starch accumulation in maize endosperms is demonstrated by the severely reduced starch content in bt1 mutant kernels (Tobias et al, 1992). Assessment of the relative importance of the hexose-P/Pi antiporter for starch accumulation in vivo awaits isolation of a mutant genotype defective in the hexose-P/Pi antiporter.…”