Polygalacturonase produced in apple tissue decayed by Botrytis cinerea.

An exo-polygalacturonase with an isoelectric point of 4.6 and an apparent molecular weight of 45 kDa was isolated from apple tissue decayed by Botrytis cinerea. This isozyme had a similar isoelectric point, optimum pH, and mode of action as an isozyme produced in liquid culture by B. cinerea. The enzyme produced in the decayed tissue was less sensitive to lower pH and less inhibited by CaCl2, MgCl2, or NaCl than the enzyme produced in culture. Such changes in the properties of the enzyme produced in infected tissue could have been essential for the pathogen's successful colonization of the host tissue. Among the cations studied, calcium was the best inhibitor of PG activity.

Enzymes Catalyzing the Reversible Conversion of Fructose-6-Phosphate and Fructose-1,6-Bisphosphate in Maize (Zea mays L.) Kernels.

The significance of the glycolytic and gluconeogenic conversion of fructose-6-phosphate and fructose-1,6-bisphosphate on sugar metabolism was investigated in maize (Zea mays L.) kernels. Maximum extractable activities of the pyrophosphate (PPi) dependent phosphofructokinase, fructose-1,6-bisphosphatase, and the ATP-dependent phosphofructokinase were measured in normal and four maize genotypes, which accumulate relatively more sugars and less starch, to determine how these enzymes are affected by the genetic lesions. Normal endosperm accumulated more dry matter than the high sugar/low starch genotypes, but protein contents did not differ greatly among the genotypes. Mutation of several starch biosynthetic enzymes had little impact on the activities of PPi-dependent phosphofructokinase, fructose-1,6-bisphosphatase, and ATP-dependent phosphofructokinase, despite the altered capacity of the cell to synthesize starch. The PPi-dependent phosphofructokinase appeared to be more active toward glycolysis in all genotypes studied. Activity of the PPi-dependent phosphofructokinase in shrunken (low sucrose synthase genotype) did not differ from the activity in other genotypes, suggesting that the gluconeogenic production of PPi may not be the primary role of the enzyme. As expected, shrunken kernels contained more sugars and less starch than normal kernels throughout kernel development except at the very early stages. Developmental profiles of normal kernels also showed marked changes in the PPi-dependent phosphofructokinase activity, whereas the level of ATP-dependent phosphofructokinase activity remained relatively steady during kernel development. In addition, the ATP-dependent phosphofructokinase, and not the PPi-dependent phosphofructokinase, appeared to correlate more closely with respiration rate. These findings suggest that glycolysis catalyzed by the ATP-dependent phosphofructokinase may serve primarily to support energy production, and glycolysis catalyzed by the PPi-dependent phosphofructokinase may contribute mainly to generation of biosynthetic intermediates.

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

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.