A Starch Deficient Mutant of Arabidopsis thaliana with Low ADPglucose Pyrophosphorylase Activity Lacks One of the Two Subunits of the Enzyme.

A starch deficient mutant of Arabidopsis thaliana (L.) Heynh. has been isolated in which leaf extracts contain only about 5% as much activity of ADPglucose pyrophosphorylase (EC 2.7.7.27) as the wild type. A single, nuclear mutation at a previously undescribed locus designated adg2 is responsible for the mutant phenotype. Although the mutant contained only 5% as much ADPglucose pyrophosphorylase activity as the wild type, it accumulated 40% as much starch when grown in a 12 hour photoperiod. The mutant also contained about 40% as much starch as the wild type when grown in continuous light, suggesting that the rate of synthesis regulates its steady state accumulation. Immunological analysis of leaf extracts using antibodies against the spinach 54 and 51 kilodalton (kD) ADPglucose pyrophosphorylase subunits indicated that the mutant is deficient in a cross-reactive 54 kD polypeptide and has only about 4% as much as the wild type of a cross-reactive 51 kD polypeptide. This result and genetic studies suggested that adg2 is a structural gene which codes for the 54 kD polypeptide, and provides the first functional evidence that the 54 kD polypeptide is a required component of the native ADPglucose pyrophosphorylase enzyme.