INFLUENCE OF GENE DOSAGE ON CARBOHYDRATE SYNTHESIS AND ENZYMATIC-ACTIVITIES IN ENDOSPERM OF STARCH-DEFICIENT MUTANTS OF MAIZE

In cereals, starch is synthesized in endosperm cells, which have a plo idy level of three. By studying the allelic dosage of mutants affectin g starch formation in maize (Zea mays L.) kernels, we determined the e ffect of down-regulated enzyme activity on starch accumulation and the activity of associated enzymes of carbohydrate metabolism. We found a direct relationship between the amount of starch produced in the endo sperm and the gene dosage of amylose extender-1, brittle-2, shrunken1, and sugary-1 mutant alleles. Changes in starch content were found to be caused by changes in the duration as well as the rate of starch syn thesis, depending on the mutant. Branching enzyme, ADP-glucose pyropho sphorylase, and sucrose synthase activities were linearly reduced in e ndosperm containing increasing dosages of amylose extender-1, brittle- 2, and shrunken-1 alleles, respectively. Debranching enzyme activity d eclined only in the presence of two or three copies of sugary-1. No en zyme-dosage relationship occurred with the dull1 mutant allele. All mu tants except sugary-1 displayed large increases (approximately 2- to 5 -fold) in activity among various enzymes unrelated to the structural g ene. This occurred in homozygous recessive genotypes, as did elevated concentrations of soluble sugars, and differed in magnitude and distri bution among enzymes according to the particular mutation.