THE MOLECULAR-BASIS OF QUANTITATIVE GENETIC-VARIATION IN CENTRAL AND SECONDARY METABOLISM IN ARABIDOPSIS

To find the genes controlling quantitative variation, we need model sy stems where functional information on physiology, development, and gen e regulation can guide evolutionary inferences. We mapped quantitative trait loci (QTLs) influencing quantitative levels of enzyme activity in primary and secondary metabolism in Arabidopsis. All 10 enzymes sho wed highly significant quantitative genetic variation. Strong positive genetic correlations were found among activity levels of 5 glycolytic enzymes, PGI, PGM, GPD, FBP, and G6P, suggesting that enzymes with cl osely related metabolic functions are coregulated. Significant QTLs we re found influencing activity of most enzymes. Some enzyme activity QT Ls mapped very close to known enzyme-encoding loci (e.g., hexokinase, PGI, and PGM). A hexokinase QTL is attributable to cis-acting regulato ry variation at the AtHXK1 locus or a closely linked regulatory locus, rather than polypeptide sequence differences. We also found a QTL on chromosome IV that may be a joint regulator of GPD, PGI, and G6P activ ity. In addition, a QTL affecting PGM activity maps within 700 kb of t he PGM-encoding locus. This QTL is predicted to alter starch biosynthe sis by 3.4%, corresponding dth theoretical models, suggesting that QTL s reflect pleiotropic effects of mutant alleles.