Duplicate Loci as QTL

The C-glycosyl flavone maysin is an important component of corn earworm (Helicoverpa zea Boddie) (CEW) resistance in maize (Zea mays L.) silks. Chlorogenic acid (CGA), a product of the phenylpropanoid pathway, has also been implicated in CEW antibiosis. The transition from the 9-carbon phenylpropanoid to the 15-carbon flavonoid pathway represents a potentially important regulation point in C-glycosyl flavone biosynthesis. The enzyme chalcone synthase (CHS) catalyzes this reaction and is represented by a set of duplicate loci in maize, colorless2 (c2) and white pollen1 (whp1). Chromosomal regions of the maize genome have been identified as quantitative trait loci (QTL) for maysin synthesis that correspond to these two loci. Our objective was to investigate the role of CHS in flavone and CGA biosynthesis. We tested c2 and whp1 as candidate loci using three related F₂ populations segregating for both structural and regulatory loci. Silks were harvested and analyzed from each population for flavonoid and phenylpropanoid content. Phenotypic data was combined with linkage maps generated for each population to conduct statistical and QTL analysis. Our results (i) support earlier findings of the requirement of a functional pericarp color1 (p1) allele for C-glycosyl flavone biosynthesis, (ii) demonstrate that variation at the c2 region on chromosome 4 influences both maysin and CGA levels in silks, (iii) reveal a dosage-dependent relationship between c2 and whp1 in which the whp1 region acts cooperatively with functional c2 to increase flavones and partially compensates for nonfunctional c2, and (iv) demonstrate that mutations in c2 and whp1 shunt intermediates from maysin synthesis to CGA. These results demonstrate that CHS function is a regulatory focal point of substrate flow between the flavone and phenylpropanoid pathways.