The genetic basis of chlorogenic acid synthesis in maize

Chlorogenic acid (3-O-caffeoylquinate; CGA) is a secondary metabolic product of the phenylpropanoid biochemical pathway synthesized in nearly all plants. Chlorogenic acid confers a broad range of biological activities, but our interest in CGA stems from its potential antibiosis activity against corn earworm (Helicoverpa zea Boddie; CEW) larvae feeding on maize (Zea mays L.) silks. Although extensive research has been performed on the general phenylpropanoid pathway and much is known about broad range of products synthesized, the genetic basis of CGA variation is still ambiguous. To shed light on the genes involved in CGA biosynthesis, quantitative trait locus (QTL) analyses were performed on three F₂ populations derived from maize inbred lines. Two QTLs located in consistent positions across populations exerted large effects; one corresponded to the location of the p gene and the other had no phenylpropanoid candidate gene. These QTLs interacted epistatically to increase CGA and to concomitantly decrease flavones.; To develop a model explaining the genetic effects of these QTLs, relative transcript abundance was measured for five genes in the phenylpropanoid pathway: phenylalanine ammonia-lyase (pal), cinnamate 4-hydroxylase (c4h), 4-coumaroyl-CoA ligase (4cl), cinnamyl alcohol dehydrogenase (cad), and chalcone synthase (chs ). Transcript levels were compared in plants with contrasting alleles of the two QTLs. Alleles of the p locus that conditioned high CGA expression in the silk, strongly induced transcript levels of chs. One allele, p-wrb, also induced pal transcripts. The second QTL, named qtl2, increased cad transcripts in the absence of functional p alleles, and decreased chs transcripts two-fold in the presence of functional p alleles. These results demonstrate that the p gene induces the phenylpropanoid and flavonoid pathways through pal and chs transcription regulation, and qtl2 diverts substrates from flavonoid synthesis into CGA. These transcript differences were consistent with the epistatic interaction detected by QTL analysis.; Another objective of this research was to clarify the role of CGA as an antibiotic agent of CEW larval feeding in maize. Chlorogenic acid has a structure that is related to maysin, a C-glycosylflavone active against CEW in resistant maize genotypes. Larval growth bioassays demonstrated that CGA was less effective than maysin for antibiosis against CEW.