Quantitative trait loci and metabolic pathways: genetic control of the concentration of maysin, a corn earworm resistance factor, in maize silks.

Abstract

Interpretation of quantitative trait locus (QTL) studies of agronomic traits is limited by lack of knowledge of biochemical pathways leading to trait expression. To more fully elucidate the biological significance of detected QTL, we chose a trait that is the product of a well-characterized pathway, namely the concentration of maysin, a C-glycosyl flavone, in silks of maize, Zea mays L. Maysin is a host-plant resistance factor against the corn earworm, Helicoverpa zea (Boddie). We determined silk maysin concentrations and restriction fragment length polymorphism genotypes at flavonoid pathway loci or linked markers for 285 F2 plants derived from the cross of lines GT114 and GT119.Single-factor analysis of variance indicated that the p1 region on chromosome 1 accounted for 58.0%o of the phenotypic variance and showed additive gene action. The pi locus is a transcription activator for portions of the flavonoid pathway. A second QTL, represented by marker umclO5a near the brown pericarpi locus on chromosome 9, accounted for 10.8% of the variance. Gene action of this region was dominant for low maysin, but was only expressed in the presence of a functional pi allele. The model explaining the greatest proportion of phenotypic variance (75.9%) included pi, umclOSa, umcl66b (chromosome 1), ri (chromosome 10), and two epistatic interaction terms, pi x umclO5a and pi x ri. Our results provide evidence that regulatory loci have a central role and that there is a complex interplay among different branches of the flavonoid pathway in the expression of this trait.Development of molecular-marker linkage maps in many species facilitates the identification of chromosome regions associated with variation in quantitative traits (1). By dissecting the continuous phenotypic variation typical of many traits into contributions from discrete genetic factors, quantitative trait locus (QTL) studies provide insights into trait inheritance and genome organization, and often are sufficient to initiate marker-assisted selection. However, the biological interpretation of QTL data is generally limited by lack of knowledge of the genetics, biochemistry, and physiology underlying trait expression. To advance the level of QTL interpretation, we analyzed variation in an economically important trait that is determined by a well-characterized genetic and biochemical pathway.The corn earworm (CEW) is a major silk-and kernelfeeding insect pest of maize in the United States and parts of Latin America (2, 3). Host-plant resistance to CEW results from both antibiosis due to chemical factors in silks (stylar/ stigmatic tissue), and morphological features such as tight covering of the ear by husk leaves (4). Understanding of the nature of antibiosis to CEW was advanced when maysin (Fig. 1), a C-glycosyl flavone that inhibits CEW larval growth, was isolated from silks of the Mexican maize landrace "Zapalote Chico" (5). Later, Wiseman et al. (6) found a highly significant relationship between increased silk maysin concentration and reduced earworm ...

A Segmental Deletion Series Generated by Sister-Chromatid Transposition of Ac Transposable Elements in Maize

“…Although our data do not rule out the possibility of an additional factor involved in maysin biosynthesis located between p1 and p2, the simplest interpretation of our results is that the p2 gene is sufficient to confer weak maysin levels, while p1 and p2 together produce higher maysin levels and stronger silk browning. These results further support the hypothesis that the p1 and p2 genes are essential coregulators of maysin biosynthesis (Byrne et al 1996;P. Zhang et al 2003;Szalma et al 2005).…”

“…The region in the vicinity of p1 has been identified as a major QTL for the control of levels of silk maysin, a C-glycosyl flavone that deters feeding by corn earworm (Byrne et al 1996;Lee et al 1998;McMullen et al 1998). Maysin accumulation is correlated with a phenotype termed silk browning, in which the cut ends of silks turn brown due to the oxidation of flavones (Byrne et al 1996;Lee et al 1998;McMullen et al 1998;Guo et al 2001;Rector et al 2003).…”

“…Maysin accumulation is correlated with a phenotype termed silk browning, in which the cut ends of silks turn brown due to the oxidation of flavones (Byrne et al 1996;Lee et al 1998;McMullen et al 1998;Guo et al 2001;Rector et al 2003). Both p1 and p2 genes are expressed in maize silk (Zhang et al 2000), and both encode highly similar R2R3-Myb proteins with a similar potential to activate flavonoid biosynthesis in transgenic cell lines (P. .…”

“…In the following generation, plants were screened for the presence of colorless tassel glume margins to distinguish homozygous p1-ww plants from sibling plants heterozygous or homozygous for the P1-wr allele, as described previously . In addition, putative mutant plants were also screened for the occurrence of browning at the cut ends of silks (Levings and Stuber 1971), an indicator of the presence of maysin, a C-glycosyl flavone whose synthesis is coregulated by the p1 and p2 genes (Byrne et al 1996;P. Zhang et al 2003;Szalma et al 2005).…”

Candidate gene analysis of organ pigmentation loci in the Solanaceae

“…In maize, the only case to date where the complete set of genes from a pathway has been examined, the maysin content of corn silks was associated with alleles of both regulatory and structural loci (1)(2)(3)(4). Other results from both maize (5,6) and Arabidopsis thaliana (7) have shown that variability in quantitative measurements of overall enzymatic activity can be attributed to structural loci or closely linked elements.…”