Genetics of the B-ring modifications of the C-glycosyl flavones in maize silks

Natural resistance to corn earworm (CEW) has been attributed to the presence of the C-glycosyl flavones maysin, apimaysin and methoxymaysin in corn silk tissues. These compounds can be distinguished by their 3′ modifications of the B-ring. Maysin has been demonstrated, in bioassays, to be twice as effective than apimaysin or methoxymaysin against CEW. Higher efficacy of maysin has been attributed to the presence of the two hydroxyl groups, present at the 3′ and 4′ position, in the B-ring. For a better understanding of the genetic mechanisms underlying the synthesis of these compounds, we conducted a quantitative trait locus (QTL) study with three populations. The objectives of this study were (i) to determine the chromosome regions in the maize genome associated with the synthesis of maysin, apimaysin, methoxymaysin, and chlorogenic acid, (ii) to analyze the genetic effects of the QTLs, (iii) to map the candidate genes for the main QTLs, and (iv) to establish a model for the genetic control of the synthesis of maysin, apimaysin and methoxymaysin.; Our results show that the largest QTL responsible for the synthesis of apimaysin mapped within the pr1 region (on chromosome 5). We mapped the flavonoid 3′hydroxylase (f3′h) gene in this area, demonstrating that the pr1 locus encodes the f3′h gene. We also detected a second copy of this gene on the short arm of chromosome 4. A second major QTL with pleiotropic effects for the synthesis of chlorogenic acid, maysin, and methoxymaysin mapped in the short arm of chromosome 4 (we designated this locus as mxm1). Our best candidate gene for the genetic effects of mxm1 locus is the second hydroxylase gene that mapped to the same region.; Because of the genetic effects of the mxm1 locus on the synthesis of these compounds, we have concluded that the synthesis of C-glycosyl flavones does not follow a single linear pathway as thought before, but rather by multiple, interacting pathways. Our data indicate that the B-ring modifications for the synthesis of C-glycosyl flavones in maize silks are likely to occur in the general phenylpropanoid pathway.