From QTLs for enzyme activity to candidate genes in maize

In order to facilitate the search for genes underlying QTLs (Quantitative T rait Loci), the activities of key enzymes of the carbohydrate metabolism in maize, and the concentration of their substrates or products were used as quantitative traits. For each of the chosen enzyme, i.e. ADPglucose pyropho sphorylase, sucrose-phosphate-synthase and invertases, the corresponding cD NA was available. Since biochemical traits are more closely related to gene expression than agronomic traits, cc-locations could be expected between a n enzyme structural gene and a QTL for its enzyme activity, and/or the corr esponding product or substrate content. This approach was applied using rec ombinant inbred lines on leaves at 3- or 4-leaf stage, under control and wa ter stress conditions and on grain, at maturity. Several QTLs were detected for each trait, particularly for two enzyme activities measured in mature leaves. Apparent cc-locations between QTL for activity and structural locus were observed for sucrose-phosphate-synthase (chromosome 8) and acid-solub le invertase (chromosomes 2 and 5). Leaf acid-soluble (vacuolar) invertase provided an interesting case since a QTL, on chromosome 5, explaining 17% o f variability was apparently cc-located with the Ivr2 gene encoding a vacuo lar invertase protein which was strongly water-stress inducible. Similarly, in grain, an amylose QTL co-located with the Sh2 gene of ADPglucose pyroph osphorylase. The reliability of this candidate was further tested through t he examination of Sh2 DNA polymorphism in 46 genetically unrelated lines. A correlation was obtained between this polymorphism and kernel starch conte nt, which further validated Sh2 as a candidate. Some improvements or altern atives to this strategy are briefly discussed.