From QTLs for enzyme activity to candidate genes in maize

In order to facilitate the search for genes underlying QTLs (QuantitativeTrait Loci), the activities of key enzymes of the carbohydrate metabolismin maize, and the concentration of their substrates or products were usedas quantitative traits. For each of the chosen enzyme, i.e. ADPglucosepyrophosphorylase, sucrose-phosphate-synthase and invertases, thecorresponding cDNA was available. Since biochemical traits are more closelyrelated to gene expression than agronomic traits, co-locations could beexpected between an enzyme structural gene and a QTL for its enzymeactivity, and/or the corresponding product or substrate content. Thisapproach was applied using recombinant inbred lines on leaves at 3- or4-leaf stage, under control and water stress conditions and on grain, atmaturity. Several QTLs were detected for each trait, particularly for twoenzyme activities measured in mature leaves. Apparent co-locations betweenQTL for activity and structural locus were observed forsucrose-phosphate-synthase (chromosome 8) and acid-soluble invertase(chromosome 2 and 5). Leaf acid-soluble (vacuolar) invertase provided aninteresting case since QTL, on chromosome 5, explaining 17% of variabilitywas apparently co-located with the Ivr2 gene encodinga vacuolar invertase protein which was strongly water-stress inducible.Similarly, in grain, an amylose QTL co-located with theSh2 gene of ADPglucose pyrophosphorylase. Thereliability of this candidate was further tested through the examination ofSh2 DNA polymorphism in 46 genetically unrelatedlines. A correlation was obtained between this polymorphism and kernelstarch content, which further validated Sh2 as acandidate. Some improvements or alternatives to this strategy are brieflydiscussed.