Locating QTLs for carbon metabolism and early growth, using candidate gene approach
--Mathilde Causse and Jean Paul Rocher

Genetic determinism of physiological and early growth traits in maize has been studied in a population of recombinant inbred lines using the QTL/RFLP methodology. Sixty five F6 lines, derived from the cross between F2, an early flint line and an Iodent line, were grown in controlled conditions in a greenhouse, until the third expanded leaf stage. Growth measures concerned leaf size, growth duration and dry matter weight. The carbon metabolism was characterized by the concentrations of carbohydrates and the activities of four key-enzymes : sucrose phosphate synthase (SPS, which regulates sucrose synthesis), ADP-glucose pyrophosphorylase (AGPase, which regulates starch synthesis), and invertase and sucrose synthase (INV and SuS, which both hydrolyze sucrose in sink organs). Differences between parental mean values, and a wide range of variation in RILs have been found for growth as well as for the physiological traits. Strong correlations were found between growth traits and invertase activity, which reflects sink organ strength.

The population has been genotyped for more than 100 RFLP marker loci and a genetic map was constructed (see companion paper). QTLs, located on thirteen chromosomal regions, were detected (by one-way ANOVAs, p<0.01) for every trait. Between one and four QTLs were detected for every trait, with R2 values (determination coefficient) between 0.07 and 0.35. Each chromosomal region frequently concerned more than one trait, and common locations of QTLs for growth traits and activity of enzymes was observed in 3 of the 13 regions (in 8 of these 13 regions when decreasing the probability threshold to 0.05). For instance, a segment on chromosome 8 exhibited QTLs for invertase activity (with R2=0.35) and dry matter weight (with R2=0.20). QTLs common to these traits also appeared on chromosome 10. On chromosome 9, a region was found where QTLs were detected for growth duration until the 3rd expanded leaf stage, SuS and AGPase activities. These common locations possibly reflect the impact of the physiological traits on growth characteristics.

We mapped loci corresponding to the structural genes of 3 of the 4 studied enzymes. Some of the genes coding for the key-enzymes were located close to or at the most likely position of the QTL for the activity of the enzyme. This emphasizes the role of these candidate genes in physiological processes. For AGPase, the gene L2 coding for the enzyme form expressed in leaves (cloned by Prioul et al, Plant Physiol., in press), unmapped until now, mapped on chromosome 1, near umc58. Hybridization with Sh2 and Bt2 clones, the two isoforms expressed in endosperm, revealed homology with 7 other loci, 3 with Bt2 and 4 with Sh2. The only QTL detected for AGPase activity did not map near one of these loci. For the sucrose synthase, the two genes Sh1 and Css1 mapped as expected on chromosome 9. A QTL for the activity of this enzyme was found in the Sh1 region, suggesting a possible involvement of this gene in the expression of its activity. For SPS, three loci were mapped on chromosomes 3, 6 and 8. A QTL for the SPS activity was found near the QTL on chromosome 8. The role of an allelic variation at these candidate loci in the activity of their enzyme still remains to be proven, as confidence intervals of QTL location are very large. Mapping the invertase gene would be also of a great interest. Finally, the carbohydrate enzyme loci were found to be involved in epistatic interactions more frequently than anonymous loci, suggesting their implication in regulation networks. 

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