The possible role of the sucrose-splitting enzymes sucrose synthase and invertase in elongating roots and hypocotyls of Arabidopsis was tested by using a combination of histochemical methods and quantitative trait locus (QTL) analysis. Lengths of roots and hypocotyls correlated better with invertase activities than with sucrose synthase activities. The highest correlations were observed with activities in the elongating zones of roots. The genetic basis of these correlations was studied by using QTL analysis. Several loci, affecting invertase activity, colocated with loci that had an effect on root or hypocotyl length. Further fine mapping of a major locus for root length, but not for hypocotyl length (top chromosome 1), consistently showed colocation with the locus for invertase activity containing a gene coding for a vacuolar invertase. The analysis of a functional knockout line confirmed the role of this invertase in root elongation, whereas other invertase genes might play a role in hypocotyl elongation. Thus, we show the power of QTL analysis, combined for morphological and biochemical traits, followed by fine-mapping and mutant analysis, in unraveling the function of genes and their role in growth and development.Arabidopsis natural variation ͉ sucrose synthase ͉ hypocotyls T otal plant yield depends on the acquisition of raw material, i.e., photosynthesis and mineral (plus water) uptake, and on the ability of the plant to cope with stress. However, the economic yield of a crop is to a large extent also determined by the partitioning of dry matter over the harvestable and nonharvestable parts of the plant. The molecular and physiological basis of the regulation of assimilate partitioning in plants is still poorly understood. In terms of biomass, the most important components in assimilate partitioning and in total yield are carbohydrates. There is increasing evidence that a limited number of key enzymes, involved in primary (carbohydrate) metabolism, might be pivotal in this process (1).Functionally, a plant can be divided into sources (the sites of assimilate production) and sinks (the sites of use and͞or storage). Sinks can either be rapidly growing, expanding organs, such as elongating stems and roots, or storage sinks accumulating reserves, such as fruits, seeds, or tubers (2).In most plant species, carbon is transported from source to sink in the form of the disaccharide sucrose. Upon arrival in the sink, sucrose has to be hydrolyzed. In plants, two pathways are available for sucrose cleaving: via invertase (Inv), yielding glucose and fructose, and via sucrose synthase (Susy), yielding fructose and UDP-glucose. In several cases, it has been suggested that sink strength might depend on the activities of these sucrose-splitting enzymes. There is increasing evidence that in storage sinks, the predominant pathway is via Susy, whereas in growing sinks, the Inv route is most important. In potatoes, the elongating rhizomes (stolons) exhibit high Inv activity, whereas a switch toward Susycatalyzed sucrose bre...