Other fruit characteristics

Fruit development is promoted by auxin produced by seeds developing within the ovary walls. Parthenocarpic fruit or fruit which develop in the absence of ovule fertilization, have the obvious advantage of being seedless and in tomato also have been reported to have elevated soluble solids content. Parthenocarpic tomato fruit were generated by the transgenic expression of a microbial gene (iaaM) that encodes a tryptophan monoxygenase used in synthesizing an auxin precursor (Ficcadenti et al., 1999). The iaaM gene from Pseudomonassavastanoi was linked to the  DefH9 promoter from Anthirrhinum majus that specifies expression in placental and ovule tissues, resulting in auxin production targeted to the ovary tissues destined for fruit development. As in other plants, the additional IAA in the developing fruit promoted fruit formation in the absence of pollination or ovule fertilization. The composition, size and abundance of the fruit from these plants are indistinguishable from control fruit.

Similar strategies have been used to alter the levels of other plant hormones in developing tomato fruit. The fruit-specific expression of a bacterial ipt gene specifying elevated cytokinin biosynthesis in ovary tissues of tomato resulted in increased total and soluble solids (Martineau et al., 1995). The fruit from these plants had islands of green pericarp tissue and excess cytokinin from fruit was exported to the leaves causing increased accumulation of genes, such as PR-1 and chitinase, induced by cytokinin (Martineau et al., 1994).

Transgenic approaches to alter fruit size and shape have not been reported. However, the identification of the QTL locus, fw2.2, in the introgressed population between L. esculentum and L. pimpinellifolium (Frary et al., 2000), may suggest candidate genes that could be introduced transgenically into tomato varieties which will influence tomato fruit size.