Fruit size

In some self-sterile species such as kiwi or strawberry and also in other seed-fruit species, like table grape or Citrus spp. where the seeds are disliked by consumers, the introduction of the parthenocarpy trait may allow control of fruit development even under environmentally prohibitive conditions for pollination and may be used in fruit crops to standardise and increase the fruit size. However, it is well known that in some fruits the quality of the parthenocarpy fruits are inferior to fruit containing seeds. Parthenocarpy trait is often polygenic and therefore more difficult to deal with breeding programmes. Methods for achieving parthenocarpy include spraying of growth regulators, genetic mutation, or altering plant ploidy level. Transgenic parthenocarpic tobacco, eggplant and tomato have been successfully obtained (Rotino et al. 1997, 1999). These plants contain the coding region of the iaaM gene for the enzyme tryptophanmonoxigenase in their genome which converts tryptophan to indolacetamide, a precursor of IAA, under the control of the placental-ovule-specific defh9 gene regulator sequence. The expression of chimeric defh9-iaaM starts during early flower development producing marketable fruit. These genes mimic the hormonal effects of pollination and embryo development by increasing the content and/or the activity of auxin specifically in the ovule. Among fruit crops, kiwi fruit, grape and strawberry are already transformed and awaiting field evaluation (Mezzetti, pers. com.).

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