Fruits, Seedless
The absence of developed seeds in fruit improves its eating quality. Moreover, it may allow more uniform fruit production in different environments. Seedless fruit occurs when seed (embryo and endosperm) growth is inhibited or the seed dies early, while the remainder of the fruit continues to grow. Bananas and grapes are the most commonly available seedless fruits. Bananas are seedless because the parent banana tree is triploid (3X chromosome sets) even though pollination is normal. Generally, species with a chromosome set number divisible by two (e.g., 2X or 4X chromo-some sets) are capable of seed production while uneven sets of chromosomes (e.g., 3X or 5X) are either sterile or do not produce seeds. After fertilization,banana fruit development can proceed normally but seed development is arrested because of the genetic imbalance. Plant breeders have also produced seedless watermelons (Citrullus lanatus). The first practical system for producing seedless watermelons was the 4X-2X hybridization method. Hybrid seed is produced from crossing a tetraploid (4X) female and diploid (2X) male. Seedless fruit is produced on the resulting triploid (3X) hybrids. Pollination occurs, but just as in bananas, the fruit continues to grow while seed growth is reduced or absent because of uneven sets of chromosomes.
Parthenocarpy, fruit development without pollen fertilization and seed set, can result in seedless fruits such as grapes, squash (Cucurbita pepo), and eggplants (Solanum melongena). The majority of table grapes and raisins are seedless. Thompson seedless grapes have a normal chromosome constitution and pollination but have specific genes causing seedlessness. In addition, seedless grapes are treated with the hormone gibberellin, which is applied early in fruit development. The application of this hormone increases the size and consistency of the fruit.
Squash and eggplants can exhibit facultative parthenocarpy, that is, parthenocarpy that occurs under environmental conditions where polli-nation and seed would not occur normally. Cool weather (early spring or late fall) or greenhouse growth conditions are the most common environments where this type of parthenocarpy occurs. Commercial-quality fruit can be produced in cool environments or in greenhouse winter production locations where pollination is limiting. Under summer growth conditions, normal pollination, fruit production, and seed set occurs. Hormone treatments cannot be reliably used under these conditions to induce parthenocarpy or increase fruit size. Seedless summer squash, zucchini, has been obtained by crossing two varieties, DG4 and Striata. This parthenocarpic variety is stable and has been used to produce additional commercial squash hybrids.
Genetic engineering has been used to produce facultative parthenocarpic eggplants. A two-part gene transferred into eggplants consists of DefH9 from snapdragon (Antirrhinum majus) and iaaM from a fungus (Pseudomonas syringae). The iaaM gene produces the plant hormone auxin, while the DefH9 component restricts expression of the hormone gene to the immature fruit of the eggplants. Thus, the eggplants apply their own hormone treatment at the appropriate time and place. Conventional application of auxin on the surface of the fruit is ineffective in this case. Normal eggplant fruit are produced even when pollination does not occur, such as in winter greenhouse production. Fruit set and seed set are normal under favorable pollination conditions. In the future, this method could also be used for producing seedless fruits of many different species.
Breeding; Fruits; Hormones; Polyploidy; Propagation; Reproduction, Sexual.
Bibliography
Mohr, H. C. "Watermelon Breeding." In Breeding Vegetable Crops. Edited by Mark J. Bassett. Westport, CT: Avi Publishing Company, Inc., 1986.
Rotino, G. L., E. M. Perri, M. Zottini, H. Sommer, A. Spena. "Genetic Engineering of Parthenocarpic Plants." Nature Biotechnology 15 (1997): 1398-1401.
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