COMING SOON: 

Wider Selection of (Genetically Engineered) Seedless Fruit 

Douglas A Powell's Agriculture Network  / Feb98

Information Systems for Biotechnology News Report How many times have you picked up a piece of fruit, anticipating the first flavorful bite, only to be faced with the tedious task of picking out the seeds? The commercial success of seedless oranges and grapes shows consumers' eagerness for such easy-to- eat produce, but currently few fruits come in seedless varieties, and when available they may be more expensive. The higher cost reflects the difficulties of making a marketable seedless fruit, which requires either mutant lines, infertile hybrids, or costly and labor-intensive treatment of flowers with phyto-hormones.

But consumers should take heart, as Italian researcher Angelo Spena and colleagues in Italy and Germany have developed a new and elegant method for tricking plants into making fruits with no seeds. In normal plant reproduction, a fruit is formed only after successful fertilization of the ovule in the flower ovary. Levels of auxin, a plant hormone, rise in response to fertilization, and stimulate seed growth and formation of fruit tissue surrounding the seed. In some cases fruit development can occur in the absence of fertilization, but this is the exception to the rule. Spena and coworkers produced transgenic eggplant and tobacco plants that set seedless fruit by engineering them to produce auxin in the unfertilized ovary.

To do this, the researchers took advantage of two unique genes. The first, isolated from the plant pathogenic bacterium Pseudomonas syringae pv. savastanoi, is the coding region of the iaaM gene which leads to the production of auxin in plant tissue. The second gene sequence is the promoter region of DefH9, a gene from snapdragons which is expressed specifically in ovules. They linked the two together and transformed tobacco and eggplant, thinking that localized expression of the auxin-producing gene in the ovule would mimic auxin synthesis in fertilized ovules, and thus lead to fruit development. Indeed, this is exactly what seems to happen.

Transgenic tobacco plants that expressed the new gene construct grew normally, but in the absence of fertilization produced smaller-than-normal capsules that contained only aborted seeds. However, if flowers of these plants were self-pollinated, they set normal capsules and fertile seeds. Similarly, transgenic eggplants showed vegetative development identical to that of untransformed control plants, but when emasculated to prevent fertilization, they set fruit that was equal in size and shape to fruit of fertilized flowers, yet contained no seeds. The weight of a typical fertilized eggplant fruit was about 250g. Fruits of transgenic, unfertilized eggplants were indistinguishable from this, while those few fruits that formed from unfertilized, untransformed plants weighed only 60g.

The transgenic seedless fruits were therefore of marketable size and quality. Transgenic plants had an additional important advantage over untransformed plants - they produced fruit under unfavorable weather conditions.Fertilization of most plants is dependent on favorable environmental conditions such as temperature, light intensity, and wind, and the absence of an optimal environment may result in incomplete fertilization. Eggplant is a warm weather crop and does not set fruit well under cool, short-day conditions. However, when grown under such unfavorable conditions the transgenic plants produced fruit where none was obtained from untransformed control plants. The ability to set fruit under conditions adverse for pollination adds a significant agronomic advantage to these plants and offers hope of extended growing seasons and increased yields along with the value of a seedless product.

It is also significant that seedless fruit is only obtained from flowers that have not been fertilized. Pollination of transgenic plants results in fruit with viable seed through which the genes for seedlessness may be passed to subsequent generations of plants. However, this means that seedless fruits may only be obtained when flowers are not pollinated. Incorporation of a male sterility gene will be required in most crops before seedless fruit can be produced under large-scale field conditions. Nevertheless, the results of these experiments are very encouraging and one can only hope that Spena and coworkers quickly turn their attention to cherries, plums, raspberries, pomegranates . . .

Reference

Rotino, GL, E. Perri, M. Zottini, H. Sommer, and A. Spena. 1997. Genetic engineering of parthenocarpic plants. Nature Biotechnology 15:1398-1401. Jim Westwood Dept. of Plant Pathology, Physiology and Weed Science, Virginia Tech westwood@vt.edu

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