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Consumers have a special liking for seedless (parthenocarpic) fruits because seeds of some fruits are hard, taste bad or otherwise cause moderate to severe digestive problems if ingested. In addition, since seed cavities in seedless fruits are filled with fruit tissues, seedless fruits provide more product to the consumer. Moreover, the shelf life of seedless fruits is typically longer than seeded cultivars and they can be grown out of season since pollination is not required.
In an article published in the June 2000 issue of Trends in Biotechnology (18(6):233-42.) Dr. Patrick Gallois and his associates review recent advances in genetic engineering of seedless fruit. Dr. Patrick addresses in this review the question of whether parthenocarpy (fruit development without fertilization) can be induced in non-pathenocarpic species and whether parthenocarpy can be improved where seedlessness is only partial or accompanied by misshapen and undersized fruits.
The review begins by reporting studies involving isolation of genes responsible for or associated with parthenocarpy. The comparison of a parthenocarpic tomato cultivar and near-isogenic (genetically of the same constitution except for the alien gene) non-parthenocarpic line (pat-2), revealed a differential expression for at least six gene products. The hope is that insertion of such candidate genes will generate seedless fruits in non-parthenocarpic plant species. This strategy will gain further momentum when more genes are identified in Arabidopsis mutants with parthenocarpic fruits and transgenics are produced using these genes.
The authors then describe the methods of a number of patents based on the principle that there is an increased amount of auxin in the ovary of parthenocarpic cultivars and that external application of auxin, gibberellins, and cytokinins induces parthenocarpy. The patents which are featured in this review are: fusion of an ovary-specific promoter with the Agrobacterium RolB gene to enhance auxin production; fusion of PDefH9, a snapdragon promoter gene with RolB gene to produce parthenocarpy; and insertion of a gene encoding an isopentenyl transferase/ tryptophan oxygenase (which acts on auxin precursor) in nonparthenocarpic plants to induce seedlessness. The methods described in the above patents have been successfully employed only in obtaining seedless eggplants and tobacco. Some of these methods have certain limitations in that emasculation has been found necessary to produce seedlessness in eggplants.
The other two important patents described in the review, deal with methods of combining two genes, which by themselves are nontoxic but lethal when brought together in the hybrid. In one of these procedures, two nontoxic genes, IamS and IamH , which are expressed in the seed coat of separate F1 individuals, when combined in F2s produce lethal effects, destroying the seeds. In the other procedure, a site-specific recombination gene, Cre (causes recombination) and a recombination site loxP (locus for crossing over) from bateriophase P1 are used. One of the parents, used in this procedure, contains the toxic gene (barnase) flanked by two loxP genes, one on each side and the second parent contains Cre gene which causes crossover. As a result of crossover in the F1 at the specific site, the toxic gene barnase is activated with the elimination of one of the two loxPs. The activated barnase destroys the seed coat causing seed abortion.
The authors conclude that with continual improvements of methods, in the near future, it may be possible to bioengineer parthenocarpy into a wider range of fruits and vegetables.