Biotechnology of Papaya

Papaya has become a model for other fruit crops with respect to biotechnology applications. This is due largely to excellent control of in vitro development gained over two decades of research. Micropropagation has been highly developed in subtropical regions to stabilize selected dioecious genotypes. Embryogenic regeneration, obtained efficiently from several kinds of immature tissues in the presence of auxins, is nonetheless often difficult to achieve on selective media employed in transformation procedures; however, improved methods of preparing embryogenic cultures are likely to overcome this bottleneck. Anther culture for production of dihaploid inbred lines is an under-researched objective that merits more attention. The relative ease with which Carica interspecific hybrids can be produced by embryo or ovule rescue has directed research activity away from protoplast fusion. Papaya has been hybridized with six other Carica species, and some hybrids inherit useful disease resistance genes, but serious fertility barriers limit breeding efforts. Phylogenetic analyses based on isozyme and DNA polymorphisms indicate that papaya is a distant relative of other members in the genus. The greatest economic impact of biotechnology on papaya industries in the near term will result from creation of virus-resistant cultivars through genetic transformation with coat protein (cp) genes. Multi-year field tests in Hawaii have demonstrated the effectiveness of translatable cp genes against local strains of papaya ringspot virus (PRSV). Greenhouse inoculations in New York and Taiwan have revealed narrow specificity and broad range resistance in different transformants, with gene dosage being a major determining factor. Untranslatable cp genes and replicase genes have also been introduced into papaya and provide protection against PRSV in greenhouse tests. Research to extend postharvest shelf life with sense and antisense versions of the ACC synthase gene is in progress in Hawaii and Australia, and transgenic plants are under observation in the greenhouse. A RAPD (randomly amplified polymorphic DNA) map of the papaya genome has been generated and used to identify markers for sex-determination, flowering height, and fruit carpellody.