Advances in papaya biotechnology

Papaya (Carica papaya L.) is an important tropical fruit crop. The fruit is consumed fresh and used in the pharmaceutical, rayon and food industries. Papaya improvement for stress tolerance and qualitative traits using conventional breeding has been difficult due to the narrow germplasm pool in the Carica genus and sexual incompatibility problems encountered during intergeneric hybridization with other genera in the Caricaceae family. Genetic engineering is an important tool in papaya improvement for modifying one or more traits in elite cultivars without altering existing characteristics. Advances in genetic engineering have been facilitated by concerted efforts for genome sequencing of papaya, development of papaya regeneration systems and efficient gene insertion techniques for transfer of desirable traits.Papaya regeneration via organogenesis and somatic embryogenesis has been refined during the past 3 decades. Early efforts to optimize gene insertion protocols utilized a number of reporter and selectable marker genes, viral- and bacterial-derived regulatory sequences and functional genes for biotic and abiotic stress tolerance. Transgenic plants were routinely produced with several cultivars. One of the best success stories in the commercialization of a genetically modified fruit crop has involved the development of transgenic papaya ring spot virus (PRSV) resistant Rainbow and SunUp cultivars, which saved the Hawaiian papaya industry. Additionally, genetically modified papayas with traits for disease resistance and extended shelf life have been extensively screened in field tests.The papaya genome sequence was published in 2008 and has opened new avenues for papaya improvement by precision breeding, which involves the use of regulatory and functional gene sequences from related genera of the Caricaceae family, and is a logical extension of conventional breeding and genetic transformation. The application of precision breeding technology for papaya can pave the way for the development of consumer and eco-friendly cultivars that would be developed in ways similar to conventional breeding while causing fewer GMO-related concerns.