Background: Papaya is a tropical-subtropical crop that is grown in back yards as well as in large-scale farms, it produces year round, many people in lesser-developed countries eat it, and it is arguably one of the most nutritious fruit crops, rich in vitamins A and C. The crop is easily grown, except for the fact that it can be severely infected by papaya ringspot virus (PRSV). PRSV is the most damaging virus of papaya worldwide. Trees infected at a young stage almost never bear fruit, and the fruit of trees that do bear show characteristics ringspots, can be deformed, and generally have lower sugar contents. Yields of infected trees are severely affected. Papaya in Bangladesh, like nearly all tropical countries, are severely affected by PRSV; in fact, PRSV is the main factor that limits papaya production in Bangladesh. The long-term goal of this project is to increase the health status of people in Bangladesh by making PRSV-resistant transgenic papaya readily available and affordable to households and farmers. Incidence to night blindness is over 3% of the population- all due to lack of a good source of vitamin A. This could be solved through increase consumption of this delicious fruit.

The dramatic impact that PRSV can have on a papaya industry was recently shown in Hawaii. Up until 1992, Hawaii enjoyed steady production of papaya with greater then 80% of their crop being exported out of state. In 1992, Hawaii accounted for more then 75% of the papaya imported to mainland US. In May 1992, PRSV was discovered in Puna, the main papaya growing area where 95% of Hawaii’s papaya was being grown, and by 1995, PRSV was widespread throughout the area. The papaya production steadily dropped from 53 millions pounds in 1992 to 26 million pounds in 1998. The papaya industry in Hawaii was on the way to virtual extinction. However, the impact of PRSV resistant transgenic papaya on restoring papaya production to the devastated areas is also dramatically shown in Hawaii. Using the concept of pathogen-derived resistance, a transgenic papaya was developed in 1991 that showed resistance to PRSV strains in Hawaii. Initial field trials in 1992 showed that the transgenic papaya was resistant under field conditions and that the papaya had excellent horticultural properties. The transgenic cultivars ‘Rainbow’ and ‘SunUp’ were developed and showed excellent resistance in field trials in 1995. These cultivars received regulatory approval and licenses were obtained for commercial release to growers in Hawaii in May 1998. The transgenic papaya has replaced infected papaya in Puna and other areas of Hawaii, such that the production of papaya in Hawaii is predicted to reach 50 million pounds in 2001, only three years after the release of the transgenic papaya in Hawaii.

In many tropical regions, PRSV is already well established and thus households or farmers cannot routinely grow healthy, high-yielding papaya. This is the case in Northeast Thailand and in Bangladesh. It has been suggested that in Bangladesh PRSV causes a yield reduction of 70-100%. Unfortunately, efforts to control PRSV through development of resistant cultivars have failed because resistance is not known in papaya (Carica papaya) and efforts to transfer resistant genes from other Carica species have failed because the resulting crosses are sterile. Cross protection, a form of immunizing plants with a mild strain of papaya ringspot virus to protect against infection with a severe strain, showed some measure of success in Hawaii. However, the mild mutant isolate developed in Hawaii could not provide sustained control of PRSV in other countries. The seriousness of the PRSV in Bangladesh has also prompted cross protection research as a possible means to control PRSV. Tolerant cultivars developed for Thailand are being used in Northeast Thailand. However, these cultivars become infected by PRSV and seeds must be produced under stringent conditions and progenies continually screened for tolerance because the multigenic nature of the tolerance trait makes it difficult to transfer to progenies. Furthermore, data from Thailand demonstrated that production by PRSV-resistant papaya is over 10 times greater than lines bred for tolerance.

The control of PRSV in Hawaii clearly shows that the pathogen-derived resistance approach is the most effective way to control PRSV. Bangladesh as a developing country requires biosafety and regulatory standards for the introduction, testing, and ultimately release of transgenics. Though papaya is not within the mandated crops, CIMMYT Bangladesh is offering to its partners at Cornell University and USDA, Hilo Hawaii, project management and leadership in biosafety and regulatory issues, using the current CIMMYT Agronomist with 20 years of experience in Bangladesh.

Objectives:

1. Characterize PRSV isolates in Bangladesh,
2. Engineer CP transgene constructs of representative PRSV isolates from Bangladesh,
3. Develop transgenic papaya with the PRSV transgene constructs and identify R0 lines that are resistant to PRSV,
4. Evaluate progenies of R0 plants for resistance and horticultural characteristics, and select promising lines for advanced testing,
5. Obtain regulatory and intellectual property rights approval for selected lines, and
6. Increase the seeds of selected transgenic papaya lines or cultivars and subsequently release seeds to farmers and households.

Target Germplasm: Shahee variety of papaya released by the Bangladesh Agriculture Research Institute

Gene(s): Coat protein gene of the virus

Partner(s): Cornell University, Bangladesh Agriculture Research Institute (BARI), Bangabandhu Sheikh Mozibur Rahman Agricultural University (BSMRAU)

Status: Viral strains have been collected within Bangladesh and are being studied for homology to a synthetic gene being produced in the USDA lab in Hawaii. If found homologous, then transformation with the synthetic gene will be made and tested. CIMMYT has assisted the Bangladesh government in developing biosafety and regulation of transgenics in collaboration with USAID-funded projects ABSP-II and PBS within the country.

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