Progress 10/01/03 to 09/30/06
The goal of this project was to develop and produce transgenic papayas with Papaya Ringspot Virus (PRSV) resistance for use in Guam, Saipan and the Northern Marianas, the most important island groups in the American Pacific producing a significant amount of papayas for their local economy. This virus, PRSV, and regularly occurring typhoons are the most serious constraints to papaya production (and papaya field experiments) and in the region. Originally, we had intended to produce transgenic lines utilizing the coat protein gene (CP) of a PRSV isolate from Guam because transgenic PRSV resistance is related to the degree of homology between the transgene and the virus strain targeted for control. As CP gene homology decreases below 96%, transgenic resistance decreases in a direct manner. In addition, we conducted experiments jointly with G. Wall in Guam to evaluate the potential for PRSV control in Guam using the Rainbow and SunUp transgenic papaya lines developed for
use in Hawaii. We postulated that PRSV isolates or strains from the Pacific region were likely to share enough CP gene homology to confer at least some transgenic PRSV resistance. The first trial to evaluate Rainbow and SunUp was destroyed by a hurricane before any substantial data was obtained. It was reinstalled the following year, and we determined that Rainbow and SunUp conferred substantial resistance, though not complete immunity, to PRSV in Guam. Rainbow infection rates were 8.2% while SunUp plants did not become infected after 14 months in the field. Consequently, the project objectives were modified to use conventional breeding approaches for developing papaya lines for use in Guam. To accomplish this modified objective, crosses were made with SunUp (homozygous for the resistant transgene) and Saipan Red, a red-flesh papaya line grown locally in Guam and Saipan. The F1 lines were selfed and backcrossed to Saipan Red to develop backcross lines that were 75 and 87.5% similar to
Saipan Red. These backcross generations are yet to be evaluated for adequacy of the transgenic resistance and to confirm the horticultural quality Saipan Red. In addition, results of another project of the PIs on broadening transgenic resistance in papaya indicates that a segmented-gene approach utilizing segments of the CP gene from different PRSV strains provides resistance to a broad range of PRSV isolates from around the world. These segmented gene lines ought to be evaluated for their potential in Guam. Both the backcross lines described above and the selected segmented gene lines will need to be evaluated for their utility in American Pacific region. This proposed work is being developed into a new project.
Growers in Guam, Saipan and the Northern Marianas can now produce papayas without serious risk of loss from PRSV, a virus that, in addition to damage by hurricanes, seriously limits papaya production in the region. Both of the Hawaii transgenic lines, Rainbow and SunUp, as well as backcross lines with the local papaya line, Saipan Red, have been developed for use in the American Pacific. Results from a related project, which utilizes a segmented gene approach, suggests that even broader PRSV resistance will be available for use in this pacific island region.
- No publications reported this period
Progress 10/01/04 to 09/30/05
The purpose of this project is to develop transgenic papaya varieties for use in Guam. Because resistance is related to the degree of homology to coat protein (CP) transgene used to transform papaya, we have developed a transformation construct utilizing the CP gene of PRSV from a PRSV isolate from Guam. This construct is currently being used to transform Kapoho, Sunrise, and Saipan (a papaya variety grown in Guam). The initial transformation experiments have been conducted, and cultures are just being subjected to selection in tissue culture. We expect to have transgenic plants growing in the greenhouse by the end of the next reporting period. We anticipate that these transgenic lines will be used directly (after selfing to produce a homozygous transgenic line) or used for crossing with local varieties to select local transgenic papaya lines for Guam. In addition, a field trial has been established in Guam to determine the utility of Rainbow and SunUp transgenic
resistance (which utilizes the Hawaii CP gene) in Guam. The trial is about 3 months old, and plants have not become infected by PRSV. This result suggests that the Hawaii CP gene might provide an appropriate level of PRSV resistance in Guam. In addition, we are arranging to evaluate several multi-segmented CP gene transformant lines in Guam. These multi-segmented transformant lines utilize segments of the transgene from 3 different PRSV isolates from Thailand, Taiwan, and Hawaii in order to increase CP gene homology to most if not all isolates of PRSV, thus conferring broad PRSV resistance. Preliminary findings in another project has confirmed this approach to broad PRSV resistance, and we therefore intend to evaluate some of these multi-segmented gene transformants for their utility in Guam. Permits are being obtained and this trial will be installed in the next few months. Evaluations should be well underway by the end of the next reporting period.
At the conclusion of these studies, we will have several transgenic papaya lines for use in Guam and the surrounding pacific area that are highly resistant to PRSV which has caused serious losses to papaya growers in the region. We expect that the resistance will be broad and growers should not be threatened by PRSV isolates from elsewhere in the world.
- No publications reported this period