Analysis of the resistance in genetically engineered papaya against papaya ringspot potyvirus, partial characterization of the PRSV.Brazil.Bahia isolate, and development of transgenic papaya for Brazil

Papaya ringspot is the most important virus disease of papaya ( Carica papaya L.) worldwide and is a limiting factor for the papaya industry in Brazil. The disease is caused by papaya ringspot potyvirus (PRSV). Control of papaya ringspot is difficult because PRSV is rapidly transmitted by several aphids species and genetic resistance is not present in Carica paypaya. PRSV-resistant transgenic papayas expressing the coat protein (cp) gene of a PRSV strain from Hawaii were first developed in the early 1990's. The PRSV resistant transgenic line 55-1 was the parent of Rainbow and SunUp, the first PRSV-resistant transgenic varieties released commercially. Rainbow and SunUp are hemizygous and homozygous for the cp gene, respectively. Experiments suggested that resistance was due to posttranscriptional gene silencing (PTGS), an RNA-mediated type of protection. Furthermore, Rainbow was effective against PRSV isolates from Hawaii but not from other geographic regions.; This project was undertaken to develop deeper knowledge of the PRSV:transgenic papaya resistance system and to develop transgenic papaya that are resistant to PRSV from Brazil. The effects of five factors in this resistance system were studied; two from the plant side (transgene copy number and plant developmental stage), and three from the virus side (virulence, inoculum strength, and sequence homology) of the interaction. Transgenic lines 55-1 and 63-1, and five PRSV isolates (PRSV.HA, HA 5-1, Brazil.Bahia, TH, and HA/TH), were used in these studies. The results suggest that all but the virulence factor affect the fate of the transgenic papaya:PRSV interaction.; Developing transgenic papaya with the cp gene of PRSV.Brazil.Bahia was the approach used to get resistance to Brazilian isolates of PRSV. Translatable and untranslatable cp genes were introduced into papaya somatic embryos by microprojectile bombardment. A total of 54 transgenic lines were obtained (26 translatable and 28 untranslatable). Initial characterization revealed resistance to PRSV in some lines. These transgenic lines are the result of a technology transfer program developed between Cornell University and Embrapa, and should provide an avenue for controlling PRSV in Brazil.