5.3.12
TRANSFORMATION OF THAI PAPAYA FOR RESISTANCE TO PAPAYA RINGSPOT VIRUS

S CHOWPONGPANG1, M BATESON1, T BURNS1, R LINES1, S ATTATHOM2 and J DALE1

1Centre for Molecular Biotechnology, Queensland University of Technology, 2 George Street, Brisbane, Qld 4001, Australia; 2Plant Genetic Engineering Unit, Kasetsart University, Kamphaengsaen, Nakhon Pathom 73140, Thailand

Background and objectives
Papaya (Carica papaya) is one of the major fruit crops in Thailand, and is consumed in both the green and ripe states. However, papaya is susceptible to papaya ringspot virus (PRSV) which causes a severe disease in both papaya and cucurbits, resulting in major losses to production. There are two types of PRSV, PRSV-P which infects both papaya and cucurbits and PRSV-W which infects cucurbits only. There is no evidence of naturally occurring PRSV resistance in domestic cultivars of papaya. There have been many attempts to overcome PRSV, including the use of mild strain cross-protection and the crossing of commercial varieties of papaya with wild Carica species, with limited success. Pathogen-derived resistance (PDR) is the most promising method for control of PRSV and has been demonstrated against the PRSV-P isolate in Hawaii [1]. However, this PRSV resistance is strain-specific and therefore PDR must be targeted to PRSV strains from specific geographic regions. The objective of this Australian Centre for International Agricultural Research (ACIAR)-funded project is to generate transgenic Thai papaya incorporating the coat protein (cp) gene of the Thai isolate of PRSV-P (Th) for resistance to PRSV.

Results and conclusions
An efficient somatic embryo liquid induction system was developed using immature zygotic embryos [2]. Somatic embryos were bombarded with plasmid constructs conferring resistance to kanamycin (NPTII) and including the full-length PRSV-P (Th) cp gene. Germinating embryos were selected on kanamycin media and screened by PCR for the presence of the PRSV-P (Th) cp and NPTII genes. Of 13 individual lines of Thai papaya selected, 10 contained the cp gene. Individual lines were clonally multiplied and acclimatized in soil in a growth room. Transgenic papaya will be initially challenged using the Australia strain of PRSV-P (Aus) prior to further challenges in Thailand. It is expected that the transgenic Thai papaya will be overcome by the PRSV-P (Aus) strain. These and other results of the characterization of the transgenic Thai papaya will be presented.

References
1. Luis S, Manshardt RM, Fitch MMM et al., 1997. Molecular Breeding 3, 161-168.
2. Fitch MMM, Manshardt RM, 1990. Plant Cell Reports 9, 320-324.