1.12.12
RESISTANCE OF TRANSGENIC PAPAYA WITH COAT PROTEIN (CP) GENE OF PAPAYA RINGSPOT VIRUS IS AFFECTED BY THE CP GENE OF THE ATTACKING VIRUS HYBRID

CH CHIANG1, FJ JAN1, SD YEH2 and D GONSALVES1

1Department of Plant Pathology, Cornell University NYSAES, Geneva, NY 14456, USA; 2Department of Plant Pathology, National Chung University, Taichung, Taiwan, ROC

Background and objectives
Papaya ringspot potyvirus (PRSV) is the major limiting factor for economic papaya production throughout the tropics and subtropics. Efforts to control PRSV have had limited success either by conventional breeding or by selecting tolerant varieties. However, the strategy of pathogen-derived resistance has recently provided a papaya line, designated line 55-1, with effective resistance against PRSV from Hawaii [1]. Line 55-1 contains the CP transgene of PRSV HA 5-1 which is a mild nitrous acid mutant of the severe PRSV HA isolate from Hawaii. 'SunUp' and 'Rainbow' were subsequently developed from line 55-1, with SunUp being homozygous for the CP transgene and Rainbow being hemizygous. Furthermore, although Rainbow is resistant to PRSV isolates from Hawaii, it is susceptible to a number of PRSV isolates from different geographical regions of the world. Since the results suggested that resistance was RNA-mediated, differential reactions of the transgenic papaya to PRSV isolates may be due to differences in the CP sequences of the isolates and the CP transgene. In this study, we used a laboratory-generated PRSV CP recombinant to determine if the specific protection of Rainbow is influenced by the CP gene of the attacking virus.

Materials and methods
Rainbow is a hybrid derived from a cross of transgenic SunUp and nontransgenic 'Kapoho'. It contains the CP transgene of PRSV HA 5-1 which has 99.8% nucleotide identity to PRSV HA. A full-length cDNA clone of PRSV HA that generated in vitro infectious transcripts from a T3 promoter was recently developed [2]. Additionally, we constructed a recombinant hybrid of PRSV HA that contained a substituted CP gene of PRSV YK, a mosaic strain from Taiwan. The hybrid, designated pHA-3'YK, contained the sequence of PRSV HA except for the extreme 3' terminus which consists of 136 nt of NIb, the CP gene and the 3'-non-translatable region of PRSV YK. Capped PRSV HA-3'YK transcripts were mechanically applied to non-transgenic papaya. Tissue extracts from the originally infected non-transgenic papaya or from subsequently infected non-transgenic papaya were inoculated to SunUp and Rainbow. PRSV HA and YK viruses were also inoculated to transgenic papaya as controls.

Results and conclusions
In a number of greenhouse experiments, transgenic Rainbow showed typical systemic mosaic symptoms when inoculated with PRSV YK or PRSV HA-3'YK, but were resistant to PRSV HA. Transgenic SunUp reacted similarly; however, symptom development was delayed as compared to that of Rainbow. These results demonstrated that the specific resistance of the transgenic papaya Rainbow and SunUp is dependent on the CP sequence of the invading virus, since the hybrid virus genome was comprised of PRSV HA except for the CP gene of YK. Efforts are being made to determine the minimum differences in CP homologies and lengths that are required for the attacking virus to overcome the resistance of SunUp and Rainbow.

References
1. Fitch MMM, Manshardt RM, Gonsalves D et al., 1992. Bio/Technology 10, 1466-1472.
2. Chiang CH, Yeh SD, 1997. Botanical Bulletin of Academia Sinica (Taipei) 38, 153-163.