Botany and Improvement 43
fields have been replaced by the resistant transgenic papaya and because many aban-
doned infected fields have since been destroyed. These conditions, along with judi-
cious isolation and rouging of infected plants, have enabled growers to continue to
produce non-transgenic papaya, especially to supply the Japan market, which does
not yet allow the importation of transgenic papaya. In January 2003, Canada allowed
the importation of transgenic papaya. Yet another benefit is that papaya acreage
has expanded on Oahu due to the use of PRSV-resistant transgenic ‘Rainbow’ (or
new hybrids that have been derived from ‘Rainbow’). Efforts for the development
of transgenic papaya have recently started with Bangladesh, Uganda and Tanzania.
Efforts in the first four countries (Brazil, Jamaica, Venezuela and Thailand) have
resulted in the development of resistant transgenic papaya that is suitable for their
country (Cai et al. 1999; Tennant et al. 2002). In fact, some of the transgenic papaya
are well advanced in field trials and are moving through the process of deregula-
tion. How fast the process will move, in the light of the current GMO climate, is not
known. However, it is clear that the PRSV-resistant transgenic papaya is a practical
solution for controlling PRSV, as has been shown in Hawaii. Conventional inter-
specific hybridisation is being used to cross papayas with resistant species like, C.
cauliflora (Chen 1992; Louw 1994; Lehane 1996), cultivars like, Cariflora and F 1
hybrid of the cross, Cariflorax Sunrise Solo (Conover et al. 1986; Escudero et al.
1994). Attempts are also under way to create genetically transformed plants resistant
to the PRSV and other viruses.
Mahon et al. (1996) have developed a method for the stable transformation and
regeneration of a dioecious papaya cultivar by micro-projectile bombardment. The
method was developed after investigating both zygotic and somatic embryos as tar-
get tissue and optimisation of a number of parameters using transient expression of
the uidA reporter gene. Caberera-Ponce et al. (1995) have also produced herbicide
resistant transgenic papaya plants using zygotic embryos and embryogenic callus as
target cells for particle bombardment. More emphasis has been laid on coat protein-
mediated protection through the transfer and expression of the PRSV coat protein
(cp) gene in papaya (Fitch et al. 1992; Lehane 1996; Fitch et al. 1998; Gonsalves
1998; Yeh et al. 1998). In their studies, Gonsalves et al. (1998) and Cai et al. (1999)
used 4-week-old somatic embryos for bombardment with particles containing the
non-translatable form of the coat protein (cp) gene of PRSV HA5-1 and observed
that the nine bombarded plates produced as many as 207 kanamycin-resistant clus-
ters over a period of 7 months. A total of 83 transgenic lines expressing the non-
translatable coat protein gene of PRSV were obtained from the somatic embryo
clusters that originated from immature zygotic embryos. Twenty-five transgenic
lines (out of 83) were resistant to the homologous PRSV isolate from Hawaii and
some of these were resistant to PSRV isolates outside of Hawaii, including Australia,
Taiwan, Mexico, Jamaica, Bahamas and Brazil. Transgenic plants possessing desir-
able characters particularly cp gene have also been produced through Agrobacterium
tumefaciens mediated genetic transformation (Yang et al. 1996; McCandless 1997;
Yeh et al. 1998). The primary results of the field tests of the cp transgenic plants have
indicated a great potential for the control of PRSV. Such cultivars are expected to
give the papaya production a big fillip. Another area which has drawn attention of
the biotechnologists is extension of postharvest life with sense and antisense versions