Papaya
Ringspot Virus, (PRSV) is a major constraint in papaya production and
marketing in the Caribbean. This disease weakens the plant, stunting its growth
and causes disfigured mottled leaves and fruit. The productive life of a tree
is reduced from three years to one year or less. Tolerance, but not resistance,
has been observed in a few papaya varieties. Papaya varieties have been
selected that begin fruiting within three feet from the ground which allows for
production before this virus takes its toll. Since natural resistance to PRSV
does not exist in Carica papaya, to control this viral disease,
papayas are being genetically engineered with the coat protein (CP) gene from
local PRSV strains. Six gene constructs of PRSV-CP have been made from two PRSV
strains on St. Croix, USVI that are being used to genetically engineer
cultivars of papaya. In vitro somatic embryos, derived from immature zygotic
papaya embryos, has been successful in 20 papaya cultivars. Through the application
of recombinant DNA technology and in vitro selection, transgenic somatic papaya
embryos are being propagated, matured and germinated. Transgenic seedling are
being evaluated for stable incorporation of PRSV-CP and resistance to PRSV
under greenhouse and field conditions. Controlled papaya breeding is being used
to incorporate PRSV resistance into other papaya cultivars of importance to the
USVI.