The Dialectic of Pro-Poor Papaya - The Oxford Topic of Food, Politics, and Society - page 756

Development of Transgenic
Virus-resistant Papaya
Given the devastating effects of PRSV on papaya productivity and its increasingly global
spread, scientists began more than forty years ago to investigate the potential for devel-
oping resistant papaya varieties. Early attempts focused on the possibility of “cross
protection,” whereby plants would be deliberately infected with a mild strain of the
virustoprotectagainstlaterinfectionbyamorevirulentstrain—aprocessanalogous
to vaccination with live but weakened virus in humans. These attempts met with only
mixed results, however: The milder strains of PRSV still produced symptoms of infec-
tion,markingthefruitandmakingfarmersreluctanttodeployit.Italsofailedtoconfer
total protection against virulent PRSV when deployed in the field in both Taiwan and
hailandfrom1985onward(Gonsalves1998).
Coincidentally, at around this time, the science of molecular biology began to yield
new tools for transforming plants and other organisms, including the use of recombi-
nant DNA technology, which offered the potential to move genes between unrelated
organisms. Simultaneous advances in understanding of the mechanisms whereby plant
virusesparasitizetheirhostsledtotheproposalin1985—byJohnSanfordatCornell
UniversityandStephenJohnston,thenatDukeUniversity—of“pathogen-derivedresis-
tance”(PDR),namelythatresistancetoapathogencouldbeconferredbyintroducinga
genefromthatpathogenintothehost(SanfordandJohnston1985).
SanfordandJohnstonrecognizedthatthismechanismalreadyexistsinnatureinthe
phenomenonofcrossprotection(discussedabove).hechallengewouldthereforebeto
refine the technique by introducing the necessary genes without the need to infect the
plant(orotherorganism)witharelatedvirus,andtodosoreliablywiththegenesbeing
heritableasnecessary.Laterthisphenomenonofpathogen-derivedresistancewasiden-
tiiedasresultingfrom“RNAinterference”(RNAi),aprocessbywhichRNAmolecules
inhibit gene expression. The two scientists who identified RNAi and coined the term
(AndrewFireandCraigMello)sharedthe2006NobelPrizeinPhysiologyorMedicine
for their 1998 Nature publication identifying RNAi in the nematode worm C. elegans
(Fireet al.1998).
A plant virus is a relatively simple structure composed primarily of its hereditary
material(eitherRNAorDNA),whichistypicallyhousedinaproteinstructuretermed
a“coatprotein.”Evenbackin1985,SanfordandJohnstonhadproposedthatoneofthe
waysresistancecouldbederivedmightbefromutilizingtheviralgeneticsequences
involved in producing this coat protein. Just a year later, a separate team showed
that transgenic tobacco expressing the coat protein gene of tobacco mosaic virus did
indeed exhibit resistance to infection by the virus (Abel 1986). his appeared to be
pathogen-derived resistance in action; in the case of tobacco the genes were introduced
by the newly discovered method of using Agrobacterium tumefaciens to insert viral
DNA into the genome of the target plant.