During lab experiments the researchers who developed the GMO papayanoticed
that the more mRNA for the coat protein was
produced, the more resistance the plants showed to the virus.31 Carica papayais
a diploid plant, meaning that each gene is present
in two forms (alleles). If the two forms are identical then one is dealing with a homozygous characteristic. This can best be
compared to a person’s blood group. Humans are also diploid organisms. Humans with the O blood group are homozygous for
that characteristic: both forms of the blood group gene are O (OO). By contrast the blood group AB is determined by the A allele
and the B allele. The ‘Sunup’ papayais
homozygous for the virus resistance characteristic. In other words, it has two identical
alleles. But the ‘Rainbow’ papayais
a cross between the GMO papaya‘
Sunup’ and the non-GMO papaya‘
Kapoho’. As a result
‘Rainbow’ has only received virus resistance from one parent (‘Sunup’). As a result ‘Rainbow’ produces less mRNA for the coat
protein than ‘Sunup’ and exhibits a lower resistance against PRSV.31
As mentioned earlier the silencing process is based on double stranded RNA. Therefore two RNA molecules must bind to
each other before the process is activated (Figure 5, page 16). Greater similarity between the molecules allows for easier and
stronger binding and a more efficient RNA degradation process. In Hawaiithe
coat protein from PRSV HA 5-1 was used. The GMO
papayasare
protected against other PRS viruses as long as the coat protein’s genetic code does not differ too much from that
of PRSV HA 5-1. For example, the GMO papaya‘
Sunup’ was also resistant to PRS viruses from Jamaica and Brazil.
However,
protection against the Thai PRSV was much less effective because the Thai PRSV’s DNA coding has less in common with PRSV
HA 5-1 compared to the Jamaican and Brazilianviruses.
31