Potatoes with transgene for virus resistance were variably resistant, and
some lines without the target gene nevertheless became highly
Potato (Solanum tuberosum), varieties Russet Burbank and Ranger
Three different gene constructs: (1) coat protein (CP) gene derived from
the potato leaf luteovirus and fused to the cauliflower mosaic virus
(CaMV-35S) promoter so that the gene would be expressed in all parts of
the plant; (2) a modified form of the CP gene fused to the same promoter;
and (3) as a control, a gene construct identical to the above-mentioned
ones, but without the CP or modified CP gene. Using Agrobacterium
as a vector, potato plants of both varieties (Russet Burbank and
Ranger Russet) were transformed separately with the three constructs.
Make potato plants resistant to the potato leaf luteovirus, which
is transmitted by aphids that feed on potato plants. Potato leaf
luteovirus infections can cause severe losses in potato crops. Previous
experiments had shown that transgenic potatoes with the CP gene
had some resistance to the virus, although they did not make the coat
protein. By modifying the CP gene, the researchers hoped that the
plants would make the coat protein and show increased resistance to the
virus. They created different lines of potatoes each containing one of
the three constructs.
All Russet Burbank transgenic lines containing the CP gene or the
modified CP gene showed significantly lower virus infection than
did unmanipulated potato plants.
Although the Russet Burbank transgenic lines containing the CP
gene or the modified CP gene showed significantly lower virus
infection than did unmanipulated potato plants, they did not actually
produce the coat protein itself. The cause of the increased resistance
- The Ranger Russet transgenic lines did contain
the transgenes and produced transgenic RNA for the coat protein, but
only 4 of 15 lines showed an increase in virus resistance, indicating
some unknown variety-dependent effect.
Remarkably, the most resistant Ranger Russet line was one that had been
transformed with the construct that did not contain the CP
gene or the modified CP gene. This resistance was subsequently
inherited as a dominant trait.
There were "statistically significant differences in virus resistance
among independently derived transformants of the same variety-construct
combination. . . . It thus appears that the variability associated with
generating an individual clone [transgenic line] is larger than the
effect of the construct" (p. 439).
This study illustrates how a target effect in a transgenic plant -
virus resistance in this case - can arise although the relation of that
phenotypic change to the genetic manipulation remains a complete riddle.
Presting, G. G., O. P. Smith, and C. R. Brown (1995). "Resistance
to Potato Leafroll Virus Transformed with the Coat Protein Gene or With
Vector Control Constructs," Phytopathology vol. 85, pp. 436-42.
USDA, Agricultural Research Service.
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