Insect-resistant transgenic potatoes had less foliage and altered
levels of leaf-glycoalkaloids.
Potato (Solanum tuberosum), cultivar Desiree.
Inserted Transgenes and Intended Effect:
Five lines of transgenic potatoes were genetically altered to produce
insecticidal substances. Three lines contained the gene for the
production of the the protein GNA (snowdrop lectin), derived from the
snowdrop (Galanthus nivalis); one line contained the CpTi
gene for the production of a cowpea trypsin inhibitor (CpTI) derived
from the cowpea (Vigna unguiculata); and one line contained the
Con A gene for the production of jackbean lectin, derived from
the jackbean (Canavalia ensiformis). Each of the gene constructs
contained the cauliflower mosaic virus (CaMV-35S) promoter so that the
target gene would be expressed continuously in all parts of the plant.
Goal of This Study:
The researchers wanted to determine whether the genetic transformation
of potatoes to produce insecticide resulted in any unintended changes in
the amounts of glycoalkaloids in the leaves of the plants. Glycoalkaloids
are normally present in the leaves and stems of potato plants and are
highly toxic to mammals. Ecologically, glycoalkaloid-containing leaves
may be feeding deterrents to browsing mammals and some insects due
to their toxicity and bitterness, so there is some concern about the
possible unintended alteration of glycoalkaloid levels in genetically
modified potatoes. "It is incorrect to assume that the current methods
of genetic engineering used to express single transgenes in plants are
completely targeted and will have no, or minimal, effects on unrelated
biosynthetic pathways in transformed plants" (p. 144).
Results of This Study and Unintended Effects:
The five lines of transgenic potatoes were compared with normal
nontransgenic controls and nontransgenic controls that were derived from
tissue cultures. The plants were grown for 48 days and then the levels
of glycoalkaloids in stems and leaves were determined. In addition,
in five plants of each line the lower, middle, and upper leaves were
tested. The following unintended effects were found:
Four of five transgenic lines produced significantly less foliage than
the control plants.
In three of the five transgenic lines, stem production was significantly
reduced, while in one of the lines stem production was significantly
higher, which was reflected in the bushy growth form of the plants in
that transgenic line (ConA4-1).
All transgenic lines had significantly lower levels of leaf-glycoalkaloids
than both the normal controls and the tissue culture controls. For
example, the line with the Con A gene had a 24% reduction in glycoalkaloid
levels, while the three lines with the GNA gene had on average 44%
less leaf-glycoalkaloid content.
Four of five transgenic lines had statistically significant lowered
levels of glycoalkaloids in their stems.
In two of the five transgenic lines, the ratios of two different
glycoalkaloids (alpha-chaconine and alpha-solanine) were significantly
altered in the leaves.
These results indicate that since the transgenic lines differed on the
whole from the normal controls and the tissue culture controls, the lower
glycoalkaloid levels were not due to tissue culturing alone. The genetic
transformation itself had an effect on the plants' physiology. Whether
this was directly related to the target genes or some other aspect of
the genetic manipulation is not clear.
The authors remark: "Thus, any inadvertent lowering of foliar
glycoalkaloids in transgenic potato plants could cause an undesired
increase in susceptibility to those pests which are sensitive to threshold
concentrations of glycoalkaloids for insect deterrence or toxicity,
potentially reducing the benefits of expressing anti-insect transgenes
in plants. . . . We think that it is as important to monitor unintended
changes in the levels of such secondary plant compounds as it is to
evaluate the potential risks and benefits of the intended transgene
product (anti-insect gene products) in the agro-ecosystem" (p. 148).
Birch, A. N. E., I. E. Geoghegan, D. W. Griffiths, and J. W. McNicol
(2002). "The Effect Of Genetic Transformations for Pest Resistance
on Foliar Solanidine-Based Glycoalkaloids of Potato (Solanum
tuberosum)," Annals of Applied Biology vol 140, pp. 143-9.
Scottish Crop Research Institute, Dundee, Scotland; Biomathematics and
Statistics Scotland, Dundee, Scotland.
Scottish Executive Environment and Rural Affairs Department.
Not on the market as of 2008.
Copyright 2008 The Nature