gm-potato/report-4

Insect-resistant transgenic potatoes had less foliage and altered levels of leaf-glycoalkaloids.

Manipulated Organism: 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.

Additional Comments: 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).

Source: 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.

Author Affiliations: Scottish Crop Research Institute, Dundee, Scotland; Biomathematics and Statistics Scotland, Dundee, Scotland.

Funding: Scottish Executive Environment and Rural Affairs Department.

Product Status: Not on the market as of 2008.

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Seth JordanAugust 26, 2020