gm-barley/report-2

Transgenic barley plants over-expressing a zinc transport protein had smaller seeds and did not accumulate more zinc when grown in zinc-deficient soil.

Manipulated Organism: Barley (Hordeum vulgare).

Inserted Transgenes: AtZIP1 and AtZIP3, which are genes for zinc transport proteins from the mustard plant Arabidopsis thaliana. The DNA constructs contained one of the AtZIP genes under the control of the maize ubiquitin promoter as well as a gene for resistance to the antibiotic hygromycin under the control of the cauliflower mosaic virus 35S promoter.

Goal of This Study: "Increasing the zinc content of cereal grains will be important for improving human nutrition. Improved plant zinc efficiency will lead to increased yields when available zinc is limiting plant growth. The aim of our work was to test how the over-expression of zinc transporters in cereals affects plant growth, seed mineral content, and zinc transport rates" (p. 373).

Intended Effect:

A number of transgenic barley lines were created with levels of ZIP1 RNA much higher than in the parent. (With the techniques they used, the authors couldn't determine how much of the ZIP1 RNA was produced by the transgene and how much was produced by barley's own version of the AtZIP1 gene.)
When deprived of zinc for 12-24 hours under hydroponic conditions and then resupplied, the transgenic lines over-expressing ZIP1 took up zinc 2-3 times more quickly than the parent.
Some transgenic lines grown in a zinc-sufficient potting mix produced seeds with a zinc concentration 2-3 times greater than in seeds from the parent line.

Unintended Effects:

While both the AtZIP1 and AtZIP3 genes were introduced into the barley cultivar Golden Promise, the authors state that only AtZIP1 yielded a transgenic line of barley (p. 380). No rationale was given for this result.
In the second transgenic generation, some of the descendants of the trangenic lines had little to no detectable ZIP1 RNA. According to the authors, "These low levels were either due to silencing of the transgene and endogenous gene or due to loss of the transgene from segregation" (p. 377). Despite the appearance of low ZIP1 expression, these plants also showed elevated levels of zinc uptake following zinc deprivation, although not as much as the ZIP1 over-expressing plants.
Under zinc-sufficient conditions in the hydroponic experiment, no differences in zinc uptake were observed between the parent and transgenic lines.
Seeds from several different transgenic lines were 20-30% lighter than seeds from the parent line.
After 40 days of growing in zinc-deficient sand, the transgenic lines suffered just as much reduction in their foliar zinc level as the parent line.

Source:

Ramesh, S. A., S. Choimes, and D. P. Schachtman (2004). "Over-Expression of an Arabidopsis Zinc Transporter in Hordeum vulgare Increases Short-Term Zinc Uptake after Zinc Deprivation and Seed Zinc Content," Plant Molecular Biology vol. 54, pp. 373-85.

Author Affiliations: CSIRO Plant Industry, Australia.

Funding: Not mentioned.

Product Status: Not on the market as of 2009.

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Seth JordanAugust 15, 2020