gm-sugar-cane/report-2

Sugarcane engineered to reduce polyphenol oxidase (PPO) activity had greater PPO activity, even without the transgene.

Manipulated Organism: Sugarcane (Saccharum sp.), cultivar Q117.

Inserted Transgenes: Sense or antisense constructs of a gene for polyphenol oxidase (PPO), derived from a sugarcane cDNA library. The PPO gene was fused to the maize ubiquitin promoter so that it would be expressed throughout the plant. The PPO construct was introduced via microprojectile bombardment along with a construct containing the antibiotic resistance gene nptII under control of the Emu promoter.

Goal: Produce sugarcane plants with lower levels of PPO, which is an enzyme that contributes to the darkening of raw sugar. "[L]ighter colored raw sugar can command a premium in the market place. A means of reducing the level of these [dark] compounds would be of great interest to the sugar industry worldwide" (Vickers et al. 2005a, p. 354). Both sense and antisense constructs were used with the hope that one or both of these strategies would silence expression of the native PPO gene.

Intended Effect: Of the dozens of transgenic clones evaluated over five experiments, only a few had lower PPO activity or lighter juice than the line (Q117) used for the transformation.

Unintended Effects:

In most cases, insertion of either the sense or antisense PPO gene led to higher levels of PPO activity and darker juice compared with Q117 and other commercial cultivars, which was the opposite of the desired result.
Plants subjected to tissue culture but not genetic transformation also had significantly higher levels of PPO activity.
In field trials, the genetically engineered plants had significantly lower cane yields than Q117 and other commercial cultivars. In several experiments, the average yield of the transgenic plants was less than 50% of the commercial lines.
Yield was also reduced, but not as strongly, in plants subjected to tissue culture but not genetic transformation.

Additional Comments: This article is one of many illustrating the unintended effects of tissue culture and the techniques of genetic transformation, quite apart from the inserted transgenes. These techniques are integral to the overall process of genetic engineering.

Sources:

Vickers, J. E., C. P .L. Grof, G. D. Bonnett, P. A. Jackson et al. (2005a). "Overexpression of Polyphenol Oxidase in Transgenic Sugarcane Results in Darker Juice and Raw Sugar," Crop Science vol. 45, pp. 354-62.

Vickers, J. E., C. P .L. Grof, G. D. Bonnett, P. A. Jackson et al. (2005b). "Effects of Tissue Culture, Biolistic Transformation, and Introduction of PPO and SPS Gene Constructs on Performance of Sugarcane Clones in the Field," Australian Journal of Agricultural Research vol. 56, pp.57-68.

Author Affiliations: CSIRO Plant Industry (Australia), University of Southern Queensland (Australia).

Funding: CSIRO Tropical Agri-exports Multidivisional Program (Australia), Sugar Research and Development Corporation (Australia).

Product Status: Not on the market as of April, 2009.

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Seth JordanAugust 26, 2020