D. G Lindsay, CEBAS-CSIC, Murcia
9.4 Current and future potential for GM potato
The following text provides examples of where future trends in GM potato production may lie, taking into account the potential for enhancing quality parameters, nutritional value and non-food uses. The examples selected are not meant to be comprehensive but illustrative.
9.4.1 Antinutritional and nutritional compounds
The USDA-ARS has developed transgenic lines with reduced glycoalkaloid content by down-regulating the expression of a gene encoding solanidine UDP- glucosyltransferase (http://www.ars.usda.gov/is/pr/1999/991115.htm). Glyco- alkaloids are natural compounds which can be harmful to humans and animals when consumed in high concentration. Transgenic tubers show up to a 40%
reduction in glycoalkaloid levels in field trials. This provides opportunities to rescue advanced breeding selections with excellent commercial traits but which were previously discarded due to unacceptable glycoalkaloid levels. Further benefits would accrue from reduced glycoalkaloids in potato starch wastes, as a high residue content renders the wastes unsuitable for use as fodder.
The production of transgenic crops containing proteins with improved amino acid composition should be of benefit to humans as well as to monogastric animals (pig, poultry, etc.) unable to synthesise all of the amino acids needed to sustain life. The potato is the most important non-cereal food crop in the world.
However, it contains limited amounts of the essential amino acids lysine, tryptophan, methionine, and cysteine. Improvements in the nutritional value of food crops such as potato are especially important for people subsisting on a vegetarian diet in which the main source of protein comes from seeds, grains, tubers, etc., which contain limiting amounts of essential amino acids.
Chakraborty et al. (2000) reported improvements in the nutritive value of transgenic potato through the expression of a non-allergenic seed albumin gene (AmA1) from Amaranthus hypochondriacus. As a donor gene, theAmA1 gene has several advantages for genetic transformation experiments. First, this seed protein has a well-balanced amino acid composition, making it nutritionally superior to other proteins recommended by the WHO. Second, the purified protein has no known allergenic properties. Finally, the protein is controlled by a single gene, which facilitates integration into other species. The team showed a five- to ten-fold increase inAmA1transcript levels in tubers of transgenic lines using the tuber-specific granule bound starch synthase (GBSS) promoter compared with the 35S CaMV promoter. Transgenic lines contained a significant two- to four-fold increase in lysine, methionine, cysteine, and tyrosine content in their protein amino acids. Data collected for two consecutive years revealed a 35 to 45% increase in total protein content in transgenic tubers, which corresponded to an increase in most essential amino acids. Grain amaranth is used in many foods throughout the world and amaranth forage has been used for centuries as an important component of the human diet throughout the tropics. The authors presented these facts as evidence of the non-allergenic nature of amaranth.
Fructans, or fructose-oligosaccharides, consist of short chains of fructose molecules. Inulin is a mixture of linear fructose-polymers with different chain- length and a glucose molecule at each C2-end. In over 30,000 plants (e.g.
chicory, onion, asparagus, artichoke) inulin serves as a storage carbohydrate.
Compounds such as inulin reduce the energy density of food and are used to enrich food with dietary fibre or to replace sugar and fat. When fructans are consumed, the undigested portion is reported to support growth of ‘friendly’
bacteria, such as Bifidobacteria and Lactobacillus species. Other benefits noted include increased production of beneficial short-chain fatty acids such as butyrate, increased absorption of calcium and magnesium and improved elimination of toxic compounds (van den Heuvel, 1999). Hellwege et al.
(2000) have developed transgenic potato tubers which synthesise the full spectrum of inulin molecules naturally occurring in globe artichoke (Cynara scolymus). High molecular weight inulins have been produced by expressing the sucrose:sucrose 1-fructosyl transferase and the fructan:fructan 1- fructosylhydrolase genes from globe artichoke. Inulin made up 5% of the dry weight of the transgenic tuber. This approach has the potential to enhance the value of staple foods such as potato with compounds giving additional benefits.
9.4.2 Food processing and industrial uses
Starch is the primary storage compound in tubers and starchy foods are the world’s most abundant staples. It is the most important source of calories in the animal and human diet and provides a starter material for the preparation of more than 500 different commercial products. The physical properties of starch
vary with plant source but there are considerable opportunities to generate novel starches for use in food and non-food market sectors. Genetic engineering has already generated novel potato starch, including high amylopectin starch (with no apparent yield penalty) through the down-regulation of the granule bound starch synthase gene which controls amylose synthesis (Visser et al. 1991;
Kossman and Lloyd, 2000 and references therein). High amylose starch is also in great demand by the starch industry for its unique functional properties, but very few high amylose crops are available. Scwall et al. (2000) showed that concurrent down-regulation of two starch branching enzymes, A and B, in potato tubers modifies both starch grain morphology and composition and produces a significant increase in amylose content.
Starket al.(1992) increased the starch content of tubers by expressing anE.
coli glgC16 gene which encodes for the enzyme ADPglucose pyro- phosphorylase. The corresponding potato enzyme resides in the starch granule and plays a key role in starch biosynthesis. TheE. colienzyme is not regulated by the same fine control mechanisms which operate on the endogenous potato enzyme and is therefore able to increase the production of ADPglucose which becomes incorporated into the growing starch granule. Tuber starch content can be increased by up to 25% in some glgC16 expressing lines but the response appears to be genotype dependent. These high starch potatoes also accumulate lower levels of reducing sugars (glucose and fructose) in stored tubers which is highly relevant to the requirements of the processing sector. The processing industry requires low reducing sugar levels in tubers as these sugars are primarily responsible for non-enzymic browning through a typical Maillard reaction which occurs at the temperatures required to generate potato chips (crisps) and French fries.
Ideally, the industry would like to store tubers at low temperature (ca.4ºC) to minimise sprout growth and eliminate the need to use chemicals to suppress the sprouting process. However, low temperatures induce glucose and fructose accumulation. Success in minimising sugar accumulation using transgenic approaches have come from the use of the glgC16 gene and from modifying the expression of genes in pathways of primary carbohydrate metabolism, e.g., by minimising the conversion of sucrose to glucose and fructose by expressing invertase inhibitor protein (Greineret al.1999). More detail on the control of sugar accumulation starch biosynthesis and potato quality can be found in Davies and Viola (1992), Davies and Mackay (1994), Davies (1996) and Davies (1998).
9.4.3 Pharmaceutical uses
Tacket et al. (2000) reported a new approach for delivering vaccine antigens using inexpensive plant-based oral vaccines generated in potato. Norwalk virus capsid protein (NVCP) assembled into virus-like particles, was used as a test antigen to determine immune responses in healthy adults eating GM potato containing NVCP. Overall, 19 out of 20 volunteers developed an immune
response of some kind. Similarly, Chong and Langridge (2000) demonstrated expression of bioactive antimicrobial human lactoferrin in potato plants. This was the first report of synthesis of full length biologically active hLF in edible plants. Expression was significant (up to 0.1% of total soluble protein) and antimicrobial activity against four different human pathogenic bacterial strains was detected in extracts of tuber tissues.
At the time of writing this chapter, livestock and related industries in the UK are undergoing torrid times due to a severe outbreak of foot and mouth disease.
Implementation of an expensive vaccination programme has been hotly debated.
It is of some interest, therefore, that Carrillo et al. (2001) demonstrated the induction of a virus-specific antibody response to foot and mouth disease virus using the structural protein VP1 expressed in transgenic potato plants. The group previously reported the oral and parental immunogenicity of the structural protein VP1 of foot and mouth disease virus (FMDV) expressed in different transgenic plants. Their recent report indicates that transgenic potatoes containing the VP1 gene cloned under the regulatory activity of either a single or a double copy of the 35S CaMV promoter, represents a viable strategy for increasing the level of VP1 gene expression. Furthermore, immunised animals presented a FMDV VP1 specific antibody response and showed protection against the experimental challenge. These results clearly show the potential of using plants as antigen expression systems.