Increased nutritive value of transgenic potato by expressing a nonallergenic seed albumin gene from <em>Amaranthus hypochondriacus</em>

Abstract

One of the goals of plant genetic engineering has been to create crops that are tailored to provide better nutrition for humans and their domestic animals. A major target has been the improvement of the amino acid composition of seed proteins, because animals, including humans, are incapable of synthesizing 10 of the 20 amino acids needed for protein synthesis, and these “essential” amino acids must therefore be obtained from the diet. Advances in plant tissue culture techniques and gene transfer technology have opened up possibilities for modifying the amino acid contents of plants. One approach has been to manipulate the regulation of amino acid biosynthesis to increase the abundance of a particular amino acid. Mutant selection and engineering genes encoding key enzymes of amino acid biosynthetic pathways have been used to increase amino acids in crop plants (1). However, an increase in the free essential amino acids does not lead to an increase in the fixed content, and the amino acids could be leached out from the plant tissue and lost during boiling and other processing (2, 3). An alternative approach is the insertion and the expression of gene(s) encoding essential amino acid-rich proteins in transgenic plants.

Potato is the most important noncereal food crop and ranks fourth in terms of total global food production, besides being used as animal feed and as raw material for manufacture of starch, alcohol, and other food products. The essential amino acids that limit the nutritive value of potato protein are lysine, tyrosine, and the sulfur-containing amino acids methionine and cysteine (4). In quality improvement programs of potato, the priorities have been to improve disease and pest resistance (5–8), to increase yields (9), and to increase adaptability to biotic and abiotic conditions (10–12), whereas nutritional status has remained secondary. In an attempt to develop economically important crop plants with improved nutritional quality, our laboratory earlier reported the cloning of a gene that encodes a seed-specific protein, amaranth seed albumin (AmA1) from Amaranthus hypochondriacus (13). This gene has been patented (14). The AmA1 protein has great potential as a donor protein for the following reasons: (i) unlike most seed proteins, it is a well-balanced protein in terms of amino acid composition and even better than the values recommended by the World Health Organization for a nutritionally rich protein; (ii) it is a nonallergenic protein in its purified form; and (iii) unlike many seed storage proteins, AmA1 is encoded by a single gene and thus would facilitate gene transfer into target plants with less difficulty. In this paper, we report the tuber-specific as well as constitutive expression of AmA1 in potato by using granule-bound starch synthase (GBSS) and cauliflower mosaic virus (CaMV) 35S promoters, respectively. The expression of AmA1 in transgenic tubers resulted in a significant increase in most essential amino acids. Unexpectedly, the transgenic plants also contained more total protein in tubers compared to control plants. These findings demonstrate the feasibility of using the AmA1 gene in genetic engineering to improve the nutritive value of other nonseed and grain crops.

Wild-type and transgenic potato plants in restricted experimental plots were grown to maturity in the winter season and tubers were harvested. Protein was extracted from the tubers and used for amino acid analysis. Values are presented as the mean ± SE for three each of wild-type and transgenic plants.

Potato plants in restricted experimental plots were grown to maturity in the winter season and tubers were harvested. The tuber number, fresh weight, and protein content were determined. Total tuber protein was extracted with 20% trichloroacetic acid and estimated spectrophotometrically. Values are presented as the mean ± SE for four wild-type and three or four transgenic plants.

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