Alternative Sweetener Produced in Transgenic Tomatoes

Transgenic tomatoes expressing the taste modifying protein, miraculin, was developed by a group of Japanese researchers. Miraculin was first isolated from the miracle fruit, the red berries (Richadella dulcifica), a shrub native to West Africa. Indigenous peoples often use these berries to improve the palatability of their acidic maize dishes and to sweeten sour beverages. Miraculin itself is not sweet, but can turn a sour taste to a sweet taste. The sweetness induced by citric acid after exposure to miraculin has been estimated to be around 3000 times that of sucrose on a weight basis. This unique property, including the possibility of being used as an alternative low-calorie sweetener for diabetic and obese individuals, led to increasing interests in this protein. Nine taste modifying proteins are known, but as with miraculin, the commercial feasibility of these proteins is limited, since their natural sources are tropical plants that are difficult to grow outside their normal environments. Although previous attempts have been made to produce miraculin in foreign hosts such as E. coli, S. cerevisiae and transgenic tobacco, resulting recombinant miraculins do not have taste-modifying activity.  

The miraculin gene was introduced to tomato cotyledons and recombinant miraculin accumulated at high levels in both leaves and fruits of up to 102.5 and 90.7 µg/g fresh weight, respectively. They found  that the choice of plant species is an important factor in the stable production of miraculin, with tomato being a more suitable host than lettuce for miraculin production. This successful production of recombinant miraculin in transgenic tomatoes is thus a new method to make it more available for mass production of low-calorie sweetener and flavor enhancer.  

The paper published by the Plant Biotechnology Journal is available to subscribers at http://www.blackwell-synergy.com/doi/full/10.1111/j.1467-7652.2007.00283.x