Bt (European corn borer resistance), Roundup Ready, and Liberty herbicide resistance are all examples of input traits that have been introduced into crop plants with genetic engineering. Provitamin A rice is an example of a transgenic crop with a new output trait. This rice has been genetically engineered to produce beta-carotene, which is converted by the human body into vitamin A.
Although rice plants naturally produce beta-carotene, none is present in the endosperm, the part of the plant that is consumed. In many countries where rice is a staple food people are vitamin A deficient, which may lead to blindness or susceptibility to other diseases. By increasing the nutritional value of rice, scientists hope to prevent millions from suffering these afflictions.
Scientists knew that 3 additional steps needed to occur to convert an existing precursor into beta-carotene in the endosperm. However, they discovered that introducing two genes encoding enzymes for the first two steps gave the same result because the gene encoding the enzyme for the third and final step was naturally expressed in endosperm cells.
The first gene construct used the psy coding region, which encodes a protein for phytoene synthase enzyme from daffodil. This enzyme performs the first additional step toward making beta-carotene. Since the goal is to produce beta-carotene in the endosperm, the phytoene synthase enzyme only needs to be produced in that part of the cell. Therefore, a promoter from the gene for glutelin (Gt1), the most abundant storage protein in the endosperm, was used.
The gene for the second enzyme used the CrtI coding region, which encodes a bacterial phytoene desaturase enzyme. The 35S promoter was used in the construct resulting in production of the CrtI protein in all cells. A sequence encoding a transit peptide was also added to the construct to direct the phytoene desaturase enzyme to the endosperm plastids where it is needed for the second reaction.
The product of the second step is then converted to beta-carotene by the action of a third enzyme that is naturally produced in the rice endosperm. The presence of beta-carotene in the endosperm gives the rice grains a yellow color leading to it being nicknamed ‘Golden Rice’.
Provitamin A rice is still in the developmental stages. Scientists must work to create a line that is able to supply enough beta-carotene to meet daily nutritional needs. Also, testing will be needed to determine if the alteration of this biosynthetic pathway has any negative affects in yield potential or other important compounds produced in the cell.
Guerinot, M. (2000) The Green Revolution Strikes Gold. Science. Science 287:241, 243.
Ye, X et al. (2000) Engineering the Provitamin A (beta-Carotene) Biosynthetic Pathway into (Carotenoid-Free) Rice Endosperm. Science 287:303-305.