Golden rice is a variety of rice (Oryza sativa) produced through genetic engineering to biosynthesize beta-carotene, a precursor of pro-vitamin A in the edible parts of rice. The scientific details of the rice were first published in Science in 2000. Golden rice was developed as a fortified food to be used in areas where there is a shortage of dietary vitamin A. In 2005 a new variety called Golden Rice 2 was announced which produces up to 23 times more beta-carotene than the original variety of golden rice. Neither variety is currently available for human consumption. Although golden rice was developed as a humanitarian tool, it has met with significant opposition from environmental and anti-globalization activists.

Creation of golden rice

A simplified overview of the carotenoid biosynthesis pathway in golden rice. The enzymes expressed in the endosperm of golden rice, shown in red, catalyze the biosyntheis of beta-carotene from geranylgeranyl diphosphate. Beta-carotene is assumed to be converted to retinal and subsequently retinol (vitamin A) in the animal gut.

Golden rice was created by Ingo Potrykus of the Institute of Plant Sciences at the Swiss Federal Institute of Technology, working with Peter Beyer of the University of Freiburg. The project started in 1992 and at the time of publication in 2000, golden rice was considered a significant breakthrough in biotechnology as the researchers had engineered an entire biosynthetic pathway.

Golden rice was designed to produce beta-carotene, a precursor of Vitamin A, in the part of rice that people eat, the endosperm. The rice plant can naturally produce beta-carotene, which is a carotenoid pigment that occurs in the leaves and is involved in photosynthesis. However, the plant does not normally produce the pigment in the endosperm since photosynthesis does not occur in the endosperm.

Golden rice was created by transforming rice with two beta-carotene biosynthesis genes:

• 1. psy (phytoene synthase) from daffodil (Narcissus pseudonarcissus)
• 2. crt1 from the soil bacterium Erwinia uredovora

(The insertion of a lyc (lycopene cyclase) gene was thought to be needed but further research showed that it is already being produced in wild-type rice endosperm.)

The psy and crt1 genes were transformed into the rice nuclear genome and placed under the control of an endosperm specific promoter, so that they are only expressed in the endosperm. The exogenous lyc gene has a transit peptide sequence attached so that it is targeted to the plastid, where geranylgeranyl diphosphate formation occurs. The bacterial crt1 gene was an important inclusion to complete the pathway, since it can catalyze multiple steps in the synthesis of carotenoids, while these steps require more than one enzyme in plants. The end product of the engineered pathway is lycopene, but if the plant accumulated lycopene the rice would be red. Recent analysis has shown that the plant's endogenous enzymes process the lycopene to beta-carotene in the endosperm, giving the rice the distinctive yellow colour for which it is named.The original Golden rice was called SGR1, and under greenhouse conditions it produced 1.6 µg/g of carotenoids.

 Subsequent development

Golden rice has been bred with local rice cultivars in the Philippines, Taiwan and with the American rice variety Cocodrie. The first field trials of these golden rice cultivars were conducted by Louisiana State University Agricultural Center in 2004. Field testing will allow a more accurate measurement of the nutritional value of golden rice and will enable feeding tests to be performed. Preliminary results from the field tests have shown that field-grown Golden rice produces 4 to 5 times more beta-carotene than Golden rice grown under greenhouse conditions. In 2005, a team of researchers at biotechnology company Syngenta produced a variety of golden rice called "Golden Rice 2". They combined the phytoene synthase gene from maize with crt1 from the original golden rice. Golden rice 2 produces 23 times more carotenoids than golden rice (up to 37 µg/g), and preferentially accumulates beta-carotene (up to 31 µg/g of the 37 µg/g of carotenoids). To receive the Recommended Dietary Allowance (RDA), it is estimated that 144 g of this rice would have to be eaten. Bioavailiability of the carotene from either variety has not been tested in any modelIn June 2005, researcher Peter Beyer received funding from the Bill and Melinda Gates Foundation to further improve Golden rice by increasing the levels of or the bioavailability of pro-vitamin A, vitamin E, iron, and zinc, and to improve protein quality through genetic modification.

Golden rice and vitamin A deficiency

Prevalence of vitamin A deficiency. Red is most severe (clinical), green least severe. Countries not reporting data are coded blue. Source: WHO

The research that led to golden rice was conducted with the goal of helping children who suffer from Vitamin A Deficiency (VAD). At the beginning of the 21st century, 124 million people, in 118 countries in Africa and South East Asia, were estimated to be affected by VAD. VAD is responsible for 1-2 million deaths, 500,000 cases of irreversible blindness and millions of cases of xerophthalmia annually.

Children and   pregnant women are at highest risk. Vitamin A is supplemented orally and by injection in areas where the diet is deficient in Vitamin A. As of 1999 there were 43 countries that had vitamin A supplementation programs for children under 5; in 10 of these countries, two high dose supplements are available per year, which according to UNICEF could effectively eliminate VAD. However UNICEF and a number of NGOs involved in supplementation note that more frequent low-dose supplementation should be a goal where feasible. Because many children in countries where there is a dietary deficiency in Vitamin A rely on rice as a staple food, the genetic modification to make rice produce provitamin A (beta-carotene) is seen as a simple and less expensive alternative to vitamin supplements or an increase in the consumption of green vegetables or animal products. It can be considered as the genetically engineered equivalent of fluoridated water or iodized salt.

Theoretical analyses of the potential nutritional benefits of golden rice show that consumption of golden rice would not eliminate the problems of blindness and increased mortality, but should be seen as a complement to other methods of Vitamin A supplementation.