To meet the unabated increase in demand for nutritious food, geneticists are reinventing plant varieties that contain higher amounts of essential vitamins than their conventional counterparts. Recognizing that rice, corn and sweet potatoes are dietary staples for people in developing countries that also have high rates of malnutrition, scientists are creating nutrient-dense varieties of these crops. As shown below, genetic modification or selective breeding are being used to make beta-carotene rich varieties of rice, corn and sweet potatoes that can be grown in nations with high rates of vitamin A deficiency.
The first beta-carotene containing rice variety (Golden Rice) was developed in 2000, based on the fact that while rice plants synthesize β-carotene in vegetative tissues, they do not in the endosperm (the edible part) because two steps of the biosynthetic pathway are absent. By addition of only two genes, phytoene synthase (psy) and phytoene desaturase (crt I), the pathway is reconstituted and β-carotene is synthesized in the endosperm (1). Improved versions (Golden Rice 1 and 2) have been created, with Golden Rice 2 announced in 2004. One eight ounce cup of cooked Golden Rice-2 provides 450 micrograms of retinol, which is 50-60% of the adult Recommended Dietary Allowance (RDA) of vitamin A. It has been estimated that it will take at least until 2011 before the first Golden Rice variety obtains final regulatory approval in target countries (2).
Approval of genetically modified plants has been a daunting task, because every new transgenic event must comply with regulations to guarantee the safety of a product derived from it. Hurdles that must be overcome are shown in the following table, which is derived from Koenig et al., Food Chem Tox. 42: 1047-1088, 2004.
Table 1. Requirements for genetically modified plants to comply with U.S. regulations, derived from Koenig et al., Food Chem Tox. 42: 1047-1088, 2004
Another means by which concentrations of vitamins can be increased in plants is selective breeding. Increasing beta-carotene content of corn has been difficult, because corn is one of the world’s most genetically diverse food crops. Each ear has a slightly different genetic makeup, resulting in slightly different characteristics. Kernel color does not necessarily indicate beta-carotene levels, and direct nutrient screening techniques for beta-carotene are expensive, making them cost-prohibitive for many breeders. However, scientists have been working diligently to identify genes involved in carotenoid synthesis to ultimately identify beta-carotene-containing strains quicker and at lower cost. Through crossing high beta-carotene-containing lines with local varieties containing regional hardiness, it is hoped that high beta-carotene-containing corn can be grown throughout the globe. A near-term goal is to develop corn that contains 15 micrograms of beta-carotene per gram, which is over 150 times higher than levels currently present in South African Maize (3).
Sweet potatoes are also being selectively bred to increase beta carotene content. In developing countries with high rates of vitamin A deficiency, white sweet potatoes with low beta carotene and high dry matter content may make up a majority of the diet. The International Potato Center (also known as CIP) has developed high yielding varieties of orange-fleshed sweet potatoes (OFSP) with high beta carotene and dry matter content using landraces or improved germplasm that are adapted to various stresses (4,5). Varieties that have shown promise in Uganda are Ejumula, SPK004 (Kakamega) and Sowola-6. Studies have shown that daily addition of 100 grams of OFSF to the diet can prevent vitamin A deficiency (6).
Although nutrient-rich varieties of standard plant crops may be considered “conventional” to consumers, new plant varieties produced by selective breeding must undergo an approval procedure similar to that of genetically modified plants, with the exception of the steps required for the donor, transgene(s) and delivery processes. The Food and Drug Administration (FDA) will consider any new plant variety (regardless of how it is created) as being different from its predecessor and will demand an assurance of safety when consumed at anticipated levels.
References
O’ Brien, D. Agricultural Research/May-June, 2010, p. 12.
http://www.istrc.org/Symposiums/Tanzania_03/vitaa_masumba.pdf.
http://www.cipotato.org/vitaa/pubs2008/Chronica-Vitamin-A-partnership-for-Africa.pdfncountered in differing geographies