Regulating Phytonutrient Levels in Plants – Toward Modification of Plant Metabolism for Human Health

Abstract

Plants constitute a major component of our diet, providing pigments and additional phytonutrients that are thought to be essential for maintenance of human health and are therefore also referred to as functional metabolites. Several fruit and vegetable species already contain high levels of several of these ingredients, while others do not. Nevertheless, efforts have been devoted to increasing and diversifying the content of phytonutrients, such as carotenoids, flavonoids, and vitamins, even in plants that normally produce high levels of such nutritional components. These efforts rely on transgenic and non-transgenic approaches which have exposed complex regulation mechanisms required for increasing the levels of functional metabolites in plants. The study of these regulatory mechanisms is essential to expedite improvement of levels of these metabolites in fruits, vegetables, cereals, legumes, and starchy roots or tubers. Such improvement is important for the following reasons: (1) to increase the efficiency of the industrial extraction of these compounds that are later being used as natural food supplements or fortifiers and as a source of natural colors to replace the chemical alternatives; (2) to improve and diversify the diet in populations of developing countries, where malnutrition may occur through lack of variety in the diet; (3) to provide fresh agricultural products such as fruits and vegetables highly enriched with certain phytonutrients to possibly substitute the chemically synthesized food supplements and vitamins; and (4) to provide an array of new and attractive colors to our diet.

Three basic approaches to modifying a biosynthetic pathway to increase amounts of desirable phytonutrients are available: (1) manipulation of pathway flux, including increasing, preventing, or redirecting flux into or within the pathway; (2) introduction of novel biosynthetic activities from other organisms via genetic engineering; and (3) manipulation of metabolic sink to efficiently sequester the end-products of particular metabolic pathways. These approaches have been effectively demonstrated in relation to the flavonoid and carotenoid biosynthetic pathways in tomato (Solanum lycopersicum). This chapter is therefore focused on carotenoids and flavonoids, their importance to human nutrition, and approaches used to induce, regulate, and diversify their content in tomato fruits. In addition, several examples of outstanding approaches employed to modulate carotenoid content in other plant species will also be given.