The Role Of Plant Breeding In Improving Phytochemical Content

Traditionally, the primary goal of plant breeders has been to improve yield by developing varieties resistant to diseases, insects, and abiotic stress. Only recently have breeders attempted to change the phytochemical composition of plants for human health. The number of species that have had their nutritional content altered is few, and most of the attention within this area has been directed toward micronu-trient (defined as most minerals and all vitamins) content, not phytochemicals

(Graham et al., 1999). The major emphasis has been on increasing the iron, zinc, and P-carotene content in wheat, maize, rice, beans, and cassava (Bouis, 1996). The FAO and WHO have identified micronutrient malnutrition as a primary health concern, affecting more than two billion people worldwide (Welch and Graham, 1999). In developed countries, however, caloric intake generally exceeds needs, and there has been an increased emphasis on improving micronutrient and phytochemical content of food.

Allard (1999) defined plant breeding as the "controlled evolution of plants ... by humans with the goal of producing populations that have superior agricultural and economic characteristics." Plant improvement by breeding is limited by naturally occurring variation and the restriction of reproductive barriers between distantly related plants. Although these conditions limit the breadth of genetic transfer that is possible, the relatively low cost of implementing and conducting a breeding program ensures that this method will continue to be used. Variation is key to plant improvement through breeding, and results from mutations are estimated to occur at a rate of 10-5 to 10-7/locus/generation (Allard, 1999). Random mutations that affect amino acid sequences, and therefore protein function, normally have deleterious effects on plant survival. It is not likely, therefore, that either naturally occurring or chemically induced random mutations would generate useful plants with respect to phytochemical content. Further, identifying and characterizing the mutation is slow and is arguably the bottleneck of this technology, especially as it relates to developing useful screening procedures (Zhu, 2000). Recently developed methods such as T-DNA insertional mutagenesis offers an alternative to random mutations, and are used to introduce mutations and quickly characterize the function and location of the gene (Azpirox-Leehan and Feldmann, 1997; Krysan et al., 1999).

Most of our major, and many minor, domesticated crops are well represented in seed banks around the world. Within these collections are named cultivars and varieties, numbered accessions, and land races. A much smaller number of wild (nondo-mesticated) relatives are housed in seed banks, and these offer a source of rich genetic variation that has often been overlooked. Many of these species can be used as gene donors and crossed with the domesticated species. Potentially, there is great genetic diversity within a species having wide geographic distribution. Over time, in response to geographic and reproductive isolation, different multilocus assemblages of alleles would have accumulated in these populations in response to natural selection. It is likely that these accessions would differ significantly in many traits including vitamin, micronutrient, and phytochemical expression. Accessions from wild populations have commonly been used as sources for disease resistance (Tanksley and McCouch, 1997), and they should be screened for nutrient and phytochemical content. It is these traits in unimproved accessions and land races that breeders must identify and introgress into breeding lines. This task is a substantial undertaking because very little assessment of genetic diversity has been made of the germplasm held within gene banks.

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