Breeding for desirable glucosinolate in Brassica vegetables
Authors: Hanuman Ram and Gograj Singh Jat
Division of Vegetable Science, IARI, New Delhi-110 012
The Brassicaceae family, also called Cruciferae, includes around 375 genera and about 3,200 species including many economically important plants such as leaf and root vegetables, oilseed and condiment crops, and the model plant Arabidopsis thaliana. Brassicaceae vegetables are widely cultivated, with many genera, species, and cultivars including Brassica rapa (Chinese cabbage, Chinese mustard, bok choy and turnip), B. oleracea (cabbage, broccoli, cauliflower, kale, Brussels sprouts and kohlrabi), B. napus (rapeseed and rutabaga), B. juncea (mustard green), and Raphanus sativus (radish). Glucosinolates, secondary metabolites found in Brassicaceae and related families, have three moieties: a Beta- thioglucose moiety, a sulfonated oxime moiety, and a variable aglycone side chain derived from a a-amino acid. Glucosinolates, of which nearly 200 types having different substituents have been identified, are classifiable into three classes based on the structure of different amino acid precursors: aliphatic glucosinolates, indole glucosinolates, and aromatic glucosinolates. Physical tissue or cell injury leads to the breakdown of glucosinolates through the hydrolytic action of the enzyme myrosinase, resulting in the production of compounds including isothiocynates, thiocyanates and nitriles. Derivative compounds of glucosinolates have a wide range of biological functions including anticarcinogenic properties in humans, anti-nutritional effects of seed meal in animals, insect pest repellent and fungal disease suppression (Brader et al., 2006). Consumption of crucifers vegetables, reduces the incidence of cancer in humans and other mammals (Block et al., 1992; Fahey and Talalay 1995). This seems to be due to the presence of inducers of phase II enzymes, that detoxify carcinogens and mutagens in various mammalian organs (Prestera et al., 1996). Two genes, GSL-PRO and GSL-ELONG, regulate sidechain length. The action of the former results in 3-carbon GSL, whereas action of the latter produces 4-carbon GSL. The double recessive genotype produces only trace amounts of aliphatic GSL. The third gene, GSL-ALK controls sidechain desaturation. GSL-OH, that is responsible for sidechain hydroxylation. Elucidation of the inheritance of these major genes controlling biosynthesis of GSL will allow for manipulation of these genes and facilitate development of lines with specific GSL profiles. This capability will be important for improvement of Brassica breeding lines with high content of desirable GSL, like glucoraphanin, a demonstrated precursor of anticarcinogenic compounds. Consumption of broccoli (Brassica oleracea var. italica) is associated with a reduction in risk of prostate cancer lung cancer and colorectal adenomas (Lin et al. 1998). The anticarcinogenic activity is most likely to be due to activity of the isothiocyanates iberin and sulforaphane derived, respectively from 3-methylsulphinylpropyl (3-MSP) and 4-methylsulphinylbutyl (4-MSB) glucosinolates that accumulate in the florets of broccoli. To obtain enhanced levels of either 3-MSP or 4-MSB glucosinolate it is necessary to have B. Villosa alleles in either a homozygous or heterozygous state at a single quantitative trait locus (QTL). The ratio of these two glucosinolates, and thus whether Iberin or SF is generated upon hydrolysis, is determined by the presence or absence of B. villosa alleles at this QTL, but also at an additional QTL2. Commercial freezing processes and storage of high glucosinolate broccoli maintains the high level of glucosinolates compared to standard cultivars, although the blanching process denatures the endogenous myrosinase activity.
Certain glucosinolates such as sinigrin and progoitrin, and their respective breakdown products are often bitter or astringent. Humans often reject foods that taste excessively bitter. This instinctive rejection is believed to have been important for human safety because it can protect people from consuming some potential toxins. The removal of specific glucosinolates and their breakdown products is thought to reduce bitterness and to increase consumer acceptance. Consumer reports have described that taste, rather than recognized nutrition or health value, is the key to food selection. For this reason, expectations of consumer willingness to compromise on the taste of glucosinolate enriched vegetables for health value are risky. Moreover, those efforts might only appeal to a niche market. Improvement of glucosinolate compounds for human health or processing fitness requires not only the pursuit of breeding efficiency by marker-assisted selection or new analytical methods but also careful consideration of the taste of Brassicaceae vegetables.
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Ph D Scholar, Division of Vegetable Science, IARI, New Delhi - 110012
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