Broccoli is receiving increased attention because its florets are rich in many chemoprotective phytochemicals such as glucosinolates (GSs). Epidemiological studies have shown that diets rich in cruciferous (Brassicaceae) vegetables, especially broccoli, may provide protection against various forms of cancer, including colon and prostate cancer, particularly in the early initiation stages (Higdon et al., 2007). In addition, protective effects of broccoli against cardiovascular diseases have also been noted (Zhang et al., 2011).
Current research has demonstrated that the chemoprotective properties of cruciferous vegetables are attributed to GS hydrolysis products, primarily, isothiocyanates [ITCs (Traka and Mithen, 2009; Verkerk et al., 2009)]. Glucosinolates, consisting of β-thioglucoside N-hydroxysulfate, a side chain, and a β-d-glucopyranose moiety, are divided into three groups based on their amino acid precursors: aliphatic GSs (methionine, leucine, iso-leucine), indole GSs (tryptophan), and aromatic GSs (phenylalanine) (Table 1) (Sønderby et al., 2010b). On tissue damage, GSs can be hydrolyzed to several classes of bioactive breakdown products, including ITCs, thiocyanates, and nitriles, by typical plant myrosinases [β-thioglucoside glucohydrolase (EC 22.214.171.124)], which are compartmentalized either in specialized myrosin cells in the phloem parenchyma or in stomata cells (Burow and Wittstock, 2010). In addition, some bacteria in the gastrointestinal tract show myrosinase activity (Traka and Mithen, 2009). Among the hydrolysis products, many ITCs, particularly sulforaphane (SF), derived from the hydrolysis of aliphatic GSs, have shown high anticarcinogenic activity in mammalian cells (Traka and Mithen, 2009). SF has been studied extensively; this ITC is hydrolyzed from glucoraphanin, which is abundant in broccoli florets, seeds, and sprouts. The anticarcinogenic mechanism of ITCs, and particularly of SF, have been reviewed in previous studies (Traka and Mithen, 2009; Verkerk et al., 2009). Recent studies have suggested that SF prevents vascular diseases by acting as an indirect antioxidant through the activation of Nrf-2 (Xue et al., 2008) and protects against ischemic–reperfusion injury of the heart through the antioxidant pathway and mitochondrial KATP channels (Piao et al., 2010). It is notable that hepatic, colonic mucosal, and pancreatic quinone reductase and glutathione S-transferase activities were induced by high doses of SF but not by SF nitrile (Matusheski and Jeffery, 2001). Moreover, the activity of specifier proteins such as epithiospecifier protein in broccoli could inhibit the formation of SF (Matusheski et al., 2004). However, proper cooking (e.g., blanching for 2 min, short-term boiling, or microwave treatment) without macerating broccoli florets can efficiently inactivate endogenous S-glycosyl hydrolases and specifier proteins; intact GSs can then be degraded by colonic microflora to promote the formation of health-benefitting ITCs (Matusheski et al., 2004; Sarikamis et al., 2006).
Trivial and chemical names of main glucosinolates identified in broccoli.
Some hydrolysis products derived from alkenyl GSs, including 2-(R)-hydroxy-3-butenyl GS, 2-propenyl GS (sinigrin), and 3-butenyl GS (gluconapin), also possess a degree of anticarcinogenic activity (Fahey et al., 1997), but these compounds may have adverse health effects (Verkerk et al., 2009). For example, oxazolidine-2-thiones formed from progoitrin and found in rapeseed have been associated with goiter and other negative effects in animals, including depressed growth, poor egg production, and liver damage (Tripathi and Mishra, 2007). To date, despite a lack of evidence for a goitrogenic effect of progoitrin and its breakdown products in humans (Traka and Mithen, 2009), the potential health risks may discourage people from consuming foods containing relatively high concentrations of this chemical. The alkenyl GSs and neoglucobrassicin also contribute to the pungent, bitter taste of some Brassica vegetables (Drewnowski and Gomez-Carneros, 2000; Schonhof et al., 2004). Some broccoli lines (e.g., ‘Shogun’, Eu8-1, and ‘Lvxiong90’) with relatively higher concentrations of alkenyl GSs (particularly progoitrin) were identified in previous studies (Brown et al., 2002; Kushad et al., 1999; Rosa and Rodrigues, 2001; Schonhof et al., 2004; Wang et al., 2012).
Indole-3-carbinol and diindolylmethane, two major hydrolysis products of glucobrassicin that are abundant in cruciferous plants, exhibit protective activities against many types of cancer, particularly hormone-responsive conditions, including breast, prostate, and ovarian cancers (Higdon et al., 2007). However, these compounds might also promote carcinogenesis by inducing phase-I enzymes, which can oxidize inert polyaromatic hydrocarbons to DNA-binding products (Baird et al., 2005). A recent study further demonstrated that the neoglucobrassicin/myrosinase complex showed strongly mutagenic properties in bacterial and mammalian cells (Glatt et al., 2011). Considering these findings, consumption of large amounts of indole glucosinolates should be approached with caution.
The composition and concentration of aliphatic GSs are mainly determined by the genotype of B. oleracea vegetables, although they can also be affected by many exogenous factors (Brown et al., 2002; Farnham et al., 2004; Ku et al., 2013; Schonhof et al., 2004). Compared with other Brassica vegetables, broccoli florets contain higher concentrations of glucoraphanin and comparatively low concentrations of other methionine-derived GSs (Verkerk et al., 2009). Some pure lines high in glucoraphanin were found in previous investigations of GSs in broccoli germplasm (Farnham et al., 2000; Kushad et al., 1999; Wang et al., 2012). Attempts have been made to breed broccoli with enhanced concentrations of 3-methylsulfinylpropyl GS (glucoiberin) and glucoraphanin to enhance health benefits (Faulkner et al., 1998; Mithen et al., 2003; Sarikamis et al., 2006). Sarikamis et al. (2006) bred a hybrid (48-13-4 × Br9) that shows an ≈2-fold increase in glucoraphanin concentration compared with commercial cultivars.
Various strategies for enhancing concentrations of chemoprotective GSs and decreasing concentrations of antinutritional GSs in Brassica vegetables were proposed by Verkerk et al. (2009). Previously, 14 pure broccoli lines with glucoraphanin concentrations more than 2-fold higher than those of commercial hybrids were obtained as candidates for breeding high-GS cultivars (Wang et al., 2012). In the present study, 10 parental lines—eight high-glucoraphanin lines screened previously and two lines with good agronomic characteristics—were used to breed high-glucoraphanin F1 hybrids (Table 2). We first evaluated two major commercial cultivars, Youxiu and Lvxiong90, which exhibited the highest concentrations of glucoraphanin and progoitrin, respectively, among commercial cultivars in our previous investigation (Wang et al., 2012), and six trial cultivars (Table 2). The reference cultivar with the highest concentration of glucoraphanin was determined as the final control, and the high-glucoraphanin hybrids were evaluated in comparison with this control. We also identified and analyzed 16 F1 hybrids with more than 3-fold higher glucoraphanin content than the controls for individual GSs and groups and two parental lines were validated as high-glucoraphanin types.
Code, name, source, planting location, and type of commercial cultivars, trial cultivars, and parental lines used in this study of high-glucoraphanin broccoli.
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Comparison of individual glucosinolates and each group of glucosinolates among high-glucoraphanin broccoli hybrids.z
The head diameter and head weight of broccoli lines planted in the field.z