Submitted to: Plant Breeding
Publication Type: Peer Reviewed Journal
Publication Acceptance Date: 7/11/2003
Publication Date: 2/23/2004
Citation: Farnham, M.W., Wilson, P.E., Stephenson, K.K., Fahey, J.W. 2004. Genetic and environmental effects on glucosinolate content and chemoprotective potential of broccoli. Plant Breeding 123:60-65.
Interpretive Summary: Broccoli is well known as a source of plant compounds known as glucosinolates. One of these, glucoraphanin, is known to form a compound called sulforaphane. In research experiments, rats fed sulforaphane or broccoli extracts have a lower risk of cancer than rats not fed these materials. Evidence also indicates that sulforaphane protects human cells from cancer-causing agents by inducing those cells to detoxify those harmful agents. Thus, a diet rich in broccoli may protect humans against certain cancers. We are interested in knowing if glucoraphanin levels and the detoxifying activity of broccoli can be altered. If we can increase these factors, this might enhance the capacity of broccoli to protect people against cancer. Specific goals of this research were to determine how stable glucoraphanin content and detoxifying activity of different broccoli varieties are when grown in different environments and how heritable these traits are. Results of this research indicate that genetic control of glucoraphanin expression is relatively high for broccoli while the environment has a smaller influence on expression of this trait. These results support the idea that glucoraphanin is the primary agent in broccoli that induces detoxifying activity and indicate that plant breeders should be able to readily increase expression of these traits to improve added value in broccoli. Commercial and public broccoli breeders will use this information in their efforts to genetically enhance healthful attributes of broccoli. Identified varieties that produce stable levels of glucoraphanin will be useful to medical researchers interested in fully testing the capacity of broccoli to protect cells against cancer.
Technical Abstract: Glucoraphanin (GR) is the most abundant glucosinolate present in broccoli and its cognate isothiocyanate is sulforaphane. Sulforaphane is a potent inducer of mammalian detoxication and antioxidant (Phase 2) enzyme activity and it has been shown to inhibit chemically-induced tumorigenesis in animal models. Only a few studies have examined the effects of plant growth environment on the concentration of glucoraphanin in broccoli florets, or the resulting enhancement of Phase 2 enzyme activity when mammalian cells are exposed to extracts of broccoli florets. Thus, this study was designed to: 1) measure glucosinolate levels in broccoli florets from an array of genotypes grown in several environments; 2) measure the elevation of a key Phase 2 enzyme, quinone reductase (QR), in mammalian cells when they are exposed to extracts of these broccoli florets; 3) estimate broad sense heritability for the concentration of specific glucosinolates and for Phase 2 enzyme induction potential; and 4) evaluate total broccoli head GR content as a tool for future breeding for enhanced GR. There were significant environmental and genotype-by-environment effects on the levels of GR and Quinone Reductase Induction Potential (QRIP) of extracts of broccoli heads, but the effect of genotype was more prominent than the environmental factors. Broad sense heritability estimates for GR and QRIP were moderately high (0.63 and 0.49, respectively), while broad sense heritability estimates for days from transplant to maturity (DTH) were very high (0.85) and estimates for each of the three indole glucosinolates measured were very low (all less than 0.2). When GR and QRIP were compared among genotypes, comparisons were different depending whether the focus was on concentration or total content per head. In particular, the range among genotypes widened when comparing total content per head versus concentration per gram of floret and their relative ranking changed.