Biological Control Agents That Are Good for Our Health and Bad for Pests?
Brassica plants such as winter rape, turnip, cabbage, broccoli, and kale are rich in sulfur containing compounds called glucosinolates. These plants are currently receiving renewed attention as important dietary sources of compounds (primarily glucosinolates and anti-oxidants) which protect against cancer. Glucosinolate concentrations vary within and between different species and subspecies of Brassica. For example, glucosinolate levels were examined in five subspecies and sixty-five accessions of Brassica oleracea (including 50 broccoli accessions) grown under the same cultural conditions. Among commercially grown broccoli cultivars, "Brigadier" contained 3.5 and 6.4-fold higher total glucosinolates than "Packman" and "Baccus". About 80% of the total glucosinolates in broccoli are present as glucoraphanin (4-methyl-sulphinylbutyl glucosinolate), an aliphatic glucosinolate. When glucoraphanin comes in contact with myrosinase it breaks down into sulforaphane -- one of the most potent anti-cancer compounds in Brassicas.
Glucosinolates in Brassicas also have figured prominently in the evolution of these plants' chemical defenses to herbivores, plant pathogens, and weeds. With insect herbivores, glucosinolates in short-lived (non-perennial) plants are proposed to act at low concentrations primarily as qualitative defenses against nonadapted generalist herbivores while specialist herbivores, on the other hand, are able to overcome glucosinolates, even to the point of using them as feeding and oviposition stimulants. Nevertheless, the effects of glucosinolates on both generalist and specialist herbivores may be determined by the specific glucosinolates present and their concentrations. For example, maximum herbivory on naturalized mustard plants (Brassica rapa) by two crucifer insect specialists, the flea beetle Phyllotreta cruciferae and the diamondback moth, occurred at intermediate glucosinolate levels. Thus the specific glucosinolates and their concentrations present in certain Brassica cultivars, in combination with correlated resistance factors, might be used to affect populations of some insect pests specializing on crucifers.
Glucosinolates in plant material decomposing in the soil may be converted into organic isothiocyanates, thiocyanates, and nitriles which reduce weed seed germination. In potatoes, with fall-planted and spring-incorporated "Juniper" winter rape (Brassica napas), weed biomass was reduced 96% when common lambsquarter (Chenopodium album) and 50% when redroot pigweed (Amaranthus retroflexus) were the major weed species. In one study total glucosinolate levels were higher in Brassica napas roots than in shoots. But Brassica shoots have been generally found to be more active than roots against weeds. One reason may be the specific glucosinolate found in each portion of the plants. In roots, phenylethyl glucosinolate made up to 80% of the total glucosinolates, while in shoots four compounds (4-pentenyl glucosinolate and 3-butenyl glucosinolate and their 2- hydroxy derivatives) made up to 90% of the total glucosinolates. The later compounds may have more activity against seed germination or may persist longer. The concept of incorporating Brassica residues for weed control and to suppress other soil-borne pests is biofumigation. The incorporation of Brassica may even have a role in replacing methyl bromide as that product is phased out.
Plant breeders are developing Brassica genotypes that produce high levels of glucosinolates to help fight cancer. They should also consider the pest suppressive capabilities of glucosinolates. In the future, not only will it be important to eat your broccoli to remain healthy, but growing broccoli high in glucosinolates might also be an important tactic in biological control.
- John Masiunas and Catherine Eastman, University of Illinois
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