|dc.description.abstract|| Glucosinolate myrosinase system is well-established defense mechanism against insect herbivores feed on plants from Brassicaceae family. This defense mechanism is also known as the mustard oil bomb which bring myrosinase enzyme and glucosinolate together upon insect attack and tissue disruption and cause glucosinolate hydrolysis by myrosinase enzyme activity. Glucosinolate and its hydrolysis products have a deterring effect on herbivores, and protect plant parts from insect damage. The modified plants have been named MINELESS due to lack of myrosin cells.
The aim of this study was to get an insight into defense responses of MINELESS and wild- type Brassica napus plants after being challenged by insects Delia radicum, Delia floralis, Mamestra brassicae and Meligethes aeneus. The hypothesis behind this work were due to lack of myrosin cells and the enzyme myrosinase, the glucosinolate-myrosinase system is modulated and MINELESS plants possess more glucosinolates in plant tissues. Glucosinolate have adverse and deterring effects on insect herbivores, and larvae of generalist herbivore, Mamestra brassicae preferred to feed on wild-type cotyledons of Brassica napus plants, which contain lower amounts of glucosinolates than those of MINELESS. Role of glucosinolates on attraction of pollen beetles, Meligethes aeneus, is not exactly known, however, pollen beetles are attracted by glucosinolates hydrolysis products, isothyocyanates. Wild-type plants were preferred by pollen beetles for feeding over MINELESS plants which possess high levels of glucosinolates, but low levels of isothiocyanates due to lack of myrosinase activity.
Four glucosinolates were detected with the highest amount of indol-3-ylmethyl- (I3M) glucosinolate both in control and insect challenged aboveground tissues, with slightly higher amounts in MINELESS plants than wild-type. However total glucosinolate levels did not show exact difference between MINELESS and wild-type plants. I3M glucosinolates levels increased upon herbivore feeding in all treatment days. Increasing days of treatment increased levels of aliphatic glucosinolate, 5-methylsulphinylpentyl- (5MSOP) glucosinolate and decreased levels of indole glucosinolate, I3M glucosinolate.
In both no-choice and free choice experiments, M. brassicae larvae consumed more wild-type cotyledons than those of MINELESS. In no-choice experiments consumed area of cotyledons increased in correlation with increasing number of days of feeding, and wild-type plants were consumed more than MINELESS plants.
The ablation of myrosin cells and consequent lack of myrosinase activity did not affect feeding by pollen beetles in both feeding experiments. Wild-type buds were damaged more than MINELESS buds, however the difference was not significant. Also in Y-tube olfactometer experiment beetles moved mostly towards odour of oilseed rape flowers rather than fresh air. Due to the small number of individuals tested, we did not consider this result significant.||