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STUDIES ON INSECT PESTS OF BRASSICA NAPUS AND THE ROLE OF THE MYROSINASE-GLUCOSINOLATE SYSTEM


STUDIES ON INSECT PESTS OF BRASSICA NAPUS AND THE ROLE OF THE MYROSINASE-GLUCOSINOLATE SYSTEM

BO PONTOPPIDAN, RICHARD HOPKINS 1 , BARBAPA EKBOM 1 , SUSANNA EPIKSSON, BO EK, LARS PASK2, JOHAN MEIJER.

Dept. of Plant Biology, Uppsala Genetic Center, SLU, S-750 07 Uppsala, Sweden 1 Dept. of Entomology, SLU, S-750 07 Uppsala, Sweden 2 Dept. of Medical and Physiological Chemistry, Biomedical Center, Uppsala University, S-751 23 Uppsala, Sweden. .

Background and Objectives

Upon tissue damage glucosinolates in cruciferous plants are hydrolysed by the J3-thioglucosidase myrosinase. Certain myrosinase isoforms occur in large complexes with myrosinase-associated protein (MYAP) and myrosinase binding proteins (MBPS) and other as yet unidentified proteins. The myrosinase-glucosinolate system has long been suggested to be involved in the defence against insect herbivores due to the toxic nature of the released hydrolysis products [ 1 ]. We have found that isoforms of MBP and MYAP are induced by mechanical wounding or jasmonate treatment. The physiological role of MYAP and MBP is not clear, however. Despite the presence of the myrosinase-glucosinolate system a number of insect species have become specialised on crucifers as hosts. These insects have either developed a way of handling the plants defence system or they manage to avoid triggering it. Previous studies on the induction of these proteins have been carried out using purely mechanical damage. We suggest that this treatment may not be a good general mimic of insect damage since different insects have different feeding behaviour and furthermore the insect may liberate compounds that affect the response of the plant [2]. We are thus conducting studies on the effects on the wound response of oilseed rape Brassica napus to different insect pests as well as the insects ability to cope with glucosinolates.

Results and Conclusions

Oilseed rape plants at the stage of two true leaves was subjected to herbivory for one hour by last instar larvae of Diamondback Moth (Plutella xylostella) or to mechanical damage of the first leaf. Samples were collected at several times after challenge and analysed by Northern blotting. Preliminary experiments indicated that wound induction by Diamondback moth larvae is more rapid but weaker than the purely mechanical damage. We will also initiate wounding experiments on oilseed rape plants during the summer with pollen beetles (Meligethes aeneus) and flea beetles (Phyllotreta undulata ). Analysis of insects have revealed significant myrosinase activity in some species. We have purified a myrosinase from the mustard aphid, which will be analysed further by peptide sequencing and for catalytic properties.

This study was supported by SLU, the Foundation for Strategic Research and the Swedish Research Council for Forestry and Agriculture.

References

[1] Bones AM and Rossiter JT (1996) Physiol Plantarum 97: 194-208
[2] Ekbom B (1995) Brassica oilseeds (Kimber and McGregor, eds) CAB Int, Cambridge, pp. 141-152