1.9.4
THE ROLE OF GLUCOSINOLATES IN PLANT DEFENCE RESPONSES TO PHOMA LINGAM

S WAHLBERG AND C DIXELIUS

Department of Plant Biology, Swedish University of Agricultural Sciences, Box 7080, S-750 07 Uppsala, Sweden

Background and objectives
Glucosinolates are secondary metabolites with a common basic structure of a glucose molecule linked with a thioglucoside bond to an N-sulphated amino acid [1]. They are found in plants within the order Capparales. About 120 different glucosinolates exist in nature, and each species has a unique profile comprising between one and 30 different compounds in different relative amounts. The profile is influenced by the developmental stage, tissue and physiological status of the plant. Upon tissue disruption, glucosinolates come into contact with the beta-thioglucosidase enzymes called myrosinases, and are subsequently broken down to form toxic and volatile hydrolysis products which have biological effects on pathogens, insects and herbivores [1, 2]. The exact role of glucosinolates in plant defence is, however, not fully understood. Our aim is to more accurately study the defence response of Brassicaceae to the fungal pathogen Phoma lingam and thus learn more about the role of glucosinolates in correlation to other defence mechanisms in this particular system. P.*nbsplingam causes blackleg in many economically important crops, and it is of great importance to gain more knowledge of the mechanisms behind resistance to this pathogen.

Materials and methods
Plant material resistant or sensitive to P. lingam was inoculated with the fungus, and leaf samples surrounding the infection site were collected at intervals post-infection ranging from 4 h to 17 days after inoculation. Leaf samples were analysed for glucosinolate profile, and for expression patterns of known defence-related genes. This facilitated comparisons between the responses in susceptible and resistant material, and correlations between glucosinolate profiles and defence gene expression. The glucosinolate studies were performed by micellar electrokinetic capillary chromatography (MECC) of desulphorylated glucosinolates, and the expression patterns analysed with Northern blots of plant RNA, using probes for cloned defence-related genes. The plant material used was Brassica napus (cv. Hanna) (sens.), B. nigra (res.) and Arabidopsis thaliana (res.). The latter species were studied in most detail, due to the potential use of mutants in the glucosinolate biosynthetic pathways.

Results and conclusions
Preliminary analyses indicate an increase in the amount of indole-glucosinolates in response to fungal infection in B. napus. This increase culminates 8-11 days after infection, and thereafter the total glucosinolate content declines. The decline at the end of infection is probably an effect of the large extent of tissue damage, and thus breakdown of glucosinolates by myrosinase. Results obtained with B. napus will be compared with data from experiments with B. nigra and A. thaliana.

References
1. Bennet RN, Wallsgrove RM, 1994. New Phytologist 127, 617-633.
2. Louda S, Mole S, 1991. In Rosenthal and Berenbaum (eds) Herbivores: Their Interactions with Secondary Plant Metabolites, 2E, Vol 1: The Chemical Participants. Academic Press, pp. 123-164.