5.4.6S
A JASMONATE-DEPENDENT PATHWAY IN ARABIDOPSIS THALIANA IS REQUIRED FOR RESISTANCE AGAINST PARTICULAR PATHOGENS

WF BROEKAERT1, BPA THOMMA1, IAMA PENNINCKX1, A BUCHALA2, K EGGERMONT1 and BPA CAMMUE1

1FA Janssens Laboratory of Genetics, Katholieke Universiteit Leuven, K. Mercierlaan 92, B-3001 Heverlee-Leuven, Belgium; 2Institut de Biologie Vegetale, Universite de Fribourg, Rue Albert-Gockel 3, CH-1700 Fribourg, Switzerland

Background and objectives
Upon infection, with fungal pathogens, Arabidopsis plants have previously been shown to produce higher levels of the signalling compound salicylic acid (SA), which in turn activates the antimicrobial protein genes PR-1, PR-2, and PR-5 [1]. The importance of this pathway in natural resistance against pathogens has been demonstrated by a number of observations. Application of SA to Arabidopsis plants leads to protection against infection by the downy mildew fungus Peronospora parasitica [2]. Mutants blocked in their ability to induce PR-1, PR-2 and PR-3 upon pathogen challenge or SA treatment show increased susceptibility to P. parasitica, whereas mutants that overproduce SA show enhanced resistance to this fungus [3-5].

Results and conclusions
We have found that fungal infection of Arabidopsis results in a systemic increase in the levels of the lipid-derived hormone jasmonic acid (JA). Exogenous treatment of Arabidopsis with JA leads to the induction of a set of antimicrobial protein genes which is distinct from that induced upon treatment wit SA. Pathogen-induced activation of JA-inducible antimicrobial protein genes was shown to be completely independent of SA, but on the other hand was abolished in an Arabidopsis mutant affected in its ability to respond to JA. The JA-insensitive mutant was found to be highly susceptible to the pathogens Altemaria brassicicola and Botrytis cinerea

but not to >P. parasitica, whereas the reverse was observed for mutants blocked in their response to SA. Hence, Arabidopsis appears to have both an SA-dependent and a JA-dependent defence response pathway leading to induction of different sets of antimicrobial proteins. The two pathways play an essential role for resistance against distinct, possibly partially overlapping, sets of pathogens. These findings may have implications for deploying plant protection strategies based on induction of natural defence mechanisms.

References
1. Uknes et al., 1992. Plant Cell 4, 645-656.
2. Lawton et al., 1995. Molecular Plant-Microbe Interactions 8, 863-870.
3. Delaney et al., 1994. Science 266, 1247-1250.
4. Cao et al., 1994. Plant Cell 6, 1583-1592.
5. Bowling et al., 1994. Plant Cell 6, 1845-1857