L SCHELLENBAUM¹, L ZIMMERLI¹, JP METRAUX¹ and B MAUCH-MANI¹

¹Department of Biology, Plant Biology, University of Fribourg, route A. Gockel 3, 1700 Fribourg, Switzerland

Background and objectives
The general phenomenon of plant immunization has been known for several decades. It consists of inducing genetically susceptible plants to defend themselves against attack by a virulent pathogen. In nature, immunization can occur when plants come into contact with a necrotizing pathogen. Immunization takes place not only at the primary infection site, but also in distant (systemic) parts of the plant. The necrosis-related immunization process seems to follow a salicylic acid (SA)-dependent pathway. Strong support for the implication of SA in this pathway comes from experiments with transgenic plants (expressing the NahG gene for salicylate hydroxylase) unable to accumulate SA. Such plants become susceptible to avirulent pathogens and cannot be immunized.

Treatment of plants with SA or chemicals such as benzo (1,2,3) thiadiazole-7-carbothioic acid S-methyl ester (BTH) and 2,6 dichloroisonicotinic acid (INA) also leads to subsequent immunization. These compounds can induce resistance through the accumulation of pathogenesis-related proteins.

Necrotizing agents and chemicals are not the only way to immunize plants against pathogens. Plant-growth-promoting rhizobacteria have also been shown to induce systemic resistance in plants. For example, Pseudomonas fluorescens strains can induce resistance against viruses, bacteria and fungi. However, little is known about the underlying mechanisms and the molecular basis involved.

Results and conclusions
We present a comparison of these different immunization processes by using the pathosystem Arabidopsis thaliana-Peronospora parasitica. Similarities and differences are shown at the molecular, biochemical and microscopical levels.