Departement de Biologie, Université de Fribourg, 3 Rte A Gockel, 1700 Fribourg, Switzerland

Background and objectives
In many plants, a long-lasting, broad-spectrum resistance can be induced locally and systemically after a local infection with necrotrophic microorganisms. This resistance is termed systemic acquired resistance (SAR) [1]. The onset of resistance is correlated with the accumulation of salicylic acid (SA) as well as the expression of a number of pathogenesis-related (PR) proteins. The most convincing evidence for a central role of SA in SAR is the fact that transgenic tobacco and Arabidopsis plants over-expressing the salicylic acid hydroxylase from Pseudomonas putida (encoded by the nahG gene), and therefore not accumulating SA, are not able to establish an SAR response [2]. Although SA has been shown to play an important role in the onset of SAR, the biosynthetic pathway of SA and its regulation are poorly understood. Furthermore, very little is known about the signal transduction pathway linking pathogen attack to SA biosynthesis [3]. In order to gain more information on the SA biosynthetic pathway and its regulation, a screen for Arabidopsis mutants with altered levels of SA after inoculation with a bacterial pathogen has been undertaken.

Materials and methods
A high-throughput screening for SA was designed to monitor SA levels in EMS mutants of Arabidopsis thaliana ecotype Columbia after inoculation with Pseudomonas syringae DC 3000. The analytical procedures and the conditions for inoculation are described elsewhere.

Results and conclusions
Among 4500 chemically mutagenized Arabidopsis plants inoculated with an avirulent P. syringae DC3000 strain, four mutants were isolated that have approximately 20 times reduced levels of free and conjugated SA in comparison to the wild type. These mutants were designated SA induction-deficient (sid).

Physiological characterization of the sid mutants showed that they have a block in a pathway leading to SA accumulation after attack with avirulent bacteria and biotrophic fungi, and confirmed the importance of SA in the pathway leading to SAR. In contrast, the signal transduction pathway downstream of SA leading to resistance is not affected in sid mutants. However, the last step of SA biosynthesis, the hydroxylation of benzoic acid, and the signal transduction pathway downstream of SA leading to the establishment of SAR, are not affected in sid mutants.

The genetic characterization of the sid mutants revealed that the traits are recessive and that the four mutants fall in two complementation groups. Both mutations have been mapped: sid1 is located on the lower arm of chromosome IV, whereas sid2 is located at the lower end of chromosome I. Current experiments are aimed at a detailed characterization of the sid mutants.

1. Sticher L, Mauch-Mani B, Métraux JP, 1997. Annual Review of Phytopathology 35, 235-270.
2. Ryals J, Neuenschwander U, Willits M et al., 1996. Plant Cell 8 1899-1819.
3. Yang Y, Shah J, Klessig D, 1997. Genes and Development 11, 1621-1639.