Cornell University, Department of Plant Pathology, Ithaca NY, USA

Background and objectives
Pathogen-induced resistance mechanisms are essential for prevention of plant disease. These include systemic acquired resistance (SAR) and other inducible systems that are independent of SAR. To gain an understanding of the regulation of SAR and SAR-independent resistance (SIR), we are employing genetic and molecular methods to identify and isolate genes important to these processes.

In previous work with J Ryals and collaborators, we showed that salicylic acid (SA) was an essential signalling molecule for activation of SAR. We subsequently identified an Arabidopsis gene called NIM1 (non-inducible immunity-1), required for transduction of SA signals in activation of SAR [1]. Upon cloning, NIM1 was found to encode a protein that contains ankyrin repeats, and appeared homologous to vertebrate Ik-B and Drosophila Cactus, repressors of transcription factors in those animals [2]. The NIM1 gene was independently identified by X Dong and co-workers (called NPR1), and by D Klessig and co-workers (called SAI1): Dong's group also cloned NPR1. Our present objectives are to increase understanding of SAR regulation by analysing NIM1 protein function and by identifying proteins that physically interact with NIM1. We are also seeking to identify other genes that control expression of SAR by searching for second site suppressor mutations that mask the nim1 mutant phenotype.

Our work to identify the determinants of SIR stems from observations that SA-depleted plants and nim1 mutants, which are defective in expression of SAR, show a partial breakdown in race-specific resistance yet still express partial resistance against avirulent pathogen races. [1, 3]. We hope to identify the determinants of resistance that are still functional in these SAR-compromised plants, through genetic screens for enhanced susceptibility to avirulent pathogens and by identification of mRNA species that are independent of SAR, yet induced by pathogen. Both approaches involve nim1 mutant and/or SA-degrading genotypes to eliminate responses due to SAR. Genes identified in these experiments are candidate SIR loci.

Results and conclusions
The predicted structure of NIM1 predicts its interaction with other proteins, via ankyrin repeats. We have conducted yeast two-hybrid screens, and have identified candidate NIM1-partner proteins. Tests are under way to validate their significance. Suppressor screens have revealed several mutations that confer resistance to compatible pathogens in nim1 plants. Screens for SIR mutants have led to the identification of several nim1 lines with enhanced susceptibility to avirulent races of Peronospora parasitica.

Greater understanding of the regulation of SAR and SIR will provide important new knowledge about plant signal transduction pathways. Findings in Arabidopsis are likely to be applicable to agricultural species, because the pathways underlying induced resistance appear to be conserved in diverse plant groups. In addition to enhancing basic understanding of plant disease resistance, studies of induced resistance may also permit development of novel strategies for creating disease-resistant crop plants.

1. Delaney TP, Friedrich L, Ryals JA, 1995. Proceedings of the National Academy of Sciences, USA 92, 6602-6606.
2. Ryals J, Weymann K, Lawton KA et al., 1997. Plant Cell 9, 425-439.
3. Delaney TP, 1997. Plant Physiology 106, 5-12.