Section of Plant Pathology, Department of Plant Ecology and Evolutionary Biology, Utrecht University, PO Box 800-84, 3508 TB Utrecht, The Netherlands

Background and objectives
Plants have the ability to acquire an enhanced level of resistance against pathogen attack after appropriate stimulation. Classic systemic acquired resistance (SAR) is a pathogen-inducible defence mechanism that is dependent on salicylic acid (SA) and is associated with the systemic accumulation of pathogenesis-related (PR) proteins. Selected non-pathogenic, root-colonizing Pseudomonas fluorescens bacteria are able to elicit a phenotypically similar systemic resistance response. Using Arabidopsis thaliana as a model, it was shown that this rhizobacteria-mediated induced systemic resistance (ISR) follows a signalling pathway that, unlike SAR, is independent of SA and PR gene activation [1, 2]. In this study the genetic background of ISR and the relationship between ISR and basal resistance against the challenging pathogen P. syringae pv. tomato (Pst) is investigated.

Results and conclusions
Different Arabidopsis ecotypes were screened for their potential to express ISR against infection by Pst, after root treatment with the non-pathogenic P. fluorescens strain WCS417r. From the 10 ecotypes tested, two ecotypes (RLD and Ws-0) did not develop ISR after treatment with P. fluorescens WCS417r. This ISR-non-responsive phenotype was correlated with a remarkably high level of susceptibility to Pst, suggesting that the ability to express ISR is dependent on a threshold level of basal resistance. Subsequently, crosses were made between ISR-responsive (Col-0 and Ler) and ISR-non-responsive ecotypes (RLD and Ws-0). The F1 hybrids were, like their ISR-responsive parents, fully capable of expressing ISR and exhibited a relatively high level of basal resistance. This indicates that the potential to express ISR and basal resistance against Pst are both inherited as dominant traits. The ISR-non-responsive ecotypes RLD and Ws-0 failed to complement each other for the potential to express ISR and basal resistance in their F1 progeny, indicating that RLD and Ws-0 are allelic. Analysis of F2 plants derived from the RLDxCol-O cross, revealed that the potential to express ISR and basal resistance against Pst co-segregate in a 3:1 fashion, indicating that both characteristics are monogenically determined and genetically linked. To further investigate the molecular basis underlying the relationship between ISR and basal resistance, Arabidopsis mutants with enhanced disease susceptibility to several virulent bacterial pathogens are currently investigated for their ability to express SR.

1. Pieterse CMJ, van Wees SCM, Hoffland E et al., 1996. Plant Cell 8, 1225-1237.
2. van Wees SCM, Pieterse CMJ, Trijssenaar A et al., 1997. Molecular Plant-Microbe Interactions 10, 716-724.