Section of Plant Pathology, Department of Plant Ecology and Evolutionary Biology, Utrecht University, PO Box 80084, 3508 TB Utrecht, The Netherlands

Background and objectives
Plants can develop elevated levels of disease resistance after being exposed to specific biotic or abiotic stimuli. Infection by an avirulent pathogen triggers systemic acquired resistance (SAR), rendering uninfected plant parts resistant to subsequent infection. Also, selected non-pathogenic fluorescent Pseudomonas bacteria that colonize the rhizosphere are able to trigger an induced systemic resistance (ISR) response. Using Arabidopsis, it was shown that the rhizobacterium-mediated ISR response is, unlike pathogen-induced SAR, independent of salicylic acid (SA) and activation of pathogenesis-related (PR) genes, which are markers for SAR [1, 2]. To further compare the signalling pathways controlling ISR and SAR, Arabidopsis plants, impaired in their response to the defence-related signalling molecules jasmonic acid (JA) and ethylene, were tested on their ability to express ISR or SAR against infection by the challenging pathogen P. syringae pv. tomato (Pst). ISR induced by P. fluorescens WCS4I7r was blocked in the JA-response mutant jar1 and the ethylene-response mutant etr1, but not in SA-non-accumulating NahG plants. This indicates that for the expression of ISR, responsiveness to JA and ethylene is required, but that SA is not necessary. In contrast, SAR induced by the avirulent pathogen Pst(avrRpt2) was blocked in NahG plants but not in jar1 or etr1, indicating that SAR is independent of the JA- and ethylene-response, but instead requires SA. Apparently, ISR and SAR are two distinct inducible defence responses. We investigated whether elicitation of both ISR and SAR leads to a higher level of protection.

Results and discussion
Plants were treated with a combination of WCS4I7r and Pst(avrRpt2). Indeed, significantly better protection was established by the combination treatment than by either inducer alone. This additive effect was absent in jar1, etr1 and NahG plants, in which either the ISR or the SAR response was blocked. Northern blot analyses showed that PR genes were activated only after infection by Pst(avrRpt2) and not after treatment with WCS4I7r. The combination of the two inducers of resistance did not result in an enhancement of PR gene expression, suggesting that the ISR pathway triggered by WCS4I7r does not affect the one leading to SAR. On the other hand, the results strongly indicate that the ISR- and SAR-signalling pathways leading to the enhanced defensive capacity are dissimilar. Also the signalling molecules SA, JA and ethylene were tested for additional effects on WCS4I7r-mediated ISR. WCS4I7r, SA, methyl jasmonate (MeJA) and ACC (converted into ethylene in the plant) all induced resistance. Only the combination of WCS417r with SA increased the level of protection compared to the effect of the inducers alone. WCS4I 7r did not enhance MeJA- or ACC-induced protection, indicating that WCS417r saturates the MeJA- and ACC-responsive signalling pathway. These results demonstrate that the enhanced protection resulting from the WCS4I7r-stimulated ISR-pathway is at least partly additive to that elicited by SA.

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 Pant-Microbe Interactions 10, 716-724.