1.4.25
A NOVEL SIGNALLING PATHWAY CONTROLLING INDUCED SYSTEMIC RESISTANCE IN ARABIDOPSIS

CMJ PIETERSE, SCM VAN WEES, JA VAN PELT and LC VAN LOON

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
Selected non-pathogenic, root-colonizing Pseudomonas fluorescens bacteria are able to protect Arabidopsis plants against pathogen attack by triggering an induced systemic resistance (ISR) response [1, 2]. Phenotypically, ISR appears similar to classic, pathogen-induced systemic acquired resistance (SAR) of which the signalling pathway is relatively well studied. To investigate whether pathogen-induced SAR and non-pathogenic rhizobacteria-mediated ISR are controlled by the same signalling pathway, Arabidopsis mutants and transgenics that are altered in their response to the signalling molecules salicylic acid (SA), ethylene or jasmonic acid (JA) were tested on their ability to express ISR and SAR, respectively, against the challenging pathogen P. syringae pv. tomato.

Results and conclusions
SA-non-accumulating NahG plants expressed rhizobacteria-mediated ISR to the same level as wild-type Col-0 plants, whereas pathogen-induced SAR was blocked in these plants. In contrast, the ethylene response mutant etr1 and the JA response mutant jar1 readily expressed SAR, whereas ISR was impaired in these mutants. Moreover, ISR did not coincide with the activation of genes encoding pathogenesis-related (PR) proteins, which is a typical characteristic of SAR. These results clearly demonstrate that non-pathogenic rhizobacteria trigger a signalling pathway different from that controlling SAR, leading to a form of systemic resistance which, in contrast to SAR, is independent of SA accumulation and PR gene expression, but instead requires responsiveness to the plant hormones ethylene and JA. Interestingly, mutant npr1, which was originally isolated for its inability to express PR genes and as a result is unable to express SAR, was also not responsive to induction of ISR. This suggests that NPR1 plays a crucial role in ISR as well.

To elucidate the sequence of signalling events involved in ISR, the resistance-inducing abilities of methyl jasmonate (MEJA) and ACC were tested. ACC, which is readily converted into ethylene in the plant, induced protection in Col-0, NahG and jar1 plants but not in etr1 plants. Furthermore, MEJA induced resistance in Col-0 and NahG plants but not in jar1 and etr1, suggesting that components from the ethylene response are required to facilitate MEJA-induced protection. In npr1 plants, responsiveness to MEJA or ACC was highly diminished, suggesting that the essential components from the JA and ethylene responses act upstream of NPR1. Based on the striking resemblance to rhizobacteria-mediated ISR, we postulate that in ISR signal transduction, the JA and ethylene responses are successively engaged to trigger a defence response that is regulated by NPR1.

References
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.