Laboratory of Phytopathology, Faculty of Agricultural and Applied Biological Sciences, University of Gent, Coupure links, 653, B-9000 Gent, Belgium

Background and objectives
Root colonization by some non-pathogenic rhizobacteria can increase resistance throughout the whole plant. This kind of induced systemic resistance was observed for Pseudomonas aeruginosa 7NSK2 in bean, tomato and tobacco. Using salicylic acid (SA)-deficient mutants it was demonstrated that P. aeruginosa 7NSK2-induced resistance depends on the production of SA by this strain [1]. SA is a known modulator of plant defence responses to pathogens. After pathogen infection, SA accumulates locally and to a lesser extent systemically. The systemic increase in SA, in the order of 1 痢/g leaf, is associated with an increase in resistance termed systemic acquired resistance (SAR) [2]. P. aeruginosa 7NSK2-induced resistance cannot easily be explained as SAR, because the amount of SA produced by this strain is estimated to be only in the nanogram range. In this work we tried to link both phenomena by applying SA to plants in amounts that occur after pathogen infection and amounts that are likely to be produced by P. aeruginosa 7NSK2 during root colonization.

Materials and methods
Surface-disinfected beans were germinated in autoclaved sand. After 6 days, plantlets were transferred to test tubes with autoclaved perlite. Plants were grown in these tubes for 12 days and received 90 ml of a half-strength Hoagland solution with 100 然 to 1 nM SA. The first pair of leaves was challenged with Botrytis cinerea to assay for induced resistance. In the same set-up, the activity of soluble root peroxidases was assayed 4 days after transfer to the test tubes.

Results and conclusions
Plants grown in nutrient solutions with 1-100 然 SA were more resistant to B. cinerea than control plants and plants grown with 100 nM SA. Given the applied SA doses, this seems to mimic pathogen-induced SAR. In roots, induction of peroxidase activity, a marker for the activation of plant defence mechanisms, was observed only in the 100 然 SA treatment. By consequence, 1 and 10 然 SA doses seem to work by affecting signalling for induced resistance rather than indirectly through mimicking a pathogen infection. Surprisingly, plants grown with 1 and 10 nM SA displayed the same resistance to B. cinerea as plants grown with 1-100 然 SA. This indicates that low amounts of SA, in the range of those produced by P. aeruginosa 7NSK2, induce a resistance similar to SAR. The absence of induced resistance in plants grown in 100 nM SA could be explained by host-plant regulation of SA levels through glucosylation. We are currently testing the hypothesis that, at nM levels, SA escapes glucosylation and serves as a signal for induced resistance. This includes the tracking of radiolabelled SA after root application and the measuring of free and bound SA in roots and leaves.

1. De Meyer G, H鐪te M, 1997. Phytopathology 87, 588-593.
2. Durner J, Shah J, Klessig DF, 1997. Trends in Plant Science 2, 266-274.