1.4.6S
TRACING THE PATH OF THE MOBILE SIGNAL OF SYSTEMIC ACQUIRED RESISTANCE IN CUCUMBER

J SMITH-BECKER, SL MIDLAND, JJ SIMS and NT KEEN

University of California, Department of Plant Pathology, Riverside, California

Backround and objectives
In the classic model of systemic acquired resistance (SAR), a signal produced by an infected leaf moves through the vascular tissue to uninfected leaves, where it induces PR proteins and associated resistance to subsequent pathogen attack. The isolation of a mobile signal for SAR has eluded researchers since efforts first began more than 15 years ago. Salicylic acid has emerged as a key intermediary in the signalling pathway between pathogen-induced necrosis and the expression of resistance in uninfected tissues. However, several lines of evidence suggest that salicylic acid is not the only biochemical signal for SAR. For example, the majority of salicylic acid accumulating in phloem exudates of cucumber inoculated with P. syringae does not originate in the inoculated leaf [1]. Our long-term objective is the identification of SAR signalling components that are upstream of salicylic acid synthesis.

Results and conclusions
The production of mobile signal by inoculated cucumber leaves is followed by a transient increase in phenylalanine ammonia-lyase (PAL) activity in petioles and stems; with peaks in activity at 9 and 12 h, respectively, after inoculation. Salicylic acid begins to accumulate in phloem fluids at about the time PAL activity begins to increase, reaching maximum concentrations 15 h after inoculation. The accumulation of salicylic acid in phloem fluids occurs even if leaves and roots are excised from plants 6 h after the inducing inoculation. These results suggest that the primary target of the initial signal is the vascular tissue, where salicylic acid synthesis is induced for transport throughout the plant. Consequently, the vascular tissue may be actively involved in the perception and relay of the SAR signal, rather than simply a conduit for signal transport to uninfected tissues.

Reference
1. Rasmussen J, Hammerschmidt R, Zook M, 1991. Plant Physiology 97, 1342-1347.