Pre-treatment of cucumber plants with acibenzolar-S-methyl systemically primes a phenylalanine ammonia lyase gene (PAL1) for enhanced expression upon attack with a pathogenic fungus

Systemic acquired resistance (SAR) of plants induced upon infection with a necrotizing pathogen, is characterised by an enhanced capacity to induce cellular defence responses to subsequent pathogen attack. Pre-treating the first leaves of cucumber plants with the synthetic SAR activator acibenzolar-S-methyl (ASM; S-methylbenzo[1,2,3] thiadiazole-7-carbothiate) protects whole plants from infection with the virulent fungal pathogen Colletotrichum orbiculare. Using a sensitive fluorescence-based RT-PCR system to follow the expression of three defence-related genes in the third leaves of treated cucumber plants, we have demonstrated, in non-inoculated plants, that ASM treatment alone systemically induces the expression of an acidic peroxidase-encoding gene (POX) and pathogenesis-related protein I gene homologue (PRl-1a). In contrast, expression of a gene encoding a phenylalanine ammonia lyase homologue (PAL1) is not directly responsive to ASM treatment. However, expression of POX, PRl-1a, and PAL1 is primed for ASM-dependent enhancement upon subsequent inoculation with C. orbiculare. The results suggest the systemic potentiation of defence-related gene expression is a key component of SAR. (C) 2003 Elsevier Science Ltd. All rights reserved.