Salicylic acid in plant defense responses.

The importance of salicylic acid (SA) in both local resistance to pathogens and the subsequent establishment of systemic acquired resistance has been investigated in tobacco. In order to assess the role of SA in the plant defense response, it was decided to try to stop the accumulation of SA by expression of an enzyme able to degrade it. A gene encoding salicylate hydroxylase (SH-L) was cloned from Pseudomonas putida and shown to be functional in E. coli. This gene was fused to various plant promoters which should allow temporal and spatial alteration of salicylate accumulation in pathogen-challenged tobacco. The promoters chosen included the CaMV35S promoter which allowed constitutive expression of SH-L and thus total inhibition of SA accumulation. An AoPR1 promoter was used to inhibit the early accumulation of SA around developing lesions. The tobacco acidic PR1a gene promoter was used to drive SH-L expression in response to endogenous SA accumulation both locally and systemically following localised pathogen attack. Two pathogen systems, one viral, tobacco mosaic virus (TMV) and one bacterial. Pseudomonas syringae, were used to analyse how their interactions with tobacco were altered in various SH-L backgrounds by examining lesion phenotypes, defence gene expression and SA levels. It was found that local PR protein (at least PR1a) induction is dependent on salicylate-mediated signalling and that SA is absolutely required in the development of HR to limit virus spread and kill bacteria. It was also confirmed that SA is required for the establishment of systemic acquired resistance (SAR). A recent hypothesis on the mechanism of action of SA is that SA may function in plants by inhibiting catalase thus allowing the accumulation of hydrogen peroxide (H2O2) which can then act as a second messenger to switch on defence gene expression and activate SAR. The theory was tested using transgenic plants unable to accumulate salicylate. It was concluded that SA does not function downstream of hydrogen peroxide in the induction of PR proteins.