Salicylic acid and its location in response to biotic and abiotic stress.

Salicylic acid (SA) is an important signal involved in the activation of plant defence responses against abiotic and biotic stress. SA may derive from the phenylpropanoid pathway or via isochorismate synthase as demonstrated in Nicotiana benthamiana, tomato and Arabidopsis thaliana. The phenylpropanoid pathway as well as isochorismate synthase are localized in the chloroplasts but it remains unknown if the end product SA is in the same organelle. We have studied the localization of SA in A. thaliana using the salicylate hydroxylase (NahG) gene expressed with a chloroplast targeting sequence. Plants expressing NahG in the chloroplasts are unable to accumulate SA induced after pathogen or UV exposure. Our data infer that SA is initially located in the chloroplasts.