Intercellular salicylic acid accumulation is important for age-related resistance in Arabidopsis to Pseudomonas syringae

As Arabidopsis plants mature, they develop age-related resistance (ARR) to virulent Pseudomonas syringae. As for other defense responses, salicylic acid (SA) accumulation is required for ARR [Kus JV, Zaton K, Sarkar R, Cameron RK. Age-related resistance in Arabidopsis is a developmentally regulated defense response to P. syringae. Plant Cell 2002;14:479–90]. SA levels in the intercellular space increased during ARR and correlated in some experiments with anti-bacterial activity detection in intercellular washing fluids. Growth of Pseudomonas in culture was inhibited by 1mM SA, providing support for an anti-microbial role for SA during ARR. Application of SA to the intercellular space rescued the ARR-defective mutant sid2, but not the SA-hydrolyzing transgenic line, NahG. The protection afforded by SA application in sid2 was observed only while SA levels remained elevated in the intercellular space. Destruction of SA in the intercellular space, by salicylate hydroxylase application, reduced the ARR response in wild type plants. These results strongly suggest that SA accumulation in the intercellular space is critical for manifestation of ARR.