Are nitric oxide or its derivatives the phloematic systemic signals responsible for systemic acquired resistance in Arabidopsis thaliana?

Using a specific and sensitive solid-state amperometric sensor assay for NO and its derivatives, we observed significant increases in nitrite in phloem sap collected from Arabidopsis thaliana (ecotype Columbia) plants in which SAR had been induced with an avirulent strain of Pseudomonas syringae pv. maculicola. The accumulation was detected in both inoculated and non-inoculated leaves from 3 to 12 hours post-inoculation. Phloematic increases in nitrite were also detected in plants treated with the NO-releasing compound NOC-18 [2,2’-(hydroxynitrosohydrazino)bis-ethanamine], which systemically protected them against a virulent strain of Pseudomonas syringae pv. maculicola. The protection was detected as reduction in bacterial growth in planta. Further investigations are in progress to verify whether NOC-18 treatment induces transcript changes in SAR genes and to establish whether NO derivatives are the systemic signals of SAR.