LOCAL AND SYSTEMIC BIOSYNTHESIS OF SALICYLIC-ACID IN INFECTED CUCUMBER PLANTS

Radiolabeling studies showed that salicylic acid (SA), an essential co mponent in the signal transduction pathway leading to systemic acquire d resistance, is synthesized from phenylalanine (Phe) and benzoic acid in cucumber (Cucumis sativus L.) plants inoculated with pathogens. Le af discs from plants inoculated with either tobacco necrosis virus or Pseudomonas lachrymans incorporated more [C-14]Phe into [C-14]SA than mock-inoculated controls. The identity of SA was confirmed by gas chro matography-mass spectrometry. No reduction in specific activity of [C- 14]SA was observed for either free or bound SA between control and inf ected plants after feeding [C-14]phe. A specific inhibitor of Phe ammo nia-lyase, 2-aminoindan-2-phosphonic acid, completely inhibited the in corporation of [C-14]Phe into [C-14]SA, although plants treated with 2 -aminoindan-2-phosphonic acid could still produce [C-14]SA from [C-14] benzoic acid. Biosynthesis of SA in tissue inoculated with tobacco nec rosis virus followed a transient pattern with the highest induction oc curring 72 h postinoculation. Uninfected tissues from an infected plan t synthesized de novo more SA than did controls. This suggests the inv olvement of a systemic signal triggering SA synthesis in tissue distan t from the site of infection that display systemic acquired resistance .