Infection Structures and Localization of the Pathogenesis-related Protein AP24 in Leaves of Tomato Plants Exhibiting Systemic Acquired Resistance against Phytophthora infestans after Pre-treatment with 3-Aminobutyric acid or Tobacco Necrosis Virus

In tomato plants systemic acquired resistance (SAR) against late blight caused by Phytophthora infestans was induced by pre-treatment with a chemical inducer, DL-3- aminobutyric acid (3-ABA), or by pre-inoculation with a biotic inducer, Tobacco necrosis virus (TNV). Ultra- structural studies revealed that haustoria of the late blight fungus in tomato leaves expressing SAR were morphologically changed and some of them were severely damaged. Both inducers eventually caused a signi®cant inhibition of haustorium development and hyphal growth. To investigate the possible role of pathogenesis-related proteins (PR-proteins) in the defence of leaves of tomato plants expressing SAR, the accumulation and localization of the tomato PR-protein AP24 were studied by immunocytochemical methods. Immuno¯uorescence labelling investigations demonstra- ted a systemic accumulation of AP24 in leaves of tomato plants either pre-treated with 3-ABA or pre-inoculated with TNV. In the corresponding leaves of control plants without SAR induction this protein was accumulated only after inoculation with P. infestans. Furthermore, AP24 became detectable by immunogold labelling in starch granules of chloroplasts of untreated upper leaves expressing SAR. After invasion of the late blight fungus, AP24 was also detected in fungal cell walls and the space formed between fungal cell walls and the invaginated plasma membrane. A more dense accumulation of AP24 was observed in fungal cell walls as well as in cell wall appositions (papillae) in leaves expressing SAR. These ®ndings indicate that pre-treatment with the chemical inducers 3-ABA and pre-inoculation with the biotic inducer TNV activated a similar accumulation pattern of AP24 in leaves of tomato plants exhibiting SAR.