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 ortobacco necrosis virus

In tomato plants systemic acquired resistance (SAR) against late blight cau sed by Phytophthora infestans was induced by pre-treatment with a chemical inducer, DL-3-aminobutyric acid (3-ABA), or by pre-inoculation with a bioti c inducer, Tobacco necrosis virus (TNV). Ultrastructural studies revealed t hat haustoria of the late blight fungus in tomato leaves expressing SAR wer e morphologically changed and some of them were severely damaged. Both indu cers eventually caused a significant inhibition of haustorium development a nd 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. Immunofluorescence labelling investi gations demonstrated a systemic accumulation of AP24 in leaves of tomato pl ants either pre-treated with 3-ABA or pre-inoculated with TNV. In the corre sponding leaves of control plants without SAR induction this protein was ac cumulated only after inoculation With P. infestans. Furthermore, AP24 becam e detectable by immunogold labelling in starch granules of chloroplasts of untreated upper leaves expressing SAR. After invasion of the late blight fu ngus, AP24 was also detected in fungal cell walls and the space formed betw een fungal cell walls and the invaginated plasma membrane. A more dense acc umulation of AP24 was observed in fungal cell walls as well as in cell wall appositions (papillae) in leaves expressing SAR. These findings indicate t hat 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 giants exhibiting SAR.