Biochemical and cytological studies on mechanisms of systemically induced resistance to Phytophthora infestans in tomato plants

Plants can respond to exogenous stimuli, for example biotic or abiotic factors, with physiological changes triggering a systemic resistance (systemic acquired resistance; SAR) against plant diseases. In tomato plants, syllabication against late blight caused by Phytophthora infestans could be induced by pretreatment with the chemical inducer DL-3-aminobutyric acid (BABA) and also by pre-inoculation with the biological inducer tobacco necrosis virus (TNV). In this study, the mechanisms of SAR were compared in tomato plants pretreated with BABA and pie-inoculated with TNV using biochemical and cytological methods. Biochemical stud ies revealed increased contents of free salicylic acid (SA) in tomato leaves treated with both inducers. However, no enhancement of SA contents was found in the untreated upper leaves expressing SAR at 3 and 7 days after treatment with BABA and inoculation with TNV, respectively. Thus, the locally elevated SA levels might be necessary for the synthesis of the still unknown signal responsible for SAR expression. Inoculation with P. infestans also caused local SA accumulation in tomato leaves. However, the elevated SA contents of leaves exhibiting SAR were comparable with those of control plants after fungal infection. indicating that no further increase of the se level is necessary for SAR expression. In fluorescence-microscopic studies no inhibition of pathogen development was observed on the leaf surfaces of tomato plants exhibiting SAR mediated by BABA or TNV. In contrast to the leaf surfaces, the penetration sites of P. infestans in SAP-expressing plants showed intensive fluorescence after aniline blue staining which indicates the formation of callose. At these sites a significant growth inhibition and morphological changes of intercellular hyphae were detected. In addition, hypersensitive reactions were frequently observed in leaves exhibiting SAR. The results of the biochemical and cytological studies suggest that the chemical activator BABA and the biological inducer TNV trigger similar defence mechanisms in tomato plants.