Accumulation of cell-wall bound phenolic compounds and phytoalexin in Arabidopsis thaliana leaves following inoculation with pathovars of Pseudomonas syringae

Here we have examined genetically well-defined interactions in leaves of Arabidopsis ecotype Col-5 during compatible and incompatible interactions with isogenic pairs of Pseudomonas syringae pathovars tomato (Pst) DC3000 and phaseolicola (Pph) 1448A strains differing only in the presence or absences of cloned avr genes that determine the compatibility and incompatibility of the reaction. Strain Pst DC3000 was virulent causing formation of a spreading lesion surrounded by a chlorotic halo. The bean pathogen Pph 1448A (as a non-host pathogen) caused no visible symptoms apart from very patchy discolouration after 3 days. The presence of the avirulence genes avrPpiA and avrPphB in Pph 1448A led to a rapid or slow HR owing to interaction with the resistance genes RPM1 and RPS5 respectively. The hrpL mutant of Pph failed to produce symptoms. Microscopical and biochemical studies were used for in situ localization of plant molecules involved in plant resistance such as autofluorescent phenolics, lignin and camalexin. A strong correlation was observed between the timing and extent of cell death and high levels of phenolic and lignin accumulation in cell-walls and cytoplasm of cells undergoing the HR, whereas no staining of lignin-like material was observed during the compatible interaction. Accumulation of phytoalexin, camalexin, was associated with tissue necrosis whether during the HR or a susceptible response. Although high levels of camalexin were found within tissues inoculated with the virulent strain DC3000, the earliest detection of camalexin occurred during HR following inoculation with avirulent strain. The results indicate that accumulation of phenolics but not camalexin may be involved in resistance against bacterial pathogens used in this study.