Relationship of bacterial growth and chemical events in the leaf apoplast during susceptible and resistant interactions with Pseudomonas syringae species and tobacco (Nicotiana tabacum)

The leaf apoplast is the first battleground for plant pathogens invading via stomata. The apoplast is the border just outside the plant cell membrane and in a leaf, much of the apoplast is in direct contact or near the air space within the leaf. It is one of the most accessible parts of the plant for the study of early events in the bacteria/plant interactions and likewise it offers lucrative possibilities for plant disease control. Recent studies have shown that several apoplast phenolics respond to bacterial infection within the first few hours after inoculation. Two phenolics in particular, acetovanillone and acetosyringone, vary in their timing and concentrations depending on the type of interaction: saprophytic (Pseudomonas fluorescens), susceptible (P. syringae pv. tabaci), or resistant (P. s. pv. syringae). In pathogenic interactions, a second phase occurs which involves a dramatic increase of chlorogenic acid into the apoplast. Chlorogenic acid is stored in the plant vacuole and its appearance indicates leakage across the apoplast/symplast (A/S) barrier. All of these changes in apoplast phenolics could affect bacterial growth. Therefore we have tried to correlate bacterial multiplication with these chemical events in the three types of interactions. The oxidative environment of the apoplast that is thought to exist during these early periods suggests that these phenolic compounds would influence the redox environment and could directly affect bacterial multiplication.