The entry of bacteria into the plant during the infection process leads to various types of interaction, observable at the level of the whole plant, constituent tissues or individual cells. These interactions have been investigated experimentally by artificial infiltration of intact plants (‘Inoculation of intact plants’, this page) or by the use of in vitro systems (including micropropagates, excised organs and cell suspensions; see ‘Use of in vitro systems’, p. 132).

Inoculation of intact plants

The effect of different bacteria in determining the nature of the plant response was initially demonstrated by Klement et al. (1964), who artificially infiltrated leaves of tobacco with a range of bacterial species (Fig. 6.1) and observed three main types of result:

1. Hypersensitive reaction (HR): where there is typically a rapid death of the plant cells, with no spread of bacteria to surrounding tissues. This reaction was induced by a range of bacteria comprising various pathovars of Pseudomonas syringae.

2. Disease reaction: involving a delayed host cell response, with spread of bacteria to other parts of the plant. This reaction was induced by Pseudomonas syringae pv. tabaci and resulted in wildfire disease.

3. No observable reaction, after infiltration of the saprophytic bacterium Pseudomonas fluorescens.

The results obtained by Klement et al.