The Hypersensitive Reaction and Other Responses of Bean Leaves to Bacteria

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Abstract
Responses of leaves of bean (Phaseolus vulgaris) to inoculation were classified as hypersensitive with Pseudomonas mors-prunorum, hypersensitive or susceptible with P. phaseolicola Races 1 and 2 depending on race, or none with P. fluorescens. In hypersensitive responses numbers of bacteria increased little after the first 1–2 days. In susceptible tissues numbers continued to increase for 4 days to values about a thousand-fold higher than in resistant tissues. Numbers of P. fluorescens did not increase after inoculation. Hypersensitive responses were characterized visibly, microscopically and by increases in the permeability of leaf cells to ions. Light and humidity had little effect on responses which were affected by temperature; responses were most rapid at 25 °C and still occurred at 37 °C. There were none in resistant leaves inoculated with suspensions containing less than 1.2 ± 106 viable cells ml−1 from one-day cultures or with suspensions of c. 105 viable cells ml−1 from 8-day cultures of P. mors-prunorum or P. phaseolicola Race 1. Time for induction of hypersensitive responses was 60–90 min at the end of which marked increases in permeability of cells of inoculated leaves could be detected; permeability continued to increase with time. Corresponding changes in susceptible tissues at first were delayed by 1–2 days but later were much more pronounced. Inoculation with P. fluorescens caused little or no change in permeability. In tissues involved in hypersensitive responses, cytological changes were readily detected about 2 h after inoculation; they were followed by extensive disorganization of cytoplasm which was partially characterized by transmission electron-microscopy. The following did not induce the characteristic hypersensitive response caused by bacteria: bacteria killed in various ways or the liquid in which they were killed; extracts of, or intercellular fluids from leaves in which hypersensitive responses were developing; cell walls from bacteria; cell-free filtrates from cultures of bacteria in various media, including intercellular fluid from leaves.

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