Bacterial infection and pre-treatment with 24-epibrassinolide markedly affect the heat emission and membrane permeability of rape cotyledons

Changes in metabolic activity in spring oilseed rape (Brassica napus L.) seedlings during bacterial infection were determined by measuring heat emission with an isothermal calorimeter. Heat emission was markedly increased in cotyledons infected with the incompatible bacterium Pseudomonas syringae pv. syringae. One day after inoculation, visible signs of necrosis were detected on the infected cotyledons. This indicates that the hypersensitive response had been activated. Heat emission continued to increase as the hypersensitive response progressed, reflecting an increase in the metabolic rate in the infected tissue. With both of the cultivars tested heat emission by infected cotyledons was increased by pre-treatment with BR27. Membrane permeability as measured by ion leakage also rapidly increased during the first 3 days of infection. In uninfected cotyledons pre-treated with BR27, membrane permeability was slightly increased. In infected cotyledons not pre-treated with BR27, both visible necrosis and membrane permeability rapidly increased during the first 2 or 3 days after inoculation. In infected cotyledons pre-treated with BR27, membrane permeability was significantly reduced by the second or third day after inoculation. This confirms that BR27 plays a role in preventing damage to plant cell membranes caused by bacterial infection during the hypersensitive response.