INCOMPATIBLE PATHOGEN INFECTION RESULTS IN ENHANCED REACTIVE OXYGEN AND CELL-DEATH RESPONSES IN TRANSGENIC TOBACCO EXPRESSING A HYPERACTIVEMUTANT CALMODULIN

Transgenic tobacco (Nicotiana tabacum L. cv. Wisconsin 38) lines expre ssing a mutant calmodulin (VU-3) that hyperactivates NAD kinase exhibi t an enhanced elicitor-stimulated oxidative-burst reaction (S.A. Hardi ng et al., 1997, EMBO J. 16. 1137-1144). VU-3 transgenic tobacco was u sed in the present study to investigate the relationship between calmo dulin signalling, the production of active oxygen species and cell dea th in response to infection with an incompatible pathogen. Following P . syringae pv. syringae 61 infection, suspension cells derived from VU -3 transgenic plants exhibited a stronger oxidative burst (3- to 4-fol d higher primary and secondary burst reactions), greater media alkalin ization (3-fold) and more rapid cell death (4-fold greater mortality a t 20 h post infection) than did infected control tobacco cells. Infect ion of leaf tissues with P. syringae pv. syringae 61 also resulted in an enhanced cell death response compared to control tobacco tissues. T his cell death response of VU-3 leaf tissues, but not control leaf tis sues, was further enhanced by the presence of 50 mu M salicylic acid, suggesting that this transgenic line is more sensitive to the effects of this agent. Overall, the data support the model that calmodulin sig nalling pathways are involved in the plant oxidative burst and contrib ute to the regulation of cell death in infected plant tissues undergoi ng the hypersensitive response.