Transgenic tobacco expressing a foreign calmodulin gene shows an enhanced production of active oxygen species

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Abstract
A strategy for elucidating specific molecular targets of calcium and calmodulin in plant defense responses has been developed. We have used a dominant‐acting calmodulin mutant (VU‐3, Lys to Arg115) to investigate the oxidative burst and nicotinamide co‐enzyme fluxes after various stimuli (cellulase, harpin, incompatible bacteria, osmotic and mechanical) that elicit plant defense responses in transgenic tobacco cell cultures. VU‐3 calmodulin differs from endogenous plant calmodulin in that it cannot be methylated post‐translationally, and as a result it hyperactivates calmodulin‐dependent NAD kinase. Cells expressing VU‐3 calmodulin exhibited a stronger active oxygen burst that occurred more rapidly than in normal control cells challenged with the same stimuli. Increases in NADPH level were also greater in VU‐3 cells and coincided both in timing and magnitude with development of the active oxygen species (AOS) burst. These data show that calmodulin is a target of calcium fluxes in response to elicitor or environmental stress, and provide the first evidence that plant NAD kinase may be a downstream target which potentiates AOS production by altering NAD(H)/NADP(H) homeostasis.