Light Suppresses 3-Hydroxy-3-methylglutaryl Coenzyme A Reductase Gene Expression in Arabidopsis thaliana

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Abstract
3-Hydroxy-3-methylglutaryl (HMG) coenzyme A reductase mRNA accumulates preferentially in dark-grown Arabidopsis plants. As one step toward understanding the role that light plays in the regulation of the mevalonate pathway in plants, we characterized the suppression of HMG1 gene expression in response to illumination wavelength, duration, and fluence rate. The accumulation of HMG1 mRNA by dark treatment is suppressed by continuous exposure to white light and is dependent on the amount of light perceived during the period of illumination. By using promoter/reporter gene fusions we also demonstrate that this reaction is mediated by cis-acting elements that reside in the Arabidopsis HMG1 promoter and, therefore, is likely to be controlled at the transcriptional level. HMG1 expression is differentially responsive to continuous blue and red light but not to far-red light. In contrast, changes in HMG1 mRNA levels were not observed in response to brief light pulses of any spectrum, suggesting that continuous illumination is required for sustained and maximal suppression of HMG coenzyme A reductase expression. Taken together, these data indicate that light-mediated control of the HMG1 gene is mediated by a regulatory circuit that monitors aspects of both spectral quality and fluence and involves either multiple photoreceptors or a single photoreceptor that is differentially sensitive to both blue and red light.