Although calmodulin (CaM) is known to play multiple regulatory roles in eukaryotes, its direct function as transcriptional regulator is unknown. Furthermore, the physiological functions of CaM are largely unknown in plants. Here, we show that one of the four Arabidopsis thaliana CaM isoforms, CAM7, is a transcriptional regulator that directly interacts with the promoters of light-inducible genes and promotes photomorphogenesis. CAM7 overexpression causes hyperphotomorphogenic growth and an increase in the expression of light-inducible genes. Mutations in CAM7 produce no visible effects on photomorphogenic growth, indicating likely redundant gene functions. However, cam7 mutants display reduced expression of light-inducible genes, and cam7 hy5 double mutants show an enhancement of the hy5 phenotype. Moreover, overexpression of CAM7 can partly suppress the hy5 phenotype, indicating that the two factors work together to control lightinduced seedling development. The mutational and transgenic studies, together with physiological analyses, illustrate the concerted function of CAM7 and HY5 basic leucine zipper transcription factor in Arabidopsis seedling development.

Light is an important factor for plant growth and development. We have identified and functionally characterized a regulatory gene SHORT HYPOCOTYL IN WHITE LIGHT1 (SHW1) involved in Arabidopsis (Arabidopsis thaliana) seedling development. SHW1 encodes a unique serine-arginine-aspartate-rich protein, which is constitutively localized in the nucleus of hypocotyl cells. Transgenic analyses have revealed that the expression of SHW1 is developmentally regulated and is closely associated with the photosynthetically active tissues. Genetic and molecular analyses suggest that SHW1 acts as a negative regulator of light-mediated inhibition of hypocotyl elongation, however, plays a positive regulatory role in light-regulated gene expression. The shw1 mutants also display shorter hypocotyl in dark, and analyses of shw1 cop1 double mutants reveal that SHW1 acts nonredundantly with COP1 to control hypocotyl elongation in the darkness. Taken together, this study provides evidences that SHW1 is a regulatory protein that is functionally interrelated to COP1 and plays dual but opposite regulatory roles in photomorphogenesis.

The unique member of the calmodulin gene family, Calmodulin7 ( CAM 7), plays a crucial role as transcriptional regulator to promote Arabidopsis seedling development. CAM 7 regulates the expression of HY 5 , which is intimately involved in the promotion of photomorphogenic growth and light‐regulated gene expression. COP 1 ubiquitin ligase suppresses photomorphogenesis by degrading multiple photomorphogenesis promoting factors including HY 5 in darkness. Genetic interaction studies, in this report, reveal that CAM 7 and COP 1 co‐ordinately work to promote photomorphogenic growth and light‐regulated gene expression at lower intensity of light. CAM 7 physically interacts with COP 1 in the nucleus. Further, in vivo study suggests that CAM 7 and COP 1 interaction is light intensity dependent. We have also shown that functional COP 1 is required for optimum accumulation of CAM 7 at lower fluences of light. Taken together, this study demonstrates the coordinated function of CAM 7 and COP 1 in Arabidopsis seedling development.