Editors' ChoicePlant biology

Plants Sense a Blue Light Special

Sci. Signal.  29 Oct 2013:
Vol. 6, Issue 299, pp. ec261
DOI: 10.1126/scisignal.2004851

Daily and seasonal changes in the period and spectral composition of sunlight can elicit physiological changes in plants and animals by stimulation of cryptochromes (CRYs). In plants, CRYs are stimulated by blue light to activate the DNA-binding transcription factor CIB1 to promote expression of the FT gene encoding FLOWERING LOCUS T. The light-oxygen-voltage (LOV) domain F-box protein family, which includes FKF1, ZTL, and LKP2, also senses blue light and promotes expression of FT. Liu et al. showed that blue light prevents the degradation of CIB1 in a process that depends on ZTL and LKP2. In experiments with Arabidopsis transgenic for either the 35S promoter or the endogenous CIB1 promoter coupled to expression of myc-CIB1, blue light increased CIB1 abundance. Three-week-old plants grown in continuous white light then switched to dark or red light for 16 hours showed increased abundance of CIB1 within 1 hour of exposure to blue light. CIB1 was abundant in 3-week-old plants or 8-day-old seedlings grown in blue light for 16 hours, and CIB1 abundance rapidly decreased when plants were switched to dark, red, or far red light. Pharmacological inhibition of the proteasome prevented the decrease in abundance of CIB1 in plant tissues grown in the dark, whereas inhibition of transcription did not prevent the increased abundance of CIB1 induced by exposure to blue light, suggesting that CIB1 is actively degraded in the absence of blue light. Analysis of plants genetically deficient for various components of light signaling indicated that the LOV proteins ZTL and LKP2 were each necessary for blue light to stabilize CIB1, whereas CRY1 and CRY2 were not. CIB1 overexpression triggers accelerated flowering time, which requires cry1 and cry2. Flowering time in CIB1-overexpressing plants was delayed in the ztl-3 or lkp2 mutant background. Thus, blue light acts through a coordinated mechanism involving ZTL- and LKP2-dependent stabilization of CIB1and CRY-dependent activation of CIB1 to promote flowering.

H. Liu, Q. Wang, Y. Liu, X. Zhao, T. Imaizumi, D. E. Somers, E. M. Tobin, C. Lin, Arabidopsis CRY2 and ZTL mediate blue-light regulation of the transcription factor CIB1 by distinct mechanisms. Proc. Natl. Acad. Sci. U.S.A. 110, 17582–17587 (2013). [Abstract] [Full]