Box 2 | The plant photoreceptors

From the following article:

Living by the calendar: how plants know when to flower

Marcelo J. Yanovsky and Steve A. Kay

Nature Reviews Molecular Cell Biology 4, 265-276 (April 2003)

doi:10.1038/nrm1077

Phytochromes are red-light/far-red-light (R/FR) photoreceptors that perceive light through a tetrapyrrole chromophore that is bound covalently to their amino-terminal photosensory domain30. The carboxy-terminal domain contains two PAS (for period circadian protein, Ah receptor nuclear translocator protein and single-minded protein) repeats, which initiate a signalling cascade by mediating direct interactions with molecules such as the basic-helix-loop-helix transcription factor PIF3, and a histidine-kinase-related domain (HKRD), which might phosphorylate direct targets such as phytochrome kinase substrate 1 (a protein that negatively regulates phytochrome signalling)30. The light-labile phytochrome (phy)A is more active in FR, whereas phyB and other light-stable phytochromes are more active in R. This difference is due in part to their differential light-stability, but also to other properties that are specific to the phyA domain103.

Living by the calendar: how plants know when to flower 

Cryptochromes are blue/UV-A photoreceptors that bind pterin and flavin chromophores at their amino-terminal domain104. Blue-light activation of cryptochromes initiates a signalling cascade through their carboxy-terminal domain104. This signalling cascade operates in part through the direct inactivation of constitutive photomorphogenic 1 (COP1), which is a general repressor of photomorphogenic responses104.

Phototropins have two PAS/LOV domains that bind a flavin mononucleotide (FMN) chromophore38. The absorption of blue light triggers the formation of covalent adducts between FMN and cysteine residues in the PAS/LOV domains, which induce a conformational change that is thought to initiate a signalling cascade through activation of the serine/threonine kinase activity at the carboxy-terminal domain38.

Zeitlupe (ZTL), flavin-binding kelch repeat F-box 1 (FKF1) and LOV kelch protein 2 (LKP2) share a unique combination of motifs, which includes an amino-terminal PAS/LOV domain, an F-box domain that probably recruits proteins for ubiquitylation and subsequent degradation, and six kelch repeats that mediate protein–protein interactions16, 39, 40, 41, 42. The PAS/LOV domain of this family of proteins might bind FMN, allowing these molecules to target specific proteins for degradation in a light-dependent manner.