RED AND FAR-RED INSENSITIVE 2, a RING-Domain Zinc Finger Protein, Mediates Phytochrome-Controlled Seedling Deetiolation Responses

摘要

Light is arguably the most important resource for plants, and an array of photosensory pigments enables plants to develop optimally in a broad range of ambient-light conditions. The red- and far-red-light-absorbing photosensory pigments or phytochromes (phy) regulate seedling deetiolation responses, photoperiodic flowering, and circadian rhythm. We have identified a long hypocotyl mutant under red and far-red light, rfi2-1 (red and far-red insensitive 2 to 1). rfi2-1 was also impaired in phytochrome-mediated end-of-day far-red light response, cotyledon expansion, far-red light block of greening, and light-induced expression of CHLOROPHYLL A/B BINDING PROTEIN 3 and CHALCONE SYNTHASE. Introduction of rfi2-1 mutation into phyB-9 or phyA-211 did not enhance or suppress the long hypocotyl phenotype of phyB-9 or phyA-211 under red or far-red light, respectively, and RFI2 likely functions downstream of phyB or phyA. RFI2 was identified through the segregation of two T-DNA insertions into different recombinant lines, genetic rescue, and phenotypic characterization of a second mutant allele rfi2-2. RFI2 encodes a protein with a C3H2C3-type zinc finger or RING domain known to mediate protein-protein or protein-DNA interactions, and RFI2 is localized to the nucleus. RFI2 therefore reveals a signaling step that mediates phytochrome control of seedling deetiolation. nnnn--------------------------------------------------------------------------------nPerception of light signals from their natural environment plays an important role for all living organisms, especially for plants, which are unable to migrate to more favorable locations. As a result, light influences many aspects of plant development from seed germination, seedling deetiolation, phototropism, shade avoidance, to photoperiodic flowering. Plants have evolved a number of informational photoreceptors to monitor their ambient-light signals, and these photoreceptors include red/far-red light-absorbing phytochromes (phy) and UV-A/blue light-absorbing cryptochromes and phototropins. There are five phytochromes, phyA to phyE, in Arabidopsis (Arabidopsis thaliana); phyA and phyB play predominant roles in control of seedling deetiolation responses. phyA is the receptor for far-red light-mediated deetiolation responses, whereas phyB is the major photoreceptor for red-light-mediated deetiolation responses (Aukerman et al., 1997; Whitelam and Devlin, 1997; Devlin et al., 1998). Analysis of the phyA/phyB double mutants clearly demonstrated a positive effect of phyA on phyB in control of seedling deetiolation responses (Neff and Chory, 1998). A negative interaction between phyA and phyB has also been described (Cerdán et al., 1999; Hennig et al., 2001). For example, overexpression of phyB in Arabidopsis decreased the far-red light-mediated inhibition of hypocotyl elongation (Wagner et al., 1996). nGenetic screens have identified mutants that are defective in far-red light-mediated deetiolation responses, and the mutated genes encode phyA-specific signaling components such as FAR1, FHY1, FHY3, FIN2, FIN219, LAF1, LAF6, PAT1, and HFR1 (Whitelam et al., 1993; Hoecker et al., 1998; Soh et al., 1998; Hudson et al., 1999; Bolle et al., 2000; Fairchild et al., 2000; Hsieh et al., 2000; Ballesteros et al., 2001). Genetic screens have also identified several red light-specific mutants including gi, elf3, srr1, aprr1, ztl, pef2, pef3, and red1 (Ahmad and Cashmore, 1996; Wagner et al., 1997; Huq et al., 2000a; Makino et al., 2000; Somers et al., 2000; Liu et al., 2001; Staiger et al., 2003). A few of the mutated genes such as GI, ELF3, SRR1, APRR1, and ZTL have been molecularly characterized (Huq et al., 2000b; Makino et al., 2000; Somers et al., 2000; Liu et al., 2001; Staiger et al., 2003). Five other mutants have defective deetiolation responses under both red and far-red light and include cog1, pef1, psi2, pft1, and prr7 (Ahmad and Cashmore, 1996; Genoud et al., 1998; Cerdán and Chory, 2003; Kaczorowski and Quail, 2003; Park et al., 2003). Both cog1 and psi2 show hypersensitive hypocotyl growth response to red and far-red light, and COG1 and PSI2 therefore negatively regulate phyA and phyB signaling (Genoud et al., 1998; Park et al., 2003). COG1 encodes a Dof family member of transcription factors. In contrast, pef1 and prr7 have hyposensitive hypocotyl growth response to red and far-red light, suggesting that PEF1 and PRR7 regulate phyA and phyB signaling positively (Cerdán and Chory, 2003; Kaczorowski and Quail, 2003). PRR7 encodes a PSEUDO-RESPONSE REGULATOR and the absence of PRR7 also causes a coordinated 3-to-6-h shift in the phasing of the oscillatory expression of CCA1, LHY, and TOC1, the central components of the circadian clock (Kaczorowski and Quail, 2003). Interestingly, pft1 was hyporesponsive to far-red light but hyperresponsive to red light (Cerdán and Chory, 2003). PFT1 may therefore function at a phytochrome-signaling node where antagonistic interactions between phyA and phyB occur. nnCombined biochemical and genetic approaches have also identified a few light-signaling components that function in both phyA and phyB pathways. PIF3, a bHLH transcription factor, was initially isolated by its interaction with phyA and phyB (Ni et al., 1999). A T-DNA knockout allele of PIF3 showed a hypersensitive hypocotyl growth response only to red light. Although PIF3 does not function in phyA-mediated inhibition of hypocotyl elongation, it negatively regulates both phyA- and phyB-mediated cotyledon opening and expansion responses (Kim et al., 2003). Another phyA and phyB interactor, NDPK2, functions positively in phytochrome signaling (Choi et al., 1999). Recombinant NDPK2 preferentially binds to the red light-activated form of phytochrome, and this interaction increases the activity of the recombinant NDPK2 protein. Mutation in NDPK2 showed a partial defect in red and far-red light-mediated cotyledon opening and greening responses (Choi et al., 1999). In contrast, the overexpression lines of PKS1, a phytochrome phosphorylation substrate, were less sensitive only to red light, but retained normal sensitivity to blue and far-red light, indicating that PKS1 acts as an inhibitor of phyB signaling (Fankhauser et al., 1999). phyA and phyB also interact in vitro with PIF1, PIF4, ARR4, ELF3, FyPP, and PAPP5 (Liu et al., 2001; Sweere et al., 2001; Huq and Quail, 2002, 2005; Kim et al., 2002; Ryu et al., 2005). nnAlthough a number of phytochrome-signaling components have been identified, the mechanisms by which phytochromes regulate seedling deetiolation and flowering responses are still largely unknown (Quail, 2002; Simpson and Dean, 2002). We isolated a new long hypocotyl mutant, rfi2-1 (red and far-red insensitive 2 to 1), under both red and far-red light. rfi2-1 also showed other defects in phyA- and phyB-mediated deetiolation responses such as cotyledon expansion and light-induced gene expression. We uncovered two T-DNA insertions on chromosome 2 in rfi2-1, and the recombinant line carrying a T-DNA insertion in front of At2g47700 retained the mutant phenotype. Genetic rescue of rfi2-1 and phenotypic characterization of rfi2-2 further confirmed the identity of RFI2 gene. RFI2 defines a novel step in phytochrome signaling that controls seedling deetiolation responses.