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BACKGROUND: The plant-specific transcriptional activator LEAFY (LFY) is a central regulator of the transition to reproductive development in Arabidopsis. LFY has a second, later role in the induction of floral homeotic gene expression. Available data suggests that, while LFY activity is controlled via interaction with tissue-specific coactivators, other mechanisms exist that regulate LFY activity, the identity of which are not known. RESULTS: We have identified a novel component in the temporal control of the switch from vegetative to reproductive development in Arabidopsis thaliana. The SPLAYED (SYD) gene product acts with LFY to regulate shoot apical meristem identity. SYD is also involved in the regulation of floral homeotic gene expression. In addition, mutations in SYD cause LFY-independent phenotypes that indicate that SYD is necessary for meristem maintenance during reproductive development and that SYD is required for proper carpel and ovule development. SYD encodes a presumptive Arabidopsis homolog of the yeast Snf2p ATPase, which is implicated in transcriptional control via chromatin remodeling. CONCLUSIONS: SYD acts as a LFY-dependent repressor of the meristem identity switch in the floral transition, most likely by altering the activity of the LFY transcription factor. That SYD regulates flowering in response to environmental stimuli suggests that the effect of environmental cues on plant development may be achieved in part by regulating transcription factor activity via alteration of the chromatin state.

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0960-9822 (Print)Journal ArticleResearch Support, Non-U.S. Gov’tResearch Support, U.S. Gov’t, Non-P.H.S.

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Plants produce new appendages reiteratively from groups of stem cells called shoot apical meristems. LEAFY (LFY) and APETALA1 (AP1) are pivotal for the switch to the reproductive phase, where instead of leaves the shoot apical meristem produces flowers. Use of steroid- inducible activation of LFY demonstrated that early expression of AP1 is a result of transcriptional induction by LFY. This AP1 induction is independent of protein synthesis and occurs specifically in the tissues and at the developmental stage in which floral fate is assumed. Later expression of AP1 appears to be only indirectly affected by LFY.

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Seedlings of a transgenic Arabidopsis line (ABO) that overexpresses phytochrome B (phyB) display enhanced deetiolation specifically in red light. To identify genetic loci necessary for phytochrome signal transduction in red light, we chemically mutagenized ABO seeds and screened M2 seedlings for revertants of the enhanced deetiolation response. One recessive, red light-specific extragenic revertant, designated red1, was isolated. The mutant phenotype was expressed in the original ABO background as well as in the nontransgenic Nossen (No-0) progenitor background. red1 is also deficient in several other aspects of red light-induced responses known to be mediated by phyB, such as inhibition of petiole elongation and the shade avoidance response. red1 was mapped to the bottom of chromosome 4 at a position distinct from all known photoreceptor loci. Together with complementation analysis, the data show that red1 is a novel photomorphogenic mutant. The evidence suggests that red1 represents a putative phytochrome signal transduction mutant potentially specific to the phyB pathway.

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Wagner, DHoecker, UQuail, P HPlant Cell. 1997 May;9(5):731-43.

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We used a series of in vitro-generated deletion and amino acid substitution derivatives of phytochrome B (phyB) expressed in transgenic Arabidopsis to identify regions of the molecule important for biological activity. Expression of the chromophore-bearing N-terminal domain of phyB alone resulted in a fully photoactive, monomeric molecule lacking normal regulatory activity. Expression of the C-terminal domain alone resulted in a photoinactive, dimeric molecule, also lacking normal activity. Thus, both domains are necessary, but neither is sufficient for phyB activity. Deletion of a small region on each major domain (residues 6 to 57 and 652 to 712, respectively) was shown to compromise phyB activity differentially without interfering with spectral activity or dimerization. Deletion of residues 6 to 57 caused a large increase in the fluence rate of continuous red light (Rc) required for maximal seedling responsiveness, indicating a marked decrease in efficiency of light signal perception or processing per mole of mutant phyB. In contrast, deletion of residues 652 to 712 resulted in a photoreceptor that retained saturation of seedling responsiveness to Rc at low fluence rates but at a response level much below the maximal response elicited by the parent molecule. This deletion apparently reduces the maximal biological activity per mole of phyB without a major decrease in efficiency of signal perception, thus suggesting disruption of a process downstream of signal perception. In addition, certain phyB constructs caused dominant negative interference with endogenous phyA activity in continuous far-red light, suggesting that the two photoreceptors may share reaction partners.

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Wagner, D.Koloszvari, M.Quail, P. H.Plant Cell. 1996 May;8(5):859-871.

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In early seedling development, far-red-light-induced deetiolation is mediated primarily by phytochrome A (phyA), whereas red-light-induced deetiolation is mediated primarily by phytochrome B (phyB). To map the molecular determinants responsible for this photosensory specificity, we tested the activities of two reciprocal phyA/phyB chimeras in diagnostic light regimes using overexpression in transgenic Arabidopsis. Although previous data have shown that the NH2-terminal halves of phyA and phyB each separately lack normal activity, fusion of the NH2-terminal half of phyA to the COOH-terminal half of phyB (phyAB) and the reciprocal fusion (phyBA) resulted in biologically active phytochromes. The behavior of these two chimeras in red and far-red light indicates: (i) that the NH2-terminal halves of phyA and phyB determine their respective photosensory specificities; (ii) that the COOH-terminal halves of the two photoreceptors are necessary for regulatory activity but are reciprocally inter-changeable and thus carry functionally equivalent determinants; and (iii) that the NH2-terminal halves of phyA and phyB carry determinants that direct the differential light lability of the two molecules. The present findings suggest that the contrasting photosensory information gathered by phyA and phyB through their NH2-terminal halves may be transduced to downstream signaling components through a common biochemical mechanism involving the regulatory activity of the COOH-terminal domains of the photoreceptors.

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Wagner, DFairchild, C DKuhn, R MQuail, P HGM47475/GM/NIGMS NIH HHS/Proc Natl Acad Sci U S A. 1996 Apr 30;93(9):4011-5.

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Overexpression of phytochrome B (phyB) in transgenic Arabidopsis results in enhanced deetiolation in red light. To define domains of phyB functionally important for its regulatory activity, we performed chemical mutagenesis of a phyB-overexpressing line and screened for phenotypic revertants in red light. Four phyB-transgene-linked revertants that retain parental levels of full-length, dimeric, and spectrally normal overexpressed phyB were identified among 101 red-light-specific revertants. All carry single amino acid substitutions in the transgene-encoded phyB that reduce activity by 40- to 1000-fold compared to the nonmutagenized parent. The data indicate that the mutant molecules are fully active in photosignal perception but defective in the regulatory activity responsible for signal transfer to downstream components. All four mutations fall within a 62-residue region in the COOH-terminal domain of phyB, with two independent mutations occurring in a single amino acid, Gly-767. Accumulating evidence indicates that the identified region is a critical determinant in the regulatory function of both phyB and phyA.

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Wagner, DQuail, P HProc Natl Acad Sci U S A. 1995 Sep 12;92(19):8596-600.

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The long-day plant Arabidopsis thaliana (L.) Heynh. flowers early in response to brief end-of-day (EOD) exposures to far-red light (FR) following a fluorescent short day of 8 h. FR promotion of flowering was nullified by subsequent brief red light (R) EOD exposure, indicating phytochrome involvement. The EOD response to R or FR is a robust measure of phytochrome action. Along with their wild-type (WT) parents, mutants deficient in either phytochrome A or B responded similarly to the EOD treatments. Thus, neither phytochrome A nor B exclusively regulated flowering, although phytochrome B controlled hypocotyl elongation. Perhaps a third phytochrome species is important for the EOD responses of the mutants and/or their flowering is regulated by the amount of the FR-absorbing form of phytochrome, irrespective of the phytochrome species. Overexpression of phytochrome A or phytochrome B resulted in differing photoperiod and EOD responses among the genotypes. The day-neutral overexpressor of phytochrome A had an EOD response similar to all of the mutants and WTs, whereas R EOD exposure promoted flowering in the overexpressor of phytochrome B and FR EOD exposure inhibited this promotion. The comparisons between relative flowering times and leaf numbers at flowering of the over-expressors and their WTs were not consistent across photoperiods and light treatments, although both phytochromes A and B contributed to regulating flowering of the transgenic plants.

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Bagnall, D JKing, R WWhitelam, G CBoylan, M TWagner, DQuail, P HGM 47475/GM/NIGMS NIH HHS/Plant Physiol. 1995 Aug;108(4):1495-503.

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The phytochrome family of photoreceptors monitors the light environment and dictates patterns of gene expression that enable the plant to optimize growth and development in accordance with prevailing conditions. The enduring challenge is to define the biochemical mechanism of phytochrome action and to dissect the signaling circuitry by which the photoreceptor molecules relay sensory information to the genes they regulate. Evidence indicates that individual phytochromes have specialized photosensory functions. The amino-terminal domain of the molecule determines this photosensory specificity, whereas a short segment in the carboxyl-terminal domain is critical for signal transfer to downstream components. Heterotrimeric GTP-binding proteins, calcium-calmodulin, cyclic guanosine 5′-phosphate, and the COP-DET-FUS class of master regulators are implicated as signaling intermediates in phototransduction.

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Quail, P HBoylan, M TParks, B MShort, T WXu, YWagner, DGM47475/GM/NIGMS NIH HHS/New York, N.Y.Science. 1995 May 5;268(5211):675-80.

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