phyB Mutants Research Articles

A novel Phytochrome B allele in Arabidopsis thaliana exhibits partial mutant phenotype: a short deletion in N-terminal extension reduces Phytochrome B activity

During analysis of an Arabidopsis thaliana line possessing a Phytochrome A epiallele (phyA’), a partial Phytochrome B-deficient phenotype was observed, consisting of elongated hypocotyls in seedlings grown under constant white light or red light (660 nm). The observed hypocotyls were twice the length (8 mm) of wild-type (4 mm), but approximately half the length of a null phyB-9 mutant (14 mm). Several analyses were performed to characterize this apparent partial phyB mutant. Sequencing of the entire exonic region revealed three point mutations that altered codon usage, and one in-frame 12 base pair deletion. Each of the point mutations has been described in other lines that display wild-type phenotype, and therefore their effect is thought to be minimal, if any. The N-terminal deletion of amino acids 9 through 12 (GGGR) is a unique mutation found in this line. This deletion most likely contributes to the phyB mutant phenotype by lowering the binding affinity of the active form of Phytochrome B (Pfr) with Phytochrome Interacting Factor 3 (PIF3).

Involvement of phytochromes in gibberellin-mediated photomorphogenesis in rice seedlings

Gibberellin (GA) is an important plant hormone, which, together with phytochrome-mediated light signals, regulates Arabidopsis thaliana development. However, roles of GA and its interaction with phytochromes in mediating skotomorphogenesis and photomorphogenesis remain unclear in rice. In the present study, the lengths of coleoptiles, seminal roots and up-ground parts were measured in either dark- or white light-grown seedlings treated with GA biosynthesis inhibitor PAC in wild type and rice phytochrome mutants. Meantime, transcript levels of light-regulated genes LHCB were also analyzed. It was observed that PAC treatment caused the inhibition of coleoptile elongation and induction of LHCB genes in dark-grown seedlings. Moreover phyA mutant exhibited less sensitive to PAC than wild type and phyB mutant in dark-grown seedlings. In white light-grown seedlings, growth of up-ground parts was inhibited by PAC while phyB mutant exhibited less sensitive than wild type and phyA mutant did. In addition, phyB antagonistically affect the inductive responses of root elongation triggered by PAC in light-grown seedlings. These results suggest that GA is necessary to maintain skotomorphogenesis and to suppress photomorphogenesis in dark-grown rice seedlings. In addition, phyA and phyB either negatively or positively affect responses caused by PAC treatment in rice seedlings. For the first time, our findings reveal the interaction between phytochrome- and GA-mediated pathways which lays the foundation for dissecting the molecular mechanism of coordinated regulation of rice development by light and GA.

Vegetative axillary bud dormancy induced by shade and defoliation signals in the grasses

Vegetative axillary bud dormancy and outgrowth is regulated by several hormonal and environmental signals. In perennials, the dormancy induced by hormonal and environmental signals has been categorized as eco-, endo- or para-dormancy. Over the past several decades para-dormancy has primarily been investigated in eudicot annuals. Recently, we initiated a study using the monoculm phyB mutant (phyB-1) and the freely branching near isogenic wild type (WT) sorghum (Sorghum bicolor) to identify molecular mechanisms and signaling pathways regulating dormancy and outgrowth of axillary buds in the grasses. In a paper published in the January 2010 issue of Plant Cell and Environment, we reported the role of branching genes in the inhibition of bud outgrowth by phyB, shade and defoliation signals. Here we present a model that depicts the molecular mechanisms and pathways regulating axillary bud dormancy induced by shade and defoliation signals in the grasses.

A role for ABCB19-mediated polar auxin transport in seedling photomorphogenesis mediated by cryptochrome 1 and phytochrome B

During seedling establishment, blue and red light suppress hypocotyl growth through the cryptochrome 1 (cry1) and phytochrome B (phyB) photosensory pathways, respectively. How these photosensory pathways integrate with growth control mechanisms to achieve the appropriate degree of stem elongation was investigated by combining cry1 and phyB photoreceptor mutations with genetic manipulations of a multidrug resistance-like membrane protein known as ABCB19 that influenced auxin distribution within the plant, as evidenced by a combination of reporter gene assays and direct auxin measurements. Auxin signaling and ABCB19 protein levels, hypocotyl growth rates, and apical hook opening were measured in mutant and wild-type seedlings exposed to a range of red and blue light conditions. Ectopic/overexpression of ABCB19 (B19OE) greatly increased auxin in the hypocotyl, which reduced the sensitivity of hypocotyl growth specifically to blue light in long-term assays and red light in high-resolution, short-term assays. Loss of ABCB19 partially suppressed the cry1 hypocotyl growth phenotype in blue light. Hypocotyl growth of B19OE seedlings in red light was very similar to phyB mutants. Altered auxin distribution in B19OE seedlings also affected the opening of the apical hook. The cry1 and phyB photoreceptor mutations both increased ABCB19 protein levels at the plasma membrane, as measured by confocal microscopy. The B19OE plant proved to be a useful tool for determining aspects of the mechanism by which light, acting through cry1 or phyB, influences the auxin transport process to control hypocotyl growth during de-etiolation.

Phytochrome B Is Involved in Mediating Red Light-Induced Stomatal Opening in Arabidopsis thaliana

The stomatal pores of higher plants enable gaseous exchange into and out of leaves for photosynthesis and evaporation. Stomatal opening is induced by both blue and red lights. It is shown that blue light-induced stomatal opening is mediated by the blue light receptor phototropins (PHOT1 and PHOT2) and cryptochromes (CRY1 and CRY2). However, whether phytochrome B (phyB) is involved in red light regulation of stomatal opening remains largely unclear. Here, we report a positive role for Arabidopsis (Arabidopsis thaliana) phyB in the regulation of red light-induced stomatal opening. The phyB mutant stomata displayed a reduced red light response, whereas stomata of the phyB-overexpressing plants displayed a hypersensitive response to red light. In addition, stomata of the cry1 cry2 phyB, phot1 phot2 phyB, and cry1 phyA phyB triple mutant plants showed more reduced light response than those of the single or double mutant plants under white light, implying that phyB acts in concert with phyA, CRY, and PHOT in light regulation of stomatal opening. Stomata of phyB cop1 mutant opened less wide than those of the cop1 mutant, and stomata of the pif3 pif4 mutant opened wider than those of the wild-type, indicating that COP1, together with the PIFs (phytochrome interacting factors), may act downstream of PHYB in regulating stomatal opening. Furthermore, quantitative RT-PCR analysis showed that the expression of MYB60 was reduced in the cry1 cry2 and phyA phyB mutants under blue and red lights, respectively, but induced in the CRY1- and phyB-overexpressing plants. These results demonstrate that phyB and CRY might regulate stomatal opening, at least in part, by regulating MYB60 expression.

Light exaggerates apical hook curvature through phytochrome actions in tomato seedlings

Contrary to the established notion that the apical hook of dark-grown dicotyledonous seedlings opens in response to light, we found in tomato (Solanum lycopersicum L.) that the apical hook curvature is exaggerated by light. Experiments with several tomato cultivars and phytochrome mutants, irradiated with red and far-red light either as a brief pulse (Rp, FRp) or continuously (Rc, FRc), revealed: the hook-exaggeration response is maximal at the emergence of the hypocotyl from the seed; the effect of Rp is FRp-reversible; fluence-response curves to a single Rp or FRp show an involvement of low and very low fluence responses (LFR, VLFR); the effect of Rc is fluence-rate dependent, but that of FRc is not; the phyA mutant (phyA hp-1) failed to respond to an Rp of less than 10(-2) micromol m(-2) and to an FRp of all fluences tested as well as to FRc, thus indicating that the hook-exaggeration response involves phyA-mediated VLFR. The Rp fluence-response curve with the same mutant also confirmed the presence of an LFR mediated by phytochrome(s) other than phyA, although the phyB1 mutant (phyB1 hp-1) still showed full response probably due to other redundant phytochrome species (e.g., phyB2). Simulation experiments led to the possible significance of hook exaggeration in the field that the photoresponse may facilitate the release of seed coat when seeds germinate at some range of depth in soil. It was also observed that seed coat and/or endosperm are essential to the hook exaggeration.

The Dof protein DAG1 mediates PIL5 activity on seed germination by negatively regulating GA biosynthetic gene AtGA3ox1

We have previously shown that inactivation of the gene encoding the Arabidopsis thaliana transcription factor DOF AFFECTING GERMINATION 1 (DAG1) renders seed germination more sensitive to both phytochrome B (phyB) and gibberellins (GA). dag1 mutant seeds require less red (R) light fluence and a lower GA concentration than WT to germinate. Here, we show that inactivation of the gene PHYTOCHROME INTERACTING FACTOR 3-LIKE 5 (PIL5) results in down-regulation of DAG1. Inactivation of PIL5 in the dag1 mutant background further increased the germination potential of dag1 mutant seeds, supporting the suggestion that DAG1 is under the positive control of PIL5. Germination of dag1phyB seeds showed a reduced requirement of gibberellins as compared with phyB mutant seeds, both in the presence and in the absence of GA biosynthesis. Furthermore, the GA biosynthetic gene AtGA3ox1 is upregulated in dag1 seeds as compared with the WT, and DAG1 actually binds to the AtGA3ox1 promoter, as shown by chromatin immunoprecipitation experiments. Expression analysis at different time points confirms that AtGA3ox1 is directly regulated by DAG1, while suggesting that DAG1 is not a direct regulatory target of PIL5. Our data indicate that in the phyB pathway leading to seed germination, DAG1 negatively regulates GA biosynthesis and suggest that DAG1 acts downstream of PIL5. In addition, the analysis of hypocotyls of dag1 and phyB mutant plantlets, of plantlets overexpressing phyB in the dag1 mutant, as well as of dag1phyB double mutant suggests that DAG1 may act as a negative regulatory element downstream of phyB also in hypocotyl elongation.

A complementary role for ELF3 and TFL1 in the regulation of flowering time by ambient temperature

Plants regulate their time to flowering by gathering information from the environment. Photoperiod and temperature are among the most important environmental variables. Sub-optimal, but not near-freezing, temperatures regulate flowering through the thermosensory pathway, which overlaps with the autonomous pathway. Here we show that ambient temperature regulates flowering by two genetically distinguishable pathways, one requiring TFL1 and another requiring ELF3. The delay in flowering time observed at lower temperatures was partially suppressed in single elf3 and tfl1 mutants, whereas double elf3 tfl1 mutants were insensitive to temperature. tfl1 mutations abolished the temperature response in cryptochrome mutants that are deficient in photoperiod perception, but not in phyB mutants, which have a constitutive photoperiodic response. In contrast to tfl1, elf3 mutations were able to suppress the temperature response in phyB mutants, but not in cryptochrome mutants. Gene expression profiles revealed that the tfl1 and elf3 effects are due to the activation of different sets of genes, and identified CCA1 and SOC1/AGL20 as being important cross-talk points. Finally, genome-wide gene expression analysis strongly suggests a general and complementary role for ELF3 and TFL1 in temperature signalling.

Phytochrome B enhances photosynthesis at the expense of water-use efficiency in Arabidopsis.

In open places, plants are exposed to higher fluence rates of photosynthetically active radiation and to higher red to far-red ratios than under the shade of neighbor plants. High fluence rates are known to increase stomata density. Here we show that high, compared to low, red to far-red ratios also increase stomata density in Arabidopsis (Arabidopsis thaliana). High red to far-red ratios increase the proportion of phytochrome B (phyB) in its active form and the phyB mutant exhibited a constitutively low stomata density. phyB increased the stomata index (the ratio between stomata and epidermal cells number) and the level of anphistomy (by increasing stomata density more intensively in the adaxial than in the abaxial face). phyB promoted the expression of FAMA and TOO MANY MOUTHS genes involved in the regulation of stomata development in young leaves. Increased stomata density resulted in increased transpiration per unit leaf area. However, phyB promoted photosynthesis rates only at high fluence rates of photosynthetically active radiation. In accordance to these observations, phyB reduced long-term water-use efficiency estimated by the analysis of isotopic discrimination against (13)CO(2). We propose a model where active phyB promotes stomata differentiation in open places, allowing plants to take advantage of the higher irradiances at the expense of a reduction of water-use efficiency, which is compensated by a reduced leaf area.

The role of phytochrome B, D and E in thermoperiodic responses of Arabidopsis thaliana

The objective of this work was to study the role of the phytochromes (phy) B, D and E in the thermoperiodic control of elongation and flowering time in Arabidopsis thaliana. WT, and phyB, phyD and phyE single mutants, and phyB phyD and phyB phyE double mutants, were grown under day/night temperatures (DT/NT) of 12/22°C, 17/17°C or 22/12°C (negative, zero and positive DIF, respectively) for inflorescence stem length measurements, and under DT/NT 17/25°C or 25/17°C (negative and positive DIF, respectively) for leaf morphology and flowering time measurements. In WT final length of the stem, petiole and leaf blade were longer under positive DIF compared to negative DIF. The temperature effect was stronger in the leaf petiole than the stem, whereas only a slight change was seen in the leaf blade length direction and none in the width direction. The temperature effect on stem and petiole elongation was reduced or nearly eliminated in the genotypes lacking phyB, while a phyD or a phyE mutation had no influence or a slightly positive influence on the temperature effect, respectively. These results suggest that phyB, and not phyD or phyE, is needed for a complete thermoperiodic control of elongation growth in A. thaliana. For all genotypes tested, plants flowered earlier at negative DIF than positive DIF, suggesting that none of the three phytochromes B, D, or E is needed for a thermoperiodic control of flowering time in A. thaliana.

Phytochrome‐mediated inhibition of shade avoidance involves degradation of growth‐promoting bHLH transcription factors

Plant growth and development are particularly sensitive to changes in the light environment and especially to vegetational shading. The shade-avoidance response is mainly controlled by the phytochrome photoreceptors. In Arabidopsis, recent studies have identified several related bHLH class transcription factors (PIF, for phytochrome-interacting factors) as important components in phytochrome signaling. In addition to a related bHLH domain, most of the PIFs contain an active phytochrome binding (APB) domain that mediates their interaction with light-activated phytochrome B (phyB). Here we show that PIF4 and PIF5 act early in the phytochrome signaling pathways to promote the shade-avoidance response. PIF4 and PIF5 accumulate to high levels in the dark, are selectively degraded in response to red light, and remain at high levels under shade-mimicking conditions. Degradation of these transcription factors is preceded by phosphorylation, requires the APB domain and is sensitive to inhibitors of the proteasome, suggesting that PIF4 and PIF5 are degraded upon interaction with light-activated phyB. Our data suggest that, in dense vegetation, which is rich in far-red light, shade avoidance is triggered, at least partially, as a consequence of reduced phytochrome-mediated degradation of transcription factors such as PIF4 and PIF5. Consistent with this idea, the constitutive shade-avoidance phenotype of phyB mutants partially reverts in the absence of PIF4 and PIF5.

HY5 and HYH are positive regulators of nitrate reductase in seedlings and rosette stage plants

HY5 and HYH are bZIP transcription factors well known to be involved in photomorphogenesis and light signalling. Loss-of-function mutants of HY5 and HYH revealed that these genes are essential for induction of a key enzyme in nitrogen assimilation, nitrate reductase (EC 1.7.1.1). In Arabidopsis thaliana seedlings nitrate reductase was expressed under low irradiance far-red or red light at the same level as under higher irradiance, photosynthetic active, white light. However, high NR expression at low light levels occurred only in the presence of sucrose in the growth medium. Sucrose did not promote expression in darkness. Whereas HY5 was necessary for high nitrate reductase expression in far-red light, HYH was important in red light. COP1 is known to promote degradation of HY5 and HYH, and in the cop1 mutant, nitrate reductase activity was relatively high also in darkness. PhyA and PhyB mutants were tested, and confirmed the phytochrome dependency for far-red and red light induction of nitrate reductase in seedlings. In rosette leaves of 3-week-old green plants the daily increase in nitrate reductase expression in response to light-on was abolished in the hyh and hy5 hyh double mutant. The hy5 hyh double mutant had lower nitrate reductase activity than any of the single mutants in photosynthetic active light in both seedlings and rosette leaves.

PhyB‐1 sorghum maintains responsiveness to simulated shade, irradiance and red light : far‐red light

The sorghum [Sorghum bicolor (L.) Moench] phyB-1 mutant exhibits a constitutive shade-avoidance phenotype including excessive shoot elongation. It was previously shown that this mutant also overproduces ethylene. Although phytochrome B (phyB) is assumed to be the pigment most important in sensing and transducing shade signals, the sorghum phyB-1 mutant still responds to light signals characteristic of shade. Specifically, it was determined that the leaf blade : leaf sheath elongation of phyB-1 is responsive to red : far red (R : FR), but this response is opposite that of wild type (WT). Reducing the photosynthetic photon flux density (PPFD) strongly reduced the leaf blade : leaf sheath of WT but did not affect phyB-1, demonstrating a role for phyB in sensing PPFD. Using light-emitting diode (LED) lighting, it was found that WT ethylene production was increased with low R : FR while PPFD had no effect. Conversely, phyB-1 ethylene production increased only with high PPFD, high R : FR which was the treatment resulting in the least ethylene production by WT. Elevated ethylene production inhibits shoot elongation, but may contribute to shade avoidance by reducing leaf blade : leaf sheath elongation. Ethylene responses to light treatments designed to promote or reduce phytochrome A (phyA) activity, and the analysis of PHYA levels in the two cultivars suggests that phyA could be involved in transducing shade signals in light-grown sorghum. Responses potentially tranduced by phyA are elevated in phyB-1 which also over-expresses PHYA.

Phytochrome-Mediated Inhibition of Coleoptile Growth in Rice: Age-dependency and Action Spectra†

Phytochrome has been shown to be the major photoreceptor involved in the photo-inhibition of coleoptile growth in Japonica-type rice (Oryza sativa L.). We have characterized this typical photomorphogenetic response of rice using mutants deficient in phytochrome A (phyA) and phytochrome B (phyB) and with respect to age-dependency and action spectra. Seedlings were irradiated with a pulse of light 40 h or 80 h after germination (i.e. at an early or late developmental stage) and the final coleoptile length of these seedlings was determined. A saturating pulse of red light (R) had a stronger effect when it was given in the late stage than in the early stage. It was found that the photoinhibition is mediated by both the phyA and the phyB in the late stage but predominantly by phyB in the early stage. Consistent with many other reported responses, the photo-inhibition in the phyA mutant, which was observed in the early and late developmental stages and is thought to be mediated mainly by phyB, occurred in the low-fluence range (10(1)-10(3) micromol m(-2)) of R and was far-red-light (FR)-reversible; the photo-inhibition in the phyB mutant, which was observed in the late developmental stage and is thought to be mediated mainly by phyA, occurred in the very-low-fluence range (10(-2)-10(0) micromol m(-2)) and was FR-irreversible. The action spectra (350-800 nm at 50 nm intervals) obtained at the two developmental stages using phyA and phyB mutants indicated that both the phyB-mediated low-fluence response and the phyA-mediated very-low-fluence response have a major peak at 650 nm and a minor peak at 400 nm.

Effect of Red Light Irradiance on Circumnutation of Arabidopsis thaliana Inflorescence Stems

The nutation frequency of Arabidopsis inflorescence stems gradually decreased when cultivated under dark or irradiation of weak white light (<5 μmol m-2s-1). Re-irradiation of white, blue, red and far-red light during dark treatment had different effects on the nutation frequency. The nutation frequency of inflorescence stems increased soon after irradiation with a red light. This quick response in the nutation frequency to red light irradiation was observed in wild-type plant and phyB mutant, but not in either phyA or phyAphyB mutants. These results suggest that the light intensity and the red light photoperception via phytochrome A play important roles in the regulation of circumnutation.

Two Native Pools of Phytochrome A in Monocots: Evidence from Fluorescence Investigations of Phytochrome Mutants of Rice

Fluorescence investigations of phytochrome (phy) in rice (Oryza sativa L. cv. Nipponbare) mutants deficient in phyA, phyB and phyA plus phyB were performed. Total content of the pigment (P(tot)) and its spectroscopic and photochemical characteristics were determined in different parts of the dark-grown and far-red light (FR)-grown coleoptiles. Spectroscopically, phyA in the phyB mutant was identical to phyA in the wild-type (WT) and the extent of the conversion from Pr to lumi-R at 85 K was the same for phyA in both lines and varied similarly, depending on the part of the coleoptile used. The latter finding proved that phyA in rice is heterogeneous and comprises two phyA populations, phyA' and phyA". Functional properties of phyA were also determined. In the dark the phyB mutant had a higher content of phyA, inactive protochlorophyllide (Pchlide633) and active protochlorophyllide (Pchlide655) than WT and its coleoptile was longer, indicating that phyB may affect the development of WT seedlings in the dark. Constant FR drastically reduced the content of phyA, Pchlide633 and Pchlide655 and brought about coleoptile shortening and appearance of the first leaf, whereas pulsed FR of equal fluence was less effective. This suggested that the reactions were primarily of the high irradiance responses type, which are likely to be mediated by phyA'. The effects on protochlorophyllide biosynthesis and growth responses type were more pronounced in the phyB mutant than in the WT seedlings, which can be connected with the higher phyA' content in the phyB mutant and/or phyB interference with its action in WT seedlings. In the phyA mutant induction of Pchlide633 and Pchlide655 biosynthesis was observed under constant FR, indicating that phyC may be responsible for this effect.

A role for ethylene in the phytochrome-mediated control of vegetative development.

Members of the phytochrome family of photoreceptors play key roles in vegetative plant development, including the regulation of stem elongation, leaf development and chlorophyll accumulation. Hormones have been implicated in the control of these processes in de-etiolating seedlings. However, the mechanisms by which the phytochromes regulate vegetative development in more mature plants are less well understood. Pea (Pisum sativum) mutant plants lacking phytochromes A and B, the two phytochromes present in this species, develop severe defects later in development, including short, thick, distorted internodes and reduced leaf expansion, chlorophyll content and CAB gene transcript level. Studies presented here indicate that many of these defects in phyA phyB mutant plants appear to be due to elevated ethylene production, and suggest that an important role of the phytochromes in pea is to restrict ethylene production to a level that does not inhibit vegetative growth. Mutant phyA phyB plants produce significantly more ethylene than WT plants, and application of an ethylene biosynthesis inhibitor rescued many aspects of the phyA phyB mutant phenotype. This deregulation of ethylene production in phy-deficient plants appears likely to be due, at least in part, to the elevated transcript levels of key ethylene-biosynthesis genes. The phytochrome A photoreceptor appears to play a prominent role in the regulation of ethylene production, as phyA, but not phyB, single-mutant plants also exhibit a phenotype consistent with elevated ethylene production. Potential interactions between ethylene and secondary plant hormones in the control of the phy-deficient mutant phenotype were explored, revealing that ethylene may inhibit stem elongation in part by reducing gibberellin levels.

Phytochrome B Represses Teosinte Branched1 Expression and Induces Sorghum Axillary Bud Outgrowth in Response to Light Signals

Light is one of the environmental signals that regulate the development of shoot architecture. Molecular mechanisms regulating shoot branching by light signals have not been investigated in detail. Analyses of light signaling mutants defective in branching provide insight into the molecular events associated with the phenomenon. It is well documented that phytochrome B (phyB) mutant plants display constitutive shade avoidance responses, including increased plant height and enhanced apical dominance. We investigated the phyB-1 mutant sorghum (Sorghum bicolor) and analyzed the expression of the sorghum Teosinte Branched1 gene (SbTB1), which encodes a putative transcription factor that suppresses bud outgrowth, and the sorghum dormancy-associated gene (SbDRM1), a marker of bud dormancy. Buds are formed in the leaf axils of phyB-1; however, they enter into dormancy soon after their formation. The dormant state of phyB-1 buds is confirmed by the high level of expression of the SbDRM1 gene. The level of SbTB1 mRNA is higher in the buds of phyB-1 compared to wild type, suggesting that phyB mediates the growth of axillary shoots in response to light signals in part by regulating the mRNA abundance of SbTB1. These results are confirmed by growing wild-type seedlings with supplemental far-red light that induces shade avoidance responses. We hypothesize that active phyB (Pfr) suppresses the expression of the SbTB1 gene, thereby inducing bud outgrowth, whereas environmental conditions that inactivate phyB allow increased expression of SbTB1, thereby suppressing bud outgrowth.

Arabidopsis Response Regulators ARR3 and ARR4 Play Cytokinin-Independent Roles in the Control of Circadian Period

Light and temperature are potent environmental signals used to synchronize the circadian oscillator with external time and photoperiod. Phytochrome and cryptochrome photoreceptors integrate light quantity and quality to modulate the pace and phase of the clock. PHYTOCHROME B (phyB) controls period length in red light as well as the phase of the clock in white light. phyB interacts with ARABIDOPSIS RESPONSE REGULATOR4 (ARR4) in a light-dependent manner. Accordingly, we tested ARR4 and other members of the type-A ARR family for roles in clock function and show that ARR4 and its closest relative, ARR3, act redundantly in the Arabidopsis thaliana circadian system. Loss of ARR3 and ARR4 lengthens the period of the clock even in the absence of light, demonstrating that they do so independently of active phyB. In addition, in white light, arr3,4 mutants show a leading phase similar to phyB mutants, suggesting that circadian light input is modulated by the interaction of phyB with ARR4. Although type-A ARRs are involved in cytokinin signaling, the circadian defects appear to be independent of cytokinin, as exogenous cytokinin affects the phase but not the period of the clock. Therefore, ARR3 and ARR4 are critical for proper circadian period and define an additional level of regulation of the circadian clock in Arabidopsis.

Suppression of the Floral ActivatorHd3aIs the Principal Cause of the Night Break Effect in Rice[W

A short exposure to light in the middle of the night causes inhibition of flowering in short-day plants. This phenomenon is called night break (NB) and has been used extensively as a tool to study the photoperiodic control of flowering for many years. However, at the molecular level, very little is known about this phenomenon. In rice (Oryza sativa), 10 min of light exposure in the middle of a 14-h night caused a clear delay in flowering. A single NB strongly suppressed the mRNA of Hd3a, a homolog of Arabidopsis thaliana FLOWERING LOCUS T (FT), whereas the mRNAs of OsGI and Hd1 were not affected. The NB effect on Hd3a mRNA was maximal in the middle of the 14-h night. The phyB mutation abolished the NB effect on flowering and Hd3a mRNA, indicating that the NB effect was mediated by phytochrome B. Because expression of the other FT-like genes was very low and not appreciably affected by NB, our results strongly suggest that the suppression of Hd3a mRNA is the principal cause of the NB effect on flowering in rice.