phyA phyB Research Articles

Phytochrome contributes to blue-light-mediated stem elongation and flower initiation in mature Arabidopsis thaliana plants

To examine whether phytochromes contribute to blue-light-mediated stem elongation, plant phenotypic responses were investigated in wild type Arabidopsis thaliana (Col-0), and its quintuple phytochrome (phyA phyB phyC phyD phyE) mutant plants under the following light treatments: (1) R, a pure red light from 660-nm LED; (2) B, a pure blue light from 455-nm LED; (3) BR, a impure blue light from LED combination of 94% B and 6% R; and (4) BRF, another impure blue light from LED combination of BR and 6 μmol·m−2·s−1 of FR (735 nm). A photosynthetic photon flux density of ≈100 μmol·m−2·s−1 was provided for all the light treatments. The calculated phytochrome photoequilibrium was 0.89, 0.50, 0.69, and 0.60 for R, B, BR, and BRF, respectively, indicating a higher phytochrome activity under R and BR than B and BRF. After 18 days of light treatment, B or BRF increased main stem length in wild-type plants compared with R, but BR had an inhibition effect similar to R. Also, B and BRF relative to R or BR induced earlier flowering and reduced leaf size in wild type plants, showing typical shade-avoidance responses. In phytochrome-deficient mutant plants, the above shade-avoidance responses were inhibited under B or BRF. However, hypocotyl length, a growth trait characterizing the de-etiolation stage, was reduced under B, BR and BRF vs. R regardless of phytochrome absence. These findings suggest that for mature Arabidopsis plants, phytochrome plays a role in blue-light-mediated stem elongation and the associated shade-avoidance responses.

The cold response regulator CBF1 promotes Arabidopsis hypocotyl growth at ambient temperatures.

Light and temperature are two core environmental factors that coordinately regulate plant growth and survival throughout their entire life cycle. However, the mechanisms integrating light and temperature signaling pathways in plants remain poorly understood. Here, we report that CBF1, an AP2/ERF-family transcription factor essential for plant cold acclimation, promotes hypocotyl growth under ambient temperatures in Arabidopsis. We show that CBF1 increases the protein abundance of PIF4 and PIF5, two phytochrome-interacting bHLH-family transcription factors that play pivotal roles in modulating plant growth and development, by directly binding to their promoters to induce their gene expression, and by inhibiting their interaction with phyB in the light. Moreover, our data demonstrate that CBF1 promotes PIF4/PIF5 protein accumulation and hypocotyl growth at both 22°C and 17°C, but not at 4°C, with a more prominent role at 17°C than at 22°C. Together, our study reveals that CBF1 integrates light and temperature control of hypocotyl growth by promoting PIF4 and PIF5 protein abundance in the light, thus providing insights into the integration mechanisms of light and temperature signaling pathways in plants.

Response of photosynthetic apparatus in Arabidopsis thaliana L. mutant deficient in phytochrome A and B to UV-B

The effects of UV-B radiation (1 W m–2, 1 and 2 h) on PSII activity, chloroplast structure, and H2O2 contents in leaves of 26-d-old Arabidopsis thaliana phyA phyB double mutant (DMut) compared with the wild type (WT) were investigated. UV-B decreased PSII activity and affected the H2O2 content in WT and DMut plants grown under white light (WL). The chloroplast structure changes in DMut plants exposed to UV were more significant than that in WT. Reductions in maximal and real quantum photochemical yields and increase in the value of thermal dissipation of absorbed light energy per PSII RC and the amount of QB-nonreducing centers of PSII were bigger in mutant compared to WT plants grown both under WL and red light. Such difference in action of UV-B on WT and DMut can be explained by higher content of UV-absorbing pigments and carotenoids in WT leaves compared with DMut.

The Rice Phytochrome Genes, PHYA and PHYB, Have Synergistic Effects on Anther Development and Pollen Viability

Phytochromes are the main plant photoreceptors regulating multiple developmental processes. However, the regulatory network of phytochrome-mediated plant reproduction has remained largely unexplored. There are three phytochromes in rice, phyA, phyB and phyC. No changes in fertility are observed in the single mutants, whereas the seed-setting rate of the phyA phyB double mutant is significantly reduced. Histological and cytological analyses showed that the reduced fertility of the phyA phyB mutant was due to defects in both anther and pollen development. The four anther lobes in the phyA phyB mutant were developed at different stages with fewer pollen grains, most of which were aborted. At the mature stage, more than one lobe in the double mutant was just consisted of several cell layers. To identify genes involved in phytochrome-mediated anther development, anther transcriptomes of phyA, phyB and phyA phyB mutants were compared to that of wild-type rice respectively. Analysis of 2,241 double-mutant-specific differentially expressed transcripts revealed that the metabolic profiles, especially carbohydrate metabolism, were altered greatly, and heat-shock responses were activated in the double mutant. This study firstly provides valuable insight into the complex regulatory networks underlying phytochrome-mediated anther and pollen development in plants, and offers novel clues for hybrid rice breeding.

Resistance of Arabidopsis thaliana L. photosynthetic apparatus to UV-B is reduced by deficit of phytochromes B and A

The photosynthetic responses of 25-day-old Arabidopsis phyA phyB double mutant (DM) compared with the wild type (WT) to UV-B radiation (1Wm−2, 30min) were investigated. UV-B irradiation led to reduction of photosystem 2 (PS-2) activity and the photosynthetic rate. In plants grown under both white and red light (λm - 660nm) the reduction was greater in DM plants compared to the WT. Without UV-B irradiation a decrease in PS-2 activity was observed in DM grown under RL only. It is assumed that the lower content of UV-absorbing pigments and carotenoids observed in DM may be one of the reasons of reduced PS-2 resistance to UV-B. Higher decrease in activities under UV in DM plants grown under RL compared to DM plants grown under white light is likely due to the lack of activity of cryptochromes in plants grown under red light. Rates of post-stress recovery of photosynthetic activity of DM compared with WT plants under white and red light of low intensity were studied. Almost complete recovery of the activity was found which was not observed under dark conditions and in the presence of a protein synthesis inhibitor, chloramphenicol. It is assumed that phytochrome system participates in stress-protective mechanisms of the photosynthetic apparatus to UV-radiation.

A proteome map of a quadruple photoreceptor mutant sustains its severe photosynthetic deficient phenotype

Light is the environmental factor that most affects plant growth and development through its impact on photomorphogenesis and photosynthesis. A quadruple photoreceptor mutant lacking four of the most important photoreceptors in plants, phytochromes A and B (phyA, phyB) and cryptochromes 1 and 2 (cry1, cry2), is severely affected in terms of growth and development. Previous studies have suggested that in addition to a photomorphogenic disorder, the phyA phyB cry1 cry2 quadruple mutant might have severe alterations in photosynthetic ability. Here, we investigated the photosynthetic processes altered in the quadruple mutant and performed a proteomic profiling approach to identify some of the proteins involved. The phyA phyB cry1 cry2 quadruple mutant showed reduced leaf area and total chlorophyll content. Photosynthetic rates at high irradiances were reduced approximately 65% compared to the wild type (WT). Light-saturated photosynthesis and the response of net CO2 exchange to low and high internal CO2 concentrations suggest that the levels or activity of the components of the Calvin cycle and electron transport might be reduced in the quadruple mutant. Most of the under-expressed proteins in the phyA phyB cry1 cry2 quadruple mutant consistently showed a chloroplastic localization, whereas components of the Calvin cycle and light reaction centers were overrepresented. Additionally, Rubisco expression was reduced threefold in the phyA phyB cry1 cry2 quadruple mutant.Together, these results highlight the importance of the phytochrome and cryptochrome families in proper autotrophy establishment in plants. They also suggest that an overall limitation in the chlorophyll levels, expression of Rubisco, and enzymes of the Calvin Cycle and electron transport that affect ribulose-1,5-biphosphate (RuBP) regeneration reduced photosynthetic capacity in the phyA phyB cry1 cry2 quadruple mutant.

The Phytochrome B/Phytochrome C Heterodimer Is Necessary for Phytochrome C-Mediated Responses in Rice Seedlings

BackgroundPhyC levels have been observed to be markedly lower in phyB mutants than in Arabidopsis or rice wild type etiolated seedlings, but the mechanism of this phenomenon has not been fully elucidated.ResultsIn the present study, we investigated the mechanism by which phyB affects the protein concentration and photo-sensing abilities of phyC and demonstrated that rice phyC exists predominantly as phyB/phyC heterodimers in etiolated seedlings. PHYC-GFP protein was detected when expressed in phyA phyC mutants, but not in phyA phyB mutants, suggesting that phyC requires phyB for its photo-sensing abilities. Interestingly, when a mutant PHYB gene that has no chromophore binding site, PHYB(C364A), was introduced into phyB mutants, the phyC level was restored. Moreover, when PHYB(C364A) was introduced into phyA phyB mutants, the seedlings exhibited de-etiolation under both far-red light (FR) and red light (R) conditions, while the phyA phyB mutants were blind to both FR and R. These results are the first direct evidence that phyC is responsible for regulating seedling de-etiolation under both FR and R. These findings also suggest that phyB is indispensable for the expression and function of phyC, which depends on the formation of phyB/phyC heterodimers.SignificanceThe present report clearly demonstrates the similarities and differences in the properties of phyC between Arabidopsis and rice and will advance our understanding of phytochrome functions in monocots and dicots.

Expression of enzymes involved in chlorophyll catabolism in Arabidopsis is light controlled.

We found that the levels of mRNA of two enzymes involved in chlorophyll catabolism in Arabidopsis (Arabidopsis thaliana), products of two chlorophyllase genes, AtCLH1 and AtCLH2, dramatically increase (by almost 100- and 10-fold, respectively) upon illumination with white light. The measurements of photosystem II quantum efficiency in 3-(3,4-dichlorophenyl)-1,1-dimethylurea-inhibited leaves show that their expression is not related to photosynthesis but mediated by photoreceptors. To identify the photoreceptors involved, we used various light treatments and Arabidopsis photoreceptor mutants (cry1, cry2, cry1cry2, phot1, phot2, phot1phot2, phyA phyB, phyAphyB). In wild-type Columbia, the amount of transcripts of both genes increase after white-light irradiation but their expression profile and the extent of regulation differ considerably. Blue and red light is active in the case of AtCLH1, whereas only blue light raises the AtCLH2 mRNA level. The fundamental difference is the extent of up-regulation, higher by one order of magnitude in AtCLH1. Both blue and red light is active in the induction of AtCLH1 expression in all mutants, pointing to a complex control network and redundancy between photoreceptors. The blue-specific up-regulation of the AtCLH2 transcript is mediated by cryptochromes and modulated by phototropin1 and phytochromes. Individually darkened leaves were used to test the effects of senescence on the expression of AtCLH1 and AtCLH2. The expression profile of AtCLH1 remains similar to that found in nonsenescing leaves up to 5 d after darkening. In contrast, the light induction of AtCLH2 mRNA declines during dark treatment. These results demonstrate that the expression of enzymes involved in chlorophyll catabolism is light controlled.

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.

Comprehensive metabolite profiling of phyA phyB phyC triple mutants to reveal their associated metabolic phenotype in rice leaves

The phytochrome photoreceptors regulate plant growth and development throughout their life cycle. Rice (Oryza sativa) possesses three phytochromes, phyA, phyB, and phyC. Physiological, genetic, and biochemical analyses of null mutants of each phytochrome have revealed the function of each in rice. However, few studies have investigated the relationship between phytochrome signaling and metabolism. In the present study, non-targeted metabolite analysis by gas chromatography time-of-flight mass spectrometry (GC/TOF-MS) and targeted metabolite analysis by capillary electrophoresis electrospray ionization mass spectrometry (CE/ESI-MS) were employed to investigate metabolic changes in rice phyA phyB phyC triple mutants. Distinct metabolic profiles between phyA phyB phyC triple mutants and the wild type (WT), as well as those between young and mature leaf blades, could be clearly observed by principal component analysis (PCA). The metabolite profiles indicated high accumulation of amino acids, organic acids, sugars, sugar phosphates, and nucleotides in the leaf blades of phyA phyB phyC triple mutants, especially in the young leaves, compared with those in the WT. Remarkable overaccumulation of monosaccharide, such as glucose (53.4-fold), fructose (42.5-fold), and galactose (24.5-fold), was observed in young leaves of phyA phyB phyC triple mutants. These metabolic phenotypes suggest that sugar metabolism, carbon partitioning, sugar transport, or some combination of these is impaired in the phyA phyB phyC triple mutants, and conversely, that phytochromes have crucial roles in sugar metabolism.

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.

Distinct and Cooperative Functions of Phytochromes A, B, and C in the Control of Deetiolation and Flowering in Rice

We have isolated phytochrome B (phyB) and phyC mutants from rice (Oryza sativa) and have produced all combinations of double mutants. Seedlings of phyB and phyB phyC mutants exhibited a partial loss of sensitivity to continuous red light (Rc) but still showed significant deetiolation responses. The responses to Rc were completely canceled in phyA phyB double mutants. These results indicate that phyA and phyB act in a highly redundant manner to control deetiolation under Rc. Under continuous far-red light (FRc), phyA mutants showed partially impaired deetiolation, and phyA phyC double mutants showed no significant residual phytochrome responses, indicating that not only phyA but also phyC is involved in the photoperception of FRc in rice. Interestingly, the phyB phyC double mutant displayed clear R/FR reversibility in the pulse irradiation experiments, indicating that both phyA and phyB can mediate the low-fluence response for gene expression. Rice is a short-day plant, and we found that mutation in either phyB or phyC caused moderate early flowering under the long-day photoperiod, while monogenic phyA mutation had little effect on the flowering time. The phyA mutation, however, in combination with phyB or phyC mutation caused dramatic early flowering.

Cryptochrome 1 contributes to blue-light sensing in pea.

Cryptochromes are widespread in higher plants but their physiological roles as blue-light photoreceptors have been examined in relatively few species. Screening in a phyA null mutant background has identified several blue-light response mutants in pea (Pisum sativum), including one that carries a substitution of a highly conserved glycine residue in the N-terminal photolyase-homologous domain of the pea CRY1 gene. Analyses of cry1, phyA, and phyB mutants show that all three photoreceptors contribute to seedling photomorphogenesis under high-irradiance blue light, whereas phyA is the main photoreceptor active under low irradiances. Triple phyA phyB cry1 mutants grown under high-irradiance blue light are indistinguishable from dark-grown wild-type plants in length and leaf expansion but show a small residual response to higher-irradiance white light. Monogenic cry1 mutants have little discernable phenotype at the seedling stage, but later in development are more elongated than wild-type plants. In addition, the loss of cry1 moderates the short-internode phenotype of older phyA mutants, suggesting an antagonism between phyA and cry1 under some conditions. Pea cry1 has a small inhibitory effect on flowering under long and short days. However, the phyA cry1 double mutant retains a clear promotion of flowering in response to blue-light photoperiod extensions, indicating a role for one or more additional blue-light photoreceptors in the control of flowering in pea.

포도 세포현탁배양계에서 Phytochrome 신호전달에 의해 조절되는 안토시아닌 생합성

포도 현탁배양계를 이용하여 안토시아닌 생합성 경로에 관여하는 phytochrome 신호 전달에 관하여 연구하였다. 그 결과 세포의 생중량과 세포수의 생장은 배양 시 광의 유무 및 단색광의 종류와 관계없이 배양 4일째부터 8일째까지 급격히 증가하였고 8일 이후부터 정상기에 들어서서 최대 생장을 보였다. 그러나 안토시아닌 색소 축적은 청색광에서 가장 높은 효과를 나타내었으며 적색광에서도 높았으나 원적색광하에서는 암소같이 색소생성이 억제되었다. 배양시기에 따른 색소생성에 미치는 광 조사의 효과는 대수증가기인 배양4일째에 조사한 단색광간에 차이가 인정되지 않았으나, 정상기인 배양 7일째 조사한 단색광의 효과는 그 종류에 따라 색소 축적 패턴에 큰 차이를 나타내는 것으로 보아 배양 7일째가 효과적이었다. Phytochrome의 광가역적 효과를 알아보기 위하여 암배양 7일째세포에 다양한 조건의 단색광 (B/R, B/FR, B/R/FR)을 가역적으로 처리했을 경우, 청색광 단독 처리한 구와 비슷한 양의 색소 축적양상을 보였으며, B/R/R 조건의 단색광이 가역적으로 처리된 세포주에서 가장 많은 색소 축적을 보였다. 그러나 배양 7일째 R/FR 광가역 처리시에는 적색광 단독 처리에 비해 색소 축적이 억제되었으나, R/FR/R 가역처리시에는 적색광의 색소 축적양과 동일한 수준으로 회복되었다. 안토시아닌 생합성의 주요효소인 PAL과 CHS의 발현분석에서 PAL의 발현은 7일째 청색광이 처리된 세포주에서 조금 증가하는 현상을 보였으나, 적색광과 근적외광 처리구에서는 거의 반응이 없었다. CHS의 발현은 단색광 처리에서 높은 변화를 보였으며, 광을 처리한 7일째 높은 발현을 보이다 12일 이후 다시 증가하는 패턴을 보였다. 또한 적색광과 원적색광이 가역적으로 처리된 세포주에서 CHS 발현양도 가역적으로 증가하였다. PHYA와 PHYB는 포도세포에서 단일유전자로 존재하였으며, 단색광 처리에 따라 색소축적이 이루어질 때 이들 이들 두 유전자의 발현도 증가하였다. This experiment was carried out to confirm that phytochrome regulates anthocyanin bio-synthesis during cell suspension culture system of grape or not. In suspension culture of grape, maximum accumulation of anthocyanin was observed at the stationary phase under continuous white light condition. From mono-chromatic light interruption for 24h at the 4th or 7th day on the suspension cultured cells, the anthocyanin accumulation was highly enhanced at the light interruption at 7th day than 4th day under all monochromatic light treatment. However, the cell growth patterns were not affected by any light treatment. In the darkness, the anthocyanin synthesis was very low but remarkably increased by blue light or red light irradiation. However, the increase of anthocyanin accumulation by blue or red light was suppressed by far-red light in the suspension cells of grape. This suppression by far-red light on the anthocyanin synthesis also observed on the cells treated red or far-red light alternatively. These results implied that phytochrome regulation system may be involved in the anthocyanin biosynthesis of the suspension grape cells. By RNA expression analysis, chalcone synthase (CHS) gene was expressed highly by blue and red light but low by far-red light. The synergistic increase of CHS gene expression was also observed at the treatment of blue light followed by red for 24h. This result may explain the increase of anthocyanin accumulation in B/R treatment. Although the expression of phytochrome gene (PHYA or PHYB) was not highly increased by all light treatment (blue, red, and far-red light) the expression of both PHYA gene and PHYB gene was increased a little in cells treated red or far-red light. In grape suspension cells, the red light enhanced the anthocyanin synthesis, whereas the far-red light was suppressed. Although it was not confirmed whether or not phytochrome gene is activated in anthocyanin accumulating grape cells, we believed that anthocyanin biosynthesis in grape cells may be regulated under phytochrome signal transduction system.

A genomic analysis of the shade avoidance response in Arabidopsis.

Plants respond to the proximity of neighboring vegetation by elongating to prevent shading. Red-depleted light reflected from neighboring vegetation triggers a shade avoidance response leading to a dramatic change in plant architecture. These changes in light quality are detected by the phytochrome family of photoreceptors. We analyzed global changes in gene expression over time in wild-type, phyB mutant, and phyA phyB double mutant seedlings of Arabidopsis in response to simulated shade. Using pattern fitting software, we identified 301 genes as shade responsive with patterns of expression corresponding to one of various physiological response modes. A requirement for a consistent pattern of expression across 12 chips in this way allowed more subtle changes in gene expression to be considered meaningful. A number of previously characterized genes involved in light and hormone signaling were identified as shade responsive, as well as several putative, novel shade-specific signal transduction factors. In addition, changes in expression of genes in a range of pathways associated with elongation growth and stress responses were observed. The majority of shade-responsive genes demonstrated antagonistic regulation by phyA and phyB in response to shade following the pattern of many physiological responses. An analysis of promoter elements of genes regulated in this way identified conserved promoter motifs potentially important in shade regulation.

Mutations in the huge Arabidopsis gene BIG affect a range of hormone and light responses.

In independent genetic screens, for shade-avoidance response and cytokinin sensitivity, we identified two Arabidopsis mutants, attenuated shade avoidance 1 (asa1) and umbrella1 (umb1), which have very similar pleiotropic phenotypes. asa1 and umb1 are allelic to tir3-1, and are caused by mutations in BIG, which is required for normal auxin efflux. They have a compact rosette, fewer lateral roots, delayed flowering, more secondary inflorescence, smaller seeds and, in the Laer-0 background, much shorter internodes between adjacent flowers, suggesting an interaction between BIG and ERECTA. These mutants have organ-specific defects in response to cytokinins, ethylene, N-1-naphthylphthalamic acid (NPA) and gibberellin (GA). The phenotype of the asa1 ga1-3 double mutant is consistent with defects in GA signalling. There are subtle effects in responses to auxins, abscisic acid and brassinolide. Elongation growth associated with shade avoidance in phyA phyB null mutants is suppressed by asa1 in all organs other than the hypocotyl. Therefore, we here provide evidence that BIG is a key player not just in auxin signalling, but in a multitude of light and hormone pathways.

Resetting of the circadian clock by phytochromes and cryptochromes in Arabidopsis.

The authors sought to investigate the role of phytochromes A and B (phyA and phyB) and cryptochromes 1 and 2 (cryl and cry2) in the synchronization of the leaf position rhythm in Arabidopsis thaliana. The seedlings were transferred from white light-dark cycles to free-running conditions with or without exposure to a light treatment during the final hours of the last dark period. The phase advance caused by a far-red light treatment was absent in the phyA mutant, deficient in the fhy1 and fhy3 mutants involved in phyA signaling, and normal in the cryl and cryl cry2 mutants. The phase shift caused by blue light was normal in the cry2 mutant; reduced in the phyA, cryl, phyA cry1, and cry1 cry2 mutants; and abolished in the phyA cryl cry2 triple mutant. The phase shift caused by red light was partially retained by the phyA phyB double mutant. The authors conclude that cryl and cry2 participate as photoreceptors in the blue light input to the clock but are not required for the phyA-mediated effects on the phase of the circadian rhythm of leaf position. The signaling proteins FHY1 and FHY3 are shared by phyA-mediated photomorphogenesis and phyA input to the clock.

Shl, a New set of Arabidopsis mutants with exaggerated developmental responses to available red, far-red, and blue light.

The interaction of light perception with development is the subject of intensive genetic analysis in the model plant Arabidopsis. We performed genetic screens in low white light-a threshold condition in which photomorphogenetic signaling pathways are only partially active-for ethyl methane sulfonate-generated mutants with altered developmental phenotypes. Recessive mutants with exaggerated developmental responses were obtained in eight complementation groups designated shl for seedlings hyperresponsive to light. shl1, shl2, shl5, and shl3 shl4 (double mutant) seedlings showed limited or no phenotypic effects in darkness, but showed significantly enhanced inhibition of hypocotyl elongation in low-white, red, far-red, blue, and green light across a range of fluences. These results reflect developmental hyper-responsiveness to signals generated by both phytochrome and cryptochrome photoreceptors. The shl11 mutant retained significant phenotypic effects on hypocotyl length in both the phyA mutant and phyB mutant backgrounds but may be dependent on CRY1 for phenotypic expression in blue light. The shl2 phenotype was partially dependent on PHYB, PHYA, and CRY1 in red, far-red, and blue light, respectively. shl2 and, in particular, shl1 were partially dependent on HY5 activity for their light-hyperresponsive phenotypes. The SHL genes act (genetically) as light-dependent negative regulators of photomorphogenesis, possibly in a downstream signaling or developmental pathway that is shared by CRY1, PHYA, and PHYB and other photoreceptors (CRY2, PHYC, PHYD, and PHYE).

Hierarchical coupling of phytochromes and cryptochromes reconciles stability and light modulation of Arabidopsis development.

In plants, development is a continuing process that takes place under strong fluctuations of the light environment. Here we show that in Arabidopsis thaliana plants grown under intense white light, coupling of the photoreceptor cryptochrome 2 to developmental processes is broader than previously appreciated. Compared to the wild type, the cry2 mutant showed reduced activity of a Lhcb1*2 promoter fused to a reporter, and delayed flowering. The cry2 mutation also reduced the inhibition of hypocotyl growth, the unfolding of the cotyledons, the rate of leaf production during the vegetative phase, and the pace of development after transition to the reproductive stage; but these effects were obvious only in the absence of cryptochrome 1 and in some cases phytochrome A and/or phytochrome B. Complementary, the cry2 mutation uncovered novel roles for cryptochrome 1 and phytochrome A. The activity of the Lhcb1*2 promoter was higher in the cry1 cry2 mutant than in the cry2 mutant, suggesting that cry1 could be involved in blue-light repression of photosynthetic genes. Surprisingly, the phyA cry1 cry2 triple mutant flowered earlier and showed better response to photoperiod than the cry1 cry2 double mutant, indicating that phyA is involved in light repression of flowering. Growth and development were severely impaired in the quadruple phyA phyB cry1 cry2 mutant. We propose that stability and light modulation of development are achieved by simultaneous coupling of phytochrome A, phytochrome B, cryptochrome 1 and cryptochrome 2 to developmental processes, in combination with context-dependent hierarchy of their relative activities.

Interaction of phytochromes A and B in the control of de-etiolation and flowering in pea.

The interactions of phytochrome A (phyA) and phytochrome B (phyB) in the photocontrol of vegetative and reproductive development in pea have been investigated using null mutants for each phytochrome. White-light-grown phyA phyB double mutant plants show severely impaired de-etiolation both at the seedling stage and later in development, with a reduced rate of leaf production and swollen, twisted internodes, and enlarged cells in all stem tissues. PhyA and phyB act in a highly redundant manner to control de-etiolation under continuous, high-irradiance red light. The phyA phyB double mutant shows no significant residual phytochrome responses for either de-etiolation or shade-avoidance, but undergoes partial de-etiolation in blue light. PhyB is shown to inhibit flowering under both long and short photoperiods and this inhibition is required for expression of the promotive effect of phyA. PhyA is solely responsible for the promotion of flowering by night-breaks with white light, whereas phyB appears to play a major role in detection of light quality in end-of-day light treatments, night breaks and day extensions. Finally, the inhibitory effect of phyB is not graft-transmissible, suggesting that phyB acts in a different manner and after phyA in the control of flower induction.