Phytochrome B affects responsiveness to gibberellins in Arabidopsis

Interpretation of the difference in shade tolerance of two subtropical forest tree species of different successional stages at the transcriptome and physiological levels

Differences in plant shade tolerance constitute a major mechanism driving the succession of forest communities in subtropical forests. However, the indirect effects of differences in light requirements on the growth of mid- and late-successional tree species are unclear, and this potential growth effect has not been explained at the transcriptome level. Here, a typical mid-successional dominant tree species, Schima superba Gardn. et Champ, and a typical late-successional dominant tree species, Cryptocarya concinna Hance were used as materials and planted under 100% full light (FL) and 30% FL (low light, LL) to explore the responses of tree species in different successional stages of subtropical forests to different light environments. Transcriptome sequencing was used to analyze the expression changes in genes related to growth and photoprotection under different light environments. The young leaves of S. superba accumulated more malondialdehyde (MDA) and superoxide radicals (${\mathrm{O}}_2^{{{}^{\bullet}}^{-}}$) under LL. A lower hormone content (auxin, cytokinin, gibberellin) in the young leaves, a weaker photosynthetic capacity in the mature leaves and significant downregulation of related gene expression were also found under LL, which resulted in the total biomass of S. superba under LL being lower than that under FL. The young leaves of C. concinna had less MDA and ${\mathrm{O}}_2^{{{}^{\bullet}}^{-}}$, and a higher hormone contents under LL than those under FL. There was no significant difference in photosynthetic capacity between mature leaves in contrasting light environments. Although the biomass of C. concinna under LL was less than that under FL, the height of C. concinna under LL was higher than that under FL, indicating that C. concinna could grow well under the two light environments. Our results describing the acclimatization of light at the physiological, molecular and transcriptome levels are important for a complete understanding of successional mechanisms.

Progress of Research on the Regulatory Pathway of the Plant Shade-Avoidance Syndrome

When subject to vegetational shading, shade-avoiding plants detect neighbors by perceiving reduced light quantity and altered light quality. The former includes decreases in the ratio of red to far-red wavelengths (low R:FR) and low blue light ratio (LBL) predominantly detected by phytochromes and cryptochromes, respectively. By integrating multiple signals, plants generate a suite of responses, such as elongation of a variety of organs, accelerated flowering, and reduced branching, which are collectively termed the shade-avoidance syndrome (SAS). To trigger the SAS, interactions between photoreceptors and phytochrome-interacting factors are the general switch for activation of downstream signaling pathways. A number of transcription factor families and phytohormones, especially auxin, gibberellins, ethylene, and brassinosteroids, are involved in the SAS processes. In this review, shade signals, the major photoreceptors involved, and the phenotypic characteristics of the shade-intolerant plant Arabidopsis thaliana are described in detail. In addition, integration of the signaling mechanisms that link photoreceptors with multiple hormone signaling pathways is presented and future research directions are discussed.

Organ-specific phytohormone synthesis in two Geranium species with antithetical responses to far-red light enrichment

Plants growing in high densities experience a reduced red (R) to far-red (FR) light ratio and shade-intolerant species respond with accelerated elongation growth to reach the top of the canopy: the shade avoidance syndrome (SAS). FR-enriched light inactivates phytochrome photoreceptors, which results in subsequent action of several plant hormones regulating growth. SAS is adaptive for shade-intolerant plants, but is suppressed in shade-tolerant plant species. Inspired by a previously published transcriptome analysis, we use two species of the genus Geranium here to study the involvement of auxin, brassinosteroids (BRs), and gibberellins (GAs) in supplemental FR-induced elongation growth. G. pyrenaicum, a shade-avoiding species, strongly induces auxin and gibberellin levels, but not BR, in elongating petioles. We show that, in this species, FR light perception, hormone synthesis, and growth are local and restricted to the petiole, and not the leaf lamina. Using chemical hormone inhibitors, we confirm the essential role of auxin and GAs in supplemental FR-induced elongation growth. Shade-tolerant G. robertianum does not display the change in hormone levels upon FR light enrichment, resulting in the lack of a shade avoidance response.

Transcriptome and phenotyping analyses support a role for chloroplast sigma factor 2 in red-light-dependent regulation of growth, stress, and photosynthesis

Sigma factor (SIG) proteins contribute to promoter specificity of the plastid-encoded RNA polymerase during chloroplast genome transcription. All six members of the SIG family, that is, SIG1-SIG6, are nuclear-encoded proteins targeted to chloroplasts. Sigma factor 2 (SIG2) is a phytochrome-regulated protein important for stoichiometric control of the expression of plastid- and nuclear-encoded genes that impact plastid development and plant growth and development. Among SIG factors, SIG2 is required not only for transcription of chloroplast genes (i.e., anterograde signaling), but also impacts nuclear-encoded, photosynthesis-related, and light signaling-related genes (i.e., retrograde signaling) in response to plastid functional status. Although SIG2 is involved in photomorphogenesis in Arabidopsis, the molecular bases for its role in light signaling that impacts photomorphogenesis and aspects of photosynthesis have only recently begun to be investigated. Previously, we reported that SIG2 is necessary for phytochrome-mediated photomorphogenesis specifically under red (R) and far-red light, thereby suggesting a link between phytochromes and nuclear-encoded SIG2 in light signaling. To explore transcriptional roles of SIG2 in R-dependent growth and development, we performed RNA sequencing analysis to compare gene expression in sig2-2 mutant and Col-0 wild-type seedlings at two developmental stages (1- and 7-day). We identified a subset of misregulated genes involved in growth, hormonal cross talk, stress responses, and photosynthesis. To investigate the functional relevance of these gene expression analyses, we performed several comparative phenotyping tests. In these analyses, strong sig2 mutants showed insensitivity to bioactive GA 3, high intracellular levels of hydrogen peroxide (H2O2) indicative of a stress response, and specific defects in photosynthesis, including elevated levels of cyclic electron flow (CEF) and nonphotochemical quenching (NPQ). We demonstrated that SIG2 regulates a broader range of physiological responses at the molecular level than previously reported, with specific roles in red-light-mediated photomorphogenesis.

MLK1 and MLK2 Coordinate RGA and CCA1 Activity to Regulate Hypocotyl Elongation in Arabidopsis thaliana

Gibberellins (GAs) modulate diverse developmental processes throughout the plant life cycle. However, the interaction between GAs and the circadian rhythm remains unclear. Here, we report that MUT9p-LIKE KINASE1 (MLK1) and MLK2 mediate the interaction between GAs and the circadian clock to regulate hypocotyl elongation in Arabidopsis thaliana DELLA proteins function as master growth repressors that integrate phytohormone signaling and environmental pathways in plant development. MLK1 and MLK2 interact with the DELLA protein REPRESSOR OF ga1-3 (RGA). Loss of MLK1 and MLK2 function results in plants with short hypocotyls and hyposensitivity to GAs. MLK1/2 and RGA directly interact with CIRCADIAN CLOCK ASSOCIATED1 (CCA1), which targets the promoter of DWARF4 (DWF4) to regulate its roles in cell expansion. MLK1/2 antagonize the ability of RGA to bind CCA1, and these factors coordinately regulate the expression of DWF4 RGA suppressed the ability of CCA1 to activate expression from the DWF4 promoter, but MLK1/2 reversed this suppression. Genetically, MLK1/2 act in the same pathway as RGA and CCA1 in hypocotyl elongation. Together, our results provide insight into the mechanism by which MLK1 and MLK2 antagonize the function of RGA in hypocotyl elongation and suggest that MLK1/2 coordinately mediate the regulation of plant development by GAs and the circadian rhythm in Arabidopsis.

Key players associated with tuberization in potato: potential candidates for genetic engineering

Tuberization in potato (Solanum tuberosum L.) is a complex biological phenomenon which is affected by several environmental cues, genetic factors and plant nutrition. Understanding the regulation of tuber induction is essential to devise strategies to improve tuber yield and quality. It is well established that short-day photoperiods promote tuberization, whereas long days and high-temperatures inhibit or delay tuberization. Worldwide research on this complex biological process has yielded information on the important bio-molecules (proteins, RNAs, plant growth regulators) associated with the tuberization process in potato. Key proteins involved in the regulation of tuberization include StSP6A, POTH1, StBEL5, StPHYB, StCONSTANS, Sucrose transporter StSUT4, StSP5G, etc. Biomolecules that become transported from "source to sink" have also been suggested to be important signaling candidates regulating the tuberization process in potatos. Four molecules, namely StSP6A protein, StBEL5 RNA, miR172 and GAs, have been found to be the main candidates acting as mobile signals for tuberization. These biomolecules can be manipulated (overexpressed/inhibited) for improving the tuberization in commercial varieties/cultivars of potato. In this review, information about the genes/proteins and their mechanism of action associated with the tuberization process is discussed.

DELLA-mediated PIF degradation contributes to coordination of light and gibberellin signalling in Arabidopsis

Light and gibberellins (GAs) antagonistically regulate hypocotyl elongation in plants. It has been demonstrated that DELLAs, which are negative regulators of GA signalling, inhibit phytochrome-interacting factors 3 and 4 (PIF3 and PIF4) by sequestering their DNA-recognition domains. However, it is unclear whether there are other mechanisms of regulatory crosstalk between DELLAs and PIFs. Here, we demonstrate that DELLAs negatively regulate the abundance of four PIF proteins through the ubiquitin–proteasome system. Reduction of PIF3 protein abundance by DELLAs correlates closely with reduced hypocotyl elongation. Both sequestration and degradation of PIF3 by DELLAs contribute to a reduction in PIF3 binding to its target genes. Thus, we show that promotion of PIF degradation by DELLAs is required to coordinate light and GA signals, and the dual regulation of transcription factors by DELLAs by both sequestration and degradation may be a general mechanism.

Gibberellins (GA) negatively regulate light-mediated suppression of hypocotyl elongation in plants. Here, Li et al. show that GA-mediated destabilization of DELLA proteins promotes accumulation of the light-regulated PIF transcription factors thus contributing to the crosstalk between light and GA signalling.

H3K36ac Is an Evolutionary Conserved Plant Histone Modification That Marks Active Genes

In eukaryotic cells, histones are subject to a large number of posttranslational modifications whose sequential or combinatorial action affects chromatin structure and genome function. We identified acetylation at Lys-36 in histone H3 (H3K36ac) as a new chromatin modification in plants. The H3K36ac modification is evolutionary conserved in seed plants, including the gymnosperm Norway spruce (Picea abies) and the angiosperms rice (Oryza sativa), tobacco (Nicotiana tabacum), and Arabidopsis (Arabidopsis thaliana). In Arabidopsis, H3K36ac is highly enriched in euchromatin but not in heterochromatin. Genome-wide chromatin immunoprecipitation sequencing experiments revealed that H3K36ac peaks at the 5' end of genes, mainly on the two nucleosomes immediately distal to the transcription start site, independently of gene length. H3K36ac overlaps with H3K4me3 and the H2A.Z histone variant. The histone acetyl transferase GCN5 and the histone deacetylase HDA19 are required for H3K36ac homeostasis. H3K36ac and H3K36me3 show negative crosstalk, which is mediated by GCN5 and the histone methyl transferase SDG8. Although H3K36ac is associated with gene activity, we did not find a linear relationship between H3K36ac and transcript levels, suggesting that H3K36ac is a binary indicator of transcription.

Roles of proteome dynamics and cytokinin signaling in root to hypocotyl ratio changes induced by shading roots of Arabidopsis seedlings

In nature, root systems of most terrestrial plants are protected from light exposure by growing in a dark soil environment. Hence, in vitro cultivation in transparent Petri dishes leads to physiological perturbations, but the mechanisms underlying root-mediated light perception and responses have not been fully elucidated. Thus, we compared Arabidopsis thaliana seedling development in transparent and darkened Petri dishes at low light intensity (20 µmol m(-2) s(-1)), allowing us to follow (inter alia) hypocotyl elongation, which is an excellent process for studying interactions of signals involved in the regulation of growth and developmental responses. To obtain insights into molecular events underlying differences in seedling growth under these two conditions, we employed liquid chromatography-mass spectrometry (LC-MS) shotgun proteomics (available via the PRIDE deposit PXD001612). In total, we quantified the relative abundances of peptides representing 1,209 proteins detected in all sample replicates of LC-MS analyses. Comparison of MS spectra after manual validation revealed 48 differentially expressed proteins. Functional classification, analysis of available gene expression data and literature searches revealed alterations associated with root illumination (inter alia) in autotrophic CO2 fixation, C compound and carbohydrate metabolism, and nitrogen metabolism. The results also indicate a previously unreported role for cytokinin plant hormones in the escape-tropism response to root illumination. We complemented these results with reverse transcription followed by quantitative PCR (RT-qPCR), chlorophyll fluorescence and detailed cytokinin signaling analyses, detecting in the latter a significant increase in the activity of the cytokinin two-component signaling cascade in roots and implicating the cytokinin receptor AHK3 as the major mediator of root to hypocotyl signaling in responses to root illumination.

Arabidopsis DET1 represses photomorphogenesis in part by negatively regulating DELLA protein abundance in darkness

Arabidopsis De-etiolated 1 (DET1) is one of the key repressors that maintain the etiolated state of seedlings in darkness. The plant hormone gibberellic acid (GA) also participates in this process, and plants deficient in GA synthesis or signaling show a partially de-etiolated phenotype in darkness. However, how DET1 and the GA pathway work in concert in repressing photomorphogenesis remains largely unknown. In this study, we found that the abundance of DELLA proteins in det1-1 was increased in comparison with that in the wild-type plants. Mutation in DET1 changed the sensitivity of hypocotyl elongation of mutant seedlings to GA and paclobutrazol (PAC), an inhibitor of GA synthesis. However, we did not find obvious differences between det1-1 and wild-type plants with regard to the bioactive GA content or the GA signaling upstream of DELLAs. Genetic data showed that removal of several DELLA proteins suppressed the det1-1 mutant phenotype more obviously than GA treatment, indicating that DET1 can regulate DELLA proteins via some other mechanisms. In addition, a large-scale transcriptomic analysis revealed that DET1 and DELLAs play antagonistic roles in regulating expression of photosynthetic and cell elongation-related genes in etiolated seedlings. Taken together, our results show that DET1 represses photomorphogenesis in darkness in part by reducing the abundance of DELLA proteins.

Heterosis, stress, and the environment: a possible road map towards the general improvement of crop yield

Contemporary plant breeding is under pressure to improve crop productivity at a rate surpassing past achievements. Different research groups dealing with this issue reached similar conclusions that the solution lies in improved biomass production by way of enhanced light capture and use efficiency, modified photosystem biochemistry, and improved partitioning of assimilates to the economic part of the plant. There seems to be a consensus of sorts. This 'opinion paper' calls attention to the phenomenon of heterosis, as expressed in maize, sorghum, and other crops where, depending on the case and the trait, larger biomass and greater yield have been achieved without a change in growth duration, photosystem biochemistry, or harvest index. This discussion maintains that there is no consensus about the genetics or the genomics of heterosis in regulating yield under diverse environments. Therefore, in a search for the basis of heterosis in yield and adaptation, the discussion bypasses the genetics and searches for answers in the phenomics of heterosis. The heterotic phenotype in itself provides challenging and important hints towards improving the yield of open-pollinated crops in general. These hints are linked to the homeostasis of photosynthesis with respect to temperature, the photobiology of the plant as mediated by phytochrome, the architectural foundations of the formation of a large sink, and the associated hormones and signals in controlling sink differentiation and source-sink communication. This discussion does not lay out plans and protocols but provides clues to explore within and beyond the current thinking about breeding for high yield.

Differences and similarities in the photoregulation of gibberellin metabolism between rice and dicots

In rice seedlings, elongation of leaf sheaths is suppressed by light stimuli. The response is mediated by two classes of photoreceptors, phytochromes and cryptochromes. However, it remains unclear how these photoreceptors interact in the process. Our recent study using phytochrome mutants and novel cryptochrome RNAi lines revealed that cryptochromes and phytochromes function cooperatively, but independently to reduce active GA contents in seedlings in visible light. Blue light captured by cryptochrome 1 (cry1a and cry1b) induces robust expression of GA 2-oxidase genes (OsGA2ox4-7). In parallel, phytochrome B with auxiliary action of phytochrome A mediates repression of GA 20-oxidase genes (OsGA20ox2 and OsGA20ox4). The independent effects cumulatively reduce active GA contents, leading to a suppression of leaf sheath elongation. These regulatory mechanisms are distinct from phytochrome B function in dicots. We discuss reasons why the distinct system appeared in rice, and advantages of the rice system in early photomorphogenesis.

Cryptochrome and phytochrome cooperatively but independently reduce active gibberellin content in rice seedlings under light irradiation

In contrast to a wealth of knowledge about the photoregulation of gibberellin metabolism in dicots, that in monocots remains largely unclear. In this study, we found that a blue light signal triggers reduction of active gibberellin content in rice seedlings with simultaneous repression of two gibberellin 20-oxidase genes (OsGA20ox2 and OsGA20ox4) and acute induction of four gibberellin 2-oxidase genes (OsGA2ox4-OsGA2ox7). For further examination of the regulation of these genes, we established a series of cryptochrome-deficient lines through reverse genetic screening from a Tos17 mutant population and construction of knockdown lines based on an RNA interference technique. By using these lines and phytochrome mutants, we elucidated that cryptochrome 1 (cry1), consisting of two species in rice plants (cry1a and cry1b), is indispensable for robust induction of the GA2ox genes. On the other hand, repression of the GA20ox genes is mediated by phytochromes. In addition, we found that the phytochromes also mediate the repression of a gibberellin 3-oxidase gene (OsGA3ox2) in the light. These results imply that, in rice seedlings, phytochromes mediate the repression of gibberellin biosynthesis capacity, while cry1 mediates the induction of gibberellin inactivation capacity. The cry1 action was demonstrated to be dominant in the reduction of active gibberellin content, but, in rice seedlings, the cumulative effects of these independent actions reduced active gibberellin content in the light. This pathway design in which different types of photoreceptors independently but cooperatively regulate active gibberellin content is unique from the viewpoint of dicot research. This redundancy should provide robustness to the response in rice plants.

Blue light regulated shade avoidance

Most plants grow in dense vegetation with the risk of being out-competed by neighboring plants. These neighbors can be detected not only through the depletion in light quantity that they cause, but also through the change in light quality, which plants perceive using specific photoreceptors. Both the reduction of the red:far-red ratio and the depletion of blue light are signals that induce a set of phenotypic traits, such as shoot elongation and leaf hyponasty, which increase the likelihood of light capture in dense plant stands. This set of phenotypic responses are part of the so called shade avoidance syndrome (SAS). This addendum discusses recent findings on the regulation of the SAS of Arabidopsis thaliana upon blue light depletion. Keller et al. and Keuskamp et al. show that the low blue light attenuation induced shade avoidance response of seedling and rosette-stage A. thaliana plants differ in their hormonal regulation. These studies also show there is a regulatory overlap with the R:FR-regulated SAS.

Gibberellin and auxin influence the diurnal transcription pattern of photoreceptor genes via CRY1a in tomato

BACKGROUND

Plant photoreceptors, phytochromes and cryptochromes, regulate many aspects of development and growth, such as seed germination, stem elongation, seedling de-etiolation, cotyledon opening, flower induction and circadian rhythms. There are several pieces of evidence of interaction between photoreceptors and phyto-hormones in all of these physiological processes, but little is known about molecular and genetic mechanisms underlying hormone-photoreceptor crosstalk.

METHODOLOGY/PRINCIPAL FINDINGS

In this work, we investigated the molecular effects of exogenous phyto-hormones to photoreceptor gene transcripts of tomato wt, as well as transgenic and mutant lines with altered cryptochromes, by monitoring day/night transcript oscillations. GA and auxin alter the diurnal expression level of different photoreceptor genes in tomato, especially in mutants that lack a working form of cryptochrome 1a: in those mutants the expression of some (IAA) or most (GA) photoreceptor genes is down regulated by these hormones.

CONCLUSIONS/SIGNIFICANCE

Our results highlight the presence of molecular relationships among cryptochrome 1a protein, hormones, and photoreceptors' gene expression in tomato, suggesting that manipulation of cryptochromes could represent a good strategy to understand in greater depth the role of phyto-hormones in the plant photoperceptive mechanism.

Shade avoidance

The presence of neighboring vegetation modifies the light environment experienced by plants, generating signals that are perceived by phytochromes and cryptochromes. These signals cause large changes in plant body form and function, including enhanced growth of the hypocotyl and petioles, a more erect position of the leaves and early flowering in Arabidopsis thaliana. Collectively, these so-called shade-avoidance responses tend to reduce the degree of current or future shade by neighbors. Shade light signals increase the abundance of PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and PIF5 proteins, promote the synthesis and redirection of auxin, favor the degradation of DELLA proteins and increase the expression of auxin, gibberellins and brassinosteroid-promoted genes, among other events downstream the photoreceptors. Selectively disrupting these events by genetic or pharmacological approaches affects shade-avoidance responses with an intensity that depends on the developmental context and the environment. Shade-avoidance responses provide a model to investigate the signaling networks used by plants to take advantage of the cues provided by the environment to adjust to the challenges imposed by the environment itself.

Physiological regulation and functional significance of shade avoidance responses to neighbors

Plants growing in dense vegetations compete with their neighbors for resources such as water, nutrients and light. The competition for light has been particularly well studied, both for its fitness consequences as well as the adaptive behaviors that plants display to win the battle for light interception. Aboveground, plants detect their competitors through photosensory cues, notably the red:far-red light ratio (R:FR). The R:FR is a very reliable indicator of future competition as it decreases in a plant-specific manner though red light absorption for photosynthesis and is sensed with the phytochrome photoreceptors. In addition, also blue light depletion is perceived for neighbor detection. As a response to these light signals plants display a suite of phenotypic traits defined as the shade avoidance syndrome (SAS). The SAS helps to position the photosynthesizing leaves in the higher zones of a canopy where light conditions are more favorable. In this review we will discuss the physiological control mechanisms through which the photosensory signals are transduced into the adaptive phenotypic responses that make up the SAS. Using this mechanistic knowledge as a starting point, we will discuss how the SAS functions in the context of the complex multi-facetted environments that plants usually grow in.

Molecular interactions between light and hormone signaling to control plant growth

As sessile organisms, plants modulate their growth rate and development according to the continuous variation in the conditions of their surrounding environment, an ability referred to as plasticity. This ability relies on a web of interactions between signaling pathways triggered by endogenous and environmental cues. How changes in environmental factors are interpreted by the plant in terms of developmental or growth cues or, in other words, how they contribute to plant plasticity is a current, major question in plant biology. Light stands out among the environmental factors that shape plant development. Plants have evolved systems that allow them to monitor both quantitative and qualitative differences in the light that they perceive, that render important changes in their growth habit. In this review we focus on recent findings about how information from this environmental cue is integrated during de-etiolation and in the shade-avoidance syndrome, and modulated by several hormone pathways-the endogenous cues. In some cases the interaction between a hormone and the light signaling pathways is reciprocal, as is the case of the gibberellin pathway, whereas in other cases hormone pathways act downstream of the environmental cue to regulate growth. Moreover, the circadian clock adds an additional layer of regulation, which has been proposed to integrate the information provided by light with that provided by hormone pathways, to regulate daily growth.

The hormonal regulation of de-etiolation

De-etiolation involves a number of phenotypic changes as the plants shift from a dark-grown (etiolated) to a light-grown (de-etiolated) morphology. Whilst these light-induced, morphological changes are thought to be mediated by plant hormones, the precise mechanism/s are not yet fully understood. Here we provide further direct evidence that gibberellins (GAs) may play an important role in de-etiolation, because a similar light-induced reduction in bioactive GA levels was detected in barley (Hordeum vulgare L.), Arabidopsis (Arabidopsis thaliana L.), and pea (Pisum sativum L.). This is indicative of a highly conserved, negative-regulatory role for GAs in de-etiolation, in a range of taxonomically diverse species. In contrast, we found no direct evidence of a reduction in brassinosteroid (BR) levels during de-etiolation in any of these species.

Sucrose transporter StSUT4 from potato affects flowering, tuberization, and shade avoidance response

Sucrose (Suc) transporters belong to a large gene family. The physiological role of SUT1 proteins has been intensively investigated in higher plants, whereas that of SUT4 proteins is so far unknown. All three known Suc transporters from potato (Solanum tuberosum), SUT1, SUT2, and SUT4, are colocalized and their RNA levels not only follow a diurnal rhythm, but also oscillate in constant light. Here, we examined the physiological effects of transgenic potato plants on RNA interference (RNAi)-inactivated StSUT4 expression. The phenotype of StSUT4-RNAi plants includes early flowering, higher tuber production, and reduced sensitivity toward light enriched in far-red wavelength (i.e. in canopy shade). Inhibition of StSUT4 led to tuber production of the strict photoperiodic potato subsp. andigena even under noninductive long-day conditions. Accumulation of soluble sugars and Suc efflux from leaves of transgenic plants are modified in StSUT4-RNAi plants, leading to modified Suc levels in sink organs. StSUT4 expression of wild-type plants is induced by gibberellins and ethephon, and external supply of gibberellic acid leads to even more pronounced differences between wild-type and StSUT4-RNAi plants regarding tuber yield and internode elongation, indicating a reciprocal regulation of StSUT4 and gibberellins.

Shade avoidance

The threat to plant survival presented by light limitation has driven the evolution of highly plastic adaptive strategies to either tolerate or avoid shading by neighbouring vegetation. When subject to vegetational shading, plants are exposed to a variety of informational signals, which include altered light quality and a reduction in light quantity. The former includes a decrease in the ratio of red to far-red wavelengths (low R : FR) and is detected by the phytochrome family of plant photoreceptors. Monitoring of R : FR ratio can provide an early and unambiguous warning of the presence of competing vegetation, thereby evoking escape responses before plants are actually shaded. The molecular mechanisms underlying physiological responses to alterations in light quality have now started to emerge, with major roles suggested for the PIF (PHYTOCHROME INTERACTING FACTOR) and DELLA families of transcriptional regulators. Such studies suggest a complex interplay between endogenous and exogenous signals, mediated by multiple photoreceptors. The phenotypic similarities between physiological responses habitually referred to as 'the shade avoidance syndrome' and other abiotic stress responses suggest plants may integrate common signalling mechanisms to respond to multiple perturbations in their natural environment.

A study of gibberellin homeostasis and cryptochrome-mediated blue light inhibition of hypocotyl elongation

Cryptochromes mediate blue light-dependent photomorphogenic responses, such as inhibition of hypocotyl elongation. To investigate the underlying mechanism, we analyzed a genetic suppressor, scc7-D (suppressors of cry1cry2), which suppressed the long-hypocotyl phenotype of the cry1cry2 (cryptochrome1/cryptochrome2) mutant in a light-dependent but wavelength-independent manner. scc7-D is a gain-of-expression allele of the GA2ox8 gene encoding a gibberellin (GA)-inactivating enzyme, GA 2-oxidase. Although scc7-D is hypersensitive to light, transgenic seedlings expressing GA2ox at a level higher than scc7-D showed a constitutive photomorphogenic phenotype, confirming a general role of GA2ox and GA in the suppression of hypocotyl elongation. Prompted by this result, we investigated blue light regulation of mRNA expression of the GA metabolic and catabolic genes. We demonstrated that cryptochromes are required for the blue light regulation of GA2ox1, GA20ox1, and GA3ox1 expression in transient induction, continuous illumination, and photoperiodic conditions. The kinetics of cryptochrome induction of GA2ox1 expression and cryptochrome suppression of GA20ox1 or GA3ox1 expression correlate with the cryptochrome-dependent transient reduction of GA(4) in etiolated wild-type seedlings exposed to blue light. Therefore we propose that in deetiolating seedlings, cryptochromes mediate blue light regulation of GA catabolic/metabolic genes, which affect GA levels and hypocotyl elongation. Surprisingly, no significant change in the GA(4) content was detected in the whole shoot samples of the wild-type or cry1cry2 seedlings grown in the dark or continuous blue light, suggesting that cryptochromes may also regulate GA responsiveness and/or trigger cell- or tissue-specific changes of the level of bioactive GAs.

DELLA protein function in growth responses to canopy signals

Plants can sense neighbour competitors through light-quality signals and respond with shade-avoidance responses. These include increased shoot elongation, which enhances light capture and thus competitive power. Such plant-plant interactions therefore profoundly affect plant development in crowded populations. Shade-avoidance responses are tightly coordinated by interactions between light signals and hormones, with essential roles for the phytochrome B photoreceptor [sensing the red:far red (R:FR) ratio] and the hormone gibberellin (GA). The family of growth-suppressing DELLA proteins are targets for GA signalling and are proposed to integrate signals from other hormones. However, the importance of these regulators has not been studied in the ecologically relevant, complex realm of plant canopies. Here we show that DELLA abundance is regulated during growth responses to neighbours in dense Arabidopsis stands. This occurs in a R:FR-dependent manner in petioles, depends on GA, and matches the induction kinetics of petiole elongation. Similar interactions were observed in the growth response of seedling hypocotyls and are general for a second canopy signal, reduced blue light. Enhanced DELLA stability in the gai mutant inhibits shade-avoidance responses, indicating that DELLA proteins constrain shade-avoidance. However, using multiple DELLA knockout mutants, we show that the observed DELLA breakdown is not sufficient to induce shade-avoidance in petioles, but plays a more central role in hypocotyls. These data provide novel information on the regulation of shade-avoidance under ecologically important conditions, defining the importance of DELLA proteins and GA and unravelling the existence of GA- and DELLA-independent mechanisms.

Reciprocal influence of ethylene and gibberellins on response-gene expression in Arabidopsis thaliana

The complexity of hormonal responses and their functional overlap support the presence of an intensive cross-talk between hormone signalling pathways. A detailed analysis of responses induced by ethylene and gibberellin (GA) in a GA-insensitive mutant (gai), an ethylene-resistant mutant (etr1-3), the gai etr1-3 double-mutant, and in wild-type Arabidopsis thaliana plants, revealed multiple interactions between ethylene and GA signal transduction pathways. Ethylene insensitive mutants and wild-type plants treated with 1-methylcyclopropene (1-MCP), an ethylene perception inhibitor, displayed a stronger responsiveness of genes differentially regulated by GA. In addition, microarray-analysis showed that the GA-response in an ethylene-insensitive background is different from that in the wild-type, confirming the importance of ethylene in a plant's response towards GA. In this paper, we present a number of genes with an altered response-pattern as a direct consequence of cross-talk between ethylene and GA.

Regulation of brassinosteroid responses by phytochrome B in rice

Plant growth and development are coordinately controlled by environmental signals and internal factors. Light signals, mediated by phytochromes, regulate photomorphogenesis by interacting with endogenous programmes that involve multiple phytohormones. Brassinosteroids (BRs) are a group of growth-promoting phytohormones with a crucial role in the light-dependent development of plants. However, the interaction between light-signalling pathways and BR signalling is not well understood. Here, we examined the responses of lamina joint inclination and coleoptile elongation to exogenous brassinolide (BL) under light or dark conditions. Both responses were more pronounced under darkness, implying that BR signalling is inhibited by light. To elucidate which phytochrome is involved in this interaction, we isolated rice phytochrome-deficient mutants (osphyA, osphyB and osphyC) from a T-DNA insertional population. Whereas the osphyA and osphyC knockout mutants did not differ from the wild-type plants in their BL responses, osphyB mutants were more sensitive. In addition, RT-PCR analysis revealed enhanced expression of BR-inducible genes and decreased transcript levels of BR-biosynthetic genes in osphyB plants. These results suggest that Phytochrome B acts as a negative regulator of BL-regulated growth and development processes in rice.

DELLAs contribute to plant photomorphogenesis

Plant morphogenesis is profoundly influenced by light (a phenomenon known as photomorphogenesis). For example, light inhibits seedling hypocotyl growth via activation of phytochromes and additional photoreceptors. Subsequently, information is transmitted through photoreceptor-linked signal transduction pathways and used (via previously unknown mechanisms) to control hypocotyl growth. Here we show that light inhibition of Arabidopsis (Arabidopsis thaliana) hypocotyl growth is in part dependent on the DELLAs (a family of nuclear growth-restraining proteins that mediate the effect of the phytohormone gibberellin [GA] on growth). We show that light inhibition of growth is reduced in DELLA-deficient mutant hypocotyls. We also show that light activation of phytochromes promotes the accumulation of DELLAs. A green fluorescent protein (GFP)-tagged DELLA (GFP-RGA) accumulates in elongating cells of light-grown, but not dark-grown, transgenic wild-type hypocotyls. Furthermore, transfer of seedlings from light to dark (or vice versa) results in rapid changes in hypocotyl GFP-RGA accumulation, changes that are paralleled by rapid alterations in the abundance in hypocotyls of transcripts encoding enzymes of GA metabolism. These observations suggest that light-dependent changes in hypocotyl GFP-RGA accumulation are a consequence of light-dependent changes in bioactive GA level. Finally, we show that GFP accumulation and quantitative modulation of hypocotyl growth is proportionate with light energy dose (the product of exposure duration and fluence rate). Hence, DELLAs inhibit hypocotyl growth during the light phase of the day-night cycle via a mechanism that is quantitatively responsive to natural light variability. We conclude that DELLAs are a major component of the adaptively significant mechanism via which light regulates plant growth during photomorphogenesis.

Diurnal regulation of the brassinosteroid-biosynthetic CPD gene in Arabidopsis

Plant steroid hormones, brassinosteroids (BRs), are essential for normal photomorphogenesis. However, the mechanism by which light controls physiological functions via BRs is not well understood. Using transgenic plants carrying promoter-luciferase reporter gene fusions, we show that in Arabidopsis (Arabidopsis thaliana) the BR-biosynthetic CPD and CYP85A2 genes are under diurnal regulation. The complex diurnal expression profile of CPD is determined by dual, light-dependent, and circadian control. The severely decreased expression level of CPD in phytochrome-deficient background and the red light-specific induction in wild-type plants suggest that light regulation of CPD is primarily mediated by phytochrome signaling. The diurnal rhythmicity of CPD expression is maintained in brassinosteroid insensitive 1 transgenic seedlings, indicating that its transcriptional control is independent of hormonal feedback regulation. Diurnal changes in the expression of CPD and CYP85A2 are accompanied by changes of the endogenous BR content during the day, leading to brassinolide accumulation at the middle of the light phase. We also show that CPD expression is repressed in extended darkness in a BR feedback-dependent manner. In the dark the level of the bioactive hormone did not increase; therefore, our data strongly suggest that light also influences the sensitivity of plants to BRs.

Diurnal regulation of plant growth

Life occurs in an ever-changing environment. Some of the most striking and predictable changes are the daily rhythms of light and temperature. To cope with these rhythmic changes, plants use an endogenous circadian clock to adjust their growth and physiology to anticipate daily environmental changes. Most studies of circadian functions in plants have been performed under continuous conditions. However, in the natural environment, diurnal outputs result from complex interactions of endogenous circadian rhythms and external cues. Accumulated studies using the hypocotyl as a model for plant growth have shown that both light signalling and circadian clock mutants have growth defects, suggesting strong interactions between hypocotyl elongation, light signalling and the circadian clock. Here, we review evidence suggesting that light, plant hormones and the circadian clock all interact to control diurnal patterns of plant growth.

Thermoperiodic stem elongation involves transcriptional regulation of gibberellin deactivation in pea

The physiological basis of thermoperiodic stem elongation is as yet poorly understood. Thermoperiodic control of gibberellin (GA) metabolism has been suggested as an underlying mechanism. We have investigated the influence of different day and night temperature combinations on GA levels, and diurnal steady-state expression of genes involved in GA biosynthesis (LS, LH, NA, PSGA20ox1, and PsGA3ox1) and GA deactivation (PsGA2ox1 and PsGA2ox2), and related this to diurnal stem elongation in pea (Pisum sativum L. cv Torsdag). The plants were grown under a 12-h light period with an average temperature of 17 degrees C. A day temperature/night temperature combination of 13 degrees C/21 degrees C reduced stem elongation after 12 d by 30% as compared to 21 degrees C/13 degrees C. This was correlated with a 55% reduction of GA1. Although plant height correlated with GA1 content, there was no correlation between diurnal growth rhythms and GA1 content. NA, PsGA20ox1, and PsGA2ox2 showed diurnal rhythms of expression. PsGA2ox2 was up-regulated in 13 degrees C/21 degrees C (compared to 21 degrees C/13 degrees C), at certain time points, by up to 19-fold. Relative to PsGA2ox2, the expression of LS, LH, NA, PSGA20ox1, PsGA3ox1, and PsGA2ox1 was not or only slightly affected by the different temperature treatments. The sln mutant having a nonfunctional PsGA2ox1 gene product showed the same relative stem elongation response to temperature as the wild type. This supports the importance of PsGA2ox2 in mediating thermoperiodic stem elongation responses in pea. We present evidence for an important role of GA catabolism in thermoperiodic effect on stem elongation and conclude that PsGA2ox2 is the main mediator of this effect in pea.

The involvement of gibberellin 20-oxidase genes in phytochrome-regulated petiole elongation of Arabidopsis

Long day (LD) exposure of rosette plants causes rapid stem/petiole elongation, a more vertical growth habit, and flowering; all changes are suggestive of a role for the gibberellin (GA) plant growth regulators. For Arabidopsis (Arabidopsis thaliana) L. (Heynh), we show that enhancement of petiole elongation by a far-red (FR)-rich LD is mimicked by a brief (10 min) end-of-day (EOD) FR exposure in short day (SD). The EOD response shows red (R)/FR photoreversibility and is not affected in a phytochrome (PHY) A mutant so it is mediated by PHYB and related PHYs. FR photoconversion of PHYB to an inactive form activates a signaling pathway, leading to increased GA biosynthesis. Of 10 GA biosynthetic genes, expression of the 20-oxidase, AtGA20ox2, responded most to FR (up to a 40-fold increase within 3 h). AtGA20ox1 also responded but to a lesser extent. Stimulation of petiole elongation by EOD FR is reduced in a transgenic AtGA20ox2 hairpin gene silencing line. By contrast, it was only in SD that a T-DNA insertional mutant of AtGA20ox1 (ga5-3) showed reduced response. Circadian entrainment to a daytime pattern provides an explanation for the SD expression of AtGA20ox1. Conversely, the strong EOD/LD FR responses of AtGA20ox2 may reflect its independence of circadian regulation. While FR acting via PHYB increases expression of AtGA20ox2, other GA biosynthetic genes are known to respond to R rather than FR light and/or to other PHYs. Thus, there must be different signal transduction pathways, one at least showing a positive response to active PHYB and another showing a negative response.

SPINDLY and GIGANTEA interact and act in Arabidopsis thaliana pathways involved in light responses, flowering, and rhythms in cotyledon movements

SPINDLY (SPY) is a negative regulator of gibberellin signaling in Arabidopsis thaliana that also functions in previously undefined pathways. The N terminus of SPY contains a protein-protein interaction domain consisting of 10 tetratricopeptide repeats (TPRs). GIGANTEA (GI) was recovered from a yeast two-hybrid screen for proteins that interact with the TPR domain. GI and SPY also interacted in Escherichia coli and in vitro pull-down assays. The phenotypes of spy and spy-4 gi-2 plants support the hypothesis that SPY functions with GI in pathways controlling flowering, circadian cotyledon movements, and hypocotyl elongation. GI acts in the long-day flowering pathway upstream of CONSTANS (CO) and FLOWERING LOCUS T (FT). Loss of GI function causes late flowering and reduces CO and FT RNA levels. Consistent with SPY functioning in the long-day flowering pathway upstream of CO, spy-4 partially suppressed the reduced abundance of CO and FT RNA and the late flowering of gi-2 plants. Like gi, spy affects the free-running period of cotyledon movements. The free-running period was lengthened in spy-4 mutants and shortened in plants that overexpress SPY under the control of the 35S promoter of Cauliflower mosaic virus. When grown under red light, gi-2 plants have a long hypocotyl. This hypocotyl phenotype was suppressed in spy-4 gi-2 double mutants. Additionally, dark-grown and far-red-light-grown spy-4 seedlings were found to have short and long hypocotyls, respectively. The different hypocotyl length phenotypes of spy-4 seedlings grown under different light conditions are consistent with SPY acting in the GA pathway to inhibit hypocotyl elongation and also acting as a light-regulated promoter of elongation.

Modulation of flowering responses in different Nicotiana varieties

We have identified and characterized a FLOWERING PROMOTING FACTOR 1 ( FPF1 ) gene from tobacco ( NtFPF1 ). Over-expression of NtFPF1 leads to early flowering in the day-neutral tobacco Nicotiana tabacum cv. Hicks, and under inductive photoperiods also in the short-day Nicotiana tabacum cv. Hicks Maryland Mammoth ( MM ) tobacco and the long-day plant Nicotiana sylvestris . N. sylvestris wild-type plants remained in the rosette stage and never flowered under non-inductive short-days, whereas 35S:: NtFPF1 transgenic plants bolted but did not flower. However, if treated with gibberellins, transgenic N. sylvestris plants flowered much faster under non-inductive short days than corresponding wild type plants, indicating an additive effect of gibberellins and the NtFPF1 protein in flowering time control. The day-neutral wild type cv. Hicks and the short-day cv. Hicks MM plants exhibit an initial rosette stage, both under short- and long-days. In the transgenic lines, this rosette stage was completely abolished. Wild-type plants of cv. Hicks MM never flowered under long days; however, all transgenic lines over-expressing NtFPF1 flowered under this otherwise non-inductive photoperiod.

Gibberellins repress photomorphogenesis in darkness

Plants undergo two different developmental programs depending on whether they are growing in darkness (skotomorphogenesis) or in the presence of light (photomorphogenesis). It has been proposed that the latter is the default pathway followed by many plants after germination and before the seedling emerges from soil. The transition between the two pathways is tightly regulated. The conserved COP1-based complex is central in the light-dependent repression of photomorphogenesis in darkness. Besides this control, hormones such as brassinosteroids (BRs), cytokinins, auxins, or ethylene also have been shown to regulate, to different extents, this developmental switch. In the present work, we show that the hormone gibberellin (GA) widely participates in this regulation. Studies from Arabidopsis show that both chemical and genetic reductions of endogenous GA levels partially derepress photomorphogenesis in darkness. This is based both on morphological phenotypes, such as hypocotyl elongation and hook and cotyledon opening, and on molecular phenotypes, such as misregulation of the light-controlled genes CAB2 and RbcS. Genetic studies indicate that the GA signaling elements GAI and RGA participate in these responses. Our results also suggest that GA regulation of this response partially depends on BRs. This regulation seems to be conserved across species because lowering endogenous GA levels in pea (Pisum sativum) induces full de-etiolation in darkness, which is not reverted by BR application. Our results, therefore, attribute an important role for GAs in the establishment of etiolated growth and in repression of photomorphogenesis.

Molecular mechanism of gibberellin signaling in plants

The hormone gibberellin (GA) plays an important role in modulating diverse processes throughout plant development. In recent years, significant progress has been made in the identification of upstream GA signaling components and trans- and cis-acting factors that regulate downstream GA-responsive genes in higher plants. GA appears to derepress its signaling pathway by inducing proteolysis of GA signaling repressors (the DELLA proteins). Recent evidence indicates that the DELLA proteins are targeted for degradation by an E3 ubiquitin ligase SCF complex through the ubiquitin-26S proteasome pathway.

The novel MYB protein EARLY-PHYTOCHROME-RESPONSIVE1 is a component of a slave circadian oscillator in Arabidopsis

Using fluorescent differential display, we identified, from approximately 8000 displayed bands, a DNA fragment showing rapid induction in response to red light irradiation. This EARLY-PHYTOCHROME-RESPONSIVE1 gene (EPR1) encodes a novel nucleus-localized MYB protein harboring a single MYB domain that is highly similar to the circadian oscillator proteins CCA1 and LHY. EPR1 is regulated by both phytochrome A and phytochrome B, and the red-light induction of EPR1 is not inhibited by cycloheximide, demonstrating that EPR1 represents a primary phytochrome-responsive gene. Our results show that EPR1 overexpression results in enhanced far-red light-induced cotyledon opening and delayed flowering. In wild-type Arabidopsis plants grown in continuous light, the EPR1 transcript exhibits circadian rhythmicity similar to that of CCA1 and LHY. Moreover, EPR1 suppresses its own expression, suggesting that this protein is part of a regulatory feedback loop. Constitutive expression of CCA1 and LHY results in the loss of EPR1 rhythmicity, whereas increased levels of EPR1 have no effect on the central oscillator. We propose that EPR1 is a component of a slave oscillator that contributes to the refinement of output pathways, ultimately mediating the correct oscillatory behavior of target genes.

Genomic and physiological studies of early cryptochrome 1 action demonstrate roles for auxin and gibberellin in the control of hypocotyl growth by blue light

Blue light inhibits elongation of etiolated Arabidopsis thaliana hypocotyls during the first 30 min of irradiation by a mechanism that depends on the phototropin 1 (phot1) photoreceptor. The cryptochrome 1 (cry1) photoreceptor begins to exert control after 30 min. To identify genes responsible for the cry1 phase of growth inhibition, mRNA expression profiles of cry1 and wild-type seedlings were compared using DNA microarrays. Of the roughly 420 genes found to be differentially expressed at the point of cry1 response incipience, approximately half were expressed higher and half lower in cry1 relative to the wild type. Many of the cry1-dependent genes encoded kinases, transcription factors, cell cycle regulators, cell wall metabolism enzymes, gibberellic acid (GA) biosynthesis enzymes, and auxin response factors. High-resolution growth studies supported the hypothesis that genes in the last two categories were indeed relevant to cry1-mediated growth control. Inhibiting GA4 biosynthesis with a 3beta-hydroxylase inhibitor (Ca-prohexadione) restored wild-type response kinetics in cry1 and completely suppressed its long-hypocotyl phenotype in blue light. Co-treatment of cry1 seedlings with Ca-prohexadione plus GA4 completely reversed the effects of the inhibitor, restoring the long-hypocotyl phenotype typical of the mutant. Treatment of wild-type seedlings with GA4 was not sufficient to phenocopy cry1 seedlings, but co-treatment with IAA plus GA4 produced cry1-like growth kinetics for a period of approximately 5 h. The genomic and physiological data together indicate that blue light acting through cry1 quickly affects the expression of many genes, a subset of which suppresses stem growth by repressing GA and auxin levels and/or sensitivity.

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.

Phytochrome-hormonal signalling networks

Through time, plants have evolved an extraordinary ability to interpret environmental cues. One of the most reliable of these cues is light, and plants are particularly adept at sensing and translating environmental light signals. The phytochrome family of photoreceptors monitor cues such as daylength or vegetative shade and adjust development to reflect change in these parameters. Indeed, it is their ability to coordinate these complex developmental changes that underpins the remarkable success of plants. Evidence is mounting that hormones control many of these light-mediated changes. Therefore, if we are to understand how light manipulates development we need to explore the interplay between light and hormonal signalling. Toward this goal, this review highlights the known convergence points of the phytochrome and the hormonal networks and explores their interactions. Contents Summary 449 I. Introduction 449 II. The phytochrome protein 450 III. Bacteriophytochromes 450 IV. IBacteriophytochrome signalling 450 V. Plant phytochrome signalling 451 VI. Ethylene perception and signalling 451 VII. Cytokinin perception and signalling 452 VIII. Brassinosteroid perception and signalling 453 IX. Gibberellin signalling 455 X. Auxin signalling 456 XI. Proteolysis in light and hormonal signalling 458 XII. Conclusion 459 Acknowledgements 459 References 459.

Growth and stomata development of Arabidopsis hypocotyls are controlled by gibberellins and modulated by ethylene and auxins

The plant hormones gibberellin (GA), ethylene and auxin can promote hypocotyl elongation of Arabidopsis seedlings grown in the light on a low nutrient medium (LNM). In this study, we used hypocotyl elongation as a system to investigate interactions between GA and ethylene or auxin and analysed their influence on the development of stomata in the hypocotyl. When applied together, GA and ethylene or auxin exerted a synergistic effect on hypocotyl elongation. Stimulated cell elongation is the main cause of hypocotyl elongation. Furthermore, hypocotyls treated with GA plus either ethylene or auxin show an increased endoreduplication. In addition, a small but significant increase in cell number was observed in the cortical cell files of hypocotyls treated with ethylene and GA together. However, studies with transgenic seedlings expressing CycB1::uidA genes revealed that cell division in the hypocotyl occurs only in the epidermis and mainly to form stomata, a process strictly regulated by hormones. Stomata formation in the hypocotyl is induced by the treatment with either GA or ethylene. The effect of GA could be strongly enhanced by the simultaneous addition of ethylene or auxin to the growth medium. Gibberellin is the main signal inducing stomata formation in the hypocotyl. In addition, this signal regulates hypocotyl elongation and is modulated by ethylene and auxin. The implication of these three hormones in relation to cell division and stomata formation is discussed.

Independent control of gibberellin biosynthesis and flowering time by the circadian clock in Arabidopsis

Flowering of the facultative long-day plant Arabidopsis is controlled by several endogenous and environmental factors, among them gibberellins (GAs) and day length. The promotion of flowering by long days involves an endogenous clock that interacts with light cues provided by the environment. Light, and specifically photoperiod, is also known to regulate the biosynthesis of GAs, but the effects of GAs and photoperiod on flowering are at least partially separable. Here, we have used a short-period mutant, toc1, to investigate the role of the circadian clock in the control of flowering time by GAs and photoperiod. We show that toc1 affects expression of several floral regulators and a GA biosynthetic gene, but that these effects are independent.

In vitro bulb development in shallot (Allium cepa L. Aggregatum Group): effects of anti-gibberellins, sucrose and light

Bulbing was studied in shallot plants cultured in vitro. Bulbing occurred under a 16 h photoperiod with fluorescent + incandescent light and 30-50 g 1(-1) sucrose in the culture medium. Exogenous gibberellin (10 microM GA3) inhibited leaf and root growth and bulbing. When added to the medium at a concentration of 10 microM, three inhibitors of gibberellin biosynthesis (ancymidol, flurprimidol and paclobutrazol) promoted bulb formation and the percentage of bulbing. When ancymidol was used in combination with GA3, it did not reverse the effect of GA3 applied alone. Under treatments with 30-70 g l(-1) sucrose, bulbing ratios greater than those found in control plants were achieved by addition of ancymidol, and bulb fresh weight was increased in the same way. Ancymidol caused a 66% decrease in sucrose content in leaf bases but greatly increased the glucose, fructose and fructan contents. The increase in fructan content by ancymidol could result from the three-fold rise in total [14C]sucrose uptake per plant from the culture medium associated with a marked increase in leaf base labelling at the expense of root labelling. The possible role of ancymidol is discussed and evidence supports a major regulatory role for gibberellins in bulbing.

Quantitative trait loci controlling light and hormone response in two accessions of Arabidopsis thaliana

We have mapped quantitative trait loci (QTL) responsible for natural variation in light and hormone response between the Cape Verde Islands (Cvi) and Landsberg erecta (Ler) accessions of Arabidopsis thaliana using recombinant inbred lines (RILs). Hypocotyl length was measured in four light environments: white, blue, red, and far-red light and in the dark. In addition, white light plus gibberellin (GA) and dark plus the brassinosteroid biosynthesis inhibitor brassinazole (BRZ) were used to detect hormone effects. Twelve QTL were identified that map to loci not previously known to affect light response, as well as loci where candidate genes have been identified from known mutations. Some QTL act in all environments while others show genotype-by-environment interaction. A global threshold was established to identify a significant epistatic interaction between two loci that have few main effects of their own. LIGHT1, a major QTL, has been confirmed in a near isogenic line (NIL) and maps to a new locus with effects in all light environments. The erecta mutation can explain the effect of the HYP2 QTL in the blue, BRZ, and dark environments, but not in far-red. LIGHT2, also confirmed in an NIL, has effects in white and red light and shows interaction with GA. The phenotype and map position of LIGHT2 suggest the photoreceptor PHYB as a candidate gene. Natural variation in light and hormone response thus defines both new genes and known genes that control light response in wild accessions.

Photocontrol of stem growth

Rapid and measurable growth rate changes that occur in seedling stems upon illumination serve as an excellent means to analyze signal transduction. Growth kinetic studies have shown how red, far-red and blue light signals are transduced via the solitary and/or coordinated action of known plant photoreceptors. These reports are consistent with current findings describing light-induced photoreceptor interaction and compartmentation.