Persistent effects of changes in phytochrome status on internode growth in light-grown mustard: Occurrence, kinetics and locus of perception

PHYTOCHROME-INTERACTING FACTOR 7 and RELATIVE OF EARLY FLOWERING 6 act in shade avoidance memory in Arabidopsis

Shade avoidance helps plants maximize their access to light for growth under crowding. It is unknown, however, whether a priming shade avoidance mechanism exists that allows plants to respond more effectively to successive shade conditions. Here, we show that the shade-intolerant plant Arabidopsis can remember a first experienced shade event and respond more efficiently to the next event on hypocotyl elongation. The transcriptional regulator PHYTOCHROME-INTERACTING FACTOR 7 (PIF7) and the histone H3K27-demethylase RELATIVE OF EARLY FLOWERING 6 (REF6) are identified as being required for this shade avoidance memory. RNA-sequencing analysis reveals that shade induction of shade-memory-related genes is impaired in the pif7 and ref6 mutants. Based on the analyses of enrichments of H3K27me3, REF6 and PIF7, we find that priming shade treatment induces PIF7 accumulation, which further recruits REF6 to demethylate H3K27me3 on the chromatin of certain shade-memory-related genes, leading to a state poised for their transcription. Upon a second shade treatment, enhanced shade-mediated inductions of these genes result in stronger hypocotyl growth responses. We conclude that the transcriptional memory mediated by epigenetic modification plays a key role in the ability of primed plants to remember previously experienced shade and acquire enhanced responses to recurring shade conditions.

Improving the Predictive Value of Phytochrome Photoequilibrium: Consideration of Spectral Distortion Within a Leaf

The ratio of active phytochrome (Pfr) to total phytochrome (Pr + Pfr), called phytochrome photo-equilibrium (PPE; also called phytochrome photostationary state, PSS) has been used to explain shade avoidance responses in both natural and controlled environments. PPE is commonly estimated using measurements of the spectral photon distribution (SPD) above the canopy and photoconversion coefficients. This approach has effectively predicted morphological responses when only red and far-red (FR) photon fluxes have varied, but controlled environment research often utilizes unique ratios of wavelengths so a more rigorous evaluation of the predictive ability of PPE on morphology is warranted. Estimations of PPE have rarely incorporated the optical effects of spectral distortion within a leaf caused by pigment absorbance and photon scattering. We studied stem elongation rate in the model plant cucumber under diverse spectral backgrounds over a range of one to 45% FR (total photon flux density, 400-750 nm, of 400 μmol m^-2 s^-1) and found that PPE was not predictive when blue and green varied. Preferential absorption of red and blue photons by chlorophyll results in an SPD that is relatively enriched in green and FR at the phytochrome molecule within a cell. This can be described by spectral distortion functions for specific layers of a leaf. Multiplying the photoconversion coefficients by these distortion functions yields photoconversion weighting factors that predict phytochrome conversion at the site of photon perception within leaf tissue. Incorporating spectral distortion improved the predictive value of PPE when phytochrome was assumed to be homogeneously distributed within the whole leaf. In a supporting study, the herbicide norflurazon was used to remove chlorophyll in seedlings. Using distortion functions unique to either green or white cotyledons, we came to the same conclusions as with whole plants in the longer-term study. Leaves of most species have similar spectral absorbance so this approach for predicting PPE should be broadly applicable. We provide a table of the photoconversion weighting factors. Our analysis indicates that the simple, intuitive ratio of FR (700-750 nm) to total photon flux (far-red fraction) is also a reliable predictor of morphological responses like stem length.

Location Matters: Canopy Light Responses over Spatial Scales

Plants use light as a signal to determine neighbour proximity in dense vegetation. Far-red (FR) light reflected from neighbour plants elicits an array of growth responses throughout the plant. Recently, various light quality-induced signals have been discovered that travel between organs and tissue layers. These signals share upstream and downstream components, but can have opposing effects on cell growth. The question is how plants can coordinate these spatial signals into various growth responses in remote tissues. This coordination allows plants to adapt to the environment, and understanding the underlying mechanisms could allow precision engineering of crops. To achieve this understanding, plant photobiology research will need to focus increasingly on spatial signalling at the whole-plant level.

Subtle variation in shade avoidance responses may have profound consequences for plant competitiveness

BACKGROUND AND AIMS

Although phenotypic plasticity has been shown to be beneficial for plant competitiveness for light, there is limited knowledge on how variation in these plastic responses plays a role in determining competitiveness.

METHODS

A combination of detailed plant experiments and functional-structural plant (FSP) modelling was used that captures the complex dynamic feedback between the changing plant phenotype and the within-canopy light environment in time and 3-D space. Leaf angle increase (hyponasty) and changes in petiole elongation rates in response to changes in the ratio between red and far-red light, two important shade avoidance responses in Arabidopsis thaliana growing in dense population stands, were chosen as a case study for plant plasticity. Measuring and implementing these responses into an FSP model allowed simulation of plant phenotype as an emergent property of the underlying growth and response mechanisms.

KEY RESULTS

Both the experimental and model results showed that substantial differences in competitiveness may arise between genotypes with only marginally different hyponasty or petiole elongation responses, due to the amplification of plant growth differences by small changes in plant phenotype. In addition, this study illustrated that strong competitive responses do not necessarily have to result in a tragedy of the commons; success in competition at the expense of community performance.

CONCLUSIONS

Together, these findings indicate that selection pressure could probably have played a role in fine-tuning the sensitive shade avoidance responses found in plants. The model approach presented here provides a novel tool to analyse further how natural selection could have acted on the evolution of plastic responses.

Characterization of photomorphogenic responses and signaling cascades controlled by phytochrome-A expressed in different tissues

The photoreceptor phytochrome A acts as a light-dependent molecular switch and regulates responses initiated by very low fluences of light (VLFR) and high fluences (HIR) of far-red light. PhyA is expressed ubiquitously, but how phyA signaling is orchestrated to regulate photomorphogenesis is poorly understood. To address this issue, we generated transgenic Arabidopsis thaliana phyA-201 mutant lines expressing the biologically active phyA-YFP photoreceptor in different tissues, and analyzed the expression of several reporter genes, including ProHY5:HY5-GFP and Pro35S:CFP-PIF1, and various FR-HIR-dependent physiological responses. We show that phyA action in one tissue is critical and sufficient to regulate flowering time and root growth; control of cotyledon and hypocotyl growth requires simultaneous phyA activity in different tissues; and changes detected in the expression of reporters are not restricted to phyA-containing cells. We conclude that FR-HIR-controlled morphogenesis in Arabidopsis is mediated partly by tissue-specific and partly by intercellular signaling initiated by phyA. Intercellular signaling is critical for many FR-HIR induced responses, yet it appears that phyA modulates the abundance and activity of key regulatory transcription factors in a tissue-autonomous fashion.

Spatiotemporal Phytochrome Signaling during Photomorphogenesis: From Physiology to Molecular Mechanisms and Back

Light exposure results in distinct responses in specific seedling tissues during photomorphogenesis. Light promotes growth of cotyledons and leaves, as well as development and elongation of roots, whereas light inhibits elongation of hypocotyls. For distinct plant responses such as shade avoidance, far-red light or shifts in spectral light quality similarly have disparate impacts on distinct plant tissues, resulting in elongation of stems or petioles and a reduction in growth of leaf blades for many species. The physiological bases of such tissue- and organ-specific light responses were initially studied using localized irradiation of specific tissues and organs, or irradiation of dissected plant parts. These historical approaches were used to identify spatial-specific pools of photoreceptors responsible for regulating local, i.e., tissue- or organ-specific, or distal, i.e., interorgan, plant responses. The red/far-red responsive phytochromes have been the most widely studied among photoreceptors in this regard. Whereas, the spatial localization of photoreceptors regulating many tissue- or organ-specific light responses were identified, the underlying signaling networks responsible for mediating the observed responses have not been well defined. Recent approaches used to investigate the molecular bases of spatiotemporal light responses include selective irradiation of plants harboring mutations in specific photoreceptors, tissue-specific expression of photoreceptors, primarily in photoreceptor mutant backgrounds, or tissue-specific biochemical ablation of photoreceptor accumulation. Progressive integration of such approaches for regulating the availability of localized pools of phytochromes with the use of transcriptomic or proteomic analyses for assessing the genes or proteins which these spatially discrete pools of phytochrome regulate is yielding emergent insight into the molecular bases of spatiotemporal phytochrome signaling pathways responsible for regulating spatiotemporal light responses of which we have been aware of at the physiological level for decades. Here, I discuss historical and emerging approaches to elucidating spatiotemporal signaling mediated by phytochromes during photomorphogenesis.

Plant Responses to Vegetation Proximity: A Whole Life Avoiding Shade

In high density of vegetation, plants detect neighbors by perceiving changes in light quality through phytochrome photoreceptors. Close vegetation proximity might result in competition for resources, such as light. To face this challenge, plants have evolved two alternative strategies: to either tolerate or avoid shade. Shade-avoiding species generally adapt their development by inducing hypocotyl, stem, and petiole elongation, apical dominance and flowering, and decreasing leaf expansion and yield, a set of responses collectively known as the shade avoidance syndrome (SAS). The SAS responses have been mostly studied at the seedling stage, centered on the increase of hypocotyl elongation. After compiling the main findings about SAS responses in seedlings, this review is focused on the response to shade at adult stages of development, such as petioles of adult leaves, and the little information available on the SAS responses in reproductive tissues. We discuss these responses based on the knowledge about the molecular mechanisms and components with a role in regulating the SAS response of the hypocotyls of Arabidopsis thaliana. The transcriptional networks involved in this process, as well as the communication among the tissues that perceive the shade and the ones that respond to this stimulus will also be briefly commented.

Gene expression changes triggered by end-of-day far-red light treatment on early developmental stages of Eustoma grandiflorum (Raf.) Shinn

To better understand the molecular mechanisms related to growth promotion in the early developmental stages of Eustoma grandiflorum (Raf.) Shinn. under end-of-day far-red light (EOD-FR) treatment, we analyzed the leaf transcriptome of treated (EOD) and untreated plants (Cont) by using RNA-seq technology. EOD-FR treatment for only about 2 weeks in regions with limited sunshine during winter resulted in significantly higher internode length between the 3rd and 4th nodes on the main stem in EOD than in Cont. Among the differentially expressed genes (DEGs) related to synthesis or transport of auxin, higher levels of YUCCA (CL6581) and PIN4 (CL6181) were noted after treatment in EOD than in Cont in the leaf. In addition, high expression levels of GA20ox (Unigene11862) related to gibberellin (GA) synthesis and transcription factor bHLH 135 (CL7761) were observed in the stem of EOD, 3 h after treatment. A vertical section of the stem showed that the pith length of cells at the 4th node was longer in EOD than in Cont. Collectively, these results suggested that EOD-FR treatment increased the expression of DEGs related to GA and auxin biosynthesis, bHLH transcription factor, and internodal cell elongation along the longitudinal axis of Eustoma plants.

Phytochrome signaling: time to tighten up the loose ends

Phytochromes are red and far-red light photoreceptors that play fundamental roles in controlling many aspects of plant growth and development in response to light. The past two decades have witnessed the mechanistic elucidation of the action mode of phytochromes, including their regulation by external and endogenous factors and how they exert their function as transcriptional regulators. More importantly, recent advances have substantially deepened our understanding on the integration of the phytochrome-mediated signal into other cellular and developmental processes, such as elongation of hypocotyls, shoot branching, circadian clock, and flowering time, which often involves complex intercellular and interorgan signaling. Based on these advances, this review illustrates a blueprint of our current understanding of phytochrome signaling and its crosstalk with other signaling pathways, and also points out still open questions that need to be addressed in the future.

Cotyledon-Generated Auxin Is Required for Shade-Induced Hypocotyl Growth in Brassica rapa

Plant architecture is optimized for the local light environment. In response to foliar shade or neighbor proximity (low red to far-red light), some plant species exhibit shade-avoiding phenotypes, including increased stem and hypocotyl growth, which increases the likelihood of outgrowing competitor plants. If shade persists, early flowering and the reallocation of growth resources to stem elongation ultimately affect the yield of harvestable tissues in crop species. Previous studies have shown that hypocotyl growth in low red to far-red shade is largely dependent on the photoreceptor phytochrome B and the phytohormone auxin. However, where shade is perceived in the plant and how auxin regulates growth spatially are less well understood. Using the oilseed and vegetable crop species Brassica rapa, we show that the perception of low red to far-red shade by the cotyledons triggers hypocotyl cell elongation and auxin target gene expression. Furthermore, we find that following shade perception, elevated auxin levels occur in a basipetal gradient away from the cotyledons and that this is coincident with a gradient of auxin target gene induction. These results show that cotyledon-generated auxin regulates hypocotyl elongation. In addition, we find in mature B. rapa plants that simulated shade does not affect seed oil composition but may affect seed yield. This suggests that in field settings where mutual shading between plants may occur, a balance between plant density and seed yield per plant needs to be achieved for maximum oil yield, while oil composition might remain constant.

Stem transcriptome reveals mechanisms to reduce the energetic cost of shade-avoidance responses in tomato

While the most conspicuous response to low red/far-red ratios (R:FR) of shade light perceived by phytochrome is the promotion of stem growth, additional, less obvious effects may be discovered by studying changes in the stem transcriptome. Here, we report rapid and reversible stem transcriptome responses to R:FR in tomato (Solanum lycopersicum). As expected, low R:FR promoted the expression of growth-related genes, including those involved in the metabolism of cell wall carbohydrates and in auxin responses. In addition, genes involved in flavonoid synthesis, isoprenoid metabolism, and photosynthesis (dark reactions) were overrepresented in clusters showing reduced expression in the stem of low R:FR-treated plants. Consistent with these responses, low R:FR decreased the levels of flavonoids (anthocyanin, quercetin, kaempferol) and selected isoprenoid derivatives (chlorophyll, carotenoids) in the stem and severely reduced the photosynthetic capacity of this organ. However, lignin contents were unaffected. Low R:FR reduced the stem levels of jasmonate, which is a known inducer of flavonoid synthesis. The rate of stem respiration was also reduced in low R:FR-treated plants, indicating that by downsizing the stem photosynthetic apparatus and the levels of photoprotective pigments under low R:FR, tomato plants reduce the energetic cost of shade-avoidance responses.

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.

Automated analysis of hypocotyl growth dynamics during shade avoidance in Arabidopsis

Plants that are adapted to environments where light is abundant are especially sensitive to competition for light from neighboring vegetation. As a result, these plants initiate a series of changes known as the shade avoidance syndrome, during which plants elongate their stems and petioles at the expense of leaf development. Although the developmental outcomes of exposure to prolonged shade are known, the signaling dynamics during the initial exposure of seedlings to shade is less well studied. Here, we report the development of a new software-based tool, called HyDE (Hypocotyl Determining Engine) to measure hypocotyl lengths of time-resolved image stacks of Arabidopsis wild-type and mutant seedlings. We show that Arabidopsis grows rapidly in response to the shade stimulus, with measurable growth after just 45 min shade exposure. Similar to other mustard species, this growth response occurs in multiple distinct phases, including two phases of rapid growth and one phase of slower growth. Using mutants affected in shade avoidance phenotypes, we demonstrate that most of this early growth requires new auxin biosynthesis via the indole-3-pyruvate pathway. When activity of this pathway is reduced, the first phase of elongation growth is absent, and this is correlated with reduced activity of auxin-regulated genes. Finally, we show that varying shade intensity and duration can affect the shape and magnitude of the growth response, indicating a broad range of the elongation response to shade.

Involvement of auxin and brassinosteroid in the regulation of petiole elongation under the shade

Plants grown under a canopy recognize changes in light quality and modify their growth patterns; this modification is known as shade avoidance syndrome. In leaves, leaf blade expansion is suppressed, whereas petiole elongation is promoted under the shade. However, the mechanisms that control these responses are largely unclear. Here, we demonstrate that both auxin and brassinosteroid (BR) are required for the normal leaf responses to shade in Arabidopsis (Arabidopsis thaliana). The microarray analysis of leaf blades and petioles treated with end-of-day far-red light (EODFR) revealed that almost half of the genes induced by the treatment in both parts were previously identified as auxin-responsive genes. Likewise, BR-responsive genes were overrepresented in the EODFR-induced genes. Hence, the auxin and BR responses were elevated by EODFR treatment in both leaf blades and petioles, although opposing growth responses were observed in these two parts. The analysis of the auxin-deficient doc1/big mutant and the BR-deficient rot3/cyp90c1 mutant further indicates that auxin and BR were equally required for the normal petiole elongation response to the shade stimulus. In addition, the spotlight irradiation experiment revealed that phytochrome in leaf blades but not that in petioles regulated petiole elongation, which was probably mediated through regulation of the auxin/BR responses in petioles. On the basis of these findings, we conclude that auxin and BR cooperatively promote petiole elongation in response to the shade stimulus under the control of phytochrome in the leaf blade.

Right place, right time: Spatiotemporal light regulation of plant growth and development

Recent advances in our understanding of the roles of photoreceptors in light-dependent regulation of plant growth and development have been rapid and significant. Developments have been reported for numerous plant photoreceptor signaling pathways, yet researchers have made the most progress in increasing our comprehension of the roles of phytochrome family members, as well as the intracellular roles of phytochromes and phytochrome-interacting proteins in light-dependent signaling. An understudied, but vitally important, area of phytochrome biology centers on the roles phytochromes play in intercellular and interorgan signaling at the molecular level that results in the coordination of growth responses between distinct tissues and organs. This frontier of research into the spatiotemporal roles of phytochromes, and more generally plant photoreceptors, which is only beginning to be investigated and understood at the molecular genetic level, has a rich history of physiological data.

Light signaling: back to space

Recent work has increased our understanding of the molecular and cellular mechanisms of the phytochrome family of photoreceptors in controlling plant photomorphogenesis. However, the importance of long-distance communication in controlling light responses has received relatively little attention and is poorly understood. In this article, by taking a closer look at old and new experiments that extend the analysis of light signaling beyond the limits of the plant cell, we offer to look at the field in a new light. Furthermore, we discuss how intercellular and inter-organ communication might integrate with the transcriptional networks controlling light-regulated responses in plants, a novel view that might help to re-assess the parameters by which we screen for photomorphogenic mutants in the future.

Phytochrome in cotyledons regulates the expression of genes in the hypocotyl through auxin-dependent and -independent pathways

To elucidate the mechanism of plant responses to shading, we identified three promoter/enhancer trap lines (M812, J53, J59) that exhibited reporter expression in the hypocotyl in response to the end-of-day far-red light treatment. Interestingly, we found auxin-responsive genes in the vicinities of the reporter insertion sites in M812 and J53. We examined the effects of auxin on the reporter expression in these lines together with a previously identified N35 line. The results indicated that the reporter expression was induced by exogenous auxin in N35 and J53. Furthermore, an auxin transport inhibitor inhibited the responses of these lines to the end-of-day far-red light treatment, suggesting the involvement of auxin in the responses of plants to shading. By contrast, neither auxin nor the transport inhibitor affected the response in M812 and J59. Interestingly, J59 responded to ABA. Hence, ABA might be involved in the response as well. Analysis of the photoreceptive sites for the responses revealed the cotyledons, not the hypocotyl, are the major photoreceptive sites both in the auxin-responsive and ABA-responsive lines. Hence, some signals appeared to be transmitted from the cotyledons to the hypocotyl.

Regulation of phytochrome B signaling by phytochrome A and FHY1 in Arabidopsis thaliana

Phytochrome A (phyA) and phytochrome B (phyB) share the control of many processes but little is known about mutual signaling regulation. Here, we report on the interactions between phyA and phyB in the control of the activity of an Lhcb1*2 gene fused to a reporter, hypocotyl growth and cotyledon unfolding in etiolated Arabidopsis thaliana. The very-low fluence responses (VLFR) induced by pulsed far-red light and the high-irradiance responses (HIR) observed under continuous far-red light were absent in the phyA and phyA phyB mutants, normal in the phyB mutant, and reduced in the fhy1 mutant that is defective in phyA signaling. VLFR were also impaired in Columbia compared to Landsberg erecta. The low-fluence responses (LFR) induced by red-light pulses and reversed by subsequent far-red light pulses were small in the wild type, absent in phyB and phyA phyB mutants but strong in the phyA and fhy1 mutants. This indicates a negative effect of phyA and FHY1 on phyB-mediated responses. However, a pre-treatment with continuous far-red light enhanced the LFR induced by a subsequent red-light pulse. This enhancement was absent in phyA, phyB, or phyA phyB and partial in fhy1. The levels of phyB were not affected by the phyA or fhy1 mutations or by far-red light pre-treatments. We conclude that phyA acting in the VLFR mode (i.e. under light pulses) is antagonistic to phyB signaling whereas phyA acting in the HIR mode (i.e. under continuous far-red light) operates synergistically with phyB signaling, and that both types of interaction require FHY1.

Light-grown plants of transgenic tobacco expressing an introduced oat phytochrome A gene under the control of a constitutive viral promoter exhibit persistent growth inhibition by far-red light

A comparison of the photoregulation of development has been made for etiolated and light-grown plants of wild-type (WT) tobacco (Nicotiana tabacun L.) and an isogenic transgenic line which expresses an introduced oat phytochrome gene (phyA) under the control of a constitutive viral promoter. Etiolated seedlings of both the WT and transgenic line showed irradiance-dependent inhibition of hypocotyl growth under continuous far-red (FR) light; transgenic seedlings showed a greater level of inhibition under a given fluence rate and this is considered to be the result of the heterologous phytochrome protein (PhyA) functioning in a compatible manner with the native etiolated phytochrome. Deetiolation of WT seedlings resulted in a loss of responsiveness to prolonged FR. Light-grown transgenic seedlings, however, continued to respond in an irradiance-dependent manner to prolonged FR and it is proposed that this is a specific function of the constitutive PhyA. Mature green plants of the WT and transgenic lines showed a qualitatively similar growth promotion to a brief end-of-day FR-treatment but this response was abolished in the transgenic plants under prolonged irradiation by this same FR source. Growth inhibition (McCormac et al. 1991, Planta 185, 162-170) and enhanced levels of nitrate-reductase activity under irradiance of low red:far-red ratio, as achieved by the FR-supplementation of white light, emphasised that the introduced PhyA was eliciting an aberrant mode of photoresponse compared with the normal phytochrome population of light-grown plants. Total levels of the oat-encoded phytochrome in the etiolated transgenic tobacco were shown to be influenced by the wavelength of continuous irradiation in a manner which was qualitatively similar to that seen for the native, etiolated tobacco phytochrome, and distinct from that seen in etiolated oat tissues. These results are discussed in terms of the proposal that the constitutive oat-PhyA pool in the transgenic plants leads to a persistence of a mode of response normally restricted to the situation in etiolated plants.

Far-Red Radiation Reflected from Adjacent Leaves: An Early Signal of Competition in Plant Canopies

When individual seedlings of Datura ferox and Sinapis alba were transferred to populations formed by plants of similar stature, they responded with an increase in the rate of stem elongation. The reaction was detected within 3 days after transplanting and occurred well before shading among neighbors became important. This rapid response, which may be crucial for success in the competition for light, was reduced or abolished when individual internodes were "blinded" to the far-red radiation scattered by the surrounding seedlings. These results show the operation of a localized, photomorphogenetic control of stem elongation that may play a central role in the plastic adjustment of plants during the early stages of canopy development.

The loci of perception for phytochrome control of internode growth in light-grown mustard: Promotion by low phytochrome photoequilibria in the internode is enhanced by blue light perceived by the leaves

Under continuous white light (WL), extension growth of the first internode in Sinapis alba L. was promoted by low red (R): far-red (FR) ratios reaching the stem and-or the leaves. Conversely, the growth promotion by end-of-day light treatments was only triggered by FR perceived by the leaves and cotyledons, while FR given to the growning internode alone was tatally ineffective. Continuous WL+FR given to the internode was also in-effective if the rest of the shoot remained in darkness. Both the background stem growth, and the growth promotion caused by either an end-of-day FR pulse or continuous WL+FR given to the internode, increased with increasing fluence rates of WL given to the rest of the shoot. The increase by WL of the growth-stimulatory effect of low phytochrome photoequilibria in the internode appears to be mediated by a specific blue-light-absorbing photoreceptor, as blue-deficient light from sodium-discharge lamps, or from filtered fluorescent tubes, promoted background stem growth similarly to WL but did not amplify the response to the R:FR ratio in the internode. Supplementing the blue-deficient light (94 μmol·m(-2)·s(-1)) with low fluence rates of blue (<9 μmol·m(-2)·s(-1)) restored the promotive effect of low R:FR reaching the internode.