Interaction of red to far red light ratio and ethylene in regulating stem elongation of <Emphasis Type="Italic">Helianthus annuus</Emphasis>

Sunflower (Helianthus annuus L.) stems showed increased elongation under two types of vegetative shade: canopy shade (low red to far red [R/FR] ratio) and neighbouring proximity shade (FR enrichment). Hypocotyls also elongated more under narrow-band FR light than under narrow-band R light. Ethylene levels were determined in actively elongating 7-day-old hypocotyls and 17-day-old internodes under three R/FR ratios. Ethylene levels were lower in both sunflower hypocotyls and internodes when the R/FR ratio was reduced. Both FR enrichment of normal R/FR ratio and narrow-band FR light with very low light irradiance resulted in reduction in ethylene levels in 7-day-old hypocotyls. Further, in application experiments, sunflower stems grown under low R/FR ratio were more sensitive to ethephon and less sensitive to aminoethoxyvinylglycine (AVG) than stems grown under high R/FR ratio. Low R/FR ratio appears to initiate reduction in ethylene levels in sunflower seedlings, allowing maximum stem elongation. These results, and findings of other authors, suggest that various plant species may have developed different ways of regulating stem elongation and ethylene levels in response to low R/FR ratio.     

Phytochrome A, phytochrome B and HY4 are involved in hypocotyl growth responses to natural radiation in Arabidopsis: weak de-etiolation of the phyA mutant under dense canopies

The roles of phytochrome A (phyA), phytochrome B (phyB) and a putative blue-light (BL) photoreceptor (HY4) in the control of hypocotyl growth by natural radiation were investigated using phyA, phyB and hy4 mutants of Arabidopsis thaliana. Full sunlight inhibited hypocotyl growth to a larger extent in wild-type (WT) than in phyA, phyB and, particularly, hy4 seedlings. In WT seedlings, hypocotyl growth was promoted by selectively lowering BL irradiance, lowering red-light (R) plus far-red-light (FR) irradiance or lowering the R/FR ratio (which was achieved either by increasing FR or by reducing R). The effects of lowering BL were reduced in hy4 and exaggerated in phyA seedlings. The effects of lowering R+FR were reduced in phyA and exaggerated in hy4 seedlings. Neither phyB nor hy4 mutants responded to low R/FR ratios. Neighbouring plants reflecting FR without shading caused subtle reductions of the R/FR ratio. This signal promoted hypocotyl growth in WT but not in phyA, phyB or hy4 seedlings. Intermediate canopy shade produced similar effects in all genotypes. Under deep shade, de-etiolation was severely impaired in phyA seedlings, which died prematurely. Thus, the FR ‘high-irradiance reaction’ mediated by phyA could be important for seedling survival under dense canopies.     

Auxin and Ethylene Regulate Elongation Responses to Neighbor Proximity Signals Independent of Gibberellin and DELLA Proteins in Arabidopsis

Plants modify growth in response to the proximity of neighbors. Among these growth adjustments are shade avoidance responses, such as enhanced elongation of stems and petioles, that help plants to reach the light and outgrow their competitors. Neighbor detection occurs through photoreceptor-mediated detection of light spectral changes (i.e. reduced red:far-red ratio [R:FR] and reduced blue light intensity). We recently showed that physiological regulation of these responses occurs through light-mediated degradation of nuclear, growth-inhibiting DELLA proteins, but this appeared to be only part of the full mechanism. Here, we present how two hormones, auxin and ethylene, coregulate DELLAs but regulate shade avoidance responses through DELLA-independent mechanisms in Arabidopsis (Arabidopsis thaliana). Auxin appears to be required for both seedling and mature plant shoot elongation responses to low blue light and low R:FR, respectively. Auxin action is increased upon exposure to low R:FR and low blue light, and auxin inhibition abolishes the elongation responses to these light cues. Ethylene action is increased during the mature plant response to low R:FR, and this growth response is abolished by ethylene insensitivity. However, ethylene is also a direct volatile neighbor detection signal that induces strong elongation in seedlings, possibly in an auxin-dependent manner. We propose that this novel ethylene and auxin control of shade avoidance interacts with DELLA abundance but also controls independent targets to regulate adaptive growth responses to surrounding vegetation.     

Temperature‐dependent shade avoidance involves the receptor‐like kinase ERECTA

Plants detect the presence of neighbouring vegetation by monitoring changes in the ratio of red (R) to far‐red (FR) wavelengths (R:FR) in ambient light. Reductions in R:FR are perceived by the phytochrome family of plant photoreceptors and initiate a suite of developmental responses termed the shade avoidance syndrome. These include increased elongation growth of stems and petioles, enabling plants to overtop competing vegetation. The majority of shade avoidance experiments are performed at standard laboratory growing temperatures (>20°C). In these conditions, elongation responses to low R:FR are often accompanied by reductions in leaf development and accumulation of plant biomass. Here we investigated shade avoidance responses at a cooler temperature (16°C). In these conditions, Arabidopsis thaliana displays considerable low R:FR‐mediated increases in leaf area, with reduced low R:FR‐mediated petiole elongation and leaf hyponasty responses. In Landsberg erecta, these strikingly different shade avoidance phenotypes are accompanied by increased leaf thickness, increased biomass and an altered metabolite profile. At 16°C, low R:FR treatment results in the accumulation of soluble sugars and metabolites associated with cold acclimation. Analyses of natural genetic variation in shade avoidance responses at 16°C have revealed a regulatory role for the receptor‐like kinase ERECTA.     

The Shade Avoidance Syndrome in Arabidopsis: The Antagonistic Role of Phytochrome A and B Differentiates Vegetation Proximity and Canopy Shade

Light limitation caused by dense vegetation is one of the greatest threats to plant survival in natural environments. Plants detect such neighboring vegetation as a reduction in the red to far-red ratio (R:FR) of the incoming light. The low R:FR signal, perceived by phytochromes, initiates a set of responses collectively known as the shade avoidance syndrome, intended to reduce the degree of current or future shade from neighbors by overtopping such competitors or inducing flowering to ensure seed production. At the seedling stage these responses include increased hypocotyl elongation. We have systematically analyzed the Arabidopsis seedling response and the contribution of phyA and phyB to perception of decreased R:FR, at three different levels of photosynthetically active radiation. Our results show that the shade avoidance syndrome, induced by phyB deactivation, is gradually antagonized by phyA, operating through the so-called FR-High Irradiance Response, in response to high FR levels in a range that simulates plant canopy shade. The data indicate that the R:FR signal distinguishes between the presence of proximal, but non-shading, neighbors and direct foliar shade, via a intrafamily photosensory attenuation mechanism that acts to suppress excessive reversion toward skotomorphogenic development under prolonged direct vegetation shade.     

Photomorphogenetic responses to UV radiation and short-term red light in lettuce seedlings

Effects of red light (R), far-red light (FR) and UV radiation on growth and greening of lettuce seedlings (Latuca sativa L., cv. Berlinskii) have been investigated. UV-B and UV-C inhibited hypocotyl elongation and stimulated cotyledonary growth. R in combination with UV-B and UV-C partly eliminated these effects, but FR increased those and reversed the R effect. Chlorophyll accumulation was inhibited by UV-B and UV-C. In comparison with cotyledonary growth, R strengthened the UV inhibitory effect, and FR reversed this effect of R. Thus, UV and phytochrome system modify the effects of each other on hypocotyl and leaf growth in lettuce seedlings depending on the level of active phytochrome formed.     

Coaction of blue/ultraviolet-A light and light absorbed by phytochrome in controlling growth of pine (Pinus sylestris L.) seedlings

Photomorphogenesis is a conspicuous feature in conifers. In the case of the shade-intolerant Scots pine (Pinus sylvestris L.), control of stem growth by light is well expressed at the seedling stage and can readily be studied. The present data show that hypocotyl growth is controlled by the far-red-absorbing form of phytochrome (Pfr). However, the Scots pine seedling requires blue or ultraviolet (UV-A) light to become fully responsive to Pfr. Blue/UV-A light has no direct effect on hypocotyl growth and its action appears to be limited to establishing the responsiveness of the seedling to Pfr. This type of coaction between phytochrome and blue/UV-A light has been observed previously in a number of angiosperm seedlings. With regard to the high irradiance reaction of phytochrome in long-term far-red light the pine seedling deviates totally from what has been observed in etiolated angiosperms since continuous far-red light has no effect on stem growth.Abbreviations B light of wavelength between 500 and 400 nm - FR standard far-red light - HIR high irradiance reaction of phytochrome - R high-fluence-rate red light (_R = 0.8) - RG9-light long-wavelength far-red light defined by the properties of the Schott RG9 glass filter (_RG₉<0.01) - = Pfr/Ptot wavelength-dependent photoequilibrium of the phytochrome system (far-red-absorbing form of phytochrome/total phytochrome) - UV-A near ultraviolet light of wavelength between 400 and 320 nm - W white light Research supported by a grant from the Deutsche Forschungsgemeinschaft (Schwerpunkt Physiologie der Bäume).     

Characterization of Shade Avoidance Responses in Lotus japonicus

Sessile plants must continuously adjust their growth and development to optimize photosynthetic activity under ever-fluctuating light conditions. Among such light responses in plants, one of the best-characterized events is the so-called shade avoidance, for which a low ratio of the red (R):far-red (FR) light intensities is the most prominent stimulus. Such shade avoidance responses enable plants to overtop their neighbors, thereby enhancing fitness and competitiveness in their natural habitat. Considerable progress has been achieved during the last decade in understanding the molecular mechanisms underlying the shade avoidance responses in the model rosette plant, Arabidopsis thaliana. We characterize here the fundamental aspects of the shade avoidance responses in the model legume, Lotus japonicus, based on the fact that its phyllotaxis (or morphological architecture) is quite different from that of A. thaliana. It was found that L. japonicus displays the characteristic shade avoidance syndrome (SAS) under defined laboratory conditions (a low R:FR ratio, low light intensity, and low blue light intensity) that mimic the natural canopy. In particular, the outgrowth of axillary buds (i.e., both aerial and cotyledonary shoot branching) was severely inhibited in L. japonicus grown in the shade. These results are discussed with special emphasis on the unique aspects of SAS observed with this legume.     

Light quantity and quality interactions in the control of elongation growth in light-grown Chenopodium rubrum L. seedlings

The spectral control of hypocotyl elongation in light-grown Chenopodium rubrum L. seedlings has been studied. The results showed that although the seedlings responded to changes in the quantity of combined red and far-red radiation, they were also very sensitive to changes in the quantity of blue radiation reaching the plant. Altering the proportion of red: far-red radiation in broad waveband white light caused marked differences in hypocotyl extension. Comparison of the responses of green and chlorophyll-free seedlings indicated no qualitative difference in the response to any of the light sources used, although photosynthetically incompetent plants were more sensitive to all wavelengths. Blue light was found to act primarily of a photoreceptor which is different from phytochrome. It is concluded that hypocotyl extension rate in vegetation shade is photoregulated by the quantity of blue light and the proportion of red: far-red radiation. In neutral shade, such as that caused by stones or overlying soil, hypocotyl extension appears to be regulated primarily by the quantity of light in the blue waveband and secondarily by the quantity of light in the red and far-red wavebands.Abbreviations B blue - FR far-red - k ₁, k ₂ rate constants for photoconverison of Pr to Pfr and Pfr to Pr, respective - k ₁/k ₁ +k ₂= phytochrome photoequilibrium - k ₁ +k ₂= phytochrome cycling rate - Pr=R absorbing form of phytochrome - Pfr=FR absorbing form of phytochrome - P_tot Pr+Pfr - PAR photosynthetically active radiation = 400–700 nm - R red - WL white light     

Action spectra for the inhibition of growth in radish hypocotyls

In etiolated seedlings of Raphanus sativus L. the inhibition of hypocotyl elongation by continuous light showed a major bimodal peak of action in the red and far-red, and two minor peaks in the blue regions of the spectrum. It is argued that, under conditions of prolonged irradiation, phytochrome is the pigment controlling the inhibition of hypocotyl elongation by red and far-red light, but that its mode of action in far-red is different from that in red. A distinct pigment is postulated for blue light.Abbreviations B blue - FR far red - G green - R red - HIR high irradiance reaction - P_r and P_fr red and far red absorbing forms of phytochrome - R red     

Interactions of temperature and light quality on phytohormone-mediated elongation of <Emphasis Type="Italic">Helianthus annuus</Emphasis> hypocotyls

Two important environmental signals, shade light, where the red/far-red (R/FR) light ratio is reduced, and elevated temperatures can each promote shoot growth. We examined their interactions using hypocotyl elongation of young sunflower (Helianthus annuus) seedlings, and we did this in the context of a possible hormonal mechanism for the growth increases that were induced by each environmental signal. Seedlings were subjected to combinations of six different temperatures (10, 15, 20, 25, 30 and 35°C) and four R/FR ratios (normal at 1.2 and reduced at 0.9, 0.6 and 0.3). Hypocotyl length was significantly increased by each of elevated temperature and FR enrichment. The magnitude of elongation induced by FR enrichment (low R/FR ratios) was dependent on temperature, with maximal effects of FR enrichment being seen at 20°C. Hypocotyl tissue concentrations of four endogenous gibberellins (GAs) and abscisic acid (ABA) were measured using the stable isotope dilution method. Hypocotyl ethylene evolution was also assessed. Thus, hypocotyl growth in both normal and shade light is highly dependent on temperature, with the most significant increases in FR-induced growth occurring at 20 and 25°C. A causal involvement of endogenous hormones, especially the GAs, in the growth that is induced by elevated temperatures, as well as in FR-induced growth, is strongly implied, with temperature being the stronger signal.     

Regulation by light of chlorophyll synthesis in the cotyledons of Scots pine (Pinus sylvestris) seedlings

Seedlings of gymnosperms, unlike angiosperms, synthesize chlorophyll(ide) (Chl) in darkness (D). In Scots pine cotyledons ( Pinus sylvestris L.) Chl accumulation ceases in D at a low level but Chl accumulation is strongly increased by light, red light (R) being more effective than blue light (B), whereas in Pinus maritima Chi synthesis is almost light-independent. In Scots pine the capacity to form Chl can be increased by R pulses, fully reversible by far-red light, demonstrating the involvement of phytochrome. However, when B- or R–grown seedlings were transferred to D, Chl accumulation stopped immediately irrespective of the level of P_fr (far-red light absorbing form of phytochrome), indicating that the conversion of protochlorophyllide (PChl) is light-dependent. Dose response curves in R and B and simultaneous irradiation with R and B show that R and B are perceived by separate photoreceptors. The immunodetected NADPH-dependent protochlorophyllide oxidoreductase (POR, EC 1.6.99.1), assumed to regulate light-dependent Chl synthesis in angiosperms, is not correlated with the capacity of gymnosperm Chi accumulation in darkness. While two FOR bands could be separated in extracts from dark grown material (38 and 36 kDa) of Pinus sylvestris and P. maritima , only the 38 kDa band disappeared consistently in the light. However. the significance of the more light resistant 36 kDa band for chlorophyll synthesis remains unclear as well.     

A genome-wide association study identifies variants underlying the Arabidopsis thaliana shade avoidance response

Shade avoidance is an ecologically and molecularly well-understood set of plant developmental responses that occur when the ratio of red to far-red light (R:FR) is reduced as a result of foliar shade. Here, a genome-wide association study (GWAS) in Arabidopsis thaliana was used to identify variants underlying one of these responses: increased hypocotyl elongation. Four hypocotyl phenotypes were included in the study, including height in high R:FR conditions (simulated sun), height in low R:FR conditions (simulated shade), and two different indices of the response of height to low R:FR. GWAS results showed that variation in these traits is controlled by many loci of small to moderate effect. A known PHYC variant contributing to hypocotyl height variation was identified and lists of significantly associated genes were enriched in a priori candidates, suggesting that this GWAS was capable of generating meaningful results. Using metadata such as expression data, GO terms, and other annotation, we were also able to identify variants in candidate de novo genes. Patterns of significance among our four phenotypes allowed us to categorize associations into three groups: those that affected hypocotyl height without influencing shade avoidance, those that affected shade avoidance in a height-dependent fashion, and those that exerted specific control over shade avoidance. This grouping allowed for the development of explicit hypotheses about the genetics underlying shade avoidance variation. Additionally, the response to shade did not exhibit any marked geographic distribution, suggesting that variation in low R:FR-induced hypocotyl elongation may represent a response to local conditions.     

Nicotiana plumbaginifolia hlg mutants have a mutation in a PHYB-type phytochrome gene: they have elongated hypocotyls in red light, but are not elongated as adult plants

Two new allelic mutants of Nicotiana plumbaginifolia have been isolated which display a hypocotyl which is long (hlg) when seedlings are grown in continuous white light (W). This can be accounted for by the decreased response to red light (R) of the hypocotyl elongation rate in these mutants. Responses to other wavelengths are unaffected in the mutants. When grown in white light, mature hlg mutants are not elongated with respect to the wild-type; they also bolt and flower later. The shade-avoidance responses to red/far red ratio (R:FR) are intact in these mutants. Both mutants are deficient in a phyB-like polypeptide that is immunodetectable in the wild-type; both have wild-type levels of a phyA-like polypeptide. These alleles are inherited in a partially dominant manner, and correspond to single-base missense mutations in a gene highly homologous to N. tabacum PHYB, which codes for a phytochrome B-type photoreceptor. One allele, hlg-1, has an introduced amino acid substitution; this may define a residue essential for phytochrome protein stability. The other allele, hlg-2, has a stop codon introduced C-terminal to the chromophore binding domain. As these phyB mutants are unaffected in shade-avoidance responses, but deficient in perception of R, it is concluded that the phyB absent in these mutants is responsible for R perception in the N. plumbaginifolia seedling, but is not a R:FR sensor in light-grown 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.     

Interaction of Brassinosteroids with Light Quality and Plant Hormones in Regulating Shoot Growth of Young Sunflower and <Emphasis Type="Italic">Arabidopsis</Emphasis> Seedlings

Sunflower hypocotyls elongate as light quality changes from the normal red to far-red (R/FR) ratio of sunlight to a lower R/FR ratio. This low R/FR ratio-induced elongation significantly increases endogenous concentrations of indole-3-acetic acid (IAA) and also of three gibberellins (GAs): GA₂₀, GA₁, and GA₈. Of these, it is likely GA₁ that drives low R/FR-induced growth. Brassinosteroids are also involved in shoot growth. Here we tested three R/FR ratios: high, normal, and low. Significant hypocotyl elongation occurred with this stepwise reduction in R/FR ratio, but endogenous castasterone concentrations in the hypocotyls remained unchanged. Brassinolide was also applied to the seedlings and significantly increased hypocotyl growth, though one that was uniform across all three R/FR ratios. Applied brassinolide increased hypocotyl elongation while significantly reducing (usually) levels of IAA, GA₂₀, and GA₈, but not that of GA₁, which remained constant. Given the above, we conclude that endogenous castasterone does not mediate the hypocotyl growth that is induced by enriching FR light, relative to R light. Similarly, we conclude that the hypocotyl growth that is induced by applied brassinolide does not result from an interaction of brassinolide with changes in light quality. The ability of applied brassinolide to influence IAA, GA₂₀, and GA₈ content, yet have no significant effect on GA₁, is hard to explain. One speculative hypothesis, though, could involve the brassinolide-induced reductions that occurred for endogenous IAA, given IAA’s known ability to differentially influence the expression levels of GA20ox, GA3ox, and GA2ox, key genes in GA biosynthesis.