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.     

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 through 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, which plants usually grow in.Key words: competition, shade avoidance, hormones, cell wall, adaptive plasticity, photoreceptor, light     

Interactions between ethylene and gibberellins in phytochrome-mediated shade avoidance responses in tobacco

Plants respond to proximate neighbors with a suite of responses that comprise the shade avoidance syndrome. These phytochrome-mediated responses include hyponasty (i.e. a more vertical orientation of leaves) and enhanced stem and petiole elongation. We showed recently that ethylene-insensitive tobacco (Nicotiana tabacum) plants (Tetr) have reduced responses to neighbors, showing an important role for this gaseous plant hormone in shade avoidance. Here, we investigate interactions between phytochrome signaling and ethylene action in shade avoidance responses. Furthermore, we investigate if ethylene acts in these responses through an interaction with the GA class of hormones. Low red to far-red light ratios (R:FR) enhanced ethylene production in wild-type tobacco, resulting in shade avoidance responses, whereas ethylene-insensitive plants showed reduced shade avoidance responses. Plants with inhibited GA production showed hardly any shade avoidance responses at all to either a low R:FR or increased ethylene concentrations. Furthermore, low R:FR enhanced the responsiveness of hyponasty and stem elongation in both wild-type and Tetr plants to applied GA(3), with the stem elongation process being more responsive to GA(3) in the wild type than in Tetr. We conclude that phytochrome-mediated shade avoidance responses involve ethylene action, at least partly by modulating GA action.     

Blue light-dependent interactions of CRY1 with GID1 and DELLA proteins regulate gibberellin signaling and photomorphogenesis in Arabidopsis

Cryptochromes are blue light photoreceptors that mediate various light responses in plants and mammals. In Arabidopsis (Arabidopsis thaliana), cryptochrome 1 (CRY1) mediates blue light-induced photomorphogenesis, which is characterized by reduced hypocotyl elongation and enhanced anthocyanin production, whereas gibberellin (GA) signaling mediated by the GA receptor GA-INSENSITIVE DWARF1 (GID1) and DELLA proteins promotes hypocotyl elongation and inhibits anthocyanin accumulation. Whether CRY1 control of photomorphogenesis involves regulation of GA signaling is largely unknown. Here, we show that CRY1 signaling involves the inhibition of GA signaling through repression of GA-induced degradation of DELLA proteins. CRY1 physically interacts with DELLA proteins in a blue light-dependent manner, leading to their dissociation from SLEEPY1 (SLY1) and the inhibition of their ubiquitination. Moreover, CRY1 interacts directly with GID1 in a blue light-dependent but GA-independent manner, leading to the inhibition of the interaction between GID1 with DELLA proteins. These findings suggest that CRY1 controls photomorphogenesis through inhibition of GA-induced degradation of DELLA proteins and GA signaling, which is mediated by CRY1 inhibition of the interactions of DELLA proteins with GID1 and SCF^SLY1, respectively.

Blue light-dependent interactions of CRY1 with GID1 and DELLA proteins inhibit gibberellin (GA)-induced degradation of DELLA proteins to regulate GA signaling and photomorphogenesis.     

The adaptive evolution of plasticity: phytochrome-mediated shade avoidance responses

Many plants display a characteristic suite of developmental"shade avoidance" responses, such as stem elongation and acceleratedreproduction, to the low ratio of red to far-red wavelengths(R:FR) reflected or transmitted from green vegetation. ThisR:FR cue of crowding and vegetation shade is perceived by thephytochrome family of photoreceptors. Phytochrome-mediated responsesprovide an ideal system for investigating the adaptive evolutionof phenotypic plasticity in natural environments. The molecularand developmental mechanisms underlying shade avoidance responsesare well studied, and testable ecological hypotheses exist fortheir adaptive significance. Experimental manipulation of phenotypesdemonstrates that shade avoidance responses may be adaptive,resulting in phenotypes with high relative fitness in the environmentsthat induce those phenotypes. The adaptive value of shade avoidancedepends upon the competitive environment, resource availability,and the reliability of the R:FR cue for predicting the selectiveenvironment experienced by an induced phenotype. Comparativestudies and a reciprocal transplant experiment with Impatienscapensis provide evidence of adaptive divergence in shade avoidanceresponses between woodland and clearing habitats, which mayresult from population differences in the frequency of selectionon shade avoidance traits, as well as differences in the reliabilityof the R:FR cue. Recent rapid progress in elucidating phytochromesignaling pathways in the genetic model Arabidopsis thalianaand other species now provides the opportunity for studyinghow selection on shade avoidance traits in natural environmentsacts upon the molecular mechanisms underlying natural phenotypicvariation.     

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.     

Proteolysis-independent downregulation of DELLA repression in Arabidopsis by the gibberellin receptor GIBBERELLIN INSENSITIVE DWARF1

This article presents evidence that DELLA repression of gibberellin (GA) signaling is relieved both by proteolysis-dependent and -independent pathways in Arabidopsis thaliana. DELLA proteins are negative regulators of GA responses, including seed germination, stem elongation, and fertility. GA stimulates GA responses by causing DELLA repressor degradation via the ubiquitin-proteasome pathway. DELLA degradation requires GA biosynthesis, three functionally redundant GA receptors GIBBERELLIN INSENSITIVE DWARF1 (GID1a, b, and c), and the SLEEPY1 (SLY1) F-box subunit of an SCF E3 ubiquitin ligase. The sly1 mutants accumulate more DELLA proteins but display less severe dwarf and germination phenotypes than the GA biosynthesis mutant ga1-3 or the gid1abc triple mutant. Interestingly, GID1 overexpression rescued the sly1 dwarf and infertility phenotypes without decreasing the accumulation of the DELLA protein REPRESSOR OF ga1-3. GID1 rescue of sly1 mutants was dependent on the level of GID1 protein, GA, and the presence of a functional DELLA motif. Since DELLA shows increasing interaction with GID1 with increasing GA levels, it appears that GA-bound GID1 can block DELLA repressor activity by direct protein-protein interaction with the DELLA domain. Thus, a SLY1-independent mechanism for GA signaling may function without DELLA degradation.     

Evolutionary conservation of plant gibberellin signalling pathway components

Background:  

Gibberellins (GA) are plant hormones that can regulate germination, elongation growth, and sex determination. They ubiquitously occur in seed plants. The discovery of gibberellin receptors, together with advances in understanding the function of key components of GA signalling in Arabidopsis and rice, reveal a fairly short GA signal transduction route. The pathway essentially consists of GID1 gibberellin receptors that interact with F-box proteins, which in turn regulate degradation of downstream DELLA proteins, suppressors of GA-controlled responses.     

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.     

Isolation and characterization of a <Emphasis Type="Italic">GAI/RGA</Emphasis>-like gene from <Emphasis Type="Italic">Gossypium hirsutum</Emphasis>

The aim of the investigation reported here was to assess the role of gibberellin in cotton fiber development. The results of experiments in which the gibberellin (GA) biosynthesis inhibitor paclobutrazol (PAC) was tested on in vitro cultured cotton ovules revealed that GA is critical in promoting cotton fiber development. Plant responses to GA are mediated by DELLA proteins. A cotton nucleotide with high sequence homology to Arabidopsis thaliana GAI (AtGAI) was identified from the GenBank database and analyzed with the BLAST program. The full-length cDNA was cloned from upland cotton (Gossypium hirsutum, Gh) and sequenced. A comparison of the putative protein sequence of this cDNA with all Arabidopsis DELLA proteins indicated that GhRGL is a putative ortholog of AtRGL. Over-expression of this cDNA in Arabidopsis plants resulted in the dwarfed phenotype, and the degrees of dwarfism were related to the expression levels of GhRGL. The deletion of 17 amino acids, including the DELLA domain, resulted in the dominant dwarf phenotype, demonstrating that GhRGL is a functional protein that affects plant growth. Real-time quantitative PCR results showed that GhRGL mRNA is highly expressed in the cotton ovule at the elongation stage, suggesting that GhRGL may play a regulatory role in cotton fiber elongation.     

Gibberellin regulates Arabidopsis floral development via suppression of DELLA protein function

The phytohormone gibberellin (GA) regulates the development and fertility of Arabidopsis flowers. The mature flowers of GA-deficient mutant plants typically exhibit reduced elongation growth of petals and stamens. In addition, GA-deficiency blocks anther development, resulting in male sterility. Previous analyses have shown that GA promotes the elongation of plant organs by opposing the function of the DELLA proteins, a family of nuclear growth repressors. However, it was not clear that the DELLA proteins are involved in the GA-regulation of stamen and anther development. We show that GA regulates cell elongation rather than cell division during Arabidopsis stamen filament elongation. In addition, GA regulates the cellular developmental pathway of anthers leading from microspore to mature pollen grain. Genetic analysis shows that the Arabidopsis DELLA proteins RGA and RGL2 jointly repress petal, stamen and anther development in GA-deficient plants, and that this function is enhanced by RGL1 activity. GA thus promotes Arabidopsis petal, stamen and anther development by opposing the function of the DELLA proteins RGA, RGL1 and RGL2.     

Early developmental responses to seedling environment modulate later plasticity to light spectral quality

Correlations between developmentally plastic traits may constrain the joint evolution of traits. In plants, both seedling de-etiolation and shade avoidance elongation responses to crowding and foliage shade are mediated by partially overlapping developmental pathways, suggesting the possibility of pleiotropic constraints. To test for such constraints, we exposed inbred lines of Impatiens capensis to factorial combinations of leaf litter (which affects de-etiolation) and simulated foliage shade (which affects phytochrome-mediated shade avoidance). Increased elongation of hypocotyls caused by leaf litter phenotypically enhanced subsequent elongation of the first internode in response to low red:far red (R:FR). Trait expression was correlated across litter and shade conditions, suggesting that phenotypic effects of early plasticity on later plasticity may affect variation in elongation traits available to selection in different light environments.     

Population differentiation for plasticity to light in an annual herb: Adaptation and cost

Phenotypic plasticity allows plants to cope with environmental heterogeneity. Environmental variation among populations may select for differentiation in plasticity. To test this idea, we used the annual plant Geranium carolinianum, which inhabits old fields that are densely vegetated and lack canopy cover and wood margins with tree shade but less neighbor shade. Individuals from three populations of each habitat were planted in natural low and high light environments, and morphological traits important for light acquisition were measured. Old-field plants were more plastic, with greater elongation of petioles and internodes in low light than those from wood margins. This larger shade avoidance response suggests evolution of greater plasticity to neighbor shade than to the tree canopy. Fitness of old-field plants was high across both light environments, whereas fitness of wood-margin plants was reduced in low light. Selection favored longer internodes in low than high light. Finally, plasticity for internode length was negatively associated with fitness in high light, suggesting a cost of plasticity for this trait. Together these results indicate that shade-avoidance plasticity of petiole and internode length is adaptive. However, greater elongation of internode length may be constrained by the cost of plasticity expressed in high light. The evolution of plasticity appears to reflect a balance between its adaptive nature and its cost to fitness.     

Testing adaptive plasticity to UV: costs and benefits of stem elongation and light-induced phenolics

On exposure to ultraviolet radiation (UV), many plant species both reduce stem elongation and increase production of phenolic compounds that absorb in the UV region of the spectrum. To demonstrate that such developmental plasticity to UV is adaptive, it is necessary to show that the induced phenotype is both beneficial in inductive environments and maladaptive in non-inductive environments. We measured selection on stem elongation and phenolic content of seedlings of Impatiens capensis transplanted into ambient-UV and UV-removal treatments. We extended the range of phenotypes expressed, and thus the opportunity for selection in each UV treatment, by pretreating seedlings with either a low ratio of red:far-red wavelengths (R:FR), which induced stem elongation and reduced phenolic concentrations, or high R:FR, which had the opposite effect on these two phenotypic traits. Reduced stem length relative to biomass was advantageous for elongated plants under ambient UV, whereas increased elongation was favored in the UV-removal treatment. Selection favored an increase in the level of phenolics induced by UV in the ambient-UV treatment, but a decrease in phenolics in the absence of UV. These results are consistent with the hypotheses that reduced elongation and increased phenolic concentrations serve a UV-protective function and provide the first explicit demonstration in a wild species that plasticity of these traits to UV is adaptive. The observed cost to phenolics in the absence of UV may explain why many species plastically upregulate phenolic production when exposed to UV, rather than evolve constitutively high levels of these compounds. Finally, pretreatment with low R:FR simulating foliar shade did not exacerbate the fitness impact of UV exposure when plants had several weeks to acclimate to UV. This observation suggests that the evolution of adaptive shade avoidance responses to low R:FR in crowded stands will not be constrained by increased sensitivity to UV in elongated plants when they overtop their neighbors.     

Gibberellin-mediated proteasome-dependent degradation of the barley DELLA protein SLN1 repressor

DELLA proteins are nuclear repressors of plant gibberellin (GA) responses. Here, we investigate the properties of SLN1, a DELLA protein from barley that is destabilized by GA treatment. Using specific inhibitors of proteasome function, we show that proteasome-mediated protein degradation is necessary for GA-mediated destabilization of SLN1. We also show that GA responses, such as the aleurone alpha-amylase response and seedling leaf extension growth, require proteasome-dependent GA-mediated SLN1 destabilization. In further experiments with protein kinase and protein phosphatase inhibitors, we identify two additional signaling steps that are necessary for GA response and for GA-mediated destabilization of SLN1. Thus, GA signaling involves protein phosphorylation and dephosphorylation steps and promotes the derepression of GA responses via proteasome-dependent destabilization of DELLA repressors.     

Plasticity versus canalization: population differences in the timing of shade-avoidance responses

The reliability of environmental cues and costs of a fixed phenotype are two factors determining whether selection favors phenotypic plasticity or environmental specialization. This study examines the relationship between these two factors and the evolution of plant competitive strategies (plastic vs. fixed morphologies). In natural plant populations, shifts in light quality associated with foliar shade reliably indicate the presence of neighbors. These cues mediate plastic stem-elongation responses that often increase competitive ability and access to light. Using experimental light treatments (full sun, neutral shade, and foliar shade), genetic differences among populations of Abutilon theophrasti (velvetleaf) in average elongation and plasticity to foliar-shade cues were examined. Six populations, two from each of three site types (fields in continuous corn cultivation, fields undergoing corn-soy rotation, and weedy sites), were exposed to the light treatments at two stages in their life history. At the seedling stage, populations derived from cornfield sites exhibited higher, average elongation than populations from either rotating corn-soy fields or weedy areas. Because seedling elongation may delay shading of velvetleaf by corn, population differences may reflect adaptive responses to directional selection imposed by competitive conditions. However, the effects of simulated foliar shade on elongation were three times as great as the average population differences, and these comparatively higher levels of elongation were associated with an allocation cost. These results are consistent with the hypothesis that phenotypic plasticity may limit the evolution of specialists; reliable environmental cues enable individuals to facultatively adopt highly elongated, costly phenotypes in crowded patches while avoiding the costs of that phenotype in less crowded microsites. At later life-history stages, populations experiencing competition with corn exhibited lower plasticity to light quality than populations derived from weedy areas. Elongation at later nodes is maladaptive in cornfields because velvetleaf is ultimately incapable of overtopping corn; individuals that elongate therefore experience the cost of allocating to stems but fail to improve leaf exposure. The decreased responsiveness of cornfield populations to light quality is consistent with theoretical predictions in which reduced plasticity is favored when environmental cues fail to mediate an adaptive response.     

Evidence that the Arabidopsis nuclear gibberellin signalling protein GAI is not destabilised by gibberellin

Plant growth is regulated by bioactive gibberellin (GA), although there is an unexplained diversity in the magnitude of the GA responses exhibited by different plant species. GA acts via a group of orthologous proteins known as the DELLA proteins. The Arabidopsis genome contains genes encoding five different DELLA proteins, the best known of which are GAI and RGA. The DELLA proteins are thought to act as repressors of GA-regulated processes, whilst GA is thought to act as a negative regulator of DELLA protein function. Recent experiments have shown that GA induces rapid disappearance of nuclear RGA, SLR1 and SLN1 (DELLA proteins from rice and barley), suggesting that GA signalling and degradation of DELLA proteins are coupled. However, RGL1, another Arabidopsis DELLA protein, does not disappear from the nucleus in response to GA treatment. Here, we present evidence suggesting that GAI, like RGL1, is stable in response to GA treatment, and show that transgenic Arabidopsis plants containing constructs that enable high-level expression of GAI exhibit a dwarf, GA non-responsive phenotype. Thus, GAI appears to be less affected by GA than RGA, SLR1 or SLN1. We also show that neither of the two putative nuclear localisation signals contained in DELLA proteins are individually necessary for nuclear localisation of GAI. The various DELLA proteins have different properties, and we suggest that this functional diversity may explain, at least in part, why plant species differ widely in their GA response magnitudes.     

Illuminated behaviour: phytochrome as a key regulator of light foraging and plant anti-herbivore defence

In many ecological scenarios, the success of an individual plant is defined by the behavioural decisions that it makes when confronted with the risks of competition with other plants, and biomass losses to insect herbivores. These decisions involve expression of shade avoidance responses and induced chemical defences. Because these responses are costly, they frequently engender resource allocation dilemmas. In this review, I discuss the mechanisms that trigger adaptive responses to competitors and herbivores, highlighting the role of phytochromes as central organizers of the overall resource allocation strategy of plants. Phytochromes sense the reduction in the red to far-red (R : FR) ratio of sunlight caused by the proximity of other plants. Shade-intolerant plants respond to low R : FR ratios with shade avoidance behaviours and reduced investment in defence. Pfr depletion leads to increased stability of growth-promoting phytochrome-interacting factors (PIFs), and results in the production of auxins and gibberellins, degradation of DELLA proteins, which are repressors of PIFs, and reduced sensitivity to jasmonates. Thus, phytochrome appears to fulfil its organizational role by regulating the relative strength of the signalling circuits controlled by growth-related and defence-related hormones. I point out cases of signalling redundancy and discuss the significance of recent work on hormone signalling for our understanding of the mechanisms that control adaptive plant behaviour.     

The suppressive function of the rice DELLA protein SLR1 is dependent on its transcriptional activation activity

When the gibberellin (GA) receptor GIBBERELLIN INSENSITIVE DWARF 1 (GID1) binds to GA, GID1 interacts with DELLA proteins, repressors of GA signaling. This interaction inhibits the suppressive function of DELLA protein and thereby activates the GA response. However, how DELLA proteins exert their suppressive function and how GID1s inhibit suppressive function of DELLA proteins is unclear. By yeast one-hybrid experiments and transient expression of the N-terminal region of rice DELLA protein (SLR1) in rice callus, we established that the N-terminal DELLA/TVHYNP motif of SLR1 possesses transactivation activity. When SLR1 proteins with various deletions were over-expressed in rice, the severity of dwarfism correlated with the transactivation activity observed in yeast, indicating that SLR1 suppresses plant growth through transactivation activity. This activity was suppressed by the GA-dependent GID1-SLR1 interaction, which may explain why GA responses are induced in the presence of GA. The C-terminal GRAS domain of SLR1 also exhibits a suppressive function on plant growth, possibly by directly or indirectly interacting with the promoter region of target genes. Our results indicate that the N-terminal region of SLR1 has two roles in GA signaling: interaction with GID1 and transactivation activity.