Shade Avoidance Research Articles

Genome-wide survey of B-box proteins in potato (Solanum tuberosum)—Identification, characterization and expression patterns during diurnal cycle, etiolation and de-etiolation

Plant B-box domain proteins (BBX) mediate many light-influenced developmental processes including seedling photomorphogenesis, seed germination, shade avoidance and photoperiodic regulation of flowering. Despite the wide range of potential functions, the current knowledge regarding BBX proteins in major crop plants is scarce. In this study, we identify and characterize the StBBX gene family in potato, which is composed of 30 members, with regard to structural properties and expression profiles under diurnal cycle, etiolation and de-etiolations. Based on domain organization and phylogenetic relationships, StBBX genes have been classified into five groups. Using real-time quantitative PCR, we found that expression of most of them oscillates following a 24-h rhythm; however, large differences in expression profiles were observed between the genes regarding amplitude and position of the maximal and minimal expression levels in the day/night cycle. On the basis of the time-of-day/time-of-night, we distinguished three expression groups specifically expressed during the light and two during the dark phase. In addition, we showed that the expression of several StBBX genes is under the control of the circadian clock and that some others are specifically associated with the etiolation and de-etiolation conditions. Thus, we concluded that StBBX proteins are likely key players involved in the complex diurnal and circadian networks regulating plant development as a function of light conditions and day duration.

Differential response of Scots pine seedlings to variable intensity and ratio of red and far‐red light

We investigated the response to increasing intensity of red (R) and far-R (FR) light and to a decrease in R:FR ratio in Pinus sylvestris L. (Scots pine) seedling. The results showed that FR high-irradiance response for hypocotyl elongation may be present in Scots pine and that this response is enhanced by increasing light intensity. However, both hypocotyl inhibition and pigment accumulation were more strongly affected by the R light compared with FR light. This is in contrast to previous reports in Arabidopsis thaliana (L.) Heynh. In the angiosperm, A. thaliana R light shows an overall milder effect on inhibition of hypocotyl elongation and on pigment biosynthesis compared with FR suggesting conifers and angiosperms respond very differently to the different light regimes. Scots pine shade avoidance syndrome with longer hypocotyls, shorter cotyledons and lower chlorophyll content in response to shade conditions resembles the response observed in A. thaliana. However, anthocyanin accumulation increased with shade in Scots pine, which again differs from what is known in angiosperms. Overall, the response of seedling development and physiology to R and FR light in Scots pine indicates that the regulatory mechanism for light response may differ between gymnosperms and angiosperms.

Identification of a Novel Gene in the Shade Avoidance Response in Plants

Plants rely on light as their source of food and elicit a series of coordinated responses known as the shade avoidance syndrome (SAS) when faced with light challenge. These molecular and developmental responses include elongation of petioles, accelerated flowering, and decreased germination. Here we examine seven candidate genes in the model plant Arabidopsis thaliana for their role in the SAS. We conducted qPCR in mutant lines lacking each of these genes to observe any alteration in expression of pathways including light perception, transcriptional regulation, plant growth, and immune response. Preliminary results indicate that At1g677960, a gene of previously unknown function, may play a role in the SAS. Loss of this gene leads to impaired induction of shade avoidance genes and plants fail to show increased petiole length under shade conditions. Mutants are still able to induce early flowering, suggesting that this novel gene is required in the shade avoidance response but may act downstream or in parallel to the primary response. Further investigation of the role of At1g677960 will allow us to better understand the molecular mechanics of shade avoidance, and potentially offer insight into improving the success of vital crop species.Support or Funding InformationResearch reported here was supported by an Institutional Development Award (IDeA) from the National Institute of General Medical Sciences of the National Institutes of Health under grant number P20GM103449. Its contents are solely the responsibility of the authors and do not necessarily represent the official views of NIGMS or NIH.

Modification of CUL3 by Related to Ubiquitin (RUB) is partially regulated by the Phytochrome B pathway in Arabidopsis thaliana.

Plant growth and development depends on detecting the different wavelengths in the electromagnetic spectrum in order to perceive light conditions and grow accordingly. Phytochrome B, a red and far‐red light photoreceptor in plants, plays an integral role in shade avoidance, flowering time, seed germination, and de‐etiolation. Proper degradation of this photoreceptor and the immediate downstream signaling components, the Phytochrome Interacting Factors (PIFs), is crucial to the function of these processes and occurs via the ubiquitin‐proteasome system. The Light Regulating Bric‐a‐Brac/Tramtrack/Broad Complex proteins (LRB1 and LRB2) interact with CUL3 to form an E3 ligase complex which facilitates the attachment of ubiquitin to phytochrome B and PIF proteins. In plants, cullin‐based E3 ligase activity is dependent on the attachment of RUB (the plant ortholog of NEDD8 in human and other species) to cullin. We found that the LRB proteins contain two highly conserved regions near the N‐terminus which are similar to regions on CUL1/2 and RBX1, and these regions may play a role in the Rubylation process. Further investigation determined that the levels of Rub‐modified CUL3 are modulated by red light, and the PhyB signaling pathway plays a role in this process. To determine if the N‐terminal region of the LRBs play a role in rubylation of CUL3, a truncated version of the LRB gene missing this highly conserved region was inserted into wild type and a lrb1 lrb2 double mutant in Arabidopsis thaliana. Phenotypic, genetic, and immunoblot analysis of homozygous lines featuring the truncated LRBs suggest that the N‐terminal region may affect the ability of the LRBs to regulate red light responses. Future analysis of this region will provide insight into its role in phytochrome degradation and rubylation of CUL3.Support or Funding InformationThis research is funded by an S3 Summer Scholars Grant through the Office of Undergraduate Research at Grand Valley State University

Repertoire of traits in the sapling of a dwarf Mediterranean shrub confers withstanding the combined stress of drought and shade

ABSTRACTThe ability to cope with combinations of stresses is crucial for the establishment and development of young plants and determines processes of population dynamics. Seedlings and saplings of Mediterranean woody species in woody ecosystems often experience both drought and shade stresses. Our study aims at understanding the effect of the combined stresses of drought and shade on young sapling survival and development. We tested three alternative hypotheses: (1) Drought decreases plant performance in shade. (2) The impact of drought is lessened in shade or shade may even ameliorate plant performance under drought. (3) Plant response to shade is independent of plant response to water availability. The first and second hypotheses imply an interaction between drought and shade, while the third implies an orthogonal impact of drought and shade. We also ask what are the functional morphological traits that are involved in tolerating shade and drought stresses. We tested the hypotheses and questions focusing on Sarcopoterium spinosum as a model species in a fully factorial container experiment with three shading levels and two levels of water availability. The results demonstrate that moderate shading (50%) ameliorates seedling performance (measured by final biomass) under dry conditions. In contrast, when water was available, shading had a monotonic negative effect on seedling performance. Conversely, shade and drought had an independent effect on biomass partitioning and under drought – allocation to roots increased, while under shade allocation to leaves increased. The dry conditions did not cause a reduction in shade tolerance, and both shade avoidance and tolerance morphology were evidenced under moderate (50%) and deep (70%) shading (stem elongation, increased leaf area). Deep rooting – a drought-avoidance trait – was similar in high and low water availability. Our overall results support the hypothesis that shade ameliorates drought stress. Moreover, S. spinosum demonstrates that woody species inhabiting water-limited ecosystems may have a repertoire of traits that allows simultaneous tolerance to both drought and shade.

Flowering and Growth Responses of Cultivated Lentil and Wild Lens Germplasm toward the Differences in Red to Far-Red Ratio and Photosynthetically Active Radiation.

Understanding environmental responses of pulse crop species and their wild relatives will play an important role in developing genetic strategies for crop improvement in response to changes in climate. This study examined how cultivated lentil and wild Lens germplasm responded to different light environments, specifically differences in red/far-red ratio (R/FR) and photosynthetically active radiation (PAR). Three genotypes of each the seven Lens species were grown in environmentally controlled growth chambers equipped to provide light treatments consisting of different R/FR ratios and PAR values. Our results showed that overall, days to flower of Lens genotypes were mainly influenced by the R/FR induced light quality change but not by the PAR related light intensity change. The cultivated lentil (L. culinaris) showed consistent, accelerated flowering in response to the low R/FR light environment together with three wild lentil genotypes (L. orientalis IG 72611, L. tomentosus IG 72830, and L. ervoides IG 72815) while most wild lentil genotypes had reduced responses and flowering time was not significantly affected. The longest shoot length, longest internode length, and largest leaflet area were observed under the low R/FR low PAR environment for both cultivated and wild lentils. The distinctly different responses between flowering time and elongation under low R/FR conditions among wild Lens genotypes suggests discrete pathways controlling flowering and elongation, which are both components of shade avoidance responses. The yield and above-ground biomass of Lens genotypes were the highest under high R/FR high PAR conditions, intermediate under low R/FR low PAR conditions, and lowest under high R/FR low PAR light conditions. Three L. lamottei genotypes (IG 110809, IG 110810, and IG 110813) and one L. ervoides genotype (IG 72646) were less sensitive in their time to flower responses while maintaining similar yield, biomass, and harvest index across all three light environments; these are indications of better adaptability toward changes in light environment.

Exploring growth-defence trade-offs in Arabidopsis: phytochrome B inactivation requires JAZ10 to suppress plant immunity but not to trigger shade-avoidance responses.

Under conditions that involve a high risk of competition for light among neighbouring plants, shade-intolerant species often display increased shoot elongation and greater susceptibility to pathogens and herbivores. The functional links between morphological and defence responses to crowding are not well understood. In Arabidopsis, the protein JAZ10 is thought to play a key role connecting the inactivation of the photoreceptor phytochrome B (phyB), which takes place under competition for light, with the repression of jasmonate-mediated plant defences. Here, we show that a null mutation of the JAZ10 gene in Arabidopsis did not affect plant growth nor did it suppress the shade-avoidance responses elicited by phyB inactivation. However, the jaz10 mutation restored many of the defence traits that are missing in the phyB mutant, including the ability to express robust responses to jasmonate and to accumulate indolic glucosinolates. Furthermore, the jaz10phyB double mutant showed a significantly increased resistance to the pathogenic fungus Botrytis cinerea compared with the phyB parental line. Our results demonstrate that, by inactivating JAZ10, it is possible to partially uncouple shade avoidance from defence suppression in Arabidopsis. These findings may provide clues to improve plant resistance to pathogens in crops that are planted at high density.

Development of transgenic crops based on photo-biotechnology.

The phenotypes associated with plant photomorphogenesis such as the suppressed shade avoidance response and de-etiolation offer the potential for significant enhancement of crop yields. Of many light signal transducers and transcription factors involved in the photomorphogenic responses of plants, this review focuses on the transgenic overexpression of the photoreceptor genes at the uppermost stream of the signalling events, particularly phytochromes, crytochromes and phototropins as the transgenes for the genetic engineering of crops with improved harvest yields. In promoting the harvest yields of crops, the photoreceptors mediate the light regulation of photosynthetically important genes, and the improved yields often come with the tolerance to abiotic stresses such as drought, salinity and heavy metal ions. As a genetic engineering approach, the term photo-biotechnology has been coined to convey the idea that the greater the photosynthetic efficiency that crop plants can be engineered to possess, the stronger the resistance to biotic and abiotic stresses. Development of GM crops based on photoreceptor transgenes (mainly phytochromes, crytochromes and phototropins) is reviewed with the proposal of photo-biotechnology that the photoreceptors mediate the light regulation of photosynthetically important genes, and the improved yields often come with the added benefits of crops' tolerance to environmental stresses.

Light quality regulates plant architecture in different genotypes of Chrysanthemum morifolium Ramat

Specific light spectra play an important role in plant photomorphogenic responses. Plants are sensitive to light ranging from UV (280–400nm) to far-red light (700–800nm). The shoot architecture or plant shape is an important quality trait in ornamental plants and can be altered under specific light spectra. The shade avoidance syndrome is a well-documented response of plants to canopy shading and low R:FR conditions, characterized by shoot elongation and inhibited branching. Treatments with LED light combinations to obtain different spectral compositions were tested on rooted cuttings of 3 chrysanthemum genotypes (a pot chrysanthemum, a cut flower and a disbud chrysanthemum genotype) to assess the effect on shoot architecture. Red light treatment generally showed increased bud outgrowth and increased average bud length while blue+far-red light treatment resulted in decreased bud outgrowth and bud length. Some effects were genotype dependent, such as plant height, which increased under blue+far-red light treatments compared to red light treatment, only for the pot chrysanthemum genotype.Treatment with blue+far-red light in 25 decapitated cuttings showed a strong elongation of the topmost axillary bud and inhibition of underlying buds for the pot chrysanthemum and cut flower genotypes. This effect also persisted in greenhouse conditions.

Karrikins: Regulators Involved in Phytohormone Signaling Networks during Seed Germination and Seedling Development.

Seed germination and early seedling establishment are critical stages during a plant’s life cycle. These stages are precisely regulated by multiple internal factors, including phytohormones and environmental cues such as light. As a family of small molecules discovered in wildfire smoke, karrikins (KARs) play a key role in various biological processes, including seed dormancy release, germination regulation, and seedling establishment. KARs show a high similarity with strigolactone (SL) in both chemical structure and signaling transduction pathways. Current evidence shows that KARs may regulate seed germination by mediating the biosynthesis and/or signaling transduction of abscisic acid (ABA), gibberellin (GA) and auxin [indoleacetic acid (IAA)]. Interestingly, KARs regulate seed germination differently in different species. Furthermore, the promotion effect on seedling establishment implies that KARs have a great potential application in alleviating shade avoidance response, which attracts more and more attention in plant molecular biology. In these processes, KARs may have complicated interactions with phytohormones, especially with IAA. In this updated review, we summarize the current understanding of the relationship between KARs and SL in the chemical structure, signaling pathway and the regulation of plant growth and development. Further, the crosstalk between KARs and phytohormones in regulating seed germination and seedling development and that between KARs and IAA during shade responses are discussed. Finally, future challenges and research directions for the KAR research field are suggested.

Arabidopsis DNA topoisomerase I alpha is required for adaptive response to light and flower development.

ABSTRACTDNA topoisomerase I alpha (TOP1α) plays a specific role in Arabidopsis thaliana development and is required for stem cell regulation in shoot and floral meristems. Recently, a new role independent of meristem functioning has been described for TOP1α, namely flowering time regulation. The same feature had been detected by us earlier for fas5, a mutant allele of TOP1α. In this study we clarify the effects of fas5 on bolting initiation and analyze the molecular basis of its role on flowering time regulation. We show that fas5 mutation leads to a constitutive shade avoidance syndrome, accompanied by leaf hyponasty, petiole elongation, lighter leaf color and early bolting. Other alleles of TOP1α demonstrate the same shade avoidance response. RNA sequencing confirmed the activation of shade avoidance gene pathways in fas5 mutant plants. It also revealed the repression of many genes controlling floral meristem identity and organ morphogenesis. Our research further expands the knowledge of TOP1α function in plant development and reveals that besides stem cell maintenance TOP1α plays an important new role in regulating the adaptive plant response to light stimulus and flower development.

Auxin biosynthetic intermediate genes and their role in developmental growth and plasticity in higher plants

Auxins are a group of phytohormones that regulate several aspects of plant growth and development. Indole-3-acetic acid (IAA) is the predominant form of auxin in plants and several IAA biosynthetic pathways have been previously proposed but remain genetically uncharacterized. One of the proposed pathways is the indole-3-pyruvic acid (IPyA) pathway, which is inferred to regulate key developmental processes such as apical hook formation and shade avoidance. Recent molecular evidence suggests the existence of the pathway in higher plants but remains unverified due to the elusive nature of IPyA in vitro. Extending on these recent advances, this research was aimed at investigating aspects of IPyA-dependent auxin biology in Pisum sativum (pea) using reverse genetics, expression profiling, and analytical techniques. As a result the genes PsTAR2, PsTAR 5g Mt 80, and PsTAR 5g Mt 90, which are inferred to encode key enzymes in the IPyA pathway, were cloned. On expression analysis PsTAR2 was found to be slightly heightened in response to IPyA-inducing conditions (shade) while IAA levels remained unaltered contrary to previous reports. Moreover, the inferred homologs PsTAR 5g Mt 80 and PsTAR 5g Mt 90 appeared down-regulated in the same conditions suggesting functional divergence in the gene family. Thus, PsTAR2 was thought to be solely responsible for regulating IPyA-dependent auxin synthesis. Consequently, using a reverse genetic approach, called TILLING, the PsTAR2 gene was mutated in order to study the down-stream effects of IPyA deficiency. The procedure is currently underway and in the process of isolating two novel pstar2 (IPyA) mutant lines consisting of a missense mutation (pstar2 4280) and a highly desired knockout mutation (pstar2 918). On completion the novel mutants are anticipated to be indispensable to future IPyA-auxin investigations in higher plants. In light of the unstable nature of IPyA, a protocol has been formulated using UPLC for fractioning followed by MS/MS analysis. This technique appears to be very promising as a robust IPyA detection protocol in plant extracts.

Light distribution in Chinese solar greenhouse and its effect on plant growth

Chinese solar greenhouse (CSG) is universally applied in northern China for producing horticultural products. CSG is characterized by the unbalanced structures with an arched front roof face to the south side and a thick wall as well as back roof in the north side. Such structures affect light distribution in the greenhouse. This study aims to investigate the light distribution properties in CSG from north to south sections, and to investigate tomato plant growth performance in the corresponding locations. Experiments were carried out in a CSG which was divided into three equal sections from north to south side. Tomato was grown in the greenhouse. Results showed that PAR intensity in the south and middle sections of CSG was permanently higher than the north section. This resulted in a distinct plant growth performance in CSG. Specifically, plants grown in the north section of CSG exhibited a shade avoidance response with stem elongation phenotype and leaf expansion. Furthermore, the north-plants showed lower leaf photosynthetic capacity which correlated with a lower total nitrogen and chlorophyll contents in comparison with the plants grown in the middle and south sections. Taken together, plants in the north section of CSG produced less total biomass than the middle and south section plants. We conclude that plant growth is not uniform in CSG due to heterogeneous light distribution which was caused by unbalanced greenhouse structures. This study may provide sound evidence for exploring a proper lighting strategy as well as fine crop management in CSG.

Leaf photosynthetic rate, growth, and morphology of lettuce under different fractions of red, blue, and green light from light-emitting diodes (LEDs)

Current LED-based artificial lights for crop cultivation consist of red and blue lights because these spectra effectively promote leaf photosynthesis. However, the absence of green light could be disadvantageous for crop production, as green light plays an important role in plant development. The objective of this study was to investigate whether adding green light to different proportions of red and blue light would affect the leaf photosynthetic rate, growth, and morphology of lettuce plants. Plants were transplanted and grown hydroponically for 25 days under different combinations of red, blue (0, 10, 20, and 30%), and green (0 and 10%) light at 150 ± 15 μmol•m-2•s-1 of photosynthetic photon flux density (PPFD). The leaf photosynthetic rate was highest under 80% red and 20% blue light and decreased significantly with the addition of green light and the absence of blue light. As the fraction of blue light increased, leaf size and plant growth decreased significantly. However, while the addition of green light considerably reduced the leaf photosynthetic rate, it did not reduce plant growth. In the absence of blue light, the plants showed symptoms of the shade avoidance response, which possibly enhanced their growth by improving their light interception. Therefore, the addition of 10% (15 μmol•m-2•s-1) green light did not have a positive effect on the growth of lettuce. Further study using higher intensities of green light is required to investigate the effects of green light on plant growth.

Integration of Phytochrome and Cryptochrome Signals Determines Plant Growth during Competition for Light

Plants in dense vegetation perceive their neighbors primarily through changes in light quality. Initially, the ratio between red (R) and far-red (FR) light decreases due to reflection of FR by plant tissue well before shading occurs. Perception of low R:FR by the phytochrome photoreceptors induces the shade avoidance response [1], of which accelerated elongation growth of leaf-bearing organsisan important feature. Low R:FR-induced phytochrome inactivation leads to the accumulation and activation of the transcription factors PHYTOCHROME-INTERACTING FACTORs (PIFs) 4, 5, and 7 and subsequent expression of their growth-mediating targets [2, 3]. When true shading occurs, transmitted light is especially depleted in red and blue (B) wavelengths, due to absorption by chlorophyll [4]. Although the reduction of blue wavelengths alone does not occur in nature, long-term exposure to low B light induces a shade avoidance-like response that is dependent on the cryptochrome photoreceptors and the transcription factors PIF4 and PIF5 [5-7]. We show in Arabidopsis thaliana that low B in combination with lowR:FR enhances petiole elongation similar to vegetation shade, providing functional context fora low B response in plant competition. Low Bpotentiates the low R:FR response through PIF4,PIF5, and PIF7, and it involves increased PIF5 abundance and transcriptional changes. LowB attenuates a low R:FR-induced negative feedback loop through reduced gene expression of negative regulators and reduced HFR1 levels. The enhanced response to combined phytochromeand cryptochrome inactivation shows how multiple light cues can be integrated to fine-tune the plant's response to a changing environment.

SHADE AVOIDANCE 4 Is Required for Proper Auxin Distribution in the Hypocotyl

The phytohormone auxin is involved in virtually every aspect of plant growth and development. Through polar auxin transport, auxin gradients can be established, which then direct plant differentiation and growth. Shade avoidance responses are well-known processes that require polar auxin transport. In this study, we have identified a mutant, shade avoidance 4 (sav4), defective in shade-induced hypocotyl elongation and basipetal auxin transport. SAV4 encodes an unknown protein with armadillo repeat- and tetratricopeptide repeat-like domains known to provide protein-protein interaction surfaces. C terminally yellow fluorescent protein-tagged SAV4 localizes to both the plasma membrane and the nucleus. Membrane-localized SAV4 displays a polar association with the shootward plasma membrane domain in hypocotyl and root cells, which appears to be necessary for its function in hypocotyl elongation. Cotransfection of SAV4 and ATP-binding cassette B1 (ABCB1) auxin transporter in tobacco (Nicotiana benthamiana) revealed that SAV4 blocks ABCB1-mediated auxin efflux. We thus propose that polarly localized SAV4 acts to inhibit ABCB-mediated auxin efflux toward shoots and facilitates the establishment of proper auxin gradients.

Bottom-up Assembly of the Phytochrome Network.

Plants have developed sophisticated systems to monitor and rapidly acclimate to environmental fluctuations. Light is an essential source of environmental information throughout the plant’s life cycle. The model plant Arabidopsis thaliana possesses five phytochromes (phyA-phyE) with important roles in germination, seedling establishment, shade avoidance, and flowering. However, our understanding of the phytochrome signaling network is incomplete, and little is known about the individual roles of phytochromes and how they function cooperatively to mediate light responses. Here, we used a bottom-up approach to study the phytochrome network. We added each of the five phytochromes to a phytochrome-less background to study their individual roles and then added the phytochromes by pairs to study their interactions. By analyzing the 16 resulting genotypes, we revealed unique roles for each phytochrome and identified novel phytochrome interactions that regulate germination and the onset of flowering. Furthermore, we found that ambient temperature has both phytochrome-dependent and -independent effects, suggesting that multiple pathways integrate temperature and light signaling. Surprisingly, none of the phytochromes alone conferred a photoperiodic response. Although phyE and phyB were the strongest repressors of flowering, both phyB and phyC were needed to confer a flowering response to photoperiod. Thus, a specific combination of phytochromes is required to detect changes in photoperiod, whereas single phytochromes are sufficient to respond to light quality, indicating how phytochromes signal different light cues.

YUCCA auxin biosynthetic genes are required for Arabidopsis shade avoidance.

Plants respond to neighbor shade by increasing stem and petiole elongation. Shade, sensed by phytochrome photoreceptors, causes stabilization of PHYTOCHROME INTERACTING FACTOR proteins and subsequent induction of YUCCA auxin biosynthetic genes. To investigate the role of YUCCA genes in phytochrome-mediated elongation, we examined auxin signaling kinetics after an end-of-day far-red (EOD-FR) light treatment, and found that an auxin responsive reporter is rapidly induced within 2 hours of far-red exposure. YUCCA2, 5, 8, and 9 are all induced with similar kinetics suggesting that they could act redundantly to control shade-mediated elongation. To test this hypothesis we constructed a yucca2, 5, 8, 9 quadruple mutant and found that the hypocotyl and petiole EOD-FR and shade avoidance responses are completely disrupted. This work shows that YUCCA auxin biosynthetic genes are essential for detectable shade avoidance and that YUCCA genes are important for petiole shade avoidance.

Sensitivity of Seven Diverse Species to Blue and Green Light: Interactions with Photon Flux

Despite decades of research, the effects of spectral quality on plant growth, and development are not well understood. Much of our current understanding comes from studies with daily integrated light levels that are less than 10% of summer sunlight thus making it difficult to characterize interactions between light quality and quantity. Several studies have reported that growth is increased under fluorescent lamps compared to mixtures of wavelengths from LEDs. Conclusions regarding the effect of green light fraction range from detrimental to beneficial. Here we report the effects of eight blue and green light fractions at two photosynthetic photon fluxes (PPF; 200 and 500 μmol m-2 s-1; with a daily light integral of 11.5 and 29 mol m-2 d-1) on growth (dry mass), leaf expansion, stem and petiole elongation, and whole-plant net assimilation of seven diverse plant species. The treatments included cool, neutral, and warm white LEDs, and combinations of blue, green and/or red LEDs. At the higher PPF (500), increasing blue light in increments from 11 to 28% reduced growth in tomato, cucumber, and pepper by 22, 26, and 14% respectively, but there was no statistically significant effect on radish, soybean, lettuce or wheat. At the lower PPF (200), increasing blue light reduced growth only in tomato (41%). The effects of blue light on growth were mediated by changes in leaf area and radiation capture, with minimal effects on whole-plant net-assimilation. In contrast to the significant effects of blue light, increasing green light in increments from 0 to 30% had a relatively small effect on growth, leaf area and net assimilation at either low or high PPF. Surprisingly, growth of three of the seven species was not reduced by a treatment with 93% green light compared to the broad spectrum treatments. Collectively, these results are consistent with a shade avoidance response associated with either low blue or high green light fractions.

Tomato phyE Is Required for Shade Avoidance in the Absence of phyB1 and phyB2.

The phytochrome (phy) family of red and far-red photoreceptors provides plants with critical information about their surrounding environment and can signal downstream developmental and physiological changes. Neighboring plants compete for limited light resources, and their presence is detected by the phytochrome photoreceptors as a reduced ratio of red: far-red light. One common response to shade is increased elongation of petioles and internodes to compete with their neighbors. While the phytochrome family, phyB in particular, has been well studied in Arabidopsis, information about the other phytochrome family members is limited, especially in sympodial crop plants such as tomato, that have a very different architecture from that of the model plant. To study the tomato phytochrome family we took advantage of several existing mutants and generated an artificial miRNA (amiRNA) line to target SlPHYE, the remaining phytochrome B subfamily member with no currently available mutant line. Here, we characterize internode elongation and shade avoidance phenotypes of the SlPHYE amiRNA line (PHYE amiRNA). In addition, higher order phytochrome subfamily B mutants were generated with the PHYE amiRNA line to investigate the role of SlphyE within the phyB subfamily. We find that the PHYE amiRNA line has no detectable phenotype on its own, however in higher order combinations with SlphyB1 and/or SlphyB2 there are notable defects in shade avoidance. Most notably, we find that the triple mutant combination of SlPHYE amiRNA, SlphyB1, and SlphyB2 has a phenotype that is much stronger than the SlphyB1 SlphyB2 double, showing constitutive shade avoidance and little to no response to shade. This indicates that SlphyE is required for the shade avoidance response in the absence of SlphyB1 and SlphyB2.