Photoreceptor signaling networks in plant responses to shade

Transcriptome Signatures of Selection, Drift, Introgression, and Gene Duplication in the Evolution of an Extremophile Endemic Plant

Plants on serpentine soils provide extreme examples of adaptation to environment, and thus offer excellent models for the study of evolution at the molecular and genomic level. Serpentine outcrops are derived from ultramafic rock and have extremely low levels of essential plant nutrients (e.g., N, P, K, and Ca), as well as toxic levels of heavy metals (e.g., Ni, Cr, and Co) and low moisture availability. These outcrops provide habitat to a number of endemic plant species, including the annual mustard Caulanthus amplexicaulis var. barbarae (Cab) (Brassicaceae). Its sister taxon, C. amplexicaulis var. amplexicaulis (Caa), is intolerant to serpentine soils. Here, we assembled and annotated comprehensive reference transcriptomes of both Caa and Cab for use in protein coding sequence comparisons. A set of 29,443 reciprocal best Blast hit (RBH) orthologs between Caa and Cab was compared with identify coding sequence variants, revealing a high genome-wide dN/dS ratio between the two taxa (mean = 0.346). We show that elevated dN/dS likely results from the composite effects of genetic drift, positive selection, and the relaxation of negative selection. Further, analysis of paralogs within each taxon revealed the signature of a period of elevated gene duplication (∼10 Ma) that is shared with other species of the tribe Thelypodieae, and may have played a role in the striking morphological and ecological diversity of this tribe. In addition, distribution of the synonymous substitution rate, dS, is strongly bimodal, indicating a history of reticulate evolution that may have contributed to serpentine adaptation.

Light Controls Protein Localization through Phytochrome-Mediated Alternative Promoter Selection

Alternative promoter usage is a proteome-expanding mechanism that allows multiple pre-mRNAs to be transcribed from a single gene. The impact of this mechanism on the proteome and whether it is positively exploited in normal organismal responses remain unclear. We found that the plant photoreceptor phytochrome induces genome-wide changes in alternative promoter selection in Arabidopsis thaliana. Through this mechanism, protein isoforms with different N termini are produced that display light-dependent differences in localization. For instance, shade-grown plants accumulate a cytoplasmic isoform of glycerate kinase (GLYK), an essential photorespiration enzyme that was previously thought to localize exclusively to the chloroplast. Cytoplasmic GLYK constitutes a photorespiratory bypass that alleviates fluctuating light-induced photoinhibition. Therefore, phytochrome controls alternative promoter selection to modulate protein localization in response to changing light conditions. This study suggests that alternative promoter usage represents another ubiquitous layer of gene expression regulation in eukaryotes that contributes to diversification of the proteome.

Algal light sensing and photoacclimation in aquatic environments

Anoxygenic photosynthetic prokaryotes arose in ancient oceans ~3.5 billion years ago. The evolution of oxygenic photosynthesis by cyanobacteria followed soon after, enabling eukaryogenesis and the evolution of complex life. The Archaeplastida lineage dates back ~1.5 billion years to the domestication of a cyanobacterium. Eukaryotic algae have subsequently radiated throughout oceanic/freshwater/terrestrial environments, adopting distinctive morphological and developmental strategies for adaptation to diverse light environments. Descendants of the ancestral photosynthetic alga remain challenged by a typical diurnally fluctuating light supply ranging from ~0 to ~2000 μE m^-2  s^-1 . Such extreme changes in light intensity and variations in light quality have driven the evolution of novel photoreceptors, light-harvesting complexes and photoprotective mechanisms in photosynthetic eukaryotes. This minireview focuses on algal light sensors, highlighting the unexpected roles for linear tetrapyrroles (bilins) in the maintenance of functional chloroplasts in chlorophytes, sister species to streptophyte algae and land plants.

Light-controlled motility in prokaryotes and the problem of directional light perception

The natural light environment is important to many prokaryotes. Most obviously, phototrophic prokaryotes need to acclimate their photosynthetic apparatus to the prevailing light conditions, and such acclimation is frequently complemented by motility to enable cells to relocate in search of more favorable illumination conditions. Non-phototrophic prokaryotes may also seek to avoid light at damaging intensities and wavelengths, and many prokaryotes with diverse lifestyles could potentially exploit light signals as a rich source of information about their surroundings and a cue for acclimation and behavior. Here we discuss our current understanding of the ways in which bacteria can perceive the intensity, wavelength and direction of illumination, and the signal transduction networks that link light perception to the control of motile behavior. We discuss the problems of light perception at the prokaryotic scale, and the challenge of directional light perception in small bacterial cells. We explain the peculiarities and the common features of light-controlled motility systems in prokaryotes as diverse as cyanobacteria, purple photosynthetic bacteria, chemoheterotrophic bacteria and haloarchaea.

Evolutionary origin of phytochrome responses and signaling in land plants

Phytochromes comprise one of the major photoreceptor families in plants, and they regulate many aspects of plant growth and development throughout the plant life cycle. A canonical land plant phytochrome originated in the common ancestor of streptophytes. Phytochromes have diversified in seed plants and some basal land plants because of lineage-specific gene duplications that occurred during the course of land plant evolution. Molecular genetic analyses using Arabidopsis thaliana suggested that there are two types of phytochromes in angiosperms, light-labile type I and light-stable type II, which have different signaling mechanisms and which regulate distinct responses. In basal land plants, little is known about molecular mechanisms of phytochrome signaling, although red light/far-red photoreversible physiological responses and the distribution of phytochrome genes are relatively well documented. Recent advances in molecular genetics using the moss Physcomitrella patens and the liverwort Marchantia polymorpha revealed that basal land plants show far-red-induced responses and that the establishment of phytochrome-mediated transcriptional regulation dates back to at least the common ancestor of land plants. In this review, we summarize our knowledge concerning functions of land plant phytochromes, especially in basal land plants, and discuss subfunctionalization/neofunctionalization of phytochrome signaling during the course of land plant evolution.

The shade-avoidance syndrome: multiple signals and ecological consequences

Plants use photoreceptor proteins to detect the proximity of other plants and to activate adaptive responses. Of these photoreceptors, phytochrome B (phyB), which is sensitive to changes in the red (R) to far-red (FR) ratio of sunlight, is the one that has been studied in greatest detail. The molecular connections between the proximity signal (low R:FR) and a model physiological response (increased elongation growth) have now been mapped in considerable detail in Arabidopsis seedlings. We briefly review our current understanding of these connections and discuss recent progress in establishing the roles of other photoreceptors in regulating growth-related pathways in response to competition cues. We also consider processes other than elongation that are controlled by photoreceptors and contribute to plant fitness under variable light conditions, including photoresponses that optimize the utilization of soil resources. In examining recent advances in the field, we highlight emerging roles of phyB as a major modulator of hormones related to plant immunity, in particular salicylic acid and jasmonic acid (JA). Recent attempts to manipulate connections between light signals and defence in Arabidopsis suggest that it might be possible to improve crop health at high planting densities by targeting links between phyB and JA signalling.

A non-DNA-binding activity for the ATHB4 transcription factor in the control of vegetation proximity

In plants, perception of vegetation proximity by phytochrome photoreceptors activates a transcriptional network that implements a set of responses to adapt to plant competition, including elongation of stems or hypocotyls. In Arabidopsis thaliana, the homeodomain-leucine zipper (HD-Zip) transcription factor ARABIDOPSIS THALIANA HOMEOBOX 4 (ATHB4) regulates this and other responses, such as leaf polarity. To better understand the shade regulatory transcriptional network, we have carried out structure-function analyses of ATHB4 by overexpressing a series of truncated and mutated forms and analyzing three different responses: hypocotyl response to shade, transcriptional activity and leaf polarity. Our results indicated that ATHB4 has two physically separated molecular activities: that performed by HD-Zip, which is involved in binding to DNA-regulatory elements, and that performed by the ETHYLENE-RESPONSIVE ELEMENT BINDING FACTOR-associated amphiphilic repression (EAR)-containing N-terminal region, which is involved in protein-protein interaction. Whereas both activities are required to regulate leaf polarity, DNA-binding activity is not required for the regulation of the seedling responses to plant proximity, which indicates that ATHB4 works as a transcriptional cofactor in the regulation of this response. These findings suggest that transcription factors might employ alternative mechanisms of action to regulate different developmental processes.

New insights of red light-induced development

The red/far-red light absorbing photoreceptors phytochromes regulate development and growth and thus play an essential role in optimizing adaptation of the sessile plants to the ever-changing environment. Our understanding of how absorption of a red/far-red photon by phytochromes initiates/modifies diverse physiological responses has been steadily improving. Research performed in the last 5 years has been especially productive and led to significant conceptual changes about the mode of action of these photoreceptors. In this review, we focus on the phytochrome B photoreceptor, the major phytochrome species active in light-grown plants. We discuss how its light-independent inactivation (termed dark/thermal reversion), post-translational modification, including ubiquitination, phosphorylation and sumoylation, as well as heterodimerization with other phytochrome species modify red light-controlled physiological responses. Finally, we discuss how photobiological properties of phytochrome B enable this photoreceptor to function also as a thermosensor.

Photomorphogenic responses to ultraviolet-B light

Exposure to ultraviolet B (UV-B) light regulates numerous aspects of plant metabolism, morphology and physiology through the differential expression of hundreds of genes. Photomorphogenic responses to UV-B are mediated by the photoreceptor UV RESISTANCE LOCUS8 (UVR8). Considerable progress has been made in understanding UVR8 action: the structural basis of photoreceptor function, how interaction with CONSTITUTIVELY PHOTOMORPHOGENIC 1 initiates signaling and how REPRESSOR OF UV-B PHOTOMORPHOGENESIS proteins negatively regulate UVR8 action. In addition, recent research shows that UVR8 mediates several responses through interaction with other signaling pathways, in particular auxin signaling. Nevertheless, many aspects of UVR8 action remain poorly understood. Most research to date has been undertaken with Arabidopsis, and it is important to explore the functions and regulation of UVR8 in diverse plant species. Furthermore, it is essential to understand how UVR8, and UV-B signaling in general, regulates processes under natural growth conditions. Ultraviolet B regulates the expression of many genes through UVR8-independent pathways, but the activity and importance of these pathways in plants growing in sunlight are poorly understood.

Biological activity and dimerization state of modified phytochrome A proteins

To assess potential physical interactions of type I phyA with the type II phyB-phyE phytochromes in vivo, transgenes expressing fusion gene forms of phyA were introduced into the Arabidopsis phyA mutant background. When a single c-Myc (myc) epitope is added to either the N- or C-terminus of phyA, the constructs completely complement phyA mutant phenotypes. However, addition of larger tags, such as six consecutive myc epitopes or the yellow fluorescent protein sequence, result in fusion proteins that show reduced activity. All the tagged phyA proteins migrate as dimers on native gels and co-immunoprecipitation reveals no binding interaction of phyA to any of the type II phys in the dark or under continuous far-red light. Dimers of the phyA 1–615 amino acid N-terminal photosensory domain (NphyA), generated in vivo with a yeast GAL4 dimerization domain and attached to a constitutive nuclear localization sequence, are expressed at a low level and, although they cause a cop phenotype in darkness and mediate a very low fluence response to pulses of FR, have no activity under continuous FR. It is concluded that type I phyA in its Pr form is present in plants predominantly or exclusively as a homodimer and does not stably interact with type II phys in a dimer-to-dimer manner. In addition, its activity in mediating response to continuous FR is sensitive to modification of its N- or C-terminus.

COP1 regulates plant growth and development in response to light at the post-translational level

Photoreceptors perceive different wavelengths of light and transduce light signals downstream via a range of proteins. COP1, an E3 ubiquitin ligase, regulates light signaling by mediating the ubiquitination and subsequent proteasomal degradation of photoreceptors such as phytochromes and cryptochromes, as well as various development-related proteins including other light-responsive proteins. COP1 is itself regulated by direct interactions with several signaling molecules that modulate its activity. The control of photomorphogenesis by COP1 is also regulated by its localization to the cytoplasm in response to light. COP1 thus acts as a tightly regulated switch that determines whether development is skotomorphogenic or photomorphogenic. In this review, we discuss the effects of COP1 on the abundance and activity of various development-related proteins, including photoreceptors, and summarize the regulatory mechanisms that influence COP1 activity and stability in plants.

Differential Sensitivity of Fruit Pigmentation to Ultraviolet Light between Two Peach Cultivars

Anthocyanins provide nutritional benefits and are responsible for red coloration in many fruits. Light affects anthocyanin biosynthesis in peach (Prunus persica). However, some cultivars show differential sensitivity to light. In the present study, 'Hujingmilu (HJ),' a naturally deeply colored cultivar, and 'Yulu (YL),' showing low pigmentation, were used to study the mechanism underlying UV-light-induced anthocyanin biosynthesis. Both UVA and UVB induced fruit pigmentation of 'HJ,' but 'YL' was only sensitive to UVB. Transcriptomic analyses showed over 5000 genes were differentially expressed by pairwise comparisons of RNA libraries isolated from tissue of each cultivar treated with darkness, UVA and UVB. Twenty-three genes related to anthocyanin biosynthesis were identified from the transcriptome data, which were coordinately up-regulated during accumulation of anthocyanins, and down-regulated in the dark. Altered expression of several light receptors, as well as CONSTITUTIVE PHOTOMORPHOGENIC10 (COP10) and ELONGATED HYPOCOTYL 5 homolog (HYH), and a specific anthocyanin transporter glutathione S-transferase (GST), in 'YL' fruit appears to be responsible for the insensitivity to UVA of this cultivar. Expression profiles of several transcription factors of the families MYB, bHLH, bZIP and NAC were highly correlated with those of the anthocyanin biosynthesis genes. The study provides a valuable overview of the underlying molecular mechanisms of UV-light induced anthocyanin response using peach cultivars with differing light sensitivities.

Plant dual-specificity tyrosine phosphorylation-regulated kinase optimizes light-regulated growth and development in Arabidopsis

Light controls vegetative and reproductive development of plants. For a plant, sensing the light input properly ensures coordination with the ever-changing environment. Previously, we found that LIGHT-REGULATED WD1 (LWD1) and LWD2 regulate the circadian clock and photoperiodic flowering. Here, we identified Arabidopsis YET ANOTHER KINASE1 (AtYAK1), an evolutionarily conserved protein and a member of dual-specificity tyrosine phosphorylation-regulated kinases (DYRKs), as an interacting protein of LWDs. Our study revealed that AtYAK1 is an important regulator for various light responses, including the circadian clock, photomorphogenesis and reproductive development. AtYAK1 could antagonize the function of LWDs in regulating the circadian clock and photoperiodic flowering. By examining phenotypes of atyak1, we found that AtYAK1 regulated light-induced period-length shortening and photomorphogenic development. Moreover, AtYAK1 mediated plant fertility especially under inferior light conditions including low light and short-day length. This study discloses a new regulator connecting environmental light to plant growth.

Shade Inhibits Leaf Size by Controlling Cell Proliferation and Enlargement in Soybean

To gain more insight into the physiological function of shade and how shade affects leaf size, we investigated the growth, leaf anatomical structure, hormones and genes expressions in soybean. Soybean seeds were sown in plastic pots and were allowed to germinate and grow for 30 days under shade or full sunlight conditions. Shade treated plants showed significantly increase on stem length and petiole length, and decrease on stem diameters, shoot biomass and its partition to leaf also were significantly lower than that in full sunlight. Smaller and thinner on shade treated leaves than corresponding leaves on full sunlight plants. The decreased leaf size caused by shade was largely attributable to cell proliferation in young leaves and both cell proliferation and enlargement in old leaves. Shade induced the expression of a set of genes related to cell proliferation and/or enlargement, but depended on the developmental stage of leaf. Shade significantly increased the auxin and gibberellin content, and significantly decreased the cytokinin content in young, middle and old leaves. Taken together, these results indicated that shade inhibited leaf size by controlling cell proliferation and enlargement, auxin, gibberellin and cytokinin may play important roles in this process.

Plant Phenotypic and Transcriptional Changes Induced by Volatiles from the Fungal Root Pathogen Rhizoctonia solani

Beneficial soil microorganisms can affect plant growth and resistance by the production of volatile organic compounds (VOCs). Yet, little is known on how VOCs from soil-borne plant pathogens affect plant growth and resistance. Here we show that VOCs released from mycelium and sclerotia of the fungal root pathogen Rhizoctonia solani enhance growth and accelerate development of Arabidopsis thaliana. Seedlings briefly exposed to the fungal VOCs showed similar phenotypes, suggesting that enhanced biomass and accelerated development are primed already at early developmental stages. Fungal VOCs did not affect plant resistance to infection by the VOC-producing pathogen itself but reduced aboveground resistance to the herbivore Mamestra brassicae. Transcriptomics of A. thaliana revealed that genes involved in auxin signaling were up-regulated, whereas ethylene and jasmonic acid signaling pathways were down-regulated by fungal VOCs. Mutants disrupted in these pathways showed similar VOC-mediated growth responses as the wild-type A. thaliana, suggesting that other yet unknown pathways play a more prominent role. We postulate that R. solani uses VOCs to predispose plants for infection from a distance by altering root architecture and enhancing root biomass. Alternatively, plants may use enhanced root growth upon fungal VOC perception to sacrifice part of the root biomass and accelerate development and reproduction to survive infection.

Local auxin production underlies a spatially restricted neighbor-detection response in Arabidopsis

Competition for light triggers numerous developmental adaptations known as the "shade-avoidance syndrome" (SAS). Important molecular events underlying specific SAS responses have been identified. However, in natural environments light is often heterogeneous, and it is currently unknown how shading affecting part of a plant leads to local responses. To study this question, we analyzed upwards leaf movement (hyponasty), a rapid adaptation to neighbor proximity, in Arabidopsis We show that manipulation of the light environment at the leaf tip triggers a hyponastic response that is restricted to the treated leaf. This response is mediated by auxin synthesized in the blade and transported to the petiole. Our results suggest that a strong auxin response in the vasculature of the treated leaf and auxin signaling in the epidermis mediate leaf elevation. Moreover, the analysis of an auxin-signaling mutant reveals signaling bifurcation in the control of petiole elongation versus hyponasty. Our work identifies a mechanism for a local shade response that may pertain to other plant adaptations to heterogeneous environments.

Light-mediated self-organization of sunflower stands increases oil yield in the field

Here, we show a unique crop response to intraspecific interference, whereby neighboring sunflower plants in a row avoid each other by growing toward a more favorable light environment and collectively increase production per unit land area. In high-density stands, a given plant inclined toward one side of the interrow space, and the immediate neighbors inclined in the opposite direction. This process started early as an incipient inclination of pioneer plants, and the arrangement propagated gradually as a "wave" of alternate inclination that persisted until maturity. Measurements and experimental manipulation of light spectral composition indicate that these responses are mediated by changes in the red/far-red ratio of the light, which is perceived by phytochrome. Cellular automata simulations reproduced the patterns of stem inclination in field experiments, supporting the proposition of self-organization of stand structure. Under high crop population densities (10 and 14 plants per m²), as yet unachievable in commercial farms with current hybrids due to lodging and diseases, self-organized crops yielded between 19 and 47% more oil than crops forced to remain erect.

Neighbor detection at the leaf tip adaptively regulates upward leaf movement through spatial auxin dynamics

Vegetation stands have a heterogeneous distribution of light quality, including the red/far-red light ratio (R/FR) that informs plants about proximity of neighbors. Adequate responses to changes in R/FR are important for competitive success. How the detection and response to R/FR are spatially linked and how this spatial coordination between detection and response affects plant performance remains unresolved. We show in Arabidopsis thaliana and Brassica nigra that localized FR enrichment at the lamina tip induces upward leaf movement (hyponasty) from the petiole base. Using a combination of organ-level transcriptome analysis, molecular reporters, and physiology, we show that PIF-dependent spatial auxin dynamics are key to this remote response to localized FR enrichment. Using computational 3D modeling, we show that remote signaling of R/FR for hyponasty has an adaptive advantage over local signaling in the petiole, because it optimizes the timing of leaf movement in response to neighbors and prevents hyponasty caused by self-shading.

Genome-wide mapping of DNase I hypersensitive sites reveals chromatin accessibility changes in Arabidopsis euchromatin and heterochromatin regions under extended darkness

Light, as the energy source in photosynthesis, is essential for plant growth and development. Extended darkness causes dramatic gene expression changes. In this study, we applied DNase-seq (DNase I hypersensitive site sequencing) to study changes of chromatin accessibility in euchromatic and heterochromatic regions under extended darkness in Arabidopsis. We generated 27 Gb DNase-seq and 67.6 Gb RNA-seq data to investigate chromatin accessibility changes and global gene expression under extended darkness and control condition in Arabidopsis. We found that ~40% DHSs (DNaseI hypersensitive sites) were diminished under darkness. In non-TE regions, the majority of DHS-changed genes were DHS-diminished under darkness. A total of 519 down-regulated genes were associated with diminished DHSs under darkness, mainly involved in photosynthesis process and retrograde signaling, and were regulated by chloroplast maintenance master regulators such as GLK1. In TE regions, approximately half of the DHS-changed TEs were DHS-increased under darkness and were primarily associated with the LTR/Gypsy retrotransposons in the heterochromatin flanking the centromeres. In contrast, DHS-diminished TEs under darkness were enriched in Copia, LINE, and MuDR dispersed across chromosomes. Together, our results indicated that extended darkness resulted in more increased chromatin compaction in euchromatin and decompaction in heterochromatin, thus further leading to gene expression changes in Arabidopsis.

On the induction of injury in tomato under continuous light: circadian asynchrony as the main triggering factor

Unlike other species, when tomato plants (Solanum lycopersicum L.) are deprived of at least 8h of darkness per day, they develop a potentially lethal injury. In an effort to understand why continuous light (CL) is injurious to tomato, we tested five factors, which potentially could be responsible for triggering the injury in CL-grown tomato: (i) differences in the light spectral distribution between sunlight and artificial light, (ii) continuous light signalling, (iii) continuous supply of light for photosynthesis, (iv) continuous photo-oxidative pressure and (v) circadian asynchrony - a mismatch between the internal circadian clock frequency and the external light/dark cycles. Our results strongly suggest that continuous-light-induced injury does not result from the unnatural spectral distribution of artificial light nor from the continuity of light per se. Instead, circadian asynchrony seems to be the main factor inducing the CL-induced injury, but the mechanism is not by the earlier hypothesised circadian pattern in sensitivity for photoinhibition. Here, however, we show for the first time diurnal fluctuations in sensitivity to photoinhibition during normal photoperiods. Similarly, we also report for the first time diurnal and circadian rhythms in the maximum quantum efficiency of PSII (Fv/Fm) and the parameter F0.

Phytochrome diversification in cyanobacteria and eukaryotic algae

Phytochromes control almost every aspect of plant biology, including germination, growth, development, and flowering, in response to red and far-red light. These photoreceptors thus hold considerable promise for engineering crop plant responses to light. Recently, structural research has shed new light on how phytochromes work. Genomic and transcriptomic studies have improved our understanding of phytochrome loss, retention, and diversification during evolution. We are also beginning to understand phytochrome function in cyanobacteria and eukaryotic algae.

Illuminating colors: regulation of carotenoid biosynthesis and accumulation by light

Light stimulates the biosynthesis of carotenoids and regulates the development of plastid structures to accommodate these photoprotective pigments. Work with Arabidopsis revealed molecular factors coordinating carotenoid biosynthesis and storage with photosynthetic development during deetiolation, when underground seedlings emerge to the light. Some of these factors also adjust carotenoid biosynthesis in response to plant proximity (i.e., shade), a mechanism that was readapted in tomato to monitor fruit ripening progression. While light positively impacts carotenoid production and accumulation in most cases, total carotenoid levels decrease in roots of colored carrot cultivars when illuminated. The recent discovery that such cultivars might be photomorphogenic mutants provides an explanation for this striking phenotype.

The phycocyanobilin chromophore of streptophyte algal phytochromes is synthesized by HY2

Land plant phytochromes perceive red and far-red light to control growth and development, using the linear tetrapyrrole (bilin) chromophore phytochromobilin (PΦB). Phytochromes from streptophyte algae, sister species to land plants, instead use phycocyanobilin (PCB). PCB and PΦB are synthesized by different ferredoxin-dependent bilin reductases (FDBRs): PΦB is synthesized by HY2, whereas PCB is synthesized by PcyA. The pathway for PCB biosynthesis in streptophyte algae is unknown. We used phylogenetic analysis and heterologous reconstitution of bilin biosynthesis to investigate bilin biosynthesis in streptophyte algae. Phylogenetic results suggest that PcyA is present in chlorophytes and prasinophytes but absent in streptophytes. A system reconstituting bilin biosynthesis in Escherichia coli was modified to utilize HY2 from the streptophyte alga Klebsormidium flaccidum (KflaHY2). The resulting bilin was incorporated into model cyanobacterial photoreceptors and into phytochrome from the early-diverging streptophyte alga Mesostigma viride (MvirPHY1). All photoreceptors tested incorporate PCB rather than PΦB, indicating that KflaHY2 is sufficient for PCB synthesis without any other algal protein. MvirPHY1 exhibits a red-far-red photocycle similar to those seen in other streptophyte algal phytochromes. These results demonstrate that streptophyte algae use HY2 to synthesize PCB, consistent with the hypothesis that PΦB synthesis arose late in HY2 evolution.

Perception and signalling of light and temperature cues in plants

Light and temperature patterns are often correlated under natural plant growth conditions. In this review, we analyse the perception and signalling mechanisms shared by both these environmental cues and discuss the functional implications of their convergence to control plant growth. The first point of integration is the phytochrome B (phyB) receptor, which senses light and temperature. Downstream of phyB, the signalling core comprises two branches, one involving PHYTOCHROME INTERACTING FACTOR 4 (PIF4) and the other CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) and ELONGATED HYPOCOTYL 5 (HY5). The dynamics of accumulation and/or localization of each of these core signalling components depend on light and temperature conditions. These pathways are connected through COP1, which enhances the activity of PIF4. The circadian clock modulates this circuit, since EARLY FLOWERING 3 (ELF3), an essential component of the evening complex (EC), represses expression of the PIF4 gene and PIF4 transcriptional activity. Phytochromes are probably not the only entry point of temperature into this network, but other sensors remain to be established. The sharing of mechanisms of action for two distinct environmental cues is to some extent unexpected, as it renders these responses mutually dependent. There are nonetheless many ecological contexts in which such a mutual influence could be beneficial.

Release of resource constraints allows greater carbon allocation to secondary metabolites and storage in winter wheat

The atmospheric CO₂ concentration ([CO₂ ]) is rapidly increasing, and this may have substantial impact on how plants allocate metabolic resources. A thorough understanding of allocation priorities can be achieved by modifying [CO₂ ] over a large gradient, including low [CO₂ ], thereby altering plant carbon (C) availability. Such information is of critical importance for understanding plant responses to global environmental change. We quantified the percentage of daytime whole-plant net assimilation (A) allocated to night-time respiration (R), structural growth (SG), nonstructural carbohydrates (NSC) and secondary metabolites (SMs) during 8 weeks of vegetative growth in winter wheat (Triticum aestivum) growing at low, ambient and elevated [CO₂ ] (170, 390 and 680 ppm). R/A remained relatively constant over a large gradient of [CO₂ ]. However, with increasing C availability, the fraction of assimilation allocated to biomass (SG + NSC + SMs), in particular NSC and SMs, increased. At low [CO₂ ], biomass and NSC increased in leaves but decreased in stems and roots, which may help plants achieve a functional equilibrium, that is, overcome the most severe resource limitation. These results reveal that increasing C availability from rising [CO₂ ] releases allocation constraints, thereby allowing greater investment into long-term survival in the form of NSC and SMs.

Double overexpression of DREB and PIF transcription factors improves drought stress tolerance and cell elongation in transgenic plants

Although a variety of transgenic plants that are tolerant to drought stress have been generated, many of these plants show growth retardation. To improve drought tolerance and plant growth, we applied a gene-stacking approach using two transcription factor genes: DEHYDRATION-RESPONSIVE ELEMENT-BINDING 1A (DREB1A) and rice PHYTOCHROME-INTERACTING FACTOR-LIKE 1 (OsPIL1). The overexpression of DREB1A has been reported to improve drought stress tolerance in various crops, although it also causes a severe dwarf phenotype. OsPIL1 is a rice homologue of Arabidopsis PHYTOCHROME-INTERACTING FACTOR 4 (PIF4), and it enhances cell elongation by activating cell wall-related gene expression. We found that the OsPIL1 protein was more stable than PIF4 under light conditions in Arabidopsis protoplasts. Transactivation analyses revealed that DREB1A and OsPIL1 did not negatively affect each other's transcriptional activities. The transgenic plants overexpressing both OsPIL1 and DREB1A showed the improved drought stress tolerance similar to that of DREB1A overexpressors. Furthermore, double overexpressors showed the enhanced hypocotyl elongation and floral induction compared with the DREB1A overexpressors. Metabolome analyses indicated that compatible solutes, such as sugars and amino acids, accumulated in the double overexpressors, which was similar to the observations of the DREB1A overexpressors. Transcriptome analyses showed an increased expression of abiotic stress-inducible DREB1A downstream genes and cell elongation-related OsPIL1 downstream genes in the double overexpressors, which suggests that these two transcription factors function independently in the transgenic plants despite the trade-offs required to balance plant growth and stress tolerance. Our study provides a basis for plant genetic engineering designed to overcome growth retardation in drought-tolerant transgenic plants.

Effects of Light-Emitting Diode Irradiation on Growth Characteristics and Regulation of Porphyrin Biosynthesis in Rice Seedlings

We examined the effects of light quality on growth characteristics and porphyrin biosynthesis of rice seedlings grown under different wavelengths from light emitting diodes (LEDs). After 10 days of exposure to various wavelengths of LEDs, leaf area and shoot biomass were greater in seedlings grown under white and blue LEDs than those of green and red LEDs. Both green and red LED treatments drastically decreased levels of protoporphyrin IX (Proto IX) and Mg-porphyrins compared to those of white LED, while levels of Mg-Proto IX monomethyl ester and protochlorophyllide under blue LED were decreased by 21% and 49%, respectively. Transcript levels of PPO1 were greatly upregulated in seedlings grown under red LED compared to white LED, whereas transcript levels of HO2 and CHLD were upregulated under blue LED. Overall, most porphyrin biosynthetic genes in the Fe-porphyrin branch remained almost constant or upregulated, while most genes in the Mg-porphyrin branch were downregulated. Expression levels of nuclear-encoded photosynthetic genes Lhcb and RbcS noticeably decreased after exposure to blue and red LEDs, compared to white LED. Our study suggests that specific wavelengths of LED greatly influence characteristics of growth in plants partly through altering the metabolic regulation of the porphyrin biosynthetic pathway, and possibly contribute to affect retrograde signaling.

The developmental and environmental regulation of gravitropic setpoint angle in Arabidopsis and bean

Root and shoot branches are major determinants of plant form and critical for the effective capture of resources below and above ground. These branches are often maintained at specific angles with respect to gravity, known as gravitropic set point angles (GSAs). We have previously shown that the mechanism permitting the maintenance of non-vertical GSAs is highly auxin-dependent and here we investigate the developmental and environmental regulation of root and shoot branch GSA. We show that nitrogen and phosphorous deficiency have opposing, auxin signalling-dependent effects on lateral root GSA in Arabidopsis: while low nitrate induces less vertical lateral root GSA, phosphate deficiency results in a more vertical lateral root growth angle, a finding that contrasts with the previously reported growth angle response of bean adventitious roots. We find that this root-class-specific discrepancy in GSA response to low phosphorus is mirrored by similar differences in growth angle response to auxin treatment between these root types. Finally we show that both shaded, low red/far-red light conditions and high temperature induce more vertical growth in Arabidopsis shoot branches. We discuss the significance of these findings in the context of efforts to improve crop performance via the manipulation of root and shoot branch growth angle.

Functional specialization of Nicotiana attenuata phytochromes in leaf development and flowering time

Phytochromes mainly function in photoautotrophic organisms to adjust growth in response to fluctuating light signals. The different isoforms of plant phytochromes often display both conserved and divergent roles, presumably to fine-tune plant responses to environmental signals and optimize fitness. Here we describe the distinct, yet partially redundant, roles of phytochromes NaPHYA, NaPHYB1 and NaPHYB2 in a wild tobacco species, Nicotiana attenuata using RNAi-silenced phytochrome lines. Consistent with results reported from other species, silencing the expression of NaPHYA or NaPHYB2 in N. attenuata had mild or no influence on plant development as long as NaPHYB1 was functional; whereas silencing the expression of NaPHYB1 alone strongly altered flowering time and leaf morphology. The contribution of NaPHYB2 became significant only in the absence of NaPHYB1; plants silenced for both NaPHYB1 and NaPHYB2 largely skipped the rosette-stage of growth to rapidly produce long, slender stalks that bore flowers early: hallmarks of the shade-avoidance responses. The phenotyping of phytochrome-silenced lines, combined with sequence and transcript accumulation analysis, suggest the independent functional diversification of the phytochromes, and a dominant role of NaPHYB1 and NaPHYB2 in N. attenuata's vegetative and reproductive development.

UV-A radiation effects on higher plants: Exploring the known unknown

Ultraviolet-A radiation (UV-A: 315-400nm) is a component of solar radiation that exerts a wide range of physiological responses in plants. Currently, field attenuation experiments are the most reliable source of information on the effects of UV-A. Common plant responses to UV-A include both inhibitory and stimulatory effects on biomass accumulation and morphology. UV-A effects on biomass accumulation can differ from those on root: shoot ratio, and distinct responses are described for different leaf tissues. Inhibitory and enhancing effects of UV-A on photosynthesis are also analysed, as well as activation of photoprotective responses, including UV-absorbing pigments. UV-A-induced leaf flavonoids are highly compound-specific and species-dependent. Many of the effects on growth and development exerted by UV-A are distinct to those triggered by UV-B and vary considerably in terms of the direction the response takes. Such differences may reflect diverse UV-perception mechanisms with multiple photoreceptors operating in the UV-A range and/or variations in the experimental approaches used. This review highlights a role that various photoreceptors (UVR8, phototropins, phytochromes and cryptochromes) may play in plant responses to UV-A when dose, wavelength and other conditions are taken into account.

Increasing carbon availability stimulates growth and secondary metabolites via modulation of phytohormones in winter wheat

Phytohormones play important roles in plant acclimation to changes in environmental conditions. However, their role in whole-plant regulation of growth and secondary metabolite production under increasing atmospheric CO2 concentrations ([CO2]) is uncertain but crucially important for understanding plant responses to abiotic stresses. We grew winter wheat (Triticum aestivum) under three [CO2] (170, 390, and 680 ppm) over 10 weeks, and measured gas exchange, relative growth rate (RGR), soluble sugars, secondary metabolites, and phytohormones including abscisic acid (ABA), auxin (IAA), jasmonic acid (JA), and salicylic acid (SA) at the whole-plant level. Our results show that, at the whole-plant level, RGR positively correlated with IAA but not ABA, and secondary metabolites positively correlated with JA and JA-Ile but not SA. Moreover, soluble sugars positively correlated with IAA and JA but not ABA and SA. We conclude that increasing carbon availability stimulates growth and production of secondary metabolites via up-regulation of auxin and jasmonate levels, probably in response to sugar-mediated signalling. Future low [CO2] studies should address the role of reactive oxygen species (ROS) in leaf ABA and SA biosynthesis, and at the transcriptional level should focus on biosynthetic and, in particular, on responsive genes involved in [CO2]-induced hormonal signalling pathways.

The acclimation of Tilia cordata stomatal opening in response to light, and stomatal anatomy to vegetational shade and its components

Stomatal anatomical traits and rapid responses to several components of visible light were measured in Tilia cordata Mill. seedlings grown in an open, fully sunlit field (C-set), or under different kinds of shade. The main questions were: (i) stomatal responses to which visible light spectrum regions are modified by growth-environment shade and (ii) which separate component of vegetational shade is most effective in eliciting the acclimation effects of the full vegetational shade. We found that stomatal opening in response to red or green light did not differ between the plants grown in the different environments. Stomatal response to blue light was increased (in comparison with that of C-set) in the leaves grown in full vegetational shade (IABW-set), in attenuated UVAB irradiance (AB-set) or in decreased light intensity (neutral shade) plus attenuated UVAB irradiance (IAB-set). In all sets, the addition of green light-two or four times stronger-into induction light barely changed the rate of the blue-light-stimulated stomatal opening. In the AB-set, stomatal response to blue light equalled the strong IABW-set response. In attenuated UVB-grown leaves, stomatal response fell midway between IABW- and C-set results. Blue light response by neutral shade-grown leaves did not differ from that of the C-set, and the response by the IAB-set did not differ from that of the AB-set. Stomatal size was not modified by growth environments. Stomatal density and index were remarkably decreased only in the IABW- and IAB-sets. It was concluded that differences in white light responses between T. cordata leaves grown in different light environments are caused only by their different blue light response. Differences in stomatal sensitivity are not dependent on altered stomatal anatomy. Attenuated UVAB irradiance is the most efficient component of vegetational shade in stimulating acclimation of stomata, whereas decreased light intensity plays a minor role.

Approaches to Study Light Effects on Brassinosteroid Sensitivity

Light perception and hormone signaling in plants are likely connected at multiple points. Light conditions, perceived by photoreceptors, control plant responses by altering hormone concentration, tissue sensitivity, or a combination of both. Whereas it is relatively straightforward to assess the light effects on hormone levels, hormone sensitivity is subjected to interpretation. In Arabidopsis thaliana seedlings, hypocotyl length is strongly affected by light conditions. As hypocotyl elongation also depends on brassinosteroids (BRs), assaying this response provides a valuable and easy way to measure the responsiveness of seedlings to BRs and the impact of light. We describe a simple protocol to evaluate the responsiveness of hypocotyls to commercial BRs and/or BR inhibitors under a range of light conditions. These assays can be used to establish whether light affects BR sensitivity or whether BRs affect light sensitivity. Overall, our protocol can be easily applied for deetiolation (under polychromatic or monochromatic light) and simulated shade treatments combined with BR treatments.

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.

Hormonal Regulation in Shade Avoidance

At high vegetation density, shade-intolerant plants sense a reduction in the red (660 nm) to far-red (730 nm) light ratio (R/FR) in addition to a general reduction in light intensity. These light signals trigger a spectrum of morphological changes manifested by growth of stem-like tissue (hypocotyl, petiole, etc.) instead of harvestable organs (leaves, fruits, seeds, etc.)-namely, shade avoidance syndrome (SAS). Common phenotypical changes related to SAS are changes in leaf hyponasty, an increase in hypocotyl and internode elongation and extended petioles. Prolonged shade exposure leads to early flowering, less branching, increased susceptibility to insect herbivory, and decreased seed yield. Thus, shade avoidance significantly impacts on agronomic traits. Many genetic and molecular studies have revealed that phytochromes, cryptochromes and UVR8 (UV-B photoreceptor protein) monitor the changes in light intensity under shade and regulate the stability or activity of phytochrome-interacting factors (PIFs). PIF-governed modulation of the expression of auxin biosynthesis, transporter and signaling genes is the major driver for shade-induced hypocotyl elongation. Besides auxin, gibberellins, brassinosteroids, and ethylene are also required for shade-induced hypocotyl or petiole elongation growth. In leaves, accumulated auxin stimulates cytokinin oxidase expression to break down cytokinins and inhibit leaf growth. In the young buds, shade light promotes the accumulation of abscisic acid to repress branching. Shade light also represses jasmonate- and salicylic acid-induced defense responses to balance resource allocation between growth and defense. Here we will summarize recent findings relating to such hormonal regulation in SAS in Arabidopsis thaliana, Brassica rapa, and certain crops.

Integration of light and circadian signals that regulate chloroplast transcription by a nuclear-encoded sigma factor

We investigated the signalling pathways that regulate chloroplast transcription in response to environmental signals. One mechanism controlling plastid transcription involves nuclear-encoded sigma subunits of plastid-encoded plastid RNA polymerase. Transcripts encoding the sigma factor SIG5 are regulated by light and the circadian clock. However, the extent to which a chloroplast target of SIG5 is regulated by light-induced changes in SIG5 expression is unknown. Moreover, the photoreceptor signalling pathways underlying the circadian regulation of chloroplast transcription by SIG5 are unidentified. We monitored the regulation of chloroplast transcription in photoreceptor and sigma factor mutants under controlled light regimes in Arabidopsis thaliana. We established that a chloroplast transcriptional response to light intensity was mediated by SIG5; a chloroplast transcriptional response to the relative proportions of red and far red light was regulated by SIG5 through phytochrome and photosynthetic signals; and the circadian regulation of chloroplast transcription by SIG5 was predominantly dependent on blue light and cryptochrome. Our experiments reveal the extensive integration of signals concerning the light environment by a single sigma factor to regulate chloroplast transcription. This may originate from an evolutionarily ancient mechanism that protects photosynthetic bacteria from high light stress, which subsequently became integrated with higher plant phototransduction networks.

Functional Characterization of the Maize Phytochrome-Interacting Factors PIF4 and PIF5

Phytochrome-interacting factors (PIFs) play important roles in photomorphogenesis, the shade avoidance response, and other aspects of plant growth and development. PIF family proteins have been well-studied in Arabidopsis thaliana, but little is known about their physiological functions and molecular mechanisms in maize (Zea mays). In this study, we investigated the physiological functions of ZmPIF4 and ZmPIF5, two highly conserved members of the PIF gene family. RT-qPCR and western blot analyses revealed that ZmPIF4 and ZmPIF5 expression and ZmPIF4 and ZmPIF5 levels peak at night and remain low during the day. Overexpression of ZmPIF4 and ZmPIF5 in Arabidopsis partially rescued the reduced hypocotyl elongation and defective response to gravity in pif1 pif3 pif4 pif5 quadruple mutants (pifq). In addition, under high red: far-red light conditions, Arabidopsis lines overexpressing ZmPIF4 exhibited a constitutive shade avoidance response, including early flowering, slender leaves and inflorescences, plant lodging and precocious leaf senescence. Furthermore, ZmPIF4 physically interacted with the Arabidopsis DELLA protein REPRESSOR OF GA1-3 (RGA), indicating a potential interaction between ZmPIF4 and gibberellin signaling pathway on plant growth. Taken together, our results revealed that ZmPIF4 and ZmPIF5 are functionally conserved proteins that may play conserved roles in the response to phytochrome signaling in plants.

HIGHLIGHTS

In this study, the functions of ZmPIF4 and ZmPIF5 were characterized by expression in Arabidopsis, revealing conserved roles of PIF family proteins in photomorphogenesis and the shade avoidance response in land plants.

Clathrin regulates blue light-triggered lateral auxin distribution and hypocotyl phototropism in Arabidopsis

Phototropism is the process by which plants grow towards light in order to maximize the capture of light for photosynthesis, which is particularly important for germinating seedlings. In Arabidopsis, hypocotyl phototropism is predominantly triggered by blue light (BL), which has a profound effect on the establishment of asymmetric auxin distribution, essential for hypocotyl phototropism. Two auxin efflux transporters ATP-binding cassette B19 (ABCB19) and PIN-formed 3 (PIN3) are known to mediate the effect of BL on auxin distribution in the hypocotyl, but the details for how BL triggers PIN3 lateralization remain poorly understood. Here, we report a critical role for clathrin in BL-triggered, PIN3-mediated asymmetric auxin distribution in hypocotyl phototropism. We show that unilateral BL induces relocalization of clathrin in the hypocotyl. Loss of clathrin light chain 2 (CLC2) and CLC3 affects endocytosis and lateral distribution of PIN3 thereby impairing BL-triggered establishment of asymmetric auxin distribution and consequently, phototropic bending. Conversely, auxin efflux inhibitors N-1-naphthylphthalamic acid and 2,3,5-triiodobenzoic acid affect BL-induced relocalization of clathrin, endocytosis and lateralization of PIN3 as well as asymmetric distribution of auxin. These results together demonstrate an important interplay between auxin and clathrin function that dynamically regulates BL-triggered hypocotyl phototropism in Arabidopsis.

Loss of photosynthetic efficiency in the shade. An Achilles heel for the dense modern stands of our most productive C4 crops?

The wild progenitors of major C₄ crops grew as individuals subjected to little shading. Today they are grown in dense stands where most leaves are shaded. Do they maintain photosynthetic efficiency in these low light conditions produced by modern cultivation? The apparent maximum quantum yield of CO₂ assimilation (ΦCO2max,app), a key determinant of light-limited photosynthesis, has not been systematically studied in field stands of C₄ crops. ΦCO2max,app was derived from the initial slope of the response of leaf CO₂ uptake (A) to photon flux (Q). Leaf fractional light absorptance (α) was measured to determine the absolute maximum quantum yield of CO₂ assimilation on an absorbed light basis (ΦCO2max,abs). Light response curves were determined on sun and shade leaves of 49 field plants of Miscanthus × giganteus and Zea mays following canopy closure. ΦCO2max,app and ΦCO2max,abs declined significantly by 15-27% (P<0.05) with canopy depth. Experimentally, leaf age was shown unlikely to cause this loss. Modeling canopy CO₂ assimilation over diurnal courses suggested that the observed decline in ΦCO2max,app with canopy depth costs 10% of potential carbon gain. Overcoming this limitation could substantially increase the productivity of major C₄ crops.

FAR-RED INSENSITIVE 219/JAR1 Contributes to Shade Avoidance Responses of Arabidopsis Seedlings by Modulating Key Shade Signaling Components

To receive an ample amount of light, plants use elongation growth in response to vegetation shade. The combined interaction of light and hormones, including jasmonic acid (JA) signaling controls this elongation. However, the detailed molecular mechanisms underlying the response are still emerging. FAR-RED INSENSITIVE 219/JASMONATE RESISTANCE 1 (FIN219/JAR1), a cytoplasmic localized JA-conjugating enzyme, integrates far-red light and JA signaling. Here, we report that FIN219/JAR1 negatively regulates shade-induced hypocotyl elongation and gene expression in Arabidopsis seedlings in response to shade. In turn, simulated shade reduces FIN219 protein accumulation. Analysis of phyA 211 fin219-2 double mutants indicated that FIN219 and phyA are synergistic in regulating shade-induced hypocotyl elongation and gene expression. Moreover, FIN219 differentially affected the expression of the shade-signaling bHLH factors PIF5 and PAR1, thereby increasing the expression of the auxin-response genes IAA29 and SAUR68 on exposure to shade. Furthermore, the protein level of CONSTITUTIVE PHOTOMORPHOGENIC 1 (COP1) was affected in both fin219 mutants and overexpression lines as compared with the wild type under shade. Intriguingly, ectopic expression of FIN219 inhibited the nuclear accumulation of COP1 in response to shade. Further co-immunoprecipitation studies revealed that FIN219 interacted with COP1 and phyA under shade. Therefore, FIN219/JAR1 may play a vital role in modulating the Arabidopsis response to simulated shade via multiple layers of molecular mechanisms.

Morphology and Hydraulic Architecture of Vitis vinifera L. cv. Syrah and Torrontés Riojano Plants Are Unaffected by Variations in Red to Far-Red Ratio

Plants have evolved an array of specific photoreceptors to acclimate to the light environment. By sensing light signals, photoreceptors modulate plant morphology, carbon- and water-physiology, crop yield and quality of harvestable organs, among other responses. Many cultural practices and crop management decisions alter light quantity and quality perceived by plants cultivated in the field. Under full sunlight, phytochromes perceive high red to far red ratios (R:FR; 1.1), whereas overhead or lateral low R:FR (below 1.1) are sensed in the presence of plant shade or neighboring plants, respectively. Grapevine is one of the most important fruit crops in the world. To date, studies on grapevine response to light focused on different Photosynthetic Active Radiation (PAR) levels; however, limited data exist about its response to light quality. In this study we aimed to investigate morphological, biochemical, and hydraulic responses of Vitis vinifera to variations in R:FR. Therefore, we irradiated Syrah and Torrontés Riojano plants, grown in a glasshouse, with lateral FR light (low lateral R:FR treatment), while others, that were kept as controls, were not irradiated (ambient lateral R:FR treatment). In response to the low lateral R:FR treatment, grapevine plants did not display any of the SAS morphological markers (i.e. stem length, petiole length and angle, number of lateral shoots) in any of the cultivars assessed, despite an increase in gibberelins and auxin concentrations in leaf tissues. Low lateral R:FR did not affect dry matter partitioning, water-related traits (stomata density and index, wood anatomy), or water-related physiology (plant conductance, transpiration rate, stem hydraulic conductivity, stomatal conductance). None of the Vitis vinifera varieties assessed displayed the classical morphological and hydraulic responses associated to SAS induced by phytochromes. We discuss these results in the context of natural grapevine environment and agronomical relevance.