Ethylene-induced hyponastic growth in Arabidopsis thaliana is controlled by ERECTA

Ethylene signaling modulates air humidity responses in plants

Air humidity significantly impacts plant physiology. However, the upstream elements that mediate humidity sensing and adaptive responses in plants remain largely unexplored. In this study, we define high humidity-induced cellular features of Arabidopsis plants and take a quantitative phosphoproteomics approach to obtain a high humidity-responsive landscape of membrane proteins, which we reason are likely the early checkpoints of humidity signaling. We found that a brief high humidity exposure (i.e., 0.5 h) is sufficient to trigger extensive changes in membrane protein abundance and phosphorylation. Enrichment analysis of differentially regulated proteins reveals high humidity-sensitive processes such as 'transmembrane transport', 'response to abscisic acid', and 'stomatal movement'. We further performed a targeted screen of mutants, in which high humidity-responsive pathways/proteins are disabled, to uncover genes mediating high humidity sensitivity. Interestingly, ethylene pathway mutants (i.e., ein2 and ein3eil1) display a range of altered responses, including hyponasty, reactive oxygen species level, and responsive gene expression, to high humidity. Furthermore, we observed a rapid induction of ethylene biosynthesis genes and ethylene evolution after high humidity treatment. Our study sheds light on the potential early signaling events in humidity perception, a fundamental but understudied question in plant biology, and reveals ethylene as a key modulator of high humidity responses in plants.

Multifaceted roles of the ERECTA family in plant organ morphogenesis

Receptor-like kinases (RLKs) can participate in multiple signalling pathways and are considered one of the most critical components of the early events of intercellular signalling. As an RLK, the ERECTA family (ERf), which comprises ERECTA (ER), ERECTA-Like1 (ERL1), and ERECTA-Like2 (ERL2) in Arabidopsis, regulates multiple signalling pathways in plant growth and development. Despite its indispensability, detailed information on ERf-manipulated signalling pathways remains elusive. In this review, we attempt to summarize the essential roles of the ERf in plant organ morphogenesis, including shoot apical meristem, stem, and reproductive organ development.

Phosphatidic Acid in Plant Hormonal Signaling: From Target Proteins to Membrane Conformations

Cells sense a variety of extracellular signals balancing their metabolism and physiology according to changing growth conditions. Plasma membranes are the outermost informational barriers that render cells sensitive to regulatory inputs. Membranes are composed of different types of lipids that play not only structural but also informational roles. Hormones and other regulators are sensed by specific receptors leading to the activation of lipid metabolizing enzymes. These enzymes generate lipid second messengers. Among them, phosphatidic acid (PA) is a well-known intracellular messenger that regulates various cellular processes. This lipid affects the functional properties of cell membranes and binds to specific target proteins leading to either genomic (affecting transcriptome) or non-genomic responses. The subsequent biochemical, cellular and physiological reactions regulate plant growth, development and stress tolerance. In the present review, we focus on primary (genome-independent) signaling events triggered by rapid PA accumulation in plant cells and describe the functional role of PA in mediating response to hormones and hormone-like regulators. The contributions of individual lipid signaling enzymes to the formation of PA by specific stimuli are also discussed. We provide an overview of the current state of knowledge and future perspectives needed to decipher the mode of action of PA in the regulation of cell functions.

2D morphometric analysis of Arabidopsis thaliana nuclei reveals characteristic profiles of different cell types and accessions

Functional changes of cells upon developmental switches and in response to environmental cues are often reflected in nuclear phenotypes, showing distinctive chromatin states corresponding to transcriptional changes. Such characteristic nuclear shapes have been microscopically monitored and can be quantified after differential staining of euchromatin and heterochromatin domains. Here, we examined several nuclear parameters (size, DNA content, DNA density, chromatin compaction, relative heterochromatin fraction (RHF), and number of chromocenters) in relation to spatial distribution of genes and transposon elements (TEs), using standard 2D fluorescence microscopy. We provide nuclear profiles for different cell types and different accessions of Arabidopsis thaliana. A variable, yet significant, fraction of TEs was found outside chromocenters in all cell types, except for guard cells. The latter cell type features nuclei with the highest level of chromatin compaction, while their chromocenters seem to contain gene-rich regions. The highest number of parameter correlations was found in the accession Cvi, whereas Ler showed only few correlations. This may point at differences in phenotype robustness between accessions. The significantly high association of NOR chromocenters in accessions Ws and Cvi corresponds to their low RHF level.

The Receptor-Like Kinase ERECTA Confers Improved Water Use Efficiency and Drought Tolerance to Poplar via Modulating Stomatal Density

Poplar is one of the most important tree species in the north temperate zone, but poplar plantations are quite water intensive. We report here that CaMV 35S promoter-driven overexpression of the PdERECTA gene, which is a member of the LRR-RLKs family from Populus nigra × (Populus deltoides × Populus nigra), improves water use efficiency and enhances drought tolerance in triploid white poplar. PdERECTA localizes to the plasma membrane. Overexpression plants showed lower stomatal density and larger stomatal size. The abaxial stomatal density was 24-34% lower and the stomatal size was 12-14% larger in overexpression lines. Reduced stomatal density led to a sharp restriction of transpiration, which was about 18-35% lower than the control line, and instantaneous water use efficiency was around 14-63% higher in overexpression lines under different conditions. These phenotypic changes led to increased drought tolerance. PdERECTA overexpression plants not only survived longer after stopping watering but also performed better when supplied with limited water, as they had better physical and photosynthesis conditions, faster growth rate, and higher biomass accumulation. Taken together, our data suggest that PdERECTA can alter the development pattern of stomata to reduce stomatal density, which then restricts water consumption, conferring enhanced drought tolerance to poplar. This makes PdERECTA trees promising candidates for establishing more water use efficient plantations.

A Peptide Pair Coordinates Regular Ovule Initiation Patterns with Seed Number and Fruit Size

Ovule development in Arabidopsis thaliana involves pattern formation, which ensures that ovules are regularly arranged in the pistils to reduce competition for nutrients and space. Mechanisms underlying pattern formation in plants, such as phyllotaxis, flower morphogenesis, or lateral root initiation, have been extensively studied, and genes controlling the initiation of ovules have been identified. However, the fundamental patterning mechanism that determines the spacing of ovule anlagen within the placenta remained unexplored. Using natural variation analysis combined with quantitative trait locus analysis, we found that the spacing of ovules in the developing gynoecium and fruits is controlled by two secreted peptides, EPFL2 and EPFL9 (also known as Stomagen), and their receptors from the ERECTA (ER) family that act from the carpel wall and the placental tissue. We found that a signaling pathway controlled by EPFL9 acting from the carpel wall through the LRR-receptor kinases ER, ERL1, and ERL2 promotes fruit growth. Regular spacing of ovules depends on EPFL2 expression in the carpel wall and in the inter-ovule spaces, where it acts through ERL1 and ERL2. Loss of EPFL2 signaling results in shorter gynoecia and fruits and irregular spacing of ovules or even ovule twinning. We propose that the EPFL2 signaling module evolved to control the initiation and regular, equidistant spacing of ovule primordia, which may serve to minimize competition between seeds or facilitate equal resource allocation. Together, EPFL2 and EPFL9 help to coordinate ovule patterning and thereby seed number with gynoecium and fruit growth through a set of shared receptors.

Genetic and signalling pathways of dry fruit size: targets for genome editing-based crop improvement

Fruit is seed-bearing structures specific to angiosperm that form from the gynoecium after flowering. Fruit size is an important fitness character for plant evolution and an agronomical trait for crop domestication/improvement. Despite the functional and economic importance of fruit size, the underlying genes and mechanisms are poorly understood, especially for dry fruit types. Improving our understanding of the genomic basis for fruit size opens the potential to apply gene-editing technology such as CRISPR/Cas to modulate fruit size in a range of species. This review examines the genes involved in the regulation of fruit size and identifies their genetic/signalling pathways, including the phytohormones, transcription and elongation factors, ubiquitin-proteasome and microRNA pathways, G-protein and receptor kinases signalling, arabinogalactan and RNA-binding proteins. Interestingly, different plant taxa have conserved functions for various fruit size regulators, suggesting that common genome edits across species may have similar outcomes. Many fruit size regulators identified to date are pleiotropic and affect other organs such as seeds, flowers and leaves, indicating a coordinated regulation. The relationships between fruit size and fruit number/seed number per fruit/seed size, as well as future research questions, are also discussed.

Celery and Celeriac: A Critical View on Present and Future Breeding

Cultivated for the crispy petioles and round, fleshy, and flavored hypocotyl celery and celeriac have over two centuries of breeding history in Europe. In this review paper we summarized the most recent advances touching when necessary the historical context of celery and celeriac breeding. In the post genomic era of research, the genome sequence of celery is only partially available. We comprised however in this paper the most important aspects of celery genetics that are available today and have applicability in celery modern cultivars development. We discussed the problems and traits that drive the main celery and celeriac breeding goals, like hybrid seed production, disease resistance, and interesting enlarged hypocotyl and petiole characteristics. Besides the classical breeding traits we covered the potential of integration of existing cultivars as sources for consumer oriented traits like nutraceuticals and health promoting substances. Sustainability is a subject that is continuously growing in popularity and we looked at the genetic base of celery and celeriac that makes them sources for abiotic stress resistance and candidates for phytoremediation. We explored the fundamental concepts gained in various fields of celery and related species research, as resources for future improvement of celery and celeriac germplasm. We forecast what the next years will bring to Apium breeding.

Stem development through vascular tissues: EPFL-ERECTA family signaling that bounces in and out of phloem

Plant cells communicate with each other using a variety of signaling molecules. Recent studies have revealed that various types of secreted peptides, as well as phytohormones known since long ago, mediate cell-cell communication in diverse contexts of plant life. These peptides affect cellular activities, such as proliferation and cell fate decisions, through their perception by cell surface receptors located on the plasma membrane of target cells. ERECTA (ER), an Arabidopsis thaliana receptor kinase gene, was first identified as a stem growth regulator, and since then an increasing number of studies have shown that ER is involved in a wide range of developmental and physiological processes. In particular, molecular functions of ER have been extensively studied in stomatal patterning. Furthermore, the importance of ER signaling in vascular tissues of inflorescence stems, especially in phloem cells, has recently been highlighted. In this review article, first we briefly summarize the history of ER research including studies on stomatal development, then introduce ER functions in vascular tissues, and discuss its interactions with phytohormones and other receptor kinase signaling pathways. Future questions and challenges will also be addressed.

Time-Lapse Imaging to Examine the Growth Kinetics of Arabidopsis Seedlings in Response to Ethylene

Ethylene is well known to inhibit the growth of dark-grown eudicot seedlings. Most studies examine this inhibition after several days of exposure to ethylene. However, such end-point analysis misses transient responses and the dynamic nature of growth regulation. Here, high-resolution, time-lapse imaging is described as a method to gather data about ethylene growth kinetics and movement responses of the hypocotyls of dark-grown seedlings of Arabidopsis thaliana. These methods allow for the characterization of short-term kinetic responses and can be modified for the analysis of roots and seedlings from other species.

A Comparative Study of Ethylene Emanation upon Nitrogen Deficiency in Natural Accessions of Arabidopsis thaliana

An original approach to develop sustainable agriculture with less nitrogen fertilizer inputs is to tackle the cross-talk between nitrogen nutrition and plant growth regulators. In particular the gaseous hormone, ethylene, is a prime target for that purpose. The variation of ethylene production in natural accessions of the model species Arabidopsis thaliana was explored in response to the nitrate supply. Ethylene was measured with a laser-based photoacoustic detector. First, experimental conditions were established with Columbia-0 (Col-0) accession, which was grown in vitro on horizontal plates across a range of five nitrate concentrations (0.5, 1, 2.5, 5, or 10 mM). The concentrations of 1 and 10 mM nitrate were retained for further characterization. Along with a decrease of total dry biomass and higher biomass allocation to the roots, the ethylene production was 50% more important at 1 mM than at 10 mM nitrate. The total transcript levels of 1-AMINOCYCLOPROPANE-1-CARBOXYLIC ACID SYNTHASES (ACS) in roots and those of ACC OXIDASES (ACO) in shoots increased by 100% between the same treatments. This was mainly due to higher transcript levels of ACS6 and of ACO2 and ACO4 respectively. The assumption was that during nitrogen deficiency, the greater biomass allocation in favor of the roots was controlled by ethylene being released in the shoots after conversion of ACC originating from the roots. Second, biomass and ethylene productions were measured in 20 additional accessions. Across all accessions, the total dry biomass and ethylene production were correlated negatively at 1 mM but positively at 10 mM nitrate. Furthermore, polymorphism was surveyed in ACC and ethylene biosynthesis genes and gene products among accessions. Very few substitutions modifying the amino acids properties in conserved motifs of the enzymes were found in the accessions. Natural variation of ethylene production could be further explored to improve Nitrogen Use Efficiency (NUE), in particular by manipulating features like the biomass production and the timing of senescence upon nitrogen limitation.

Analysis of Network Topologies Underlying Ethylene Growth Response Kinetics

Most models for ethylene signaling involve a linear pathway. However, measurements of seedling growth kinetics when ethylene is applied and removed have resulted in more complex network models that include coherent feedforward, negative feedback, and positive feedback motifs. The dynamical responses of the proposed networks have not been explored in a quantitative manner. Here, we explore (i) whether any of the proposed models are capable of producing growth-response behaviors consistent with experimental observations and (ii) what mechanistic roles various parts of the network topologies play in ethylene signaling. To address this, we used computational methods to explore two general network topologies: The first contains a coherent feedforward loop that inhibits growth and a negative feedback from growth onto itself (CFF/NFB). In the second, ethylene promotes the cleavage of EIN2, with the product of the cleavage inhibiting growth and promoting the production of EIN2 through a positive feedback loop (PFB). Since few network parameters for ethylene signaling are known in detail, we used an evolutionary algorithm to explore sets of parameters that produce behaviors similar to experimental growth response kinetics of both wildtype and mutant seedlings. We generated a library of parameter sets by independently running the evolutionary algorithm many times. Both network topologies produce behavior consistent with experimental observations, and analysis of the parameter sets allows us to identify important network interactions and parameter constraints. We additionally screened these parameter sets for growth recovery in the presence of sub-saturating ethylene doses, which is an experimentally-observed property that emerges in some of the evolved parameter sets. Finally, we probed simplified networks maintaining key features of the CFF/NFB and PFB topologies. From this, we verified observations drawn from the larger networks about mechanisms underlying ethylene signaling. Analysis of each network topology results in predictions about changes that occur in network components that can be experimentally tested to give insights into which, if either, network underlies ethylene responses.

Insights into the Role of the Berry-Specific Ethylene Responsive Factor VviERF045

During grape ripening, numerous transcriptional and metabolic changes are required in order to obtain colored, sweet, and flavored berries. There is evidence that ethylene, together with other signals, plays an important role in triggering the onset of ripening. Here, we report the functional characterization of a berry-specific Ethylene Responsive Factor (ERF), VviERF045, which is induced just before véraison and peaks at ripening. Phylogenetic analysis revealed it is close to the SHINE clade of ERFs, factors involved in the regulation of wax biosynthesis and cuticle morphology. Transgenic grapevines lines overexpressing VviERF045 were obtained, in vitro propagated, phenotypically characterized, and analyzed for the content of specific classes of metabolites. The effect of VviERF045 was correlated with the level of transgene expression, with high-expressing lines showing stunted growth, discolored and smaller leaves, and a lower level of chlorophylls and carotenoids. One line with intermediate expression, L15, was characterized at the transcriptomic level and showed 573 differentially expressed genes compared to wild type plants. Microscopy and gene expression analyses point toward a major role of VviERF045 in epidermis patterning by acting on waxes and cuticle. They also indicate that VviERF045 affects phenolic secondary metabolism and induces a reaction resembling a plant immune response with modulation of receptor like-kinases and pathogen related genes. These results suggest also a possible role of this transcription factor in berry ripening, likely related to changes in epidermis and cuticle of the berry, cell expansion, a decrease in photosynthetic capacity, and the activation of several defense related genes as well as from the phenylpropanoid metabolism. All these processes occur in the berry during ripening.

Ethylene-Mediated Regulation of A2-Type CYCLINs Modulates Hyponastic Growth in Arabidopsis

Upward leaf movement (hyponastic growth) is frequently observed in response to changing environmental conditions and can be induced by the phytohormone ethylene. Hyponasty results from differential growth (i.e. enhanced cell elongation at the proximal abaxial side of the petiole relative to the adaxial side). Here, we characterize Enhanced Hyponasty-d, an activation-tagged Arabidopsis (Arabidopsis thaliana) line with exaggerated hyponasty. This phenotype is associated with overexpression of the mitotic cyclin CYCLINA2;1 (CYCA2;1), which hints at a role for cell divisions in regulating hyponasty. Indeed, mathematical analysis suggested that the observed changes in abaxial cell elongation rates during ethylene treatment should result in a larger hyponastic amplitude than observed, unless a decrease in cell proliferation rate at the proximal abaxial side of the petiole relative to the adaxial side was implemented. Our model predicts that when this differential proliferation mechanism is disrupted by either ectopic overexpression or mutation of CYCA2;1, the hyponastic growth response becomes exaggerated. This is in accordance with experimental observations on CYCA2;1 overexpression lines and cyca2;1 knockouts. We therefore propose a bipartite mechanism controlling leaf movement: ethylene induces longitudinal cell expansion in the abaxial petiole epidermis to induce hyponasty and simultaneously affects its amplitude by controlling cell proliferation through CYCA2;1. Further corroborating the model, we found that ethylene treatment results in transcriptional down-regulation of A2-type CYCLINs and propose that this, and possibly other regulatory mechanisms affecting CYCA2;1, may contribute to this attenuation of hyponastic growth.

Involvement of cotton gene GhFPF1 in the regulation of shade avoidance responses in Arabidopsis thaliana

Phytochrome system perceives the reduction in the ratio of red to far-red light when plants are grown under dense canopy. This signal, regarded as a warning of competition, will trigger a series of phenotypic changes to avoid shade. Progress has been made for several phytochrome signaling intermediates acting as positive regulators of accelerated elongation growth and promotion of flowering in shade-avoidance has been identified. Recently, a FPF1 homolog GhFPF1 was identified in upland cotton. Our data supported that transgenic Arabidopsis of over-expressing GhFPF1 displayed a constitutive shade-avoiding phenotype resembling phyB mutants in several respects such as accelerated elongation of hypocotyl and petioles, upward of leaf movement, and promoted flowering. In this addendum, by dissection of GhFPF1 acting as a component of shade-avoidance responses we suppose that GhFPF1 might influence the timing of the floral transition independently of shade-mediated early flowering. Furthermore, the opposite changes of IAA content in transgenic leaves and stems suggested that alteration of IAA storage and release took place during shade-avoidance responses.

Diverse roles of ERECTA family genes in plant development

Multiple receptor-like kinases (RLKs) enable intercellular communication that coordinates growth and development of plant tissues. ERECTA family receptors (ERfs) are an ancient family of leucine-rich repeat RLKs that in Arabidopsis consists of three genes: ERECTA, ERL1, and ERL2. ERfs sense secreted cysteine-rich peptides from the EPF/EPFL family and transmit the signal through a MAP kinase cascade. This review discusses the functions of ERfs in stomata development, in regulation of longitudinal growth of aboveground organs, during reproductive development, and in the shoot apical meristem. In addition the role of ERECTA in plant responses to biotic and abiotic factors is examined. Elena D. Shpak (Corresponding author).

The receptor-like kinase ERECTA contributes to the shade-avoidance syndrome in a background-dependent manner

BACKGROUND AND AIMS

Plants growing at high densities perceive a decrease in the red to far-red (R/FR) ratio of incoming light. These changes in light quality trigger a suite of responses collectively known as the shade-avoidance syndrome (SAS) including hypocotyl and stem elongation, inhibition of branching and acceleration of flowering.

METHODS

Quantitative trait loci (QTLs) were mapped for hypocotyl length to end-of-day far-red (EOD), a simulated shade-avoidance response, in recombinant inbred line (RIL) populations of Arabidopsis thaliana seedlings, derived from Landsberg erecta (Ler) and three accessions (Columbia, Col; Nossen, No-0; and Cape Verde Islands, Cvi-0).

KEY RESULTS

Five loci were identified as being responsible for the EOD response, with a positive contribution of Ler alleles on the phenotype independently of the RIL population. Quantitative complementation analysis and transgenic lines showed that PHYB is the candidate gene for EODRATIO5 in the Ler × Cvi-0 RIL population, but not for two co-localized QTLs, EODRATIO1 and EODRATIO2 mapped in the Ler × No-0 and Ler × Col RIL populations, respectively. The ERECTA gene was also implicated in the SAS in a background-dependent manner. For hypocotyl length EOD response, a positive contribution of erecta alleles was found in Col and Van-0, but not in Ler, Cvi-0, Hir-1 or Ws. Furthermore, pleiotropic effects of ERECTA in the EOD response were also detected for petiole and lamina elongation, hyponastic growth, and flowering time.

CONCLUSIONS

The results show that the analysis of multiple mapping populations leads to a better understanding of the SAS genetic architecture. Moreover, the background- and trait-dependent contribution of ERECTA in the SAS suggest that its function in shaded natural environments may be relevant for some populations in different phases of plant development. It is proposed that ERECTA is involved in canalization processes buffering the genetic variation of the SAS against environmental light fluctuations.

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

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

Natural variation identifies multiple loci controlling petal shape and size in Arabidopsis thaliana

Natural variation in organ morphologies can have adaptive significance and contribute to speciation. However, the underlying allelic differences responsible for variation in organ size and shape remain poorly understood. We have utilized natural phenotypic variation in three Arabidopsis thaliana ecotypes to examine the genetic basis for quantitative variation in petal length, width, area, and shape. We identified 23 loci responsible for such variation, many of which appear to correspond to genes not previously implicated in controlling organ morphology. These analyses also demonstrated that allelic differences at distinct loci can independently affect petal length, width, area or shape, suggesting that these traits behave as independent modules. We also showed that ERECTA (ER), encoding a leucine-rich repeat (LRR) receptor-like serine-threonine kinase, is a major effect locus determining petal shape. Allelic variation at the ER locus was associated with differences in petal cell proliferation and concomitant effects on petal shape. ER has been previously shown to be required for regulating cell division and expansion in other contexts; the ER receptor-like kinase functioning to also control organ-specific proliferation patterns suggests that allelic variation in common signaling components may nonetheless have been a key factor in morphological diversification.

Sensing the environment: key roles of membrane-localized kinases in plant perception and response to abiotic stress

Adverse environmental conditions have negative effects on plant growth and development. Receptor proteins on the plasma membrane sense various environmental stimuli and transduce them to downstream intra- and intercellular signalling networks. Receptor-like kinases (RLKs) play important roles in perceiving the extracellular ligands and activating the downstream pathway via phosphorylation of intracellular serine/threonine kinase domains. The Arabidopsis genome possesses >600 RLK-encoding genes, some of which are implicated in the perception of environmental signals during the life cycle of the sessile plants. Histidine kinases are also membrane-localized kinases and perceive osmotic stress and plant hormones. In this review, we focus on the RLKs and histidine kinases that play a role in plant response to abiotic stresses. We summarize our recent understanding of their specific roles in stress responses and absicisic acid (ABA) regulation. Elucidation of the functions of these kinases in the osmotic stress response will provide a better understanding of stress-sensing mechanisms in plants and help to identify potential candidate genes for genetic engineering of improved stress-tolerant crops.

A comparative study of ethylene growth response kinetics in eudicots and monocots reveals a role for gibberellin in growth inhibition and recovery

Time-lapse imaging of dark-grown Arabidopsis (Arabidopsis thaliana) hypocotyls has revealed new aspects about ethylene signaling. This study expands upon these results by examining ethylene growth response kinetics of seedlings of several plant species. Although the response kinetics varied between the eudicots studied, all had prolonged growth inhibition for as long as ethylene was present. In contrast, with continued application of ethylene, white millet (Panicum miliaceum) seedlings had a rapid and transient growth inhibition response, rice (Oryza sativa 'Nipponbare') seedlings had a slow onset of growth stimulation, and barley (Hordeum vulgare) had a transient growth inhibition response followed, after a delay, by a prolonged inhibition response. Growth stimulation in rice correlated with a decrease in the levels of rice ETHYLENE INSENSTIVE3-LIKE2 (OsEIL2) and an increase in rice F-BOX DOMAIN AND LRR CONTAINING PROTEIN7 transcripts. The gibberellin (GA) biosynthesis inhibitor paclobutrazol caused millet seedlings to have a prolonged growth inhibition response when ethylene was applied. A transient ethylene growth inhibition response has previously been reported for Arabidopsis ethylene insensitive3-1 (ein3-1) eil1-1 double mutants. Paclobutrazol caused these mutants to have a prolonged response to ethylene, whereas constitutive GA signaling in this background eliminated ethylene responses. Sensitivity to paclobutrazol inversely correlated with the levels of EIN3 in Arabidopsis. Wild-type Arabidopsis seedlings treated with paclobutrazol and mutants deficient in GA levels or signaling had a delayed growth recovery after ethylene removal. It is interesting to note that ethylene caused alterations in gene expression that are predicted to increase GA levels in the ein3-1 eil1-1 seedlings. These results indicate that ethylene affects GA levels leading to modulation of ethylene growth inhibition kinetics.

OSCILLATOR: A system for analysis of diurnal leaf growth using infrared photography combined with wavelet transformation

BACKGROUND

Quantification of leaf movement is an important tool for characterising the effects of environmental signals and the circadian clock on plant development. Analysis of leaf movement is currently restricted by the attachment of sensors to the plant or dependent upon visible light for time-lapse photography. The study of leaf growth movement rhythms in mature plants under biological relevant conditions, e.g. diurnal light and dark conditions, is therefore problematic.

RESULTS

Here we present OSCILLATOR, an affordable system for the analysis of rhythmic leaf growth movement in mature plants. The system contains three modules: (1) Infrared time-lapse imaging of growing mature plants (2) measurement of projected distances between leaf tip and plant apex (leaf tip tracking growth-curves) and (3) extraction of phase, period and amplitude of leaf growth oscillations using wavelet analysis. A proof-of-principle is provided by characterising parameters of rhythmic leaf growth movement of different Arabidopsis thaliana accessions as well as of Petunia hybrida and Solanum lycopersicum plants under diurnal conditions. The amplitude of leaf oscillations correlated to published data on leaf angles, while amplitude and leaf length did not correlate, suggesting a distinct leaf growth profile for each accession. Arabidopsis mutant accession Landsberg erecta displayed a late phase (timing of peak oscillation) compared to other accessions and this trait appears unrelated to the ERECTA locus.

CONCLUSIONS

OSCILLATOR is a low cost and easy to implement system that can accurately and reproducibly quantify rhythmic growth of mature plants for different species under diurnal light/dark cycling.

Modulation of ethylene- and heat-controlled hyponastic leaf movement in Arabidopsis thaliana by the plant defence hormones jasmonate and salicylate

Upward leaf movement (hyponastic growth) is adopted by several plant species including Arabidopsis thaliana, as a mechanism to escape adverse growth conditions. Among the signals that trigger hyponastic growth are, the gaseous hormone ethylene, low light intensities, and supra-optimal temperatures (heat). Recent studies indicated that the defence-related phytohormones jasmonic acid (JA) and salicylic acid (SA) synthesized by the plant upon biotic infestation repress low light-induced hyponastic growth. The hyponastic growth response induced by high temperature (heat) treatment and upon application of the gaseous hormone ethylene is highly similar to the response induced by low light. To test if these environmental signals induce hyponastic growth via parallel pathways or converge downstream, we studied here the roles of Methyl-JA (MeJA) and SA on ethylene- and heat-induced hyponastic growth. For this, we used a time-lapse camera setup. Our study includes pharmacological application of MeJA and SA and biological infestation using the JA-inducing caterpillar Pieris rapae as well as mutants lacking JA or SA signalling components. The data demonstrate that MeJA is a positive, and SA, a negative regulator of ethylene-induced hyponastic growth and that both hormones repress the response to heat. Taking previous studies into account, we conclude that SA is the first among many tested components which is repressing hyponastic growth under all tested inductive environmental stimuli. However, since MeJA is a positive regulator of ethylene-induced hyponastic growth and is inhibiting low light- and heat-induced leaf movement, we conclude that defence hormones control hyponastic growth by affecting stimulus-specific signalling pathways.

Phytochrome B control of total leaf area and stomatal density affects drought tolerance in rice

We report that phytochrome B (phyB) mutants exhibit improved drought tolerance compared to wild type (WT) rice (Oryza sativa L. cv. Nipponbare). To understand the underlying mechanism by which phyB regulates drought tolerance, we analyzed root growth and water loss from the leaves of phyB mutants. The root system showed no significant difference between the phyB mutants and WT, suggesting that improved drought tolerance has little relation to root growth. However, phyB mutants exhibited reduced total leaf area per plant, which was probably due to a reduction in the total number of cells per leaf caused by enhanced expression of Orysa;KRP1 and Orysa;KRP4 (encoding inhibitors of cyclin-dependent kinase complex activity) in the phyB mutants. In addition, the developed leaves of phyB mutants displayed larger epidermal cells than WT leaves, resulting in reduced stomatal density. phyB deficiency promoted the expression of both putative ERECTA family genes and EXPANSIN family genes involved in cell expansion in leaves, thus causing greater epidermal cell expansion in the phyB mutants. Reduced stomatal density resulted in reduced transpiration per unit leaf area in the phyB mutants. Considering all these findings, we propose that phyB deficiency causes both reduced total leaf area and reduced transpiration per unit leaf area, which explains the reduced water loss and improved drought tolerance of phyB mutants.

Changes in light intensity reveal a major role for carbon balance in Arabidopsis responses to high temperature

High temperature (HT) is a major limiting factor for plant productivity. Because some responses to HT, notably hyponasty, resemble those encountered in low light (LL), we hypothesized that plant responses to HT are under the control of carbon balance. We analysed the interactive effects of HT and irradiance level on hyponasty and a set of traits related to plant growth in natural accessions of Arabidopsis thaliana and mutants affected in heat dissipation through transpiration (NCED6-OE, ost2) and starch metabolism (pgm). HT induced hyponasty, reduced plant growth and modified leaf structure. LL worsened the effects of HT, while increasing light restored trait values close to levels observed at control temperature. Leaf temperature per se did not play a major role in the observed responses. By contrast, a major role of carbon balance was supported by hyponastic growth of pgm, as well as morphological, physiological (photosynthesis, sugar and starch contents) and transcriptional data. Carbon balance could be a common sensor of HT and LL, leading to responses specific of the shade avoidance syndrome. Hyponasty and associated changes in plant traits could be key traits conditioning plant performance under competition for light, particularly in warm environments.

Natural variation in stomatal abundance of Arabidopsis thaliana includes cryptic diversity for different developmental processes

BACKGROUND AND AIMS

Current understanding of stomatal development in Arabidopsis thaliana is based on mutations producing aberrant, often lethal phenotypes. The aim was to discover if naturally occurring viable phenotypes would be useful for studying stomatal development in a species that enables further molecular analysis.

METHODS

Natural variation in stomatal abundance of A. thaliana was explored in two collections comprising 62 wild accessions by surveying adaxial epidermal cell-type proportion (stomatal index) and density (stomatal and pavement cell density) traits in cotyledons and first leaves. Organ size variation was studied in a subset of accessions. For all traits, maternal effects derived from different laboratory environments were evaluated. In four selected accessions, distinct stomatal initiation processes were quantitatively analysed.

KEY RESULTS AND CONCLUSIONS

Substantial genetic variation was found for all six stomatal abundance-related traits, which were weakly or not affected by laboratory maternal environments. Correlation analyses revealed overall relationships among all traits. Within each organ, stomatal density highly correlated with the other traits, suggesting common genetic bases. Each trait correlated between organs, supporting supra-organ control of stomatal abundance. Clustering analyses identified accessions with uncommon phenotypic patterns, suggesting differences among genetic programmes controlling the various traits. Variation was also found in organ size, which negatively correlated with cell densities in both organs and with stomatal index in the cotyledon. Relative proportions of primary and satellite lineages varied among the accessions analysed, indicating that distinct developmental components contribute to natural diversity in stomatal abundance. Accessions with similar stomatal indices showed different lineage class ratios, revealing hidden developmental phenotypes and showing that genetic determinants of primary and satellite lineage initiation combine in several ways. This first systematic, comprehensive natural variation survey for stomatal abundance in A. thaliana reveals cryptic developmental genetic variation, and provides relevant relationships amongst stomatal traits and extreme or uncommon accessions as resources for the genetic dissection of stomatal development.

Arabidopsis ERECTA-family receptor kinases mediate morphological alterations stimulated by activation of NB-LRR-type UNI proteins

Shoot apical meristems (SAMs), which maintain stem cells at the tips of stems, and axillary meristems (AMs), which arise at leaf axils for branch formation, play significant roles in the establishment of plant architecture. Previously, we showed that, in Arabidopsis thaliana, activation of NB-LRR (nucleotide-binding site-leucine-rich repeat)-type UNI proteins affects plant morphology through modulation of the regulation of meristems. However, information about genes involved in the processes was still lacking. Here, we report that ERECTA (ER) receptor kinase family members cooperatively mediate the morphological alterations that are stimulated by activation of UNI proteins. uni-1D is a gain-of-function mutation in the UNI gene and uni-1D mutants exhibit early termination of inflorescence stem growth and also formation of extra AMs at leaf axils. The former defect involves modulation of the SAM activity and is suppressed by er mutation. Though the AM phenotype is not affected by a single er mutation, it is suppressed by simultaneous mutations of ER-family members. It was previously shown that trans-zeatin (tZ)-type cytokinins were involved in the morphological phenotypes of uni-1D mutants and that expression of CYP735A2, which is essential for biosynthesis of tZ-type cytokinins, was modulated in uni-1D mutants. We show that this modulation of CYP735A2 expression requires activities of ER-family members. Moreover, the ER activity in UNI-expressing cells contributes to all morphological phenotypes of uni-1D mutants, suggesting that a cross-talk between ER-family-dependent and UNI-triggered signaling pathways plays a significant role in the morphological alterations observed in uni-1D mutants.

Regulatory network identification by genetical genomics: signaling downstream of the Arabidopsis receptor-like kinase ERECTA

Gene expression differences between individuals within a species can be largely explained by differences in genetic background. The effect of genetic variants (alleles) of genes on expression can be studied in a multifactorial way by the application of genetical genomics or expression quantitative trait locus mapping. In this paper, we present a strategy to construct regulatory networks by the application of genetical genomics in combination with transcript profiling of mutants that are disrupted in single genes. We describe the network identification downstream of the receptor-like kinase ERECTA in Arabidopsis (Arabidopsis thaliana). Extending genetical genomics on the Landsberg erecta/Cape Verde Islands (Ler/Cvi) recombinant inbred population with expression profiling of monogenic mutants enabled the identification of regulatory networks in the so far elusive ERECTA signal transduction cascade. We provide evidence that ERECTA is the causal gene for the major hotspot for transcript regulation in the Arabidopsis Ler/Cvi recombinant inbred population. We further propose additional genetic variation between Ler and Cvi in loci of the signaling pathway downstream of ERECTA and suggest candidate genes underlying these loci. Integration of publicly available microarray expression data of other monogenic mutants allowed us to link ERECTA to a downstream mitogen-activated protein kinase signaling cascade. Our study shows that microarray data of monogenic mutants can be effectively used in combination with genetical genomics data to enhance the identification of genetic regulatory networks.

ERECTA controls low light intensity-induced differential petiole growth independent of phytochrome B and cryptochrome 2 action in Arabidopsis thaliana

Plants can respond quickly and profoundly to changes in their environment. Several species, including Arabidopsis thaliana, are capable of differential petiole growth driven upward leaf movement (hyponastic growth) to escape from detrimental environmental conditions. Recently, we demonstrated that the leucine-rich repeat receptor-like Ser/Thr kinase gene ERECTA, explains a major effect Quantitative Trait Locus (QTL) for ethylene-induced hyponastic growth in Arabidopsis. Here, we demonstrate that ERECTA controls the hyponastic growth response to low light intensity treatment in a genetic background dependent manner. Moreover, we show that ERECTA affects low light-induced hyponastic growth independent of Phytochrome B and Cryptochrome 2 signaling, despite that these photoreceptors are positive regulators of low light-induced hyponastic growth.

Out of the mouths of plants: the molecular basis of the evolution and diversity of stomatal development

Stomata are microscopic valves on the plant epidermis that played a critical role in the evolution of land plants. Studies in the model dicot Arabidopsis thaliana have identified key transcription factors and signaling pathways controlling stomatal patterning and differentiation. Three paralogous Arabidopsis basic helix-loop-helix proteins, SPEECHLESS (SPCH), MUTE, and FAMA, mediate sequential steps of cell-state transitions together with their heterodimeric partners SCREAM (SCRM) and SCRM2. Cell-cell signaling components, including putative ligands, putative receptors, and mitogen-activated protein kinase cascades, orient asymmetric cell divisions and prevent overproduction and clustering of stomata. The recent availability of genome sequence and reverse genetics tools for model monocots and basal land plants allows for the examination of the conservation of genes important in stomatal patterning and differentiation. Studies in grasses have revealed that divergence of SPCH-MUTE-FAMA predates the evolutionary split of monocots and dicots and that these proteins show conserved and novel roles in stomatal differentiation. By contrast, specific asymmetric cell divisions in Arabidopsis and grasses require unique molecular components. Molecular phylogenetic analysis implies potential conservation of signaling pathways and prototypical functions of the transcription factors specifying stomatal differentiation.