Teaching an old hormone new tricks: cytosolic Ca2+ elevation involvement in plant brassinosteroid signal transduction cascades

Kinase CPK10 regulates low light-induced tomato flower drop downstream of IDL6 in a calcium-dependent manner

Flower drop is a major cause for yield loss in many crops. Previously, we found that tomato (Solanum lycopersicum) INFLORESCENCE DEFICIENT IN ABSCISSION-Like (SlIDL6) contributes to flower drop induced by low light. However, the molecular mechanisms by which SlIDL6 acts as a signal to regulate low light-induced abscission remain unclear. In this study, SlIDL6 was found to elevate cytosolic Ca2+ concentrations ([Ca2+]cyt) in the abscission zone (AZ), which was required for SlIDL6-induced flower drop under low light. We further identified that one calcium-dependent protein kinase gene (SlCPK10) was highly expressed in the AZ and up-regulated by SlIDL6-triggered [Ca2+]cyt. Over-expression and knockout of SlCPK10 in tomato resulted in accelerated and delayed abscission, respectively. Genetic evidence further indicated that knockout of SlCPK10 significantly impaired the function of SlIDL6 in accelerating abscission. Furthermore, Ser-371 phosphorylation in SlCPK10 dependent on SlIDL6 was necessary and sufficient for its function in regulating flower drop, probably by stabilizing the SlCPK10 proteins. Taken together, our findings reveal that SlCPK10, as a downstream component of the IDL6 signaling pathway, regulates flower drop in tomato under low light stress.

The Sustainable Use of Halophytes in Salt-Affected Land: State-of-the-Art and Next Steps in a Saltier World

Salinization is a major cause of soil degradation that affects several million hectares of agricultural land, threatening food security and the sustainability of agricultural systems worldwide. Nevertheless, despite the negative impact of salinity, salt-affected land also provides several important ecosystem services, from providing habitats and nurseries for numerous species to sustainable food production. This opinion paper, written in the framework of the EU COST Action CA22144 SUSTAIN on the sustainable use of salt-affected land, therefore, focuses on the potential of halophytes and saline agriculture to transform and restore key functions of these salt-affected and marginal lands. As the current knowledge on sustainable saline agriculture upscaling is fragmented, we highlight (i) the research gaps in halophyte and salinity research and (ii) the main barriers and potentials of saline agriculture for addressing food security and environmental sustainability in terms of population growth and climate change.

Functionality of Reactive Oxygen Species (ROS) in Plants: Toxicity and Control in Poaceae Crops Exposed to Abiotic Stress

Agriculture and changing environmental conditions are closely related, as weather changes could adversely affect living organisms or regions of crop cultivation. Changing environmental conditions trigger different abiotic stresses, which ultimately cause the accumulation of reactive oxygen species (ROS) in plants. Common ROS production sites are the chloroplast, endoplasmic reticulum, plasma membrane, mitochondria, peroxisomes, etc. The imbalance in ROS production and ROS detoxification in plant cells leads to oxidative damage to biomolecules such as lipids, nucleic acids, and proteins. At low concentrations, ROS initiates signaling events related to development and adaptations to abiotic stress in plants by inducing signal transduction pathways. In plants, a stress signal is perceived by various receptors that induce a signal transduction pathway that activates numerous signaling networks, which disrupt gene expression, impair the diversity of kinase/phosphatase signaling cascades that manage the stress response in the plant, and result in changes in physiological responses under various stresses. ROS production also regulates ABA-dependent and ABA-independent pathways to mitigate drought stress. This review focuses on the common subcellular location of manufacturing, complex signaling mechanisms, and networks of ROS, with an emphasis on cellular effects and enzymatic and non-enzymatic antioxidant scavenging mechanisms of ROS in Poaceae crops against drought stress and how the manipulation of ROS regulates stress tolerance in plants. Understanding ROS systems in plants could help to create innovative strategies to evolve paths of cell protection against the negative effects of excessive ROS in attempts to improve crop productivity in adverse environments.

Paradigms of receptor kinase signaling in plants

Plant receptor kinases (RKs) function as key plasma-membrane localized receptors in the perception of molecular ligands regulating development and environmental response. Through the perception of diverse ligands, RKs regulate various aspects throughout the plant life cycle from fertilization to seed set. Thirty years of research on plant RKs has generated a wealth of knowledge on how RKs perceive ligands and activate downstream signaling. In the present review, we synthesize this body of knowledge into five central paradigms of plant RK signaling: (1) RKs are encoded by expanded gene families, largely conserved throughout land plant evolution; (2) RKs perceive many different kinds of ligands through a range of ectodomain architectures; (3) RK complexes are typically activated by co-receptor recruitment; (4) post-translational modifications fulfill central roles in both the activation and attenuation of RK-mediated signaling; and, (5) RKs activate a common set of downstream signaling processes through receptor-like cytoplasmic kinases (RLCKs). For each of these paradigms, we discuss key illustrative examples and also highlight known exceptions. We conclude by presenting five critical gaps in our understanding of RK function.

Phenotypes of cyclic nucleotide-gated cation channel mutants: probing the nature of native channels

Plant cyclic nucleotide gated channels (CNGCs) facilitate cytosolic Ca²⁺ influx as an early step in numerous signaling cascades. CNGC-mediated Ca²⁺ elevations are essential for plant immune defense and high temperature thermosensing. In the present study, we evaluated phenotypes of CNGC2, CNGC4, CNGC6, and CNGC12 null mutants in these two pathways. It is shown CNGC2, CNGC4, and CNGC6 physically interact in vivo, whereas CNGC12 does not. CNGC involvement in immune signaling was evaluated by monitoring mutant response to elicitor peptide Pep3. Pep3 response cascades involving CNGCs included mitogen-activated kinase activation mediated by Ca²⁺ -dependent protein kinase phosphorylation. Pep3-induced reactive oxygen species generation was impaired in cngc2, cngc4, and cngc6, but not in cngc12, suggesting that CNGC2, CNGC4, and CNGC6 (which physically interact) may be components of a multimeric CNGC channel complex for immune signaling. However, unlike cngc2 and cngc4, cngc6 is not sensitive to high Ca²⁺ and displays no pleiotropic dwarfism. All four cngc mutants showed thermotolerance compared to wild-type, although CNGC12 does not interact with the other three CNGCs. These results imply that physically interacting CNGCs may, in some cases, function in a signaling cascade as components of a heteromeric channel complex, although this may not be the case in other signaling pathways.

BdGUCD1 and Cyclic GMP Are Required for Responses of Brachypodium distachyon to Fusarium pseudograminearum in the Mechanism Involving Jasmonate

Guanosine 3′,5′-cyclic monophosphate (cGMP) is an important signaling molecule in plants. cGMP and guanylyl cyclases (GCs), enzymes that catalyze the synthesis of cGMP from GTP, are involved in several physiological processes and responses to environmental factors, including pathogen infections. Using in vitro analysis, we demonstrated that recombinant BdGUCD1 is a protein with high guanylyl cyclase activity and lower adenylyl cyclase activity. In Brachypodium distachyon, infection by Fusarium pseudograminearum leads to changes in BdGUCD1 mRNA levels, as well as differences in endogenous cGMP levels. These observed changes may be related to alarm reactions induced by pathogen infection. As fluctuations in stress phytohormones after infection have been previously described, we performed experiments to determine the relationship between cyclic nucleotides and phytohormones. The results revealed that inhibition of cellular cGMP changes disrupts stress phytohormone content and responses to pathogen. The observations made here allow us to conclude that cGMP is an important element involved in the processes triggered as a result of infection and changes in its levels affect jasmonic acid. Therefore, stimuli-induced transient elevation of cGMP in plants may play beneficial roles in priming an optimized response, likely by triggering the mechanisms of feedback control.

GLUTAMATE RECEPTOR-like gene OsGLR3.4 is required for plant growth and systemic wound signaling in rice (Oryza sativa)

Recent studies have revealed the physiological roles of glutamate receptor-like channels (GLRs) in Arabidopsis; however, the functions of GLRs in rice remain largely unknown. Here, we show that knockout of OsGLR3.4 in rice leads to brassinosteroid (BR)-regulated growth defects and reduced BR sensitivity. Electrophoretic mobility shift assays and transient transactivation assays indicated that OsGLR3.4 is the downstream target of OsBZR1. Further, agonist profile assays showed that multiple amino acids can trigger transient Ca²⁺ influx in an OsGLR3.4-dependent manner, indicating that OsGLR3.4 is a Ca²⁺ -permeable channel. Meanwhile, the study of internode cells demonstrated that OsGLR3.4-mediated Ca²⁺ flux is required for actin filament organization and vesicle trafficking. Following root injury, the triggering of both slow wave potentials (SWPs) in leaves and the jasmonic acid (JA) response are impaired in osglr3.4 mutants, indicating that OsGLR3.4 is required for root-to-shoot systemic wound signaling in rice. Brassinosteroid treatment enhanced SWPs and OsJAZ8 expression in root-wounded plants, suggesting that BR signaling synergistically regulates the OsGLR3.4-mediated systemic wound response. In summary, this article describes a mechanism of OsGLR3.4-mediated cell elongation and long-distance systemic wound signaling in plants and provides new insights into the contribution of GLRs to plant growth and responses to mechanical wounding.

A Molecular Pinball Machine of the Plasma Membrane Regulates Plant Growth-A New Paradigm

Novel molecular pinball machines of the plasma membrane control cytosolic Ca²⁺ levels that regulate plant metabolism. The essential components involve: 1. an auxin-activated proton pump; 2. arabinogalactan glycoproteins (AGPs); 3. Ca²⁺ channels; 4. auxin-efflux "PIN" proteins. Typical pinball machines release pinballs that trigger various sound and visual effects. However, in plants, "proton pinballs" eject Ca²⁺ bound by paired glucuronic acid residues of numerous glycomodules in periplasmic AGP-Ca²⁺. Freed Ca²⁺ ions flow down the electrostatic gradient through open Ca²⁺ channels into the cytosol, thus activating numerous Ca²⁺-dependent activities. Clearly, cytosolic Ca²⁺ levels depend on the activity of the proton pump, the state of Ca²⁺ channels and the size of the periplasmic AGP-Ca²⁺ capacitor; proton pump activation is a major regulatory focal point tightly controlled by the supply of auxin. Auxin efflux carriers conveniently known as "PIN" proteins (null mutants are pin-shaped) pump auxin from cell to cell. Mechanosensitive Ca²⁺ channels and their activation by reactive oxygen species (ROS) are yet another factor regulating cytosolic Ca²⁺. Cell expansion also triggers proton pump/pinball activity by the mechanotransduction of wall stress via Hechtian adhesion, thus forming a Hechtian oscillator that underlies cycles of wall plasticity and oscillatory growth. Finally, the Ca²⁺ homeostasis of plants depends on cell surface external storage as a source of dynamic Ca²⁺, unlike the internal ER storage source of animals, where the added regulatory complexities ranging from vitamin D to parathormone contrast with the elegant simplicity of plant life. This paper summarizes a sixty-year Odyssey.

Screening for Arabidopsis mutants with altered Ca2+ signal response using aequorin-based Ca2+ reporter system

Environmental stimuli evoke transient increases of the cytosolic Ca²⁺ level. To identify upstream components of Ca²⁺ signaling, we have optimized two forward genetic screening systems based on Ca²⁺ reporter aequorin. AEQsig6 and AEQub plants were used for generating ethyl methanesulfonate (EMS)-mutagenized libraries. The AEQsig6 EMS-mutagenized library was preferably used to screen the mutants with reduced Ca²⁺ signal response due to its high effectiveness, while the AEQub EMS-mutagenized library was used for screening of the mutants with altered Ca²⁺ signal response.

For complete details on the use and execution of this protocol, please refer to Chen et al. (2020) and Zhu et al. (2013).

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Highly efficient systems for screening of Ca²⁺ signal mutants in Arabidopsis

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Step-by-step instructions to analyze Ca²⁺ signal response using Ca²⁺ reporter aequorin

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AEQsig6 system for identifying mutants impaired in shoot-based Ca²⁺ signaling

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FAS system for isolating tissue- or stimuli-specific Ca²⁺ responsive mutants

Brassinosteroids application induces phosphatidic acid production and modify antioxidant enzymes activity in tobacco in calcium-dependent manner

Brassinosteroids (BRs) are steroid hormones regulating various aspects of plant metabolism, including growth, development and stress responses. However, little is known about the mechanism of their impact on antioxidant systems and phospholipid turnover. Using tobacco plants overexpressing H+/Ca2+vacuolar Arabidopsis antiporter CAX1, we showed the role of Ca2+ ion balance in the reactive oxygen species production and rapid phosphatidic acid accumulation induced by exogenous BR. Combination of our experimental results with public transcriptomic and proteomic data revealed a particular role of Ca2+-dependent phospholipid metabolizing enzymes in BR signaling. Here we provide novel insights into the role of calcium balance and lipid-derived second messengers in plant responses to exogenous BRs and propose a complex model integrating BR-mediated metabolic changes with phospholipid turnover.

Crosstalk between Brassinosteroid and Redox Signaling Contributes to the Activation of CBF Expression during Cold Responses in Tomato

Brassinosteroids (BRs) play a critical role in plant responses to stress. However, the interplay of BRs and reactive oxygen species signaling in cold stress responses remains unclear. Here, we demonstrate that a partial loss of function in the BR biosynthesis gene DWARF resulted in lower whilst overexpression of DWARF led to increased levels of C-REPEAT BINDING FACTOR (CBF) transcripts. Exposure to cold stress increased BR synthesis and led to an accumulation of brassinazole-resistant 1 (BZR1), a central component of BR signaling. Mutation of BZR1 compromised the cold- and BR-dependent increases in CBFs and RESPIRATORY BURST OXIDASE HOMOLOG 1(RBOH1) transcripts, as well as preventing hydrogen peroxide (H₂O₂) accumulation in the apoplast. Cold- and BR-induced BZR1 bound to the promoters of CBF1, CBF3 and RBOH1 and promoted their expression. Significantly, suppression of RBOH1 expression compromised cold- and BR-induced accumulation of BZR1 and related increases in CBF transcripts. Moreover, RBOH1-dependent H₂O₂ production regulated BZR1 accumulation and the levels of CBF transcripts by influencing glutathione homeostasis. Taken together, these results demonstrate that crosstalk between BZR1 and reactive oxygen species mediates cold- and BR-activated CBF expression, leading to cold tolerance in tomato (Solanum lycopersicum).

Roles of Brassinosteroids in Mitigating Heat Stress Damage in Cereal Crops

Heat stress causes huge losses in the yield of cereal crops. Temperature influences the rate of plant metabolic and developmental processes that ultimately determine the production of grains, with high temperatures causing a reduction in grain yield and quality. To ensure continued food security, the tolerance of high temperature is rapidly becoming necessary. Brassinosteroids (BR) are a class of plant hormones that impact tolerance to various biotic and abiotic stresses and regulate cereal growth and fertility. Fine-tuning the action of BR has the potential to increase cereals' tolerance and acclimation to heat stress and maintain yields. Mechanistically, exogenous applications of BR protect yields through amplifying responses to heat stress and rescuing the expression of growth promoters. Varied BR compounds and differential signaling mechanisms across cereals point to a diversity of mechanisms that can be leveraged to mitigate heat stress. Further, hormone transport and BR interaction with other molecules in plants may be critical to utilizing BR as protective agrochemicals against heat stress. Understanding the interplay between heat stress responses, growth processes and hormone signaling may lead us to a comprehensive dogma of how to tune BR application for optimizing cereal growth under challenging environments in the field.

Integration of metabolomics and existing omics data reveals new insights into phytoplasma-induced metabolic reprogramming in host plants

Phytoplasmas are cell wall-less bacteria that induce abnormal plant growth and various diseases, causing severe economic loss. Phytoplasmas are highly dependent on nutrients imported from host cells because they have lost many genes involved in essential metabolic pathways during reductive evolution. However, metabolic crosstalk between phytoplasmas and host plants and the mechanisms of phytoplasma nutrient acquisition remain poorly understood. In this study, using metabolomics approach, sweet cherry virescence (SCV) phytoplasma-induced metabolite alterations in sweet cherry trees were investigated. A total of 676 metabolites were identified in SCV phytoplasma-infected and mock inoculated leaves, of which 187 metabolites were differentially expressed, with an overwhelming majority belonging to carbohydrates, fatty acids/lipids, amino acids, and flavonoids. Available omics data of interactions between plant and phytoplasma were also deciphered and integrated into the present study. The results demonstrated that phytoplasma infection promoted glycolysis and pentose phosphate pathway activities, which provide energy and nutrients, and facilitate biosynthesis of necessary low-molecular metabolites. Our findings indicated that phytoplasma can induce reprograming of plant metabolism to obtain nutrients for its own replication and infection. The findings from this study provide new insight into interactions of host plants and phytoplasmas from a nutrient acquisition perspective.

Transcriptomic analyses show that 24-epibrassinolide (EBR) promotes cold tolerance in cotton seedlings

In plants, brassinosteroids (BRs) are a class of steroidal hormones that are involved in numerous physiological responses. However, the function of BRs in cold tolerance in cotton has not been explored. In this study, cotton seedlings were treated with five concentrations (0, 0.05, 0.1, 0.2, 0.5 and 1.0 mg/L) of 24-Epibrassinolide (EBR) at 4°C. We measured the electrolyte leakage, malondialdehyde (MDA) content, proline content, and net photosynthesis rate (Pn) of the seedlings, which showed that EBR treatment increased cold tolerance in cotton in a dose-dependent manner, and that 0.2 mg/L is an optimum concentration for enhancing cold tolerance. The function of EBR in cotton cotyledons was investigated in the control 0 mg/L (Cold+water) and 0.2 mg/L (Cold+EBR) treatments using RNA-Seq. A total of 4,001 differentially expressed genes (DEGs), including 2,591 up-regulated genes and 1,409 down-regulated genes were identified. Gene Ontology (GO) and biochemical pathway enrichment analyses showed that EBR is involved in the genetic information process, secondary metabolism, and also inhibits abscisic acid (ABA) and ethylene (ETH) signal transduction. In this study, physiological experiments showed that EBR can increase cold tolerance in cotton seedlings, and the comprehensive RNA-seq data shed light on the mechanisms through which EBR increases cold tolerance in cotton seedlings.

Moonlighting Proteins Shine New Light on Molecular Signaling Niches

Plants as sessile organisms face daily environmental challenges and have developed highly nuanced signaling systems to enable suitable growth, development, defense, or stalling responses. Moonlighting proteins have multiple tasks and contribute to cellular signaling cascades where they produce additional variables adding to the complexity or fuzziness of biological systems. Here we examine roles of moonlighting kinases that also generate 3',5'-cyclic guanosine monophosphate (cGMP) in plants. These proteins include receptor like kinases and lipid kinases. Their guanylate cyclase activity potentiates the development of localized cGMP-enriched nanodomains or niches surrounding the kinase and its interactome. These nanodomains contribute to allosteric regulation of kinase and other molecules in the immediate complex directly or indirectly modulating signal cascades. Effects include downregulation of kinase activity, modulation of other members of the protein complexes such as cyclic nucleotide gated channels and potential triggering of cGMP-dependent degradation cascades terminating signaling. The additional layers of information provided by the moonlighting kinases are discussed in terms of how they may be used to provide a layer of fuzziness to effectively modulate cellular signaling cascades.

Species-independent analytical tools for next-generation agriculture

Innovative approaches are urgently required to alleviate the growing pressure on agriculture to meet the rising demand for food. A key challenge for plant biology is to bridge the notable knowledge gap between our detailed understanding of model plants grown under laboratory conditions and the agriculturally important crops cultivated in fields or production facilities. This Perspective highlights the recent development of new analytical tools that are rapid and non-destructive and provide tissue-, cell- or organelle-specific information on living plants in real time, with the potential to extend across multiple species in field applications. We evaluate the utility of engineered plant nanosensors and portable Raman spectroscopy to detect biotic and abiotic stresses, monitor plant hormonal signalling as well as characterize the soil, phytobiome and crop health in a non- or minimally invasive manner. We propose leveraging these tools to bridge the aforementioned fundamental gap with new synthesis and integration of expertise from plant biology, engineering and data science. Lastly, we assess the economic potential and discuss implementation strategies that will ensure the acceptance and successful integration of these modern tools in future farming practices in traditional as well as urban agriculture.

γ-Aminobutyric acid and related amino acids in plant immune responses: Emerging mechanisms of action

The entanglement between primary metabolism regulation and stress responses is a puzzling and fascinating theme in plant sciences. Among the major metabolites found in plants, γ-aminobutyric acid (GABA) fulfils important roles in connecting C and N metabolic fluxes through the GABA shunt. Activation of GABA metabolism is known since long to occur in plant tissues following biotic stresses, where GABA appears to have substantially different modes of action towards different categories of pathogens and pests. While it can harm insects thanks to its inhibitory effect on the neuronal transmission, its capacity to modulate the hypersensitive response in attacked host cells was proven to be crucial for host defences in several pathosystems. In this review, we discuss how plants can employ GABA's versatility to effectively deal with all the major biotic stressors, and how GABA can shape plant immune responses against pathogens by modulating reactive oxygen species balance in invaded plant tissues. Finally, we discuss the connections between GABA and other stress-related amino acids such as BABA (β-aminobutyric acid), glutamate and proline.

Molecular Mechanisms of Brassinosteroid-Mediated Responses to Changing Environments in Arabidopsis

Plant adaptations to changing environments rely on integrating external stimuli into internal responses. Brassinosteroids (BRs), a group of growth-promoting phytohormones, have been reported to act as signal molecules mediating these processes. BRs are perceived by cell surface receptor complex including receptor BRI1 and coreceptor BAK1, which subsequently triggers a signaling cascade that leads to inhibition of BIN2 and activation of BES1/BZR1 transcription factors. BES1/BZR1 can directly regulate the expression of thousands of downstream responsive genes. Recent studies in the model plant Arabidopsis demonstrated that BR biosynthesis and signal transduction, especially the regulatory components BIN2 and BES1/BZR1, are finely tuned by various environmental cues. Here, we summarize these research updates and give a comprehensive review of how BR biosynthesis and signaling are modulated by changing environments and how these changes regulate plant adaptive growth or stress tolerance.

Comparative Proteomic Analysis Provides Insights into the Regulatory Mechanisms of Wheat Primary Root Growth

Plant roots are vital for acquiring nutrients and water from soil. However, the mechanisms regulating root growth in hexaploid wheat remain to be elucidated. Here, an integrated comparative proteome study on the roots of two varieties and their descendants with contrasting root phenotypes was performed. A total of 80 differentially expressed proteins (DEPs) associated with the regulation of primary root growth were identified, including two plant steroid biosynthesis related proteins and nine class III peroxidases. Real-time PCR analysis showed that brassinosteroid (BR) biosynthesis pathway was significantly elevated in long-root plants compared with those short-root plants. Moreover, O2.- and H2O2 were distributed abundantly in both the root meristematic and elongation zones of long root plants, but only in the meristematic zone of short-root plants. The differential distribution of reactive oxygen species (ROS) in the root tips of different genotypes may be caused by the differential expression of peroxidases. Taken together, our results suggest that the regulation of wheat primary root growth is closely related to BR biosynthesis pathway and BR-mediated ROS distribution.

Effects of exogenous 24-epibrassinolide and brassinazole on negative gravitropism and tension wood formation in hybrid poplar (Populus deltoids × Populus nigra)

Exogenous 24-epibrassinolide (BL) and brassinazole (BRZ) have regulatory roles in G-fiber cell wall development and secondary xylem cell wall carbohydrate biosynthesis during tension wood formation in hybrid poplar. Brassinosteroids (BRs) play important roles in regulating gravitropism and vasculature development. Here, we report the effect of brassinosteroids on negative gravitropism and G-fiber cell wall development of the stem in woody angiosperms. We applied exogenous 24-epibrassinolide (BL) or its biosynthesis inhibitor brassinazole (BRZ) to slanted hybrid poplar trees (Populus deltoids × Populus nigra) and measured the morphology of gravitropic stems, anatomy and chemistry of secondary cell wall. We furthermore analyzed the expression levels of auxin transport and cellulose biosynthetic genes after 24-epibrassinolide (BL) or brassinazole (BRZ) application. The BL-treated seedlings showed no negative gravitropism bending, whereas application of BRZ dramatically enhanced negative gravitropic bending. BL treatment stimulated secondary xylem fiber elongation and G-fiber formation on the upper side of stems but delayed G-fiber maturation. BRZ inhibited xylem fiber elongation but induced the production of more mature G-fibers on the upper side of stems. Wood chemistry analyses and immunolocalization demonstrated that BL and BRZ treatments increased the cellulose content and modified the deposition of cell wall carbohydrates including arabinose, galactose and rhamnose in the secondary xylem. The expression of cellulose biosynthetic genes, especially those related to cellulose microfibril deposition (PtFLA12 and PtCOBL4) was significantly upregulated in BL- and BRZ-treated TW stems compared with control stems. The significant differences of G-fibers development and negative gravitropism bending between 24-epibrassinolide (BL) and brassinazole (BRZ) application suggest that brassinosteroids are important for secondary xylem development during tension wood formation. Our findings provide potential insights into the mechanism by which BRs regulate G-fiber cell wall development to accomplish negative gravitropism in TW formation.

Crosstalk of the Brassinosteroid Signalosome with Phytohormonal and Stress Signaling Components Maintains a Balance between the Processes of Growth and Stress Tolerance

Brassinosteroids (BRs) are a class of phytohormones, which regulate various processes during plant life cycle. Intensive studies conducted with genetic, physiological and molecular approaches allowed identification of various components participating in the BR signaling—from the ligand perception, through cytoplasmic signal transduction, up to the BR-dependent gene expression, which is regulated by transcription factors and chromatin modifying enzymes. The identification of new components of the BR signaling is an ongoing process, however an emerging view of the BR signalosome indicates that this process is interconnected at various stages with other metabolic pathways. The signaling crosstalk is mediated by the BR signaling proteins, which function as components of the transmembrane BR receptor, by a cytoplasmic kinase playing a role of the major negative regulator of the BR signaling, and by the transcription factors, which regulate the BR-dependent gene expression and form a complicated regulatory system. This molecular network of interdependencies allows a balance in homeostasis of various phytohormones to be maintained. Moreover, the components of the BR signalosome interact with factors regulating plant reactions to environmental cues and stress conditions. This intricate network of interactions enables a rapid adaptation of plant metabolism to constantly changing environmental conditions.

The Role of the Primary Cell Wall in Plant Morphogenesis

Morphogenesis remains a riddle, wrapped in a mystery, inside an enigma. It remains a formidable problem viewed from many different perspectives of morphology, genetics, and computational modelling. We propose a biochemical reductionist approach that shows how both internal and external physical forces contribute to plant morphogenesis via mechanical stress⁻strain transduction from the primary cell wall tethered to the plasma membrane by a specific arabinogalactan protein (AGP). The resulting stress vector, with direction defined by Hechtian adhesion sites, has a magnitude of a few piconewtons amplified by a hypothetical Hechtian growth oscillator. This paradigm shift involves stress-activated plasma membrane Ca2+ channels and auxin-activated H⁺-ATPase. The proton pump dissociates periplasmic AGP-glycomodules that bind Ca2+. Thus, as the immediate source of cytosolic Ca2+, an AGP-Ca2+ capacitor directs the vectorial exocytosis of cell wall precursors and auxin efflux (PIN) proteins. In toto, these components comprise the Hechtian oscillator and also the gravisensor. Thus, interdependent auxin and Ca2+ morphogen gradients account for the predominance of AGPs. The size and location of a cell surface AGP-Ca2+ capacitor is essential to differentiation and explains AGP correlation with all stages of morphogenetic patterning from embryogenesis to root and shoot. Finally, the evolutionary origins of the Hechtian oscillator in the unicellular Chlorophycean algae reflect the ubiquitous role of chemiosmotic proton pumps that preceded DNA at the dawn of life.

Ca2+/Calmodulin-Dependent AtSR1/CAMTA3 Plays Critical Roles in Balancing Plant Growth and Immunity

During plant-pathogen interactions, plants have to relocate their resources including energy to defend invading organisms; as a result, plant growth and development are usually reduced. Arabidopsis signal responsive1 (AtSR1) has been documented as a negative regulator of plant immune responses and could serve as a positive regulator of plant growth and development. However, the mechanism by which AtSR1 balances plant growth and immunity is poorly understood. Here, we performed a global gene expression profiling using Affymetrix microarrays to study how AtSR1 regulates defense- and growth-related genes in plants with and without bacterial pathogen infection. Results revealed that AtSR1 negatively regulates most of the immune-related genes involved in molecular pattern-triggered immunity (PTI), effector-triggered immunity (ETI), and in salicylic acid (SA)- and jasmonate (JA)-mediated signaling pathways. AtSR1 may rigidly regulate several steps of the SA-mediated pathway, from the activation of SA synthesis to the perception of SA signal. Furthermore, AtSR1 may also regulate plant growth through its involvement in regulating auxin- and BRs-related pathways. Although microarray data revealed that expression levels of defense-related genes induced by pathogens are higher in wild-type (WT) plants than that in atsr1 mutant plants, WT plants are more susceptible to the infection of virulent pathogen as compared to atsr1 mutant plants. These observations indicate that the AtSR1 functions in suppressing the expression of genes induced by pathogen attack and contributes to the rapid establishment of resistance in WT background. Results of electrophoretic mobility shift assay (EMSA) and chromatin immunoprecipitation (ChIP)-PCR assays suggest that AtSR1 acts as transcription factor in balancing plant growth and immunity, through interaction with the “CGCG” containing CG-box in the promotors of its target genes.

A Plant Phytosulfokine Peptide Initiates Auxin-Dependent Immunity through Cytosolic Ca2+ Signaling in Tomato

Phytosulfokine (PSK) is a disulfated pentapeptide that is an important signaling molecule. Although it has recently been implicated in plant defenses to pathogen infection, the mechanisms involved remain poorly understood. Using surface plasmon resonance and gene silencing approaches, we showed that the tomato (Solanum lycopersicum) PSK receptor PSKR1, rather than PSKR2, functioned as the major PSK receptor in immune responses. Silencing of PSK signaling genes rendered tomato more susceptible to infection by the economically important necrotrophic pathogen Botrytis cinerea Analysis of tomato mutants defective in either defense hormone biosynthesis or signaling demonstrated that PSK-induced immunity required auxin biosynthesis and associated defense pathways. Here, using aequorin-expressing tomato plants, we provide evidence that PSK perception by tomato PSKR1 elevated cytosolic [Ca²⁺], leading to auxin-dependent immune responses via enhanced binding activity between calmodulins and the auxin biosynthetic YUCs. Thus, our data demonstrate that PSK acts as a damage-associated molecular pattern and is perceived mainly by PSKR1, which increases cytosolic [Ca²⁺] and activates auxin-mediated pathways that enhance immunity of tomato plants to B. cinerea.

A Plant Phytosulfokine Peptide Initiates Auxin-Dependent Immunity through Cytosolic Ca2+ Signaling in Tomato

Phytosulfokine (PSK) is a disulfated pentapeptide that is an important signaling molecule. Although it has recently been implicated in plant defenses to pathogen infection, the mechanisms involved remain poorly understood. Using surface plasmon resonance and gene silencing approaches, we showed that the tomato (Solanum lycopersicum) PSK receptor PSKR1, rather than PSKR2, functioned as the major PSK receptor in immune responses. Silencing of PSK signaling genes rendered tomato more susceptible to infection by the economically important necrotrophic pathogen Botrytis cinerea Analysis of tomato mutants defective in either defense hormone biosynthesis or signaling demonstrated that PSK-induced immunity required auxin biosynthesis and associated defense pathways. Here, using aequorin-expressing tomato plants, we provide evidence that PSK perception by tomato PSKR1 elevated cytosolic [Ca²⁺], leading to auxin-dependent immune responses via enhanced binding activity between calmodulins and the auxin biosynthetic YUCs. Thus, our data demonstrate that PSK acts as a damage-associated molecular pattern and is perceived mainly by PSKR1, which increases cytosolic [Ca²⁺] and activates auxin-mediated pathways that enhance immunity of tomato plants to B. cinerea.

Brassinosteroids regulate root growth by controlling reactive oxygen species homeostasis and dual effect on ethylene synthesis in Arabidopsis

The brassinosteroids (BRs) represent a class of phytohormones, which regulate numerous aspects of growth and development. Here, a det2-9 mutant defective in BR synthesis was identified from an EMS mutant screening for defects in root length, and was used to investigate the role of BR in root development in Arabidopsis. The det2-9 mutant displays a short-root phenotype, which is result from the reduced cell number in root meristem and decreased cell size in root maturation zone. Ethylene synthesis is highly increased in the det2-9 mutant compared with the wild type, resulting in the hyper-accumulation of ethylene and the consequent inhibition of root growth. The short-root phenotype of det2-9 was partially recovered in the det2-9/acs9 double mutant and det2-9/ein3/eil1-1 triple mutant which have defects either in ethylene synthesis or ethylene signaling, respectively. Exogenous application of BR showed that BRs either positively or negatively regulate ethylene biosynthesis in a concentration-dependent manner. Different from the BR induced ethylene biosynthesis through stabilizing ACSs stability, we found that the BR signaling transcription factors BES1 and BZR1 directly interacted with the promoters of ACS7, ACS9 and ACS11 to repress their expression, indicating a native regulation mechanism under physiological levels of BR. In addition, the det2-9 mutant displayed over accumulated superoxide anions (O2-) compared with the wild-type control, and the increased O2- level was shown to contribute to the inhibition of root growth. The BR-modulated control over the accumulation of O2- acted via the peroxidase pathway rather than via the NADPH oxidase pathway. This study reveals an important mechanism by which the hormone cross-regulation between BRs and ethylene or/and ROS is involved in controlling root growth and development in Arabidopsis.

cGMP signalling in plants: from enigma to main stream

All living organisms communicate with their environment, and part of this dialogue is mediated by secondary messengers such as cyclic guanosine mono phosphate (cGMP). In plants, most of the specific components that allow production and breakdown of cGMP have now been identified apart from cGMP dependent phosphodiesterases, enzymes responsible for cGMP catabolism. Irrespectively, the role of cGMP in plant signal transductions is now firmly established with involvement of this nucleotide in development, stress response, ion homeostasis and hormone function. Within these areas, several consistent themes where cGMP may be particularly relevant are slowly emerging: these include regulation of cation fluxes, for example via cyclic nucleotide gated channels and in stomatal functioning. Many details of signalling pathways that incorporate cGMP remain to be unveiled. These include downstream targets other than a small number of ion channels, in particular cGMP dependent kinases. Improved genomics tools may help in this respect, especially since many proteins involved in cGMP signalling appear to have multiple and often overlapping functional domains which hampers identification on the basis of simple homology searches. Another open question regards the topographical distribution of cGMP signals are they cell limited? Does long distance cGMP signalling occur and if so, by what mechanisms? The advent of non-disruptive fluorescent reporters with high spatial and temporal resolution will provide a tool to accelerate progress in all these areas. Automation can facilitate large scale screens of mutants or the action of effectors that impact on cGMP signalling.

BAK1 is involved in AtRALF1-induced inhibition of root cell expansion

The rapid alkalinization factor (RALF) peptide negatively regulates cell expansion, and an antagonistic relationship has been demonstrated between AtRALF1, a root-specific RALF isoform in Arabidopsis, and brassinosteroids (BRs). An evaluation of the response of BR signaling mutants to AtRALF1 revealed that BRI1-associated receptor kinase1 (bak1) mutants are insensitive to AtRALF1 root growth inhibition activity. BAK1 was essential for the induction of AtRALF1-responsive genes but showed no effect on the mobilization of Ca2+ and alkalinization responses. Homozygous plants accumulating AtRALF1 and lacking the BAK1 gene did not exhibit the characteristic semi-dwarf phenotype of AtRALF1-overexpressors. Biochemical evidence indicates that AtRALF1 and BAK1 physically interact with a Kd of 4.6 μM and acridinium-labeled AtRALF1 was used to demonstrate that part of the specific binding of AtRALF1 to intact seedlings and to a microsomal fraction derived from the roots of Arabidopsis plants is BAK1-dependent. Moreover, AtRALF1 induces an increase in BAK1 phosphorylation, suggesting that the binding of AtRALF1 to BAK1 is functional. These findings show that BAK1 contains an additional AtRALF1 binding site, indicating that this protein may be part of a AtRALF1-containing complex as a co-receptor, and it is required for the negative regulation of cell expansion.

Constitutive cyclic GMP accumulation in Arabidopsis thaliana compromises systemic acquired resistance induced by an avirulent pathogen by modulating local signals

The infection of Arabidopsis thaliana plants with avirulent pathogens causes the accumulation of cGMP with a biphasic profile downstream of nitric oxide signalling. However, plant enzymes that modulate cGMP levels have yet to be identified, so we generated transgenic A. thaliana plants expressing the rat soluble guanylate cyclase (GC) to increase genetically the level of cGMP and to study the function of cGMP in plant defence responses. Once confirmed that cGMP levels were higher in the GC transgenic lines than in wild-type controls, the GC transgenic plants were then challenged with bacterial pathogens and their defence responses were characterized. Although local resistance was similar in the GC transgenic and wild-type lines, differences in the redox state suggested potential cross-talk between cGMP and the glutathione redox system. Furthermore, large-scale transcriptomic and proteomic analysis highlighted the significant modulation of both gene expression and protein abundance at the infection site, inhibiting the establishment of systemic acquired resistance. Our data indicate that cGMP plays a key role in local responses controlling the induction of systemic acquired resistance in plants challenged with avirulent pathogens.

Identification of putative phosphoproteins in wheat spikes induced by Fusarium graminearum

Phosphorylation and dephosphorylation events were initiated in wheat scab resistance. The putative FHB-responsive phosphoproteins are mainly involved in three functional groups and contain at least one tyrosine, serine, or threonine phosphorylation site. Fusarium head blight (FHB), caused by Fusarium graminearum, is a severe disease in wheat. Protein phosphorylation plays an important role in plant-pathogen interactions, however, a global analysis of protein phosphorylation in response to FHB infection remains to be explored. To study the effect of FHB on the phosphorylation state of wheat proteins, proteins extracted from spikes of a resistant wheat cultivar after 6 h of inoculation with F. graminearum or sterile H2O were separated by two-dimensional gel electrophoresis, and then the immunodetection of putative phosphoproteins was conducted by Western blotting using specific anti-phosphotyrosine antibody, anti-phosphothreonine antibody and anti-phosphoserine antibody. A total of 35 phosphorylated signals was detected and protein identities of 28 spots were determined. Functional categorization showed that the putative FHB-responsive phosphoproteins were mainly involved in defense/stress response, signal transduction, and metabolism. The phosphorylation status of proteins associated with signaling pathways mediated by salicylic acid, calcium ions, small GTPase, as well as with detoxification, reactive oxygen species scavenging, antimicrobial compound synthesis, and cell wall fortification was regulated in wheat spikes in response to F. graminearum infection. The present study reveals dynamics of wheat phosphoproteome in response to F. graminearum infection and suggests an important role of protein Ser/Thr/Tyr phosphorylation in fundamental mechanisms of wheat scab resistance.

Reverse function of ROS-induced CBL10 during salt and drought stress responses

Cellular levels of Ca(2+) and reactive oxygen species (ROS) are maintained at low levels in the cytosol but fluctuate greatly when acting as second messengers to decode environmental and developmental signals. Phytohormones are primary signals leading to various changes in ROS or Ca(2+) signaling during synergistic and antagonistic cross-talk. In this study, we found that brassinosteroids (BRs), hormones involved in diverse plant developmental processes, promote ROS production. To identify downstream signaling components of ROS during BR-mediated plant development, we searched for genes whose expression remained unchanged by ROS only in BR- signaling mutants and found calcineurin B-like (CBL) 10, which encodes a CBL should be changed to CBL10. protein that senses calcium. ROS-induced CBL10 expression was nullified and endogenous CBL10 expression in the shoot was low in the BR-signaling mutant. Using a cbl10 mutant and a transgenic plant overexpressing CBL10, we showed that BR sensitivity during hypocotyl growth decreased in the cbl10 mutant under salt stress, providing an additional mechanism for positive regulation of salt stress by CBL10. We also demonstrated that CBL10 negatively affects tolerance to drought and is not mediated by abscisic acid-induced signaling. Our results suggest that Ca(2+) signaling through CBL10 differently affects the response to abiotic stresses, partly by regulating BR sensitivity of plant tissues.

The CLAVATA signaling pathway mediating stem cell fate in shoot meristems requires Ca(2+) as a secondary cytosolic messenger

CLAVATA1 (CLV1) is a receptor protein expressed in the shoot apical meristem (SAM) that translates perception of a non-cell-autonomous CLAVATA3 (CLV3) peptide signal into altered stem cell fate. CLV3 reduces expression of WUSCHEL (WUS) and FANTASTIC FOUR 2 (FAF2) in the SAM. Expression of WUS and FAF2 leads to maintenance of undifferentiated stem cells in the SAM. CLV3 binding to CLV1 inhibits expression of these genes and controls stem cell fate in the SAM through an unidentified signaling pathway. Cytosolic Ca(2+) elevations, cyclic nucleotide (cGMP)-activated Ca(2+) channels, and cGMP have been linked to signaling downstream of receptors similar to CLV1. Hence, we hypothesized that cytosolic Ca(2+) elevation mediates the CLV3 ligand/CLV1 receptor signaling that controls meristem stem cell fate. CLV3 application to Arabidopsis seedlings results in elevation of cytosolic Ca(2+) and cGMP. CLV3 control of WUS was prevented in a genotype lacking a functional cGMP-activated Ca(2+) channel. In wild-type plants, CLV3 inhibition of WUS and FAF2 expression was impaired by treatment with either a Ca(2+) channel blocker or a guanylyl cyclase inhibitor. When CLV3-dependent repression of WUS is blocked, altered control of stem cell fate leads to an increase in SAM size; we observed a larger SAM size in seedlings treated with the Ca(2+) channel blocker. These results suggest that the CLV3 ligand/CLV1 receptor system initiates a signaling cascade that elevates cytosolic Ca(2+), and that this cytosolic secondary messenger is involved in the signal transduction cascade linking CLV3/CLV1 to control of gene expression and stem cell fate in the SAM.

Gene expression and functional analyses in brassinosteroid-mediated stress tolerance

The plant hormone brassinosteroid (BR) plays essential roles in plant growth and development, while also controlling plant stress responses. This dual ability of BR is intriguing from a mechanistic point of view and as a viable solution for stabilizing crop yields under the changing climatic conditions. Here we report a time course analysis of BR responses under both stress and no-stress conditions, the results of which establish that BR incorporates many stress-related features even under no-stress conditions, which are then accompanied by a dynamic stress response under unfavourable conditions. Found within the BR transcriptome were distinct molecular signatures of two stress hormones, abscisic acid and jasmonic acid, which were correlated with enhanced endogenous levels of the two hormones in BR-treated seedlings. The marked presence of genes related to protein metabolism and modification, defence responses and calcium signalling highlights the significance of their associated mechanisms and roles in BR processes. Functional analysis of loss-of-function mutants of a subset of genes selected from the BR transcriptome identified abiotic stress-related roles for ACID PHOSPHATASE5 (ACP5), WRKY33, JACALIN-RELATED LECTIN1-3 (JAC-LEC1-3) and a BR-RESPONSIVE-RECEPTOR-LIKE KINASE (BRRLK). Overall, the results of this study provide a clear link between the molecular changes impacted by BR and its ability to confer broad-range stress tolerance, emphasize the importance of post-translational modification and protein turnover as BR regulatory mechanisms and demonstrate the BR transcriptome as a repertoire of new stress-related regulatory and structural genes.

Role of the proteome in phytohormonal signaling

Phytohormones are orchestrators of plant growth and development. A lot of time and effort has been invested in attempting to comprehend their complex signaling pathways but despite success in elucidating some key components, molecular mechanisms in the transduction pathways are far from being resolved. The last decade has seen a boom in the analysis of phytohormone-responsive proteins. Abscisic acid, auxin, brassinosteroids, cytokinin, ethylene, gibberellins, nitric oxide, oxylipins, strigolactones, salicylic acid--all have been analyzed to various degrees. For this review, we collected data from proteome-wide analyses resulting in a list of over 2000 annotated proteins from Arabidopsis proteomics and nearly 500 manually filtered protein families merged from all the data available from different species. We present the currently accepted model of phytohormone signaling, highlight the contributions made by proteomic-based research and describe the key nodes in phytohormone signaling networks, as revealed by proteome analysis. These include ubiquitination and proteasome mediated degradation, calcium ion signaling, redox homeostasis, and phosphoproteome dynamics. Finally, we discuss potential pitfalls and future perspectives in the field. This article is part of a Special Issue entitled: Plant Proteomics--a bridge between fundamental processes and crop production, edited by Dr. Hans-Peter Mock.

Calcium and ZmCCaMK are involved in brassinosteroid-induced antioxidant defense in maize leaves

Brassinosteroids (BRs) have been shown to enhance stress tolerance by inducing antioxidant defense systems. However, the mechanisms of BR-induced antioxidant defense in plants remain to be determined. In this study, the role of calcium (Ca(2+)) and maize calcium/calmodulin-dependent protein kinase (CCaMK), ZmCCaMK, in BR-induced antioxidant defense, and the relationship between ZmCCaMK and Ca(2+) in BR signaling were investigated. BR treatment led to a significant increase in cytosolic Ca(2+) concentration in protoplasts from maize mesophyll, and Ca(2+) was shown to be required for BR-induced antioxidant defense. Treatment with BR induced increases in gene expression and enzyme activity of ZmCCaMK in maize leaves. Transient overexpression and silencing of ZmCCaMK in maize protoplasts demonstrated that ZmCCaMK was required for BR-induced antioxidant defense. The requirement for CCaMK was further investigated using a loss-of-function mutant of OsCCaMK, the orthologous gene of ZmCCaMK in rice. Consistent with the findings in maize, BR treatment could not induce antioxidant defense in the rice OsCCAMK mutant. Furthermore, Ca(2+) was required for BR-induced gene expression and activation of ZmCCaMK, while ZmCCaMK was shown to enhance the BR-induced increase in cytosolic Ca(2+) concentration. Moreover, our results also showed that ZmCCaMK and H2O2 influenced each other. These results indicate that Ca(2+) works together with ZmCCaMK in BR-induced antioxidant defense, and there are two positive feedback loops between Ca(2+) or H2O2 and ZmCCaMK in BR signaling in maize.

Interplay between reactive oxygen species and hormones in the control of plant development and stress tolerance

As a consequence of a sessile lifestyle, plants are continuously exposed to changing environmental conditions and often life-threatening stresses caused by exposure to excessive light, extremes of temperature, limiting nutrient or water availability, and pathogen/insect attack. The flexible coordination of plant growth and development is necessary to optimize vigour and fitness in a changing environment through rapid and appropriate responses to such stresses. The concept that reactive oxygen species (ROS) are versatile signalling molecules in plants that contribute to stress acclimation is well established. This review provides an overview of our current knowledge of how ROS production and signalling are integrated with the action of auxin, brassinosteroids, gibberellins, abscisic acid, ethylene, strigolactones, salicylic acid, and jasmonic acid in the coordinate regulation of plant growth and stress tolerance. We consider the local and systemic crosstalk between ROS and hormonal signalling pathways and identify multiple points of reciprocal control, as well as providing insights into the integration nodes that involve Ca(2+)-dependent processes and mitogen-activated protein kinase phosphorylation cascades.

Cation channels are involved in brassinosteroid signalling in higher plants

Brassinosteroids (BRs) are an important class of plant hormones with a multitude of functions. They have been intensively investigated for their biosynthesis, distribution and physiological functions. The aim of this study was to examine possible effects of BRs on the plant plasma membrane cation conductances and Ca(2+) signalling. The wheat root protoplasts (tested by patch-clamping) and excised arabidopsis roots (analysed by Ca(2+)-aequorin chemiluminometry), were used. In the whole-cell plasma membrane patches, 24-epibrassinolide, 28-homobrassionolide or 24-epicastasterone (1 μM) were applied exogenously. 24-Epicastasterone increased the activity of the K(+) efflux conductance in 50% of tested protoplasts while 24-epibrassonolide and 28-homobrassionolide did not modify the plasma membrane currents. Addition of 24-epicastasterone at the cytosolic side (to the pipette solution) resulted in dramatic stimulation of a time-dependent K(+) efflux current (in 30% of protoplasts) and an activation of Ca(2+) influx currents (in 30% of protoplasts). Gadolinium ions, which are blockers of cation channels, inhibited the 24-epicastasterone-induced cation channel activities. In Arabidopsis thaliana plants constitutively expressing aequorin, exogenous 24-epibrassonolide, 28-homobrassionolide and 24-epicastasterone induced a transient elevation of the cytosolic free Ca(2+), which was inhibited by Gd(3+) and mediated by Ca(2+) influx from the bathing solution. In Ca(2+)-aequorin tests, 10 μM of exogenous BRs was the minimal concentration at which statistically significant changes of the cytosolic Ca(2+) were observed. In conclusion, the obtained results suggest that the plasma membrane of root cells contains the brassinosteroid-activated cation-permeable channels, which can probably be involved in rapid regulation of the K(+) homeostasis and Ca(2+) signalling.

Growth control: brassinosteroid activity gets context

Brassinosteroid activity controls plant growth and development, often in a seemingly opposing or complex manner. Differential impact of the hormone and its signalling components, acting both as promoters and inhibitors of organ growth, is exemplified by meristem differentiation and cell expansion in above- and below-ground organs. Complex brassinosteroid-based control of stomata count and lateral root development has also been demonstrated. Here, mechanisms underlying these phenotypic outputs are examined. Among these, studies uncovering core brassinosteroid signalling components, which integrate with distinct peptide, hormone, and environmental pathways, are reviewed. Finally, the differential spatiotemporal context of brassinosteroid activity within the organ, as an important determinant of controlled growth, is discussed.

Developmentally distinct activities of the exocyst enable rapid cell elongation and determine meristem size during primary root growth in Arabidopsis

BACKGROUND

Exocytosis is integral to root growth: trafficking components of systems that control growth (e.g., PIN auxin transport proteins) to the plasma membrane, and secreting materials that expand the cell wall to the apoplast. Spatiotemporal regulation of exocytosis in eukaryotes often involves the exocyst, an octameric complex that tethers selected secretory vesicles to specific sites on the plasma membrane and facilitates their exocytosis. We evaluated Arabidopsis lines with mutations in four exocyst components (SEC5, SEC8, EXO70A1 and EXO84B) to explore exocyst function in primary root growth.

RESULTS

The mutants have root growth rates that are 82% to 11% of wild-type. Even in lines with the most severe defects, the organization of the quiescent center and tissue layers at the root tips appears similar to wild-type, although meristematic, transition, and elongation zones are shorter. Reduced cell production rates in the mutants are due to the shorter meristems, but not to lengthened cell cycles. Additionally, mutants demonstrate reduced anisotropic cell expansion in the elongation zone, but not the meristematic zone, resulting in shorter mature cells that are similar in shape to wild-type. As expected, hypersensitivity to brefeldin A links the mutant root growth defect to altered vesicular trafficking. Several experimental approaches (e.g., dose-response measurements, localization of signaling components) failed to identify aberrant auxin or brassinosteroid signaling as a primary driver for reduced root growth in exocyst mutants.

CONCLUSIONS

The exocyst participates in two spatially distinct developmental processes, apparently by mechanisms not directly linked to auxin or brassinosteroid signaling pathways, to help establish root meristem size, and to facilitate rapid cell expansion in the elongation zone.

Ca2+ signalling in plant immune response: from pattern recognition receptors to Ca2+ decoding mechanisms

Ca2+ is a ubiquitous second messenger for cellular signalling in various stresses and developmental processes. Here, we summarize current developments in the roles of Ca2+ during plant immunity responses. We discuss the early perception events preceding and necessary for triggering cellular Ca2+ fluxes, the potential Ca2+-permeable channels, the decoding of Ca2+ signals predominantly via Ca2+-dependent phosphorylation events and transcriptional reprogramming. To highlight the complexity of the cellular signal network, we briefly touch on the interplay between Ca2+-dependent signalling and selected major signalling mechanisms--with special emphasis on reactive oxygen species at local and systemic levels.

PAG1, a cotton brassinosteroid catabolism gene, modulates fiber elongation

Cotton (Gossypium hirsutum) is the major source of natural textile fibers. Brassinosteroids (BRs) play crucial roles in regulating fiber development. The molecular mechanisms of BRs in regulating fiber elongation, however, are poorly understood. pagoda1 (pag1) was identified via an activation tagging genetic screen and characterized by genome walking and brassinolide (BL) supplementation. RNA-Seq analysis was employed to elucidate the mechanisms of PAG1 in regulating fiber development. pag1 exhibited dwarfism and reduced fiber length due to significant inhibition of cell elongation and expansion. BL treatment rescued its growth and fiber elongation. PAG1 encodes a homolog of Arabidopsis CYP734A1 that inactivates BRs via C-26 hydroxylation. RNA-Seq analyses showed that the constitutive expression of PAG1 downregulated the expression of genes involved in very-long-chain fatty acids (VLCFA) biosynthesis, ethylene-mediated signaling, response to cadmium, cell wall development, cytoskeleton organization and cell growth. Our results demonstrate that PAG1 plays crucial roles in regulating fiber development via controlling the level of endogenous bioactive BRs, which may affect ethylene signaling cascade by mediating VLCFA. Therefore, BR may be a critical regulator of fiber elongation, a role which may in turn be linked to effects on VLCFA biosynthesis, ethylene and cadmium signaling, cell wall- and cytoskeleton-related gene expression.