Plant Cell Signaling Research Articles

Mitochondrial Complex I Disruption Causes Broad Reorchestration of Plant Lipidome Including Chloroplast Lipids

Mitochondrial complex I (CI) plays a crucial role in oxidising NADH generated by the metabolism (including photorespiration) and thereby participates in the mitochondrial electron transfer chain feeding oxidative phosphorylation that generates ATP. However, CI mutations are not lethal in plants and cause moderate phenotypes, and therefore CI mutants are instrumental to examine consequences of mitochondrial homeostasis disturbance on plant cell metabolisms and signalling. To date, the consequences of CI disruption on the lipidome have not been examined. Yet, in principle, mitochondrial dysfunction should impact on lipid synthesis through chloroplasts (via changes in photorespiration, redox homeostasis, and N metabolism) and the endoplasmic reticulum (ER) (via perturbed mitochondrion-ER crosstalk). Here, we took advantage of lipidomics technology (by LC-MS), phospholipid quantitation by 31P-NMR, and total lipid quantitation to assess the impact of CI disruption on leaf, pollen, and seed lipids using three well-characterised CI mutants: CMSII in N. sylvestris and both ndufs4 and ndufs8 in Arabidopsis. Our results show multiple changes in cellular lipids, including galactolipids (chloroplastic), sphingolipids, and ceramides (synthesised by ER), suggesting that mitochondrial homeostasis is essential for the regulation of whole cellular lipidome via specific signalling pathways. In particular, the observed modifications in phospholipid and sphingolipid/ceramide molecular species suggest that CI activity controls phosphatidic acid-mediated signalling.

Functional Diversification and Structural Origins of Plant Natural Product Methyltransferases.

In plants, methylation is a common step in specialized metabolic pathways, leading to a vast diversity of natural products. The methylation of these small molecules is catalyzed by S-adenosyl-l-methionine (SAM)-dependent methyltransferases, which are categorized based on the methyl-accepting atom (O, N, C, S, or Se). These methyltransferases are responsible for the transformation of metabolites involved in plant defense response, pigments, and cell signaling. Plant natural product methyltransferases are part of the Class I methyltransferase-superfamily containing the canonical Rossmann fold. Recent advances in genomics have accelerated the functional characterization of plant natural product methyltransferases, allowing for the determination of substrate specificities and regioselectivity and further realizing the potential for enzyme engineering. This review compiles known biochemically characterized plant natural product methyltransferases that have contributed to our knowledge in the diversification of small molecules mediated by methylation steps.

The MKK2a Gene Involved in the MAPK Signaling Cascades Enhances Populus Salt Tolerance.

Mitogen-activated protein kinase (MAPK) cascades are highly conserved signal transduction modules, which transmit environmental signals in plant cells through stepwise phosphorylation and play indispensable roles in a wide range of physiological and biochemical processes. Here, we isolated and characterized a gene encoding MKK2 protein from poplar through the rapid amplification of cDNA ends (RACE). The full-length PeMKK2a gene was 1571 bp, including a 1068 bp open reading frame (ORF) encoding 355 amino acids, and the putative PeMKK2a protein belongs to the PKc_like (protein kinase domain) family (70–336 amino acids) in the PKc_MAPKK_plant subfamily and contains 62 sites of possible phosphorylation and two conserved domains, DLK and S/T-xxxxx-S/T. Detailed information about its gene structure, sequence similarities, subcellular localization, and transcript profiles under salt-stress conditions was revealed. Transgenic poplar lines overexpressing PeMKK2a exhibited higher activities of superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD) than non-transgenic poplar under salt stress conditions. These results will provide insight into the roles of MAPK signaling cascades in poplar response to salt stress.

Overexpression of a cotton nonspecific lipid transfer protein gene, GhLTP4, enhances drought tolerance by remodeling lipid profiles, regulating abscisic acid homeostasis and improving tricarboxylic acid cycle in cotton

Drought is a major abiotic stress that causes severely crop yield loss worldwide. Lipid transfer proteins (LTPs) play an essential role during plant growth and cell signaling transduction associated with stress responses. However, the regulatory circuits involved in drought stress remain largely unresolved. Here, we reported that a nonspecific lipid transfer protein, namely GhLTP4 , positively modulated drought stress tolerance in cotton. GhLTP4 (synonym Pfs6 , GenBank: ABO42261) was preferentially expressed in fiber cells, while, it was also strongly induced by phytohormone abscisic acid (ABA) and drought stress treatments. Compared with wild type, over- and down-expression of GhLTP4 in cotton led to the increased and decreased drought tolerance, respectively. Overexpression of GhLTP4 remarkably increased lipid components and the accumulation of culticular waxes in transgenic cotton leaves under normal condition. Under drought stress, GhLTP4 -overexpressing transgenic cotton exhibited the remodeled lipid profiles, more ABA contents, and the improved tricarboxylic acid cycle process. Taken together, we reveal the function and underlying mechanisms of GhLTP4 in regulating cotton drought tolerance and create valuable genetic accessions for cotton drought-tolerant breeding. • Overexpression of GhLTP4 enhances significantly cotton drought tolerance. • Overexpression of GhLTP4 alters the lipid profiles of cotton leaf tissue. • Overexpression of GhLTP4 improves the drought tolerance by remodeling lipid profiles and ABA homeostasis. • Overexpression of GhLTP4 promotes tricarboxylic acid metabolism under drought stress.

PYR/PYL/RCAR Receptors Play a Vital Role in the Abscisic-Acid-Dependent Responses of Plants to External or Internal Stimuli

Abscisic acid (ABA) is a phytohormone that plays a key role in regulating several developmental processes as well as in response to stressful conditions such as drought. Activation of the ABA signaling cascade allows the induction of an appropriate physiological response. The basic components of the ABA signaling pathway have been recognized and characterized in recent years. Pyrabactin resistance, pyrabactin resistance-like, and the regulatory component of ABA receptors (PYR/PYL/RCAR) are the major components responsible for the regulation of the ABA signaling pathway. Here, we review recent findings concerning the PYR/PYL/RCAR receptor structure, function, and interaction with other components of the ABA signaling pathway as well as the termination mechanism of ABA signals in plant cells. Since ABA is one of the basic elements related to abiotic stress, which is increasingly common in the era of climate changes, understanding the perception and transduction of the signal related to this phytohormone is of paramount importance in further increasing crop tolerance to various stress factors.

Structural Insights into Methylated DNA Recognition by the Methyl-CpG Binding Domain of MBD6 from Arabidopsis thaliana.

Cytosine methylation is an epigenetic modification essential for formation of mature heterochromatin, gene silencing, and genomic stability. In plants, methylation occurs not only at cytosine bases in CpG but also in CpHpG and CpHpH contexts, where H denotes A, T, or C. Methyl-CpG binding domain (MBD) proteins, which recognize symmetrical methyl-CpG dinucleotides and act as gene repressors in mammalian cells, are also present in plant cells, although their structural and functional properties still remain poorly understood. To fill this gap, in this study, we determined the solution structure of the MBD domain of the MBD6 protein from Arabidopsis thaliana and investigated its binding properties to methylated DNA by binding assays and an in-depth NMR spectroscopic analysis. The AtMBD6 MBD domain folds into a canonical MBD structure in line with its binding specificity toward methyl-CpG and possesses a DNA binding interface similar to mammalian MBD domains. Intriguingly, however, the binding affinity of the AtMBD6 MBD domain toward methyl-CpG-containing DNA was found to be much lower than that of known mammalian MBD domains. The main difference arises from the absence of positively charged residues in AtMBD6 that supposedly interact with the DNA backbone as seen in mammalian MBD/methyl-CpG-containing DNA complexes. Taken together, we have established a structural basis for methyl-CpG recognition by AtMBD6 to develop a deeper understanding how MBD proteins work as mediators of epigenetic signals in plant cells.

Calcium/Calmodulin-Mediated Defense Signaling: What Is Looming on the Horizon for AtSR1/CAMTA3-Mediated Signaling in Plant Immunity.

Calcium (Ca2+) signaling in plant cells is an essential and early event during plant-microbe interactions. The recognition of microbe-derived molecules activates Ca2+ channels or Ca2+ pumps that trigger a transient increase in Ca2+ in the cytoplasm. The Ca2+ binding proteins (such as CBL, CPK, CaM, and CML), known as Ca2+ sensors, relay the Ca2+ signal into down-stream signaling events, e.g., activating transcription factors in the nucleus. For example, CaM and CML decode the Ca2+ signals to the CaM/CML-binding protein, especially CaM-binding transcription factors (AtSRs/CAMTAs), to induce the expressions of immune-related genes. In this review, we discuss the recent breakthroughs in down-stream Ca2+ signaling as a dynamic process, subjected to continuous variation and gradual change. AtSR1/CAMTA3 is a CaM-mediated transcription factor that represses plant immunity in non-stressful environments. Stress-triggered Ca2+ spikes impact the Ca2+-CaM-AtSR1 complex to control plant immune response. We also discuss other regulatory mechanisms in which Ca2+ signaling activates CPKs and MAPKs cascades followed by regulating the function of AtSR1 by changing its stability, phosphorylation status, and subcellular localization during plant defense.

Influence of change in ROS production in plant cells on resistance of agricultural plants to oxidative stress

The paper considers theoretical data on the mechanisms of the formation of reactive oxygen species (ROS) and the transduction of stress signals in plant cells. The negative effects of ROS with excessive generation are described. A method of protoplast isolation and a fluorescent microscopy were selected for the experiments. Also, in this work, a comparative analysis of the production of ROS during hypoxia and reaeration for agricultural plants that are differently resistant to the described stress is carried out. Based on the results of the experimental part of the work, conclusions were drawn and the practical application of this study was described.

Determination of Reactive Carbonyl Species, Which Mediate Reactive Oxygen Species Signals in Plant Cells.

Responses of plant cells to reactive oxygen species (ROS), e.g., reprogramming of defense genes or progression of cell death, should include the ROS signal transmission to target proteins, but the biochemistry of this process is largely unknown. Lipid peroxide-derived α,β-unsaturated aldehydes and ketones (reactive carbonyl species; RCS), downstream products of ROS stimuli, are recently emerging endogenous agents that can mediate ROS signal to proteins via covalent modification. The involvement of RCS in certain ROS signaling in plants (oxidative injury of leaves and roots, ROS-induced programmed cell death, senescence, and abscisic acid and auxin signaling) has been verified by the determination of RCS with the use of conventional HPLC. Because distinct kinds of RCS act differently in the cell and so are metabolized, identification and quantification of each RCS in plant tissues provide central information to decipher biochemical mechanisms of plant responses to ROS. This article illustrates practical methods of plant sample preparation and extraction and analysis of RCS.

Hormone mediated cell signaling in plants under changing environmental stress

Phytohormones control particular forms of plant development, maturation and response to external stimuli. Many studies have identified the specific, individual, and non-redundant role of each of these organic natural components in plant maturation and response to surrounding constraints. This work revolves around the individual role of each hormone in plant signaling. Certain hormone responses are not necessarily related to gene regulation, yet all these hormones are involved in modulating gene expression by controlling either the abundance of transcriptional factors, repressors, or their activities through post-translational modifications. Coordination between different hormone signaling, and other pathways such as ROS regulatory mechanisms could be altered by timing and types of stresses, depending on plant species under environmental stress. Hence, we describe the role of phytohormones in plants after exposure to external stimuli. The complex-mode of hormone signaling under abiotic and biotic stress will be discussed.

Transport of organic anions in root cells and its role in cell signaling in higher plants

The organic anion balance is critical for metabolic, bioenergetic, and electrochemical processes in plant cells, controlling the quality and quantity of yield and plant stress resistance. Nevertheless, the redistribution and membrane transport of these substances in plant tissues have not been investigated in detail. The mechanism of passive anion efflux from a plant cell through the ion channels has not been established so far. Here, using the patch-clamp technique, we have characterized the ion channel-mediated conductances of ascorbate, malate, gluconate, citrate, fumarate, and pronionate in the root cells of Arabidopsis thaliana, Triticum aestivum, and Helianthus annuus. These conductances showed high permeability to ascorbate, malate, and citrate, as well as low permeability to fumarate, propionate, and gluconate. Anion channel conductances of root cells showed rapid activation kinetics and low potential dependence. They were also inhibited by 9-anthracenecarboxylic acid, suggesting that they belong to the ALMT family of anion channels found only in higher plants. Aequorin chemilu minometry was used to test the effect of organic anions on the Ca2+ signaling in root cells. Among four organic anions tested, only ascorbate induced a significant increase in the cytosolic Ca2+ activity at physiological levels (1 and 10 mM). This effect may underlie the previously unknown functions of exogenous ascorbate related to short- and long-distance signaling in higher plants.

A metabolomics insight into the Cyclic Nucleotide Monophosphate signaling cascade in tomato under non-stress and salinity conditions

Cyclic Nucleotides Monophosphate (cNMP) are key signalling compounds whose role in plant cell signal transduction is still poorly understood. In this work we used sildenafil, a phosphodiesterase (PDE) inhibitor used in human, to amplify the signal cascade triggered by cNMP using tomato as model plant. Metabolomics was then used, together with plant growth and root architecture parameters, to unravel the changes elicited by PDE inhibition either under non-stress and 100 mM NaCl salinity conditions.The PDE inhibitor elicited a significant increase in biomass (+62 %) and root length (+56 %) under no stress conditions, and affected root architecture in terms of distribution over diameter classes. Together with cGMP, others cNMP were modulated by the treatment. Moreover, PDE inhibition triggered a broad metabolic reprogramming involving photosynthesis and secondary metabolism. A complex crosstalk network of phytohormones and other signalling compounds could be observed in treated plants. Nonetheless, metabolites related to redox imbalance processes and NO signalling could be highlighted in tomato following PDE application. Despite salinity damped down the growth-promoting effects of sildenafil, interesting implications in plant mitigation to stress-related detrimental effects could be observed.

Carbon dioxide-dependent signal transduction in mammalian systems.

Carbon dioxide (CO2) is a fundamental physiological gas known to profoundly influence the behaviour and health of millions of species within the plant and animal kingdoms in particular. A recent Royal Society meeting on the topic of 'Carbon dioxide detection in biological systems' was extremely revealing in terms of the multitude of roles that different levels of CO2 play in influencing plants and animals alike. While outstanding research has been performed by leading researchers in the area of plant biology, neuronal sensing, cell signalling, gas transport, inflammation, lung function and clinical medicine, there is still much to be learned about CO2-dependent sensing and signalling. Notably, while several key signal transduction pathways and nodes of activity have been identified in plants and animals respectively, the precise wiring and sensitivity of these pathways to CO2 remains to be fully elucidated. In this article, we will give an overview of the literature relating to CO2-dependent signal transduction in mammalian systems. We will highlight the main signal transduction hubs through which CO2-dependent signalling is elicited with a view to better understanding the complex physiological response to CO2 in mammalian systems. The main topics of discussion in this article relate to how changes in CO2 influence cellular function through modulation of signal transduction networks influenced by pH, mitochondrial function, adenylate cyclase, calcium, transcriptional regulators, the adenosine monophosphate-activated protein kinase pathway and direct CO2-dependent protein modifications. While each of these topics will be discussed independently, there is evidence of significant cross-talk between these signal transduction pathways as they respond to changes in CO2. In considering these core hubs of CO2-dependent signal transduction, we hope to delineate common elements and identify areas in which future research could be best directed.

Polyamines as key regulatory players in plants under metal stress—A way for an enhanced tolerance

AbstractPolyamine (PA) metabolism and functions have started to raise attention from plant scientists in the last years. PAs have been investigated for their involvement in plant cell signalling and protection, proving that alterations in their endogenous levels can affect plant growth, development and survival. The recognised roles of PAs in metal‐stressed plants are presented and discussed on a “case‐study” basis, for each metal. Bearing in mind that the contamination of soils by heavy metals (HMs) is a growing problem worldwide, it is important to find efficient mechanisms through which agricultural productivity and food quality are safeguarded in a scenario of increased pollution. Making cultivars more tolerant to HM‐stress, capable of detoxifying or accumulating them safely, is a goal that will most certainly benefit from researching the functions and applicability of PAs. To date, plant PAs have been recognised for their roles as membrane‐, protein‐ and nucleic acid‐stabilisers, as protectors of cellular integrity and photosynthetic machinery, as direct and indirect signalling agents, and as emerging members of the non‐enzymatic antioxidant system. Moreover, PAs are not only important in normal plant developmental processes but have also been suggested to induce stress priming, to act as epigenetic regulators of gene expression and to enhance the detoxification and vacuolar compartmentalization of HMs. Although the stress‐ameliorating effects of PAs have been widely studied for several abiotic stresses, not much is known regarding their effects on metal induced‐stress, except for Cd. This review summarises the available work on the effects of PAs in plants exposed to Cu, Cd, Fe, Mn, Cr, Ni, Hg, Al, Pb and Zn.

Protein Phosphorylation in Plant Cell Signaling.

Owing to their sessile nature, plants have evolved sophisticated sensory mechanisms to respond quickly and precisely to the changing environment. The extracellular stimuli are perceived and integrated by diverse receptors, such as receptor-like protein kinases (RLKs) and receptor-like proteins (RLPs), and then transmitted to the nucleus by complex cellular signaling networks, which play vital roles in biological processes including plant growth, development, reproduction, and stress responses. The posttranslational modifications (PTMs) are important regulators for the diversification of protein functions in plant cell signaling. Protein phosphorylation is an important and well-characterized form of the PTMs, which influences the functions of many receptors and key components in cellular signaling. Protein phosphorylation in plants predominantly occurs on serine (Ser) and threonine (Thr) residues, which is dynamically and reversibly catalyzed by protein kinases and protein phosphatases, respectively. In this review, we focus on the function of protein phosphorylation in plant cell signaling, especially plant hormone signaling, and highlight the roles of protein phosphorylation in plant abiotic stress responses.

Signal Integration by Cyclin-Dependent Kinase 8 (CDK8) Module and Other Mediator Subunits in Biotic and Abiotic Stress Responses.

Environmental stresses have driven plants to develop various mechanisms to acclimate in adverse conditions. Extensive studies have demonstrated that a significant reprogramming occurs in the plant transcriptome in response to biotic and abiotic stresses. The highly conserved and large multi-subunit transcriptional co-activator of eukaryotes, known as the Mediator, has been reported to play a substantial role in the regulation of important genes that help plants respond to environmental perturbances. CDK8 module is a relatively new component of the Mediator complex that has been shown to contribute to plants’ defense, development, and stress responses. Previous studies reported that CDK8 module predominantly acts as a transcriptional repressor in eukaryotic cells by reversibly associating with core Mediator. However, growing evidence has demonstrated that depending on the type of biotic and abiotic stress, the CDK8 module may perform a contrasting regulatory role. This review will summarize the current knowledge of CDK8 module as well as other previously documented Mediator subunits in plant cell signaling under stress conditions.

H2O2 signaling modulates Glycoprotein-1 induced programmed cell death in tobacco suspension cells

Glycoprotein (GP)-1 is a glycoprotein elicitor with antiviral activity found in Streptomyces kanasensis zx01. GP-1 can induce programmed cell death (PCD) in vitro; however, the underlying mechanism is unclear. In the present study, we demonstrated that GP-1 induced PCD in tobacco suspension cells, which was modulated by hydrogen peroxide (H2O2). GP-1 participated in and modulated biologically relevant signaling in plant cells. GP-1 induced tobacco cell death in a dose- and time-dependent manner; affected the expression of BRI1-associated receptor kinase 1 (BAK1) and the accumulation of salicylic acid (SA), which are related to PCD; and enzymatic activities and mitochondrial functions. In conclusion, GP-1-induced PCD in tobacco may be mediated by H2O2 which alters BAK1 and SA levels, as well as mitochondrial and gene function. This cell signal cascade played an important role in the process of GP-1 induced plant disease resistance.

Extracellular Ca2+ induces desensitized cytosolic Ca2+ rise sensitive to phospholipase C inhibitor which suppresses root growth with Ca2+ dependence

Calcium (Ca) is an essential element for all organisms. In animal cells, the plasma membrane-localized Ca receptor CaSR coupled to a phospholipase C (PLC)-dependent signaling cascade monitors extracellular Ca2+ concentrations ([Ca2+]ext) and responds with increases in cytosolic calcium concentrations ([Ca2+]cyt). Plant roots encounter variable soil conditions, but how they sense changes in [Ca2+]ext is largely unknown. In this study, we demonstrate that increasing [Ca2+]ext evokes a transient increase in [Ca2+] in the cytosol, mitochondria, and nuclei of Arabidopsis thaliana root cells. These increases were strongly desensitized to repeat applications of [Ca2+]ext, a typical feature of receptor-mediated cellular signaling in animal and plant cells. Treatment with gadolinium (Gd3+), a CaSR activator in animal cells, induced concentration-dependent increases in [Ca2+]cyt in roots, which showed self-desensitization and cross-desensitization to [Ca2+]ext-induced increases in [Ca2+]cyt (EICC). EICC was sensitive to extracellular H+, K+, Na+, and Mg2+ levels. Treatment with the PLC inhibitor neomycin suppressed EICC and Ca accumulation in roots. The inhibitory effect of neomycin on root elongation was fully rescued by increasing [Ca2+]ext but not [Mg2+] or [K+] in the growth medium. These results suggest that [Ca2+]ext and the movement of Ca2+ into the cytosol of plant roots are regulated by a receptor-mediated signaling pathway involving PLC.

Hechtian Strands Transmit Cell Wall Integrity Signals in Plant Cells.

Hechtian strands are thread-like structures in plasmolyzed plant cells that connect the cell wall to the plasma membrane. Although these strands were first observed more than 100 years ago, their physiological roles are largely unknown. Here, we used intracellular laser microdissection to examine the effects of disrupting Hechtian strands on plasmolyzed tobacco BY-2 cells. When we focused femtosecond laser pulses on Hechtian strands, targeted disruptions were induced, but no visible changes in cell morphology were detected. However, the calcofluor white signals from β-glucans was detected in plasmolyzed cells with disrupted Hechtian strands, whereas no signals were detected in untreated plasmolyzed cells. These results suggest that Hechtian strands play roles in sensing cell wall integrity.

Plant Cell Signaling: SUMO Is under the Influence of Steroids and Salt.

How do plants reduce growth when they experience high salinity? A new study provides insight into how salt stress impinges on a plant steroid hormone signaling pathway to dampen plant growth.