Fungal elicitor Pep-25 increases cytosolic calcium ions, H2O2 production and activates the octadecanoid pathway in Arabidopsis thaliana

Integration of Electrical Signals and Phytohormones in the Control of Systemic Response

Plants are constantly exposed to environmental stresses. Local stimuli sensed by one part of a plant are translated into long-distance signals that can influence the activities in distant tissues. Changes in levels of phytohormones in distant parts of the plant occur in response to various local stimuli. The regulation of hormone levels can be mediated by long-distance electrical signals, which are also induced by local stimulation. We consider the crosstalk between electrical signals and phytohormones and identify interaction points, as well as provide insights into the integration nodes that involve changes in pH, Ca²⁺ and ROS levels. This review also provides an overview of our current knowledge of how electrical signals and hormones work together to induce a systemic response.

Plant-microbial interaction: The mechanism and the application of microbial elicitor induced secondary metabolites biosynthesis in medicinal plants

Plants and microbes interact with each other via different chemical signaling pathways. At the risophere level, the microbes can secrete molecules, called elicitors, which act on their receptors located in plant cells. The so-called elicitor molecules as well as their actions differ according to the mcirobes and induce different bilogical responses in plants such as the synthesis of secondary metabolites. Microbial compounds induced phenotype changes in plants are known as elicitors and signaling pathways which integrate elicitor's signals in plants are called elicitation. In this review, the impact of microbial elicitors on the synthesis and the secretion of secondary metabolites in plants was highlighted. Moreover, biological properties of these bioactive compounds were also highlighted and discussed. Indeed, several bacteria, fungi, and viruses release elicitors which bind to plant cell receptors and mediate signaling pathways involved in secondary metabolites synthesis. Different phytochemical classes such as terpenoids, phenolic acids and flavonoids were synthesized and/or increased in medicinal plants via the action of microbial elicitors. Moreover, these compounds compounds exhibit numerous biological activities and can therefore be explored in drugs discovery.

Enhanced SA and Ca2+ signaling results in PCD-mediated spontaneous leaf necrosis in wheat mutant wsl

Leaf is the major photosynthesis organ and the key source of wheat (Triticum aestivum L.) grain. Spotted leaf (spl) mutant is a kind of leaf lesion mimic mutants (LMMs) in plants, which is an ideal material for studying the mechanisms of leaf development. In this study, we report the leaf abnormal development molecular mechanism of a spl mutant named white stripe leaf (wsl) derived from wheat cultivar Guomai 301 (WT). Histochemical observation indicated that the leaf mesophyll cells of the wsl were destroyed in the necrosis regions. To explore the molecular regulatory network of the leaf development in mutant wsl, we employed transcriptome analysis, histochemistry, quantitative real-time PCR (qRT-PCR), and observations of the key metabolites and photosynthesis parameters. Compared to WT, the expressions of the chlorophyll synthesis and photosynthesis-related homeotic genes were repressed; many genes in the WRKY transcription factor (TF) families were highly expressed; the salicylic acid (SA) and Ca²⁺ signal transductions were enhanced in wsl. Both the chlorophyll contents and the photosynthesis rate were lower in wsl. The contents of SA and reactive oxygen species (ROS) were significantly higher, and the leaf rust resistance was enhanced in wsl. Based on the experimental data, a primary molecular regulatory model for leaf development in wsl was established. The results indicated that the SA accumulation and enhanced Ca²⁺ signaling led to programmed cell death (PCD), and ultimately resulted in spontaneous leaf necrosis of wsl. These results laid a solid foundation for further research on the molecular mechanism of leaf development in wheat.

Electrical Signals, Plant Tolerance to Actions of Stressors, and Programmed Cell Death: Is Interaction Possible?

In environmental conditions, plants are affected by abiotic and biotic stressors which can be heterogenous. This means that the systemic plant adaptive responses on their actions require long-distance stress signals including electrical signals (ESs). ESs are based on transient changes in the activities of ion channels and H⁺-ATP-ase in the plasma membrane. They influence numerous physiological processes, including gene expression, phytohormone synthesis, photosynthesis, respiration, phloem mass flow, ATP content, and many others. It is considered that these changes increase plant tolerance to the action of stressors; the effect can be related to stimulation of damages of specific molecular structures. In this review, we hypothesize that programmed cell death (PCD) in plant cells can be interconnected with ESs. There are the following points supporting this hypothesis. (i) Propagation of ESs can be related to ROS waves; these waves are a probable mechanism of PCD initiation. (ii) ESs induce the inactivation of photosynthetic dark reactions and activation of respiration. Both responses can also produce ROS and, probably, induce PCD. (iii) ESs stimulate the synthesis of stress phytohormones (e.g., jasmonic acid, salicylic acid, and ethylene) which are known to contribute to the induction of PCD. (iv) Generation of ESs accompanies K⁺ efflux from the cytoplasm that is also a mechanism of induction of PCD. Our review argues for the possibility of PCD induction by electrical signals and shows some directions of future investigations in the field.

Spatial and Temporal Dynamics of Electrical and Photosynthetic Activity and the Content of Phytohormones Induced by Local Stimulation of Pea Plants

A local leaf burning causes variation potential (VP) propagation, a decrease in photosynthesis activity, and changes in the content of phytohormones in unstimulated leaves in pea plants. The VP-induced photosynthesis response develops in two phases: fast inactivation and long-term inactivation. Along with a decrease in photosynthetic activity, there is a transpiration suppression in unstimulated pea leaves, which corresponds to the long-term phase of photosynthesis response. Phytohormone level analysis showed an increase in the concentration of jasmonic acid (JA) preceding a transpiration suppression and a long-term phase of the photosynthesis response. Analysis of the spatial and temporal dynamics of electrical signals, phytohormone levels, photosynthesis, and transpiration activity showed the most pronounced changes in the more distant leaf from the area of local stimulation. The established features are related to the architecture of the vascular bundles in the pea stem.

Enhanced Senescence Process is the Major Factor Stopping Spike Differentiation of Wheat Mutant ptsd1

Complete differentiation of the spikes guarantees the final wheat (Triticum aestivum L.) grain yield. A unique wheat mutant that prematurely terminated spike differentiation (ptsd1) was obtained from cultivar Guomai 301 treated with ethyl methane sulfonate (EMS). The molecular mechanism study on ptsd1 showed that the senescence-associated genes (SAGs) were highly expressed, and spike differentiation related homeotic genes were depressed. Cytokinin signal transduction was weakened and ethylene signal transduction was enhanced. The enhanced expression of Ca²⁺ signal transduction related genes and the accumulation of reactive oxygen species (ROS) caused the upper spikelet cell death. Many genes in the WRKY, NAC and ethylene response factor (ERF) transcription factor (TF) families were highly expressed. Senescence related metabolisms, including macromolecule degradation, nutrient recycling, as well as anthocyanin and lignin biosynthesis, were activated. A conserved tae-miR164 and a novel-miR49 and their target genes were extensively involved in the senescence related biological processes in ptsd1. Overall, the abnormal phytohormone homeostasis, enhanced Ca²⁺ signaling and activated senescence related metabolisms led to the spikelet primordia absent their typical meristem characteristics, and ultimately resulted in the phenotype of ptsd1.

Long-distance electrical signals as a link between the local action of stressors and the systemic physiological responses in higher plants

Our review is devoted to the analysis of the role of long-distance electrical signals in the development of the fast systemic physiological responses in higher plants. The characteristics and mechanisms of basic electrical signals (variation potential, action potential and system potential) are analyzed, and a potential schema of the generation and propagation of the system potential is proposed. The review summarizes the physiological changes induced by the variation potential, action potential and system potential in higher plants, including changes in gene expressions, the production of phytohormones, photosynthesis, phloem mass-flow, respiration, ATP content, transpiration and plant growth. Potential mechanisms of the changes are analyzed. Finally, a hypothetical schema, which describes a hierarchy of the variation potential, action potential and system potential, in the development of the fast systemic non-specific adaptation of plants to stressors, is proposed.

The regulatory mechanism of fungal elicitor-induced secondary metabolite biosynthesis in medical plants

A wide range of external stress stimuli trigger plant cells to undergo complex network of reactions that ultimately lead to the synthesis and accumulation of secondary metabolites. Accumulation of such metabolites often occurs in plants subjected to stresses including various elicitors or signal molecules. Throughout evolution, endophytic fungi, an important constituent in the environment of medicinal plants, have known to form long-term stable and mutually beneficial symbiosis with medicinal plants. The endophytic fungal elicitor can rapidly and specifically induce the expression of specific genes in medicinal plants which can result in the activation of a series of specific secondary metabolic pathways resulting in the significant accumulation of active ingredients. Here we summarize the progress made on the mechanisms of fungal elicitor including elicitor signal recognition, signal transduction, gene expression and activation of the key enzymes and its application. This review provides guidance on studies which may be conducted to promote the efficient synthesis and accumulation of active ingredients by the endogenous fungal elicitor in medicinal plant cells, and provides new ideas and methods of studying the regulation of secondary metabolism in medicinal plants.

Pyridine nucleotides induce changes in cytosolic pools of calcium in Arabidopsis

NAD is a pyridine nucleotide that is involved in cell metabolism and signaling of plant growth and stress. Recently, we reported on the multifaceted nature of NAD-inducible immunity in Arabidopsis. We identified NAD as an integral regulator of multiple defense layers such as production of ROS, deposition of callose, stimulation of cell death and modulation of defense metabolism including the defense hormones SA, JA and ABA, and other defense-associated metabolites. Altogether, NAD-induced immune effects confer resistance to diverse pathogenic microbes. Our addendum to this work further demonstrates an impact of NAD on the cytosolic calcium pool, a well-known component of early plant defense response.

Coordinated regulation of arbuscular mycorrhizal fungi and soybean MAPK pathway genes improved mycorrhizal soybean drought tolerance

Mitogen-activated protein kinase (MAPK) cascades play important roles in the stress response in both plants and microorganisms. The mycorrhizal symbiosis established between arbuscular mycorrhizal fungi (AMF) and plants can enhance plant drought tolerance, which might be closely related to the fungal MAPK response and the molecular dialogue between fungal and soybean MAPK cascades. To verify the above hypothesis, germinal Glomus intraradices (syn. Rhizophagus irregularis) spores and potted experiments were conducted. The results showed that AMF GiMAPKs with high homology with MAPKs from Saccharomyces cerevisiae had different gene expression patterns under different conditions (nitrogen starvation, abscisic acid treatment, and drought). Drought stress upregulated the levels of fungi and soybean MAPK transcripts in mycorrhizal soybean roots, indicating the possibility of a molecular dialogue between the two symbiotic sides of symbiosis and suggesting that they might cooperate to regulate the mycorrhizal soybean drought-stress response. Meanwhile, the changes in hydrogen peroxide, soluble sugar, and proline levels in mycorrhizal soybean as well as in the accelerated exchange of carbon and nitrogen in the symbionts were contributable to drought adaptation of the host plants. Thus, it can be preliminarily inferred that the interactions of MAPK signals on both sides, symbiotic fungus and plant, might regulate the response of symbiosis and, thus, improve the resistance of mycorrhizal soybean to drought stress.

Elucidating the molecular responses of apple rootstock resistant to ARD pathogens: challenges and opportunities for development of genomics-assisted breeding tools

Apple replant disease (ARD) is a major limitation to the establishment of economically viable orchards on replant sites due to the buildup and long-term survival of pathogen inoculum. Several soilborne necrotrophic fungi and oomycetes are primarily responsible for ARD, and symptoms range from serious inhibition of growth to the death of young trees. Chemical fumigation has been the primary method used for control of ARD, and manipulating soil microbial ecology to reduce pathogen density and aggressiveness is being investigated. To date, innate resistance of apple rootstocks as a means to control this disease has not been carefully explored, partly due to the complex etiology and the difficulty in phenotyping the disease resistance. Molecular defense responses of plant roots to soilborne necrotrophic pathogens are largely elusive, although considerable progress has been achieved using foliar disease systems. Plant defense responses to necrotrophic pathogens consist of several interacting modules and operate as a network. Upon pathogen detection by plants, cellular signals such as the oscillation of Ca(2+) concentration, reactive oxygen species (ROS) burst and protein kinase activity, lead to plant hormone biosynthesis and signaling. Jasmonic acid (JA) and ethylene (ET) are known to be fundamental to the induction and regulation of defense mechanisms toward invading necrotrophic pathogens. Complicated hormone crosstalk modulates the fine-tuning of transcriptional reprogramming and metabolic redirection, resulting in production of antimicrobial metabolites, enzyme inhibitors and cell wall refortification to restrict further pathogenesis. Transcriptome profiling of apple roots in response to inoculation with Pythium ultimum demonstrated that there is a high degree of conservation regarding the molecular framework of defense responses compared with those observed with foliar tissues. It is conceivable that the timing and intensity of genotype-specific defense responses may lead to different outcomes between rootstocks in response to invasion by necrotrophic pathogens. Elucidation of host defense mechanisms is critical in developing molecular tools for genomics-assisted breeding of resistant apple rootstocks. Due to their perennial nature, use of resistant rootstocks as a component for disease management might offer a durable and cost-effective benefit to tree performance than the standard practice of soil fumigation for control of ARD.

An Arabidopsis mutant impaired in intracellular calcium elevation is sensitive to biotic and abiotic stress

BACKGROUND

Ca2+, a versatile intracellular second messenger in various signaling pathways, initiates many responses involved in growth, defense and tolerance to biotic and abiotic stress. Endogenous and exogenous signals induce cytoplasmic Ca2+ ([Ca2+]cyt) elevation, which are responsible for the appropriate downstream responses.

RESULTS

Here we report on an ethyl-methane sulfonate-mediated Arabidopsis mutant that fails to induce [Ca2+]cyt elevation in response to exudate preparations from the pathogenic mibrobes Alternaria brassicae, Rhizoctonia solani, Phytophthora parasitica var. nicotianae and Agrobacterium tumefaciens. The cytoplasmic Ca2+elevation mutant1 (cycam1) is susceptible to infections by A. brassicae, its toxin preparation and sensitive to abiotic stress such as drought and salt. It accumulates high levels of reactive oxygen species and contains elevated salicylic acid, abscisic acid and bioactive jasmonic acid iso-leucine levels. Reactive oxygen species- and phytohormone-related genes are higher in A. brassicae-treated wild-type and mutant seedlings. Depending on the analysed response, the elevated levels of defense-related compounds are either caused by the cycam mutation and are promoted by the pathogen, or they are mainly due to the pathogen infection or application of pathogen-associated molecular patterns. Furthermore, cycam1 shows altered responses to abscisic acid treatments: the hormone inhibits germination and growth of the mutant.

CONCLUSIONS

We isolated an Arabidopsis mutant which fails to induce [Ca2+]cyt elevation in response to exudate preparations from various microbes. The higher susceptibility of the mutant to pathogen infections correlates with the higher accumulation of defense-related compounds, such as phytohormones, reactive oxygen-species, defense-related mRNA levels and secondary metabolites. Therefore, CYCAM1 couples [Ca2+]cyt elevation to biotic, abiotic and oxidative stress responses.

Cuscuta reflexa invasion induces Ca release in its host

Cuscuta reflexa induces a variety of reaction in its hosts. Some of these are visual reactions, and it is clear that these morphological changes are preceded by events at the molecular level, where signal transduction is one of the early processes. Calcium (Ca(2+)) release is the major second messenger during signal transduction, and we therefore studied Ca(2+) spiking in tomato during infection with C. reflexa. Bioluminescence in aequorin-expressing tomato was monitored for 48 h after the onset of Cuscuta infestation. Signals at the attachment sites were observed from 30 to 48 h. Treatment of aequorin-expressing tomato leaf disks with Cuscuta plant extracts suggested that the substance that induced Ca(2+) release from the host was closely linked to parasite haustoria.

Early signal transduction linking the synthesis of jasmonic acid in plant

Jasmonate signaling plays a critical role in protecting plants from pathogens or insect attacks and in limiting damage from abiotic stress. Many events contribute to the regulation of jasmonic acid (JA) synthesis during abiotic or biotic stress, but the details of the underlying mechanism remain unclear. In this Mini-Review paper, we discuss the possible roles of reactive oxygen species (ROS), nitric oxide (NO), calcium influx and mitogen-activated protein kinase (MAPK) cascade during JA synthesis or JA signal transduction.