Nitric Oxide Is Critical for Inducing Phosphatidic Acid Accumulation in Xylanase-elicited Tomato Cells

Arabidopsis thaliana phosphoinositide-specific phospholipase C 2 is required for Botrytis cinerea proliferation

Phospholipase C (PLC) plays a key role in lipid signaling during plant development and stress responses. PLC activation is one of the earliest responses during pathogen perception. Arabidopsis thaliana contains seven PLC encoding genes (AtPLC1 to AtPLC7) and two pseudogenes (AtPLC8 and AtPLC9), being AtPLC2 the most abundant isoform with constitutive expression in all plant organs. PLC has been linked to plant defense signaling, in particular to the production of reactive oxygen species (ROS). Previously, we demonstrated that AtPLC2 is involved in ROS production via the NADPH oxidase isoforms RBOHD activation during stomata plant immunity. Here we studied the role of AtPLC2 on plant resistance against the necrotrophic fungus Botrytis cinerea, a broad host-range and serious agricultural pathogen. We show that the AtPLC2-silenced (amiR PLC2) or null mutant (plc2-1) plants developed smaller B. cinerea lesions. Moreover, plc2-1 showed less ROS production and an intensified SA-dependent signaling upon infection, indicating that B. cinerea uses AtPLC2-triggered responses for a successful proliferation. Therefore, AtPLC2 is a susceptibility (S) gene that facilitates B. cinerea infection and proliferation.

CRISPR/Cas9-mediated phospholipase C 2 knock-out tomato plants are more resistant to Botrytis cinerea

MAIN CONCLUSION

CRISPR/Cas9-mediated Phospholipase C2 knock-out tomato plants are more resistant to Botrytis cinerea than wild-type plants, with less ROS and an increase and reduction of (JA) and (SA)-response marker genes, respectively. Genome-editing technologies allow non-transgenic site-specific mutagenesis of crops, offering a viable alternative to traditional breeding methods. In this study we used CRISPR/Cas9 to inactivate the tomato Phospholipase C2 gene (SlPLC2). Plant PLC activation is one of the earliest responses triggered by different pathogens regulating plant responses that, depending on the plant-pathogen interaction, result in plant resistance or susceptibility. The tomato (Solanum lycopersicum) PLC gene family has six members, named from SlPLC1 to SlPLC6. We previously showed that SlPLC2 transcript levels increased upon xylanase infiltration (fungal elicitor) and that SlPLC2 participates in plant susceptibility to Botrytis cinerea. An efficient strategy to control diseases caused by pathogens is to disable susceptibility genes that facilitate infection. We obtained tomato SlPLC2-knock-out lines with decreased ROS production upon B. cinerea challenge. Since this fungus requires ROS-induced cell death to proliferate, SlPLC2-knock-out plants showed an enhanced resistance with smaller necrotic areas and reduced pathogen proliferation. Thus, we obtained SlPLC2 loss-of-function tomato lines more resistant to B. cinerea by means of CRISPR/Cas9 genome editing technology.

The role of nitric oxide and hydrogen sulfide in regulation of redox homeostasis at extreme temperatures in plants

Nitric oxide and hydrogen sulfide, as important signaling molecules (gasotransmitters), are involved in many functions of plant organism, including adaptation to stress factors of various natures. As redox-active molecules, NO and H₂S are involved in redox regulation of functional activity of many proteins. They are also involved in maintaining cell redox homeostasis due to their ability to interact directly and indirectly (functionally) with ROS, thiols, and other molecules. The review considers the involvement of nitric oxide and hydrogen sulfide in plant responses to low and high temperatures. Particular attention is paid to the role of gasotransmitters interaction with other signaling mediators (in particular, with Ca²⁺ ions and ROS) in the formation of adaptive responses to extreme temperatures. Pathways of stress-induced enhancement of NO and H₂S synthesis in plants are considered. Mechanisms of the NO and H₂S effect on the activity of some proteins of the signaling system, as well as on the state of antioxidant and osmoprotective systems during adaptation to stress temperatures, were analyzed. Possibilities of practical use of nitric oxide and hydrogen sulfide donors as inductors of plant adaptive responses are discussed.

Do photosynthetic cells communicate with each other during cell death? From cyanobacteria to vascular plants

As in metazoans, life in oxygenic photosynthetic organisms relies on the accurate regulation of cell death. During development and in response to the environment, photosynthetic cells activate and execute cell death pathways that culminate in the death of a specific group of cells, a process known as regulated cell death (RCD). RCD control is instrumental, as its misregulation can lead to growth penalties and even the death of the entire organism. Intracellular molecules released during cell demise may act as 'survival' or 'death' signals and control the propagation of cell death to surrounding cells, even in unicellular organisms. This review explores different signals involved in cell-cell communication and systemic signalling in photosynthetic organisms, in particular Ca2+, reactive oxygen species, lipid derivates, nitric oxide, and eATP. We discuss their possible mode-of-action as either 'survival' or 'death' molecules and their potential role in determining cell fate in neighbouring cells. By comparing the knowledge available across the taxonomic spectrum of this coherent phylogenetic group, from cyanobacteria to vascular plants, we aim at contributing to the identification of conserved mechanisms that control cell death propagation in oxygenic photosynthetic organisms.

DIACYLGLYCEROL KINASE 5 participates in flagellin-induced signaling in Arabidopsis

Flagellin perception is a keystone of pattern-triggered immunity in plants. The recognition of this protein by a plasma membrane (PM) receptor complex is the beginning of a signaling cascade that includes protein phosphorylation and the production of reactive oxygen species (ROS). In both Arabidopsis (Arabidopsis thaliana) seedlings and suspension cells, we found that treatment with flg22, a peptide corresponding to the most conserved domain of bacterial flagellin, caused a rapid and transient decrease in the level of phosphatidylinositol (PI) 4,5-bisphosphate along with a parallel increase in phosphatidic acid (PA). In suspension cells, inhibitors of either phosphoinositide-dependent phospholipases C (PLC) or diacylglycerol kinases (DGKs) inhibited flg22-triggered PA production and the oxidative burst. In response to flg22, receptor-like kinase-deficient fls2, bak1, and bik1 mutants (FLAGELLIN SENSITIVE 2, BRASSINOSTEROID INSENSITIVE 1-associated kinase 1, and BOTRYTIS-INDUCED KINASE 1, respectively) produced less PA than wild-type (WT) plants, whereas this response did not differ in NADPH oxidase-deficient rbohD (RESPIRATORY BURST OXIDASE HOMOLOG D) plants. Among the DGK-deficient lines tested, the dgk5.1 mutant produced less PA and less ROS after flg22 treatment compared with WT seedlings. In response to flg22, dgk5.1 plants showed lower callose accumulation and impaired resistance to Pseudomonas syringae pv. tomato DC3000 hrcC-. Transcriptomics revealed that the basal expression of defense-related genes was altered in dgk5.1 seedlings compared with the WT. A GFP-DGK5 fusion protein localized to the PM, where RBOHD and PLC2 (proteins involved in plant immunity) are also located. The role of DGK5 and its enzymatic activity in flagellin signaling and fine-tuning of early immune responses in plant-microbe interactions is discussed.

Secreted Glycoside Hydrolase Proteins as Effectors and Invasion Patterns of Plant-Associated Fungi and Oomycetes

During host colonization, plant-associated microbes, including fungi and oomycetes, deliver a collection of glycoside hydrolases (GHs) to their cell surfaces and surrounding extracellular environments. The number and type of GHs secreted by each organism is typically associated with their lifestyle or mode of nutrient acquisition. Secreted GHs of plant-associated fungi and oomycetes serve a number of different functions, with many of them acting as virulence factors (effectors) to promote microbial host colonization. Specific functions involve, for example, nutrient acquisition, the detoxification of antimicrobial compounds, the manipulation of plant microbiota, and the suppression or prevention of plant immune responses. In contrast, secreted GHs of plant-associated fungi and oomycetes can also activate the plant immune system, either by acting as microbe-associated molecular patterns (MAMPs), or through the release of damage-associated molecular patterns (DAMPs) as a consequence of their enzymatic activity. In this review, we highlight the critical roles that secreted GHs from plant-associated fungi and oomycetes play in plant-microbe interactions, provide an overview of existing knowledge gaps and summarize future directions.

Nitric oxide, gravity response, and a unified schematic of plant signaling

Plant signaling components are often involved in numerous processes. Calcium, reactive oxygen species, and other signaling molecules are essential to normal biotic and abiotic responses. Yet, the summation of these components is integrated to produce a specific response despite their involvement in a myriad of response cascades. In the response to gravity, the role of many of these individual components has been studied, but a specific sequence of signals has not yet been assembled into a cohesive schematic of gravity response signaling. Herein, we provide a review of existing knowledge of gravity response and differential protein and gene regulation induced by the absence of gravity stimulus aboard the International Space Station and propose an integrated theoretical schematic of gravity response incorporating that information. Recent developments in the role of nitric oxide in gravity signaling provided some of the final contextual pillars for the assembly of the model, where nitric oxide and the role of cysteine S-nitrosation may be central to the gravity response. The proposed schematic accounts for the known responses to reorientation with respect to gravity in roots-the most well studied gravitropic plant tissue-and is supported by the extensive evolutionary conservation of regulatory amino acids within protein components of the signaling schematic. The identification of a role of nitric oxide in regulating the TIR1 auxin receptor is indicative of the broader relevance of the schematic in studying a multitude of environmental and stress responses. Finally, there are several experimental approaches that are highlighted as essential to the further study and validation of this schematic.

The Water-Soluble Chitosan Derivative, N-Methylene Phosphonic Chitosan, Is an Effective Fungicide against the Phytopathogen Fusarium eumartii

Chitosan has been considered an environmental-friendly polymer. However, its use in agriculture has not been extended yet due to its relatively low solubility in water. N-Methylene phosphonic chitosan (NMPC) is a water-soluble derivative prepared by adding a phosphonic group to chitosan. This study demonstrates that NMPC has a fungicidal effect on the phytopathogenic fungus Fusarium solani f. sp. eumartii (F. eumartii) judged by the inhibition of F. eumartti mycelial growth and spore germination. NMPC affected fungal membrane permeability, reactive oxygen species production, and cell death. Also, this chitosan-derivative exerted antifungal effects against two other phytopathogens, Botrytis cinerea, and Phytophthora infestans. NMPC did not affect tomato cell viability at the same doses applied to these phytopathogens to exert fungicide action. In addition to water solubility, the selective biological cytotoxicity of NMPC adds value in its application as an antimicrobial agent in agriculture.

Nitro-oleic acid triggers ROS production via NADPH oxidase activation in plants: A pharmacological approach

Nitrated fatty acids (NO₂-FAs) are important signaling molecules in mammals. NO₂-FAs are formed by the addition reaction of nitric oxide- and nitrite-derived nitrogen dioxide with unsaturated fatty acid double bonds. The study of NO₂-FAs in plant systems constitutes an interesting and emerging area. The presence of NO₂-FA has been reported in olives, peas, rice and Arabidopsis. To gain a better understanding of the role of NO₂-FA on plant physiology, we analyzed the effects of exogenous application of nitro-oleic acid (NO₂-OA). In tomato cell suspensions we found that NO₂-OA induced reactive oxygen species (ROS) production in a dose-dependent manner via activation of NADPH oxidases, a mechanism that requires calcium entry from the extracellular compartment and protein kinase activation. In tomato and Arabidopsis leaves, NO₂-OA treatments induced two waves of ROS production, resembling plant defense responses. Arabidopsis NADPH oxidase mutants showed that NADPH isoform D (RBOHD) was required for NO₂-OA-induced ROS production. In addition, on Arabidopsis isolated epidermis, NO₂-OA induced stomatal closure via RBOHD and F. Altogether, these results indicate that NO₂-OA triggers NADPH oxidase activation revealing a new signaling role in plants.

Lipases associated with plant defense against pathogens

When facing microbe invaders, plants activate genetic and metabolic defense mechanisms and undergo extracellular and intracellular changes to obtain a certain level of host resistance. Dynamic adjustment and adaptation occur in structures containing lipophilic compounds and cellular metabolites. Lipids encompassing fatty acids, fatty acid-based polymers, and fatty acid derivatives are part of the fundamental architecture of cells and tissues and are essential compounds in numerous biological processes. Lipid-associated plant defense responses are mostly facilitated by the activation of lipases (lipid hydrolyzing proteins), which cleave or transform lipid substrates in various subcellular compartments. In this review, several types of plant defense-associated lipases are described, including their molecular aspects, enzymatic actions, cellular functions, and possible functional relevance in plant defense. Defensive roles are discussed considering enzyme properties, lipid metabolism, downstream regulation, and phenotypic traits in loss-of-function mutants.

Plant lipids: Key players of plasma membrane organization and function

The plasma membrane (PM) is the biological membrane that separates the interior of all cells from the outside. The PM is constituted of a huge diversity of proteins and lipids. In this review, we will update the diversity of molecular species of lipids found in plant PM. We will further discuss how lipids govern global properties of the plant PM, explaining that plant lipids are unevenly distributed and are able to organize PM in domains. From that observation, it emerges a complex picture showing a spatial and multiscale segregation of PM components. Finally, we will discuss how lipids are key players in the function of PM in plants, with a particular focus on plant-microbe interaction, transport and hormone signaling, abiotic stress responses, plasmodesmata function. The last chapter is dedicated to the methods that the plant membrane biology community needs to develop to get a comprehensive understanding of membrane organization in plants.

Knock-Down of Arabidopsis PLC5 Reduces Primary Root Growth and Secondary Root Formation While Overexpression Improves Drought Tolerance and Causes Stunted Root Hair Growth

Phospholipase C (PLC) is a well-known signaling enzyme in metazoans that hydrolyzes phosphatidylinositol 4,5-bisphosphate (PIP2) to produce inositol 1,4,5-trisphosphate and diacylglycerol as second messengers involved in mutiple processes. Plants contain PLC too, but relatively little is known about its function there. The model system Arabidopsis thaliana contains nine PLC genes. Reversed genetics have implicated several roles for PLCs in plant development and stress signaling. Here, PLC5 is functionally addressed. Promoter-β-glucuronidase (GUS) analyses revealed expression in roots, leaves and flowers, predominantly in vascular tissue, most probably phloem companion cells, but also in guard cells, trichomes and root apical meristem. Only one plc5-1 knock-down mutant was obtained, which developed normally but grew more slowly and exhibited reduced primary root growth and decreased lateral root numbers. These phenotypes could be complemented by expressing the wild-type gene behind its own promoter. Overexpression of PLC5 (PLC5-OE) using the UBQ10 promoter resulted in reduced primary and secondary root growth, stunted root hairs, decreased stomatal aperture and improved drought tolerance. PLC5-OE lines exhibited strongly reduced phosphatidylinositol 4-monophosphate (PIP) and PIP2 levels and increased amounts of phosphatidic acid, indicating enhanced PLC activity in vivo. Reduced PIP2 levels and stunted root hair growth of PLC5-OE seedlings could be recovered by inducible overexpression of a root hair-specific PIP 5-kinase, PIP5K3. Our results show that PLC5 is involved in primary and secondary root growth and that its overexpression improves drought tolerance. Independently, we provide new evidence that PIP2 is essential for the polar tip growth of root hairs.

Understanding Cytoskeletal Dynamics During the Plant Immune Response

The plant cytoskeleton is a dynamic framework of cytoplasmic filaments that rearranges as the needs of the cell change during growth and development. Incessant turnover mechanisms allow these networks to be rapidly redeployed in defense of host cytoplasm against microbial invaders. Both chemical and mechanical stimuli are recognized as danger signals to the plant, and these are perceived and transduced into cytoskeletal dynamics and architecture changes through a collection of well-recognized, previously characterized players. Recent advances in quantitative cell biology approaches, along with the powerful molecular genetics techniques associated with Arabidopsis, have uncovered two actin-binding proteins as key intermediaries in the immune response to phytopathogens and defense signaling. Certain bacterial phytopathogens have adapted to the cytoskeletal-based defense mechanism during the basal immune response and have evolved effector proteins that target actin filaments and microtubules to subvert transcriptional reprogramming, secretion of defense-related proteins, and cell wall-based defenses. In this review, we describe current knowledge about host cytoskeletal dynamics operating at the crossroads of the molecular and cellular arms race between microbes and plants.

Nitro-oxidative metabolism during fruit ripening

Pepper (Capsicum annuum L.) and tomato (Solanum lycopersicum L.), which belong to the Solanaceae family, are among the most cultivated and consumed fleshy fruits worldwide and constitute excellent sources of many essential nutrients, such as vitamins A, C, and E, calcium, and carotenoids. While fruit ripening is a highly regulated and complex process, tomato and pepper have been classified as climacteric and non-climacteric fruits, respectively. These fruits differ greatly in shape, color composition, flavor, and several other features which undergo drastic changes during the ripening process. Such ripening-related metabolic and developmental changes require extensive alterations in many cellular and biochemical processes, which ultimately leads to fully ripe fruits with nutritional and organoleptic features that are attractive to both natural dispersers and human consumers. Recent data show that reactive oxygen and nitrogen species (ROS/RNS) are involved in fruit ripening, during which molecules, such as hydrogen peroxide (H2O2), NADPH, nitric oxide (NO), peroxynitrite (ONOO-), and S-nitrosothiols (SNOs), interact to regulate protein functions through post-translational modifications. In light of these recent discoveries, this review provides an update on the nitro-oxidative metabolism during the ripening of two of the most economically important fruits, discusses the signaling roles played by ROS/RNS in controlling this complex physiological process, and highlights the potential biotechnological applications of these substances to promote further improvements in fruit ripening regulation and nutritional quality. In addition, we suggest that the term 'nitro-oxidative eustress' with regard to fruit ripening would be more appropriate than nitro-oxidative stress, which ultimately favors the consolidation of the plant species.

The sesquiterpene botrydial from Botrytis cinerea induces phosphatidic acid production in tomato cell suspensions

The phytotoxin botrydial triggers PA production in tomato cell suspensions via PLD and PLC/DGK activation. PLC/DGK-derived PA is partially required for botrydial-induced ROS generation. Phosphatidic acid (PA) is a phospholipid second messenger involved in the induction of plant defense responses. It is generated via two distinct enzymatic pathways, either via phospholipase D (PLD) or by the sequential action of phospholipase C and diacylglycerol kinase (PLC/DGK). Botrydial is a phytotoxic sesquiterpene generated by the necrotrophic fungus Botrytis cinerea that induces diverse plant defense responses, such as the production of reactive oxygen species (ROS). Here, we analyzed PA and ROS production and their interplay upon botrydial treatments, employing tomato (Solanum lycopersicum) cell suspensions as a model system. Botrydial induces PA production within minutes via PLD and PLC/DGK. Either inhibition of PLC or DGK diminishes ROS generation triggered by botrydial. This indicates that PLC/DGK is upstream of ROS production. In tomato, PLC is encoded by a multigene family constituted by SlPLC1-SlPLC6 and the pseudogene SlPLC7. We have shown that SlPLC2-silenced plants have reduced susceptibility to B. cinerea. In this work, we studied the role of SlPLC2 on botrydial-induced PA production by silencing the expression of SlPLC2 via a specific artificial microRNA. Upon botrydial treatments, SlPLC2-silenced-cell suspensions produce PA levels similar to wild-type cells. It can be concluded that PA is a novel component of the plant responses triggered by botrydial.

SlPRA1A/RAB attenuate EIX immune responses via degradation of LeEIX2 pattern recognition receptor

Pattern recognition receptors (PRR) are plasma membrane (PM) proteins that recognize microbe-associated molecular patterns (MAMPs), triggering an immune response. PRR are classified as receptor like kinases (RLKs) or receptor like proteins (RLPs). The PM localization of PRRs, which is crucial for their availability to sense MAMPs, depends on their appropriate trafficking through the endomembrane system. Recently, we have identified SlPRA1A, a prenylated RAB acceptor type-1 (PRA1) from S. lycopersicum, as a regulator of RLP-PRR localization and protein levels. SlPRA1A overexpression strongly decreases RLP-PRR protein levels, particularly those of LeEIX2, redirecting it to the vacuole for degradation. Interestingly, SlPRA1A does not affect RLK-PRRs, indicating its activity to be specific to RLP-PRR systems. As PRA1 proteins stabilize RABs on membranes, promoting RABs activity, we aimed to identify a RAB target of SlPRA1A. Screening of a set of A. thaliana RABs revealed that AtRABA1e is able to mimic SlPRA1A activity. Through live cell imaging, we observed that SlPRA1A enhances AtRABA1e localization on SlPRA1A positive punctuated structures. These results indicate that AtRABA1e is a putative target of SlPRA1, and a co-regulator of LeEIX2 trafficking and degradation.

Tomato Prenylated RAB Acceptor Protein 1 Modulates Trafficking and Degradation of the Pattern Recognition Receptor LeEIX2, Affecting the Innate Immune Response

Plants recognize microbial/pathogen associated molecular patterns (MAMP/PAMP) through pattern recognition receptors (PRRs) triggering an immune response against pathogen progression. MAMP/PAMP triggered immune response requires PRR endocytosis and trafficking for proper deployment. LeEIX2 is a well-known Solanum lycopersicum RLP-PRR, able to recognize and respond to the fungal MAMP/PAMP ethylene-inducing xylanase (EIX), and its function is highly dependent on intracellular trafficking. Identifying protein machinery components regulating LeEIX2 intracellular trafficking is crucial to our understanding of LeEIX2 mediated immune responses. In this work, we identified a novel trafficking protein, SlPRA1A, a predicted regulator of RAB, as an interactor of LeEIX2. Overexpression of SlPRA1A strongly decreases LeEIX2 endosomal localization, as well as LeEIX2 protein levels. Accordingly, the innate immune responses to EIX are markedly reduced by SlPRA1A overexpression, presumably due to a decreased LeEIX2 availability. Studies into the role of SlPRA1A in LeEIX2 trafficking revealed that LeEIX2 localization in multivesicular bodies/late endosomes is augmented by SlPRA1A. Furthermore, inhibiting vacuolar function prevents the LeEIX2 protein level reduction mediated by SlPRA1A, suggesting that SlPRA1A may redirect LeEIX2 trafficking to the vacuole for degradation. Interestingly, SlPRA1A overexpression reduces the amount of several RLP-PRRs, but does not affect the protein level of receptor-like kinase PRRs, suggesting a specific role of SlPRA1A in RLP-PRR trafficking and degradation.

From Chaos to Harmony: Responses and Signaling upon Microbial Pattern Recognition

Pathogen- or microbe-associated molecular patterns (PAMPs/MAMPs) are detected as nonself by host pattern recognition receptors (PRRs) and activate pattern-triggered immunity (PTI). Microbial invasions often trigger the production of host-derived endogenous signals referred to as danger- or damage-associated molecular patterns (DAMPs), which are also perceived by PRRs to modulate PTI responses. Collectively, PTI contributes to host defense against infections by a broad range of pathogens. Remarkable progress has been made toward demonstrating the cellular and physiological responses upon pattern recognition, elucidating the molecular, biochemical, and genetic mechanisms of PRR activation, and dissecting the complex signaling networks that orchestrate PTI responses. In this review, we present an update on the current understanding of how plants recognize and respond to nonself patterns, a process from which the seemingly chaotic responses form into a harmonic defense.

Genetic dissection of the maize (Zea mays L.) MAMP response

Loci associated with variation in maize responses to two microbe-associated molecular patterns (MAMPs) were identified. MAMP responses were correlated. No relationship between MAMP responses and quantitative disease resistance was identified. Microbe-associated molecular patterns (MAMPs) are highly conserved molecules commonly found in microbes which can be recognized by plant pattern recognition receptors. Recognition triggers a suite of responses including production of reactive oxygen species (ROS) and nitric oxide (NO) and expression changes of defense-related genes. In this study, we used two well-studied MAMPs (flg22 and chitooctaose) to challenge different maize lines to determine whether there was variation in the level of responses to these MAMPs, to dissect the genetic basis underlying that variation and to understand the relationship between MAMP response and quantitative disease resistance (QDR). Naturally occurring quantitative variation in ROS, NO production, and defense genes expression levels triggered by MAMPs was observed. A major quantitative traits locus (QTL) associated with variation in the ROS production response to both flg22 and chitooctaose was identified on chromosome 2 in a recombinant inbred line (RIL) population derived from the maize inbred lines B73 and CML228. Minor QTL associated with variation in the flg22 ROS response was identified on chromosomes 1 and 4. Comparison of these results with data previously obtained for variation in QDR and the defense response in the same RIL population did not provide any evidence for a common genetic basis controlling variation in these traits.

Silencing of the tomato phosphatidylinositol-phospholipase C2 (SlPLC2) reduces plant susceptibility to Botrytis cinerea

The tomato [Solanum lycopersicum (Sl)] phosphatidylinositol-phospholipase C (PI-PLC) gene family is composed of six members, named SlPLC1 to SlPLC6, differentially regulated on pathogen attack. We have previously shown that the fungal elicitor xylanase induces a raise of SlPLC2 and SlPLC5 transcripts and that SlPLC2, but not SlPLC5, is required for xylanase-induced expression of defense-related genes. In this work we studied the role of SlPLC2 in the interaction between tomato and the necrotrophic fungus Botrytis cinerea. Inoculation of tomato leaves with B. cinerea increases SlPLC2 transcript levels. We knocked-down the expression of SlPLC2 by virus-induced gene silencing and plant defense responses were analyzed upon B. cinerea inoculation. SlPLC2 silenced plants developed smaller necrotic lesions concomitantly with less proliferation of the fungus. Silencing of SlPLC2 resulted as well in a reduced production of reactive oxygen species. Upon B. cinerea inoculation, transcript levels of the salicylic acid (SA)-defense pathway marker gene SlPR1a were diminished in SlPLC2 silenced plants compared to non-silenced infected plants, while transcripts of the jasmonic acid (JA)-defense gene markers Proteinase Inhibitor I and II (SlPI-I and SlPI-II) were increased. This implies that SlPLC2 participates in plant susceptibility to B. cinerea.

Plant Survival in a Changing Environment: The Role of Nitric Oxide in Plant Responses to Abiotic Stress

Nitric oxide in plants may originate endogenously or come from surrounding atmosphere and soil. Interestingly, this gaseous free radical is far from having a constant level and varies greatly among tissues depending on a given plant's ontogeny and environmental fluctuations. Proper plant growth, vegetative development, and reproduction require the integration of plant hormonal activity with the antioxidant network, as well as the maintenance of concentration of reactive oxygen and nitrogen species within a narrow range. Plants are frequently faced with abiotic stress conditions such as low nutrient availability, salinity, drought, high ultraviolet (UV) radiation and extreme temperatures, which can influence developmental processes and lead to growth restriction making adaptive responses the plant's priority. The ability of plants to respond and survive under environmental-stress conditions involves sensing and signaling events where nitric oxide becomes a critical component mediating hormonal actions, interacting with reactive oxygen species, and modulating gene expression and protein activity. This review focuses on the current knowledge of the role of nitric oxide in adaptive plant responses to some specific abiotic stress conditions, particularly low mineral nutrient supply, drought, salinity and high UV-B radiation.

Phospholipase D and phosphatidic acid in plant defence response: from protein-protein and lipid-protein interactions to hormone signalling

Phospholipase Ds (PLDs) and PLD-derived phosphatidic acids (PAs) play vital roles in plant hormonal and environmental responses and various cellular dynamics. Recent studies have further expanded the functions of PLDs and PAs into plant-microbe interaction. The molecular diversities and redundant functions make PLD-PA an important signalling complex regulating lipid metabolism, cytoskeleton dynamics, vesicle trafficking, and hormonal signalling in plant defence through protein-protein and protein-lipid interactions or hormone signalling. Different PLD-PA signalling complexes and their targets have emerged as fast-growing research topics for understanding their numerous but not yet established roles in modifying pathogen perception, signal transduction, and downstream defence responses. Meanwhile, advanced lipidomics tools have allowed researchers to reveal further the mechanisms of PLD-PA signalling complexes in regulating lipid metabolism and signalling, and their impacts on jasmonic acid/oxylipins, salicylic acid, and other hormone signalling pathways that essentially mediate plant defence responses. This review attempts to summarize the progress made in spatial and temporal PLD/PA signalling as well as PLD/PA-mediated modification of plant defence. It presents an in-depth discussion on the functions and potential mechanisms of PLD-PA complexes in regulating actin filament/microtubule cytoskeleton, vesicle trafficking, and hormonal signalling, and in influencing lipid metabolism-derived metabolites as critical signalling components in plant defence responses. The discussion puts PLD-PA in a broader context in order to guide future research.

Long-chain bases, phosphatidic acid, MAPKs, and reactive oxygen species as nodal signal transducers in stress responses in Arabidopsis

Due to their sessile condition, plants have developed sensitive, fast, and effective ways to contend with environmental changes. These mechanisms operate as informational wires conforming extensive and intricate networks that are connected in several points. The responses are designed as pathways orchestrated by molecules that are transducers of protein and non-protein nature. Their chemical nature imposes selective features such as specificity, formation rate, and generation site to the informational routes. Enzymes such as mitogen-activated protein kinases and non-protein, smaller molecules, such as long-chain bases, phosphatidic acid, and reactive oxygen species are recurrent transducers in the pleiotropic responses to biotic and abiotic stresses in plants. In this review, we considered these four components as nodal points of converging signaling pathways that start from very diverse stimuli and evoke very different responses. These pleiotropic effects may be explained by the potentiality that every one of these four mediators can be expressed from different sources, cellular location, temporality, or magnitude. Here, we review recent advances in our understanding of the interplay of these four specific signaling components in Arabidopsis cells, with an emphasis on drought, cold and pathogen stresses.

Analysis of root proteome unravels differential molecular responses during compatible and incompatible interaction between chickpea (Cicer arietinum L.) and Fusarium oxysporum f. sp. ciceri Race1 (Foc1)

BACKGROUND

Vascular wilt caused by Fusarium oxysporum f. sp. ciceri Race 1 (Foc1) is a serious disease of chickpea (Cicer arietinum L.) accounting for approximately 10-15% annual crop loss. The fungus invades the plant via roots, colonizes the xylem vessels and prevents the upward translocation of water and nutrients, finally resulting in wilting of the entire plant. Although comparative transcriptomic profiling have highlighted some important signaling molecules, but proteomic studies involving chickpea-Foc1 are limited. The present study focuses on comparative root proteomics of susceptible (JG62) and resistant (WR315) chickpea genotypes infected with Foc1, to understand the mechanistic basis of susceptibility and/or resistance.

RESULTS

The differential and unique proteins of both genotypes were identified at 48 h, 72 h, and 96 h post Foc1 inoculation. 2D PAGE analyses followed by MALDI-TOF MS and MS/MS identified 100 differentially (>1.5 fold<, p<0.05) or uniquely expressed proteins. These proteins were further categorized into 10 functional classes and grouped into GO (gene ontology) categories. Network analyses of identified proteins revealed intra and inter relationship of these proteins with their neighbors as well as their association with different defense signaling pathways. qRT-PCR analyses were performed to correlate the mRNA and protein levels of some proteins of representative classes.

CONCLUSIONS

The differential and unique proteins identified indicate their involvement in early defense signaling of the host. Comparative analyses of expression profiles of obtained proteins suggest that albeit some common components participate in early defense signaling in both susceptible and resistant genotypes, but their roles and regulation differ in case of compatible and/or incompatible interactions. Thus, functional characterization of identified PR proteins (PR1, BGL2, TLP), Trypsin protease inhibitor, ABA responsive protein, cysteine protease, protein disulphide isomerase, ripening related protein and albumins are expected to serve as important molecular components for biotechnological application and development of sustainable resistance against Foc1.

Nitric oxide production by necrotrophic pathogen Macrophomina phaseolina and the host plant in charcoal rot disease of jute: complexity of the interplay between necrotroph-host plant interactions

M. phaseolina, a global devastating necrotrophic fungal pathogen causes charcoal rot disease in more than 500 host plants. With the aim of understanding the plant-necrotrophic pathogen interaction associated with charcoal rot disease of jute, biochemical approach was attempted to study cellular nitric oxide production under diseased condition. This is the first report on M. phaseolina infection in Corchorus capsularis (jute) plants which resulted in elevated nitric oxide, reactive nitrogen species and S nitrosothiols production in infected tissues. Time dependent nitric oxide production was also assessed with 4-Amino-5-Methylamino-2',7'-Difluorofluorescein Diacetate using single leaf experiment both in presence of M. phaseolina and xylanases obtained from fungal secretome. Cellular redox status and redox active enzymes were also assessed during plant fungal interaction. Interestingly, M. phaseolina was found to produce nitric oxide which was detected in vitro inside the mycelium and in the surrounding medium. Addition of mammalian nitric oxide synthase inhibitor could block the nitric oxide production in M. phaseolina. Bioinformatics analysis revealed nitric oxide synthase like sequence with conserved amino acid sequences in M. phaseolina genome sequence. In conclusion, the production of nitric oxide and reactive nitrogen species may have important physiological significance in necrotrophic host pathogen interaction.

The tomato phosphatidylinositol-phospholipase C2 (SlPLC2) is required for defense gene induction by the fungal elicitor xylanase

The tomato [Solanum lycopersicum (Sl)] phosphatidylinositol-phospholipase C (PI-PLC) gene family is composed of six members, named SlPLC1 to SlPLC6, differentially regulated upon pathogen attack. We have previously shown that the fungal elicitor xylanase rapidly induces nitric oxide (NO), which is required for PI-PLCs activity and downstream defense responses in tomato cell suspensions. Here, we show that all six SlPLC genes are expressed in tomato cell suspensions. Treatment of the cells with xylanase induces an early increase in SlPLC5 transcript levels, followed by a raise of the amount of SlPLC2 transcripts. The production of NO is required to augment SlPLC5 transcript levels in xylanase-treated tomato cells. Xylanase also induces SlPLC2 and SlPLC5 transcript levels in planta. We knocked-down the expression of SlPLC2 and SlPLC5 by virus-induced gene silencing. We found that SlPLC2 is required for xylanase-induced expression of the defense-related genes PR1 and HSR203J.

SEC14 phospholipid transfer protein is involved in lipid signaling-mediated plant immune responses in Nicotiana benthamiana

We previously identified a gene related to the SEC14-gene phospholipid transfer protein superfamily that is induced in Nicotiana benthamiana (NbSEC14) in response to infection with Ralstonia solanacearum. We here report that NbSEC14 plays a role in plant immune responses via phospholipid-turnover. NbSEC14-silencing compromised expression of defense-related PR-4 and accumulation of jasmonic acid (JA) and its derivative JA-Ile. Transient expression of NbSEC14 induced PR-4 gene expression. Activities of diacylglycerol kinase, phospholipase C and D, and the synthesis of diacylglycerol and phosphatidic acid elicited by avirulent R. solanacearum were reduced in NbSEC14-silenced plants. Accumulation of signaling lipids and activation of diacylglycerol kinase and phospholipases were enhanced by transient expression of NbSEC14. These results suggest that the NbSEC14 protein plays a role at the interface between lipid signaling-metabolism and plant innate immune responses.

Sterol-dependent induction of plant defense responses by a microbe-associated molecular pattern from Trichoderma viride

Plant-microbe interactions involve numerous regulatory systems essential for plant defense against pathogens. An ethylene-inducing xylanase (Eix) of Trichoderma viride is a potent elicitor of plant defense responses in specific cultivars of tobacco (Nicotiana tabacum) and tomato (Solanum lycopersicum). We demonstrate that tomato cyclopropyl isomerase (SlCPI), an enzyme involved in sterol biosynthesis, interacts with the LeEix2 receptor. Moreover, we examined the role of SlCPI in signaling during the LeEix/Eix defense response. We found that SlCPI is an important factor in the regulation of the induction of defense responses such as the hypersensitive response, ethylene biosynthesis, and the induction of pathogenesis-related protein expression in the case of LeEix/Eix. Our results also suggest that changes in the sterol composition reduce LeEix internalization, thereby attenuating the induction of plant defense responses.

Lipid metabolism is differentially modulated by salicylic acid and heptanoyl salicylic acid during the induction of resistance in wheat against powdery mildew

Heptanoyl salicylic acid (HSA) is a salicylic acid (SA) derivative obtained by esterification of 2-OH benzoic acid with heptanoic acid. In wheat, the protection levels obtained against Blumeria graminis f. sp. tritici (Bgt) increased from 50% with SA to 95% with HSA. Using molecular, biochemical and cytological approaches, we investigated here how wheat lipid metabolism is differentially activated by SA and HSA in both infectious and non-infectious conditions, and how Bgt infectious process is altered by both inducers. First, in the absence of Bgt, continuous lipoxygenase (LOX)-encoding gene expression and corresponding activity were specifically induced by HSA. Moreover, compared to SA, HSA treatment resulted in earlier up-regulations of the phospholipase C2-encoding gene expression and it specifically affected the expression of a lipid transfer protein-encoding gene. In infectious context, both HSA and SA sprayings impaired penetration events and therefore haustorium formation, leading to less frequent fungal colonies. While this alteration only slowed down the evolution of Bgt infectious process in SA-sprayed leaves, it completely impaired the establishment of successful infectious events in HSA-sprayed leaves. In addition, HSA induced continuous increases of a LOX-encoding gene expression and of the corresponding LOX activity when compared to SA-sprayed leaves. Lipid metabolism is therefore overall highly responsive to HSA spraying and could represent effective defence mechanism triggered during the induction of resistance in wheat toward Bgt. The concepts of priming and energy costs of the defences induced by SA and HSA are also discussed.

Differential timing of defense-related responses induced by cerato-platanin and cerato-populin, two non-catalytic fungal elicitors

The cerato-platanin (CP) family consists of fungal-secreted proteins involved in various stages of the host-fungus interaction and acting as phytotoxins and elicitors of defense responses. The founder member of this family is CP, a non-catalytic protein with a six-stranded double-ψβ-barrel fold. Cerato-populin (Pop1) is an ortholog showing low sequence identity with CP. CP is secreted by Ceratocystis platani, the causal agent of the canker stain of plane. Pop1 is secreted by Ceratocystis populicola, a pathogen of poplar. CP and Pop1 have been suggested to act as PAMPs (pathogen-associated molecular patterns) because they induce phytoalexin synthesis, transcription of defense-related genes, restriction of conidia growth and cell death in various plants. Here, we treated plane leaves with CP or Pop1, and monitored defense responses to define the role of these elicitors in the plant interactions. Both CP and Pop1 were able to induce mitogen-activated protein kinases (MAPKs) phosphorylation, production of reactive oxygen species and nitric oxide, and overexpression of defense related genes. The characteristic DNA fragmentation and the cytological features indicate that CP and Pop1 induce cell death by a mechanism of programmed cell death. Therefore, CP and Pop1 can be considered as two novel, non-catalytic fungal PAMPs able to enhance primary defense. Of particular interest is the observation that CP showed faster activity compared to Pop1. The different timing in defense activation could potentially be due to the structural differences between CP and Pop1 (i.e. different hydrophobic index and different helix content) therefore constituting a starting point in unraveling their structure-function relationships.

Nitric oxide regulation of leaf phosphoenolpyruvate carboxylase-kinase activity: implication in sorghum responses to salinity

Nitric oxide (NO) is a signaling molecule that mediates many plant responses to biotic and abiotic stresses, including salt stress. Interestingly, salinity increases NO production selectively in mesophyll cells of sorghum leaves, where photosynthetic C₄ phosphoenolpyruvate carboxylase (C₄ PEPCase) is located. PEPCase is regulated by a phosphoenolpyruvate carboxylase-kinase (PEPCase-k), which levels are greatly enhanced by salinity in sorghum. This work investigated whether NO is involved in this effect. NO donors (SNP, SNAP), the inhibitor of NO synthesis NNA, and the NO scavenger cPTIO were used for long- and short-term treatments. Long-term treatments had multifaceted consequences on both PPCK gene expression and PEPCase-k activity, and they also decreased photosynthetic gas-exchange parameters and plant growth. Nonetheless, it could be observed that SNP increased PEPCase-k activity, resembling salinity effect. Short-term treatments with NO donors, which did not change photosynthetic gas-exchange parameters and PPCK gene expression, increased PEPCase-k activity both in illuminated leaves and in leaves kept at dark. At least in part, these effects were independent on protein synthesis. PEPCase-k activity was not decreased by short-term treatment with cycloheximide in NaCl-treated plants; on the contrary, it was decreased by cPTIO. In summary, NO donors mimicked salt effect on PEPCase-k activity, and scavenging of NO abolished it. Collectively, these results indicate that NO is involved in the complex control of PEPCase-k activity, and it may mediate some of the plant responses to salinity.

Fusaric acid induction of programmed cell death modulated through nitric oxide signalling in tobacco suspension cells

Fusaric acid (FA) is a nonhost-selective toxin mainly produced by Fusarium oxysporum, the causal agent of plant wilt diseases. We demonstrate that FA can induce programmed cell death (PCD) in tobacco suspension cells and the FA-induced PCD is modulated by nitric oxide (NO) signalling. Cells undergoing cell death induced by FA treatment exhibited typical characteristics of PCD including cytoplasmic shrinkage, chromatin condensation, DNA fragmentation, membrane plasmolysis, and formation of small cytoplasmic vacuoles. In addition, caspase-3-like activity was activated upon the FA treatment. The process of FA-induced PCD was accompanied by a rapid accumulation of NO in a FA dose-dependent manner. Pre-treatment of cells with NO scavenger 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) or NO synthase inhibitor N(G)-monomethyl-arginine monoacetate (L-NMMA) significantly reduced the rate of FA-induced cell death. Furthermore, the caspase-3-like activity and the expression of PAL and Hsr203J genes were alleviated by application of cPTIO or L-NMMA to FA-treated tobacco cells. This indicates that NO is an important factor involved in the FA-induced PCD. Our results also show that pre-treatment of tobacco cells with a caspase-3-specific inhibitor, Ac-DEVD-CHO, can reduce the rate of FA-induced cell death. These results demonstrate that the FA-induced cell death is a PCD and is modulated by NO signalling through caspase-3-like activation.

There's more to the picture than meets the eye: nitric oxide cross talk with Ca2+ signaling

Calcium and nitric oxide (NO) are two important biological messengers. Increasing evidence indicates that Ca(2+) and NO work together in mediating responses to pathogenic microorganisms and microbe-associated molecular patterns. Ca(2+) fluxes were recognized to account for NO production, whereas evidence gathered from a number of studies highlights that NO is one of the key messengers mediating Ca(2+) signaling. Here, we present a concise description of the current understanding of the molecular mechanisms underlying the cross talk between Ca(2+) and NO in plant cells exposed to biotic stress. Particular attention will be given to the involvement of cyclic nucleotide-gated ion channels and Ca(2+) sensors. Notably, we provide new evidence that calmodulin might be regulated at the posttranslational level by NO through S-nitrosylation. Furthermore, we report original transcriptomic data showing that NO produced in response to oligogalacturonide regulates the expression of genes related to Ca(2+) signaling. Deeper insight into the molecules involved in the interplay between Ca(2+) and NO not only permits a better characterization of the Ca(2+) signaling system but also allows us to further understand how plants respond to pathogen attack.

Involvement of phospholipase D-related signal transduction in chemical-induced programmed cell death in tomato cell cultures

Phospholipase D (PLD) and its product phosphatidic acid (PA) are incorporated in a complex metabolic network in which the individual PLD isoforms are suggested to regulate specific developmental and stress responses, including plant programmed cell death (PCD). Despite the accumulating knowledge, the mechanisms through which PLD/PA operate during PCD are still poorly understood. In this work, the role of PLDα1 in PCD and the associated caspase-like proteolysis, ethylene and hydrogen peroxide (H(2)O(2)) synthesis in tomato suspension cells was studied. Wild-type (WT) and PLDα1-silenced cell lines were exposed to the cell death-inducing chemicals camptothecin (CPT), fumonisin B1 (FB1) and CdSO(4). A range of caspase inhibitors effectively suppressed CPT-induced PCD in WT cells, but failed to alleviate cell death in PLDα1-deficient cells. Compared to WT, in CPT-treated PLDα1 mutant cells, reduced cell death and decreased production of H(2)O(2) were observed. Application of ethylene significantly enhanced CPT-induced cell death both in WT and PLDα1 mutants. Treatments with the PA derivative lyso-phosphatidic acid and mastoparan (agonist of PLD/PLC signalling downstream of G proteins) caused severe cell death. Inhibitors, specific to PLD and PLC, remarkably decreased the chemical-induced cell death. Taken together with our previous findings, the results suggest that PLDα1 contributes to caspase-like-dependent cell death possibly communicated through PA, reactive oxygen species and ethylene. The dead cells expressed morphological features of PCD such as protoplast shrinkage and nucleus compaction. The presented findings reveal novel elements of PLD/PA-mediated cell death response and suggest that PLDα1 is an important factor in chemical-induced PCD signal transduction.

Nitric oxide-sphingolipid interplays in plant signalling: a new enigma from the Sphinx?

Nitric oxide (NO) emerged as one of the major signaling molecules operating during plant development and plant responses to its environment. Beyond the identification of the direct molecular targets of NO, a series of studies considered its interplay with other actors of signal transduction and the integration of NO into complex signaling networks. Beside the close relationships between NO and calcium or phosphatidic acid signaling pathways that are now well-established, recent reports paved the way for interplays between NO and sphingolipids (SLs). This mini-review summarizes our current knowledge of the influence NO and SLs might exert on each other in plant physiology. Based on comparisons with examples from the animal field, it further indicates that, although SL-NO interplays are common features in signaling networks of eukaryotic cells, the underlying mechanisms and molecular targets significantly differ.

The Arabidopsis DREB2 genetic pathway is constitutively repressed by basal phosphoinositide-dependent phospholipase C coupled to diacylglycerol kinase

Phosphoinositide-dependent phospholipases C (PI-PLCs) are activated in response to various stimuli. They utilize substrates provided by type III-Phosphatidylinositol-4 kinases (PI4KIII) to produce inositol triphosphate and diacylglycerol (DAG) that is phosphorylated into phosphatidic acid (PA) by DAG-kinases (DGKs). The roles of PI4KIIIs, PI-PLCs, and DGKs in basal signaling are poorly understood. We investigated the control of gene expression by basal PI-PLC pathway in Arabidopsis thaliana suspension cells. A transcriptome-wide analysis allowed the identification of genes whose expression was altered by edelfosine, 30 μM wortmannin, or R59022, inhibitors of PI-PLCs, PI4KIIIs, and DGKs, respectively. We found that a gene responsive to one of these molecules is more likely to be similarly regulated by the other two inhibitors. The common action of these agents is to inhibit PA formation, showing that basal PI-PLCs act, in part, on gene expression through their coupling to DGKs. Amongst the genes up-regulated in presence of the inhibitors, were some DREB2 genes, in suspension cells and in seedlings. The DREB2 genes encode transcription factors with major roles in responses to environmental stresses, including dehydration. They bind to C-repeat motifs, known as Drought-Responsive Elements that are indeed enriched in the promoters of genes up-regulated by PI-PLC pathway inhibitors. PA can also be produced by phospholipases D (PLDs). We show that the DREB2 genes that are up-regulated by PI-PLC inhibitors are positively or negatively regulated, or indifferent, to PLD basal activity. Our data show that the DREB2 genetic pathway is constitutively repressed in resting conditions and that DGK coupled to PI-PLC is active in this process, in suspension cells and seedlings. We discuss how this basal negative regulation of DREB2 genes is compatible with their stress-triggered positive regulation.

Plant phosphoinositide-dependent phospholipases C: variations around a canonical theme

Phosphoinositide-specific phospholipase C (PI-PLC) cleaves, in a Ca(2+)-dependent manner, phosphatidylinositol-4,5-bisphosphate (PI-4,5-P2) into diacylglycerol (DAG) and inositol triphosphate (IP3). PI-PLCs are multidomain proteins that are structurally related to the PI-PLCζs, the simplest animal PI-PLCs. Like these animal counterparts, they are only composed of EF-hand, X/Y and C2 domains. However, plant PI-PLCs do not have a conventional EF-hand domain since they are often truncated, while some PI-PLCs have no EF-hand domain at all. Despite this simple structure, plant PI-PLCs are involved in many essential plant processes, either associated with development or in response to environmental stresses. The action of PI-PLCs relies on the mediators they produce. In plants, IP3 does not seem to be the sole active soluble molecule. Inositol pentakisphosphate (IP5) and inositol hexakisphosphate (IP6) also transmit signals, thus highlighting the importance of coupling PI-PLC action with inositol-phosphate kinases and phosphatases. PI-PLCs also produce a lipid molecule, but plant PI-PLC pathways show a peculiarity in that the active lipid does not appear to be DAG but its phosphorylated form, phosphatidic acid (PA). Besides, PI-PLCs can also act by altering their substrate levels. Taken together, plant PI-PLCs show functional differences when compared to their animal counterparts. However, they act on similar general signalling pathways including calcium homeostasis and cell phosphoproteome. Several important questions remain unanswered. The cross-talk between the soluble and lipid mediators generated by plant PI-PLCs is not understood and how the coupling between PI-PLCs and inositol-kinases or DAG-kinases is carried out remains to be established.

Involvement of phospholipases C and D in the defence responses of riboflavin-treated tobacco cells

Riboflavin is an activator of defence responses in plants that increases resistance against diseases caused by fungal, oomycete, bacterial and viral pathogens. However, the mechanisms driving defence activation by riboflavin are poorly understood. We investigated the signal transduction pathways of phospholipase C (PLC) and phospholipase D (PLD) in tobacco (Nicotiana tabacum) suspension cells using a pharmacological approach to confirm whether riboflavin-mediated activation of the defence response is dependent on both PLC and PLD. The expression patterns analysed by quantitative reverse transcription-polymerase chain reaction demonstrated that the tobacco PLC and PLD gene families were differentially expressed in riboflavin-treated tobacco cells. PLC and PLD expression accompanied defence responses including the expression of defence response genes (PAL, PR-1a and PR-1b), the production of hydrogen peroxide and the accumulation of the phytoalexin scopoletin in tobacco cells treated with riboflavin. These defence responses were significantly inhibited in the presence of the PLC inhibitor U73122 and the PLD inhibitor 1-butanol; however, inhibitor analogues had no effect. Moreover, treating tobacco cells with phosphatidic acid, a signalling molecule produced by phospholipase catalysis, induced the accumulation of the phytoalexin scopoletin and compensated for the suppressive effects of U73122 and 1-butanol on riboflavin-induced accumulation of the phytoalexin. These results offer pharmacological evidence that PLC and PLD play a role in riboflavin-induced defences of tobacco.

Nitric oxide as a mediator for defense responses

Sequential recognition of invading microbes and rapid induction of plant immune responses comprise at least two recognition systems. Early basal defenses are initiated by pathogen-associated molecular patterns and pattern recognition receptors (PRR) in the plasma membrane. Pathogens produce effectors to suppress defense but plants, in turn, can sense such effectors by dominant plant resistance (R) gene products. Plant PRR and R proteins modulate signaling networks for defense responses that rely on rapid production of reactive nitrogen species (RNS) and reactive oxygen species (ROS). Recent research has shown that nitric oxide (NO) mainly mediates biological function through chemical reactions between locally controlled accumulation of RNS and proteins leading to potential alteration of protein function. Many proteins specifically regulated by NO and participating in signaling during plant defense response have been identified, highlighting the physiological relevance of these modifications in plant immunity. ROS function independently or in cooperation with NO during defense, modulating the RNS signaling functions through the entire process. This review provides an overview of current knowledge about regulatory mechanisms for NO burst and signaling, and crosstalk with ROS in response to pathogen attack.

Hrip1, a novel protein elicitor from necrotrophic fungus, Alternaria tenuissima, elicits cell death, expression of defence-related genes and systemic acquired resistance in tobacco

Here, we report the identification, purification, characterization and gene cloning of a novel hypersensitive response inducing protein secreted by necrotrophic fungus, Alternaria tenuissima, designated as hypersensitive response inducing protein 1 (Hrip1). The protein caused the formation of necrotic lesions that mimic a typical hypersensitive response and apoptosis-related events including DNA laddering. The protein-encoding gene was cloned by rapid amplification of cDNA ends (RACE) method. The sequence analysis revealed that the cDNA is 495 bp in length and the open reading frame (ORF) encodes for a polypeptide of 163 amino acids with theoretical pI of 5.50 and molecular weight of 17 562.5 Da. Hrip1 induced calcium influx, medium alkalinization, activation of salicylic acid-induced protein kinase and several defence-related genes after infiltration in tobacco leaves. Cellular damage, restricted to the infiltrated zone, occurred only several hours later, at a time when expression of defence-related genes was activated. After several days, systemic acquired resistance was also induced. The tobacco plant cells that perceived the Hrip1 generated a cascade of signals acting at local, short, and long distances, and caused the coordinated expression of specific defence responses in a way similar to hypersensitivity to tobacco mosaic virus. Thus, Hrip1 represents a powerful tool to investigate further the signals and their transduction pathways involved in induced disease resistance in necrotrophic fungi.

The plant non-specific phospholipase C gene family. Novel competitors in lipid signalling

Non-specific phospholipases C (NPCs) were discovered as a novel type of plant phospholipid-cleaving enzyme homologous to bacterial phosphatidylcholine-specific phospholipases C and responsible for lipid conversion during phosphate-limiting conditions. The six-gene family was established in Arabidopsis, and growing evidence suggests the involvement of two articles NPCs in biotic and abiotic stress responses as well as phytohormone actions. In addition, the diacylglycerol produced via NPCs is postulated to participate in membrane remodelling, general lipid metabolism and cross-talk with other phospholipid signalling systems in plants. This review summarises information concerning this new plant protein family and focusses on its sequence analysis, biochemical properties, cellular and tissue distribution and physiological functions. Possible modes of action are also discussed.

Nitric oxide exacerbates Al-induced inhibition of root elongation in rice bean by affecting cell wall and plasma membrane properties

Aluminum (Al) toxicity is one of the most widespread problems for crop production on acid soils, and nitric oxide (NO) is a key signaling molecule involved in the mediation of various biotic and abiotic stresses in plants. Here we found that exogenous application of the NO donor sodium nitroprusside (SNP) exacerbated the inhibition of Al-induced root growth in rice bean [Vigna umbellata (Thunb.) Ohwi & Ohashi 'Jiangnan', Fabaceae]. This was accompanied by an increased accumulation of Al in the root apex. However, Al treatments had no effect on endogenous NO concentrations in root apices. These results indicate that a change in NO concentration is not the cause of Al-induced root growth inhibition and the adverse effect of SNP on Al-induced root growth inhibition should result from increased Al accumulation. Al could significantly induce citrate efflux but SNP had no effects on citrate efflux either in the absence or presence of Al. On the other hand, SNP pretreatment significantly increased Al-induced malondialdehyde accumulation and Evans Blue staining, indicating an intensification of the disruption of plasma membrane integrity. Furthermore, SNP pretreatment also caused greater induction of pectin methylesterase activity by Al, which could be the cause of the increased Al accumulation. Taken together, it is concluded that NO exacerbates Al-induced root growth inhibition by affecting cell wall and plasma membrane properties.

The rice diacylglycerol kinase family: functional analysis using transient RNA interference

Diacylglycerol kinase (DGK) is a pivotal enzyme that phosphorylates diacylglycerol (DAG) to form phosphatidic acid (PA). The production of PA from phospholipase D (PLD) and the coupled phospholipase C/DGK route is an important signaling process in animal and plant cells. In this study, we report a genomic analysis of eight putative rice DGKs encoded by a gene family (OsDGKs) grouped into three clusters. To further investigate the functions of the OsDGKs, a double-stranded RNA (dsRNA)-induced RNA silencing method was established. Introduction of in vitro-synthesized dsRNAs corresponding to a unique or conserved region of OsDGKs into rice protoplasts abolished or diminished the expression of individual or multiple OsDGK genes. Suppressing the expression of OsDGKs resulted in a distinct depletion of the transcripts of the defense gene OsNPR1 and the salt-responsive gene OsCIPK15. Our primary results suggest that OsDGKs are involved in the signaling of stress responses.

Upstream and downstream signals of nitric oxide in pathogen defence

Nitric oxide (NO) is now recognised as a crucial player in plant defence against pathogens. Considerable progress has been made in defining upstream and downstream signals of NO. Recently, MAP kinases, cyclic nucleotide phosphates, calcium and phosphatidic acid were demonstrated to be involved in pathogen-induced NO-production. However, the search for inducers of NO synthesis is difficult because of the still ambiguous enzymatic source of NO. Accumulation of NO triggers signal transduction by other second messengers. Here we depict NON-EXPRESSOR OF PATHOGENESIS-RELATED 1 and glyceraldehyde-3-phosphate dehydrogenase as central redox switches translating NO redox signalling into cellular responses. Although the exact position of NO in defence signal networks is unresolved at last some NO-related signal cascades are emerging.

Endosomal signaling of the tomato leucine-rich repeat receptor-like protein LeEix2

Extracellular leucine-rich repeat (LRR) receptor-like proteins (RLPs) represent a unique class of cell-surface receptors, as they lack a functional cytoplasmic domain. Our knowledge of how RLPs that do not contain a kinase or Toll domain function is very limited. The tomato RLP receptor LeEix2 signals to induce defense responses mediated by the fungal protein ethylene-inducing xylanase (EIX). The movement of FYVE-positive endosomes before and after EIX application was examined using spinning disc confocal microscopy. We found that while FYVE-positive endosomes generally observe a random movement pattern, following EIX application a subpopulation of FYVE-positive endosomes follow a directional movement pattern. Further, cellular endosomes travel greater distances at higher speeds following EIX application. Time-course experiments conducted with specific inhibitors demonstrate the involvement of endosomal signaling in EIX-triggered defense responses. Abolishing the existence of endosomes or the endocytic event prevented EIX-induced signaling. Endocytosis/endosome inhibitors, such as Dynasore or 1-butanol, inhibit EIX-induced signaling. Moreover, treatment with Endosidin1, which inhibits an early step in plasma membrane/endosome trafficking, enhances the induction of defense responses by EIX. Our data indicate a distinct endosomal signaling mechanism for induction of defense responses in this RLP system.

Mechanisms of xylanase-induced nitric oxide and phosphatidic acid production in tomato cells

The second messenger nitric oxide (NO), phosphatidic acid (PA) and reactive oxygen species (ROS) are involved in the plant defense response during plant-pathogen interactions. NO has been shown to participate in PA production in response to the pathogen-associated molecular pattern xylanase in tomato cell suspensions. Defense responses downstream of PA include ROS production. The goal of this work was to study the signaling mechanisms involved in PA production during the defense responses triggered by xylanase and mediated by NO in the suspension-cultured tomato cells. We analyzed the participation of protein kinases, guanylate cyclase and the NO-mediated posttranslational modification S-nitrosylation, by means of pharmacology and biochemistry. We showed that NO, PA and ROS levels are significantly diminished by treatment with the general protein kinase inhibitor staurosporine. This indicates that xylanase-induced protein phosphorylation events might be the important components leading to NO formation, and hence for the downstream regulation of PA and ROS levels. When assayed, a guanylate cyclase inhibitor or a cGMP analog did not alter the PA accumulation. These results suggest that a cGMP-mediated pathway is not involved in xylanase-induced PA formation. Finally, the inhibition of protein S-nitrosylation did not affect NO formation but compromised PA and ROS production. Data collectively indicate that upon xylanase perception, cells activate a protein kinase pathway required for NO formation and that, S-nitrosylation-dependent mechanisms are involved in downstream signaling leading to PA and ROS.