Induction of plant gp91 phox homolog by fungal cell wall, arachidonic acid, and salicylic acid in potato

Redox signaling and oxidative stress in systemic acquired resistance

Plants fully depend on their immune systems to defend against pathogens. Upon pathogen attack, plants not only activate immune responses at the infection site but also trigger a defense mechanism known as systemic acquired resistance (SAR) in distal systemic tissues to prevent subsequent infections by a broad-spectrum of pathogens. SAR is induced by mobile signals produced at the infection site. Accumulating evidence suggests that reactive oxygen species (ROS) play a central role in SAR signaling. ROS burst at the infection site is one of the earliest cellular responses following pathogen infection and can spread to systemic tissues through membrane-associated NADPH oxidase-dependent relay production of ROS. It is well known that ROS ignite redox signaling and, when in excess, cause oxidative stress, damaging cellular components. In this review, we summarize current knowledge on redox regulation of several SAR signaling components. We discuss the ROS amplification loop in systemic tissues involving multiple SAR mobile signals. Moreover, we highlight the essential role of oxidative stress in generating SAR signals including azelaic acid and extracellular NAD(P) [eNAD(P)]. Finally, we propose that eNAD(P) is a damage-associated molecular pattern serving as a converging point of SAR mobile signals in systemic tissues.

The mechanism by which oriented polypropylene packaging alleviates postharvest 'Black Spot' in radish root (Raphanus sativus)

INTRODUCTION

The postharvest physiological disorder known as 'black spot' in radish roots (Raphanus sativus) poses a significant challenge to quality maintenance during storage, particularly under summer conditions. The cause of this disorder, however, is poorly understood.

OBJECTIVES

Characterize the underlying causes of 'black spot' disorder in radish roots and identify strategies to delay its onset.

METHODS

Radish roots were placed in either polyvinyl chloride (PVC) or oriented polypropylene (OPP) packaging and stored for 4 days at 30 °C. Appearance and physiological parameters were assessed and transcriptomic and metabolomic analyses were conducted to identify the key molecular and biochemical factors contributing to the disorder and strategies for delaying its onset and development.

RESULTS

OPP packaging effectively delayed the onset of 'black spot' in radishes, potentially due to changes in phenolic and lipid metabolism. Regarding phenolic metabolism, POD and PPO activity decreased, RsCCR and RsPOD expression was downregulated, genes involved in phenols and flavonoids synthesis were upregulated and their content increased, preventing the oxidative browning of phenols and generally enhancing stress tolerance. Regarding lipid metabolism, the level of alpha-linolenic acid increased, and genes regulating cutin and wax synthesis were upregulated. Notably, high flavonoid and low ROS levels collectively inhibited RsPLA2G expression, which reduced the production of arachidonic acid, pro-inflammatory compounds (LTA4 and PGG2), and ROS, alleviating the inflammatory response and oxidative stress in radish epidermal tissues.

CONCLUSION

PVC packaging enhanced the postharvest onset of 'black spot' in radishes, while OPP packaging delayed both its onset and development. Our study provides insights into the response of radishes to different packaging materials during storage, and the causes and host responses that either enhance or delay 'black spot' disorder onset. Further studies will be conducted to confirm the molecular and biochemical processes responsible for the onset and development of 'black spot' in radishes.

A pathogenesis-related protein, PRP1, negatively regulates root nodule symbiosis in Lotus japonicus

The legume-rhizobium symbiosis represents a unique model within the realm of plant-microbe interactions. Unlike typical cases of pathogenic invasion, the infection of rhizobia and their residence within symbiotic cells do not elicit a noticeable immune response in plants. Nevertheless, there is still much to uncover regarding the mechanisms through which plant immunity influences rhizobial symbiosis. In this study, we identify an important player in this intricate interplay: Lotus japonicus PRP1, which serves as a positive regulator of plant immunity but also exhibits the capacity to decrease rhizobial colonization and nitrogen fixation within nodules. The PRP1 gene encodes an uncharacterized protein and is named Pathogenesis-Related Protein1, owing to its orthologue in Arabidopsis thaliana, a pathogenesis-related family protein (At1g78780). The PRP1 gene displays high expression levels in nodules compared to other tissues. We observed an increase in rhizobium infection in the L. japonicus prp1 mutants, whereas PRP1-overexpressing plants exhibited a reduction in rhizobium infection compared to control plants. Intriguingly, L. japonicus prp1 mutants produced nodules with a pinker colour compared to wild-type controls, accompanied by elevated levels of leghaemoglobin and an increased proportion of infected cells within the prp1 nodules. The transcription factor Nodule Inception (NIN) can directly bind to the PRP1 promoter, activating PRP1 gene expression. Furthermore, we found that PRP1 is a positive mediator of innate immunity in plants. In summary, our study provides clear evidence of the intricate relationship between plant immunity and symbiosis. PRP1, acting as a positive regulator of plant immunity, simultaneously exerts suppressive effects on rhizobial infection and colonization within nodules.

NADPH oxidase-mediated reactive oxygen species, antioxidant isozymes, and redox homeostasis regulate salt sensitivity in maize genotypes

The aim of the study is to examine the relationship between oxidative bursts, their regulation with ion homeostasis, and NADPH oxidase (NOX) in different salt-sensitive maize genotypes. For this, in the first study, four differently salt-sensitive maize genotypes (BIL214 × BIL218 as tolerant, BHM-5 as sensitive, and BHM-7 and BHM-9 as moderate-tolerant) were selected on the basis of phenotype, histochemical detection of reactive oxygen species (ROS), malondialdehyde (MDA) content, and specific and in-gel activity of NOX. In the next experiment, these genotypes were further examined in 200 mM NaCl solution in half-strength Hoagland media for nine days to study salt-induced changes in NOX activity, ROS accumulation, ion and redox homeostasis, the activity of antioxidants and their isozyme responses, and to find out potential relationships among the traits. Methylglyoxal (MG) and glyoxalse enzymes (Gly I and II) were also evaluated. Fully expanded leaf samplings were collected at 0 (control), 3, 6, 9-day, and after 7 days of recovery to assay different parameters. Na+/K+, NOX, ROS, and MDA contents increased significantly with the progression of stress duration in all maize genotypes, with a significantly higher value in BHM-5 as compared to tolerant and moderate-tolerant genotypes. A continual induction of Cu/Zn-SOD was observed in BIL214 × BIL218 due to salt stress. Substantial decreases in CAT2 and CAT3 isozymes in BHM-5 might be critical for the highest H2O2 burst in that sensitive genotype under salt stress. The highest intensified POD isozymes were visualized in BHM-5, BHM-7, and BHM-9, whereas BIL214 × BIL218 showed a continual induction of POD isozymes, although GPX activity decreased in all the genotypes at 9 days. Under salt stress, the tolerant genotype BIL214 × BIL218 showed superior ASA- and GSH-redox homeostasis by keeping GR and MDHAR activity high. This genotype also had a stronger MG detoxification system by having higher glyoxalase activity. Correlation, comparative heatmap, and PCA analyses revealed positive correlations among Na+/K+, NOX, O2•-, H2O2, MG, proline, GR, GST, and Gly I activities. Importantly, the relationship depends on the salt sensitivity of the genotypes. The reduced CAT activity as well as redox homeostasis were critical to the survival of the sensitive genotype.

A novel effector RipBT contributes to Ralstonia solanacearum virulence on potato

Ralstonia solanacearum is one of the most destructive plant-pathogenic bacteria, infecting more than 200 plant species, including potato (Solanum tuberosum) and many other solanaceous crops. R. solanacearum has numerous pathogenicity factors, and type III effectors secreted through type III secretion system (T3SS) are key factors to counteract host immunity. Here, we show that RipBT is a novel T3SS-secreted effector by using a cyaA reporter system. Transient expression of RipBT in Nicotiania benthamiana induced strong cell death in a plasma membrane-localization dependent manner. Notably, mutation of RipBT in R. solanacearum showed attenuated virulence on potato, while RipBT transgenic potato plants exhibited enhanced susceptibility to R. solanacearum. Interestingly, transcriptomic analyses suggest that RipBT may interfere with plant reactive oxygen species (ROS) metabolism during the R. solanacearum infection of potato roots. In addition, the expression of RipBT remarkably suppressed the flg22-induced pathogen-associated molecular pattern-triggered immunity responses, such as the ROS burst. Taken together, RipBT acts as a T3SS effector, promoting R. solanacearum infection on potato and presumably disturbing ROS homeostasis.

Bradyrhizobium japonicum IRAT FA3 promotes salt tolerance through jasmonic acid priming in Arabidopsis thaliana

BACKGROUND

Plant growth promoting rhizobacteria (PGPR), such as Bradyrhizobium japonicum IRAT FA3, are able to improve seed germination and plant growth under various biotic and abiotic stress conditions, including high salinity stress. PGPR can affect plants' responses to stress via multiple pathways which are often interconnected but were previously thought to be distinct. Although the overall impacts of PGPR on plant growth and stress tolerance have been well documented, the underlying mechanisms are not fully elucidated. This work contributes to understanding how PGPR promote abiotic stress by revealing major plant pathways triggered by B. japonicum under salt stress.

RESULTS

The plant growth-promoting rhizobacterial (PGPR) strain Bradyrhizobium japonicum IRAT FA3 reduced the levels of sodium in Arabidopsis thaliana by 37.7%. B. japonicum primed plants as it stimulated an increase in jasmonates (JA) and modulated hydrogen peroxide production shortly after inoculation. B. japonicum-primed plants displayed enhanced shoot biomass, reduced lipid peroxidation and limited sodium accumulation under salt stress conditions. Q(RT)-PCR analysis of JA and abiotic stress-related gene expression in Arabidopsis plants pretreated with B. japonicum and followed by six hours of salt stress revealed differential gene expression compared to non-inoculated plants. Response to Desiccation (RD) gene RD20 and reactive oxygen species scavenging genes CAT3 and MDAR2 were up-regulated in shoots while CAT3 and RD22 were increased in roots by B. japonicum, suggesting roles for these genes in B. japonicum-mediated salt tolerance. B. japonicum also influenced reductions of RD22, MSD1, DHAR and MYC2 in shoots and DHAR, ADC2, RD20, RD29B, GTR1, ANAC055, VSP1 and VSP2 gene expression in roots under salt stress.

CONCLUSION

Our data showed that MYC2 and JAR1 are required for B. japonicum-induced shoot growth in both salt stressed and non-stressed plants. The observed microbially influenced reactions to salinity stress in inoculated plants underscore the complexity of the B. japonicum jasmonic acid-mediated plant response salt tolerance.

Intertwined Roles of Reactive Oxygen Species and Salicylic Acid Signaling Are Crucial for the Plant Response to Biotic Stress

One of the earliest hallmarks of plant immune response is production of reactive oxygen species (ROS) in different subcellular compartments, which regulate plant immunity. A suitable equilibrium, which is crucial to prevent ROS overaccumulation leading to oxidative stress, is maintained by salicylic acid (SA), a chief regulator of ROS. However, ROS not only act downstream of SA signaling, but are also proposed to be a central component of a self-amplifying loop that regulates SA signaling as well as the interaction balance between different phytohormones. The exact role of this crosstalk, the position where SA interferes with ROS signaling and ROS interferes with SA signaling and the outcome of this regulation, depend on the origin of ROS but also on the pathosystem. The precise spatiotemporal regulation of organelle-specific ROS and SA levels determine the effectiveness of pathogen arrest and is therefore crucial for a successful immune response. However, the regulatory interplay behind still remains poorly understood, as up until now, the role of organelle-specific ROS and SA in hypersensitive response (HR)-conferred resistance has mostly been studied by altering the level of a single component. In order to address these aspects, a sophisticated combination of research methods for monitoring the spatiotemporal dynamics of key players and transcriptional activity in plants is needed and will most probably consist of biosensors and precision transcriptomics.

Linking Reactive Oxygen Species (ROS) to Abiotic and Biotic Feedbacks in Plant Microbiomes: The Dose Makes the Poison

In nature, plants develop in complex, adaptive environments. Plants must therefore respond efficiently to environmental stressors to maintain homeostasis and enhance their fitness. Although many coordinated processes remain integral for achieving homeostasis and driving plant development, reactive oxygen species (ROS) function as critical, fast-acting orchestrators that link abiotic and biotic responses to plant homeostasis and development. In addition to the suite of enzymatic and non-enzymatic ROS processing pathways that plants possess, they also rely on their microbiota to buffer and maintain the oxidative window needed to balance anabolic and catabolic processes. Strong evidence has been communicated recently that links ROS regulation to the aggregated function(s) of commensal microbiota and plant-growth-promoting microbes. To date, many reports have put forth insightful syntheses that either detail ROS regulation across plant development (independent of plant microbiota) or examine abiotic–biotic feedbacks in plant microbiomes (independent of clear emphases on ROS regulation). Here we provide a novel synthesis that incorporates recent findings regarding ROS and plant development in the context of both microbiota regulation and plant-associated microbes. Specifically, we discuss various roles of ROS across plant development to strengthen the links between plant microbiome functioning and ROS regulation for both basic and applied research aims.

Defence-related pathways, phytohormones and primary metabolism are key players in kiwifruit plant tolerance to Pseudomonas syringae pv. actinidiae

The reasons underlying the differential tolerance of Actinidia spp. to the pandemic pathogen Pseudomonas syringae pv. actinidiae (Psa) have not yet been elucidated. We hypothesized that differential plant-defence strategies linked to transcriptome regulation, phytohormones and primary metabolism might be key and that Actinidia chinensis susceptibility results from an inefficient activation of defensive mechanisms and metabolic impairments shortly following infection. Here, 48 h postinoculation bacterial density was 10-fold higher in A. chinensis var. deliciosa than in Actinidia arguta, accompanied by significant increases in glutamine, ornithine, jasmonic acid (JA) and salicylic acid (SA) (up to 3.2-fold). Actinidia arguta showed decreased abscisic acid (ABA) (0.7-fold), no changes in primary metabolites, and 20 defence-related genes that were only differentially expressed in this species. These include GLOX1, FOX1, SN2 and RBOHA, which may contribute to its higher tolerance. Results suggest that A. chinensis' higher susceptibility to Psa is due to an inefficient activation of plant defences, with the involvement of ABA, JA and SA, leading to impairments in primary metabolism, particularly the ammonia assimilation cycle. A schematic overview on the interaction between Psa and genotypes with distinct tolerance is provided, highlighting the key transcriptomic and metabolomic aspects contributing to the different plant phenotypes after infection.

Overexpression of Solanum tuberosum Respiratory Burst Oxidase Homolog A (StRbohA) Promotes Potato Tolerance to Phytophthora infestans

Reactive oxygen species (ROSs) represent one of the first lines of plants' biochemical defense against pathogens. Plants' respiratory burst oxidase homologs (RBOHs) produce ROSs as byproducts in several cellular compartments. In potato tubers, Solanum tuberosum respiratory burst oxidase homolog (StRBOHs) are involved in suberization and healing of wounded tissues. StRbohA has been tested in the model plant Arabidopsis thaliana, which led to enhanced plant defense against the soilborne pathogen Verticillium dahliae. Here, we showed that overexpressing StRbohA in potato plants increases plant tolerance to the oomycete Phytophthora infestans, the causal agent of late blight disease. Transgenic potato plants expressing StRbohA showed reduced disease symptoms (necrosis) compared with the wild type. In parallel, the expression of pathogenesis-related genes (PRs); RBOHs; antioxidation-related genes CPRX1, PRX2, APRX1, CAT1, and CAT2; and genes involved in the biosynthesis pathways of jasmonic and salicylic acids (ICS, PAL1, PAL2, LOX1, LOX2, and LOX3) exhibited significant increases in transgenic plants in response to infection. After higher expression of RBOHs, ROSs accumulated more in inoculation sites of the transgenic plants. ROSs act as signals that activate gene expression in the salicylic acid (SA) biosynthesis pathway, leading to the accumulation of SA and triggering SA-based defense mechanisms. SA-responsive PRs showed higher expression in the transgenic plants, which resulted in the restriction of pathogen growth in plant tissues. These results demonstrate the effective role of StRbohA in increasing potato defense against P. infestans.

Transcriptional Changes in Potato Sprouts upon Interaction with Rhizoctonia solani Indicate Pathogen-Induced Interference in the Defence Pathways of Potato

Rhizoctonia solani is the causer of black scurf disease on potatoes and is responsible for high economical losses in global agriculture. In order to increase the limited knowledge of the plants' molecular response to this pathogen, we inoculated potatoes with R. solani AG3-PT isolate Ben3 and carried out RNA sequencing with total RNA extracted from potato sprouts at three and eight days post inoculation (dpi). In this dual RNA-sequencing experiment, the necrotrophic lifestyle of R. solani AG3-PT during early phases of interaction with its host has already been characterised. Here the potato plants' comprehensive transcriptional response to inoculation with R. solani AG3 was evaluated for the first time based on significantly different expressed plant genes extracted with DESeq analysis. Overall, 1640 genes were differentially expressed, comparing control (-Rs) and with R. solani AG3-PT isolate Ben3 inoculated plants (+Rs). Genes involved in the production of anti-fungal proteins and secondary metabolites with antifungal properties were significantly up regulated upon inoculation with R. solani. Gene ontology (GO) terms involved in the regulation of hormone levels (i.e., ethylene (ET) and jasmonic acid (JA) at 3 dpi and salicylic acid (SA) and JA response pathways at 8 dpi) were significantly enriched. Contrastingly, the GO term "response to abiotic stimulus" was down regulated at both time points analysed. These results may support future breeding efforts toward the development of cultivars with higher resistance level to black scurf disease or the development of new control strategies.

Ethylene-Cytokinin Interaction Determines Early Defense Response of Wheat against Stagonospora nodorum Berk

Ethylene, salicylic acid (SA), and jasmonic acid are the key phytohormones involved in plant immunity, and other plant hormones have been demonstrated to interact with them. The classic phytohormone cytokinins are important participants of plant defense signaling. Crosstalk between ethylene and cytokinins has not been sufficiently studied as an aspect of plant immunity and is addressed in the present research. We compared expression of the genes responsible for hormonal metabolism and signaling in wheat cultivars differing in resistance to Stagonospora nodorum in response to their infection with fungal isolates, whose virulence depends on the presence of the necrotrophic effector SnTox3. Furthermore, we studied the action of the exogenous cytokinins, ethephon (2-chloroethylphosphonic acid, ethylene-releasing agent) and 1-methylcyclopropene (1-MCP, inhibitor of ethylene action) on infected plants. Wheat susceptibility was shown to develop due to suppression of reactive oxygen species production and decreased content of active cytokinins brought about by SnTox3-mediated activation of the ethylene signaling pathway. SnTox3 decreased cytokinin content most quickly by its activated glucosylation in an ethylene-dependent manner and, furthermore, by oxidative degradation and inhibition of biosynthesis in ethylene-dependent and ethylene-independent manners. Exogenous zeatin application enhanced wheat resistance against S. nodorum through inhibition of the ethylene signaling pathway and upregulation of SA-dependent genes. Thus, ethylene inhibited triggering of SA-dependent resistance mechanism, at least in part, by suppression of the cytokinin signaling pathway.

OsRbohB-mediated ROS production plays a crucial role in drought stress tolerance of rice

We found that a rice NADPH oxidase gene OsRbohB contributes drought tolerance and its functions are involved in the interaction of the OsRbohB-mediated ROS production and ABA signaling. The plasma membrane NADPH oxidases, also known as respiratory burst oxidase homologs, are the key producers of ROS under both normal and stress conditions in plants. However, their functions in rice development and stress tolerance are still under investigation. Here, we found that a rice NADPH oxidase gene OsRbohB, also named OsNOX1, is expressed in all tissues examined throughout the development stages with higher transcripts in leaves. The transcriptional expression of OsRbohB is also strongly stimulated by dehydration, salt and several phytohormonal treatments. Compared with wide-type and the OsRbohB-overexpressing transgenic plants, osrbohB, a Tos17 insertion knockout mutant of OsRbohB, shows lower ROS production, abscisic acid (ABA) content and transcripts of a series of stress-related genes. The osrbohB mutant also exhibits lower seed germination rate, organ size and thousand seed weight, but higher stomatal aperture and sensitivity to drought. Moreover, a number of genes involved in plant development, stress response, transcriptional regulation, and particularly ABA signaling are differentially expressed in osrbohB plants under both normal growth and drought conditions. All these results suggest the roles of OsRbohB in drought tolerance of rice, which probably performed through the interaction of the OsRbohB-mediated ROS production and ABA signaling.

Expression profiling of the genes encoding ABA route components and the ACC oxidase isozymes in the senescing leaves of Populus tremula

Abscisic acid (ABA) triggers and regulates, while ethylene modulates autumn leaf senescence. The expression profiles of genes encoding ABA route components and the ACC oxidase isozymes were investigated in Populus tremula during the early and moderate stages of autumn leaf senescence. The targets of interest were Ptre-HAB1-like genes (Ptre-HAB1, Ptre-HAB3a and Ptre-HAB3b), the subclass 3 of Ptre-SnRK2s genes (Ptre-SnRK2.6a, Ptre-SnRK2.6b and Ptre-SnRK2.6b) and Ptre-RbohD1, Ptre-RbohF1, and Ptre-RbohF2 genes encoding the poplar components, which are counterparts of the ABA route key regulators or the counterparts of its secondary messengers, such as Homology to ABA-insensitive 1 (HAB1), Sucrose non-fermenting 1-related protein kinases 2 (SnRK2s) or Respiratory burst oxidase D and Respiratory burst oxidase F (RbohD and RbohF, respectively) in Arabidopsis, and Ptre-ACO3, Ptre-ACO5, and Ptre-ACO6 genes encoding ACC oxidase isozymes involved in ethylene biosynthesis. The fold change in their expression levels enabled to distinguish the distinct expression patterns for the following pairs of genes: Ptre-HAB3a and Ptre-SnRK2.6a, Ptre-HAB3b and Ptre-SnRK2.2, and Ptre-HAB1 and Ptre-SnRK2.6b, where each pair involves the genes encoding the negative and positive regulators of ABA route, respectively. Among the investigated genes, the fold change of expression was the highest for Ptre-ACO3, Ptre-ACO6, and Ptre-SnRK2.6b genes during both the studied stages, and additionally for Ptre-HAB1 and Ptre-RbohD1 genes during the moderate stage. In contrast, the Ptre-RbohF1 and Ptre-RbohF2 genes exhibited only the transient upregulation at the early stage of senescence. In an in vitro study, the ability of protein kinases Ptre-SnRK2.6a and Ptre-SnRK2.6b to phosphorylate the N-terminal regions of Ptre-RbohD1 and Ptre-RbohF2 was studied; the activity of Ptre-SnRK2.6b against the studied Ptre-Rbohs was noticeably lower than that exhibited by Ptre-SnRK2.6a. It seems that despite the high similarity of their polypeptides, Ptre-SnRK2.6a and Ptre-SnRK2.6b may play different biological roles; nonetheless, it requires in vivo confirmation. Surprisingly, the highest protein kinase activity against the Ptre-Rbohs was detected in the heterologous reaction with AT-SnRK2.6/OST1 which suggests that the discussed interactions are evolutionary conserved.

RNAi of the sesquiterpene cyclase gene for phytoalexin production impairs pre- and post-invasive resistance to potato blight pathogens

Potato antimicrobial sesquiterpenoid phytoalexins lubimin and rishitin have been implicated in resistance to the late blight pathogen, Phytophthora infestans and early blight pathogen, Alternaria solani. We generated transgenic potato plants in which sesquiterpene cyclase, a key enzyme for production of lubimin and rishitin, is compromised by RNAi to investigate the role of phytoalexins in potato defence. The transgenic tubers were deficient in phytoalexins and exhibited reduced post-invasive resistance to an avirulent isolate of P. infestans, resulting in successful infection of the first attacked cells without induction of cell death. However, cell death was observed in the subsequently penetrated cells. Although we failed to detect phytoalexins and antifungal activity in the extract from wild-type leaves, post-invasive resistance to avirulent P. infestans was reduced in transgenic leaves. On the other hand, A. solani frequently penetrated epidermal cells of transgenic leaves and caused severe disease symptoms presumably from a deficiency in unidentified antifungal compounds. The contribution of antimicrobial components to resistance to penetration and later colonization may vary depending on the pathogen species, suggesting that sesquiterpene cyclase-mediated compounds participate in pre-invasive resistance to necrotrophic pathogen A. solani and post-invasive resistance to hemibiotrophic pathogen P. infestans.

Genomic, molecular evolution, and expression analysis of NOX genes in soybean (Glycine max)

Reactive oxygen species (ROS) are versatile signaling molecules in sensing stresses and play critical roles in signaling and development. Plasma membrane NADPH oxidases (NOXs) are key producers of ROS, and play important roles in the regulation of plant-pathogen interactions. Here, we performed a comprehensive analysis of the NOX gene family in the soybean genome (Glycine max) and 17 NOX (GmNOX) genes were identified. Structural analysis revealed that the GmNOX proteins in soybean were as conserved as those in other plants. 8 duplicated gene pairs were formed by a Glycine-specific whole-genome duplication (WGD) event approximately 13 million years ago (Mya). The Ka/Ks ratios of GmNOX genes ranged from 0.04 to 0.28, suggesting that the GmNOX family had undergone purifying selection in soybean. Gene expression patterns showed different expression of these duplicate genes, suggesting that the GmNOXs were retained by substantial subfunctionalization during the soybean evolutionary processes. Subsequently, the expression of GmNOXs in response to drought and phytohormones were characterized via qPCR. Importantly, four GmNOXs showed strong expression in nodules, pointing to their probable involvement in nodulation. Thus, our results shed light on the evolutionary history of this family in soybean and contribute to the functional characterization of GmNOX genes in soybean.

Functional study of Csrbohs in defence response against Xanthomonas citri ssp. citri

NADPH oxidases, encoded by rbohs (respiratory burst oxidase homologues), transfer electrons from NADPH to molecular oxygen (O2) to generate superoxide anion (O2•-), which is the first step in the formation of hydrogen peroxide (H2O2) in the plant-pathogen interaction system. In the present work, six citrus rbohs (Csrbohs) genes were identified in citrus, and their possible involvement in resistance to Xanthomonas citri ssp. citri (Xcc) was examined. Inoculation with Xcc promoted the H2O2 production and induced expression of the Csrbohs, especially CsrbohD. Results showed that CsrbohD was markedly induced in the resistant genotype kumquat 'Luofu' [Fortunella margarita (Lour.) Swingle] compared with grapefruit 'Duncan' [Citrus paradisi (Linn.) Macf.]. Virus-induced gene silencing (VIGS) of CsrbohD resulted in reduced resistance to Xcc in grapefruit, but not in kumquat. Compared with non-silenced plants, canker-like symptoms were observed earlier, and they were more extensive in the CsrbohD-silenced grapefruit. Silencing of CsrbohD also suppressed the Xcc induced reactive oxygen species (ROS) burst, and resulted in accumulation of more Xcc bacterial colonies. Taken together, these data indicate that CsrbohD promotes resistance to Xcc, especially in grapefruit.

Genome-wide mining of respiratory burst homologs and its expression in response to biotic and abiotic stresses in Triticum aestivum

BACKGROUND

Membrane-bound NADPH oxidases (Nicotinamide adenine ainucleotide phosphate oxidase) also called respiratory burst oxidase homologs (Rboh) play an essential role in ROS production under normal as well as environmental stress conditions in plants.

OBJECTIVE

To identify and study respiratory burst homologs (Rboh) from the wheat genome as well as characterize their role in various biological and molecular processes along with expression in response to biotic and abiotic stresses.

METHODS

The Rboh homologs in the wheat genome were predicted based on data processing, alignment of sequences and phylogenetic analysis of sequences in numerous plant species and wheat. The conserved motifs were known followed by domain design study. The 3-D structure prediction and similarity modeling were administered for NADPH enzyme domain. Gene ontology and a functional study were done in addition to expression analysis of Triticum aestivum respiratory burst oxidase (TaRboh) gene family in response to biotic as well as abiotic stress.

RESULTS

Phylogenetic analysis of Rboh gene family members among seven plant species including wheat, classified the family into four subfamilies. Rboh genes are mainly involved in various biological processes such as Response to oxidative stress, Superoxide anion generation, Hydrogen peroxide biosynthetic process. Among the molecular functions, calcium ion binding, peroxidase activity, oxidoreductase activity, superoxide-generating NADPH oxidase activity are essential. Enzyme annotation of the family and superfamily revealed that it encodes to five structural clusters and coding to enzymes NAD(P)H oxidase (H₂O₂-forming) (EC:1.6.3.1), Ferric-chelate reductase (NADH) (EC: 1.16.1.7), Peroxidase (EC: 1.11.1.7), Ribose-phosphate diphosphokinase (EC: 2.7.6.1). The enzymes contain six membrane-spanning domains, two hemes, and conserved motifs associated with NADPH, EF-hand and FAD binding. The outcomes additionally reflect a distinct role of this enzyme in different molecular functions which are responsible for the stress signaling. Further, the transcripts of TaRboh found expressed in various plant parts such as stem, leaves, spike, seed, and roots. We also observed expression of these gene family members under drought/combination of drought + heat and important wheat pathogens such as Puccinia striformis, Blumeria graminis f.sp. tritici, Fusarium graminiarum, F. pseudograminiarum, and Zymoseptoria tritici.

CONCLUSIONS

The investigation demonstrated that identified respiratory burst homologs (Rboh) in T. aestivum were involved in pathogen activated ROS production and have regulatory functions in cell death and defense responses.

Comparative analysis of the reactive oxygen species-producing enzymatic activity of Arabidopsis NADPH oxidases

Reactive oxygen species (ROS) produced by NADPH oxidases, called respiratory burst oxidase homologs (Rbohs), play crucial roles in development as well as biotic and abiotic stress responses in plants. Arabidopsis has 10 Rboh genes, AtRbohA to AtRbohJ. Five AtRbohs (AtRbohC, -D, -F, -H and -J) are synergistically activated by Ca²⁺ -binding and protein phosphorylation to produce ROS that play various roles in planta, although the activities of the other Rbohs remain unknown. With a heterologous expression system, we found a range of ROS-producing activity among the AtRbohs with differences up to 100 times, indicating that the required amounts of ROS are different in each situation where AtRbohs act. To specify the functions of AtRbohs involved in cell growth, we focused on AtRbohC, -H and -J, which are involved in tip growth of root hairs or pollen tubes. Ectopic expression of the root hair factor AtRbohC/ROOT HAIR DEFECTIVE 2 (RHD2) in pollen tubes restored the atrbohH atrbohJ defects in tip growth of pollen tubes. However, expression of AtRbohH or -J in root hairs did not complement the tip growth defect in the atrbohC/rhd2 mutant. Our data indicate that Rbohs possess different ranges of enzymatic activity, and that some Rbohs have evolved to carry specific functions in cell growth.

LncRNA33732-respiratory burst oxidase module associated with WRKY1 in tomato- Phytophthora infestans interactions

Our previous studies indicated that tomato WRKY1 transcription factor acts as a positive regulator during tomato resistance to Phytophthora infestans. However, the molecular mechanism of WRKY1-mediated resistance regulation remains unclear. Here, we used a comparative transcriptome analysis between wild-type and WRKY1-overexpressing tomato plants to identify differentially expressed genes (DEGs) and long non-coding RNAs (DELs), and we examined long non-coding RNA (lncRNA)-gene networks. The promoter sequences of the upregulated DEGs and DELs were analyzed. Among 1073 DEGs and 199 DELs, 1 kb 5'-upstream regions of 59 DEGs and 22 DELs contain the W-box, the target sequence of the WRKY1. The results of promoter-β-glucuronidase (GUS) fusion and yeast one-hybrid assay showed that lncRNA33732 was activated by WRKY1 through sequence-specific interactions with the W-box element in its promoter. The overexpression and silencing analysis of lncRNA33732 in tomato showed that lncRNA33732 acts as a positive regulator and enhanced tomato resistance to P. infestans by induction of the expression of respiratory burst oxidase (RBOH) and increase in the accumulation of H₂ O₂ . When the expression of RBOH gene was inhibited in tomato plants, H₂ O₂ accumulation decreased and resistance were impaired. These findings suggest that lncRNA33732 activated by WRKY1 induces RBOH expression to increase H₂ O₂ accumulation in early defense reaction of tomato to P. infestans attack. Our results provide insights into the WRKY1-lncRNA33732-RBOH module involved in the regulation of H₂ O₂ accumulation and resistance to P. infestans, as well as provide candidates to enhance broad-spectrum resistance to pathogens in tomato.

H2O2 and NADPH oxidases involve in regulation of 2-(2-phenylethyl)chromones accumulation during salt stress in Aquilaria sinensis calli

2-(2-Phenylethyl)chromones are the main compounds responsible for the quality of agarwood, which is widely used in traditional medicines, incenses and perfumes. H₂O₂ and NADPH oxidases (also known as respiratory burst oxidase homologs, Rbohs) mediate diverse physiological and biochemical processes in environmental stress responses. However, little is known about the function of H₂O₂ and NADPH oxidases in 2-(2-phenylethyl)chromones accumulation. In this study, we found that salt stress induced a transient increase in content of H₂O₂ and 2-(2-phenylethyl)chromones accumulation in Aquilaria sinensis calli. Exogenous H₂O₂ remarkably decreased the production of 2-(2-phenylethyl)chromones, while dimethylthiourea (DMTU), a scavenger of H₂O₂, significantly increased 2-(2-phenylethyl)chromones accumulation in salt treated calli. Three new H₂O₂-generating genes, named AsRbohA-C, were isolated and characterized from A. sinensis. Salt stress also induced a transient increase in AsRbohA-C expression and NADPH oxidase activity. Furthermore, exogenous H₂O₂ increased AsRbohA-C expression and NADPH oxidase activity, while DMTU inhibited AsRbohA-C expression and NADPH oxidase activity under salt stress. Moreover, diphenylene iodonium (DPI), the inhibitor of NADPH oxidases, reduced AsRbohA-C expression and NADPH oxidase activity, but significantly induced 2-(2-phenylethyl)chromones accumulation during salt stress. These results clearly demonstrated the central role of H₂O₂ and NADPH oxidases in regulation of salt-induced 2-(2-phenylethyl)chromones accumulation in A. sinensis calli.

Genome-Wide Identification and Functional Analysis of NADPH Oxidase Family Genes in Wheat During Development and Environmental Stress Responses

As the key producers of reactive oxygen species (ROS), NADPH oxidases (NOXs), also known as respiratory burst oxidase homologs (RBOHs), play crucial roles in various biological processes in plants with considerable evolutionary selection and functional diversity in the entire terrestrial plant kingdom. However, only limited resources are available on the phylogenesis and functions of this gene family in wheat. Here, a total of 46 NOX family genes were identified in the wheat genome, and these NOXs could be classified into three subgroups: typical TaNOXs, TaNOX-likes, and ferric reduction oxidases (TaFROs). Phylogenetic analysis indicated that the typical TaNOXs might originate from TaFROs during evolution, and the TaFROs located on Chr 2 might be the most ancient forms of TaNOXs. TaNOXs are highly expressed in wheat with distinct tissue or organ-specificity and stress-inducible diversity. A large-scale expression and/or coexpression analysis demonstrated that TaNOXs can be divided into four functional groups with different expression patterns under a broad range of environmental stresses. Different TaNOXs are coexpressed with different sets of other genes, which widely participate in several important intracellular processes such as cell wall biosynthesis, defence response, and signal transduction, suggesting their vital but diversity of roles in plant growth regulation and stress responses of wheat.

Receptor-mediated chitin perception in legume roots is functionally separable from Nod factor perception

The ability of root cells to distinguish mutualistic microbes from pathogens is crucial for plants that allow symbiotic microorganisms to infect and colonize their internal root tissues. Here we show that Lotus japonicus and Medicago truncatula possess very similar LysM pattern-recognition receptors, LjLYS6/MtLYK9 and MtLYR4, enabling root cells to separate the perception of chitin oligomeric microbe-associated molecular patterns from the perception of lipochitin oligosaccharide by the LjNFR1/MtLYK3 and LjNFR5/MtNFP receptors triggering symbiosis. Inactivation of chitin-receptor genes in Ljlys6, Mtlyk9, and Mtlyr4 mutants eliminates early reactive oxygen species responses and induction of defense-response genes in roots. Ljlys6, Mtlyk9, and Mtlyr4 mutants were also more susceptible to fungal and bacterial pathogens, while infection and colonization by rhizobia and arbuscular mycorrhizal fungi was maintained. Biochemical binding studies with purified LjLYS6 ectodomains further showed that at least six GlcNAc moieties (CO6) are required for optimal binding efficiency. The 2.3-Å crystal structure of the LjLYS6 ectodomain reveals three LysM βααβ motifs similar to other LysM proteins and a conserved chitin-binding site. These results show that distinct receptor sets in legume roots respond to chitin and lipochitin oligosaccharides found in the heterogeneous mixture of chitinaceous compounds originating from soil microbes. This establishes a foundation for genetic and biochemical dissection of the perception and the downstream responses separating defense from symbiosis in the roots of the 80-90% of land plants able to develop rhizobial and/or mycorrhizal endosymbiosis.

Changes in ROS production and antioxidant capacity during tuber sprouting in potato

Potato dormancy is a complex process with an extensive release phase. This study investigated involvement of reactive oxygen species during tuber dormancy release. We found that tuber sprouting was delayed by treatment with diphenylene iodonium chloride, an NADPH oxidase inhibitor; NADPH oxidase catalyze the production of ROS. In situ ROS localization and ROS content estimation revealed that dormancy release was associated with an accumulation of superoxide anion and hydrogen peroxide in tuber buds. The antioxidant compounds and enzymes display important changes during the progression of dormancy. The application of Ca²⁺ induced superoxide anion production and promoted in vitro tuber bud sprouting. Among the seven homologues of NADPH oxidases in potato, the expression of StrbohA and StrbohB were detected in particular when dormancy break. In addition, the expression of key genes that function in the potato dormancy release are discussed in relation to ROS metabolism in other plants.

Extracellular Recognition of Oomycetes during Biotrophic Infection of Plants

Extracellular recognition of pathogens by plants constitutes an important early detection system in plant immunity. Microbe-derived molecules, also named patterns, can be recognized by pattern recognition receptors (PRRs) on the host cell membrane that trigger plant immune responses. Most knowledge on extracellular pathogen detection by plants comes from research on bacterial and fungal pathogens. For oomycetes, that comprise some of the most destructive plant pathogens, mechanisms of extracellular pattern recognition have only emerged recently. These include newly recognized patterns, e.g., cellulose-binding elicitor lectin, necrosis and ethylene-inducing peptide 1-like proteins (NLPs), and glycoside hydrolase 12, as well as their receptors, e.g., the putative elicitin PRR elicitin response and the NLP PRR receptor-like protein 23. Immunity can also be triggered by the release of endogenous host-derived patterns, as a result of oomycete enzymes or damage. In this review we will describe the types of patterns, both pathogen-derived exogenous and plant-derived endogenous ones, and what is known about their extracellular detection during (hemi-)biotrophic oomycete infection of plants.

The Fundamental Role of NOX Family Proteins in Plant Immunity and Their Regulation

NADPH oxidases (NOXs), also known as respiratory burst oxidase homologs (RBOHs), are the major source of reactive oxygen species (ROS), and are involved in many important processes in plants such as regulation of acclimatory signaling and programmed cell death (PCD). Increasing evidence shows that NOXs play crucial roles in plant immunity and their functions in plant immune responses are not as separate individuals but with other signal molecules such as kinases, Rac/Rop small GTPases and hormones, mediating a series of signal transmissions. In a similar way, NOX-mediated signaling also participates in abiotic stress response of plants. We summarized here the complex role and regulation mechanism of NOXs in mediating plant immune response, and the viewpoint that abiotic stress response of plants may be a kind of special plant immunity is also proposed.

The plasma membrane NADPH oxidase OsRbohA plays a crucial role in developmental regulation and drought-stress response in rice

Plasma membrane NADPH oxidases are major producers of reactive oxygen species (ROS) in plant cells under normal growth and stress conditions. In the present study the total activity of rice NADPH oxidases and the transcription of OsRbohA, which encodes an Oryza sativa plasma membrane NADPH oxidase, were stimulated by drought. OsRbohA was expressed in all tissues examined throughout development. Its mRNA was upregulated by a number of factors, including heat, drought, salt, oxidative stress and methyl jasmonate treatment. Compared with wild-type (WT), the OsRbohA-knockout mutant osrbohA exhibited upregulated expression of other respiratory burst oxidase homolog genes and multiple abnormal agronomic traits, including reduced biomass, low germination rate and decreased pollen viability and seed fertility. However, OsRbohA-overexpressing transgenic plants showed no differences in these traits compared with WT. Although osrbohA leaves and roots produced more ROS than WT, the mutant had lesser intracellular ROS. In contrast, OsRbohA-overexpressing transgenic plants exhibited higher ROS production at the intracellular level and in tissues. Ablation of OsRbohA impaired the tolerance of plants to various water stresses, whereas its overexpression enhanced the tolerance. In addition, a number of genes related to energy supply, substrate transport, stress response and transcriptional regulation were differentially expressed in osrbohA plants even under normal growth conditions, suggesting that OsRbohA has fundamental and broad functions in rice. These results indicate that OsRbohA-mediated processes are governed by complex signaling pathways that function during the developmental regulation and drought-stress response in rice.

Comprehensive Genomic Analysis and Expression Profiling of the NOX Gene Families under Abiotic Stresses and Hormones in Plants

Plasma membrane NADPH oxidases (NOXs) are key producers of reactive oxygen species under both normal and stress conditions in plants and they form functional subfamilies. Studies of these subfamilies indicated that they show considerable evolutionary selection. We performed a comparative genomic analysis that identified 50 ferric reduction oxidases (FRO) and 77 NOX gene homologs from 20 species representing the eight major plant lineages within the supergroup Plantae: glaucophytes, rhodophytes, chlorophytes, bryophytes, lycophytes, gymnosperms, monocots, and eudicots. Phylogenetic and structural analysis classified these FRO and NOX genes into four well-conserved groups represented as NOX, FRO I, FRO II, and FRO III. Further analysis of NOXs of phylogenetic and exon/intron structures showed that single intron loss and gain had occurred, yielding the diversified gene structures during the evolution of NOXs family genes and which were classified into four conserved subfamilies which are represented as Sub.I, Sub.II, Sub.III, and Sub.IV. Additionally, both available global microarray data analysis and quantitative real-time PCR experiments revealed that the NOX genes in Arabidopsis and rice (Oryza sativa) have different expression patterns in different developmental stages, various abiotic stresses and hormone treatments. Finally, coexpression network analysis of NOX genes in Arabidopsis and rice revealed that NOXs have significantly correlated expression profiles with genes which are involved in plants metabolic and resistance progresses. All these results suggest that NOX family underscores the functional diversity and divergence in plants. This finding will facilitate further studies of the NOX family and provide valuable information for functional validation of this family in plants.

A Role for Reactive Oxygen Species Produced by NADPH Oxidases in the Embryo and Aleurone Cells in Barley Seed Germination

Reactive oxygen species (ROS) promote the germination of several seeds, and antioxidants suppress it. However, questions remain regarding the role and production mechanism of ROS in seed germination. Here, we focused on NADPH oxidases, which produce ROS. After imbibition, NADPH oxidase mRNAs were expressed in the embryo and in aleurone cells of barley seed; these expression sites were consistent with the sites of ROS production in the seed after imbibition. To clarify the role of NADPH oxidases in barley seed germination, we examined gibberellic acid (GA) / abscisic acid (ABA) metabolism and signaling in barley seeds treated with diphenylene iodonium chloride (DPI), an NADPH oxidase inhibitor. DPI significantly suppressed germination, and suppressed GA biosynthesis and ABA catabolism in embryos. GA, but not ABA, induced NADPH oxidase activity in aleurone cells. Additionally, DPI suppressed the early induction of α-amylase by GA in aleurone cells. These results suggest that ROS produced by NADPH oxidases promote GA biosynthesis in embryos, that GA induces and activates NADPH oxidases in aleurone cells, and that ROS produced by NADPH oxidases induce α-amylase in aleurone cells. We conclude that the ROS generated by NADPH oxidases regulate barley seed germination through GA / ABA metabolism and signaling in embryo and aleurone cells.

Photocontrol of Elicitor Activity of PIP-1 to Investigate Temporal Factors Involved in Phytoalexin Biosynthesis

The peptide elicitor PIP-1 can induce various immune responses in tobacco cells. Previously, we showed that types of responses induced by PIP-1 are different depending on its stimulation periods; short-term stimulation induces weak responses, whereas long-term stimulation leads to strong responses including production of the phytoalexin capsidiol. However, key components that directly regulate the initiation of capsidiol biosynthesis in response to continuous stimulation with PIP-1 remain unclear. In this study, we designed a photocleavable PIP-1 analog containing 3-amino-3-(2-nitrophenyl)propionic acid as a photocleavable residue. The activity of the analog can be "switched off" using ultraviolet (UV) irradiation without undesired side effects. This analog induced a significant level of capsidiol production unless UV-irradiated, whereas no capsidiol production was observed when tobacco cells were UV-irradiated 1 h after treatment. Using this analog, we found that the elicitor-inducible 3-hydroxy-3-methylglutaryl-CoA reductase activity is regulated based on the duration of the stimulation with PIP-1, which could be associated with the initiation of capsidiol biosynthesis.

Modification of plasma membrane NADPH oxidase activity in cucumber seedling roots in response to cadmium stress

The aim of this study was to investigate the effect of cadmium on plasma membrane (PM) NADPH oxidase activity in cucumber roots. Plants were treated with cadmium for 1, 3 or 6 days. Some of the plants after 3-day exposure to cadmium were transferred to a medium without the heavy metal for the next 3 days. Treatment of plants with cadmium for 6 days stimulated the activity of NADPH oxidase. The highest stimulation of O2(•-) production by NADPH oxidase was observed in post-stressed plants, which was correlated with the stimulation of activity of PM H(+)-ATPase in the same conditions. In order to examine the effects of cadmium stresses on the expression level of genes encoding NADPH oxidase, putative cucumber homologs encoding RBOH proteins were selected and a real-time PCR assay was performed. NADPH is a substrate for oxidase; thus alterations in the activity of glucose-6-phosphate dehydrogenase, 6-phosphogluconate dehydrogenase, NADP-isocitrate dehydrogenase and NADP-malic enzyme under cadmium stress were studied. The activity of NADPH dehydrogenases was increased under cadmium stress. The results indicate that PM NADPH oxidase could be involved in plants' response to cadmium stress by affecting the activity of PM H(+)-ATPase, and NADPH-generating enzymes could play important roles in this process.

Methanol extract of mycelia from Phytophthora infestans-induced resistance in potato

Plants recognize certain microbial compounds as elicitors in their active defence mechanisms. It has been shown that a series of defence reactions are induced in potato plant cells after treatment with water-soluble hyphal wall components prepared from Phytophthora infestans. In this study, a methanol extract from mycelia of P. infestans (MEM), which contains lipophilic compounds, was used as another elicitor for the induction of the defence reactions in potato. MEM elicitor induced reactive oxygen species (ROS), especially O2(-) and H2O2 production, and nitric oxide (NO) generation in potato leaves and suspension-cultured cells. Hypersensitive cell death was detected in potato leaves within 6-8 h after MEM elicitor treatment. The accumulation of phytoalexins was detected by MEM elicitor treatment in potato tubers. In potato suspension-cultured cells, several defence-related genes were induced by MEM elicitors, namely Strboh, Sthsr203J, StPVS3, StPR1, and StNR5, which regulate various defence-related functions. Enhanced resistance against P. infestans was found in MEM-treated potato plants. These results suggested that MEM elicitor is recognized by host and enhances defence activities to produce substances inhibitory to pathogens.

β-glucans and eicosapolyenoic acids as MAMPs in plant-oomycete interactions: past and present

Branched β-1,3-glucans and the eicosapolyenoic acids (EP) are among the best characterized oomycete elicitors that trigger innate immune responses in plants. These elicitors were identified over three decades ago, and they were useful in the study of the sequence of physiological, biochemical and molecular events that induce resistance in plants. However, in spite of the cross-kingdom parallels where these molecules are well-characterized as immune system modulators in animals, their perception and modes of action in plants remains obscure. Oomycetes are among the most important plant pathogens, responsible for diseases that devastate crops, ornamentals, and tree species worldwide. With the recent interest and advances in our understanding of innate immunity in plants, and the redefining of many of the classical elicitors as microbe-associated molecular patterns (MAMPs), it seems timely and important to reexamine β-glucans and EP using contemporary approaches. In this review, we highlight early studies of β-glucans and EP, discuss their roles as evolutionarily conserved signals, and consider their action in relation to current models of MAMP-triggered immunity.

Lipids in salicylic acid-mediated defense in plants: focusing on the roles of phosphatidic acid and phosphatidylinositol 4-phosphate

Plants have evolved effective defense strategies to protect themselves from various pathogens. Salicylic acid (SA) is an essential signaling molecule that mediates pathogen-triggered signals perceived by different immune receptors to induce downstream defense responses. While many proteins play essential roles in regulating SA signaling, increasing evidence also supports important roles for signaling phospholipids in this process. In this review, we collate the experimental evidence in support of the regulatory roles of two phospholipids, phosphatidic acid (PA), and phosphatidylinositol 4-phosphate (PI4P), and their metabolizing enzymes in plant defense, and examine the possible mechanistic interaction between phospholipid signaling and SA-dependent immunity with a particular focus on the immunity-stimulated biphasic PA production that is reminiscent of and perhaps mechanistically connected to the biphasic reactive oxygen species (ROS) generation and SA accumulation during defense activation.

Magnesium availability regulates the development of root hairs in Arabidopsis thaliana (L.) Heynh

Root hairs are reported to be plastic in response to nutrient supply, but relatively little is known about their development in response to magnesium (Mg) availability. Here, we showed that development of root hairs of Arabidopsis decreased progressively with increasing Mg supply, which was related to the initiation of new trichoblast files and likelihood of trichoblasts to form hairs. Tip-focused reactive oxygen species (ROS) and cytosolic Ca(2+) concentrations [(Ca(2+) )c] during elongation of root hairs were enhanced under low Mg but decreased under high Mg. Under low Mg, application of diphenylene iodonium (DPI) or BAPTA [1,2-bis(o-aminophenoxy)ethane-N,N,N',N'-tetraacetic acid] blocked the enhanced development of root hairs and the opposite was true when the plants under high Mg were treated with phenazine methosulphate (PMS), methyl viologen (MV) or CaCl2 . Furthermore, Mg availability did not alter root hair growth in rhd2-1 mutant that contains lower levels of ROS and cytosolic [Ca(2+) ]c. Transcriptome data and qPCR results revealed a greater fraction of morphogenetic H-genes, and cell wall organization genes were up-regulated by low Mg but down-regulated by high Mg. Our data suggest a profound effect of Mg supply on the development of root hairs in Arabidopsis, through the characterized Ca(2+) and ROS signals that modulate the elongation of root hairs and the expression of root-hair morphogenetic genes.

Versatile roles of plant NADPH oxidases and emerging concepts

NADPH oxidase (NOX) is a key player in the network of reactive oxygen species (ROS) producing enzymes. It catalyzes the production of superoxide (O2(-)), that in turn regulates a wide range of biological functions in a broad range of organisms. Plant Noxes are known as respiratory burst oxidase homologs (Rbohs) and are homologs of catalytic subunit of mammalian phagocyte gp91(phox). They are unique among other ROS producing mechanisms in plants as they integrate different signal transduction pathways in plants. In recent years, there has been addition of knowledge on various aspects related to its structure, regulatory components and associated mechanisms, and its plethora of biological functions. This update highlights some of the recent developments in the field with particular reference to important members of the plant kingdom.

Genome-wide analysis of respiratory burst oxidase homologs in grape (Vitis vinifera L.)

Plant respiratory burst oxidase homolog (rboh) genes appear to play crucial roles in plant development, defense reactions and hormone signaling. In this study, a total of seven rboh genes from grape were identified and characterized. Genomic structure and predicted protein sequence analysis indicated that the sequences of plant rboh genes are highly conserved. Synteny analysis demonstrated that several Vvrboh genes were found in corresponding syntenic blocks of Arabidopsis, suggesting that these genes arose before the divergence of the respective lineages. The expression pattern of Vvrboh genes in different tissues was assessed by qRT-PCR and two were constitutively expressed in all tissues tested. The expression profiles were similarly analyzed following exposure to various stresses and hormone treatments. It was shown that the expression levels of VvrbohA, VvrbohB and VvrbohC1 were significantly increased by salt and drought treatments. VvrbohB, VvrbohC2, and VvrbohD exhibited a dramatic up-regulation after powdery mildew (Uncinula necator (Schw.) Burr.) inoculation, while VvrbohH was down-regulated. Finally, salicylic acid treatment strongly stimulated the expression of VvrbohD and VvrbohH, while abscisic acid treatment induced the expression of VvrbohB and VvrbohH. These results demonstrate that the expression patterns of grape rboh genes exhibit diverse and complex stress-response expression signatures.

NADPH oxidase inhibitor diphenyleneiodonium and reduced glutathione mitigate ethephon-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression in sweet potato (Ipomoea batatas)

Ethephon, an ethylene releasing compound, promoted leaf senescence, H2O2 elevation, and senescence-associated gene expression in sweet potato. It also affected the glutathione and ascorbate levels, which in turn perturbed H2O2 homeostasis. The decrease of reduced glutathione and the accumulation of dehydroascorbate correlated with leaf senescence and H2O2 elevation at 72h in ethephon-treated leaves. Exogenous application of reduced glutathione caused quicker and significant increase of its intracellular level and resulted in the attenuation of leaf senescence and H2O2 elevation. A small H2O2 peak produced within the first 4h after ethephon application was also eliminated by reduced glutathione. Diphenyleneiodonium (DPI), an NADPH oxidase inhibitor, delayed leaf senescence and H2O2 elevation at 72h, and its influence was effective only within the first 4h after ethephon treatment. Ethephon-induced senescence-associated gene expression was repressed by DPI and reduced glutathione at 72h in pretreated leaves. Leaves treated with l-buthionine sulfoximine, an endogenous glutathione synthetase inhibitor, did enhance senescence-associated gene expression, and the activation was strongly repressed by reduced glutathione. In conclusion, ethephon-mediated leaf senescence, H2O2 elevation and senescence-associated gene expression are all alleviated by reduced glutathione and NADPH oxidase inhibitor DPI. The speed and the amount of intracellular reduced glutathione accumulation influence its effectiveness of protection against ethephon-mediated effects. Reactive oxygen species generated from NADPH oxidase likely serves as an oxidative stress signal and participates in ethephon signaling. The possible roles of NADPH oxidase and reduced glutathione in the regulation of oxidative stress signal in ethephon are discussed.

Metabolic survey of defense responses to a compatible hemibiotroph, Phytophthora parasitica var. nicotianae, in ethylene signaling-impaired tobacco

Reactive oxygen species (ROS) and ethylene play an important role in determining the resistance or susceptibility of plants to pathogen attack. A previous study of the response of tobacco cultivar ( Nicotiana tabacum L. cv. Wisconsin 38) to a compatible hemibiotroph, Phytophthora parasitica var. nicotianae (Ppn) showed that biphasic bursts of ROS and ethylene are positively associated with disease severity. The levels of ethylene and ROS might influence the susceptibility of plants to pathogens, with changing levels of metabolite related to disease resistance or susceptibility. In this study, to obtain more detailed information on the interaction of ROS and ethylene signaling related to resistance and/or susceptibility of plants to pathogen, Ppn-induced metabolic profiles from wild type (WT) and ethylene signaling-impaired transgenic plants that expressed Ein3 antisense (Ein3-AS) were compared using ultraperformance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS). Nonredundant mass ions (576 in ESI+ mode and 336 in ESI- mode) were selected, and 56 mass ions were identified on the basis of their accurate mass ions and MS/MS spectra. Two-way hierarchical clustering analysis of the selected mass ions revealed that nicotine and phenylpropanoid-polyamine conjugates, such as caffeoyl-dihydrocaffeoyl-spermidine, dicaffeoyl-spermidine, caffeoyl-feruloyl-spermidine, and two bis(dihydrocaffeoyl)-spermine isomers, and their intermediates, such as arginine and putrecine, were present at lower levels in Ein3-AS transgenic plants during Ppn interaction than in WT, whereas galactolipid and oxidized free fatty acid levels were higher in Ein3-AS transgenic plants. Taken together, these results reveal a function for ethylene signaling in tobacco defense responses during Ppn interaction.

An S-type anion channel SLAC1 is involved in cryptogein-induced ion fluxes and modulates hypersensitive responses in tobacco BY-2 cells

Pharmacological evidence suggests that anion channel-mediated plasma membrane anion effluxes are crucial in early defense signaling to induce immune responses and hypersensitive cell death in plants. However, their molecular bases and regulation remain largely unknown. We overexpressed Arabidopsis SLAC1, an S-type anion channel involved in stomatal closure, in cultured tobacco BY-2 cells and analyzed the effect on cryptogein-induced defense responses including fluxes of Cl(-) and other ions, production of reactive oxygen species (ROS), gene expression and hypersensitive responses. The SLAC1-GFP fusion protein was localized at the plasma membrane in BY-2 cells. Overexpression of SLAC1 enhanced cryptogein-induced Cl(-) efflux and extracellular alkalinization as well as rapid/transient and slow/prolonged phases of NADPH oxidase-mediated ROS production, which was suppressed by an anion channel inhibitor, DIDS. The overexpressor also showed enhanced sensitivity to cryptogein to induce downstream immune responses, including the induction of defense marker genes and the hypersensitive cell death. These results suggest that SLAC1 expressed in BY-2 cells mediates cryptogein-induced plasma membrane Cl(-) efflux to positively modulate the elicitor-triggered activation of other ion fluxes, ROS as well as a wide range of defense signaling pathways. These findings shed light on the possible involvement of the SLAC/SLAH family anion channels in cryptogein signaling to trigger the plasma membrane ion channel cascade in the plant defense signal transduction network.

Crosstalk between intracellular and extracellular salicylic acid signaling events leading to long-distance spread of signals

It is well recognized that salicylic acid (SA) acts as a natural signaling molecule involved in both local and systemic plant defense responses upon attacks by pathogens. Recently, cellular SA receptors and a number of SA-related phloem-mobile signals were identified. Here, we compare the old and up-to-date concepts of plant defense signaling events involving SA. Finally, the crosstalk between intracellular and extracellular SA signaling events leading to long-distance spread of signals was outlined by focusing on the modes of both the short- and long-distance signaling events involving the actions of SA. For the above purpose, two distinct conceptual models for local SA perception and signaling mechanisms in the intracellular and extracellular paths (referred to as models i and ii, respectively) were proposed. In addition to two local SA perception models, we propose that the long-distance SA action could be attributed to three different modes, namely, (iii) local increase in SA followed by transport of SA and SA intermediates, (iv) systemic propagation of SA-derived signals with both chemical and electrical natures without direct movement of SA, and (v) integrated crosstalk allowing alternately repeated secondary signal propagation and biosynthesis of SA and/or conversion of inert SA intermediates to free SA finally contributing to the systemic spread of SA-derived signals. We review here that the long-distance SA signaling events (models iii-v), inevitably involve the mechanisms described in the local signaling models (models i and ii) as the key pieces of the crosstalk.

Apoplastic and chloroplastic redox signaling networks in plant stress responses

SIGNIFICANCE

Interplay among apoplastic and chloroplastic redox signaling networks is emerging as a key mechanism in plant stress responses.

RECENT ADVANCES

Recent research has revealed components involved in apoplastic and chloroplastic redox signaling. Also, the sequence of events from stress perception, activation of apoplastic reactive oxygen species (ROS) burst through NADPH oxidases, cytoplasmic and chloroplastic Ca(2+)-transients, and organellar redox signals to physiological responses is starting to emerge. Moreover, a functional overlap between light acclimation and plant immunity in photosynthetically active tissues has been demonstrated.

CRITICAL ISSUES

Any deviations from the basal cellular redox balance may induce acclimation responses that continuously readjust cellular functions. However, diversion of resources to stress responses may lead to attenuation of growth, and exaggeration of defensive reactions may thus be detrimental to the plant. The ultimate outcome of acclimation responses must therefore be tightly controlled by the redox signaling networks between organellar and apoplastic signaling systems.

FUTURE DIRECTIONS

Two major questions still remain to be solved: the sensory mechanism for ROS and the components involved in relaying the signals from the apoplast to the chloroplast. A comprehensive view of regulatory networks will facilitate the understanding on how environmental factors affect the production of phytonutrients and biomass in plants. Translation of such information from model plants to crop species will be at the cutting edge of research in the near future. These challenges give a frame for future studies on ROS and redox regulation of stress acclimation in photosynthetic organisms.

Apoplastic and chloroplastic redox signaling networks in plant stress responses

SIGNIFICANCE

Interplay among apoplastic and chloroplastic redox signaling networks is emerging as a key mechanism in plant stress responses.

RECENT ADVANCES

Recent research has revealed components involved in apoplastic and chloroplastic redox signaling. Also, the sequence of events from stress perception, activation of apoplastic reactive oxygen species (ROS) burst through NADPH oxidases, cytoplasmic and chloroplastic Ca(2+)-transients, and organellar redox signals to physiological responses is starting to emerge. Moreover, a functional overlap between light acclimation and plant immunity in photosynthetically active tissues has been demonstrated.

CRITICAL ISSUES

Any deviations from the basal cellular redox balance may induce acclimation responses that continuously readjust cellular functions. However, diversion of resources to stress responses may lead to attenuation of growth, and exaggeration of defensive reactions may thus be detrimental to the plant. The ultimate outcome of acclimation responses must therefore be tightly controlled by the redox signaling networks between organellar and apoplastic signaling systems.

FUTURE DIRECTIONS

Two major questions still remain to be solved: the sensory mechanism for ROS and the components involved in relaying the signals from the apoplast to the chloroplast. A comprehensive view of regulatory networks will facilitate the understanding on how environmental factors affect the production of phytonutrients and biomass in plants. Translation of such information from model plants to crop species will be at the cutting edge of research in the near future. These challenges give a frame for future studies on ROS and redox regulation of stress acclimation in photosynthetic organisms.

Insights into organ-specific pathogen defense responses in plants: RNA-seq analysis of potato tuber-Phytophthora infestans interactions

Background

The late blight pathogen Phytophthora infestans can attack both potato foliage and tubers. Although interaction transcriptome dynamics between potato foliage and various pathogens have been reported, no transcriptome study has focused specifically upon how potato tubers respond to pathogen infection. When inoculated with P. infestans, tubers of nontransformed ‘Russet Burbank’ (WT) potato develop late blight disease while those of transgenic ‘Russet Burbank’ line SP2211 (+RB), which expresses the potato late blight resistance gene RB (Rpi-blb1), do not. We compared transcriptome responses to P. infestans inoculation in tubers of these two lines.

Results

We demonstrated the practicality of RNA-seq to study tetraploid potato and present the first RNA-seq study of potato tuber diseases. A total of 483 million paired end Illumina RNA-seq reads were generated, representing the transcription of around 30,000 potato genes. Differentially expressed genes, gene groups and ontology bins that exhibited differences between the WT and +RB lines were identified. P. infestans transcripts, including those of known effectors, were also identified.

Conclusion

Faster and stronger activation of defense related genes, gene groups and ontology bins correlate with successful tuber resistance against P. infestans. Our results suggest that the hypersensitive response is likely a general form of resistance against the hemibiotrophic P. infestans—even in potato tubers, organs that develop below ground.

Characterization of three novel desaturases involved in the delta-6 desaturation pathways for polyunsaturated fatty acid biosynthesis from Phytophthora infestans

Phytophthora infestans is the causative agent of potato blight that resulted in the great famine in Ireland in the nineteenth century. This microbe can release large amounts of the C20 very long-chain polyunsaturated fatty acids arachidonic acid (ARA; 20:4Δ(5, 8, 11, 14)) and eicosapentaenoic acid (EPA; 20:5Δ(5, 8, 11, 14, 17)) upon invasion that is known to elicit a hypersensitive response to their host plant. In order to identify enzymes responsible for the biosynthesis of these fatty acids, we blasted the recently fully sequenced P. infestans genome and identified three novel putatively encoding desaturase sequences. These were subsequently functionally characterized by expression in Saccharomyces cerevisiae and confirmed that they encode desaturases with Δ12, Δ6 and Δ5 activity, designated here as PinDes12, PinDes6 and PinDes5, respectively. This, together with the combined fatty acid profiles and a previously identified Δ6 elongase activity, implies that the ARA and EPA are biosynthesized predominantly via the Δ6 desaturation pathways in P. infestans. Elucidation of ARA and EPA biosynthetic mechanism may provide new routes to combating this potato blight microbe directly or by means of conferring resistance to important crops.

Nontargeted metabolite profiling in compatible pathogen-inoculated tobacco (Nicotiana tabacum L. cv. Wisconsin 38) using UPLC-Q-TOF/MS

A biphasic reactive oxygen species (ROS) production has previously been observed in tobacco at 1 and 48 h after inoculation with the hemibiotrophic compatible pathogen, Phytophthora parasitica var. nicotianae (Ppn). To characterize the response of tobacco to biphasically produced ROS concerning the propagation of Ppn, ultraperformance liquid chromatography-quadrupole-time of flight/ mass spectrometry (UPLC-Q-TOF/MS) based metabolic profiling combined with multivariate statistical analysis was performed. Among the nonredundant 355 mass ions in ESI+ mode and 345 mass ions in ESI- mode that were selected as significantly changed by Ppn inoculation (|p(corr)| > 0.6 on S-plot of orthogonal partial least-squares discriminant analysis (OPLS-DA), fold-change > 2, and p < 0.05 in the independent two-sample t test), 76 mass ions were identified on the basis of their accurate mass ions and MS/MS spectra. Phenolic amino acids, phenylpropanoids, hydroxycinnamic acid amides, linoleic acid, linolenic acid, lysophospholipids, glycoglycerolipids, and trioxidized phospholipids were identified as having changed after Ppn inoculation. On the basis of their quantitative changes, the metabolic responses occurring at each phase of ROS production after Ppn inoculation were investigated in this study.

StCDPK5 confers resistance to late blight pathogen but increases susceptibility to early blight pathogen in potato via reactive oxygen species burst

• Potato (Solanum tuberosum) calcium-dependent protein kinase (StCDPK5) has been shown to phosphorylate the N-terminal region of plasma membrane RBOH (respiratory burst oxidase homolog) proteins, and participate in StRBOHB-mediated reactive oxygen species (ROS) burst. The constitutively active form, StCDPK5VK, provides a useful tool for gain-of-function analysis of RBOH in defense responses. • StCDPK5- and StCDPK5VK-green fluorescent protein fusion proteins were predominantly targeted to the plasma membrane, and conditional expression of StCDPK5VK activated StRBOHA-D. The interaction was confirmed by bimolecular fluorescence complementation assay. We generated transgenic potato plants containing StCDPK5VK under the control of a pathogen-inducible promoter to investigate the role of ROS burst on defense responses to blight pathogens. • Virulent isolates of the late blight pathogen Phytophthora infestans and the early blight pathogen Alternaria solani induced hypersensitive response-like cell death accompanied by ROS production at the infection sites of transgenic plants. Transgenic plants showed resistance to the near-obligate hemibiotrophic pathogen P. infestans and, by contrast, increased susceptibility to the necrotrophic pathogen A. solani. • These results indicate that RBOH-dependent ROS contribute to basal defense against near-obligate pathogens, but have a negative role in resistance or have a positive role in expansion of disease lesions caused by necrotrophic pathogens.