Identification and characterisation of barley (Hordeum vulgare) respiratory burst oxidase homologue family members

Genome-wide analysis of respiratory burst oxidase homolog (Rboh) genes in Aquilaria species and insight into ROS-mediated metabolites biosynthesis and resin deposition

Respiratory burst oxidase homolog (Rboh) generates reactive oxygen species (ROS) as a defense response during biotic and abiotic stress. In Aquilaria plants, wounding and fungal infection result in biosynthesis and deposition of secondary metabolites as defense responses, which later form constituents of fragrant resinous agarwood. During injury and fungal invasion, Aquilaria tree generates ROS species via the Rboh enzymes. Despite the implication of Rboh genes in agarwood formation, no comprehensive genomic-level study of the Rboh gene family in Aquilaria is present. A systematic illustration of their role during stress and involvement in initiating signal cascades for agarwood metabolite biosynthesis is missing. In this study, 14 Rboh genes were retrieved from genomes of two Aquilaria species, A. agallocha and A. sinensis, and were classified into five groups. The promoter regions of the genes had abundant of stress-responsive elements. Protein-protein network and in silico expression analysis suggested their functional association with MAPK proteins and transcription factors such as WRKY and MYC2. The study further explored the expression profiles of Rboh genes and found them to be differentially regulated in stress-induced callus and stem tissue, suggesting their involvement in ROS generation during stress in Aquilaria. Overall, the study provides in-depth insight into two Rboh genes, AaRbohC and AaRbohA, highlighting their role in defense against fungal and abiotic stress, and likely during initiation of agarwood formation through modulation of genes involved in secondary metabolites biosynthesis. The findings presented here offer valuable information about Rboh family members, which can be leveraged for further investigations into ROS-mediated regulation of agarwood formation in Aquilaria species.

PbRbohH/J mediates ROS generation to regulate the growth of pollen tube in pear

Respiratory burst oxidase homolog (Rboh) family genes play crucial functions in development and growth. However, comprehensive and systematic investigation of Rboh family members in Rosaceae and their specific functions during pear pollen development are still limited. In the study, 63 Rboh genes were identified from eight Rosaceae genomes (Malus domestica, Pyrus bretschneideri, Pyrus communis, Prunus persica, Rubus occidentalis, Fragaria vesca, Prunus mume and Prunus avium) and divided into seven main subfamilies (I-VII) according to phylogenetic and structural features. Different modes of gene duplication led to the expansion of Rboh family, with purifying selection playing a vital role in the evolution of Rboh genes. In addition, RNA sequencing and qRT-PCR results indicated that PbRbohH and PbRbohJ were specifically high-expressed in pear pollen. Subsequently, subcellular localization revealed that PbRbohH/J distributed at the plasma membrane. Furthermore, by pharmacological analysis and antisense oligodeoxynucleotide assay, PbRbohH/J were demonstrated to mediate the formation of reactive oxygen species (ROS) to manage pollen tube growth. In conclusion, our results provide useful insights into the functions, expression patterns, evolutionary history of the Rboh genes in pear and other Rosaceae species.

Comparative physiological and transcriptomic analyses reveal ascorbate and glutathione coregulation of cadmium toxicity resistance in wheat genotypes

BACKGROUND

Cadmium (Cd) is a heavy metal with high toxicity that severely inhibits wheat growth and development. Cd easily accumulates in wheat kernels and enters the human food chain. Genetic variation in the resistance to Cd toxicity found in wheat genotypes emphasizes the complex response architecture. Understanding the Cd resistance mechanisms is crucial for combating Cd phytotoxicity and meeting the increasing daily food demand.

RESULTS

Using two wheat genotypes (Cd resistant and sensitive genotypes T207 and S276, respectively) with differing root growth responses to Cd, we conducted comparative physiological and transcriptomic analyses and exogenous application tests to evaluate Cd detoxification mechanisms. S276 accumulated more H₂O₂, O₂^-, and MDA than T207 under Cd toxicity. Catalase activity and levels of ascorbic acid (AsA) and glutathione (GSH) were greater, whereas superoxide dismutase (SOD) and peroxidase (POD) activities were lower in T207 than in S276. Transcriptomic analysis showed that the expression of RBOHA, RBOHC, and RBOHE was significantly increased under Cd toxicity, and two-thirds (22 genes) of the differentially expressed RBOH genes had higher expression levels in S276 than inT207. Cd toxicity reshaped the transcriptional profiling of the genes involving the AsA-GSH cycle, and a larger proportion (74.25%) of the corresponding differentially expressed genes showed higher expression in T207 than S276. The combined exogenous application of AsA and GSH alleviated Cd toxicity by scavenging excess ROS and coordinately promoting root length and branching, especially in S276.

CONCLUSIONS

The results indicated that the ROS homeostasis plays a key role in differential Cd resistance in wheat genotypes, and the AsA-GSH cycle fundamentally and vigorously influences wheat defense against Cd toxicity, providing insight into the physiological and transcriptional mechanisms underlying Cd detoxification.

The Barley (Hordeum vulgare ssp. vulgare) Respiratory Burst Oxidase Homolog (HvRBOH) Gene Family and Their Plausible Role on Malting Quality

Controlled generation of reactive oxygen species (ROS) is pivotal for normal plant development and adaptation to changes in the external milieu. One of the major enzymatic sources of ROS in plants are the plasma-membrane localized NADPH oxidases, also called as Respiratory Burst Oxidase Homologs (RBOH). In addition to the six previously reported, seven new members of RBOH gene family were identified in barley using in silico analysis. Conservation of genomic structure and key residues important for catalytic activity and co-factor binding was observed in barley RBOH genes. Phylogenetic analysis of plant RBOHs revealed distinct clades for monocot and dicot RBOH proteins. Hence, we propose to use the rice nomenclature for naming barley RBOH genes. Temporal changes in ROS profiles were observed during barley malting and was accompanied by changes in protein carbonylation, lipid peroxidation, and antioxidant capacity. Among the nine differentially expressed HvRBOHs during various malting stages, HvRBOHA and HvRBOHC showed most significant sustained changes in expression. RNAi knockdown lines with reduced expression of HvRBOHA/C gene exhibited genetic compensation via inducible expression of other gene family members during malting. However, the physiological consequence of reduced expression of HvRBOHA/C manifested as a poor malting quality profile attributable to low alpha-amylase activity and high levels of beta-glucan. We propose that the HvRBOHs play a critical role in modulating the redox milieu during the early stages of malting, which in turn can significantly impact carbohydrate metabolism.

Comprehensive analysis of the Gossypium hirsutum L. respiratory burst oxidase homolog (Ghrboh) gene family

BACKGROUND

Plant NADPH oxidase (NOX), also known as respiratory burst oxidase homolog (rboh), encoded by the rboh gene, is a key enzyme in the reactive oxygen species (ROS) metabolic network. It catalyzes the formation of the superoxide anion (O2•-), a type of ROS. In recent years, various studies had shown that members of the plant rboh gene family were involved in plant growth and developmental processes as well as in biotic and abiotic stress responses, but little is known about its functional role in upland cotton.

RESULTS

In the present study, 26 putative Ghrboh genes were identified and characterized. They were phylogenetically classified into six subfamilies and distributed at different densities across 18 of the 26 chromosomes or scaffolds. Their exon-intron structures, conserved domains, synteny and collinearity, gene family evolution, regulation mediated by cis-acting elements and microRNAs (miRNAs) were predicted and analyzed. Additionally, expression profiles of Ghrboh gene family were analyzed in different tissues/organs and at different developmental stages and under different abiotic stresses, using RNA-Seq data and real-time PCR. These profiling studies indicated that the Ghrboh genes exhibited temporal and spatial specificity with respect to expression, and might play important roles in cotton development and in stress tolerance through modulating NOX-dependent ROS induction and other signaling pathways.

CONCLUSIONS

This comprehensive analysis of the characteristics of the Ghrboh gene family determined features such as sequence, synteny and collinearity, phylogenetic and evolutionary relationship, expression patterns, and cis-element- and miRNA-mediated regulation of gene expression. Our results will provide valuable information to help with further gene cloning, evolutionary analysis, and biological function analysis of cotton rbohs.

In Silico Genome-Wide Analysis of Respiratory Burst Oxidase Homolog (RBOH) Family Genes in Five Fruit-Producing Trees, and Potential Functional Analysis on Lignification of Stone Cells in Chinese White Pear

: The accumulation of lignin in fruit has a significant negative impact on the quality of fruit-producing trees, and in particular the lignin formation stimulates the development of stone cells in pear fruit. Reactive oxygen species (ROS) are essential for lignin polymerization. However, knowledge of the RBOH family, a key enzyme in ROS metabolism, remains unknown in most fruit trees. In this study, a total of 40 RBOHs were identified from five fruit-producing trees (Pyrusbretschneideri, Prunuspersica, Citrussinensis, Vitisvinifera, and Prunusmume), and 10 of these sequences came from Pyrusbretschneideri. Multiple sequence alignments revealed that all 10 PbRBOHs contained the NADPH_Ox domain and the six alpha-helical transmembrane domains (TM-I to TM-VI). Chromosome localization and interspecies phylogenetic tree analysis showed that 10 PbRBOHs irregularly distributed on 8 chromosomes and 3 PbRBOHs (PbRBOHA, PbRBOHB, and PbRBOHD) are closely related to known lignification-related RBOHs. Furthermore, hormone response pattern analysis showed that the transcription of PbRBOHs is regulated by SA, ABA and MeJA. Reverse transcription-quantitative real-time polymerase chain reaction (qRT-PCR) and transcriptome sequencing analysis showed that PbRBOHA, PbRBOHB, and PbRBOHD accumulated high transcript abundance in pear fruit, and the transcriptional trends of PbRBOHA and PbRBOHD was consistent with the change of stone cell content during fruit development. In addition, subcellular localization revealed that PbRBOHA and PbRBOHD are distributed on the plasma membrane. Combining the changes of apoplastic superoxide (O2.-) content and spatio-temporal expression analysis, these results indicate that PbRBOHA and PbRBOHD, which are candidate genes, may play an important role in ROS metabolism during the lignification of pear stone cells. This study not only provided insight into the molecular characteristics of the RBOH family in fruit-producing trees, but also lays the foundation for studying the role of ROS in plant lignification.

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.

Superoxide (O2.-) accumulation contributes to symptomless (type I) nonhost resistance of plants to biotrophic pathogens

Nonhost resistance is the most common form of disease resistance exhibited by plants against most pathogenic microorganisms. Type I nonhost resistance is symptomless (i.e. no macroscopically visible cell/tissue death), implying an early halt of pathogen growth. The timing/speed of defences is much more rapid during type I nonhost resistance than during type II nonhost and host ("gene-for-gene") resistance associated with a hypersensitive response (localized necrosis, HR). However, the mechanism(s) underlying symptomless (type I) nonhost resistance is not entirely understood. Here we assessed accumulation dynamics of the reactive oxygen species superoxide (O₂^.-) during interactions of plants with a range of biotrophic and hemibiotrophic pathogens resulting in susceptibility, symptomless nonhost resistance or host resistance with HR. Our results show that the timing of macroscopically detectable superoxide accumulation (1-4 days after inoculation, DAI) is always associated with the speed of the defense response (symptomless nonhost resistance vs. host resistance with HR) in inoculated leaves. The relatively early (1 DAI) superoxide accumulation during symptomless nonhost resistance of barley to wheat powdery mildew (Blumeria graminis f. sp. tritici) is localized to mesophyll chloroplasts of inoculated leaves and coupled to enhanced NADPH oxidase (EC 1.6.3.1) activity and transient increases in expression of genes regulating superoxide levels and cell death (superoxide dismutase, HvSOD1 and BAX inhibitor-1, HvBI-1). Importantly, the partial suppression of symptomless nonhost resistance of barley to wheat powdery mildew by heat shock (49 °C, 45 s) and antioxidant (SOD and catalase) treatments points to a functional role of superoxide in symptomless (type I) nonhost resistance.

Identification of NADPH oxidase family members associated with cold stress in strawberry

NADPH oxidase is encoded by a small gene family (Respiratory burst oxidase homologs, Rbohs) and plays an important role in regulating various biological processes. However, little information about this gene family is currently available for strawberry. In this study, a total of seven Rboh genes were identified from strawberry through genomewide analysis. Gene structure analysis showed the number of exons ranged from 10 to 23, implying that this variation occurred in FvRboh genes by the insertion and distribution of introns; the order and approximate size of exons were relatively conserved. FvRbohC was predicted to localize to the thylakoid membrane of the chloroplast, while other members were computed to localize to the plasma membrane, indicating different functions. Amino acid sequence alignment, conserved domain, and motif analysis showed that all identified FvRbohs had typical features of plant Rbohs. Phylogenetic analysis of Rbohs from strawberry, grape, Arabidopsis, and rice suggested that the FvRbohs could be divided into five subgroups and showed a closer relationship with those from grape and Arabidopsis than those from rice. The expression patterns of FvRboh genes in root, stem, leaf, flower, and fruit revealed robust tissue specificity. The expression levels of FvRbohA and FvRbohD were quickly induced by cold stress, followed by an increase in NADPH oxidase activity, leading to O2− accumulation and triggering the antioxidant reaction by the transient increases in SOD activity. This suggested these two genes may be involved in cold stress and defense responses in strawberry.

Structural complexity and functional diversity of plant NADPH oxidases

Plant NADPH oxidases also known as respiratory burst oxidase homologs (Rbohs) are a family of membrane-bound enzymes that play diverse roles in the defense response and morphogenetic processes via regulated generation of reactive oxygen species. Rbohs are associated with a variety of functions, although the reason for this is not clear. To evaluate using bioinformatics, the possible mechanisms for the observed functional diversity within the plant kingdom, 127 Rboh protein sequences representing 26 plant species were analyzed. Multiple clusters were identified with gene duplications that were both dicot as well as monocot-specific. The N-terminal sequences were observed to be highly variable. The conserved cysteine (equivalent of Cys890) in C-terminal of AtRbohD suggested that the redox-based modification like S-nitrosylation may regulate the activity of other Rbohs. Three-dimensional models corresponding to the N-terminal domain for Rbohs from Arabidopsis thaliana and Oryza sativa were constructed and molecular dynamics studies were carried out to study the role of Ca2+ in the folding of Rboh proteins. Certain mutations indicated possibly affect the structure and function of the plant NADPH oxidases, thereby providing the rationale for further experimental validation.

Silencing of RBOHF2 Causes Leaf Age-Dependent Accelerated Senescence, Salicylic Acid Accumulation, and Powdery Mildew Resistance in Barley

Plant RBOH (RESPIRATORY BURST OXIDASE HOMOLOGS)-type NADPH oxidases produce superoxide radical anions and have a function in developmental processes and in response to environmental challenges. Barley RBOHF2 has diverse reported functions in interaction with the biotrophic powdery mildew fungus Blumeria graminis f. sp. hordei. Here, we analyzed, in detail, plant leaf level- and age-specific susceptibility of stably RBOHF2-silenced barley plants. This revealed enhanced susceptibility to fungal penetration of young RBOHF2-silenced leaf tissue but strongly reduced susceptibility of older leaves when compared with controls. Loss of susceptibility in old RBOHF2-silenced leaves was associated with spontaneous leaf-tip necrosis and constitutively elevated levels of free and conjugated salicylic acid. Additionally, these leaves more strongly expressed pathogenesis-related genes, both constitutively and during interaction with B. graminis f. sp. hordei. Together, this supports the idea that barley RBOHF2 contributes to basal resistance to powdery mildew infection in young leaf tissue but is required to control leaf cell death, salicylic acid accumulation, and defense gene expression in older leaves, explaining leaf age-specific resistance of RBOHF2-silenced barley plants.

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.

The role of a cytosolic superoxide dismutase in barley-pathogen interactions

Reactive oxygen species (ROS), including superoxide ( O2·-/ HO2·) and hydrogen peroxide (H2 O2 ), are differentially produced during resistance responses to biotrophic pathogens and during susceptible responses to necrotrophic and hemi-biotrophic pathogens. Superoxide dismutase (SOD) is responsible for the catalysis of the dismutation of O2·-/ HO2· to H2 O2 , regulating the redox status of plant cells. Increased SOD activity has been correlated previously with resistance in barley to the hemi-biotrophic pathogen Pyrenophora teres f. teres (Ptt, the causal agent of the net form of net blotch disease), but the role of individual isoforms of SOD has not been studied. A cytosolic CuZnSOD, HvCSD1, was isolated from barley and characterized as being expressed in tissue from different developmental stages. HvCSD1 was up-regulated during the interaction with Ptt and to a greater extent during the resistance response. Net blotch disease symptoms and fungal growth were not as pronounced in transgenic HvCSD1 knockdown lines in a susceptible background (cv. Golden Promise), when compared with wild-type plants, suggesting that cytosolic O2·-/ HO2· contributes to the signalling required to induce a defence response to Ptt. There was no effect of HvCSD1 knockdown on infection by the hemi-biotrophic rice blast pathogen Magnaporthe oryzae or the biotrophic powdery mildew pathogen Blumeria graminis f. sp. hordei, but HvCSD1 also played a role in the regulation of lesion development by methyl viologen. Together, these results suggest that HvCSD1 could be important in the maintenance of the cytosolic redox status and in the differential regulation of responses to pathogens with different lifestyles.

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.

An Arabidopsis soil-salinity-tolerance mutation confers ethylene-mediated enhancement of sodium/potassium homeostasis

High soil Na concentrations damage plants by increasing cellular Na accumulation and K loss. Excess soil Na stimulates ethylene-induced soil-salinity tolerance, the mechanism of which we here define via characterization of an Arabidopsis thaliana mutant displaying transpiration-dependent soil-salinity tolerance. This phenotype is conferred by a loss-of-function allele of ethylene overproducer1 (ETO1; mutant alleles of which cause increased production of ethylene). We show that lack of ETO1 function confers soil-salinity tolerance through improved shoot Na/K homeostasis, effected via the ethylene resistant1-constitutive triple response1 ethylene signaling pathway. Under transpiring conditions, lack of ETO1 function reduces root Na influx and both stelar and xylem sap Na concentrations, thereby restricting root-to-shoot delivery of Na. These effects are associated with increased accumulation of respiratory burst oxidase homolog F (RBOHF)-dependent reactive oxygen species in the root stele. Additionally, lack of ETO1 function leads to significant enhancement of tissue K status by an RBOHF-independent mechanism associated with elevated high-affinity K(+) TRANSPORTER5 transcript levels. We conclude that ethylene promotes soil-salinity tolerance via improved Na/K homeostasis mediated by RBOHF-dependent regulation of Na accumulation and RBOHF-independent regulation of K accumulation.

RBOHF2 of barley is required for normal development of penetration resistance to the parasitic fungus Blumeria graminis f. sp. hordei

Plant respiratory burst oxidase homologs are prominent sources of reactive oxygen species (ROS) in signal transduction and in interaction with microbes. However, the function of respiratory burst oxidase homologue (RBOH) genes in interaction with microbes might differ for certain plant and pathogen species. We produced transgenic barley knock down (KD) for the HvRBOHF2 isoform of NADPH oxidases. Young HvRBOHF2 KD shoots did not show obvious morphological alterations from the wild type but adult HvRBOHF2 KD plants developed fewer tillers, were less fertile, and showed spontaneous cell death in leaf mesophyll. Additionally, HvRBOHF2 KD plants were unable to contain wound-induced cell death. Before developmental failure became obvious, young HvRBOHF2 KD seedlings were much more susceptible to penetration by the biotrophic powdery mildew fungus Blumeria graminis f. sp. hordei. Strikingly, the B. graminis f. sp. hordei-induced cell-wall-associated oxidative burst was not substantially attenuated in HvRBOHF2 KD plants but enhanced susceptibility apparently influenced the subcellular site of hydrogen peroxide accumulation. Taken together, misexpression of HvRBOHF2 caused failure of barley to normally develop penetration resistance to B. graminis f. sp. hordei and to control leaf cell death.

Role of reactive oxygen species-generating enzymes and hydrogen peroxide during cadmium, mercury and osmotic stresses in barley root tip

The effect of cadmium (Cd) on the expression and activity of NADPH oxidase, peroxidase and oxalate oxidase as well as on the expression of aquaporins and dehydrins was studied in barley root tip. The root tip represented intact apical part of the barley root containing the root cap, meristems and elongation zone. Except stress induced by Cd, barley root tips were analysed after their exposure to phytotoxic concentration of mercury (Hg)-, hydrogen peroxide (H2O2)- or polyethylene glycol (PEG)-induced water stress in order to compare the Cd-induced changes with changes induced by these other stress factors. Cd, Hg, H2O2 and with some exceptions also PEG treatments caused similar alterations in the gene expression of reactive oxygen species (ROS)-generating and water deficiency-related genes, and in the activity of ROS-generating enzymes. These evidences support our opinion that ROS accumulation and water imbalance are the common symptoms of these stress factors and that the elevated production of H2O2 plays, probably as a signal molecule, a key role in the induction of plant responses to abiotic stresses in barley root tip. On the other hand, H2O2 at permanent high concentration is probably the main toxic factor during stress conditions.