NADPH Oxidase-Dependent Superoxide Production in Plant Reproductive Tissues

Flavonols improve tomato pollen thermotolerance during germination and tube elongation by maintaining reactive oxygen species homeostasis

Elevated temperatures impair pollen performance and reproductive success, resulting in lower crop yields. The tomato (Solanum lycopersicum) anthocyanin reduced (are) mutant harbors a mutation in FLAVANONE 3-HYDROXYLASE (F3H), resulting in impaired flavonol antioxidant biosynthesis. The are mutant has reduced pollen performance and seed set relative to the VF36 parental line, phenotypes that are accentuated at elevated temperatures. Transformation of are with the wild-type F3H gene, or chemical complementation with flavonols, prevented temperature-dependent reactive oxygen species (ROS) accumulation in pollen and restored the reduced viability, germination, and tube elongation of are to VF36 levels. Overexpression of F3H in VF36 prevented temperature-driven ROS increases and impaired pollen performance, revealing that flavonol biosynthesis promotes thermotolerance. Although stigmas of are had reduced flavonol and elevated ROS levels, the growth of are pollen tubes was similarly impaired in both are and VF36 pistils. RNA-seq was performed at optimal and stress temperatures in are, VF36, and the F3H overexpression line at multiple timepoints across pollen tube elongation. The number of differentially expressed genes increased over time under elevated temperatures in all genotypes, with the greatest number in are. These findings suggest potential agricultural interventions to combat the negative effects of heat-induced ROS in pollen that lead to reproductive failure.

BR regulates wheat root salt tolerance by maintaining ROS homeostasis

MAIN CONCLUSION

Trace amounts of epibrassinolide (EpiBL) could partially rescue wheat root length inhibition in salt-stressed situation by scavenging ROS, and ectopic expression of TaDWF4 or TaBAK1 enhances root salt tolerance in Arabidopsis by balancing ROS level. Salt stress often leads to ion toxicity and oxidative stress, causing cell structure damage and root development inhibition in plants. While prior research indicated the involvement of exogenous brassinosteroid (BR) in plant responses to salt stress, the precise cytological role and the function of BR in wheat root development under salt stress remain elusive. Our study demonstrates that 100 mM NaCl solution inhibits wheat root development, but 5 nM EpiBL partially rescues root length inhibition by decreasing H2O2 content, oxygen free radical (OFR) content, along with increasing the peroxidase (POD) and catalase (CAT) activities in salt-stressed roots. The qRT-PCR experiment also shows that expression of the ROS-scavenging genes (GPX2 and CAT2) increased in roots after applying BR, especially during salt stress situation. Transcriptional analysis reveals decreased expression of BR synthesis and root meristem development genes under salt stress in wheat roots. Differential expression gene (DEG) enrichment analysis highlights the significant impact of salt stress on various biological processes, particularly "hydrogen peroxide catabolic process" and "response to oxidative stress". Additionally, the BR biosynthesis pathway is enriched under salt stress conditions. Therefore, we investigated the involvement of wheat BR synthesis gene TaDWF4 and BR signaling gene TaBAK1 in salt stress responses in roots. Our results demonstrate that ectopic expression of TaDWF4 or TaBAK1 enhances salt tolerance in Arabidopsis by balancing ROS (Reactive oxygen species) levels in roots.

Ammonium treatment inhibits cell cycle activity and induces nuclei endopolyploidization in Arabidopsis thaliana

MAIN CONCLUSION

This study determined the effect of ammonium supply on the cell division process and showed that ammonium-dependent elevated reactive oxygen species production could mediate the downregulation of the cell cycle-related gene expression. Plants grown under high-ammonium conditions show stunted growth and other toxicity symptoms, including oxidative stress. However, how ammonium regulates the development of plants remains unknown. Growth is defined as an increase in cell volume or proliferation. In the present study, ammonium-related changes in cell cycle activity were analyzed in seedlings, apical buds, and young leaves of Arabidopsis thaliana plants. In all experimental ammonium treatments, the genes responsible for regulating cell cycle progression, such as cyclin-dependent kinases and cyclins, were downregulated in the studied tissues. Thus, ammonium nutrition could be considered to reduce cell proliferation; however, the cause of this phenomenon may be secondary. Reactive oxygen species (ROS), which are produced in large amounts in response to ammonium nutrition, can act as intermediates in this process. Indeed, high ROS levels resulting from H2O2 treatment or reduced ROS production in rbohc mutants, similar to ammonium-triggered ROS, correlated with altered cell cycle-related gene expression. It can be concluded that the characteristic ammonium growth suppression may be executed by enhanced ROS metabolism to inhibit cell cycle activity. This study provides a base for future research in determining the mechanism behind ammonium-induced dwarfism in plants, and strategies to mitigate such stress.

Genomic survey of high-throughput RNA-Seq data implicates involvement of long intergenic non-coding RNAs (lincRNAs) in cytoplasmic male-sterility and fertility restoration in pigeon pea

BACKGROUND

Long-intergenic non-coding RNAs (lincRNAs) originate from intergenic regions and have no coding potential. LincRNAs have emerged as key players in the regulation of various biological processes in plant development. Cytoplasmic male-sterility (CMS) in association with restorer-of-fertility (Rf) systems makes it a highly reliable tool for exploring heterosis for producing commercial hybrid seeds. To date, there have been no reports of lincRNAs during pollen development in CMS and fertility restorer lines in pigeon pea.

OBJECTIVE

Identification of lincRNAs in the floral buds of cytoplasmic male-sterile (AKCMS11) and fertility restorer (AKPR303) pigeon pea lines.

METHODS

We employed a computational approach to identify lincRNAs in the floral buds of cytoplasmic male-sterile (AKCMS11) and fertility restorer (AKPR303) pigeon pea lines using RNA-Seq data.

RESULTS

We predicted a total of 2145 potential lincRNAs of which 966 were observed to be differentially expressed between the sterile and fertile pollen. We identified, 927 cis-regulated and 383 trans-regulated target genes of the lincRNAs. Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of the target genes revealed that these genes were specifically enriched in pathways like pollen and pollen tube development, oxidative phosphorylation, etc. We detected 23 lincRNAs that were co-expressed with 17 pollen-related genes with known functions. Fifty-nine lincRNAs were predicted to be endogenous target mimics (eTMs) for 25 miRNAs, and found to be associated with pollen development. The, lincRNA regulatory networks revealed that different lincRNA-miRNA-mRNA networks might be associated with CMS and fertility restoration.

CONCLUSION

Thus, this study provides valuable information by highlighting the functions of lincRNAs as regulators during pollen development in pigeon pea and utilization in hybrid seed production.

High temporal-resolution transcriptome landscapes of maize embryo sac and ovule during early seed development

Here we provided a high temporal-resolution transcriptome atlas of maize embryo sac and ovule to reveal the gene activity dynamic during early seed development. The early maize (Zea mays) seed development is initiated from double fertilization in the embryo sac and needs to undergo a highly dynamic and complex development process to form the differentiated embryo and endosperm. Despite the importance of maize seed for food, feed, and biofuel, many regulators responsible for controlling its early development are not known yet. Here, we reported a high temporal-resolution transcriptome atlas of embryo sac and ovule based on 44 time point samples collected within the first four days of seed development. A total of 25,187 genes including 1598 transcription factors (TFs) involved in early seed development were detected. Global comparisons of the expressions of these genes revealed five distinct development stages of early seed, which are mainly related to double fertilization, asymmetric cell division of the zygote, as well as coenocyte formation, cellularization and differentiation in endosperm. We identified 3327 seed-specific genes, which more than one thousand seed-specific genes with main expressions during early seed development were newly identified here, including 859 and 186 genes predominantly expressed in the embryo sac and ovule, respectively. Combined with the published transcriptome data of seed, we uncovered the dominant auxin biosynthesis, transport and signaling related genes at different development stages and subregions of seed. These results are helpful for understanding the genetic control of early seed development.

Dimethylthiourea Alleviates Drought Stress by Suppressing Hydrogen Peroxide-Dependent Abscisic Acid-Mediated Oxidative Responses in an Antagonistic Interaction with Salicylic Acid in Brassica napus Leaves

In plants, prolonged drought induces oxidative stress, leading to a loss of reducing potential in redox components. Abscisic acid (ABA) is a representative hormonal signal regulating stress responses. This study aimed to investigate the physiological significance of dimethylthiourea (DMTU, an H₂O₂ scavenger) in the hormonal regulation of the antioxidant system and redox control in rapeseed (Brassica napus L.) leaves under drought stress. Drought treatment for 10 days provoked oxidative stress, as evidenced by the increase in O₂^•- and H₂O₂ concentrations, and lipid peroxidation levels, and a decrease in leaf water potential. Drought-induced oxidative responses were significantly alleviated by DMTU treatment. The accumulation of O₂^•- and H₂O₂ in drought-treated plants coincided with the enhanced expression of the NADPH oxidase and Cu/Zn-SOD genes, leading to an up-regulation in oxidative signal-inducible 1 (OXI1) and mitogen-activated protein kinase 6 (MAPK6), with a concomitant increase in ABA levels and the up-regulation of ABA-related genes. DMTU treatment under drought largely suppressed the drought-responsive up-regulation of these genes by depressing ABA responses through an antagonistic interaction with salicylic acid (SA). DMTU treatment also alleviated the drought-induced loss of reducing potential in GSH- and NADPH-based redox by the enhanced expression of glutathione reductase 1 (GR1) and up-regulation of oxidoreductase genes (TRXh5 and GRXC9). These results indicate that DMTU effectively alleviates drought-induced oxidative responses by suppressing ABA-mediated oxidative burst signaling in an antagonistic regulation of SA.

Regulation of PaRBOH1-mediated ROS production in Norway spruce by Ca2+ binding and phosphorylation

Plant respiratory burst oxidase homologs (RBOHs) are plasma membrane-localized NADPH oxidases that generate superoxide anion radicals, which then dismutate to H₂O₂, into the apoplast using cytoplasmic NADPH as an electron donor. PaRBOH1 is the most highly expressed RBOH gene in developing xylem as well as in a lignin-forming cell culture of Norway spruce (Picea abies L. Karst.). Since no previous information about regulation of gymnosperm RBOHs exist, our aim was to resolve how PaRBOH1 is regulated with a focus on phosphorylation. The N-terminal part of PaRBOH1 was found to contain several putative phosphorylation sites and a four-times repeated motif with similarities to the Botrytis-induced kinase 1 target site in Arabidopsis AtRBOHD. Phosphorylation was indicated for six of the sites in in vitro kinase assays using 15 amino-acid-long peptides for each of the predicted phosphotarget site in the presence of protein extracts of developing xylem. Serine and threonine residues showing positive response in the peptide assays were individually mutated to alanine (kinase-inactive) or to aspartate (phosphomimic), and the wild type PaRBOH1 and the mutated constructs transfected to human kidney embryogenic (HEK293T) cells with a low endogenous level of extracellular ROS production. ROS-producing assays with HEK cells showed that Ca²⁺ and phosphorylation synergistically activate the enzyme and identified several serine and threonine residues that are likely to be phosphorylated including a novel phosphorylation site not characterized in other plant species. These were further investigated with a phosphoproteomic study. Results of Norway spruce, the first gymnosperm species studied in relation to RBOH regulation, show that regulation of RBOH activity is conserved among seed plants.

Multiple forms of vitamin B6 regulate salt tolerance by balancing ROS and abscisic acid levels in maize root

Salt stress causes osmotic stress, ion toxicity and oxidative stress, inducing the accumulation of abscisic acid (ABA) and excessive reactive oxygen species (ROS) production, which further damage cell structure and inhibit the development of roots in plants. Previous study showed that vitamin B6 (VB6) plays a role in plant responses to salt stress, however, the regulatory relationship between ROS, VB6 and ABA under salt stress remains unclear yet in plants. In our study, we found that salt stress-induced ABA accumulation requires ROS production, in addition, salt stress also promoted VB6 (including pyridoxamine (PM), pyridoxal (PL), pyridoxine (PN), and pyridoxal 5'-phosphate (PLP)) accumulation, which involved in ROS scavenging and ABA biosynthesis. Furthermore, VB6-deficient maize mutant small kernel2 (smk2) heterozygous is more susceptible to salt stress, and which failed to scavenge excessive ROS effectively or induce ABA accumulation in maize root under salt stress, interestingly, which can be restored by exogenous PN and PLP, respectively. According to these results, we proposed that PN and PLP play an essential role in balancing ROS and ABA levels under salt stress, respectively, it laid a foundation for VB6 to be better applied in crop salt resistance than ABA.

Receptor for Activated C Kinase1B (OsRACK1B) Impairs Fertility in Rice through NADPH-Dependent H2O2 Signaling Pathway

The scaffold protein receptor for Activated C Kinase1 (RACK1) regulates multiple aspects of plants, including seed germination, growth, environmental stress responses, and flowering. Recent studies have revealed that RACK1 is associated with NADPH-dependent reactive oxygen species (ROS) signaling in plants. ROS, as a double-edged sword, can modulate several developmental pathways in plants. Thus, the resulting physiological consequences of perturbing the RACK1 expression-induced ROS balance remain to be explored. Herein, we combined molecular, pharmacological, and ultrastructure analysis approaches to investigate the hypothesized connection using T-DNA-mediated activation-tagged RACK1B overexpressed (OX) transgenic rice plants. In this study, we find that OsRACK1B-OX plants display reduced pollen viability, defective anther dehiscence, and abnormal spikelet morphology, leading to partial spikelet sterility. Microscopic observation of the mature pollen grains from the OX plants revealed abnormalities in the exine and intine structures and decreased starch granules in the pollen, resulting in a reduced number of grains per locule from the OX rice plants as compared to that of the wild-type (WT). Histochemical staining revealed a global increase in hydrogen peroxide (H₂O₂) in the leaves and roots of the transgenic lines overexpressing OsRACK1B compared to that of the WT. However, the elevated H₂O₂ in tissues from the OX plants can be reversed by pre-treatment with diphenylidonium (DPI), an NADPH oxidase inhibitor, indicating that the source of H₂O₂ could be, in part, NADPH oxidase. Expression analysis showed a differential expression of the NADPH/respiratory burst oxidase homolog D (RbohD) and antioxidant enzyme-related genes, suggesting a homeostatic mechanism of H₂O₂ production and antioxidant enzyme activity. BiFC analysis demonstrated that OsRACK1B interacts with the N-terminal region of RbohD in vivo. Taken together, these data indicate that elevated OsRACK1B accumulates a threshold level of ROS, in this case H₂O₂, which negatively regulates pollen development and fertility. In conclusion, we hypothesized that an optimal expression of RACK1 is critical for fertility in rice plants.

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.

Development of fusarium wilt resistant mutants of Musa spp. cv.Rasthali (AAB, Silk subgroup) and comparative proteomic analysis along with its wild type

Induced mutagenesis using embryogenic cell suspension (ECS) explants with toxin based screening is an effective tool to create non-chimeral Fusarium wilt resistant mutants in banana. Global proteomics unravel the molecular mechanism behind resistance. Race 1 of Fusarium wilt is a serious threat to Musa spp. cv.Rasthali (AAB, Silk subgroup) which is a choice variety traditionally grown in most of the south East Asian countries. Resistant gene introgression into susceptible varieties through conventional breeding has several limitations and the predominant ones being sterility and long generation time. Under such circumstances, induced mutagenesis combined with toxin based in vitro screening remains as the viable alternative for the development of fusarium wilt resistant Rasthali. Therefore, induced mutagenesis was attempted by using ethylmethane sulfonate (EMS) in embryogenic cell suspension (ECS) of Rasthali followed by in vitro screening for fusarium wilt resistance using new generation toxins and pot screening through challenge inoculation with Foc race 1. This ultimately resulted in the identification of 15 resistant lines. Global proteomic analysis in one of the resistant mutant lines namely NRCBRM15 and its wild type revealed 37 proteins, of which 20 showed differential expression. Out of 20 proteins, nineteen were significantly abundant in NRCBRM15 and only one was abundant in wild Rasthali. A total of nine genes based on protein expression were further validated using quantitative real time polymerase chain reaction (qRT-PCR). Annotation results revealed that some of the genes namely Enolase, ATP synthase-alpha subunit, Actin 2, Actin 3,-glucanase, UTP-glucose-1-phosphate uridylyltransferase, Respiratory burst oxidase homolog, V type proton ATPase catalytic subunit A and DUF292 domain containing protein are involved in diverse functions such as carbohydrate metabolism, energy production, electron carrier, response to wounding, binding proteins, cytoskeleton organization, extracellular region, structural molecule and defense.

Pest and disease management by red light

Light is essential for plant life. It provides a source of energy through photosynthesis and regulates plant growth and development and other cellular processes, such as by controlling the endogenous circadian clock. Light intensity, quality, duration and timing are all important determinants of plant responses, especially to biotic stress. Red light can positively influence plant defence mechanisms against different pathogens, but the molecular mechanism behind this phenomenon is not fully understood. Therefore, we reviewed the impact of red light on plant biotic stress responses against viruses, bacteria, fungi and nematodes, with a focus on the physiological effects of red light treatment and hormonal crosstalk under biotic stress in plants. We found evidence suggesting that exposing plants to red light increases levels of salicylic acid (SA) and induces SA signalling mediating the production of reactive oxygen species, with substantial differences between species and plant organs. Such changes in SA levels could be vital for plants to survive infections. Therefore, the application of red light provides a multidimensional aspect to developing innovative and environmentally friendly approaches to plant and crop disease management.

Reactive oxygen species coordinate the transcriptional responses to iron availability in Arabidopsis

Reactive oxygen species play a central role in the regulation of plant responses to environmental stress. Under prolonged iron (Fe) deficiency, increased levels of hydrogen peroxide (H2O2) initiate signaling events, resulting in the attenuation of Fe acquisition through the inhibition of FER-LIKE IRON DEFICIENCY-INDUCED TRANSCRIPTION FACTOR (FIT). As this H2O2 increase occurs in a FIT-dependent manner, our aim was to understand the processes involved in maintaining H2O2 levels under prolonged Fe deficiency and the role of FIT. We identified the CAT2 gene, encoding one of the three Arabidopsis catalase isoforms, as regulated by FIT. CAT2 loss-of-function plants displayed severe susceptibility to Fe deficiency and greatly increased H2O2 levels in roots. Analysis of the Fe homeostasis transcription cascade revealed that H2O2 influences the gene expression of downstream regulators FIT, BHLH genes of group Ib, and POPEYE (PYE); however, H2O2 did not affect their upstream regulators, such as BHLH104 and ILR3. Our data shows that FIT and CAT2 participate in a regulatory loop between H2O2 and prolonged Fe deficiency.

Mating type specific transcriptomic response to sex inducing pheromone in the pennate diatom Seminavis robusta

Sexual reproduction is a fundamental phase in the life cycle of most diatoms. Despite its role as a source of genetic variation, it is rarely reported in natural circumstances and its molecular foundations remain largely unknown. Here, we integrate independent transcriptomic datasets to prioritize genes responding to sex inducing pheromones (SIPs) in the pennate diatom Seminavis robusta. We observe marked gene expression changes associated with SIP treatment in both mating types, including an inhibition of S phase progression, chloroplast division, mitosis, and cell wall formation. Meanwhile, meiotic genes are upregulated in response to SIP, including a sexually induced diatom specific cyclin. Our data further suggest an important role for reactive oxygen species, energy metabolism, and cGMP signaling during the early stages of sexual reproduction. In addition, we identify several genes with a mating type specific response to SIP, and link their expression pattern with physiological specialization, such as the production of the attraction pheromone diproline in mating type - (MT-) and mate-searching behavior in mating type + (MT+). Combined, our results provide a model for early sexual reproduction in pennate diatoms and significantly expand the suite of target genes to detect sexual reproduction events in natural diatom populations.

PbrPOE21 inhibits pear pollen tube growth in vitro by altering apical reactive oxygen species content

Genome-wide identification, tissue-specific expression analysis and functional characterization of selected genes containing the pear Pollen Olea europaea I domain reveal their roles in pollen tube growth. Genes containing the Pollen Olea europaea I (POE) domain play crucial roles in diverse growth and developmental processes. Nevertheless, the specific functions of POE family members in progression of pollen tube growth (PTG) remain uncharacterized. We identified 45 PbrPOE genes in the pear (Pyrus bretschneideri) genome, clustered into seven subclasses. PbrPOE genes contained 1 to 11 exons and 0 to 10 introns, with exon/intron structure mostly conserved within each subclass. Whole-genome duplication has mainly contributed to the duplication pattern of PbrPOE genes in pear. Expression profiles of 45 PbrPOE genes in 12 different pear tissues revealed that six PbrPOE genes (PbrPOE6, 12, 21, 29, 35 and 41) of subclass B were highly expressed during the growth of the pear pollen tube in vitro. PbrPOE21 was selected for further functional analysis on the basis of its high and differential expression pattern in pollen. Antisense oligodeoxynucleotide assays demonstrated that PTG was augmented in vitro when PbrPOE21 expression was significantly inhibited. Moreover, pollen tube length in vitro was reduced when PbrPOE21 was transitorily over-expressed using particle bombardment technology. Exogenous PbrPOE21 recombinant protein inhibited PTG in vitro at an optimum concentration of 1.8 µM. PbrPOE21 also affected reactive oxygen species content in the pear pollen tube apex. We suggest that PbrPOE21 inhibits PTG in vitro by altering apical reactive oxygen species content.

Superoxide Radical Metabolism in Sweet Pepper (Capsicum annuum L.) Fruits Is Regulated by Ripening and by a NO-Enriched Environment

Superoxide radical (O2 •-) is involved in numerous physiological and stress processes in higher plants. Fruit ripening encompasses degradative and biosynthetic pathways including reactive oxygen and nitrogen species. With the use of sweet pepper (Capsicum annuum L.) fruits at different ripening stages and under a nitric oxide (NO)-enriched environment, the metabolism of O2 •- was evaluated at biochemical and molecular levels considering the O2 •- generation by a NADPH oxidase system and its dismutation by superoxide dismutase (SOD). At the biochemical level, seven O2 •--generating NADPH-dependent oxidase isozymes [also called respiratory burst oxidase homologs (RBOHs) I-VII], with different electrophoretic mobility and abundance, were detected considering all ripening stages from green to red fruits and NO environment. Globally, this system was gradually increased from green to red stage with a maximum of approximately 2.4-fold increase in red fruit compared with green fruit. Significantly, breaking-point (BP) fruits with and without NO treatment both showed intermediate values between those observed in green and red peppers, although the value in NO-treated fruits was lower than in BP untreated fruits. The O2 •--generating NADPH oxidase isozymes I and VI were the most affected. On the other hand, four SOD isozymes were identified by non-denaturing electrophoresis: one Mn-SOD, one Fe-SOD, and two CuZn-SODs. However, none of these SOD isozymes showed any significant change during the ripening from green to red fruits or under NO treatment. In contrast, at the molecular level, both RNA-sequencing and real-time quantitative PCR analyses revealed different patterns with downregulation of four genes RBOH A, C, D, and E during pepper fruit ripening. On the contrary, it was found out the upregulation of a Mn-SOD gene in the ripening transition from immature green to red ripe stages, whereas a Fe-SOD gene was downregulated. In summary, the data reveal a contradictory behavior between activity and gene expression of the enzymes involved in the metabolism of O2 •- during the ripening of pepper fruit. However, it could be concluded that the prevalence and regulation of the O2 •- generation system (NADPH oxidase-like) seem to be essential for an appropriate control of the pepper fruit ripening, which, additionally, is modulated in the presence of a NO-enriched environment.

CRK2 and C-terminal Phosphorylation of NADPH Oxidase RBOHD Regulate Reactive Oxygen Species Production in Arabidopsis

Reactive oxygen species (ROS) are important messengers in eukaryotic organisms, and their production is tightly controlled. Active extracellular ROS production by NADPH oxidases in plants is triggered by receptor-like protein kinase-dependent signaling networks. Here, we show that CYSTEINE-RICH RLK2 (CRK2) kinase activity is required for plant growth and CRK2 exists in a preformed complex with the NADPH oxidase RESPIRATORY BURST OXIDASE HOMOLOG D (RBOHD) in Arabidopsis (Arabidopsis thaliana). Functional CRK2 is required for the full elicitor-induced ROS burst, and consequently the crk2 mutant is impaired in defense against the bacterial pathogen Pseudomonas syringae pv tomato DC3000. Our work demonstrates that CRK2 regulates plant innate immunity. We identified in vitro CRK2-dependent phosphorylation sites in the C-terminal region of RBOHD. Phosphorylation of S703 RBOHD is enhanced upon flg22 treatment, and substitution of S703 with Ala reduced ROS production in Arabidopsis. Phylogenetic analysis suggests that phospho-sites in the C-terminal region of RBOHD are conserved throughout the plant lineage and between animals and plants. We propose that regulation of NADPH oxidase activity by phosphorylation of the C-terminal region might be an ancient mechanism and that CRK2 is an important element in regulating microbe-associated molecular pattern-triggered ROS production.

Foliar application of 24-epibrassinolide improves Solanum nigrum L. tolerance to high levels of Zn without affecting its remediation potential

Although Solanum nigrum L. is a phytoremediator for different metals, its growth and physiology are still compromised by toxic levels of zinc (Zn). Thus, the development of eco-friendly strategies to enhance its tolerance, maintaining remediation potential is of special interest. This study evaluated the potential of 24-epibrassinolide (24-EBL) to boost S. nigrum defence against Zn towards a better growth rate and remediation potential. After 24 days of exposure, the results revealed that Zn-mediated inhibitory effects on biomass and biometry were efficiently mitigated upon application of 24-EBL, without affecting Zn accumulation. The evaluation of oxidative stress markers reported that Zn excess stimulated the accumulation of superoxide anion (O₂^.-), but reduced hydrogen peroxide (H₂O₂) levels, while not altering lipid peroxidation (LP). This was accompanied by an up-regulation of the antioxidant system, especially proline, superoxide dismutase (SOD) and ascorbate peroxidase (APX) in both organs, and ascorbate in roots of Zn-exposed plants. Foliar application of 24-EBL, however, induced distinctive effects, lowering proline levels in both organs, as well as APX activity in shoots and SOD in roots, whilst stimulating GSH and total thiols in both organs, as well as SOD and APX activity, in shoots and in roots, respectively. Probably due to a better antioxidant efficiency, levels of O₂^.- and H₂O₂ in pre-treated plants remained identical to the control, while LP further decreased in shoots. Overall, our results indicate a protective effect of 24-EBL on S. nigrum response to excess Zn, contributing for a better tolerance and growth rate, without disturbing its phytoremediation potential.

Glutathione redox state plays a key role in flower development and pollen vigour

The importance of the glutathione pool in the development of reproductive tissues and in pollen tube growth was investigated in wild-type (WT) Arabidopsis thaliana, a reporter line expressing redox-sensitive green fluorescent protein (roGFP2), and a glutathione-deficient cad2-1 mutant (cad2-1/roGFP2). The cad2-1/roGFP2 flowers had significantly less reduced glutathione (GSH) and more glutathione disulfide (GSSG) than WT or roGFP2 flowers. The stigma, style, anther, germinated pollen grains, and pollen tubes of roGFP2 flowers had a low degree of oxidation. However, these tissues were more oxidized in cad2-1/roGFP2 flowers than the roGFP2 controls. The ungerminated pollen grains were significantly more oxidized than the germinated pollen grains, indicating that the pollen cells become reduced upon the transition from the quiescent to the metabolically active state during germination. The germination percentage was lower in cad2-1/roGFP2 pollen and pollen tube growth arrested earlier than in roGFP2 pollen, demonstrating that increased cellular reduction is essential for pollen tube growth. These findings establish that ungerminated pollen grains exist in a relatively oxidized state compared with germinating pollen grains. Moreover, failure to accumulate glutathione and maintain a high GSH/GSSG ratio has a strong negative effect on pollen germination.

Reactive Oxygen Species as Mediators of Gametophyte Development and Double Fertilization in Flowering Plants

Reactive oxygen species (ROS) are toxic by-products of aerobic metabolism. In plants, they also function as important signaling molecules that regulate biotic and abiotic stress responses as well as plant growth and development. Recent studies have implicated ROS in various aspects of plant reproduction. In male gametophytes, ROS are associated with germline development as well as the developmentally associated programmed cell death of tapetal cells necessary for microspore development. ROS have a role in regulation of female gametophyte patterning and maintenance of embryo sac polarity. During pollination, ROS play roles in the generation of self-incompatibility response during pollen-pistil interaction, pollen tube growth, pollen tube burst for sperm release and fertilization. In this mini review, we provide an overview of ROS production and signaling in the context of plant reproductive development, from female and male gametophyte development to fertilization.

Sex-dependent and -independent transcriptional changes during haploid phase gametogenesis in the sugar kelp Saccharina latissima

In haplodiplontic lineages, sexual reproduction occurs in haploid parents without meiosis. Although widespread in multicellular lineages such as brown algae (Phaeophyceae), haplodiplontic gametogenesis has been little studied at the molecular level. We addressed this by generating an annotated reference transcriptome for the gametophytic phase of the sugar kelp, Saccharina latissima. Transcriptional profiles of microscopic male and female gametophytes were analysed at four time points during the transition from vegetative growth to gametogenesis. Gametogenic signals resulting from a switch in culture irradiance from red to white light activated a core set of genes in a sex-independent manner, involving rapid activation of ribosome biogenesis, transcription and translation related pathways, with several acting at the post-transcriptional or post-translational level. Additional genes regulating nutrient acquisition and key carbohydrate-energy pathways were also identified. Candidate sex-biased genes under gametogenic conditions had potentially key roles in controlling female- and male-specific gametogenesis. Among these were several sex-biased or -specific E3 ubiquitin-protein ligases that may have important regulatory roles. Females specifically expressed several genes that coordinate gene expression and/or protein degradation, and the synthesis of inositol-containing compounds. Other female-biased genes supported parallels with oogenesis in divergent multicellular lineages, in particular reactive oxygen signalling via an NADPH-oxidase. Males specifically expressed the hypothesised brown algal sex-determining factor. Male-biased expression mainly involved upregulation of genes that control mitotic cell proliferation and spermatogenesis in other systems, as well as multiple flagella-related genes. Our data and results enhance genome-level understanding of gametogenesis in this ecologically and economically important multicellular lineage.

Functional characterization of genes mediating cell wall metabolism and responses to plant cell wall integrity impairment

BACKGROUND

Plant cell walls participate in all plant-environment interactions. Maintaining cell wall integrity (CWI) during these interactions is essential. This realization led to increased interest in CWI and resulted in knowledge regarding early perception and signalling mechanisms active during CWI maintenance. By contrast, knowledge regarding processes mediating changes in cell wall metabolism upon CWI impairment is very limited.

RESULTS

To identify genes involved and to investigate their contributions to the processes we selected 23 genes with altered expression in response to CWI impairment and characterized the impact of T-DNA insertions in these genes on cell wall composition using Fourier-Transform Infrared Spectroscopy (FTIR) in Arabidopsis thaliana seedlings. Insertions in 14 genes led to cell wall phenotypes detectable by FTIR. A detailed analysis of four genes found that their altered expression upon CWI impairment is dependent on THE1 activity, a key component of CWI maintenance. Phenotypic characterizations of insertion lines suggest that the four genes are required for particular aspects of CWI maintenance, cell wall composition or resistance to Plectosphaerella cucumerina infection in adult plants.

CONCLUSION

Taken together, the results implicate the genes in responses to CWI impairment, cell wall metabolism and/or pathogen defence, thus identifying new molecular components and processes relevant for CWI maintenance.

Nitric oxide and ROS mediate autophagy and regulate Alternaria alternata toxin-induced cell death in tobacco BY-2 cells

Synergistic interaction of nitric oxide (NO) and reactive oxygen species (ROS) is essential to initiate cell death mechanisms in plants. Though autophagy is salient in either restricting or promoting hypersensitivity response (HR)-related cell death, the crosstalk between the reactive intermediates and autophagy during hypersensitivity response is paradoxical. In this investigation, the consequences of Alternaria alternata toxin (AaT) in tobacco BY-2 cells were examined. At 3 h, AaT perturbed intracellular ROS homeostasis, altered antioxidant enzyme activities, triggered mitochondrial depolarization and induced autophagy. Suppression of autophagy by 3-Methyladenine caused a decline in cell viability in AaT treated cells, which indicated the vital role of autophagy in cell survival. After 24 h, AaT facilitated Ca2+ influx with an accumulation of reactive oxidant intermediates and NO, to manifest necrotic cell death. Inhibition of NO accumulation by 2-(4-Carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO) decreased the level of necrotic cell death, and induced autophagy, which suggests NO accumulation represses autophagy and facilitates necrotic cell death at 24 h. Application of N-acetyl-L-cysteine at 3 h, confirmed ROS to be the key initiator of autophagy, and together with cPTIO for 24 h, revealed the combined effects of NO and ROS is required for necrotic HR cell death.

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.

Fine-tuning of RBOHF activity is achieved by differential phosphorylation and Ca2+ binding

RBOHF from Arabidopsis thaliana represents a multifunctional NADPH oxidase regulating biotic and abiotic stress tolerance, developmental processes and guard cell aperture. The molecular components and mechanisms determining RBOHF activity remain to be elucidated. Here we combined protein interaction studies, biochemical and genetic approaches, and pathway reconstitution analyses to identify and characterize proteins that confer RBOHF regulation and elucidated mechanisms that adjust RBOHF activity. While the Ca²⁺ sensor-activated kinases CIPK11 and CIPK26 constitute alternative paths for RBOHF activation, the combined activity of CIPKs and the kinase open stomata 1 (OST1) triggers complementary activation of this NADPH oxidase, which is efficiently counteracted through dephosphorylation by the phosphatase ABI1. Within RBOHF, several distinct phosphorylation sites (p-sites) in the N-terminus of RBOHF appear to contribute individually to activity regulation. These findings identify RBOHF as a convergence point targeted by a complex regulatory network of kinases and phosphatases. We propose that this allows for fine-tuning of plant reactive oxygen species (ROS) production by RBOHF in response to different stimuli and in diverse physiological processes.

NADPH Oxidase (Rboh) Activity is Up Regulated during Sweet Pepper (Capsicum annuum L.) Fruit Ripening

In plants, NADPH oxidase (NOX) is also known as a respiratory burst oxidase homolog (Rboh). This highly important enzyme, one of the main enzymatic sources of superoxide radicals (O₂•⁻), is involved in the metabolism of reactive oxygen and nitrogen species (ROS and RNS), which is active in the non-climacteric pepper (Capsicum annuum L.) fruit. We used sweet pepper fruits at two ripening stages (green and red) to biochemically analyze the O₂•⁻-generating Rboh activity and the number of isozymes during this physiological process. Malondialdehyde (MDA) content, an oxidative stress marker, was also assayed as an index of lipid peroxidation. In red fruits, MDA was observed to increase 2-fold accompanied by a 5.3-fold increase in total Rboh activity. Using in-gel assays of Rboh activity, we identified a total of seven CaRboh isozymes (I–VII) which were differentially modulated during ripening. CaRboh-III and CaRboh-I were the most prominent isozymes in green and red fruits, respectively. An in vitro assay showed that CaRboh activity is inhibited in the presence of nitric oxide (NO) donors, peroxynitrite (ONOO⁻) and glutathione (GSH), suggesting that CaRboh can undergo S-nitrosation, Tyr-nitration, and glutathionylation, respectively. In summary, this study provides a basic biochemical characterization of CaRboh activity in pepper fruits and indicates that this O₂•⁻-generating Rboh is involved in nitro-oxidative stress associated with sweet pepper fruit ripening.

Generation of Superoxide by OeRbohH, a NADPH Oxidase Activity During Olive (Olea europaea L.) Pollen Development and Germination

Reactive oxygen species (ROS) are produced in the olive reproductive organs as the result of intense metabolism. ROS production and pattern of distribution depend on the developmental stage, supposedly playing a broad panel of functions, which include defense and signaling between pollen and pistil. Among ROS-producing mechanisms, plasma membrane NADPH-oxidase activity is being highlighted in plant tissues, and two enzymes of this type have been characterized in Arabidopsis thaliana pollen (RbohH and RbohJ), playing important roles in pollen physiology. Besides, pollen from different species has shown distinct ROS production mechanism and patterns of distribution. In the olive reproductive tissues, a significant production of superoxide has been described. However, the enzymes responsible for such generation are unknown. Here, we have identified an Rboh-type gene (OeRbohH), mainly expressed in olive pollen. OeRbohH possesses a high degree of identity with RbohH and RbohJ from Arabidopsis, sharing most structural features and motifs. Immunohistochemistry experiments allowed us to localize OeRbohH throughout pollen ontogeny as well as during pollen tube elongation. Furthermore, the balanced activity of tip-localized OeRbohH during pollen tube growth has been shown to be important for normal pollen physiology. This was evidenced by the fact that overexpression caused abnormal phenotypes, whereas incubation with specific NADPH oxidase inhibitor or gene knockdown lead to impaired ROS production and subsequent inhibition of pollen germination and pollen tube growth.

In silico insights on diverse interacting partners and phosphorylation sites of respiratory burst oxidase homolog (Rbohs) gene families from Arabidopsis and rice

BACKGROUND

NADPH oxidase (Nox) is a critical enzyme involved in the generation of apoplastic superoxide (O₂^-), a type of reactive oxygen species (ROS) and hence regulate a wide range of biological functions in many organisms. Plant Noxes are the homologs of the catalytic subunit from mammalian NADPH oxidases and are known as respiratory burst oxidase homologs (Rbohs). Previous studies have highlighted their versatile roles in tackling different kind of stresses and in plant growth and development. In the current study, potential interacting partners and phosphorylation sites were predicted for Rboh proteins from two model species (10 Rbohs from Arabidopsis thaliana and 9 from Oryza sativa japonica). The present work is the first step towards in silico prediction of interacting partners and phosphorylation sites for Rboh proteins from two plant species.

RESULTS

In this work, an extensive range of potential partners (unique and common), leading to diverse functions were revealed from interaction networks and gene ontology classifications, where majority of AtRbohs and OsRbohs play role in stress-related activities, followed by cellular development. Further, 68 and 38 potential phosphorylation sites were identified in AtRbohs and OsRbohs, respectively. Their distribution, location and kinase specificities were also predicted and correlated with experimental data as well as verified with the other EF-hand containing proteins within both genomes.

CONCLUSIONS

Analysis of regulatory mechanisms including interaction with diverse partners and post-translational modifications like phosphorylation have provided insights regarding functional multiplicity of Rbohs. The bioinformatics-based workflow in the current study can be used to get insights for interacting partners and phosphorylation sites from Rbohs of other plant species.

Reactive oxygen species in plant development

Reactive oxygen species (ROS) are produced by metabolic pathways in almost all cells. As signaling components, ROS are best known for their roles in abiotic and biotic stress-related events. However, recent studies have revealed that they are also involved in numerous processes throughout the plant life cycle, from seed development and germination, through to root, shoot and flower development. Here, we provide an overview of ROS production and signaling in the context of plant growth and development, highlighting the key functions of ROS and their interactions with plant phytohormonal networks.

How do Plants Keep their Functional Integrity?

Unlike animals, plants possess a non-strict and sometimes very fuzzy morphology. Mutual proportions of plant parts can vary to a much greater extent than in animals, changing according to the environmental conditions and the plant needs of nutrients, water and light. Despite the existence of this fundamental difference between plants and animals, it passes almost non-reflected in most studies on plants. In this review we make a preliminary attempt to gather together the mechanisms by which plants preserve their integrity, not loosing at the same time the physiological (and morphological) flexibility which allows them adapting to the different environments they can populate.

Tapetal-Delayed Programmed Cell Death (PCD) and Oxidative Stress-Induced Male Sterility of Aegilops uniaristata Cytoplasm in Wheat

Cytoplasmic male sterility (CMS) plays a crucial role in the utilization of hybrid vigor. Pollen development is often accompanied by oxidative metabolism responses and tapetal programmed cell death (PCD), and deficiency in these processes could lead to male sterility. Aegilops uniaristata cytoplasmic male sterility (Mu-CMS) wheat is a novel male-sterile line in wheat, which possess important potential in hybrid wheat breeding. However, its CMS mechanisms remain poorly understood. In our study, U87B1-706A, with the Aegilops uniaristata cytoplasm, and the maintainer line 706B were used to explore the abortive reason. Compared with 706B, histological analysis and PCD detection of the anther demonstrated that U87B1-706A appeared as delayed tapetal PCD as well as a disorganized organelle phenotype in the early uninucleate stage. Subsequently, a shrunken microspore and disordered exine structure were exhibited in the late uninucleate stage. While the activities of antioxidase increased markedly, the nonenzymatic antioxidant contents declined obviously following overacummulation of reactive oxygen species (ROS) during pollen development in U87B1-706A. Real-time quantitative PCR testified that the transcript levels of the superoxide dismutase (SOD), catalase (CAT), and ascorbate peroxidase (APX) genes, encoding pivotal antioxidant enzymes, were up-regulated in early pollen development. Therefore, we deduce excess ROS as a signal may be related to the increased expression levels of enzyme genes, thereby breaking the antioxidative system balance, resulting in delayed tapetal PCD initiation, which finally led to pollen abortion and male sterility in U87B1-706A. These results provide evidence to further explore the mechanisms of abortive pollen in CMS wheat.

Molecular basis of autotrophic vs mixotrophic growth in Chlorella sorokiniana

In this work, we investigated the molecular basis of autotrophic vs. mixotrophic growth of Chlorella sorokiniana, one of the most productive microalgae species with high potential to produce biofuels, food and high value compounds. To increase biomass accumulation, photosynthetic microalgae are commonly cultivated in mixotrophic conditions, adding reduced carbon sources to the growth media. In the case of C. sorokiniana, the presence of acetate enhanced biomass, proteins, lipids and starch productivity when compared to autotrophic conditions. Despite decreased chlorophyll content, photosynthetic properties were essentially unaffected while differential gene expression profile revealed transcriptional regulation of several genes mainly involved in control of carbon flux. Interestingly, acetate assimilation caused upregulation of phosphoenolpyruvate carboxylase enzyme, enabling potential recovery of carbon atoms lost by acetate oxidation. The obtained results allowed to associate the increased productivity observed in mixotrophy in C. sorokiniana with a different gene regulation leading to a fine regulation of cell metabolism.

Role of cerium oxide nanoparticle-induced autophagy as a safeguard to exogenous H2O2-mediated DNA damage in tobacco BY-2 cells

The effect of cerium oxide nanoparticle (CeNP) in plants has elicited substantial controversy. While some investigators have reported that CeNP possesses antioxidant properties, others observed CeNP to induce reactive oxygen species (ROS). In spite of considerable research carried out on the effects of CeNP in metazoans, fundamental studies that can unveil its intracellular consequences linking ROS production, autophagy and DNA damage are lacking in plants. To elucidate the impact of CeNP within plant cells, tobacco BY-2 cells were treated with 10, 50 and 250 µg ml-1 CeNP (Ce10, Ce50 and Ce250), for 24 h. Results demonstrated concentration-dependent accumulation of Ca2+ and ROS at all CeNP treatment sets. However, significant DNA damage and alteration in antioxidant defence systems were noted prominently at Ce50 and Ce250. Moreover, Ce50 and Ce250 induced DNA damage, analysed by comet assay and DNA diffusion experiments, complied with the concomitant increase in ROS. Furthermore, to evaluate the antioxidant property of CeNP, treated cells were washed after 24 h (to minimise CeNP interference) and challenged with H2O2 for 3 h. Ce10 did not induce genotoxicity and H2O2 exposure to Ce10-treated cells showed lesser DNA breakage than cells treated with H2O2 only. Interestingly, Ce10 provided better protection over N-acetyl-L-cysteine against exogenous H2O2 in BY-2 cells. CeNP exposure to transgenic BY-2 cells expressing GFP-Atg8 fusion protein exhibited formation of autophagosomes at Ce10. Application of vacuolar protease inhibitor E-64c and fluorescent basic dye acridine orange, further demonstrated accumulation of particulate matters in the vacuole and occurrence of acidic compartments, the autophagolysosomes, respectively. BY-2 cells co-treated with CeNP and autophagy inhibitor 3-methyladenine exhibited increased DNA damage in Ce10 and cell death at all assessed treatment sets. Thus, current results substantiate an alternative autophagy-mediated, antioxidant and geno-protective role of CeNP, which will aid in deciphering novel phenomena of plant-nanoparticle interaction at cellular level.

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.

The role of ion disequilibrium in induction of root cell death and autophagy by environmental stresses

Environmental stresses such as salinity, drought, oxidants, heavy metals, hypoxia, extreme temperatures and others can induce autophagy and necrosis-type programmed cell death (PCD) in plant roots. These reactions are accompanied by the generation of reactive oxygen species (ROS) and ion disequilibrium, which is induced by electrolyte/K+ leakage through ROS-activated ion channels, such as the outwardly-rectifying K+ channel GORK and non-selective cation channels. Here, we discuss mechanisms of the stress-induced ion disequilibrium and relate it with ROS generation and onset of morphological, biochemical and genetic symptoms of autophagy and PCD in roots. Based on our own data and that in the literature, we propose a hypothesis on the induction of autophagy and PCD in roots by loss of cytosolic K+. To support this, we present data showing that in conditions of salt stress-induced autophagy, gork1-1 plants lacking root K+ efflux channel have fewer autophagosomes compared with the wild type. Overall, literature analyses and presented data strongly suggest that stress-induced root autophagy and PCD are controlled by the level of cytosolic potassium and ROS.

Two SUMO Proteases SUMO PROTEASE RELATED TO FERTILITY1 and 2 Are Required for Fertility in Arabidopsis

In plants, the posttranslational modification small ubiquitin-like modifier (SUMO) is involved in regulating several important developmental and cellular processes, including flowering time control and responses to biotic and abiotic stresses. Here, we report two proteases, SUMO PROTEASE RELATED TO FERTILITY1 (SPF1) and SPF2, that regulate male and female gamete and embryo development and remove SUMO from proteins in vitro and in vivo. spf1 mutants exhibit abnormal floral structures and embryo development, while spf2 mutants exhibit largely a wild-type phenotype. However, spf1 spf2 double mutants exhibit severe abnormalities in microgametogenesis, megagametogenesis, and embryo development, suggesting that the two genes are functionally redundant. Mutation of SPF1 and SPF2 genes also results in misexpression of generative- and embryo-specific genes. In vitro, SPF1 and SPF2 process SUMO1 precursors into a mature form, and as expected in vivo, spf1 and spf2 mutants accumulate SUMO conjugates. Using a yeast two-hybrid screen, we identified EMBRYO SAC DEVELOPMENT ARREST9 (EDA9) as an SPF1-interacting protein. In vivo, we demonstrate that EDA9 is sumolyated and that, in spf1 mutants, EDA9-SUMO conjugates increase in abundance, demonstrating that EDA9 is a substrate of SPF1. Together, our results demonstrate that SPF1 and SPF2 are two SUMO proteases important for plant development in Arabidopsis (Arabidopsis thaliana).

Reciprocal Interactions between Cadmium-Induced Cell Wall Responses and Oxidative Stress in Plants

Cadmium (Cd) pollution renders many soils across the world unsuited or unsafe for food- or feed-orientated agriculture. The main mechanism of Cd phytotoxicity is the induction of oxidative stress, amongst others through the depletion of glutathione. Oxidative stress can damage lipids, proteins, and nucleic acids, leading to growth inhibition or even cell death. The plant cell has a variety of tools to defend itself against Cd stress. First and foremost, cell walls might prevent Cd from entering and damaging the protoplast. Both the primary and secondary cell wall have an array of defensive mechanisms that can be adapted to cope with Cd. Pectin, which contains most of the negative charges within the primary cell wall, can sequester Cd very effectively. In the secondary cell wall, lignification can serve to immobilize Cd and create a tougher barrier for entry. Changes in cell wall composition are, however, dependent on nutrients and conversely might affect their uptake. Additionally, the role of ascorbate (AsA) as most important apoplastic antioxidant is of considerable interest, due to the fact that oxidative stress is a major mechanism underlying Cd toxicity, and that AsA biosynthesis shares several links with cell wall construction. In this review, modifications of the plant cell wall in response to Cd exposure are discussed. Focus lies on pectin in the primary cell wall, lignification in the secondary cell wall and the importance of AsA in the apoplast. Regarding lignification, we attempt to answer the question whether increased lignification is merely a consequence of Cd toxicity, or rather an elicited defense response. We propose a model for lignification as defense response, with a central role for hydrogen peroxide as substrate and signaling molecule.

Pollen wall degradation in the Brassicaceae permits cell emergence after pollination

PREMISE OF THE STUDY

Despite attempts to degrade the sporopollenin in pollen walls, this material has withstood a hundred years of experimental treatments and thousands of years of environmental attack in insects and soil. We present evidence that sporopollenin, nonetheless, locally degrades only minutes after pollination in Arabidopsis thaliana flowers, and describe here a two-part pollen germination mechanism in A. thaliana involving both chemical weakening of the exine wall and swelling of the underlying intine.

METHODS

We explored naturally occurring components from pollen and stigma surfaces and found a tripartite mix of hydrogen peroxide, peroxidase and catalase enzymes (all at high levels at the pollination interface) to be experimentally sufficient to degrade the sporopollenin of some Brassicaceae family members.

KEY RESULTS

At pollination, factors carried on the pollen surface may mix with factors on the stigma surface in a reaction that locally oxidizes the exine pollen wall. Hydrogen peroxide, catalases, and peroxidases are biologically present at the right time and place and, when mixed experimentally, are sufficient to degrade the walls of susceptible pollen.

CONCLUSIONS

Our work on native biochemistry for breaching sporopollenin suggests new research directions in pollen aperture evolution and could aid efforts to analyze sporopollenin's composition, needed for application of this corrosion-resistant, but long-intractable material.

Prolonged dark period modulates the oxidative burst and enzymatic antioxidant systems in the leaves of salicylic acid-treated tomato

Salicylic acid (SA) is an important plant growth regulator playing a role in the hypersensitive reaction (HR) and the induction of systemic acquired resistance. Since the SA-mediated signalling pathways and the formation of reactive oxygen species (ROS) are light-dependent, the time- and concentration-specific induction of oxidative stress was investigated in leaves of tomato plants kept under light and dark conditions after treatments with 0.1mM and 1mM SA. The application of exogenous SA induced early superoxide- and H₂O₂ production in the leaves, which was different in the absence or presence of light and showed time- and concentration-dependent changes. 1mM SA, which induced HR-like cell death resulted in two peaks in the H₂O₂ production in the light but the first, priming peak was not detected in the dark. Unlike 0.1mM SA, 1mM SA application induced NADPH oxidase activity leading to increased superoxide production in the first hours of SA treatments in the light. Moreover, SA treatments inhibited catalase (CAT) activity and caused a transient decline in ascorbate peroxidase (APX), the two main enzymes responsible for H₂O₂ degradation, which led to a fast H₂O₂ burst in the light. Their activity as well as the expression of some isoenzymes of SOD and APX increased only from the 12th h in the illuminated samples. The activity of NADPH oxidase and expression SlRBOH1 gene encoding a NADPH oxidase subunit was much lower in the dark. In spite of low CAT and APX activity after SA treatments in the dark, the activation of guaiacol-dependent peroxidase (POD) could partially substitute H₂O₂ scavenging activity of these enzymes in the dark, which reduced the ROS burst and development of lesion formation in the leaves.

Autophagy, programmed cell death and reactive oxygen species in sexual reproduction in plants

Autophagy is one of the major cellular processes of recycling of proteins, metabolites and intracellular organelles, and plays crucial roles in the regulation of innate immunity, stress responses and programmed cell death (PCD) in many eukaryotes. It is also essential in development and sexual reproduction in many animals. In plants, although autophagy-deficient mutants of Arabidopsis thaliana show phenotypes in abiotic and biotic stress responses, their life cycle seems normal and thus little had been known until recently about the roles of autophagy in development and reproduction. Rice mutants defective in autophagy show sporophytic male sterility and immature pollens, indicating crucial roles of autophagy during pollen maturation. Enzymatic production of reactive oxygen species (ROS) by respiratory burst oxidase homologues (Rbohs) play multiple roles in regulating anther development, pollen tube elongation and fertilization. Significance of autophagy and ROS in the regulation of PCD of transient cells during plant sexual reproduction is discussed in comparison with animals.

Transcriptomic analysis links gene expression to unilateral pollen-pistil reproductive barriers

BACKGROUND

Unilateral incompatibility (UI) is an asymmetric reproductive barrier that unidirectionally prevents gene flow between species and/or populations. UI is characterized by a compatible interaction between partners in one direction, but in the reciprocal cross fertilization fails, generally due to pollen tube rejection by the pistil. Although UI has long been observed in crosses between different species, the underlying molecular mechanisms are only beginning to be characterized. The wild tomato relative Solanum habrochaites provides a unique study system to investigate the molecular basis of this reproductive barrier, as populations within the species exhibit both interspecific and interpopulation UI. Here we utilized a transcriptomic approach to identify genes in both pollen and pistil tissues that may be key players in UI.

RESULTS

We confirmed UI at the pollen-pistil level between a self-incompatible population and a self-compatible population of S. habrochaites. A comparison of gene expression between pollinated styles exhibiting the incompatibility response and unpollinated controls revealed only a small number of differentially expressed transcripts. Many more differences in transcript profiles were identified between UI-competent versus UI-compromised reproductive tissues. A number of intriguing candidate genes were highly differentially expressed, including a putative pollen arabinogalactan protein, a stylar Kunitz family protease inhibitor, and a stylar peptide hormone Rapid ALkalinization Factor. Our data also provide transcriptomic evidence that fundamental processes including reactive oxygen species (ROS) signaling are likely key in UI pollen-pistil interactions between both populations and species.

CONCLUSIONS

Gene expression analysis of reproductive tissues allowed us to better understand the molecular basis of interpopulation incompatibility at the level of pollen-pistil interactions. Our transcriptomic analysis highlighted specific genes, including those in ROS signaling pathways that warrant further study in investigations of UI. To our knowledge, this is the first report to identify candidate genes involved in unilateral barriers between populations within a species.

Overexpression of E3 Ubiquitin Ligase Gene AdBiL Contributes to Resistance against Chilling Stress and Leaf Mold Disease in Tomato

Ubiquitination is a common regulatory mechanism, playing a critical role in diverse cellular and developmental processes in eukaryotes. However, a few reports on the functional correlation between E3 ubiquitin ligases and reactive oxygen species (ROS) or reactive nitrogen species (RNS) metabolism in response to stress are currently available in plants. In the present study, the E3 ubiquitin ligase gene AdBiL (Adi3 Binding E3 Ligase) was introduced into tomato line Ailsa Craig via Agrobacterium-mediated method. Transgenic lines were confirmed for integration into the tomato genome using PCR. Transcription of AdBiL in various transgenic lines was determined using real-time PCR. Evaluation of stress tolerance showed that T₁ generation of transgenic tomato lines showed only mild symptoms of chilling injury as evident by higher biomass accumulation and chlorophyll content than those of non-transformed plants. Compared with wild-type plants, the contents of AsA, AsA/DHA, GSH and the activity of GaILDH, γ-GCS and GSNOR were increased, while H₂O₂, [Formula: see text], MDA, NO, SNOs, and GSNO accumulations were significantly decreased in AdBiL overexpressing plants in response to chilling stress. Furthermore, transgenic tomato plants overexpressing AdBiL showed higher activities of enzymes such as G6PDH, 6PGDH, NADP-ICDH, and NADP-ME involved in pentose phosphate pathway (PPP). The transgenic tomato plants also exhibited an enhanced tolerance against the necrotrophic fungus Cladosporium fulvum. Tyrosine nitration protein was activated in the plants infected with leaf mold disease, while the inhibition could be recovered in AdBiL gene overexpressing lines. Taken together, our results revealed a possible physiological role of AdBiL in the activation of the key enzymes of AsA-GSH cycle, PPP and down-regulation of GSNO reductase, thereby reducing oxidative and nitrosative stress in plants. This study demonstrates an optimized transgenic strategy using AdBiL gene for crop improvement against biotic and abiotic stress factors.