Emerging complexity in reactive oxygen species production and signaling during the response of plants to pathogens

Augmenting Cr(VI) phytoremediation potential of Ricinus communis through rhizospheric crosstalk with multi stress tolerant plant growth promoting Bacillus altitudinis M1

Plant growth promoting rhizobacteria are cost-effective and eco-friendly alternative for bioremediation of Cr(VI). This study investigated the effects of rhizobacterial strain Bacillus altitudinis M1 on Cr(VI) reduction, plant growth promotion and Cr(VI) stress mitigation in Ricinus communis. Biosorption and bioreduction of Cr(VI) up to 300 mg/l by the strain M1 was confirmed by FTIR, Raman Spectrum and TEM-EDX analysis. Moreover, the strain M1 exhibited high tolerance to temperature (up to 40 °C), pH (up to 8.0), NaCl (up to 6%) and various heavy metals (Pb, Cd, Ni, Cu, Mn and Zn). The strain M1 produced significant IAA, ammonia and EPS under higher concentration of Cr(VI). The strain improved the growth and development of test crop R. communis under higher Cr(VI) concentration. Inoculation of the strain M1 alleviated Cr(VI)-induced oxidative stress in roots and leaves of R. communis by decreasing proline (up to 24 and 33%), H2O2 (up to 56 and 43%), and MDA (up to 42 and 40%) by regulating the activity of antioxidant enzymes. These findings suggest that the strain M1 promotes plant growth under Cr(VI) stress through multiple mechanisms, including phytohormone production, nutrient mobilization, stress metabolite modulation, and antioxidant defense system regulation. Thus the application of the strain M1 potentially reduces Cr(VI) bioavailability, making it a promising candidate for Cr(VI) bioremediation.

Evaluation of Sodium Chloride Concentrations on Growth and Phytochemical Production of Mesembryanthemum crystallinum L. in a Hydroponic System

Mesembryanthemum crystallinum L., commonly known as the ice plant, is a halophyte recognized for its exceptional salinity tolerance. This study aimed to determine the optimal NaCl concentration for promoting plant growth, D-pinitol, and other phytochemicals in M. crystallinum cultivated in a hydroponics system. Seedlings of M. crystallinum were transplanted into a hydroponic system and subjected to different NaCl concentrations (0, 100, 200, 300, 400, and 500 mM) in the nutrient solution. To evaluate the plant’s response to salinity stress, measurements were conducted on growth parameters, chlorophyll and carotenoid levels, total flavonoid and polyphenol contents, and DPPH scavenging activity. The optimal NaCl concentration for growth was found to be 200 mM, at which the shoot fresh and dry weights were highest. Additionally, total chlorophyll and carotenoid contents were maximized at 200 mM NaCl, with a subsequent decrease at higher concentrations. The highest DPPH scavenging activity was observed in the 200 mM NaCl treatment, which correlated with increased levels of total flavonoids and polyphenols. These results indicated that optimizing NaCl concentration can enhance the antioxidant activity of Mesembryanthemum crystallinum L. The D-pinitol content also peaked at 200 mM NaCl treatment, further supporting its role osmotic adjustment under salinity stress. M. crystallinum exhibited enhanced antioxidant production and cellular protective functions at 200 mM NaCl, which optimized its biochemical defense mechanisms and helped maintain physiological functions under salinity stress. These findings provide valuable insights for agricultural and biological applications, particularly in cultivating M. crystallinum for its bioactive compounds.

Wheat lesion mimic homology gene TaCAT2 enhances plant resistance to biotic and abiotic stresses

Lesion mimic mutants (LMMs) refer to the spontaneous formation of disease-like spots on leaves without any obvious pathogen infection. The LMM genes can regulate plant immunity, thus promoting the defense of crops against pathogens. However, there is a lack of systematic understanding of the regulatory mechanism of LMMs in wheat. This study identified a wheat LMM TaCAT2, a homolog of the Arabidopsis CAT2. The prediction of the cis-regulatory element revealed that TaCAT2 was involved in the response of plants to various hormones and stresses. RT-qPCR analysis indicated that TaCAT2 was significantly up-regulated by NaCl, drought, and Fusarium graminearum infection. Fluorescence microscopy showed that the TaCAT2 was localized to the peroxisome. Overexpression of TaCAT2 enhanced plant resistance to Phytophthora infestation and F. graminearum by constitutionally activating SA and JA pathways. VIGS of TaCAT2 enhanced the sensitivity of wheat to F. graminearum. Further, TaCAT2 enhanced stress resistance by scavenging the excessive ROS and increasing the activities of antioxidative enzymes. This study lays the basis for the functional identification of TaCAT2 and its applicability in the disease resistance of wheat.

Mutant noxy8 exposes functional specificities between the chloroplast chaperones CLPC1 and CLPC2 in the response to organelle stress and plant defence

Chloroplast function is essential for growth, development, and plant adaptation to stress. Organelle stress and plant defence responses were examined here using noxy8 (nonresponding to oxylipins 8) from a series of Arabidopsis mutants. The noxy8 mutation was located at the CLPC2 gene, encoding a chloroplast chaperone of the protease complex CLP. Although its CLPC1 paralogue is considered to generate redundancy, our data reveal significant differences distinguishing CLPC2 and CLPC1 functions. As such, clpc1 mutants displayed a major defect in housekeeping chloroplast proteostasis, leading to a pronounced reduction in growth and pigment levels, enhanced accumulation of chloroplast and cytosol chaperones, and resistance to fosmidomycin. Conversely, clpc2 mutants showed severe susceptibility to lincomycin inhibition of chloroplast translation and resistance to Antimycin A inhibition of mitochondrial respiration. In the response to Pseudomonas syringae pv. tomato, clpc2 but not clpc1 mutants were resistant to bacterial infection, showing higher salicylic acid levels, defence gene expression and 9-LOX pathway activation. Our findings suggest CLPC2 and CLPC1 functional specificity, with a preferential involvement of CLPC1 in housekeeping processes and of CLPC2 in stress responses.

Sulfur metabolism-mediated fungal glutathione biosynthesis is essential for oxidative stress resistance and pathogenicity in the plant pathogenic fungus Fusarium graminearum

IMPORTANCE

Fusarium graminearum is a destructive fungal pathogen that causes Fusarium head blight (FHB) on a wide range of cereal crops. To control fungal diseases, it is essential to comprehend the pathogenic mechanisms that enable fungi to overcome host defenses during infection. Pathogens require an oxidative stress response to overcome host-derived oxidative stress. Here, we identify the underlying mechanisms of the Fgbzip007-mediated oxidative stress response in F. graminearum. ChIP-seq and subsequent genetic analyses revealed that the role of glutathione in pathogenesis is not dependent on antioxidant functions in F. graminearum. Altogether, this study establishes a comprehensive framework for the Fgbzip007 regulon on pathogenicity and oxidative stress responses, offering a new perspective on the role of glutathione in pathogenicity.

Genome-Wide Investigation and Expression Analysis of the Catalase Gene Family in Oat Plants (Avena sativa L.)

Through the degradation of reactive oxygen species (ROS), different antioxidant enzymes, such as catalase (CAT), defend organisms against oxidative stress. These enzymes are crucial to numerous biological functions, like plant development and defense against several biotic and abiotic stresses. However, despite the major economic importance of Avena sativa around the globe, little is known about the CAT gene's structure and organization in this crop. Thus, a genome-wide investigation of the CAT gene family in oat plants has been carried out to characterize the potential roles of those genes under different stressors. Bioinformatic approaches were used in this study to predict the AvCAT gene's structure, secondary and tertiary protein structures, physicochemical properties, phylogenetic tree, and expression profiling under diverse developmental and biological conditions. A local Saudi oat variety (AlShinen) was used in this work. Here, ten AvCAT genes that belong to three groups (Groups I-III) were identified. All identified CATs harbor the two conserved domains (pfam00199 and pfam06628), a heme-binding domain, and a catalase activity motif. Moreover, identified AvCAT proteins were located in different compartments in the cell, such as the peroxisome, mitochondrion, and cytoplasm. By analyzing their promoters, different cis-elements were identified as being related to plant development, maturation, and response to different environmental stresses. Gene expression analysis revealed that three different AvCAT genes belonging to three different subgroups showed noticeable modifications in response to various stresses, such as mannitol, salt, and ABA. As far as we know, this is the first report describing the genome-wide analysis of the oat catalase gene family, and these data will help further study the roles of catalase genes during stress responses, leading to crop improvement.

Insights to Gossypium defense response against Verticillium dahliae: the Cotton Cancer

The soil-borne pathogen Verticillium dahliae, also referred as "The Cotton Cancer," is responsible for causing Verticillium wilt in cotton crops, a destructive disease with a global impact. To infect cotton plants, the pathogen employs multiple virulence mechanisms such as releasing enzymes that degrade cell walls, activating genes that contribute to virulence, and using protein effectors. Conversely, cotton plants have developed numerous defense mechanisms to combat the impact of V. dahliae. These include strengthening the cell wall by producing lignin and depositing callose, discharging reactive oxygen species, and amassing hormones related to defense. Despite the efforts to develop resistant cultivars, there is still no permanent solution to Verticillium wilt due to a limited understanding of the underlying molecular mechanisms that drive both resistance and pathogenesis is currently prevalent. To address this challenge, cutting-edge technologies such as clustered regularly interspaced short palindromic repeats (CRISPR)/CRISPR-associated protein 9 (Cas9), host-induced gene silencing (HIGS), and gene delivery via nano-carriers could be employed as effective alternatives to control the disease. This article intends to present an overview of V. dahliae virulence mechanisms and discuss the different cotton defense mechanisms against Verticillium wilt, including morphophysiological and biochemical responses and signaling pathways including jasmonic acid (JA), salicylic acid (SA), ethylene (ET), and strigolactones (SLs). Additionally, the article highlights the significance of microRNAs (miRNAs), circular RNAs (circRNAs), and long non-coding RNAs (lncRNAs) in gene expression regulation, as well as the different methods employed to identify and functionally validate genes to achieve resistance against this disease. Gaining a more profound understanding of these mechanisms could potentially result in the creation of more efficient strategies for combating Verticillium wilt in cotton crops.

In vivo and In vitro evaluation of the antifungal activity of the PGPR Bacillus amyloliquefaciens RaSh1 (MZ945930) against Alternaria alternata with growth promotion influences on Capsicum annuum L. plants

Alternaria alternata that threatens pepper production and causes major economic harm is responsible for the leaf spot/blight disease. Chemical fungicides have been widely employed; unfortunately, fungicidal resistance is a current concern. Therefore, finding new environmentally friendly biocontrol agents is a future challenge. One of these friendly solutions is the use of bacterial endophytes that have been identified as a source of bioactive compounds. The current study investigates the in vivo and in vitro fungicidal potential of Bacillus amyloliquefaciens RaSh1 (MZ945930) against pathogenic A. alternata. In vitro, the results revealed that RaSh1 exhibited strong antagonistic activity against A. alternata. In addition to this, we inoculated pepper (Capsicum annuum L.) plants with B. amyloliquefaciens RaSh1 and infected them with A. alternata. As a result of A. alternata infection, which generated the highest leaf spot disease incidence (DI), the plant's growth indices and physio-biochemical characteristics significantly decreased, according to our findings. Our results also showed the abnormal and deformed cell structure using light and electron microscopy of A. alternata-infected leaves compared with other treatments. However, DI was greatly reduced with B. amyloliquefaciens RaSh1 application (40%) compared to pepper plants infected with A. alternata (80%), and this led to the largest increases in all identified physio-biochemical parameters, including the activity of the defense-related enzymes. Moreover, inoculation of pepper plants with B. amyloliquefaciens RaSh1 decreased electrolyte leakage by 19.53% and MDA content by 38.60% as compared to A. alternata infected ones. Our results show that the endophyte B. amyloliquefaciens RaSh1 has excellent potential as a biocontrol agent and positively affects pepper plant growth.

IPA1 improves drought tolerance by activating SNAC1 in rice

Drought is a major abiotic stress to rice (Oryza sativa) during growth. Ideal Plant Architecture (IPA1), the first cloned gene controlling the ideal plant type in rice, has been reported to function in both ideal rice plant architecture and biotic resistance. Here, we report that the IPA1/OsSPL14, encoding a transcriptional factor, positively regulates drought tolerance in rice. The IPA1 is constitutively expressed and regulated by H₂O₂, abscisic acid, NaCl and polyethylene glycol 6000 treatments in rice. Furthermore, the IPA1-knockout plants showed much greater accumulation of H₂O₂ as measured by 3,3'-diaminobenzidine staining in leaves compared with WT plants. Yeast one-hybrid, dual-luciferase and electrophoretic mobility shift assays indicated that the IPA1 directly activates the promoter of SNAC1. Expression of SNAC1 is significantly down-regulated in IPA1 knockout plants. Further investigation indicated that the IPA1 plays a positive role in drought-stress tolerance by inducing reactive oxygen species scavenging in rice. Together, these findings indicated that the IPA1 played important roles in drought tolerance by regulating SNAC1, thus activating the antioxidant system in rice.

Proteomic Analysis of Apple Response to Penicillium expansum Infection Based on Label-Free and Parallel Reaction Monitoring Techniques

Blue mold, caused by Penicillium expansum, is the most destructive fungal disease of apples and causes great losses during the post-harvest storage of the fruit. Although some apple cultivars are resistant to P. expansum, there has been little information on the molecular mechanism of resistance. In this study, differential proteomic analysis was performed on apple samples infected and uninfected with P. expansum. Parallel reaction monitoring (PRM) technology was used to target and verify the expression of candidate proteins. The label-free technique identified 343 differentially expressed proteins, which were mainly associated with defense responses, metal ion binding, stress responses, and oxidative phosphorylation. The differential expression of enzymes related to reactive oxygen species (ROS) synthesis and scavenging, the activation of defense-related metabolic pathways, and the further production of pathogenesis-related proteins (PR proteins) during P. expansum infection in apples, and direct resistance to pathogen invasion were determined. This study reveals the mechanisms of apple response at the proteomic level with 9 h of P. expansum infection.

Crosstalk of nitro-oxidative stress and iron in plant immunity

Accumulation of oxygen and nitrogen radicals and their derivatives, known as reactive oxygen species (ROS) and reactive nitrogen species (RNS), occurs throughout various phases of plant growth in association with biotic and abiotic stresses. One of the consequences of environmental stresses is disruption of homeostasis between production and scavenging of ROS and RNS, which leads to nitro-oxidative burst and affects other defense-related mechanisms, such as polyamines levels, phenolics, lignin and callose as defense components related to plant cell wall reinforcement. Although this subject has attracted huge interest, the cross-talk between these signaling molecules and iron, as a main metal element involved in the activity of various enzymes and numerous vital processes in the living cells, remains largely unexplored. Therefore, it seems necessary to pay more in depth attention to the mechanisms of plant resistance against various environmental stimuli for designing novel and effective plant protection strategies. This review is focused on advances in recent knowledge related to the role of ROS, RNS, and association of these signaling molecules with iron in plant immunity. Furthermore, the role of cell wall fortification as a main physical barrier involved in plant defense have been discussed in association with reactive species and iron ions.

Role of Tocochromanols in Tolerance of Cereals to Biotic Stresses: Specific Focus on Pathogenic and Toxigenic Fungal Species

Fungal pathogens capable of producing mycotoxins are one of the main threats to the cultivation of cereals and the safety of the harvested kernels. Improving the resistance of crops to fungal disease and accumulation of mycotoxins is therefore a crucial issue. Achieving this goal requires a deep understanding of plant defense mechanisms, most of them involving specialized metabolites. However, while numerous studies have addressed the contribution of phenylpropanoids and carotenoids to plant chemical defense, very few have dealt with tocochromanols. Tocochromanols, which encompass tocopherols and tocotrienols and constitute the vitamin E family, are widely distributed in cereal kernels; their biosynthetic pathway has been extensively studied with the aim to enrich plant oils and combat vitamin E deficiency in humans. Here we provide strong assumptions arguing in favor of an involvement of tocochromanols in plant–fungal pathogen interactions. These assumptions are based on both direct effects resulting from their capacity to scavenge reactive oxygen species, including lipid peroxyl radicals, on their potential to inhibit fungal growth and mycotoxin yield, and on more indirect effects mainly based on their role in plant protection against abiotic stresses.

Characterization and fine mapping of a lesion mimic mutant (Lm5) with enhanced stripe rust and powdery mildew resistance in bread wheat (Triticum aestivum L.)

A novel light intensity-dependent lesion mimic mutant with enhanced disease resistance was physiologically, biochemically, and genetically characterized, and the causative gene was fine mapped to a 1.28 Mbp interval containing 17 high-confidence genes. Lesion mimic mutants are ideal for studying disease resistance and programmed cell death photosynthesis in plants to improve crop yield. In this study, a novel light intensity-dependent lesion mimic mutant (MC21) was obtained from the wheat variety Chuannong16 (CN16) by ethyl methane sulfonate treatment. The mutant initially developed tiny lesion spots on the basal part of the leaves, which then gradually proceeded down to leaf sheaths, stems, shells, and awns at the flowering stage. The major agronomic traits were significantly altered in the mutant compared to that in the wild-type CN16. Furthermore, the mutant exhibited a lesion phenotype with degenerated chloroplast structure, decreased chlorophyll content, increased level of reactive oxygen species, and increased resistance to stripe rust and powdery mildew. Genetic analysis indicated that the lesion phenotype was controlled by a novel single semi-dominant nuclear gene. The target gene was mapped on chromosome arm 2AL located between Kompetitive Allele Specific PCR (KASP) markers, KASP-4211 and KASP-5353, and tentatively termed as lesion mimic 5 (Lm5). The fine mapping suggested that Lm5 was located in a 1.28 Mbp interval between markers KASP-5825 and KASP-9366; 17 high-confidence candidate genes were included in this genomic region. This study provides an important foundational step for further cloning of Lm5 using a map-based approach.

Morphophysiological and transcriptome analysis reveal that reprogramming of metabolism, phytohormones and root development pathways governs the potassium (K+) deficiency response in two contrasting chickpea cultivars

Potassium (K⁺) is an essential macronutrient for plant growth and development. K⁺ deficiency hampers important plant processes, such as enzyme activation, protein synthesis, photosynthesis and stomata movement. Molecular mechanism of K⁺ deficiency tolerance has been partly understood in model plants Arabidopsis, but its knowledge in legume crop chickpea is missing. Here, morphophysiological analysis revealed that among five high yielding desi chickpea cultivars, PUSA362 shows stunted plant growth, reduced primary root growth and low K⁺ content under K⁺ deficiency. In contrast, PUSA372 had negligible effect on these parameters suggesting that PUSA362 is K⁺ deficiency sensitive and PUSA372 is a K⁺ deficiency tolerant chickpea cultivar. RNA-seq based transcriptome analysis under K⁺ deficiency revealed a total of 820 differential expressed genes (DEG's) in PUSA362 and 682 DEGs in PUSA372. These DEGs belongs to different functional categories, such as plant metabolism, signal transduction components, transcription factors, ion/nutrient transporters, phytohormone biosynthesis and signalling, and root growth and development. RNA-seq expression of randomly selected 16 DEGs was validated by RT-qPCR. Out of 16 genes, 13 showed expression pattern similar to RNA-seq expression, that verified the RNA-seq expression data. Total 258 and 159 genes were exclusively up-regulated, and 386 and 347 genes were down-regulated, respectively in PUSA362 and PUSA372. 14 DEGs showed contrasting expression pattern as they were up-regulated in PUSA362 and down-regulated in PUSA372. These include somatic embryogenesis receptor-like kinase 1, thaumatin-like protein, ferric reduction oxidase 2 and transcription factor bHLH93. Nine genes which were down-regulated in PUSA362 found to be up-regulated in PUSA372, including glutathione S-transferase like, putative calmodulin-like 19, high affinity nitrate transporter 2.4 and ERF17-like protein. Some important carbohydrate metabolism related genes, like fructose-1,6-bisphosphatase and sucrose synthase, and root growth related Expansin gene were exclusively down-regulated, while an ethylene biosynthesis gene 1-aminocyclopropane-1-carboxylate oxidase 1 (ACO1) was up-regulated in PUSA362. Interplay of these and several other genes related to hormones (auxin, cytokinin, GA etc.), signal transduction components (like CBLs and CIPKs), ion transporters and transcription factors might underlie the contrasting response of two chickpea cultivars to K⁺ deficiency. In future, some of these key genes will be utilized in genetic engineering and breeding programs for developing chickpea cultivars with improved K⁺ use efficiency (KUE) and K⁺ deficiency tolerance traits.

iTRAQ-Based Proteomics Analysis of Response to Solanum tuberosum Leaves Treated with the Plant Phytotoxin Thaxtomin A

Thaxtomin A (TA) is a phytotoxin secreted by Streptomyces scabies that causes common scab in potatoes. However, the mechanism of potato proteomic changes in response to TA is barely known. In this study, the proteomic changes in potato leaves treated with TA were determined using the Isobaric Tags for Relative and Absolute Quantitation (iTRAQ) technique. A total of 693 proteins were considered as differentially expressed proteins (DEPs) following a comparison of leaves treated with TA and sterile water (as a control). Among the identified DEPs, 460 and 233 were upregulated and downregulated, respectively. Based on Gene Ontology (GO) and Kyoto Encyclopedia of Genes and Genomes (KEGG) analyses, many DEPs were found to be involved in defense and stress responses. Most DEPs were grouped in carbohydrate metabolism, amino acid metabolism, energy metabolism, and secondary metabolism including oxidation-reduction process, response to stress, plant-pathogen interaction, and plant hormone signal transduction. In this study, we analyzed the changes in proteins to elucidate the mechanism of potato response to TA, and we provided a molecular basis to further study the interaction between plant and TA. These results also offer the option for potato breeding through analysis of the resistant common scab.

Mycorrhizal fungi induced activation of tomato defense system mitigates Fusarium wilt stress

The fungus Fusarium oxysporum f. sp. lycopersici (FOL) is known to cause vascular wilt on tomato almost over the world. Inoculation of FOL reduced plant growth and increased wilt of tomato. The following study examined the possible role of arbuscular mycorrhizal fungi (AMF) consortium comprising of Rhizophagus intraradices, Funneliformis mosseae and Claroideoglomus etunicatum against FOL in tomato and explored in an inducing plant systemic defense. AMF inoculation reduced the wilt disease within vascular tissue and in vivo production of fusaric acid was observed which may be responsible in reduced wilting. FOL had an antagonistic effect on AMF colonization, reduced the number of spores, arbuscules and vesicles. AMF also inhibited the damage induced by Fusarium wilt through increasing chlorophyll contents along with the activity of phosphate metabolising enzymes (acid and alkaline phosphatases). Moreover, tomato plants with mycorrhizal inoculation showed an increase in the level of antioxidant enzymes including glutathione reductase, catalase, and etc. with an ultimate influence on the elimination of reactive oxygen species. Moreover, rise in phosphatase along with antioxidant enzymatic systems and enhanced photosynthetic performance contributed to induced resistance against FOL in tomato.

Repressed OsMESL expression triggers reactive oxygen species-mediated broad-spectrum disease resistance in rice

A few reports have indicated that a single gene confers resistance to bacterial blight, sheath blight and rice blast. In this study, we identified a novel disease resistance mutant gene, methyl esterase-like (osmesl) in rice. Mutant rice with T-DNA insertion displayed significant resistance to bacterial blight caused by Xanthomonas oryzae pv. oryzae (Xoo), sheath blight caused by Rhizoctonia solani and rice blast caused by Magnaporthe oryzae. Additionally, CRISPR-Cas9 knockout mutants and RNAi lines displayed resistance to these pathogens. Complementary T-DNA mutants demonstrated a phenotype similar to the wild type (WT), thereby indicating that osmesl confers resistance to pathogens. Protein interaction experiments revealed that OsMESL affects reactive oxygen species (ROS) accumulation by interacting with thioredoxin OsTrxm in rice. Moreover, qRT-PCR results showed significantly reduced mRNA levels of multiple ROS scavenging-related genes in osmesl mutants. Nitroblue tetrazolium staining showed that the pathogens cause ROS accumulation, and quantitative detection revealed significantly increased levels of H₂ O₂ in the leaves of osmesl mutants and RNAi lines after infection. The abundance of JA, a hormone associated with disease resistance, was significantly more in osmesl mutants than in WT plants. Overall, these results suggested that osmesl enhances disease resistance to Xoo, R. solani and M. oryzae by modulating the ROS balance.

Possible role of small secreted peptides (SSPs) in immune signaling in bryophytes

Plants utilize a plethora of peptide signals to regulate their immune response. Peptide ligands and their cognate receptors involved in immune signaling share common motifs among many species of vascular plants. However, the origin and evolution of immune peptides is still poorly understood. Here, we searched for genes encoding small secreted peptides in the genomes of three bryophyte lineages-mosses, liverworts and hornworts-that occupy a critical position in the study of land plant evolution. We found that bryophytes shared common predicted small secreted peptides (SSPs) with vascular plants. The number of SSPs is higher in the genomes of mosses than in both the liverwort Marchantia polymorpha and the hornwort Anthoceros sp. The synthetic peptide elicitors-AtPEP and StPEP-specific for vascular plants, triggered ROS production in the protonema of the moss Physcomitrella patens, suggesting the possibility of recognizing peptide ligands from angiosperms by moss receptors. Mass spectrometry analysis of the moss Physcomitrella patens, both the wild type and the Δcerk mutant secretomes, revealed peptides that specifically responded to chitosan treatment, suggesting their role in immune signaling.

Reconstruction and analysis of a genome-scale metabolic model for Agrobacterium tumefaciens

The plant pathogen Agrobacterium tumefaciens causes crown gall disease and is a widely used tool for generating transgenic plants owing to its virulence. The pathogenic process involves a shift from an independent to a living form within a host plant. However, comprehensive analyses of metabolites, genes, and reactions contributing to this complex process are lacking. To gain new insights about the pathogenicity from the viewpoints of physiology and cellular metabolism, a genome-scale metabolic model (GSMM) was reconstructed for A. tumefaciens. The model, referred to as iNX1344, contained 1,344 genes, 1,441 reactions, and 1,106 metabolites. It was validated by analyses of in silico cell growth on 39 unique carbon or nitrogen sources and the flux distribution of carbon metabolism. A. tumefaciens metabolic characteristics under three ecological niches were modelled. A high capacity to access and metabolize nutrients is more important for rhizosphere colonization than in the soil, and substantial metabolic changes were detected during the shift from the rhizosphere to tumour environments. Furthermore, by integrating transcriptome data for tumour conditions, significant alterations in central metabolic pathways and secondary metabolite metabolism were identified. Overall, the GSMM and constraint-based analysis could decode the physiological and metabolic features of A. tumefaciens as well as interspecific interactions with hosts, thereby improving our understanding of host adaptation and infection mechanisms.

Elevated production of reactive oxygen species is related to host plant resistance to sugarcane aphid in sorghum

Sugarcane aphid (Melanaphis sacchari) is a phloem-feeding insect that severely affects the growth and productivity of sorghum and other related crops. While a growing body of knowledge is accumulating regarding plant, and insect interactions, the role of reactive oxygen species (ROS) against aphid infestation in sorghum has not been established yet. Here, the involvement of H2O2 and ROS detoxification enzymes in host plant resistance to sugarcane aphid in sorghum was demonstrated. The H2O2 accumulation and expression patterns of selected ROS scavenging enzymes including ascorbate peroxidase (APX), glutathione S transferase (GST), superoxide dismutase (SOD), and catalase (CAT) in response to sugarcane aphid infestation at 3, 6, 9, and 12 days post infestation (dpi) in resistant (Tx2783) and susceptible (Tx7000) sorghum genotypes were assessed, respectively. A significant increase in H2O2 accumulation was observed in resistant genotypes at all time points studied as compared to susceptible plants. Furthermore, gene expression analysis revealed that in responding to attack by sugarcane aphid, antioxidant genes were induced in both genotypes, but much stronger in the resistant line. Furthermore, aphid survival and fecundity were significantly inhibited in resistant plants compared to susceptible plants. Taken together, our results suggest that the elevated accumulation of H2O2 and the strong upregulation of the antioxidant genes in sorghum may have contributed to host plant resistance in Tx2783 against sugarcane aphid but the weak expression of those antioxidant genes in Tx7000 resulted in the failure of attempting defense against sugarcane aphid. This report also provides the experimental evidence for the role of ROS involvement in the early defensive response to an attack by sugarcane aphid in sorghum.

Overexpression of bacterial katE gene improves the resistance of modified tomato plant against Fusarium oxysporum f. sp. lycopersici

Tomato (Solanum lycopersicum L.) yield is severely affected by Fusarium fungal disease. To improve the resistance of tomato against Fusarium oxysporum f. sp. lycopersici (FOL), Escherichia coli katE gene was transformed into two tomato cultivars, namely Castle Rock and Super strain B, via Agrobacterium tumefaciens; the transformation efficiency was 5.6% and 3.5%, respectively. The integration of the katE gene into T₀, T₁, and T₂ transgenic tomato lines was confirmed using PCR. In addition, DNA dot blot technique confirmed the integration of the katE gene into T₂ transgenic tomato lines. The RT-PCR analysis confirmed that the katE gene could be expressed normally in the T₂ modified lines. Under artificial infection with FOL, the non-modified plants exhibited more severe fungal disease symptoms than those observed in katE overexpression (OE) lines. Our analysis showed that the levels of three defense enzymes, namely superoxide dismutase (SOD), catalase (CAT), and peroxidase (POD), were increased during transgenic T₂ generation pre-treated with FOL. The bioassay of modified lines revealed that an average of 52.56% of the modified Castle Rock cultivar and 50.28% of the modified Super Strain B cultivar showed resistance under Fusarium infection. These results clearly indicate that the modified tomato plants, in which the katE gene was overexpressed, became more resistant to the infection by FOL than the wild-type plants. Our study has proven that the overexpression of the E. coli katE gene in the OE lines could be utilized to develop and improve the resistance against fungal diseases in the modified crops.

CGTase, a novel antimicrobial protein from Bacillus cereus YUPP-10, suppresses Verticillium dahliae and mediates plant defence responses

Verticillium wilt is a plant vascular disease caused by the soilborne fungus Verticillium dahliae that severely limits cotton production. In a previous study, we screened Bacillus cereus YUPP-10, an efficient antagonistic bacterium, to uncover mechanisms for controlling verticillium wilt. Here, we report a novel antimicrobial cyclodextrin glycosyltransferase (CGTase) from YUPP-10. Compared to other CGTases, six different conserved domains were identified, and six mutants were constructed by gene splicing with overlap extension PCR. Functional analysis showed that domain D was important for hydrolysis activity and domains A1 and C were important for inducing disease resistance. Direct effects of recombinant CGTase on V. dahliae included reduced mycelial growth, spore germination, spore production, and microsclerotia germination. In addition, CGTase also elicited cotton's innate defence reactions. Transgenic Arabidopsis thaliana lines that overexpress CGTase showed higher resistance to verticillium wilt. Transgenic CGTase A. thaliana plants grew faster and resisted disease better. CGTase overexpression enabled a burst of reactive oxygen species production and activated pathogenesis-related gene expression, indicating that the transgenic cotton was better prepared to protect itself from infection. Our work revealed that CGTase could inhibit the growth of V. dahliae, activate innate immunity, and play a major role in the biocontrol of fungal pathogens.

A hypersensitive response-like reaction is involved in hybrid weakness in F1 plants of the cross Capsicum annuum × Capsicum chinense

Hybrid weakness in Capsicum is characterized by the termination of leaf differentiation after the development of several leaves. F₁ plants in some crosses between Capsicum annuum and Capsicum chinense show weakness; this phenomenon has not been investigated in detail since first reported. In the present study, we characterized morphologically and physiologically hybrid weakness in Capsicum. F₁ plants did not show weaker growth than their parents 20 days after germination (DAG), but at 40 DAG, the hybrid weakness phenotype was evidenced by almost complete arrest of new leaf formation, delayed increase in plant height, and reduced upper internode length. The shoot apical meristem (SAM) of F₁ plants exhibited delayed development and an abnormal structure characterized by a flat shape and the presence of fuzzy cell layers on the surface. These abnormal SAMs of F₁ plants may lead to dwarfism. Dead cells and accumulation of H₂O₂ were visually detected in leaves of F₁ plants, and cell death was considered to be programmed, as it was accompanied by internucleosomal fragmentation of DNA. The expression of immunity marker genes PR1 and PR2 was upregulated in leaves of F₁ plants. These results suggest that a hypersensitive response-like reaction is involved in Capsicum hybrid weakness.

VdNPS, a Nonribosomal Peptide Synthetase, Is Involved in Regulating Virulence in Verticillium dahliae

Nonribosomal peptide synthetases (NPS) are known for the biosynthesis of antibiotics, toxins, and siderophore production. They are also a virulence determinant in different phytopathogens. However, until now, the functional characterization of NPS in Verticillium dahliae has not been reported. Deletion of the NPS gene in V. dahliae led to the decrease of conidia, microsclerotia, and pathogenicity. ΔVdNPS strains were tolerant to H₂O₂, and the genes involved in H₂O₂ detoxification, iron/copper transport, and cytoskeleton were differentially expressed in ΔVdNPS. Interestingly, ΔVdNPS strains exhibited hypersensitivity to salicylic acid (SA), and the genes involved in SA hydroxylation were up-regulated in ΔVdNPS compared with wild-type V. dahliae under SA stress. Additionally, during infection, ΔVdNPS induced more pathogenesis-related gene expression, higher reactive oxygen species production, and stronger SA-mediated signaling transduction in host to overcome pathogen. Uncovering the function of VdNPS in pathogenicity could provide a reliable theoretical basis for the development of cultivars with durable resistance against V. dahliae-associated diseases.

Autophagy contributes to resistance to the oxidative stress induced by pine reactive oxygen species metabolism, promoting infection by Bursaphelenchus xylophilus

BACKGROUND

Autophagy plays an important role in eukaryotes. We investigated its role in the pine wood nematode (PWN), Bursaphelenchus xylophilus, the causative agent of pine wilt disease (PWD), to find promising control strategies against PWD.

RESULTS

We analysed the expression levels of PtRBOH1 and PtRBOH2, which regulate reactive oxygen species (ROS) metabolism, in Pinus thunbergii and the expression of three autophagy genes, BxATG5, BxATG9 and BxATG16, in PWN by quantitative reverse transcription-polymerase chain reaction (qRT-PCR), and measured the content of H₂ O₂ , the main product of ROS metabolism, in pine stem. There was a correlation between the expression of autophagy genes in PWN and pine ROS metabolism during early infection. We also found that oxidative stress induces autophagy in PWN according to qRT-PCR, transmission electron microscopy and Western blot analyses. Inhibition of autophagy by 3-methyladenine or silencing of the autophagy genes BxATG9 and BxATG16 in PWN showed that autophagy is essential for feeding, fecundity, egg hatching and survival of PWN under oxidative stress, confirming the importance of autophagy in the antioxidant defences of PWN. Similarly, we demonstrated that autophagy contributes to the virulence of PWN. Moreover, PWN likely ameliorates oxidative damage by enhancing the activities of the peroxidase and catalase antioxidant pathways when autophagy is inhibited.

CONCLUSION

Autophagy contributes to resistance to the oxidative stress induced by pine ROS metabolism, thus promoting infection by PWN. Our findings clarify the defence mechanisms of PWN and the pathogenesis of PWD, and provide promising hints for control of PWD by blocking autophagy. © 2020 Society of Chemical Industry.

Diversity and Function of Endo-Bacteria in Bursaphelenchus xylophilus from Pinus massoniana Lamb. in Different Regions

The pine wood nematode (PWN) Bursaphelenchus xylophilus is the pathogen that causes pine wilt disease (PWD), a devastating forest disease. PWN-associated bacteria may play a role in PWD. However, little is known about the endo-bacteria in PWN. We analyzed the diversity of endo-bacteria in nine isolates of PWNs from Pinus massoniana Lamb. in nine epidemic areas from three Chinese provinces by high-throughput sequencing of 16S rDNA and isolated and identified culturable endo-bacteria through construction of a 16S rDNA phylogenetic tree and Biolog microbial identification. We also examined the effects of endo-bacteria on PWN fecundity, antioxidant capacity, and virulence using sterile nematodes as a control. While the dominant endo-bacteria in PWNs from different regions exhibited no significant difference in the classification levels of class and genus, their proportions differed. Pseudomonas and Stenotrophomonas were highly abundant in all PWN isolates. A total of 15 endo-bacterial strains were successfully isolated and identified as six species: Stenotrophomonas maltophilia, Pseudomonas fluorescens, Kocuria palustris, Microbacterium testaceum, Rhizobium radiobacter, and Leifsonia aquatica. We also found that P. fluorescens significantly increased the egg production of PWN, and that both P. fluorescens and S. maltophilia enhanced the mobility of PWN under oxidative stress and reduced the content of reactive oxygen species by increasing antioxidant enzyme activity in PWN. These strains also accelerated the development of PWD, and P. fluorescens had a more beneficial effect on PWN than S. maltophilia. Diversity exists among the endo-bacteria in PWNs from different regions, and some endo-bacteria can promote PWN infestation by enhancing the fecundity and antioxidant capacity of the nematode. Our study contributes to clarifying the interaction between endo-bacteria and PWN.

Comparative transcriptomic analyses of powdery mildew resistant and susceptible cultivated cucumber (Cucumis sativus L.) varieties to identify the genes involved in the resistance to Sphaerotheca fuliginea infection

Background

Cucumber (Cucumis sativus L.) is a widely cultivated vegetable crop, and its yield and quality are greatly affected by various pathogen infections. Sphaerotheca fuliginea is a pathogen that causes powdery mildew (PM) disease in cucumber. However, the genes involved in the resistance to PM in cucumber are largely unknown.

Methods

In our study, a cucumber PM resistant cultivated variety “BK2” and a susceptible cultivated variety “H136” were used to screen and identify differential expressed genes (DEGs) under the S. fuliginea infection.

Results

There were only 97 DEGs between BK2 and H136 under the control condition, suggesting a similarity in the basal gene expression between the resistant and susceptible cultivated varieties. A large number of hormone signaling-related DEGs (9.2% of all DEGs) between resistant and susceptible varieties were identified, suggesting an involvement of hormone signaling pathways in the resistance to PM. In our study, the defense-related DEGs belonging to Class I were only induced in susceptible cultivated variety and the defense-related DEGs belonging to Class II were only induced in resistant cultivated variety. The peroxidase, NBS, glucanase and chitinase genes that were grouped into Class I and II might contribute to production of the resistance to PM in resistant cultivated variety. Furthermore, several members of Pathogen Response-2 family, such as glucanases and chitinases, were identified as DEGs, suggesting that cucumber might enhance the resistance to PM by accelerating the degradation of the pathogen cell walls. Our data allowed us to identify and analyze more potential genes related to PM resistance.

Proteomics analysis reveals the defense priming effect of chitosan oligosaccharides in Arabidopsis-Pst DC3000 interaction

Chitosan oligosaccharides (COS) worked effectively in multiple plant-pathogen interactions as plant immunity regulator, however, due to the complexity of the COS-induced immune signaling network, the topic requires further investigation. In the present study, quantitative analysis of proteins was performed to investigate the underlying mechanism of COS induced resistance to Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) in Arabidopsis thaliana. 4303 proteins were successfully quantified, 186, 217 and 207 proteins were differently regulated in mock + Pst, COS, and COS + Pst treated plants, respectively, compared with mock plants. From detailed functional and hierarchical clustering analysis, a priming effect of COS on plant immune system by pre-regulated the key proteins related to signaling transduction, defense response, cell wall biosynthesis and modification, plant growth and development, gene transcription and translation, which confers enhanced resistance when Pst DC3000 infection in Arabidopsis. Moreover, RACK1B which has the potential to be the key kinase receptor for COS signals was found out by protein-protein interaction network analysis of COS responsive proteins. In conclusion, COS treatment enable plant to fine-tuning its defense mechanisms for a more rapid and stronger response to future pathogen attacks, which obviously enhances plants defensive capacity that makes COS worked effectively in multiple plant-pathogen interactions.

Exogenous salicylic acid ameliorates heat stress-induced damages and improves growth and photosynthetic efficiency in alfalfa (Medicago sativa L.)

Heat stress is found to be a detrimental factor for growth and development of alfalfa (Medicago sativa L.) which is tremendously invaluable forage due to its high feed value and yield potential. Salicylic acid (SA) has been reported to play a pivotal role in the regulation of plants biotic and abiotic stress response. However, the role of exogenous SA in protecting alfalfa from heat-induced damage has rarely been studied. In this study, four-week-old alfalfa seedlings were treated with 0.25 mM or 0.5 mM SA five days prior to high stress treatment (three day), and various growth and physiological traits were measured. The results showed that exogenous SA pretreatment could improve leaf morphology, plant height, biomass, chlorophyll content, and photosynthetic efficiency of alfalfa under heat stress. Meanwhile, SA could alleviate heat-induced membrane damage by reducing electrolyte leakage (EL) and malondialdehyde (MDA) content, and regulate the activities of antioxidant enzymes including catalase (CAT), superoxide dismutase (SOD), and peroxidase (POD). The results revealed that exogenous SA application enhanced alfalfa heat tolerance by modulating various morphological and physiological characteristics under heat stress, with more prominent effect at lower concentration (0.25 mM). Overall, this study provides fundamental insights into the SA-mediated physiological adaptation of alfalfa plants to heat stress, which could have useful implication in managing other plants which are suffering global warming.

Molecular Characterization and Functional Analysis of Three Autophagy Genes, BxATG5, BxATG9, and BxATG16, in Bursaphelenchus xylophilus

The pine wood nematode (PWN), Bursaphelenchus xylophilus, is the pathogen responsible for pine wilt disease (PWD), a devastating forest disease with a pathogenic mechanism that remains unclear. Autophagy plays a crucial role in physiological and pathological processes in eukaryotes, but its regulatory mechanism and significance in PWN are unknown. Therefore, we cloned and characterized three autophagy genes, BxATG5, BxATG9, and BxATG16, in PWN. BxATG9 and BxATG16 were efficiently silenced through RNA interference, and we found that BxATG16 positively regulated the expression of BxATG5. Silencing BxATG9 and BxATG16 severely inhibited feeding and reproduction in PWN, indicating that autophagy is essential for these processes. We then examined the expression patterns of these three autophagy genes in PWN under the stresses of α-pinene and H₂O₂, the main defense substances of pine trees, and during the development of PWD using quantitative reverse transcription polymerase chain reaction. The expression levels of BxATG5, BxATG9, and BxATG16 all significantly increased after nematodes were stressed with α-pinene and H₂O₂ and inoculated into pine trees, suggesting that autophagy plays an important role in the defense and pathogenesis of PWN. In this study, the molecular characteristics and functions of the autophagy genes BxATG5, BxATG9, and BxATG16 in PWN were elucidated.

OXI1 and DAD Regulate Light-Induced Cell Death Antagonistically through Jasmonate and Salicylate Levels

Singlet oxygen produced from triplet excited chlorophylls in photosynthesis is a signal molecule that can induce programmed cell death (PCD) through the action of the OXIDATIVE STRESS INDUCIBLE 1 (OXI1) kinase. Here, we identify two negative regulators of light-induced PCD that modulate OXI1 expression: DAD1 and DAD2, homologs of the human antiapoptotic protein DEFENDER AGAINST CELL DEATH. Overexpressing OXI1 in Arabidopsis (Arabidopsis thaliana) increased plant sensitivity to high light and induced early senescence of mature leaves. Both phenomena rely on a marked accumulation of jasmonate and salicylate. DAD1 or DAD2 overexpression decreased OXI1 expression, jasmonate levels, and sensitivity to photooxidative stress. Knock-out mutants of DAD1 or DAD2 exhibited the opposite responses. Exogenous applications of jasmonate upregulated salicylate biosynthesis genes and caused leaf damage in wild-type plants but not in the salicylate biosynthesis mutant Salicylic acid induction-deficient2, indicating that salicylate plays a crucial role in PCD downstream of jasmonate. Treating plants with salicylate upregulated the DAD genes and downregulated OXI1 We conclude that OXI1 and DAD are antagonistic regulators of cell death through modulating jasmonate and salicylate levels. High light-induced PCD thus results from a tight control of the relative activities of these regulating proteins, with DAD exerting a negative feedback control on OXI1 expression.

Comparative proteomic analysis provides insights into the complex responses to Pseudoperonospora cubensis infection of cucumber (Cucumis sativus L.)

Cucumber (Cucumis sativus L.) is an important crop distributed in many countries. Downy mildew (DM) caused by the obligate oomycete Pseudoperonospora cubensis is especially destructive in cucumber production. So far, few studies on the changes in proteomes during the P. cubensis infection have been performed. In the present study, the proteomes of DM-resistant variety ‘ZJ’ and DM-susceptible variety ‘SDG’ under the P. cubensis infection were investigated. In total, 6400 peptides were identified, 5629 of which were quantified. KEGG analysis showed that a number of metabolic pathways were significantly altered under P. cubensis infection, such as terpenoid backbone biosynthesis, and selenocompound metabolism in ZJ, and starch and sucrose metabolism in SDG. For terpenoid backbone synthesis, 1-deoxy-D-xylulose-5-phosphate synthase, 1-deoxy-D-xylulose 5-phosphate reductoisomerase, 2-C-methyl-D-erythritol 2,4-cyclodiphosphate synthase, 4-hydroxy-3-methylbut-2-en-1-yl diphosphate synthase, and geranylgeranyl pyrophosphate synthase were significantly accumulated in ZJ rather than in SDG, suggesting that pathogen-induced terpenoids accumulation might play an important role in the resistance against P. cubensis infection. Furthermore, a number of pathogenesis-related proteins, such as endochitinases, peroxidases, PR proteins and heat shock proteins were identified as DAPs, suggesting that DM resistance was controlled by a complex network. Our data allowed us to identify and screen more potential proteins related to the DM resistance.

A feedback loop between CaWRKY41 and H2O2 coordinates the response to Ralstonia solanacearum and excess cadmium in pepper

WRKY transcription factors have been implicated in both plant immunity and plant responses to cadmium (Cd); however, the mechanism underlying the crosstalk between these processes is unclear. Here, we characterized the roles of CaWRKY41, a group III WRKY transcription factor, in immunity against the pathogenic bacterium Ralstonia solanacearum and Cd stress responses in pepper (Capsicum annuum). CaWRKY41 was transcriptionally up-regulated in response to Cd exposure, R. solanacearum inoculation, and H2O2 treatment. Virus-induced silencing of CaWRKY41 increased Cd tolerance and R. solanacearum susceptibility, while heterologous overexpression of CaWRKY41 in Arabidopsis impaired Cd tolerance, and enhanced Cd and zinc (Zn) uptake and H2O2 accumulation. Genes encoding reactive oxygen species-scavenging enzymes were down-regulated in CaWRKY41-overexpressing Arabidopsis plants, whereas genes encoding Zn transporters and enzymes involved in H2O2 production were up-regulated. Consistent with these findings, the ocp3 (overexpressor of cationic peroxidase 3) mutant, which has elevated H2O2 levels, displayed enhanced sensitivity to Cd stress. These results suggest that a positive feedback loop between H2O2 accumulation and CaWRKY41 up-regulation coordinates the responses of pepper to R. solanacearum inoculation and Cd exposure. This mechanism might reduce Cd tolerance by increasing Cd uptake via Zn transporters, while enhancing resistance to R. solanacearum.

Differential effects of rapamycin on Bursaphelenchus xylophilus with different virulence and differential expression of autophagy genes under stresses in nematodes

Pine wilt disease (PWD) caused by the pine wood nematode (PWN), Bursaphelenchus xylophilus, is a devastating disease for Pinus spp. The virulence and resilience of PWN are closely linked to the spread and development of PWD. Numerous studies have shown that autophagy has important physiological and pathological functions in eukaryotes. But little is known about the relationships between autophagy and PWNs' virulence and resistance. In this study, through observation under the microscope and recording, we found the induction of autophagy by rapamycin could dramatically improve movement ability of PWNs with different virulence, and the highly virulent AMA3 isolate moved more than the low virulent YW4 isolate when autophagy was over-induced. High concentrations of rapamycin substantially improved the feeding and reproduction of AMA3 but not YW4. Conserved domains of autophagy genes BxATG3, BxATG4, and BxATG7 were first cloned from PWNs by reverse transcription-polymerase chain reaction (RT-PCR). Expression profiling of these three autophagy genes under biotic and abiotic stresses in PWNs with different virulence was determined by quantitative RT-PCR. The results revealed the expression levels of these three autophagy genes in PWNs with different virulence were increased significantly when nematodes were subject to high and low temperatures, oxidative stress, and defensive responses of pine trees. The expression levels of autophagy genes under biotic and abiotic stresses in AMA3 were higher than those in YW4, and different genes showed different performance. Our study clarified that autophagy was closely related to virulence and resistance of PWN, and the ability of a highly virulent isolate to regulate autophagy activity under stresses was stronger than that of a low virulent isolate.

Dose-dependent effects of 1O2 in chloroplasts are determined by its timing and localization of production

In plants, highly reactive singlet oxygen (1O2) is known to inhibit photosynthesis and to damage the cell as a cytotoxin. However, more recent studies have also proposed 1O2 as a signal. In plants under stress, not only 1O2 but also other reactive oxygen species (ROS) are generated simultaneously, thus making it difficult to link a particular response to the release of 1O2 and establish a signaling role for this ROS. This obstacle has been overcome by the identification of conditional mutants of Arabidopsis thaliana that selectively generate 1O2 and trigger various 1O2-mediated responses. In chloroplasts of these mutants, chlorophyll or its biosynthetic intermediates may act as a photosensitizer and generate 1O2. These 1O2-mediated responses are not only dependent on the dosage of 1O2 but also are determined by the timing and suborganellar localization of its production. This spatial- and temporal-dependent variability of 1O2-mediated responses emphasizes the importance of 1O2 as a highly versatile and short-lived signal that acts throughout the life cycle of a plant.

Loss of cytosolic glucose-6-phosphate dehydrogenase increases the susceptibility of Arabidopsis thaliana to root-knot nematode infection

BACKGROUND AND AIMS

Root knot nematodes (RKNs, Meloidogyne spp.) are microscopic roundworms with a wide host range causing great economic losses worldwide. Understanding how metabolic pathways function within the plant upon RKN infection will provide insight into the molecular aspects of plant-RKN interactions. Glucose-6-phosphate dehydrogenase (G6PDH), the key regulatory enzyme of the oxidative pentose phosphate pathway (OPPP), is involved in plant responses to abiotic stresses and pathogenesis. In this study, the roles of Arabidopsis cytosolic G6PDH in plant-RKN interactions were investigated.

METHODS

Enzyme assays and western blotting were used to characterize changes in total G6PDH activity and protein abundance in wild-type Arabidopsis in response to RKN infection. The susceptibility of wild-type plants and the double mutant g6pd5/6 to RKNs was analysed and the expression of genes associated with the basal defence response was tested after RKN infection using quantitative reverse transcription PCR.

KEY RESULTS

RKN infection caused a marked increase in total G6PDH activity and protein abundance in wild-type Arabidopsis roots. However, the transcript levels of G6PDH genes except G6PD6 were not significantly induced following RKN infection, suggesting that the increase in G6PDH activity may occur at the post-transcriptional level. The double mutant g6pd5/6 with loss-of-function of the two cytosolic isoforms G6PD5 and G6PD6 displayed enhanced susceptibility to RKNs. Moreover, reactive oxygen species (ROS) production and gene expression involved in the defence response including jasmonic acid and salicylic acid pathways were suppressed in the g6pd5/6 mutant at the early stage of RKN infection when compared to the wild-type plants.

CONCLUSIONS

The results demonstrated that the G6PDH-mediated OPPP plays an important role in the plant-RKN interaction. In addition, a new aspect of G6PDH activity involving NADPH production by the OPPP in plant basal defence against RKNs is defined, which may be involved in ROS signalling.

A Cytosolic Triosephosphate Isomerase Is a Key Component in XA3/XA26-Mediated Resistance

Bacterial blight caused by Xanthomonas oryzae pv oryzae (Xoo) causes severe damage to rice (Oryza sativa) production worldwide. The major disease resistance gene, Xa3/Xa26, confers broad-spectrum and durable resistance to Xoo at both seedling and adult stages. However, the molecular mechanism of the Xa3/Xa26-initiated defense pathway against Xoo is still largely unknown. Here, we show that a triosephosphate isomerase (TPI), OsTPI1.1, is a key component in XA3/XA26-mediated resistance to Xoo OsTPI1.1 is a glycolytic enzyme that catalyzes the reversible interconversion of dihydroxyacetone phosphate to glyceraldehyde-3-phosphate. Transcriptional suppression of OsTPI1.1 in plants harboring Xa3/Xa26 largely impaired the XA3/XA26-mediated resistance to Xoo, and constitutive overexpression of OsTPI1.1 in susceptible rice plants without Xa3/Xa26 only slightly decreased the susceptibility to Xoo Therefore, both XA3/XA26 and OsTPI1.1 are required in XA3/XA26-mediated resistance. We show that OsTPI1.1 participates in the resistance through its enzymatic activity, which was enhanced significantly by its binding with XA3/XA26. Reactive oxygen species (ROS), especially hydrogen peroxide, accumulated in the OsTPI1.1-overexpressing plants, and suppression of OsTPI1.1 decreased ROS accumulation. The changes in ROS are associated with the reduction of NADP⁺ to NADPH, which may act as a redox cofactor to scavenge ROS, leading to reduced resistance to Xoo These results suggest that OsTPI1.1 modulates ROS production as a resistance mechanism against Xoo.

Antimicrobial activity of Agaricus brasiliensis on Plasmopara viticola and its effect on the induction of resistance to the control of downy mildew on ‘Isabel Precoce’

ABSTRACT: Agaricus brasiliensis include bioactive compounds that can act as antibiotics, bacteriostatic, fungistatic and nematostatic substances. In this sense, this study aimed to evaluate the effect of a single application of aqueous mycelial suspension (AMS) of A. brasiliensis in control of downy mildew (Plasmopara viticola) and resistance induction in ‘Isabel Precoce’ grapevines under greenhouse conditions. Treatments consisted of three doses of 1%, 5%, 10%, 15% and 20% AMS A. brasiliensis, as well as treatment with acibenzolar-S-methyl (ASM). The variables analyzed were: sporangiospore germination, disease severity, represented by the area under the disease progress curve (AUDPC), catalase enzyme activity, peroxidase and polyphenol. The 10%, 15% and 20% doses of AMS caused approximately 80% reduction in germination of P. viticola sporangiospores. The treatments did not show significant effects in reducing both the AUDPC of mildew and polyphenol oxidase enzyme activity. The A. brasiliensis aqueous mycelial suspension showed a fungitoxic effect on the germination of sporangiopores; however, it was not enough to reduce the severity of mildew in the ‘Isabel Precoce’ grapevines, even when acting on the catalase and peroxidase enzymes. Thus, experiments should be performed to verify the viability of the reproductive structures of the pathogen externalized in the vines when treated with A. brasiliensis AMS.

Priming plant resistance by activation of redox-sensitive genes

Priming by natural compounds is an interesting alternative for sustainable agriculture, which also contributes to explore the molecular mechanisms associated with stress tolerance. Although hosts and stress types eventually determine the mode of action of plant-priming agents, it highlights that many of them act on redox signalling. These include vitamins thiamine, riboflavin and quercetin; organic acids like pipecolic, azelaic and hexanoic; volatile organic compounds such as methyl jasmonate; cell wall components like chitosans and oligogalacturonides; H₂O₂, etc. This review provides data on how priming inducers promote stronger and faster responses to stress by modulating the oxidative environment, and interacting with signalling pathways mediated by salycilic acid, jasmonic acid and ethylene. The histone modifications involved in priming that affect the transcription of defence-related genes are also discussed. Despite the evolutionary distance between plants and animals, and the fact that the plant innate immunity takes place in each plant cell, they show many similarities in the molecular mechanisms that underlie pathogen perception and further signalling to activate defence responses. This review highlights the similarities between priming through redox signalling in plants and in mammalian cells. The strategies used by pathogens to manipulate the host´s recognition and the further activation of defences also show similarities in both kingdoms. Moreover, phytochemicals like sulforaphane and 12-oxo-phytodienoic acid prime both plant and mammalian responses by activating redox-sensitive genes. Hence research data into the priming of plant defences can provide additional information and a new viewpoint for priming mammalian defence, and vice versa.

Silicon Mechanisms to Ameliorate Heavy Metal Stress in Plants

The increased contaminants caused by anthropogenic activities in the environment and the importance of finding pathways to reduce pollution caused the silicon application to be considered an important detoxification agent. Silicon, as a beneficial element, plays an important role in amelioration of abiotic stress, such as an extreme dose of heavy metal in plants. There are several mechanisms involved in silicon mediation in plants, including the reduction of heavy metal uptake by plants, changing pH value, formation of Si heavy metals, and stimulation of enzyme activity, which can work by chemical and physical pathways. The aim of this paper is to investigate the major silicon-related mechanisms that reduce the toxicity of heavy metals in plants and then to assess the role of silicon in increasing the antioxidant enzyme and nonenzyme activities to protect the plant cell.

Hydrogen Peroxide-Induced Root Ca2+ and K⁺ Fluxes Correlate with Salt Tolerance in Cereals: Towards the Cell-Based Phenotyping

Salinity stress-induced production of reactive oxygen species (ROS) and associated oxidative damage is one of the major factors limiting crop production in saline soils. However, the causal link between ROS production and stress tolerance is not as straightforward as one may expect, as ROS may also play an important signaling role in plant adaptive responses. In this study, the causal relationship between salinity and oxidative stress tolerance in two cereal crops-barley (Hordeum vulgare) and wheat (Triticum aestivum)-was investigated by measuring the magnitude of ROS-induced net K⁺ and Ca2+ fluxes from various root tissues and correlating them with overall whole-plant responses to salinity. We have found that the association between flux responses to oxidative stress and salinity stress tolerance was highly tissue specific, and was also dependent on the type of ROS applied. No correlation was found between root responses to hydroxyl radicals and the salinity tolerance. However, when oxidative stress was administered via H₂O₂ treatment, a significant positive correlation was found for the magnitude of ROS-induced K⁺ efflux and Ca2+ uptake in barley and the overall salinity stress tolerance, but only for mature zone and not the root apex. The same trends were found for wheat. These results indicate high tissue specificity of root ion fluxes response to ROS and suggest that measuring the magnitude of H₂O₂-induced net K⁺ and Ca2+ fluxes from mature root zone may be used as a tool for cell-based phenotyping in breeding programs aimed to improve salinity stress tolerance in cereals.

Transcriptional analysis and histochemistry reveal that hypersensitive cell death and H2O2 have crucial roles in the resistance of tea plant (Camellia sinensis (L.) O. Kuntze) to anthracnose

Anthracnose causes severe losses of tea production in China. Although genes and biological processes involved in anthracnose resistance have been reported in other plants, the molecular response to anthracnose in tea plant is unknown. We used the susceptible tea cultivar Longjing 43 and the resistant cultivar Zhongcha 108 as materials and compared transcriptome changes in the leaves of both cultivars following Colletotrichum fructicola inoculation. In all, 9015 and 8624 genes were differentially expressed between the resistant and susceptible cultivars and their controls (0 h), respectively. In both cultivars, the differentially expressed genes (DEGs) were enriched in 215 pathways, including responses to sugar metabolism, phytohormones, reactive oxygen species (ROS), biotic stimuli and signalling, transmembrane transporter activity, protease activity and signalling receptor activity, but DEG expression levels were higher in Zhongcha 108 than in Longjing 43. Moreover, functional enrichment analysis of the DEGs showed that hydrogen peroxide (H₂O₂) metabolism, cell death, secondary metabolism, and carbohydrate metabolism are involved in the defence of Zhongcha 108, and 88 key genes were identified. Protein-protein interaction (PPI) network demonstrated that putative mitogen-activated protein kinase (MAPK) cascades are activated by resistance (R) genes and mediate downstream defence responses. Histochemical analysis subsequently validated the strong hypersensitive response (HR) and H₂O₂ accumulation that occurred around the hyphal infection sites in Zhongcha 108. Overall, our results indicate that the HR and H₂O₂ are critical mechanisms in tea plant defence against anthracnose and may be activated by R genes via MAPK cascades.

An unusual role for the phytyl chains in the photoprotection of the chlorophylls bound to Water-Soluble Chlorophyll-binding Proteins

Water-Soluble Chlorophyll Proteins (WSCPs) from Brassicaceae are non-photosynthetic proteins which tetramerize upon binding four chlorophyll (Chl) molecules. The bound Chls are highly photostable, despite the lack of bound carotenoids known, in Chl-containing photosynthetic proteins, to act as singlet oxygen and Chl triplet (3Chl) quenchers. Although the physiological function of WSCPs is still unclear, it is likely to be related to their biochemical stability and their resistance to photodegradation. To get insight into the origin of this photostability, the properties of the 3Chl generated in WSCPs upon illumination were investigated. We found that, unlike the excited singlet states, which are excitonic states, the triplet state is localized on a single Chl molecule. Moreover, the lifetime of the 3Chl generated in WSCPs is comparable to that observed in other Chl-containing systems and is reduced in presence of oxygen. In contrast to previous observations, we found that WSCP actually photosensitizes singlet oxygen with an efficiency comparable to that of Chl in organic solvent. We demonstrated that the observed resistance to photooxidation depends on the conformation of the phytyl moieties, which in WSCP are interposed between the rings of Chl dimers, hindering the access of singlet oxygen to the oxidizable sites of the pigments.

Silencing of AtRAP, a target gene of a bacteria-induced small RNA, triggers antibacterial defense responses through activation of LSU2 and down-regulation of GLK1

Plants fine-tune their sophisticated immunity systems in response to pathogen infections. We previously showed that AtlsiRNA-1, a bacteria-induced plant endogenous small interfering RNA, silences the AtRAP gene, which encodes a putative RNA binding protein. In this study, we demonstrate that AtRAP functions as a negative regulator in plant immunity by characterizing molecular and biological responses of the knockout mutant and overexpression lines of AtRAP upon bacterial infection. AtRAP is localized in chloroplasts and physically interacts with Low Sulfur Upregulated 2 (LSU2), which positively regulates plant defense. Our results suggest that AtRAP negatively regulates defense responses by suppressing LSU2 through physical interaction. We also detected downregulation of the transcription factor GOLDEN2-LIKE 1 (GLK1) in atrap-1 using microarray analysis. The glk1 glk2 double mutant showed enhanced resistance to Pseudomonas syringae pv. tomato, which is consistent with a previous study showing enhanced resistance of a glk1 glk2 double mutant to Hyaloperonospora arabidopsidis. Taken together, our data suggest that silencing of AtRAP by AtlsiRNA-1 upon bacterial infection triggers defense responses through regulation of LSU2 and GLK1.

Proteomics-based identification of differentially abundant proteins reveals adaptation mechanisms of Xanthomonas citri subsp. citri during Citrus sinensis infection

BACKGROUND

Xanthomonas citri subsp. citri (Xac) is the causal agent of citrus canker. A proteomic analysis under in planta infectious and non-infectious conditions was conducted in order to increase our knowledge about the adaptive process of Xac during infection.

RESULTS

For that, a 2D-based proteomic analysis of Xac at 1, 3 and 5 days after inoculation, in comparison to Xac growth in NB media was carried out and followed by MALDI-TOF-TOF identification of 124 unique differentially abundant proteins. Among them, 79 correspond to up-regulated proteins in at least one of the three stages of infection. Our results indicate an important role of proteins related to biofilm synthesis, lipopolysaccharides biosynthesis, and iron uptake and metabolism as possible modulators of plant innate immunity, and revealed an intricate network of proteins involved in reactive oxygen species adaptation during Plants` Oxidative Burst response. We also identified proteins previously unknown to be involved in Xac-Citrus interaction, including the hypothetical protein XAC3981. A mutant strain for this gene has proved to be non-pathogenic in respect to classical symptoms of citrus canker induced in compatible plants.

CONCLUSIONS

This is the first time that a protein repertoire is shown to be active and working in an integrated manner during the infection process in a compatible host, pointing to an elaborate mechanism for adaptation of Xac once inside the plant.

Cassava postharvest physiological deterioration: a complex phenomenon involving calcium signaling, reactive oxygen species and programmed cell death

Postharvest physiological deterioration (PPD) of cassava (Manihot esculenta) storage roots is a complex physiological and biochemical process which involve many regulatory networks linked with specific proteins modulation and signaling transduction pathways. However, it is poorly understood regarding biological regulation, and the interactions among protein groups and signals to determine PPD syndrome in cassava storage roots. This review sheds some light on the possible molecular mechanisms involved in reactive oxygen species (ROS), calcium signaling transduction, and programmed cell death (PCD) in cassava PPD syndrome. A model for predicting crosstalk among calcium signaling, ROS and PCD is suggested to fine-tune PPD syndrome. This would clues to cassava molecular breeding to alleviate the PPD effects on the shelf-life.

Molecular mechanisms behind the accumulation of ATP and H2O2 in citrus plants in response to 'Candidatus Liberibacter asiaticus' infection

Candidatus Liberibacter asiaticus (Las) is a fastidious, phloem-restricted pathogen with a significantly reduced genome, and attacks all citrus species with no immune cultivars documented to date. Like other plant bacterial pathogens, Las deploys effector proteins into the organelles of plant cells, such as mitochondria and chloroplasts to manipulate host immunity and physiology. These organelles are responsible for the synthesis of adenosine triphosphate (ATP) and have a critical role in plant immune signaling during hydrogen peroxide (H₂O₂) production. In this study, we investigated H₂O₂ and ATP accumulation in relation to citrus huanglongbing (HLB) in addition to revealing the expression profiles of genes critical for the production and detoxification of H₂O₂ and ATP synthesis. We also found that as ATP and H₂O₂ concentrations increased in the leaf, so did the severity of the HLB symptoms, a trend that remained consistent among the four different citrus varieties tested. Furthermore, the upregulation of ATP synthase, a key enzyme for energy conversion, may contribute to the accumulation of ATP in infected tissues, whereas downregulation of the H₂O₂ detoxification system may cause oxidative damage to plant macromolecules and cell structures. This may explain the cause of some of the HLB symptoms such as chlorosis or leaf discoloration. The findings in this study highlight important molecular and physiological mechanisms involved in the host plants' response to Las infection and provide new targets for interrupting the disease cycle.

Absence of Cu-Zn superoxide dismutase BCSOD1 reduces Botrytis cinerea virulence in Arabidopsis and tomato plants, revealing interplay among reactive oxygen species, callose and signalling pathways

Plants activate responses against pathogens, including the oxidative burst. Necrotrophic pathogens can produce reactive oxygen species (ROS) that benefit the colonization process. Previously, we have demonstrated that tomato plants challenged with Botrytis cinerea accumulate ROS and callose, together with the induction of genes involved in defence, signalling and oxidative metabolism. Here, we studied the infection phenotype of the Δbcsod1 strain in both tomato and Arabidopsis plants. This mutant lacks bcsod1, which encodes Cu-Zn superoxide dismutase (SOD). This enzyme catalyses the conversion of superoxide ion ( O2-) into hydrogen peroxide (H₂ O₂ ). ROS play a protective role and act as signals in plants. Δbcsod1 displayed reduced virulence compared with wild-type B05.10 in both species. Plants infected with Δbcsod1 accumulated less H₂ O₂ and more O2- than those infected with B05.10, which is associated with an increase in the defensive polymer callose. This supports a major role of fungal SOD in H₂ O₂ production during the plant-pathogen interaction. The early induction of the callose synthase gene PMR4 suggested that changes in ROS altered plant defensive responses at the transcriptional level. The metabolites and genes involved in signalling and in response to oxidative stress were differentially expressed on Δbcsod1 infection, supporting the notion that plants perceive changes in ROS balance and activate defence responses. A higher O₂^- /H₂ O₂ ratio seems to be beneficial for plant protection against this necrotroph. Our results highlight the relevance of callose and the oxylipin 12-oxo-phytodienoic acid (OPDA) in the response to changes in the oxidative environment, and clarify the mechanisms that underlie the responses to Botrytis in Arabidopsis and tomato plants.

Physiological Responses and Expression Profile of NADPH Oxidase in Rice (Oryza Sativa) Seedlings under Different Levels of Submergence

BACKGROUND

Flooding due to global climate change is a serious problem that frequently decreases crop yields. Rice fields in flood-prone areas often experience full or partial submergence. Submergence has an adverse effect on internal oxygen availability, sugar status and survival. Complete submergence imposes severe pressure on plants, principally because the excess water in their surroundings deprives them of certain basic resources such as oxygen, carbon dioxide and light for photosynthesis. To better understand the mechanisms involved under different levels of flooding, it is necessary to further observe physiological responses and to identify the Rboh genes involved and determine how they are regulated during submergence.

RESULTS

In this study, significant physiological changes were observed in plant height, leaf sheath elongation and chlorophyll a, b and total content under partial and full submergence treatments. Senescence-regulating genes were severely affected under full submergence. Additionally, intracellular oxidative homeostasis was disrupted by overproduction of H2O2 and O2 (-), which affected cell viability and antioxidant enzyme activity, under different levels of submergence. Quantitative RT-PCR analyses revealed that complex regulation of Rboh genes is involved under different levels of submergence.

CONCLUSION

Our results demonstrated that the effect of physiological and the transcript levels of OsRboh genes were presented different responses to different levels of submergence in rice seedlings. There have different mechanism in intracellular to response different levels of submergence. Finally we discuss effects of the regulation of OsRboh expression and ROS production which was important to maintain homeostasis to help rice seedlings face different levels of submergence.

The Physcomitrella patens Chloroplast Proteome Changes in Response to Protoplastation

Plant protoplasts are widely used for genetic manipulation and functional studies in transient expression systems. However, little is known about the molecular pathways involved in a cell response to the combined stress factors resulted from protoplast generation. Plants often face more than one type of stress at a time, and how plants respond to combined stress factors is therefore of great interest. Here, we used protoplasts of the moss Physcomitrella patens as a model to study the effects of short-term stress on the chloroplast proteome. Using label-free comparative quantitative proteomic analysis (SWATH-MS), we quantified 479 chloroplast proteins, 219 of which showed a more than 1.4-fold change in abundance in protoplasts. We additionally quantified 1451 chloroplast proteins using emPAI. We observed degradation of a significant portion of the chloroplast proteome following the first hour of stress imposed by the protoplast isolation process. Electron-transport chain (ETC) components underwent the heaviest degradation, resulting in the decline of photosynthetic activity. We also compared the proteome changes to those in the transcriptional level of nuclear-encoded chloroplast genes. Globally, the levels of the quantified proteins and their corresponding mRNAs showed limited correlation. Genes involved in the biosynthesis of chlorophyll and components of the outer chloroplast membrane showed decreases in both transcript and protein abundance. However, proteins like dehydroascorbate reductase 1 and 2-cys peroxiredoxin B responsible for ROS detoxification increased in abundance. Further, genes such as thylakoid ascorbate peroxidase were induced at the transcriptional level but down-regulated at the proteomic level. Together, our results demonstrate that the initial chloroplast reaction to stress is due changes at the proteomic level.