The role of nitric oxide in basal and induced resistance in relation with hydrogen peroxide and antioxidant enzymes

Isolation and identification of rhizospheric pseudomonads with insecticidal effects from various crops in Khuzestan Province, Iran

Pseudomonas bacteria include a variety of species with distinct characteristics. Some species within this genus are known for their ability to stimulate plant growth. Recently, the potential of these bacteria in controlling insect pests has been documented. In this study, 58 bacterial isolates were purified from rhizospheres of wheat, broad bean and canola that were collected from different fields of Khuzestan province in south-west of Iran. With biochemical tests 19 non plant pathogenic pseudomonads strains were detected and their lethal effects on the eggs and larvae of Ephestia keuhniella as an important pest that infests stored products, were evaluated under laboratory conditions. For the bioassays, two concentrations of each strain were administered, and the 5th instar larvae and eggs of the pest were subjected to treatment. Mortality rates were recorded after 24 h. The results showed that all isolated Pseudomonad strains of this study had insecticidal effects against eggs and larvae of E. keuhniella. The strains AWI1, AWI2, AWI7, ABI12, ABI15 and ABI16 displayed the highest mortality rate (91.1 %, 86.2 %, 82.3 %, 84.2, 90.5 % and 90.5 %, respectively). Molecular identification and phylogeny tree according to 16 s rRNA sequencing clarified that AWI1, AWI2 belong to P. plecoglossicida, AWI5 belongs to P. lini, ABI12, ABI15 and ABI16 belong to P. taiwanensis. Moreover, the bacterial efficacy at a suspension concentration of 0.5 OD (80 %) was significantly greater than that at a concentration of 0.2 OD (63.33 %). No significant difference was detected in the response of the pest larvae or eggs to the different strains. Furthermore, olfactory trials revealed that the female parasitoid wasp Habrabracon hebetor actively avoided the infection of the treated larvae by the strains. These findings have practical implications for the development of microbiological pest control strategies.

Plant Disease Resistance-Related Signaling Pathways: Recent Progress and Future Prospects

Plant-pathogen interactions induce a signal transmission series that stimulates the plant's host defense system against pathogens and this, in turn, leads to disease resistance responses. Plant innate immunity mainly includes two lines of the defense system, called pathogen-associated molecular pattern-triggered immunity (PTI) and effector-triggered immunity (ETI). There is extensive signal exchange and recognition in the process of triggering the plant immune signaling network. Plant messenger signaling molecules, such as calcium ions, reactive oxygen species, and nitric oxide, and plant hormone signaling molecules, such as salicylic acid, jasmonic acid, and ethylene, play key roles in inducing plant defense responses. In addition, heterotrimeric G proteins, the mitogen-activated protein kinase cascade, and non-coding RNAs (ncRNAs) play important roles in regulating disease resistance and the defense signal transduction network. This paper summarizes the status and progress in plant disease resistance and disease resistance signal transduction pathway research in recent years; discusses the complexities of, and interactions among, defense signal pathways; and forecasts future research prospects to provide new ideas for the prevention and control of plant diseases.

OsAPX1 Positively Contributes to Rice Blast Resistance

Ascorbate peroxidases (APXs) maintain cellular reactive oxygen species (ROS) homeostasis through their peroxidase activity. Here, we report that OsAPX1 also promotes ROS production such that a delicate cellular ROS homeostasis is achieved temporally after Magnaporthe oryzae infection. OsAPX1 specifically induces ROS production through increasing respiratory burst oxidase homologs (OsRBOHs) expression and can be inhibited by DPI, a ROS inhibitor. The time-course experiment data show that the simultaneous induction of OsAPX1 and OsRBOHs leads to ROS accumulation at an early stage; whereas a more durable expression of OsAPX1 leads to ROS scavenging at a later stage. By the temporal switching between ROS inducer and eliminator, OsAPX1 triggers an instant ROS burst upon M. oryzae infection and then a timely elimination of ROS toxicity. We find that OsAPX1 is under the control of the miR172a-OsIDS1 regulatory module. OsAPX1 also affects salicylic acid (SA) synthesis and signaling, which contribute to blast resistance. In conclusion, we show that OsAPX1 is a key factor that connects the upstream gene silencing and transcription regulatory routes with the downstream phytohormone and redox pathway, which provides an insight into the sophisticated regulatory network of rice innate immunity.

Plant Growth-Promoting Activity of Pseudomonas aeruginosa FG106 and Its Ability to Act as a Biocontrol Agent against Potato, Tomato and Taro Pathogens

P. aeruginosa strain FG106 was isolated from the rhizosphere of tomato plants and identified through morphological analysis, 16S rRNA gene sequencing, and whole-genome sequencing. In vitro and in vivo experiments demonstrated that this strain could control several pathogens on tomato, potato, taro, and strawberry. Volatile and non-volatile metabolites produced by the strain are known to adversely affect the tested pathogens. FG106 showed clear antagonism against Alternaria alternata, Botrytis cinerea, Clavibacter michiganensis subsp. michiganensis, Phytophthora colocasiae, P. infestans, Rhizoctonia solani, and Xanthomonas euvesicatoria pv. perforans. FG106 produced proteases and lipases while also inducing high phosphate solubilization, producing siderophores, ammonia, indole acetic acid (IAA), and hydrogen cyanide (HCN) and forming biofilms that promote plant growth and facilitate biocontrol. Genome mining approaches showed that this strain harbors genes related to biocontrol and growth promotion. These results suggest that this bacterial strain provides good protection against pathogens of several agriculturally important plants via direct and indirect modes of action and could thus be a valuable bio-control agent.

Nitric oxide signalling in roots is required for MYB72-dependent systemic resistance induced by Trichoderma volatile compounds in Arabidopsis

Volatile compounds (VCs) of Trichoderma fungi trigger induced systemic resistance (ISR) in Arabidopsis that is effective against a broad spectrum of pathogens. The root-specific transcription factor MYB72 is an early regulator of ISR and also controls the activation of iron-deficiency responses. Nitric oxide (NO) is involved in the regulation of MYB72-dependent iron-deficiency responses in Arabidopsis roots, but the role of NO in the regulation of MYB72 and ISR by Trichoderma VCs remains unexplored. Using in vitro bioassays, we applied Trichoderma VCs to Arabidopsis seedlings. Plant perception of Trichoderma VCs triggered a burst of NO in Arabidopsis roots. By suppressing this burst using an NO scavenger, we show the involvement of NO in Trichoderma VCs-mediated regulation of MYB72 expression. Using an NO scavenger and the Arabidopsis lines myb72 and nia1nia2 in in planta bioassays, we demonstrate that NO signalling is required in the roots for activation of Trichoderma VCs-mediated ISR against the leaf pathogen Botrytis cinerea. Analysis of the defence-related genes PR1 and PDF1.2 points to the involvement of root NO in priming leaves for enhanced defence. Our results support a key role of root NO signalling in the regulation of MYB72 expression during the activation of ISR by Trichoderma VCs.

The dual interplay of RAV5 in activating nitrate reductases and repressing catalase activity to improve disease resistance in cassava

Cassava bacterial blight (CBB) caused by Xanthomonas axonopodis pv. manihotis (Xam) seriously affects cassava yield. Nitrate reductase (NR) plays an important role in plant nitrogen metabolism in plants. However, the in vivo role of NR and the corresponding signalling pathway remain unclear in cassava. In this study, we isolated MeNR1/2 and revealed their novel upstream transcription factor MeRAV5. We also identified MeCatalase1 (MeCAT1) as the interacting protein of MeRAV5. In addition, we investigated the role of MeCatalase1 and MeRAV5-MeNR1/2 module in cassava defence response. MeNRs positively regulates cassava disease resistance against CBB through modulation of nitric oxide (NO) and extensive transcriptional reprogramming especially in mitogen-activated protein kinase (MAPK) signalling. Notably, MeRAV5 positively regulates cassava disease resistance through the coordination of NO and hydrogen peroxide (H2 O2 ) level. On the one hand, MeRAV5 directly activates the transcripts of MeNRs and NO level by binding to CAACA motif in the promoters of MeNRs. On the other hand, MeRAV5 interacts with MeCAT1 to inhibit its activity, so as to negatively regulate endogenous H2 O2 level. This study highlights the precise coordination of NR activity and CAT activity by MeRAV5 through directly activating MeNRs and interacting with MeCAT1 in plant immunity.

Improvement of Postharvest Anthracnose Resistance in Mango Fruit by Nitric Oxide and the Possible Mechanisms Involved

The anthracnose rot of postharvest mango fruit is a devastating fungal disease often resulting in tremendous quality deterioration and postharvest losses. Nitric oxide (NO), as an important signaling molecule, is involved in the responses to postharvest fruit diseases. In the present study, the effectiveness of NO donor sodium nitroprusside (SNP) to prevent anthracnose of "Tainong" mango fruit caused by Colletotrichum gloeosporioides was evaluated through in vivo and in vitro tests. Results from in vivo test showed that SNP treatment effectively inhibited the lesion diameter and disease incidence on inoculated mango fruit during storage. SNP treatment could regulate hydrogen peroxide levels by reinforcing the activities of catalase, peroxidase, superoxide dismutase, and ascorbate peroxidase. Furthermore, SNP elevated the accumulation of lignin, total phenolics, anthocyanin, and flavonoids and the activities of chitinase and β-1,3-glucanase. In addition, in vitro tests indicated that SNP markedly suppressed mycelial growth and spore germination of C. gloeosporioides through damaging plasma membrane integrity and increasing the leakage of soluble sugar and protein. Our results suggested that SNP could suppress anthracnose decay in postharvest mango fruit, possibly by directly suppressing pathogen growth and indirectly triggering host defense responses.

Nitric Oxide and Hydrogen Peroxide Are Involved in Methyl Jasmonate-Regulated Response against Botrytis cinerea in Postharvest Blueberries

The involvement and the relationship between nitric oxide (NO) and hydrogen peroxide (H₂O₂) in methyl jasmonate (MeJA)-induced immune responses in blueberries against Botrytis cinerea was explored using diphenylene iodonium (DPI, NADPH oxidase inhibitor) and 2-(4-carboxyphenyl)-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide (cPTIO, NO scavenger). MeJA induced NO and H₂O₂ burst and enhanced the resistance of blueberries by elevating defense-related enzymes and the phenylpropanoid pathway. However, the above impacts stimulated by MeJA were weakened by DPI and destroyed by cPTIO. Furthermore, cPTIO abolished the increment in H₂O₂ by regulating the activities of NADPH oxidase, superoxide dismutase, catalase, and ascorbate peroxidase, whereas DPI weakened the increase in H₂O₂ but barely affected the generation of NO and the activity of nitric oxide synthase elevated by MeJA. These results indicated that NO and H₂O₂ were involved in the MeJA-induced resistance in blueberries, and NO worked upstream of H₂O₂ in this process.

Epiphytic fungi induced pathogen resistance of invasive plant Ipomoea cairica against Colletotrichum gloeosporioides

Background

Ipomoea cairica (L.) Sweet is a destructive invasive weed in South China but rarely infected with pathogens in nature. Its pathogen resistance mechanism is largely unknown at present. Some non-pathogenic isolates of Fusarium oxysporum and Fusarium fujikuroi are prevalent on many plant species and function as pathogen resistance inducers of host plants. The objective of the present research is to investigate whether the symbiosis between the both fungi and I. cairica is present, and thereby induces pathogen resistance of I. cairica.

Methods

Through field investigation, we explored the occurrence rates of F. oxysporum and F. fujikuroi on leaf surfaces of I. cairica plants in natural habitats and compared their abundance between healthy leaves and leaves infected with Colletotrichum gloeosporioides, a natural pathogen. With artificial inoculation, we assessed their pathogenicity to I. cairica and studied their contribution of pathogen resistance to I. cairica against C. gloeosporioides.

Results

We found that F. oxysporum and F. fujikuroi were widely epiphytic on healthy leaf surfaces of I. cairica in sunny non-saline, shady non-saline and sunny saline habitats. Their occurrence rates reached up to 100%. Moreover, we found that the abundance of F. oxysporum and F. fujikuroi on leaves infected with C. gloeosporioides were significantly lower than that of healthy leaves. With artificial inoculation, we empirically confirmed that F. oxysporum and F. fujikuroi were non-pathogenic to I. cairica. It was interesting that colonization by F. fujikuroi, F. oxysporum alone and a mixture of both fungi resulted in a reduction of C. gloeosporioides infection to I. cairica accompanied by lower lesion area to leaf surface area ratio, increased hydrogen peroxide (H₂O₂) concentration and salicylic acid (SA) level relative to the control. However, NPR1 expression, chitinase and β-1,3-glucanase activities as well as stem length and biomass of I. cairica plant only could be significantly improved by F. oxysporum and a mixture of both fungi but not by F. fujikuroi. In addition, as compared to colonization by F. oxysporum and a mixture of both fungi, F. fujikuroi induced significantly higher jasmonic acid (JA) level but significantly lower β-1,3-glucanase activity in leaves of I. cairica plants. Thus, our findings indicated the symbiosis of epiphytic fungiF. fujikuroi and F. oxysporum induced systemic resistance of I. cairica against C. gloeosporioides. F. oxysporum played a dominant role in inducing pathogen resistance of I. cairica. Its presence alleviated the antagonism of the JA signaling on SA-dependent β-1,3-glucanase activity and enabled I. cairica plants to maintain relatively higher level of resistance against C. gloeosporioides.

Salmonella enterica Elicits and Is Restricted by Nitric Oxide and Reactive Oxygen Species on Tomato

The enteric pathogen Salmonella enterica can interact with parts of the plant immune system despite not being a phytopathogen. Previous transcriptomic profiling of S. enterica associating with tomato suggested that Salmonella was responding to oxidative and nitrosative stress in the plant niche. We aimed to investigate whether Salmonella was eliciting generation of reactive oxygen species (ROS) and nitric oxide (NO), two components of the microbe-associated molecular pattern (MAMP)-triggered immunity (MTI) of plants. We also sought to determine whether this interaction had any measurable effects on Salmonella colonization of plants. Biochemical, gene expression and on-plant challenge assays of tomato vegetative and fruit organs were conducted to assess the elicitation of ROS and NO in response to Salmonella Newport association. The counter bacterial response and the effect of NO and ROS on Salmonella colonization was also investigated. We detected H₂O₂ in leaves and fruit following challenge with live S. Newport (p < 0.05). Conversely, NO was detected on leaves but not on fruit in response to S. Newport (p < 0.05). We found no evidence of plant defense attenuation by live S. Newport. Bacterial gene expression of S. Newport associating with leaves and fruit were indicative of adaptation to biotic stress in the plant niche. The nitrosative stress response genes hmpA and yoaG were significantly up-regulated in S. Newport on leaves and fruit tissue compared to tissue scavenged of NO or ROS (p < 0.05). Chemical modulation of these molecules in the plant had a restrictive effect on bacterial populations. Significantly higher S. Newport titers were retrieved from H₂O₂ scavenged leaves and fruit surfaces compared to controls (p < 0.05). Similarly, S. Newport counts recovered from NO-scavenged leaves, but not fruit, were higher compared to control (p < 0.05), and significantly lower on leaves pre-elicited to produce endogenous NO. We present evidence of Salmonella elicitation of ROS and NO in tomato, which appear to have a restricting effect on the pathogen. Moreover, bacterial recognition of ROS and NO stress was detected. This work shows that tomato has mechanisms to restrict Salmonella populations and ROS and NO detoxification may play an important role in Salmonella adaptation to the plant niche.

Silencing OsMAPK20-5 has different effects on rice pests in the field

Mitogen-activated protein kinases (MAPKs) play important roles in plant development and adaptive responses to biotic and abiotic stresses. Recently, a rice MAPK gene, OsMAPK20-5, has been reported to protect rice plants against autotoxicity by suppressing herbivore-induced ethylene and nitric oxide signaling. In this context, we observed that silencing OsMAPK20-5 increased the percentage of leaf roll caused by leaf folder Cnaphalocrocis medinalis and the severity of rice blast caused by Magnaporthe grisea but decreased the severity of sheath blight caused by Rhizoctonia solani. These findings show that silencing OsMAPK20-5 has different effects on rice pests in the field, and these differences have important implications for the evolution and exploitation of resistance strategies in plants.

Enhanced Nicotiana benthamiana immune responses caused by heterologous plant genes from Pinellia ternata

BACKGROUND

Pinellia ternata is a Chinese traditional medicinal herb, used to cure diseases including insomnia, eclampsia and cervical carcinoma, for hundreds of years. Non-self-recognition in multicellular organisms can initiate the innate immunity to avoid the invasion of pathogens. A design for pathogen independent, heterosis based, fresh resistance can be generated in F1 hybrid was proposed.

RESULTS

By library functional screening, we found that P. ternata genes, named as ptHR375 and ptHR941, were identified with the potential to trigger a hypersensitive response in Nicotiana benthamiana. Significant induction of ROS and Callose deposition in N. benthamiana leaves along with activation of pathogenesis-related genes viz.; PR-1a, PR-5, PDF1.2, NPR1, PAL, RBOHB and ERF1 and antioxidant enzymes was observed. After transformation into N. benthamiana, expression of pathogenesis related genes was significantly up-regulated to generate high level of resistance against Phytophthora capsici without affecting the normal seed germination and morphological characters of the transformed N. benthamiana. UPLC-QTOF-MS analysis of ptHR375 transformed N. benthamiana revealed the induction of Oxytetracycline, Cuelure, Allantoin, Diethylstilbestrol and 1,2-Benzisothiazol-3(2H)-one as bioactive compounds. Here we also proved that F1 hybrids, produced by crossing of the ptHR375 and ptHR941 transformed and non-transformed N. benthamiana, show significant high levels of PR-gene expressions and pathogen resistance.

CONCLUSIONS

Heterologous plant genes can activate disease resistance in another plant species and furthermore, by generating F1 hybrids, fresh pathogen independent plant immunity can be obtained. It is also concluded that ptHR375 and ptHR941 play their role in SA and JA/ET defense pathways to activate the resistance against invading pathogens.

Lipopolysaccharide-induced priming enhances NO-mediated activation of defense responses in pearl millet challenged with Sclerospora graminicola

Lipopolysaccharide (LPS) elicitors isolated from Pseudomonas fluorescens UOM SAR 14 effectively induced systemic and durable resistance against pearl millet downy mildew disease caused by the oomycete Sclerospora graminicola. Rapid and increased callose deposition and H2O2 accumulation were evidenced in downy mildew susceptible seeds pre-treated with LPS (SLPS) in comparison with the control seedlings, which also correlated with expression of various other defense responses. Biochemical analysis of enzymes and quantitative real-time polymerase chain reaction data suggested that LPS protects pearl millet against downy mildew through the activation of plant defense mechanisms such as generation of nitric oxide (NO), increased expression, and activities of defense enzymes and proteins. Elevation of NO concentrations was shown to be essential for LPS-mediated defense manifestation in pearl millet and had an impact on the other downstream defense responses like enhanced activation of enzymes and pathogen-related (PR) proteins. Temporal expression analysis of defense enzymes and PR-proteins in SLPS seedlings challenged with the downy mildew pathogen revealed that the activity and expression of peroxidase, phenylalanine ammonia lyase, and the PR-proteins (PR-1 and PR-5) were significantly enhanced compared to untreated control. Higher gene expression and protein activities of hydroxyproline-rich glycoproteins (HRGPs) were observed in SLPS seedlings which were similar to that of the resistant check. Collectively, our results suggest that, in pearl millet-downy mildew interaction, LPS pre-treatment affects defense signaling through the central regulator NO which triggers the activities of PAL, POX, PR-1, PR-5, and HRGPs.

Antioxidant genes of plants and fungal pathogens are distinctly regulated during disease development in different Rhizoctonia solani pathosystems

Biotic stress, as a result of plant-pathogen interactions, induces the accumulation of reactive oxygen species in the cells, causing severe oxidative damage to plants and pathogens. To overcome this damage, both the host and pathogen have developed antioxidant systems to quench excess ROS and keep ROS production and scavenging systems under control. Data on ROS-scavenging systems in the necrotrophic plant pathogen Rhizoctonia solani are just emerging. We formerly identified vitamin B6 biosynthetic machinery of R. solani AG3 as a powerful antioxidant exhibiting a high ability to quench ROS, similar to CATALASE (CAT) and GLUTATHIONE S-TRANSFERASE (GST). Here, we provide evidence on the involvement of R. solani vitamin B6 biosynthetic pathway genes; RsolPDX1 (KF620111.1), RsolPDX2 (KF620112.1), and RsolPLR (KJ395592.1) in vitamin B6 de novo biosynthesis by yeast complementation assays. Since gene expression studies focusing on oxidative stress responses of both the plant and the pathogen following R. solani infection are very limited, this study is the first coexpression analysis of genes encoding vitamin B6, CAT and GST in plant and fungal tissues of three pathosystems during interaction of different AG groups of R. solani with their respective hosts. The findings indicate that distinct expression patterns of fungal and host antioxidant genes were correlated in necrotic tissues and their surrounding areas in each of the three R. solani pathosystems: potato sprout-R. solani AG3; soybean hypocotyl-R. solani AG4 and soybean leaves-R. solani AG1-IA interactions. Levels of ROS increased in all types of potato and soybean tissues, and in fungal hyphae following infection of R. solani AGs as determined by non-fluorescence and fluorescence methods using H2DCF-DA and DAB, respectively. Overall, we demonstrate that the co-expression and accumulation of certain plant and pathogen ROS-antioxidant related genes in each pathosystem are highlighted and might be critical during disease development from the plant's point of view, and in pathogenicity and developing of infection structures from the fungal point of view.

The preservation effect of Metschnikowia pulcherrima yeast on anthracnose of postharvest mango fruits and the possible mechanism

This study aimed to determine the effects of Metschnikowia pulcherrima yeast on storage quality of 'Tainong' mango, and elucidate it's possible anti-disease mechanism. The results showed that M. pulcherrima could inhibit the changes in peel colour, fruit firmness, the contents of total soluble solids, total acid and vitamin C, and maintain the storage quality of mango fruits. An investigation of the mechanism showed that M. pulcherrima competed not only for the primary carbon source, but also for living space with Colletotrichum gloeosporioides. In addition, M. pulcherrima promoted the activities of defence-related enzymes, including ß-1,3-glucanase(GLU) and chitinase (CHT), and secreted a small amount of antimicrobial substances composed of volatile and nonvolatile anti-fungal compounds. The results strongly demonstrated that antagonistic yeast M. pulcherrima could be applied as a biocontrol agent for deducing the spoilage and decay of mango fruit.