Role of nitric oxide and reactive oxygen [corrected] species in disease resistance to necrotrophic pathogens

Molecular interactions between the soilborne pathogenic fungus Macrophomina phaseolina and its host plants

Mentioned for the first time in an article 1971, the occurrence of the term "Macrophomina phaseolina" has experienced a steep increase in the scientific literature over the past 15 years. Concurrently, incidences of M. phaseolina-caused crop diseases have been getting more frequent. The high levels of diversity and plasticity observed for M. phasolina genomes along with a rich equipment of plant cell wall degrading enzymes, secondary metabolites and putative virulence effectors as well as the unusual longevity of microsclerotia, their asexual reproduction structures, make this pathogen very difficult to control and crop protection against it very challenging. During the past years several studies have emerged reporting on host defense measures against M. phaseolina, as well as mechanisms of pathogenicity employed by this fungal pathogen. While most of these studies have been performed in crop systems, such as soybean or sesame, recently interactions of M. phaseolina with the model plant Arabidopsis thaliana have been described. Collectively, results from various studies are hinting at a complex infection cycle of M. phaseolina, which exhibits an early biotrophic phase and switches to necrotrophy at later time points during the infection process. Consequently, responses of the hosts are complex and seem coordinated by multiple defense-associated phytohormones. However, at this point no robust and strong host defense mechanism against M. phaseolina has been described.

Reactive oxygen metabolism in the proliferation of Korean pine embryogenic callus cells promoted by exogenous GSH

Exogenous glutathione (GSH) promotes the proliferation of embryogenic callus (EC) cells in Korean pine in the course of somatic embryogenesis, and reactive oxygen species (ROS) may play an important role in regulating the proliferation of EC cells by exogenous GSH. However, the concrete metabolic response of ROS is unclear. In this study, two cell lines of Korean pine with high proliferative potential 001#-001 (F, Fast proliferative potential cell line is abbreviated as F) and low proliferative potential 001#-010 (S, Slow proliferative potential cell line is abbreviated as S) were used as test materials. The responses of ROS-related enzymes and substances to exogenous GSH and L-Buthionine-sulfoximine (BSO) were investigated in EC cells. The results showed that the exogenous addition of GSH increased the number of early somatic embryogenesis (SEs) in EC cells of both F and S cell lines, decreased the amount of cell death in both cell lines. Exogenous addition of GSH promoted cell division in both cell lines, increased intracellular superoxide dismutase (SOD) and catalase (CAT) activities, inhibited intracellular hydrogen peroxide (H₂O₂), malondialdehyde (MDA) and nitric oxide (NO) production, and increased NO/ROS ratio. In conclusion, the exogenous GSH promoting the proliferation of Korean pine EC cells, the activity of intracellular antioxidant enzymes was enhanced, the ROS level was reduced, and the resistance of cells to stress was enhanced.

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.

Nitric oxide signalling in plant interactions with pathogenic fungi and oomycetes

Nitric oxide (NO) and reactive nitrogen species have emerged as crucial signalling and regulatory molecules across all organisms. In plants, fungi, and fungi-like oomycetes, NO is involved in the regulation of multiple processes during their growth, development, reproduction, responses to the external environment, and biotic interactions. It has become evident that NO is produced and used as a signalling and defence cue by both partners in multiple forms of plant interactions with their microbial counterparts, ranging from symbiotic to pathogenic modes. This review summarizes current knowledge on the role of NO in plant-pathogen interactions, focused on biotrophic, necrotrophic, and hemibiotrophic fungi and oomycetes. Actual advances and gaps in the identification of NO sources and fate in plant and pathogen cells are discussed. We review the decisive role of time- and site-specific NO production in germination, oriented growth, and active penetration by filamentous pathogens of the host tissues, as well in pathogen recognition, and defence activation in plants. Distinct functions of NO in diverse interactions of host plants with fungal and oomycete pathogens of different lifestyles are highlighted, where NO in interplay with reactive oxygen species governs successful plant colonization, cell death, and establishment of resistance.

New Findings on the Resistance Mechanism of an Elite Diploid Wild Potato Species JAM1-4 in Response to a Super Race Strain of Phytophthora infestans

Late blight is a devastating potato disease worldwide, caused by Phytophthora infestans. The P. infestans strain 2013-18-306 from Yunnan is a "supervirulent race" that overcomes all 11 known late blight resistance genes (R1 to R11) from Solanum demissum. In a previous study, we identified a diploid wild-type potato JAM1-4 (S. jamesii) with high resistance to 2013-18-306. dRenSeq analysis indicated the presence of novel R genes in JAM1-4. RNA-Seq was used to analyze the late blight resistance response genes and defense regulatory mechanisms of JAM1-4 against 2013-18-306. Gene ontology enrichment and KEGG pathway analysis showed that many disease-resistant pathways were significantly enriched. Analysis of differentially expressed genes (DEGs) revealed an active disease resistance mechanism of JAM1-4, and the essential role of multiple signal transduction pathways and secondary metabolic pathways comprised of SA-JA-ET in plant immunity. We also found that photosynthesis in JAM1-4 was inhibited to promote the immune response. Our study reveals the pattern of resistance-related gene expression in response to a super race strain of potato late blight and provides a theoretical basis for further exploration of potato disease resistance mechanisms, discovery of new late blight resistance genes, and disease resistance breeding.

Role of salicylic acid glucosyltransferase in balancing growth and defence for optimum plant fitness

Salicylic acid (SA), an essential secondary messenger for plant defence responses, plays a role in maintaining a balance (trade-off) between plant growth and resistance induction, but the detailed mechanism has not been explored. Because the SA mimic benzothiadiazole (BTH) is a more stable inducer of plant defence than SA after exogenous application, we analysed expression profiles of defence genes after BTH treatment to better understand SA-mediated immune induction. Transcript levels of the salicylic acid glucosyltransferase (SAGT) gene were significantly lower in BTH-treated Nicotiana tabacum (Nt) plants than in SA-treated Nt control plants, suggesting that SAGT may play an important role in SA-related host defence responses. Treatment with BTH followed by SA suppressed SAGT transcription, indicating that the inhibitory effect of BTH is not reversible. In addition, in BTH-treated Nt and Nicotiana benthamiana (Nb) plants, an early high accumulation of SA and SA 2-O-β-d-glucoside was only transient compared to the control. This observation agreed well with the finding that SAGT-overexpressing (OE) Nb lines contained less SA and jasmonic acid (JA) than in the Nb plants. When inoculated with a virus, the OE Nb plants showed more severe symptoms and accumulated higher levels of virus, while resistance increased in SAGT-silenced (IR) Nb plants. In addition, the IR plants restricted bacterial spread to the inoculated leaves. After the BTH treatment, OE Nb plants were slightly larger than the Nb plants. These results together indicate that SAGT has a pivotal role in the balance between plant growth and SA/JA-mediated defence for optimum plant fitness.

Involvement of nitrate reductase-dependent nitric oxide production in magnetopriming-induced salt tolerance in soybean

In the present study, experiments were performed to investigate the role of nitric oxide (NO) in magnetopriming-induced seed germination and early growth characteristics of soybean (Glycine max) seedlings under salt stress. The NO donor (sodium nitroprusside, SNP), NO scavenger (2-[4-carboxyphenyl]-4,4,5,5-tetramethylimidazoline-1-oxyl-3-oxide, CPTIO), inhibitors of nitrate reductase (sodium tungstate, ST) or NO synthase (N-nitro-L-Arg-methyl ester, LNAME) and NADPH oxidase inhibitor (diphenylene iodonium, DPI) have been used to measure the role of NO in the alleviation of salinity stress by static magnetic field (SMF of 200 mT, 1 h). Salt stress (50 mM NaCl) significantly reduced germination and early growth of seedlings emerged from non-primed seeds. Pre-treatment of seeds with SMF positively stimulated the germination and consequently promoted the seedling growth. ST, LNAME, CPTIO and DPI significantly decreased the growth of seedling, activities of α-amylase, protease and nitrate reductase (NR), hydrogen peroxide (H2 O2 ), superoxide (O2 •- ) and NO content in roots of seedlings emerged from non-primed and SMF-primed seeds. However, the extent of reduction was higher with ST in seedlings of SMF-primed seeds under both conditions, whereas SNP promoted all the studied parameters. Moreover, the generation of NO was also confirmed microscopically using a membrane permanent fluorochrome (4-5-diaminofluorescein diacetate [DAF-2 DA]). Further, analysis showed that SMF enhanced the NR activity and triggered the NO production and NR was maximally decreased by ST as compared to LNAME, CPTIO and DPI. Thus, in addition to ROS, NO might be one of the important signaling molecules in magnetopriming-induced salt tolerance in soybean and NR may be responsible for SMF-triggered NO generation in roots of soybean.

NO dioxygenase- and peroxidase-like activity of Arabidopsis phytoglobin 3 and its role in Sclerotinia sclerotiorum defense

Phytoglobin 3 appears to be ubiquitous in plants, yet there has been dearth of evidence for their potent physiological functions. Previous crystallographic studies suggest a potential NO dioxygenase like activity of Arabidopsis phytoglobin 3 (AHb3). The present work examined the in vivo function of AHb3 in plant physiology and its role in biotic stress using Arabidopsis- Sclerotinia sclerotorium pathosystem. The gene was found to be ubiquitously expressed in all plant tissues, with moderately increased expression in roots. Its expression was induced upon NO, H₂O₂ and biotic stress. A C-terminal tagged GFP version of the wild type protein revealed its enhanced accumulation in the guard cells. AHb3-GFP was found to be partitioned majorly into the nucleus while residual amounts were present in the cytoplasm. The loss of function AHb3 mutant exhibited reduced root length and fresh weight. AHb3 knockout lines also displayed enhanced susceptibility towards the S. sclerotiorum. Interestingly, these lines displayed enhanced ROS accumulation upon pathogen challenge as suggested by DAB staining. Furthermore, enhanced/decreased NO accumulation in AHb3 knockout/overexpression lines upon treatment with multiple NO donors suggests a potent NO dioxygenase like activity for the protein. Taken together, our data indicate that AHb3 play a crucial role in regulating root length as well as in mediating defense response against S. sclerotiorum, possibly by modulating NO and ROS levels.

LSP1, a responsive protein from Meyerozyma guilliermondii, elicits defence response and improves glycyrrhizic acid biosynthesis in Glycyrrhiza uralensis Fisch adventitious roots

This research explored the effects of protein and polysaccharide in Meyerozyma guilliermondii on active compounds in Glycyrrhiza uralensis Fisch adventitious roots. In this study, a responsive protein LSP1 was purified from the Meyerozyma guilliermondii since the excellent induction. The contents of total flavonoids (3.46 mg · g^-1 ), glycyrrhizic acid (0.41 mg · g^-1 ), glycyrrhetinic acid (0.41 mg · g^-1 ), and polysaccharide (94.49 mg · g^-1 ) in adventitious root peaked at LSP1 group, which were 1.6, 3.4, 2.4, 2.0-fold that of control, respectively. Besides, the responsive protein LSP1 significantly activated the defense signaling, mitogen-activated protein kinases and extremely up-regulated the expression of defense-related genes and functional genes involved in glycyrrhizic acid biosynthesis.

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

Nitric oxide (NO) is one of the main signal molecules, which is involved in plant growth and development and can change regular physiological activity in biotic and abiotic stresses. In this study, the role of NO in induced resistance with Pseudomonas fluorescent (CHA0) and basal resistance against Rhizoctonia solani in bean plant was investigated. Our results revealed that P. fluorescent and R. solani can increase NO production at 6h post inoculation (hpi). Also, using the NO donor S-nitroso-N-acetyl D-penicillamine (SNAP) led to increase NO and bean plant resistance against R. solani. Utilizing the NO scavenger, 2-(4-carboxyphenyl)-4,4,5,5-tetramethy-limidazoline-1-oxyl-3-oxide (cPTIO), not only decreased basal resistance but also reduced induced resistance. In continue, the activity of antioxidant enzymes was studied in the former treatments. SNAP, CHA0 and R. solani increased the activity of peroxidase (POX), catalase (CAT) and ascorbate peroxidase (APX) at 6, 12 and 24h post inoculation (hpi). In contrast, using cPTIO and R. solani simultaneously (cPTIO+R) showed reduction in activity of POX and APX at 6 hpi. The cPTIO+R treatment increased POX, APX and CAT activity at 12 and 24 hpi. Hydrogen peroxide (H₂O₂) monitoring in the leaf discs clarified that SNAP can increase H₂O₂ production like CHA0 and R. solani. On the other hand, SNAP increased the resistance level of leaf discs against R. solani. Treating the leaf discs with cPTIO led to decrease resistance against the pathogen. These leaf discs showed reduction in H₂O₂ production at 6 hpi and suddenly enhanced H₂O₂ generation was observed at 24hpi. This study showed that CHA0 can increase NO level in bean plants. NO induced H₂O₂ generation and regulated redox state of the host plant. This interaction resulted in significant defense against the pathogen.

Reactive oxygen species and plant resistance to fungal pathogens

Reactive oxygen species (ROS) have been studied for their role in plant development as well as in plant immunity. ROS were consistently observed to accumulate in the plant after the perception of pathogens and microbes and over the years, ROS were postulated to be an integral part of the defence response of the plant. In this article we will focus on recent findings about ROS involved in the interaction of plants with pathogenic fungi. We will describe the ways to detect ROS, their modes of action and their importance in relation to resistance to fungal pathogens. In addition we include some results from works focussing on the fungal interactor and from studies investigating roots during pathogen attack.

Role of dioxygenase α-DOX2 and SA in basal response and in hexanoic acid-induced resistance of tomato (Solanum lycopersicum) plants against Botrytis cinerea

Resistance of tomato (Solanum Lycopersicum) to the fungal pathogen Botrytis cinerea requires complex interplay between hormonal signalling. In this study, we explored the involvement of new oxylipins in the tomato basal and induced response to this necrotroph through the functional analysis of the tomato α-dioxygenase2 (α-DOX2)-deficient mutant divaricata. We also investigated the role of SA in the defence response against this necrotrophic fungus using SA-deficient tomato nahG plants. The plants lacking dioxigenase α-DOX2, which catalyses oxylipins production from fatty acids, were more susceptible to Botrytis, and hexanoic acid-induced resistance (Hx-IR) was impaired; hence α-DOX2 is required for both tomato defence and the enhanced protection conferred by natural inducer hexanoic acid (Hx) against B. cinerea. The divaricata plants accumulated less pathogen-induced callose and presented lower levels of jasmonic acid (JA) and 12-oxo-phytodienoic acid (OPDA) upon infection if compared to the wild type. Glutathion-S-transferase (GST) gene expression decreased and ROS production significantly increased in Botrytis-infected divaricata plants. These results indicate that absence of α-DOX2 influences the hormonal changes, oxidative burst and callose deposition that occur upon Botrytis infection in tomato. The study of SA-deficient nahG tomato plants showed that the plants with low SA levels displayed increased resistance to Botrytis, but were unable to display Hx-IR. This supports the involvement of SA in Hx-IR. NaghG plants displayed reduced callose and ROS accumulation upon infection and an increased GST expression. This reflects a positive relationship between SA and these defensive mechanisms in tomato. Finally, Hx boosted the pathogen-induced callose in nahG plants, suggesting that this priming mechanism is SA-independent. Our results support the involvement of the oxylipins pathway and SA in tomato response to Botrytis, probably through complex crosstalk of the hormonal balance with callose and ROS accumulation, and reinforce the role of the oxidative stress in the outcome of the plant-Botrytis interaction.

Nitric oxide as a mediator for defense responses

Sequential recognition of invading microbes and rapid induction of plant immune responses comprise at least two recognition systems. Early basal defenses are initiated by pathogen-associated molecular patterns and pattern recognition receptors (PRR) in the plasma membrane. Pathogens produce effectors to suppress defense but plants, in turn, can sense such effectors by dominant plant resistance (R) gene products. Plant PRR and R proteins modulate signaling networks for defense responses that rely on rapid production of reactive nitrogen species (RNS) and reactive oxygen species (ROS). Recent research has shown that nitric oxide (NO) mainly mediates biological function through chemical reactions between locally controlled accumulation of RNS and proteins leading to potential alteration of protein function. Many proteins specifically regulated by NO and participating in signaling during plant defense response have been identified, highlighting the physiological relevance of these modifications in plant immunity. ROS function independently or in cooperation with NO during defense, modulating the RNS signaling functions through the entire process. This review provides an overview of current knowledge about regulatory mechanisms for NO burst and signaling, and crosstalk with ROS in response to pathogen attack.

The combined use of photoaffinity labeling and surface plasmon resonance-based technology identifies multiple salicylic acid-binding proteins

Salicylic acid (SA) is a small phenolic molecule that not only is the active ingredient in the multi-functional drug aspirin, but also serves as a plant hormone that affects diverse processes during growth, development, responses to abiotic stresses and disease resistance. Although a number of SA-binding proteins (SABPs) have been identified, the underlying mechanisms of action of SA remain largely unknown. Efforts to identify additional SA targets, and thereby elucidate the complex SA signaling network in plants, have been hindered by the lack of effective approaches. Here, we report two sensitive approaches that utilize SA analogs in conjunction with either a photoaffinity labeling technique or surface plasmon resonance-based technology to identify and evaluate candidate SABPs from Arabidopsis. Using these approaches, multiple proteins, including the E2 subunit of α-ketoglutarate dehydrogenase and the glutathione S-transferases GSTF2, GSTF8, GSTF10 and GSTF11, were identified as SABPs. Their association with SA was further substantiated by the ability of SA to inhibit their enzymatic activity. The photoaffinity labeling and surface plasmon resonance-based approaches appear to be more sensitive than the traditional approach for identifying plant SABPs using size-exclusion chromatography with radiolabeled SA, as these proteins exhibited little to no SA-binding activity in such an assay. The development of these approaches therefore complements conventional techniques and helps dissect the SA signaling network in plants, and may also help elucidate the mechanisms through which SA acts as a multi-functional drug in mammalian systems.

Physcomitrella patens activates reinforcement of the cell wall, programmed cell death and accumulation of evolutionary conserved defence signals, such as salicylic acid and 12-oxo-phytodienoic acid, but not jasmonic acid, upon Botrytis cinerea infection

The moss Physcomitrella patens is an evolutionarily basal model system suitable for the analysis of plant defence responses activated after pathogen assault. Upon infection with the necrotroph Botrytis cinerea, several defence mechanisms are induced in P. patens, including the fortification of the plant cell wall by the incorporation of phenolic compounds and the induced expression of related genes. Botrytis cinerea infection also activates the accumulation of reactive oxygen species and cell death with hallmarks of programmed cell death in moss tissues. Salicylic acid (SA) levels also increase after fungal infection, and treatment with SA enhances transcript accumulation of the defence gene phenylalanine ammonia-lyase (PAL) in P. patens colonies. The expression levels of the genes involved in 12-oxo-phytodienoic acid (OPDA) synthesis, including lipoxygenase (LOX) and allene oxide synthase (AOS), increase in P. patens gametophytes after pathogen assault, together with a rise in free linolenic acid and OPDA concentrations. However, jasmonic acid (JA) could not be detected in healthy or infected tissues of this plant. Our results suggest that, although conserved defence signals, such as SA and OPDA, are synthesized and are probably involved in the defence response of P. patens against B. cinerea infection, JA production appears to be missing. Interestingly, P. patens responds to OPDA and methyl jasmonate by reducing moss colony growth and rhizoid length, suggesting that jasmonate perception is present in mosses. Thus, P. patens can provide clues with regard to the evolution of different defence pathways in plants, including signalling and perception of OPDA and jasmonates in nonflowering and flowering plants.

Physcomitrella patens activates reinforcement of the cell wall, programmed cell death and accumulation of evolutionary conserved defence signals, such as salicylic acid and 12-oxo-phytodienoic acid, but not jasmonic acid, upon Botrytis cinerea infection

The moss Physcomitrella patens is an evolutionarily basal model system suitable for the analysis of plant defence responses activated after pathogen assault. Upon infection with the necrotroph Botrytis cinerea, several defence mechanisms are induced in P. patens, including the fortification of the plant cell wall by the incorporation of phenolic compounds and the induced expression of related genes. Botrytis cinerea infection also activates the accumulation of reactive oxygen species and cell death with hallmarks of programmed cell death in moss tissues. Salicylic acid (SA) levels also increase after fungal infection, and treatment with SA enhances transcript accumulation of the defence gene phenylalanine ammonia-lyase (PAL) in P. patens colonies. The expression levels of the genes involved in 12-oxo-phytodienoic acid (OPDA) synthesis, including lipoxygenase (LOX) and allene oxide synthase (AOS), increase in P. patens gametophytes after pathogen assault, together with a rise in free linolenic acid and OPDA concentrations. However, jasmonic acid (JA) could not be detected in healthy or infected tissues of this plant. Our results suggest that, although conserved defence signals, such as SA and OPDA, are synthesized and are probably involved in the defence response of P. patens against B. cinerea infection, JA production appears to be missing. Interestingly, P. patens responds to OPDA and methyl jasmonate by reducing moss colony growth and rhizoid length, suggesting that jasmonate perception is present in mosses. Thus, P. patens can provide clues with regard to the evolution of different defence pathways in plants, including signalling and perception of OPDA and jasmonates in nonflowering and flowering plants.

Sphingolipids and plant defense/disease: the "death" connection and beyond

Sphingolipids comprise a major class of structural materials and lipid signaling molecules in all eukaryotic cells. Over the past two decades, there has been a phenomenal growth in the study of sphingolipids (i.e., sphingobiology) at an average rate of ∼1000 research articles per year. Sphingolipid studies in plants, though accounting for only a small fraction (∼6%) of the total number of publications, have also enjoyed proportionally rapid growth in the past decade. Concomitant with the growth of sphingobiology, there has also been tremendous progress in our understanding of the molecular mechanisms of plant innate immunity. In this review, we (i) cross examine and analyze the major findings that establish and strengthen the intimate connections between sphingolipid metabolism and plant programmed cell death (PCD) associated with plant defense or disease; (ii) highlight and compare key bioactive sphingolipids involved in the regulation of plant PCD and possibly defense; (iii) discuss the potential role of sphingolipids in polarized membrane/protein trafficking and formation of lipid rafts as subdomains of cell membranes in relation to plant defense; and (iv) where possible, attempt to identify potential parallels for immunity-related mechanisms involving sphingolipids across kingdoms.