Activate Plant Defense Research Articles

The effect of abiotic and biotic stresses on the production of bioactive compounds in tea (Camellia sinensis (L.) O. Kuntze)

Tea (Camellia sinensis (L.) O. Kuntze) is the most famous, flavored, functional, and therapeutic non-alcoholic beverage, with two-thirds of the world's population drinking it. Tea leaves are demonstrated to contain many bioactive compounds, including polyphenols, amino acids, volatile compounds, and alkaloids, all of which have a good potential of biomedical uses. In plants under stress, including diverse elicitors or signal molecules, accumulation of such metabolites occurs frequently. In adapting plants to the ecosystem and alleviating stress conditions, phytochemicals play a significant role. Ecological factors may be categorized into biotic and abiotic factors which may be artificially induced to activate plant defense responses due to the generation of bioactive compounds. Different biotic and abiotic factors are directly linked to the productivity of tea. Abiotic and biotic factors such as temperature, cold, nutrition, shading, humidity, light intensity, water supply, mechanical, nutrition influence the growth of the plant and the production of the bioactive compounds in tea. Biotic and abiotic factors are versatile tools for the overproduction of valuable metabolites in tea. In this review, an update will be provided on current research focusing on the response of tea to an integration of various stresses. In specific, we will discuss how various stress responses are integrated and how they affect plant growth and physiological characteristics.

Heat-killed endophytic bacterium induces robust plant defense responses against important pathogens

Stress caused by pathogens strongly damages plants. Developing products to control plant disease is an important challenge in sustainable agriculture. In this study, a heat-killed endophytic bacterium (HKEB), Bacillus aryabhattai, is used to induce plant defense against fungal and bacterial pathogens, and the main defense pathways used by the HKEB to activate plant defense are revealed. The HKEB induced high protection against different pathogens through the salicylic and jasmonic acid pathways. We report the presence of gentisic acid in the HKEB for the first time. These results show that HKEBs may be a useful tool for the management of plant diseases.

The suppression effect and mechanism of arbuscular mycorrhizal fungi against plant root rot.

Root rot is a serious soil-borne disease, with negative consequences on crop yield and quality. Arbuscular mycorrhizal (AM) fungi are a group of soil microorganisms, which play important physiological and ecological functions by establishing symbionts with plant roots. AM fungi could induce plant resistance against root rot by regulating physiological and biochemical processes. As a biological agent, AM fungi are used to antagonize soil-borne diseases such as root rot, which is a hotspot in the field of plant-microorganism interaction. We comprehensively reviewed the suppression effect of AM fungi on plant root rot, and the effect of AM fungi on root morphology of host plant, plant nutrition levels, as well as their role in competing with pathogens for ecological sites, activating plant defense systems, and regulating root exudates. Finally, we discussed the potential mechanism of AM fungi inhibiting root rot, as well as the practical problems in the efficient utilization of AM fungi were discussed, in order to provide the theoretical basis for the biological control protocol to antagonize root rot with AM fungi.

Novel marine yeast strains as plant growth-promoting agents improve defense in wheat (Triticum aestivum) against Fusarium oxysporum

Yeasts are multilateral opportunistic plant symbionts that can cause intrinsic changes in the metabolism of host plants, thereby increasing plant growth and activating plant defense against various diseases. Although yeasts isolated from soil and plants have been evaluated as biological control agents, the search for new antagonists continues. Marine yeasts can be an option for the selection of new plant promoters and antagonistic agents. The objective of the present study was selected to demonstrate the potential of two marine yeast strains isolated from Red Sea water to award beneficial agronomic quality to promote wheat growth and induce defense mechanism against Fusarium oxysporum and its ability to produce zearalenone. These yeast strains were identified based on the genotypic identification as Candida orthopsilosis and Rhodotorula mucilaginosa, and their sequences were deposited in the GenBank database under the accession number MN784461 and MN784463, respectively. The results of in vitro assay showed great inhibition on mycelial growth and zearalenone production level. Also, wheat seeds primed with the two yeast strains in the presence and absence of Fusarium displayed early seedling emergence and enhanced growth compared with wheat treated by Fusarium only and untreated wheat (control). Also, under greenhouse conditions, C. orthopsilosis and R. mucilaginosa revealed remarkable improvement in the wheat growth parameters and resistance against Fusarium oxysporum and led to a complete inhibition of zearalenone production in treated wheat roots and spikes compared to those treated by Fusarium only. Our findings demonstrated the contribution of tested marine yeasts in the increase in metabolites accumulation.

Exogenous salicylic acid improves resistance of aphid-susceptible wheat to the grain aphid, Sitobion avenae (F.) (Hemiptera: Aphididae).

Salicylic acid (SA), a phytohormone, has been considered to be a key regulator mediating plant defence against pathogens. It is still vague how SA activates plant defence against herbivores such as chewing and sucking pests. Here, we used an aphid-susceptible wheat variety to investigate Sitobion avenae response to SA-induced wheat plants, and the effects of exogenous SA on some defence enzymes and phenolics in the plant immune system. In SA-treated wheat seedlings, intrinsic rate of natural increase (rm), fecundity and apterous rate of S. avenae were 0.25, 31.4 nymphs/female and 64.4%, respectively, and significantly lower than that in the controls (P < 0.05). Moreover, the increased activities of phenylalanine-ammonia-lyase, polyphenol oxidase (PPO) and peroxidase in the SA-induced seedlings obviously depended on the sampling time, whereas activities of catalase and 4-coumarate:CoA ligase were suppressed significantly at 24, 48 and 72 h in comparison with the control. Dynamic levels of p-coumaric acid at 96 h, caffeic acid at 24 and 72 h and chlorogenic acid at 24, 48 and 96 h in wheat plants were significantly upregulated by exogenous SA application. Nevertheless, only caffeic acid content was positively correlated with PPO activity in SA-treated wheat seedlings (P = 0.031). These findings indicate that exogenous SA significantly enhanced the defence of aphid-susceptible wheat variety against aphids by regulating the plant immune system, and may prove a potential application of SA in aphid control.

An efficient direct screening system for microorganisms that activate plant immune responses based on plant\u2013microbe interactions using cultured plant cells

Microorganisms that activate plant immune responses have attracted considerable attention as potential biocontrol agents in agriculture because they could reduce agrochemical use. However, conventional methods to screen for such microorganisms using whole plants and pathogens are generally laborious and time consuming. Here, we describe a general strategy using cultured plant cells to identify microorganisms that activate plant defense responses based on plant–microbe interactions. Microbial cells were incubated with tobacco BY-2 cells, followed by treatment with cryptogein, a proteinaceous elicitor of tobacco immune responses secreted by an oomycete. Cryptogein-induced production of reactive oxygen species (ROS) in BY-2 cells served as a marker to evaluate the potential of microorganisms to activate plant defense responses. Twenty-nine bacterial strains isolated from the interior of Brassica rapa var. perviridis plants were screened, and 8 strains that enhanced cryptogein-induced ROS production in BY-2 cells were selected. Following application of these strains to the root tip of Arabidopsis seedlings, two strains, Delftia sp. BR1R-2 and Arthrobacter sp. BR2S-6, were found to induce whole-plant resistance to bacterial pathogens (Pseudomonas syringae pv. tomato DC3000 and Pectobacterium carotovora subsp. carotovora NBRC 14082). Pathogen-induced expression of plant defense-related genes (PR-1, PR-5, and PDF1.2) was enhanced by the pretreatment with strain BR1R-2. This cell–cell interaction-based platform is readily applicable to large-scale screening for microorganisms that enhance plant defense responses under various environmental conditions.

Hydrolysis Kinetics of a Novel 3,4-Dichloroisothiazole Plant-Activator Candidate LY5-24-2 by UPLC-Q-TOF

Hydrolysis characteristics of a novel 3,4-dichloroisothiazole based fungicide with activating plant defense responses as a candidate plant-activator LY5-24-2 were investigated under different conditions (pH and temperature) using ultra-performance liquid chromatography (UPLC) and quadrupole Time-of-Flight (Q-TOF). The hydrolysis case complied with the first-order kinetic model, with half-lives ranging from 4.8h to 3.2days at pH 4, 7, 9 and temperature at 25 and 50℃. One of the hydrolysis metabolite 3,4-dichloroisothiazole-5-carboxylic acid (metabolite 1, M1) was determined and quantified using authentic standard. The other hydrolysate 3-chloro-5-(trifluoromethyl) pyridin-2-amine (metabolite 2, M2) was determined and identified according to accurate mass information, fragmentation patterns and principle component analysis (PCA). By utilizing high-resolution mass spectrometry and multivariate statistical analysis, hydrolysis dynamic of the metabolites was characterized and figured out. This research provided a non-target screening method to analyze hydrolysis metabolites of a new plant-activator and to find its degradation products in aqueous solution.

Nicotinamide Effectively Suppresses Fusarium Head Blight in Wheat Plants.

Pyridine nucleotides such as a nicotinamide adenine dinucleotide (NAD) are known as plant defense activators. We previously reported that nicotinamide mononucleotide (NMN) enhanced disease resistance against fungal pathogen Fusarium graminearum in barley and Arabidopsis. In this study, we reveal that the pretreatment of nicotinamide (NIM), which does not contain nucleotides, effectively suppresses disease development of Fusarium Head Blight (FHB) in wheat plants. Correspondingly, deoxynivalenol (DON) mycotoxin accumulation was also significantly decreased by NIM pretreatment. A metabolome analysis showed that several antioxidant and antifungal compounds such as trigonelline were significantly accumulated in the NIM-pretreated spikes after inoculation of F. graminearum. In addition, some metabolites involved in the DNA hypomethylation were accumulated in the NIM-pretreated spikes. On the other hand, fungal metabolites DON and ergosterol peroxide were significantly reduced by the NIM pretreatment. Since NIM is relative stable and inexpensive compared with NMN and NAD, it may be more useful for the control of symptoms of FHB and DON accumulation in wheat and other crops.

Chitosan and Chitin Deacetylase Activity Are Necessary for Development and Virulence of Ustilago maydis.

The biotrophic fungus Ustilago maydis harbors a chitin deacetylase (CDA) family of six active genes as well as one pseudogene which are differentially expressed during colonization. This includes one secreted soluble CDA (Cda4) and five putatively glycosylphosphatidylinositol (GPI)-anchored CDAs, of which Cda7 belongs to a new class of fungal CDAs. Here, we provide a comprehensive functional study of the entire family. While budding cells of U. maydis showed a discrete pattern of chitosan staining, biotrophic hyphae appeared surrounded by a chitosan layer. We purified all six active CDAs and show their activity on different chitin substrates. Single as well as multiple cda mutants were generated and revealed a virulence defect for mutants lacking cda7 We implicated cda4 in production of the chitosan layer surrounding biotrophic hyphae and demonstrated that the loss of this layer does not reduce virulence. By combining different cda mutations, we detected redundancy as well as specific functions for certain CDAs. Specifically, certain combinations of mutations significantly affected virulence concomitantly with reduced adherence, appressorium formation, penetration, and activation of plant defenses. Attempts to inactivate all seven cda genes simultaneously were unsuccessful, and induced depletion of cda2 in a background lacking the other six cda genes illustrated an essential role of chitosan for cell wall integrity.IMPORTANCE The basidiomycete Ustilago maydis causes smut disease in maize, causing substantial losses in world corn production. This nonobligate pathogen penetrates the plant cell wall with the help of appressoria and then establishes an extensive biotrophic interaction, where the hyphae are tightly encased by the plant plasma membrane. For successful invasion and development in plant tissue, recognition of conserved fungal cell wall components such as chitin by the plant immune system needs to be avoided or suppressed. One strategy to achieve this lies in the modification of chitin to chitosan by chitin deacetylases (CDAs). U. maydis has seven cda genes. This study reveals discrete as well as redundant contributions of these genes to virulence as well as to cell wall integrity. Unexpectedly, the inactivation of all seven genes is not tolerated, revealing an essential role of chitosan for viability.

Research progress in the interactions of fungal pathogens and insect pests during host plant colonization

Microbial pathogens and insect pests frequently share the same host plant, and both can cause severe damage. When surviving on the same host, the fungal pathogens and pests can have positive effects, negative effects, or no effect on each other. Plant pathogens influence feeding preference, oviposition, growth, development, dispersal, and population dynamics of pests by inducing the accumulation of volatile compounds, soluble sugars, starch, free amino acids, secondary metabolites, and antioxidant enzymes. Insect pests, as vectors, transmit pathogens; reduce the incidence of diseases by directly feeding on fungal spores, mycelium, or toxic fungal compounds; or increase plant susceptibility to pathogens by consuming plant tissues and afford access for pathogens that is enhanced by wounding. Pests and pathogens also activate plant defense systems as mediated by defense-related chemicals, such as jasmonic acid and salicylic acid, which subsequently regulate the expression of defense-related genes. The underlying mechanisms of “tripartite” interaction of pathogens, insect pests and host plants remain unclear, and detailed genomic and proteomic analyses using molecular biology techniques will provide more basic information for the integrated management of fungal pathogens and pests.

Yellow Vein Mosaic Disease in Okra (Abelmoschus esculentus L.): An Overview on Causal Agent, Vector and Management

Okra (Abelmoschus esculentus L.) belongs to the Malvaceae family and is one of the most essential and popular vegetables globally. It is rich in proteins, carbohydrates, and vitamins. Abiotic and biotic factors threaten okra productivity. Okra yellow vein mosaic disease (OYVMD) is the most destructive disease of okra. The causal agent, [(i.e., Okra yellow vein mosaic virus (OYVMV)] of this disease belongs to the family Geminiviridae and genus Begomovirus. OYVMV is a monopartite with additional ssDNA molecule. This virus has two components DNA-A for protein coding and DNA-B for symptoms induction. Whitefly transmits OYVMV in persistent manner. Characteristic symptoms of OYVMV infected okra plants are chlorosis, dwarfing, and yellowing of veins and fruits. High temperatures with moderate rainfall enhance the development of OYVMV disease and the whitefly population. However, high humidity with low temperature and rainfall has no significant role in developing the OYVMD and whitefly population. Moreover, the virus also affects the secondary metabolites in the infected okra plants. The virus can be managed through various strategies including the application of plant defense activators, the development of resistant varieties and by controlling its vector via pesticides and plant extracts. Various plant defense activators such as monopotassium phosphate (KH2PO), salicylic acid, benzoic acid, and citric acid enhance resistance in okra against OYMVD. In addition, the resistance to OYMVD can also be achieved by successfully incorporating high yielding but resistant cultivars of acceptable quality. In this review, we have discussed history, economic impact, symptomology, disease development under a natural environment, genetics and management of OYVMV.

Nickel content in field crop seeds and agricultural soil in Central Serbia

Nickel (Ni) is an essential nutrient for animals and it has an important role in many physiological and biochemical processes in higher plants. At the same time, it belongs to the group of potentially toxic elements (PTEs). The aim of this study is to determine Ni concentrations in the soil-plant relationships between the main crops and agricultural land in Central Serbia. A total of 71 bulked soil samples are taken from the topsoil at the depth of 0-30 cm in an area belonging to 6 statistical districts of Central Serbia. A total of 71 seed samples are collected during harvest as an average sample of seed from each observed plot, of which 26 are corn, 19 sunflower, 17 wheat, and 9 soybean samples. Analysis of the collected samples includs the main soil parameters and Ni total and available concentrations in soil, as well as Ni total concentration in seeds. The median value of total Ni concentration in soil is 44.8 mg kg-1, close to MAC. The median nickel concentration in wheat and corn seeds is 0.5 mg kg-1, while soybean and sunflower seeds have higher median Ni content of 8.40 and 10.26 mg kg-1, respectively. Bioaccumulation factors in seeds (BAF) in the present study ranges from 0.013 (corn) to 0.256 (soybean). According to statistically significant differences, all crops have equal total NiT concentration in soil, while the available NiA concentration differs in soils under corn and sunflower cultivars. Based on Ni concentration in seed and BAF, two groups are distinguished ? the group of soybeans and sunflowers with higher Ni content and the group of wheat and corn with lower Ni content in seed. The obtained differences confirm that plant species have a significant role in the bio?accumulation of Ni. The determined BAF parameter is in a statistically significant negative correlation with the total Ni content in soil in all observed crops except maize. However, the BAF parameter for maize alone is in a statistically significant negative correlation with the readily available Ni concentration in the soil. The obtained correlations indicate that higher Ni concentration in soil causes lower Ni concentration in seeds, which might be due to the activation of plant defense mechanism to preserve the reproductive organs ? seeds ? from harmful effects. Based on the obtained results, Ni concentration in seeds of the main field crops in Central Serbia is safe for feed and food usage. Increased content of Ni found in agricultural soils in Central Serbia requires constant monitoring for maintaining sustainable agriculture production.

Possibilities of using yeast in biological plant protection

The metabolic activity of yeasts, as well as their common occurrence in the environment make them a potential source of compounds that can be used in biological plant protection. The article presents health-promoting effects of yeast on plants. The pro-health effect of yeast is related to the ability to provide plants with dissolved nutrients. Yeasts can also indirectly activate plant defence mechanisms and improve plant health status. The bioremediation properties and antagonism of yeasts against numerous economically important phytopathogens play an important role here. The research is also indicated that yeasts (Pichia membranifaciens, Pichia fermentans and Meyrozyma guilliermondii) in vitro show an antagonistic activity against their phytopathogens (Alternaria alternata, Rhizoctonia solani and Colletotrichum coccodes). All the mentioned aspects of yeast activity can be useful in creating high-quality biofertilizers and biopesticides.

Potency of Non-Fungicide Chemicals for Maize Inducing Resistance against Downy Mildew

Downy mildew disease control is a big challenge in Indonesia due to ability of pathogen adaptation and favorable environmental condition to the disease. Self-resistance induction by activating plant defense mechanism is a valuable control method to be developed in the future due to its environmentally safety. This study aimed to determine potency of non-fungicide chemicals to suppress downy mildew of maize. The tested chemicals were benzoic acid, sodium benzoic acid, salicylic acid, thiamine, saccharin, and aspirin with a concentration of 2000 ppm. Disease incidence, disease severity, disease progress, leaf tissue lignification, plant growth, chlorophyll content, and plant dry weight were observed to find out the effect of the chemicals application in maize against downy mildew. Results showed that seed application of benzoic acid was able to reduce disease incidence and disease severity by 80%. Benzoic acid treated plants were categorized as moderately resistant until 5-week observation, while inoculated plants (positive control) were susceptible. Based on AUDPC values, plants induced by the chemicals had slower disease development rates compared to positive control. Lignification around the stomata occured in all inoculated plants treated by chemicals. Plant growth between chemical treated plants and negative control (non-inoculated plant) mainly showed no significant different, elaborated that those chemicals were applicable. Chlorophyll content in chemical treated plants was also similar to those of negative control plant. Based on these results, benzoic acid was promoted to be further investigated as maize resistance inducer against downy mildew.

The Plant Sesquiterpene Nootkatone Efficiently Reduces Heterodera schachtii Parasitism by Activating Plant Defense.

Plant parasitic nematodes, including the beet cyst nematode Heterodera schachtii, constitute a devastating problem for crops worldwide. The limited availability of sustainable management options illustrates the need for new eco-friendly control means. Plant metabolites represent an invaluable source of active compounds for the discovery of such novel antagonistic agents. Here, we evaluated the impact of eight plant terpenoids on the H. schachtii parasitism of Arabidopsis thaliana. None of the metabolites affected the plant development (5 or 10 ppm). Nootkatone decreased the number of adult nematodes on A. thaliana to 50%, with the female nematodes being smaller compared to the control. In contrast, three other terpenoids increased the parasitism and/or female size. We discovered that nootkatone considerably decreased the number of nematodes that penetrated A. thaliana roots, but neither affected the nematode viability or attraction to plant roots, nor triggered the production of plant reactive oxygen species or changed the plant’s sesquiterpene profile. However, we demonstrated that nootkatone led to a significant upregulation of defense-related genes involved in salicylic and jasmonic acid pathways. Our results indicate that nootkatone is a promising candidate to be developed into a novel plant protection agent acting as a stimulator of plant immunity against parasitic nematodes.

Oligosaccharides: Defense Inducers, Their Recognition in Plants, Commercial Uses and Perspectives.

Plants have innate immune systems or defense mechanisms that respond to the attack of pathogenic microorganisms. Unlike mammals, they lack mobile defense cells, so defense processes depend on autonomous cellular events with a broad repertoire of recognition to detect pathogens, which compensates for the lack of an adaptive immune system. These defense mechanisms remain inactive or latent until they are activated after exposure or contact with inducing agents, or after the application of the inductor; they remain inactive only until they are affected by a pathogen or challenged by an elicitor from the same. Resistance induction represents a focus of interest, as it promotes the activation of plant defense mechanisms, reducing the use of chemical synthesis pesticides, an alternative that has even led to the generation of new commercial products with high efficiency, stability and lower environmental impact, which increase productivity by reducing not only losses but also increasing plant growth. Considering the above, the objective of this review is to address the issue of resistance induction with a focus on the potential of the use of oligosaccharides in agriculture, how they are recognized by plants, how they can be used for commercial products and perspectives.

Metabolomics Reveal Induction of ROS Production and Glycosylation Events in Wheat Upon Exposure to the Green Leaf Volatile Z-3-Hexenyl Acetate.

The activation and priming of plant defense upon perception of green leaf volatiles (GLVs) have often been reported. However, information as to which metabolic pathways in plants are affected by GLVs remains elusive. We report the production of reactive oxygen species in the tip of young wheat leaves followed by activation of antioxidant-related enzyme activity. In this study, we aimed to uncover metabolic signatures upon exposure to the GLV Z-3-hexenyl acetate (Z-3-HAC). By using an untargeted metabolomics approach, we observed changes in the phenylpropanoid pathways which yield metabolites that are involved in many anti-oxidative processes. Furthermore, exposure to GLV, followed by infection with Fusarium graminearum (Fg), induced significantly greater changes in the phenylpropanoid pathway compared to a sole Z-3-HAC treatment. Fragmentation of a selection of metabolites, which are significantly more upregulated in the Z-3-HAC + Fg treatment, showed D-glucose to be present as a substructure. This suggests that Z-3-HAC induces early glycosylation processes in plants. Additionally, we identified the presence of hexenyl diglycosides, which indicates that aerial Z-3-HAC is metabolized in the leaves by glycosyltransferases. Together these data indicate that GLV Z-3-HAC is taken up by leaves and incites oxidative stress. This subsequently results in the modulation of the phenylpropanoid pathway and an induction of glycosylation processes.

Negative impact of long-term exposure of salinity and drought stress on native Tetraena mandavillei L.

Tetraena mandavillei L. is a perennial shrub native to the Middle Eastern countries of Asia, which is extensively regarded as a drought-tolerant plant. However, the plant reduces growth and biomass when grown in high concentrations of sodium chloride in the soil. We conducted a pot experiment to influence the negative impact of different levels of salinity (0, 10, and 20 dSm-1 ) and drought stress (100, 80, 60, and 40% water field capacity), to study different growth-related parameters, physiological alterations and ion uptake by T. mandavillei. Both salinity and drought stress caused a negative impact by affecting several attributes of T. mandavillei, but the plants showed some resistance against drought stress conditions in terms of growth and biomass. In addition to that, we noticed that a combinatorial and individual impact of drought and salinity stress decreased photosynthetic pigments and gas exchange parameters in T. mandavillei. Results also depicted that the combination of the abiotic stress conditions drought and salinity induced reactive oxygen species (ROS), indicating that the plants undergo oxidative damaged. However, due to the active plant defense system, the plant enhanced its performance under abiotic stress conditions, but due to the severe drought condition (40% water field capacity), a significant (P < 0.05) decrease in the activities of antioxidant compounds was caused. Furthermore, osmolytes also increased under both salinity and drought stress conditions in this study. Our results also showed that increased salinity and drought stress in the soil caused a significant increase in sodium (Na+ ) and chloride (Cl- ) ions in roots and shoots of T. mandavillei. In contrast to that, the contents of Calcium (Ca2+ ) and potassium (K+ ) were decreased in all organs of the plants with increasing levels of salinity and drought stress. Taken together, T. mandavillei can be classified as a facultative halophyte with the ability to tolerate drought stress and using salt accumulation mechanisms to tolerate salinity stress.

A Nod factor- and type III secretion system-dependent manner for Robinia pseudoacacia to establish symbiosis with Mesorhizobium amorphae CCNWGS0123.

Under nitrogen-limiting conditions, symbiotic nodulation promotes the growth of legume plants via the fixation of atmospheric nitrogen to ammonia by rhizobia in root nodules. The rhizobial Nod factor (NF) and type III secretion system (T3SS) are two key signaling pathways for establishing the legume-rhizobium symbiosis. However, whether NF signaling is involved in the nodulation of Robinia pseudoacacia and Mesorhizobium amorphae CCNWGS0123, and its symbiotic differences compared with T3SS signaling remain unclear. Therefore, to elucidate the function of NF signaling in nodulation, we mutated nodC in M. amorphae CCNWGS0123, which aborted NF synthesis. Compared with the plants inoculated with the wild type strain, the plants inoculated with the NF-deficient strain exhibited shorter shoots with etiolated leaves. These phenotypic characteristics were similar to those of the plants inoculated with the T3SS-deficient strain, which served as a Nod- (non-effective nodulation) control. The plants inoculated with both the NF- and T3SS-deficient strains formed massive root hair swellings, but no normal infection threads were detected. Sections of the nodules showed that inoculation with the NF- and T3SS-deficient strains induced small, white bumps without any rhizobia inside. Analyzing the accumulation of 6 plant hormones and the expression of 10 plant genes indicated that the NF- and T3SS-deficient strains activated plant defense reactions while suppressing plant symbiotic signaling during the perception and nodulation processes. The requirement for NF signaling appeared to be conserved in two other leguminous trees that can establish symbiosis with M. amorphae CCNWGS0123. In contrast, the function of the T3SS might differ among species, even within the same subfamily (Faboideae). Overall, this work demonstrated that nodulation of R. pseudoacacia and M. amorphae CCNWGS0123 was both NF and T3SS dependent.

Advancement of research on plant NLRs evolution, biochemical activity, structural association, and engineering.

In this review, we have included evolution of plant intracellular immune receptors, oligomeric complex formation, enzymatic action, engineering, and mechanisms of immune inspection for appropriate defense outcomes. NLR (Nucleotide binding oligomerization domain containing leucine-rich repeat) proteins are the intracellular immune receptors that recognize pathogen-derived virulence factors to confer effector-triggered immunity (ETI). Activation of plant defense by the NLRs are often conveyed through N-terminal Toll-like/ IL-1 receptor (TIR) or non-TIR (coiled-coils or CC) domains. Homodimerization or self-association property of CC/ TIR domains of plant NLRs contribute to their auto-activity and induction of in planta ectopic cell death. High resolution crystal structures of Arabidopsis thaliana RPS4TIR, L6TIR, SNC1TIR, RPP1TIR and Muscadinia rotundifolia RPV1TIR showed that interaction is mediated through one or two distinct interfaces i.e., αA and αE helices comprise AE interface and αD and αE helices were found to form DE interface. By contrast, conserved helical regions were determined for CC domains of plant NLRs. Evolutionary history of NLRs diversification has shown that paired forms were originated from NLR singletons. Plant TIRs executed NAD+ hydrolysis activity for cell death promotion. Plant NLRs were found to form large oligomeric complexes as observed in animal inflammasomes. We have also discussed different protein engineering methods includes domain shuffling, and decoy modification that increase effector recognition spectrum of plant NLRs. In summary, our review highlights structural basis of perception of the virulence factors by NLRs or NLR pairs to design novel classes of plant immune receptors.