The interactions of Trichoderma at multiple trophic levels: inter-kingdom communication

Trichoderma spp. are universal saprotrophic fungi in terrestrial ecosystems, and as rhizosphere inhabitants, they mediate interactions with other soil microorganisms, plants, and arthropods at multiple trophic levels. In the rhizosphere, Trichoderma can reduce the abundance of phytopathogenic microorganisms, which involves the action of potent inhibitory molecules, such as gliovirin and siderophores, whereas endophytic associations between Trichoderma and the seeds and roots of host plants can result in enhanced plant growth and crop productivity, as well as the alleviation of abiotic stress. Such beneficial effects are mediated via the activation of endogenous mechanisms controlled by phytohormones such as auxins and abscisic acid, as well as by alterations in host plant metabolism. During either root colonization or in the absence of physical contact, Trichoderma can trigger early defense responses mediated by Ca²⁺ and reactive oxygen species, and subsequently stimulate plant immunity by enhancing resistance mechanisms regulated by the phytohormones salicylic acid, jasmonic acid, and ethylene. In addition, Trichoderma release volatile organic compounds and nitrogen or oxygen heterocyclic compounds that serve as signaling molecules, which have effects on plant growth, phytopathogen levels, herbivorous insects, and at the third trophic level, play roles in attracting the natural enemies (predators and parasitoids) of herbivores. In this paper, we review some of the most recent advances in our understanding of the environmental influences of Trichoderma spp., with particular emphasis on their multiple interactions at different trophic levels.

Resequencing and comparative genomics of Stagonospora nodorum: sectional gene absence and effector discovery

Stagonospora nodorum is an important wheat (Triticum aestivum) pathogen in many parts of the world, causing major yield losses. It was the first species in the large fungal Dothideomycete class to be genome sequenced. The reference genome sequence (SN15) has been instrumental in the discovery of genes encoding necrotrophic effectors that induce disease symptoms in specific host genotypes. Here we present the genome sequence of two further S. nodorum strains (Sn4 and Sn79) that differ in their effector repertoire from the reference. Sn79 is avirulent on wheat and produces no apparent effectors when infiltrated onto many cultivars and mapping population parents. Sn4 is pathogenic on wheat and has virulences not found in SN15. The new strains, sequenced with short-read Illumina chemistry, are compared with SN15 by a combination of mapping and de novo assembly approaches. Each of the genomes contains a large number of strain-specific genes, many of which have no meaningful similarity to any known gene. Large contiguous sections of the reference genome are absent in the two newly sequenced strains. We refer to these differences as “sectional gene absences.” The presence of genes in pathogenic strains and absence in Sn79 is added to computationally predicted properties of known proteins to produce a list of likely effector candidates. Transposon insertion was observed in the mitochondrial genomes of virulent strains where the avirulent strain retained the likely ancestral sequence. The study suggests that short-read enabled comparative genomics is an effective way to both identify new S. nodorum effector candidates and to illuminate evolutionary processes in this species.

Functional analysis of oxidative burst in sugarcane smut-resistant and -susceptible genotypes

Smut pathogen induced an early modulation of the production and scavenging of reactive oxygen species during defence responses in resistant sugarcane that coincided with the developmental stages of fungal growth. Sporisorium scitamineum is the causal agent of sugarcane smut disease. In this study, we characterized sugarcane reactive oxygen species (ROS) metabolism in response to the pathogen in smut-resistant and -susceptible genotypes. Sporisorium scitamineum teliospore germination and appressorium formation coincided with H₂O₂ accumulation in resistant plants. The superoxide dismutase (SOD) activity was not responsive in any of the genotypes; however, a higher number of isoenzymes were detected in resistant plants. In addition, related to resistance were lipid peroxidation, a decrease in catalase (CAT), and an increase in glutathione S-transferase (GST) activities and an earlier transcript accumulation of ROS marker genes (CAT3, CATA, CATB, GST31, GSTt3, and peroxidase 5-like). Furthermore, based on proteomic data, we suggested that the source of the increased hydrogen peroxide (H₂O₂) may be due to a protein of the class III peroxidase, which was inhibited in the susceptible genotype. H₂O₂ is sensed and probably transduced through overlapping systems related to ascorbate-glutathione and thioredoxin to influence signalling pathways, as revealed by the presence of thioredoxin h-type, ascorbate peroxidase, and guanine nucleotide-binding proteins in the infected resistant plants. Altogether, our data depicted the balance of the oxidative burst and antioxidant enzyme activity in the outcome of this interaction.

Endophytic fungi associated with Monarda citriodora, an aromatic and medicinal plant and their biocontrol potential

CONTEXT

The Food and Agriculture Organization has estimated that every year considerable losses of the food crops occur due to plant diseases. Although fungicides are extensively used for management of plant diseases, they are expensive and hazardous to the environment and human health. Alternatively, biological control is the safe way to overcome the effects of plant diseases and to sustain agriculture. Since Monarda citriodora Cerv. ex Lag. (Lamiaceae/Labiatae) is known for its antifungal properties, it was chosen for the study.

OBJECTIVE

The isolation of endophytic fungi from M. citriodora and assessing their biocontrol potential.

MATERIAL AND METHODS

The isolated endophytes were characterized using ITS-5.8 S rDNA sequencing. Their biocontrol potential was assessed using different antagonistic assays against major plant pathogens.

RESULTS

Twenty-eight endophytes representing 11 genera were isolated, of which, around 82% endophytes showed biocontrol potential against plant pathogens. MC-2 L (Fusarium oxysporum), MC-14 F (F. oxysporum), MC-22 F (F. oxysporum) and MC-25 F (F. redolens) displayed significant antagonistic activity against all the tested pathogens. Interestingly, MC-10 L (Muscodor yucatanensis) completely inhibited the growth of Sclerotinia sp., Colletotrichum capsici, Aspergillus flavus and A. fumigatus in dual culture assay, whereas MC-8 L (A. oryzae) and MC-9 L (Penicillium commune) completely inhibited the growth of the Sclerotinia sp. in fumigation assay.

CONCLUSIONS

Endophytes MC-2 L, MC-14 F, MC-22 F and MC-25 F could effectively be used to control broad range of phytopathogens, while MC-10 L, MC-8 L and MC-9 L could be used to control specific pathogens. Secondly, endophytes showing varying degrees of antagonism in different assays represented the chemo-diversity not only as promising biocontrol agents but also as a resource of defensive and bioactive metabolites.

Biocontrol Potential of Siderophore Producing Heavy Metal Resistant Alcaligenes sp. and Pseudomonas aeruginosa RZS3 vis-à-vis Organophosphorus Fungicide

In present study in vitro phytopathogen suppression activity of siderophoregenic preparations of Ni and Mn resistant Alcaligenes sp. STC1 and Pseudomonas aeruginosa RZS3 SH-94B isolated from soil were found superior over the chemical pesticide. Siderophore rich culture broth and siderophore rich supernatant exerted antifungal activity against Aspergillus niger NCIM 1025, Aspergillus flavus NCIM 650, Fusarium oxysporum NCIM 1281, Alternaria alternata ARI 715, Cercospora arachichola, Metarhizium anisopliae NCIM 1311 and Pseudomonas solanacerum NCIM 5103. Siderophore rich broth and supernatant exhibited potent antifungal activity vis-à-vis oraganophosphorus chemical fungicide; kitazine. The minimum fungicidal concentration required was 25 μl for Aspergillus niger, Aspergillus flavus, Fusarium oxysporum, Cercospora arachichola, Metarhizium anisopliae, Pseudomonas solanacerum and 75 μl for A. alternata.

The N-Glycan cluster from Xanthomonas campestris pv. campestris: a toolbox for sequential plant N-glycan processing

N-Glycans are widely distributed in living organisms but represent only a small fraction of the carbohydrates found in plants. This probably explains why they have not previously been considered as substrates exploited by phytopathogenic bacteria during plant infection. Xanthomonas campestris pv. campestris, the causal agent of black rot disease of Brassica plants, possesses a specific system for GlcNAc utilization expressed during host plant infection. This system encompasses a cluster of eight genes (nixE to nixL) encoding glycoside hydrolases (GHs). In this paper, we have characterized the enzymatic activities of these GHs and demonstrated their involvement in sequential degradation of a plant N-glycan using a N-glycopeptide containing two GlcNAcs, three mannoses, one fucose, and one xylose (N2M3FX) as a substrate. The removal of the α-1,3-mannose by the α-mannosidase NixK (GH92) is a prerequisite for the subsequent action of the β-xylosidase NixI (GH3), which is involved in the cleavage of the β-1,2-xylose, followed by the α-mannosidase NixJ (GH125), which removes the α-1,6-mannose. These data, combined to the subcellular localization of the enzymes, allowed us to propose a model of N-glycopeptide processing by X. campestris pv. campestris. This study constitutes the first evidence suggesting N-glycan degradation by a plant pathogen, a feature shared with human pathogenic bacteria. Plant N-glycans should therefore be included in the repertoire of molecules putatively metabolized by phytopathogenic bacteria during their life cycle.

Fungal Competitors Affect Production of Antimicrobial Lipopeptides in Bacillus subtilis Strain B9-5

Bacillus subtilis has shown success in antagonizing plant pathogens where strains of the bacterium produce antimicrobial cyclic lipopeptides (CLPs) in response to microbial competitors in their ecological niche. To gain insight into the inhibitory role of these CLPs, B. subtilis strain B9-5 was co-cultured with three pathogenic fungi. Inhibition of mycelial growth and spore germination was assessed and CLPs produced by B. subtilis B9-5 were quantified over the entire period of microbial interaction. B. subtilis B9-5 significantly inhibited mycelial growth and spore germination of Fusarium sambucinum and Verticillium dahliae, but not Rhizopus stolonifer. LC-MS analysis revealed that B. subtilis differentially produced fengycin and surfactin homologs depending on the competitor. CLP quantification suggested that the presence of Verticillium dahliae, a fungus highly sensitive to the compounds, caused an increase followed by a decrease in CLP production by the bacterium. In co-cultures with Fusarium sambucinum, a moderately sensitive fungus, CLP production increased more gradually, possibly because of its slower rate of spore germination. With co-cultures of the tolerant fungus Rhizopus stolonifer, B. subtilis produced high amounts of CLPs (per bacterial cell) for the duration of the interaction. Variations in CLP production could be explained, in part, by the pathogens' overall sensitivities to the bacterial lipopeptides and/or the relative growth rates between the plant pathogen and B. subtilis. CLP production varied substantially temporally depending on the targeted fungus, which provides valuable insight concerning the effectiveness of B. subtilis B9-5 protecting its ecological niche against the ingress of these pathogens.

Metabolomic applications for understanding complex tripartite plant-microbes interactions: Strategies and perspectives

Phytopathogens from the Alternaria sp., Fusarium sp., Penicillium sp., and Pseudomonas sp. and their toxigenic metabolites - alternariol, fumonisin, citrinin, and coronatine respectively, negatively impact crop yields and sales by eliciting plant diseases and/or causing human and veterinary toxicoses upon the consumption of contaminated food. These phytopathogens and their associated toxins, however, are present and most likely in undetectable concentrations pre-harvest and post-harvest of many major staple crops. Metabolomic approaches have been used extensively for better characterizing and diagnosing human disease, plant disease and, their etiological agents. Their use in agro-industrial research focusing specifically on tripartite (plant - toxicogenic microbe - beneficial microbe) interactions is, however, limited. Since new approaches for eradicating food-borne pathogens, increasing crop productivity and improving agro-international trade are being sought worldwide, the consequent integration of metabolomic approaches and perspectives in crop protection strategies for better understanding plant - toxicogenic microbe - beneficial microbe interaction in tandem is discussed.

Quercivorol as a lure for the polyphagous and Kuroshio shot hole borers, Euwallacea spp. nr. fornicatus (Coleoptera: Scolytinae), vectors of Fusarium dieback

The polyphagous shot hole borer and Kuroshio shot hole borer, two members of the Euwallacea fornicatus species complex (Coleoptera: Curculionidae: Scolytinae), are invasive ambrosia beetles that harbor distinct species of Fusarium fungal symbionts. Together with the damage caused by gallery construction, these two phytopathogenic Fusarium species are responsible for the emerging tree disease Fusarium dieback, which affects over 50 common tree species in Southern California. Host trees suffer branch dieback as the xylem is blocked by invading beetles and fungi, forcing the costly removal of dead and dying trees in urban areas. The beetles are also threatening natural riparian habitats, and avocado is susceptible to Fusarium dieback as well, resulting in damage to the avocado industries in California and Israel. Currently there are no adequate control mechanisms for shot hole borers. This paper summarizes efforts to find a suitable lure to monitor shot hole borer invasions and dispersal. Field trials were conducted in two counties in Southern California over a span of two years. We find that the chemical quercivorol is highly attractive to these beetles, and perform subsequent field experiments attempting to optimize this lure. We also explore other methods of increasing trap catch and effects of other potential attractants, as well as the deterrents verbenone and piperitone.

Mycovirus associated hypovirulence, a potential method for biological control of Fusarium species

Fusarium is a large genus of filamentous fungi belongs to the division Ascomycota and was first described as Fusisporium. Innumerable members of this genus act as pathogens, endophytes and saprophytes and can be recovered from plants and soils worldwide. Many of these members are known to be phytopathogens. It is among the most diverse and widely dispersed phyto-pathogenic fungi which cause economically important blights, rots, wilts and cankers of many ornamental, field, horticultural and forest crops both in agricultural commodities and natural ecosystems. Some species, e.g. F. graminearum and F. verticillioides have a narrow host range and mainly infect the cereals, whereas F. oxysporum has effects on both monocotyledonous and dicotyledonous plants. Attempts have been made to control the diseases caused by Fusarium sp. and to minimize crop yield losses. Till date, effective and eco-friendly methods have not been devised for the control of this devastating pathogen. A new potential of using mycovirus associated hypovirulence as biocontrol method against Fusarium species has been proposed. The present review taking into account of worldwide researches to provide possible insights for Fusarium-mycovirus coevolution.

Threats and opportunities of plant pathogenic bacteria

Plant pathogenic bacteria can have devastating effects on plant productivity and yield. Nevertheless, because these often soil-dwelling bacteria have evolved to interact with eukaryotes, they generally exhibit a strong adaptivity, a versatile metabolism, and ingenious mechanisms tailored to modify the development of their hosts. Consequently, besides being a threat for agricultural practices, phytopathogens may also represent opportunities for plant production or be useful for specific biotechnological applications. Here, we illustrate this idea by reviewing the pathogenic strategies and the (potential) uses of five very different (hemi)biotrophic plant pathogenic bacteria: Agrobacterium tumefaciens, A. rhizogenes, Rhodococcus fascians, scab-inducing Streptomyces spp., and Pseudomonas syringae.

Comparative genomic analysis of Erwinia amylovora reveals novel insights in phylogenetic arrangement, plasmid diversity, and streptomycin resistance

Erwinia amylovora is a destructive pathogen of Rosaceous plants and an economic concern worldwide. Herein, we report 93 new E. amylovora genomes from North America, Europe, the Mediterranean, and New Zealand. This new genomic information demonstrates the existence of three primary clades of Amygdaloideae (apple and pear) infecting E. amylovora and suggests all three independently originate from North America. The comprehensive sequencing also identified and confirmed the presence of 7 novel plasmids ranging in size from 2.9 to 34.7 kbp. While the function of the novel plasmids is unknown, the plasmids pEAR27, pEAR28, and pEAR35 encoded for type IV secretion systems. The strA-strB gene pair and the K43R point mutation at codon 43 of the rpsL gene have been previously documented to confer streptomycin resistance. Of the sequenced isolates, rpsL-based streptomycin resistance was more common and was found with the highest frequency in the Western North American clade.

Breakpoint: Cell Wall and Glycoproteins and their Crucial Role in the Phytopathogenic Fungi Infection

The cell wall that surrounds fungal cells is essential for their survival, provides protection against physical and chemical stresses, and plays relevant roles during infection. In general, the fungal cell wall is composed of an outer layer of glycoprotein and an inner skeletal layer of β-glucans or α- glucans and chitin. Chitin synthase genes have been shown to be important for septum formation, cell division and virulence. In the same way, chitin can act as a potent elicitor to activate defense response in several plant species; however, the fungi can convert chitin to chitosan during plant infection to evade plant defense mechanisms. Moreover, α-1,3-Glucan, a non-degradable polysaccharide in plants, represents a key feature in fungal cell walls formed in plants and plays a protective role for this fungus against plant lytic enzymes. A similar case is with β-1,3- and β-1,6-glucan which are essential for infection, structure rigidity and pathogenicity during fungal infection. Cell wall glycoproteins are also vital to fungi. They have been associated with conidial separation, the increase of chitin in conidial cell walls, germination, appressorium formation, as well as osmotic and cell wall stress and virulence; however, the specific roles of glycoproteins in filamentous fungi remain unknown. Fungi that can respond to environmental stimuli distinguish these signals and relay them through intracellular signaling pathways to change the cell wall composition. They play a crucial role in appressorium formation and penetration, and release cell wall degrading enzymes, which determine the outcome of the interaction with the host. In this review, we highlight the interaction of phypatophogen cell wall and signaling pathways with its host and their contribution to fungal pathogenesis.

Coordinate regulation of Ustilago maydis ammonium transporters and genes involved in mating and pathogenicity

The dimorphic switch from budding to filamentous growth is an essential morphogenetic transition many fungi utilize to cause disease in the host. Although different environmental signals can induce filamentous growth, the developmental programs associated with transmitting these different signals may differ. Here, we explore the relationship between filamentation and expression levels of ammonium transporters (AMTs) that also sense low ammonium for Ustilago maydis, the pathogen of maize. Overexpression of the high affinity ammonium transporter, Ump2, under normally non-inducing conditions, results in filamentous growth. Furthermore, ump2 expression levels are correlated with expression of genes involved in the mating response pathway and in pathogenicity. Ump1 and Ump2 transcription levels also tracked expression of genes normally up-regulated during either filamentous growth or during growth of the fungus inside the host. Interestingly, haploid strains deleted for the b mating-type locus, like those deleted for ump2, failed to filament on low ammonium; they also shared some alterations in gene expression patterns with cells deleted for ump2 or over-expressing this gene. Deletion of ump2 either in both mating partners or in a solopathogenic haploid strain resulted in a dramatic reduction in disease severity for infected plants, suggesting some importance of this transceptor in the pathogenesis program.

TBRG-1 a Ras-like protein in Trichoderma virens involved in conidiation, development, secondary metabolism, mycoparasitism, and biocontrol unveils a new family of Ras-GTPases

Ras-GTPases are nucleotide hydrolases involved in key cellular processes. In fungi, Ras-GTPases regulate conidiation, development, virulence, and interactions with other fungi or plants. Trichoderma spp. are filamentous saprophytic fungi, widely distributed along all latitudes, characterized by their rapid growth and metabolic diversity. Many species of this genus interact with other fungi, animals or plants. Furthermore, these fungi are used as biocontrol agents due to their ability to antagonize phytopathogenic fungi and oomycetes, through competence, antibiosis, and parasitism. However, the genetic and molecular regulation of these processes is scarcely described in these fungi. In this work, we investigated the role of the gene tbrg-1 product (GenBank accession number XP_013956100; JGI ID: Tv_70852) of T. virens during its interaction with other fungi and plants. Sequence analyses predicted that TBRG-1 bears the characteristic domains of Ras-GTPases; however, its size (1011 aa) is 3- to 4-times bigger compared with classical GTPases. Interestingly, phylogenetic analyses grouped the TBRG-1 protein with hypothetical proteins of similar sizes, sharing conserved regions; whereas other known Ras-GTPases were perfectly grouped with their respective families. These facts led us to classify TBRG-1 into a new family of Ras-GTPases, the Big Ras-GTPases (BRG). Therefore, the gene was named tbrg-1 (TrichodermaBigRas-GTPase-1). Quantification of conidia and scanning electron microscopy showed that the mutants-lacking tbrg-1 produced less conidia, as well as a delayed conidiophore development compared to the wild-type (wt). Moreover, a deregulation of conidiation-related genes (con-10, con-13, and stuA) was observed in tbrg-1-lacking strains, which indicates that TBRG-1 is necessary for proper conidiophore and conidia development. Furthermore, the lack of tbrg-1 affected positively the antagonistic capability of T. virens against the phytopathogens Rhizoctonia solani, Sclerotium rolfsii, and Fusarium oxysporum, which was consistent with the expression patterns of mycoparasitism-related genes, sp1 and cht1, that code for a protease and for a chitinase, respectively. Furthermore, the antibiosis effect of mycelium-free culture filtrates of Δtbrg-1 against R. solani was considerably enhanced. The expression of secondary metabolism-related genes, particularly gliP, showed an upregulation in Δtbrg-1, which paralleled an increase in gliotoxin production as compared to the wt. These results indicate that TBRG-1 plays a negative role in secondary metabolism and antagonism. Unexpectedly, the biocontrol activity of Δtbrg-1 was ineffective to protect the tomato seeds and seedlings against R. solani. On the contrary, Δtbrg-1 behaved like a plant pathogen, indicating that TBRG-1 is probably implicated in the recognition process for establishing a beneficial relationship with plants.

The influence of a modified lipopolysaccharide O-antigen on the biosynthesis of xanthan in Xanthomonas campestris pv. campestris B100

Background

The exopolysaccharide xanthan is a natural product which is extensively used in industry. It is a thickening agent in many fields, from oil recovery to the food sector. Xanthan is produced by the Gram negative bacterium Xanthomonas campestris pv. campestris (Xcc). We analyzed the lipopolysaccharide (LPS) of three mutant strains of the Xcc wild type B100 to distinguish if the xanthan production can be increased when LPS biosynthesis is affected.

Results

The Xcc B100 O-antigen (OA) is composed of a linear main chain of rhamnose residues with N-acetylfucosamine (FucNAc) side branches at every second rhamnose. It is the major LPS constituent. The O-antigen was missing completely in the mutant strain H21012 (deficient in wxcB), since neither rhamnose nor FucNAc could be detected as part of the LPS by MALDI-TOF-MS, and only a slight amount of rhamnose and no FucNAc was found by GC analysis. The LPS of two other mutants was analyzed, Xcc H28110 (deficient in wxcK) and H20110 (wxcN). In both of them no FucNAc could be detected in the LPS fraction, while the rhamnose moieties were more abundant than in wild type LPS. The measurements were carried out by GC and confirmed by MALDI-TOF-MS analyses that indicated an altered OA in which the branches are missing, while the rhamnan main chain seemed longer than in the wild type. Quantification of xanthan confirmed our hypothesis that a missing OA can lead to an increased production of the extracellular polysaccharide. About 6.3 g xanthan per g biomass were produced by the Xcc mutant H21012 (wxcB), as compared to the wild type production of approximately 5 g xanthan per g biomass. In the two mutant strains with modified OA however, Xcc H28110 (wxcK) and Xcc H20110 (wxcN), the xanthan production of 5.5 g and 5.3 g, respectively, was not significantly increased.

Conclusions

Mutations affecting LPS biosynthesis can be beneficial for the production of the extracellular polysaccharide xanthan. However, only complete inhibition of the OA resulted in increased xanthan production. The inhibition of the FucNAc side branches did not lead to increased production, but provoked a novel LPS phenotype. The data suggests an elongation of the linear rhamnan main chain of the LPS OA in both the Xcc H28110 (wxcK) and Xcc H20110 (wxcN) mutant strains.

Electronic supplementary material

The online version of this article (doi:10.1186/s12866-016-0710-y) contains supplementary material, which is available to authorized users.

Reprogramming of phytopathogen transcriptome by a non-bactericidal pesticide residue alleviates its virulence in rice

Bacteria equipped with virulence systems based on highly bioactive small molecules can circumvent their host's defense mechanisms. Pathogens employing this strategy are currently threatening global rice production. In the present study, variations in the virulence of the highly destructive Burkholderia plantarii were observed in different rice-producing regions. The environment-linked variation was not attributable to any known host-related or external factors. Co-occurrence analyses indicated a connection between reduced virulence and 5-Amino-1,3,4-thiadiazole-2-thiol (ATT), a non-bactericidal organic compound. ATT, which accumulates in rice plants during metabolization of specific agrochemicals, was found to reduce virulence factor secretion by B. plantarii up to 88.8% and inhibit pathogen virulence by hijacking an upstream signaling cascade. Detailed assessment of the newly discovered virulence inhibitor resulted in mechanistic insights into positive effects of ATT accumulation in plant tissues. Mechanisms of virulence alleviation were deciphered by integrating high-throughput data, gene knockout mutants, and molecular interaction assays. TroK, a histidine protein kinase in a two-component system that regulates virulence factor secretion, is likely the molecular target antagonized by ATT. Our findings provide novel insights into virulence modulation in an important plant-pathogen system that relies on the host's metabolic activity and subsequent signaling interference.

Biogenic silver nanoparticles as an antibacterial agent against bacterial leaf blight causing rice phytopathogen Xanthomonas oryzae pv. oryzae

Silver nanoparticles (Ag NP) were produced utilizing leaf extract of rice cultivar Taichung native-1. Various factors like leaf extract, silver nitrate concentrations, and duration of autoclaving were standardized during synthesis. Nanoparticles were analyzed with UV-visible absorption spectroscopy (UV-vis), dynamic light scattering, zeta potential, X-ray diffraction and transmission electron microscopy techniques. The synthesis was noted at 0.4% extract, 0.6 mM silver nitrate, 30 min of autoclaving and NP formation was confirmed from 424 nm peak in UV-vis. NP showed zeta potential of - 27 mV, face-centered cubic (fcc) crystal nature and sized around 16.5 ± 5.9 nm. Biogenic NP synthesized from susceptible rice variety were used as an antibacterial agent against phytopathogen Xanthomonas oryzae pv. oryzae (Xoo), the causative agent of bacterial leaf blight (BLB) disease in rice. Antibacterial effect of Ag NP was evaluated using in vitro assays and in vivo efficacy under greenhouse conditions. Results confirmed effective inhibition of Xoo growth and colony formation by Ag NP and found to be the more powerful antibacterial agent. Besides, Ag NP treatment (10 µg/mL) caused an enhancement in seedling vigor index. Pots treated with Ag NP (15 μg/mL) in vivo in greenhouse showed disease severity of 26.6% and disease decrease over control of 49.2%, at a much lower NP concentration than earlier reported studies. Thus, the current report implies using the leaf extract synthesized Ag NP to control and BLB disease management in field conditions.

Neovascularization during leafy gall formation on Arabidopsis thaliana upon Rhodococcus fascians infection

Extensive de novo vascularization of leafy galls emerging upon Rhodococcus fascians infection is achieved by fascicular/interfascicular cambium activity and transdifferentiation of parenchyma cells correlated with increased auxin signaling. A leafy gall consisting of fully developed yet growth-inhibited shoots, induced by the actinomycete Rhodococcus fascians, differs in structure compared to the callus-like galls induced by other bacteria. To get insight into the vascular development accompanying the emergence of the leafy gall, the anatomy of infected axillary regions of the inflorescence stem of wild-type Arabidopsis thaliana accession Col-0 plants and the auxin response in pDR5:GUS-tagged plants were followed in time. Based on our observations, three phases can be discerned during vascularization of the symptomatic tissue. First, existing fascicular cambium becomes activated and interfascicular cambium is formed giving rise to secondary vascular elements in a basipetal direction below the infection site in the main stem and in an acropetal direction in the entire side branch. Then, parenchyma cells in the region between both stems transdifferentiate acropetally towards the surface of the developing symptomatic tissue leading to the formation of xylem and vascularize the hyperplasia as they expand. Finally, parenchyma cells in the developing gall also transdifferentiate to vascular elements without any specific direction resulting in excessive vasculature disorderly distributed in the leafy gall. Prior to any apparent anatomical changes, a strong auxin response is mounted, implying that auxin is the signal that controls the vascular differentiation induced by the infection. To conclude, we propose the "sidetracking gall hypothesis" as we discuss the mechanisms driving the formation of superfluous vasculature of the emerging leafy gall.

Evaluating the biocontrol potential of Canadian strain Bacillus velezensis 1B-23 via its surfactin production at various pHs and temperatures

BACKGROUND

Microorganisms, including Bacillus species are used to help control plant pathogens, thereby reducing reliance on synthetic pesticides in agriculture. Bacillus velezensis strain 1B-23 has been shown to reduce symptoms of bacterial disease caused by Clavibacter michiganensis subsp. michiganensis in greenhouse-grown tomatoes, with in vitro studies implicating the lipopeptide surfactin as a key antimicrobial. While surfactin is known to be effective against many bacterial pathogens, it is inhibitory to a smaller proportion of fungi which nonetheless cause the majority of crop diseases. In addition, knowledge of optimal conditions for surfactin production in B. velezensis is lacking.

RESULTS

Here, B. velezensis 1B-23 was shown to inhibit in vitro growth of 10 fungal strains including Candida albicans, Cochliobolus carbonum, Cryptococcus neoformans, Cylindrocarpon destructans Fusarium oxysporum, Fusarium solani, Monilinia fructicola, and Rhizoctonia solani, as well as two strains of C. michiganensis michiganensis. Three of the fungal strains (C. carbonum, C. neoformans, and M. fructicola) and the bacterial strains were also inhibited by purified surfactin (surfactin C, or [Leu7] surfactin C15) from B. velezensis 1B-23. Optimal surfactin production occurred in vitro at a relatively low temperature (16 °C) and a slightly acidic pH of 6.0. In addition to surfactin, B. velenzensis also produced macrolactins, cyclic dipeptides and minor amounts of iturins which could be responsible for the bioactivity against fungal strains which were not inhibited by purified surfactin C.

CONCLUSIONS

Our study indicates that B. velezensis 1B-23 has potential as a biocontrol agent against both bacterial and fungal pathogens, and may be particularly useful in slightly acidic soils of cooler climates.

In planta interactions of a novel bacteriophage against Pseudomonas syringae pv. tomato

The biology and biotechnology of bacteriophages have been extensively studied in recent years to explore new and environmentally friendly methods of controlling phytopathogenic bacteria. Pseudomonas syringae pv. tomato (Pst) is responsible for bacterial speck disease in tomato plants, leading to decreased yield. Disease management strategies rely on the use of copper-based pesticides. The biological control of Pst with the use of bacteriophages could be an alternative environmentally friendly approach to diminish the detrimental effects of Pst in tomato cultivations. The lytic efficacy of bacteriophages can be used in biocontrol-based disease management strategies. Here, we report the isolation and complete characterization of a bacteriophage, named Medea1, which was also tested in planta against Pst, under greenhouse conditions. The application of Medea1 as a root drenching inoculum or foliar spraying reduced 2.5- and fourfold on average, respectively, Pst symptoms in tomato plants, compared to a control group. In addition, it was observed that defense-related genes PR1b and Pin2 were upregulated in the phage-treated plants. Our research explores a new genus of Pseudomonas phages and explores its biocontrol potential against Pst, by utilizing its lytic nature and ability to trigger the immune response of plants. KEY POINTS: • Medea1 is a newly reported bacteriophage against Pseudomonas syringae pv. tomato having genomic similarities with the phiPSA1 bacteriophage • Two application strategies were reported, one by root drenching the plants with a phage-based solution and one by foliar spraying, showing up to 60- and 6-fold reduction of Pst population and disease severity in some cases, respectively, compared to control • Bacteriophage Medea1 induced the expression of the plant defense-related genes Pin2 and PR1b.

Synthesis, toxicity evaluation and determination of possible mechanisms of antimicrobial effect of arabinogalactane-capped selenium nanoparticles

BACKGROUND

The elemental selenium nanoparticles (Se⁰NPs) find application in biology and medicine due to wide spectrum of their biological activity combined with low toxicity. For instance, Se⁰NPs are promising antimicrobial agents for plant treatment against the bacterial phytopathogen Clavibacter michiganensis sepedonicus (Cms). Careful characterization of possible mechanisms of antimicrobial action of Se⁰NPs as well as the assessment of their biosafety for plant and animal organisms represents urgent challenge.

METHODS

AG-stabilized Se⁰NPs (AG/Se⁰NPs) were synthesized by oxidation of selenide-anions by molecular oxygen dissolved in the reaction medium in the presence of AG macromolecules. The antimicrobial activity of AG/Se⁰NPs against Cms was investigated both by observing the change in optical density of bacterial suspension and directly evaluating the cell death using fluorescent microscopy with propidium iodide staining. The effect of AG/Se⁰NPs on the dehydrogenase activity was studied by determination of Cms enzymes ability to reduce colorless TTC to formazan. The effect of AG/Se⁰NPs nanocomposite on the respiration rate of Cms cells was examined by polarographic method. For qualitative visualization of the potential on the inner membrane of Cms mesosomes, the potential-dependent TMRM dye and fluorescence microscopy were used. The toxicity of the AG/Se⁰NPs was investigated on white mice by the Litchfield-Wilcoxon method. The effect of AG/Se⁰NPs on plant organisms (potato plants) was studied on healthy and Cms-infected plants by determining the level of chlorophyll and lipid peroxidation products (LPO) in their leaves when treated with nanoparticles.

RESULTS

Spherical Se° nanoparticles with an average size of 94 nm were obtained using the stabilizing potential of AG. It was found that these nanoparticles exhibited the pronounced (up to 60 %) bacteriostatic action (in 6.25 μg/mL concentration) against the bacterial phytopathogen Cms. It was shown and experimentally confirmed for the first time that the probable causes of the bacteriostatic action of AG/Se°NPs against Cms are non-reversible inhibition of Cms cell respiration, a decrease of the transmembrane potential with a change in the cell wall permeability for H⁺ protons and a decrease in their dehydrogenase activity. It was revealed that the treatment of healthy and Cms-infected potato plants with an aqueous solution of AG/Se°NPs involved no significant changes in the content of LPO and negative effect on the chlorophyll content, thus contributing to the saving of these values at the level of control intact plants.

CONCLUSION

Using a complex of complementary methods, we have found that antimicrobial activity of AG/Se⁰NPs is apparently due to their ability to inhibit the dehydrogenase activity of Cms cells, as well as to disrupt the integrity of the cell membrane, resulting in a decrease of transmembrane potential and reduction of cellular respiration. The antimicrobial and antibiofilm activity of AG/Se⁰NPs, together with their nontoxicity and safety for plant and animal organisms, determine the prospects for design of AG/Se⁰NPs-based drugs for the rehabilitation of plants from the Cms.

Analysis of the in planta transcriptome expressed by the corn pathogen Pantoea stewartii subsp. stewartii via RNA-Seq

Pantoea stewartii subsp. stewartii is a bacterial phytopathogen that causes Stewart's wilt disease in corn. It uses quorum sensing to regulate expression of some genes involved in virulence in a cell density-dependent manner as the bacterial population grows from small numbers at the initial infection site in the leaf apoplast to high cell numbers in the xylem where it forms a biofilm. There are also other genes important for pathogenesis not under quorum-sensing control such as a Type III secretion system. The purpose of this study was to compare gene expression during an in planta infection versus either a pre-inoculum in vitro liquid culture or an in vitro agar plate culture to identify genes specifically expressed in planta that may also be important for colonization and/or virulence. RNA was purified from each sample type to determine the transcriptome via RNA-Seq using Illumina sequencing of cDNA. Fold gene expression changes in the in planta data set in comparison to the two in vitro grown samples were determined and a list of the most differentially expressed genes was generated to elucidate genes important for plant association. Quantitative reverse transcription PCR (qRT-PCR) was used to validate expression patterns for a select subset of genes. Analysis of the transcriptome data via gene ontology revealed that bacterial transporters and systems important for oxidation reduction processes appear to play a critical role for P. stewartii as it colonizes and causes wilt disease in corn plants.

Major phytopathogens and strains from cocoa (Theobroma cacao L.) are differentiated by MALDI-MS lipid and/or peptide/protein profiles

Phytopathogens are the main disease agents that promote attack of cocoa plantations in all tropical countries. The similarity of the symptoms caused by different phytopathogens makes the reliable identification of the diverse species a challenge. Correct identification is important in the monitoring and management of these pests. Here we show that matrix-assisted laser desorption ionization mass spectrometry (MALDI-MS) in combination with multivariate data analysis is able to rapidly and reliably differentiate cocoa phytopathogens, namely Moniliophthora perniciosa, Phytophthora palmivora, P. capsici, P. citrophthora, P. heveae, Ceratocystis cacaofunesta, C. paradoxa, and C. fimbriata. MALDI-MS reveals unique peptide/protein and lipid profiles which differentiate these phytopathogens at the level of genus, species, and single strain coming from different hosts or cocoa tissues collected in several plantations/places. This fast methodology based on molecular biomarkers is also shown to be sufficiently reproducible and selective and therefore seems to offer a suitable tool to guide the correct application of sanitary defense approaches for infected cocoa plantations. International trading of cocoa plants and products could also be efficiently monitored by MALDI-MS. It could, for instance, prevent the entry of new phytopathogens into a country, e.g., as in the case of Moniliophthora roreri fungus that is present in all cocoa plantations of countries bordering Brazil, but that has not yet attacked Brazilian plantations. Graphical Abstract Secure identification of phytopathogens attacking cocoa plantations has been demonstrated via typical chemical profiles provided by mass spectrometric screening.

Characterization and survival of broad-spectrum biocontrol agents against phytopathogenic fungi

The current study intended to isolate, characterize and identify biocontrol bacteria possessing broad-spectrum antifungal activity from the phyllosphere of different crops including maize, wheat and potato and to assess their growth-promoting activity. In this study 14/113 biocontrol bacteria showed antifungal activity. Bacterial isolates M11 and M33 from maize out of 113 were re-selected on the basis of their strong (more than 50%) broad spectrum antifungal activity after their assessment against four economically important phytopathogenic fungi including Alternaria alternata, Rhizoctonia solani, Fusarium oxysporum and Fusarium verticillioides. The isolates were further assessed for plant growth promoting traits, i.e., indole-3-acetic acid production, phosphate solubilization, production of cellulase, microbial volatile compounds, hydrogen cyanide and siderophores. All fourteen isolates showed positive results for the production of indole-3-acetic acid hormone and cellulase enzyme, 10 isolates were positive for hydrogen cyanide production; siderophores production was observed in 7 isolates while 5 isolates showed ability to solubilize inorganic phosphate. Microbial volatile compounds were only synthesized by M11 and M33, which were identified as Bacillus amyloliquefaciens and Bacillus subtilis respectively by 16S rRNA gene sequencing. The survival study revealed that biocontrol bacteria B. amyloliquefaciens and B. subtilis have the ability to survive in cost effective molasses containing carrier material up to a three-month period.