Induction of defense-related genes in tomato plants after treatments with the biocontrol agents Pseudomonas chlororaphis ToZa7 and Clonostachys rosea IK726

Plant genotype-specific modulation of Clonostachys rosea-mediated biocontrol of septoria tritici blotch disease in wheat

BACKGROUND

Beneficial microorganisms can act as biological control agents (BCAs) directly by targeting pathogens or indirectly by enhancing the plant's defense mechanisms against pathogens. However, efficiencies with which plants benefit from BCAs vary, potentially because of genetic variation in plants for plant-BCA compatibility. The aim of this study was to explore the genetic variation in winter wheat for modulation of Clonostachys rosea-mediated biocontrol of septoria tritici blotch disease caused by the fungal pathogen Zymoseptoria tritici.

RESULTS

In total, 202 winter wheat genotypes, including landraces and old cultivars grown from 1900 onwards in the Scandinavian countries, were tested under greenhouse-controlled conditions. Foliar spray applications of the pathogen and the fungal BCA in two treatments, i.e., Z. tritici (Zt) alone and Z. tritici along with C. rosea (ZtCr) were used to assess the disease progress over time. The absence and presence of C. rosea in Zt and ZtCr, respectively, allowed the dissection of variation for plant disease resistance and biocontrol efficacy. The study showed significant (P < 0.05) phenotypic variation among plant genotypes for disease progression in both Zt and ZtCr treatments. Moreover, the application of C. rosea resulted in a significant (P < 0.05) reduction in disease progression for seven genotypes and increased disease progression for eleven genotypes, indicating a plant genotype-dependent effect on the interaction between wheat, C. rosea and Z. tritici. For the phenotypic variation in disease progress and biocontrol efficacy, a genome-wide association study using a 20K single-nucleotide polymorphism (SNP) marker array was also performed. In total, five distinct SNP markers associated with disease resistance and four SNP markers associated with C. rosea biocontrol efficacy were identified.

CONCLUSIONS

This work serves as a foundation to further characterize the genetic basis of plant-BCA interactions when inoculated with Z. tritici, facilitating opportunities for simultaneous breeding for disease resistance and biocontrol efficacy.

Bacillus endophytes for sustainable management of tomato spotted wilt virus and yield production

BACKGROUND

Tomato-spotted wilt virus (TSWV) from the Tospovirus genus affects over 1000 plant species, including key crops, and traditional control methods often prove inadequate. This study investigates the effectiveness of Bacillus amyloliquefaciens and Bacillus subtilis in reducing TSWV infection, enhancing plant growth, and strengthening defense in Nicotiana benthamiana. The aim is to assess Bacillus as a sustainable biocontrol alternative, offering an eco-friendly solution for managing TSWV disease in agriculture.

RESULTS

Here, we report the efficacy of five Bacillus isolates (out of 15 tested) - B. amyloliquefaciens (DJB5, YN48, YN28, Mg6) and B. subtilis L1-21 - significantly reducing TSWV copies per gram in N. benthamiana leaves, using a half-leaf assay. In glasshouse trials, isolates DJB5, YN48, and Mg6 decreased TSWV copies per gram by 75.7%, 83.6%, and 88.2%, with biocontrol efficacy rates of 91.2%, 94.1%, and 95.7% respectively. All the isolates consistently mitigated the symptoms of TSWV, reduced the disease severity, and area under the disease progress curve (AUDPC) at 21 days post-inoculation. Additionally, these isolates enhanced plant growth parameters, including shoot and root length, leaf number, area, and biomass. The application of endophytes in the infected plants activated antioxidant defense enzymes by elevating the activities of polyphenol oxidase (PPO), peroxidase (POD), superoxide dismutase (SOD), and chitinase. However, defense-related enzymes, such as malondialdehyde (MDA), catalase (CAT), phenylalanine ammonia-lyase (PAL), total phenol, and β-1,3-glucanase decreased as TSWV infection reduced in the leaves.

CONCLUSION

Our findings indicate that B. amyloliquefaciens isolates, DJB5, YN48, and Mg6, effectively manage TSWV by activating plant defense, reducing virus load, reducing TSWV symptoms, and promoting plant growth. © 2024 Society of Chemical Industry.

Genotypic variation in winter wheat for fusarium foot rot and its biocontrol using Clonostachys rosea

Biological control to manage plant diseases is an environmentally friendly alternative to using chemical pesticides. However, little is known about the role of genetic variation in plants affecting the efficacy of biological control agents (BCAs). The aim of this study was to explore the genetic variation in winter wheat for disease susceptibility to fusarium foot rot caused by Fusarium graminearum and variation in biocontrol efficacy of the fungal BCA Clonostachys rosea to control the disease. In total, 190 winter wheat genotypes were evaluated under controlled conditions in two treatments, i.e. (i) F. graminearum (Fg) and (ii) F. graminearum infection on C. rosea treated seeds (FgCr). Alongside disease severity, plant growth-related traits such as shoot length and root length were also measured. Comparison of genotypes between the two treatments enabled the dissection of genotypic variation for disease resistance and C. rosea efficacy. The study revealed significant variation among plant genotypes for fusarium foot rot susceptibility and other growth traits in treatment Fg. Moreover, significant variation in C. rosea efficacy was also observed in genotype contrasts between the two treatments for all traits. Using a 20K marker array, a genome-wide association study was also performed. We identified a total of 18 significant marker-trait associations for disease resistance and C. rosea efficacy for all the traits. Moreover, the markers associated with disease resistance and C. rosea efficacy were not co-localized, highlighting the independent inheritance of these traits, which can facilitate simultaneous selection for cultivar improvement.

RNA silencing is a key regulatory mechanism in the biocontrol fungus Clonostachys rosea-wheat interactions

BACKGROUND

Small RNA (sRNAs)- mediated RNA silencing is emerging as a key player in host-microbe interactions. However, its role in fungus-plant interactions relevant to biocontrol of plant diseases is yet to be explored. This study aimed to investigate Dicer (DCL)-mediated endogenous and cross-kingdom gene expression regulation in the biocontrol fungus Clonostachys rosea and wheat roots during interactions.

RESULTS

C. rosea Δdcl2 strain exhibited significantly higher root colonization than the WT, whereas no significant differences were observed for Δdcl1 strains. Dual RNA-seq revealed the upregulation of CAZymes, membrane transporters, and effector coding genes in C. rosea, whereas wheat roots responded with the upregulation of stress-related genes and the downregulation of growth-related genes. The expression of many of these genes was downregulated in wheat during the interaction with DCL deletion strains, underscoring the influence of fungal DCL genes on wheat defense response. sRNA sequencing identified 18 wheat miRNAs responsive to C. rosea, and three were predicted to target the C. rosea polyketide synthase gene pks29. Two of these miRNAs (mir_17532_x1 and mir_12061_x13) were observed to enter C. rosea from wheat roots with fluorescence analyses and to downregulate the expression of pks29, showing plausible cross-kingdom RNA silencing of the C. rosea gene by wheat miRNAs.

CONCLUSIONS

We provide insights into the mechanisms underlying the interaction between biocontrol fungi and plant roots. Moreover, the study sheds light on the role of sRNA-mediated gene expression regulation in C. rosea-wheat interactions and provides preliminary evidence of cross-kingdom RNA silencing between plants and biocontrol fungi.

Characterization and control of Rhizoctonia solani affecting lucky bamboo (Dracaena sanderiana hort. ex. Mast.) using some bioagents

In a survey conducted during the period of March-May 2019 in nurseries, warehouses, and shops at three governorates (Alexandria, El-Behera, and Giza governorates, Egypt), symptoms of root rot, basal stem rot, and wilt disease complex were observed in the lucky bamboo (Dracaena sanderiana hort. ex. Mast.). The highest disease infection percentage was found in lucky bamboo collected from Alexandria City (47.67%), while the highest disease severity was in lucky bamboo collected from El-Behera Governorate (35.19%). Rhizoctonia solani, Fusarium oxysporum, F. solani, Aspergillus niger, and Alternaria alternate were isolated and identified in the infected lucky bamboo samples. R. solani isolates were the most dominant among the recovered fungal species with a percentage of 80.89% of the total isolates (246). Pathogenicity tests showed that R. solani was the most pathogen with 100% disease infection and 76.67% disease severity. Molecular identification characterized R. solani isolate as R. solani AUMC 15120, MZ723906. Meanwhile, four biological control agents (bioagents) were isolated from the healthy lucky bamboo samples and identified based on cultural, morphological, microscopic characteristics, and the molecular phylogenetic analysis as Clonostachys rosea AUMC 15121, OL461708; Bacillus circulans TAG1, MW441316; B. siamensis TAP1, MW441318 and Ochrobactrum anthropi TAM1, MW441317. The four bioagents showed potential inhibition of R. solani in vitro as well as in vivo on lucky bamboo plants in vase treatments compared to the untreated inoculated control as well as certain fungicides and biocides used (Moncut, Rizolex-T, Topsin-M, Bio-Zeid, and Bio-Arc). The bioagent O. anthropi showed the highest inhibition growth (85.11%) of the in vitro R. solani colony, which was not significantly different from the biocide Bio-Arc (83.78%). However, C. rosea, B. siamensis and B. circulans showed inhibition values of 65.33, 64.44, and 60.44%, respectively. On the other hand, the biocide Bio-Zeid showed less inhibitory effect (43.11%), while the lowest growth inhibition was recorded by Rizolex-T (34.22%) and Topsin-M (28.67%). Furthermore, the in vivo experiment supported the in vitro results for the most effective treatments, where all the treatments significantly decreased the percentage of infection and disease severity compared to the inoculated untreated control. Additionally, the bioagent O. anthropi showed the highest effect, i.e., the lowest disease incidence and disease severity being 13.33% and 10%, compared to 100% and 75%, respectively, in the untreated inoculated control. This was not significantly different from the fungicide Moncut (13.33% and 21%) and from the bioagent C. rosea (20% and 15%) treatments for both parameters, respectively. In conclusion, the bioagents O. anthropi MW441317 at 1 × 10⁸ CFU/ml as well as C. rosea AUMC15121 at 1 × 10⁷/ml proved to be efficient to control R. solani causing root rot, and basal stem rot on lucky bamboo, compared to fungicide Moncut and can be used for disease management without the negative impact of the chemical control. Furthermore, this is the first report of the isolation and identification of Rhizoctonia solani, a pathogenic fungus, and four biocontrol agents (Bacillus circulans, B. siamensis, Ochrobactrum anthropi and Clonostachys rosea) associated with the healthy lucky bamboo plants.

Biocontrol Effect of Clonostachys rosea on Fusarium graminearum Infection and Mycotoxin Detoxification in Oat (Avena sativa)

Oat (Avena sativa) is susceptible to Fusarium head blight (FHB). The quality of oat grain is threatened by the accumulation of mycotoxins, particularly the trichothecene deoxynivalenol (DON), which also acts as a virulence factor for the main pathogen Fusarium graminearum. The plant can defend itself, e.g., by DON detoxification by UGT-glycosyltransferases (UTGs) and accumulation of PR-proteins, even though these mechanisms do not deliver effective levels of resistance. We studied the ability of the fungal biocontrol agent (BCA) Clonostachys rosea to reduce FHB and mycotoxin accumulation. Greenhouse trials showed that C. rosea-inoculation of oat spikelets at anthesis 3 days prior to F. graminearum inoculation reduced both the amount of Fusarium DNA (79%) and DON level (80%) in mature oat kernels substantially. DON applied to C. rosea-treated spikelets resulted in higher conversion of DON to DON-3-Glc than in mock treated plants. Moreover, there was a significant enhancement of expression of two oat UGT-glycosyltransferase genes in C. rosea-treated oat. In addition, C. rosea treatment activated expression of genes encoding four PR-proteins and a WRKY23-like transcription factor, suggesting that C. rosea may induce resistance in oat. Thus, C. rosea IK726 has strong potential to be used as a BCA against FHB in oat as it inhibits F. graminearum infection effectively, whilst detoxifying DON mycotoxin rapidly.

Insights into the ecological generalist lifestyle of Clonostachys fungi through analysis of their predicted secretomes

Introduction

The fungal secretome comprise diverse proteins that are involved in various aspects of fungal lifestyles, including adaptation to ecological niches and environmental interactions. The aim of this study was to investigate the composition and activity of fungal secretomes in mycoparasitic and beneficial fungal-plant interactions.

Methods

We used six Clonostachys spp. that exhibit saprotrophic, mycotrophic and plant endophytic lifestyles. Genome-wide analyses was performed to investigate the composition, diversity, evolution and gene expression of Clonostachys secretomes in relation to their potential role in mycoparasitic and endophytic lifestyles.

Results and discussion

Our analyses showed that the predicted secretomes of the analyzed species comprised between 7 and 8% of the respective proteomes. Mining of transcriptome data collected during previous studies showed that 18% of the genes encoding predicted secreted proteins were upregulated during the interactions with the mycohosts Fusarium graminearum and Helminthosporium solani. Functional annotation of the predicted secretomes revealed that the most represented protease family was subclass S8A (11-14% of the total), which include members that are shown to be involved in the response to nematodes and mycohosts. Conversely, the most numerous lipases and carbohydrate-active enzyme (CAZyme) groups appeared to be potentially involved in eliciting defense responses in the plants. For example, analysis of gene family evolution identified nine CAZyme orthogroups evolving for gene gains (p ≤ 0.05), predicted to be involved in hemicellulose degradation, potentially producing plant defense-inducing oligomers. Moreover, 8-10% of the secretomes was composed of cysteine-enriched proteins, including hydrophobins, important for root colonization. Effectors were more numerous, comprising 35-37% of the secretomes, where certain members belonged to seven orthogroups evolving for gene gains and were induced during the C. rosea response to F. graminearum or H. solani. Furthermore, the considered Clonostachys spp. possessed high numbers of proteins containing Common in Fungal Extracellular Membranes (CFEM) modules, known for their role in fungal virulence. Overall, this study improves our understanding of Clonostachys spp. adaptation to diverse ecological niches and establishes a basis for future investigation aiming at sustainable biocontrol of plant diseases.

An avirulent Ralstonia solanacearum strain FJAT1458 outcompetes with virulent strain and induces tomato plant resistance against bacterial wilt

BACKGROUND

Bacterial wilt (BW) caused by Ralstonia solanacearum (RS) is considered as one of the most destructive plant diseases. An avirulent strain of RS, FJAT1458, is a potential biocontrol agent of BW. In this study, the mechanism of FJAT1458 against BW was evaluated.

RESULTS

FJAT1458 was tagged with the red fluorescent protein gene, and the resulting strain was named as FJAT1458-RFP. When FJAT1458-RFP and FJAT91-GFP (a virulent strain of RS labelled with the green fluorescent protein gene), were co-inoculated in potted tomato plants, the colonization of FJAT91-GFP reached an almost undetectable level at 7 days post-inoculation (dpi) in the roots and at 9 dpi in rhizosphere soil. When they were co-inoculated in a hydroponic tomato growing system, numbers of the two strains were similar at 3 dpi in the root tissues; however, FJAT91-GFP was not detected at 9 dpi while FJAT1458-RFP maintained 1.77 × 10⁵  CFU g^-1 . The inoculation of FJAT1458-RFP alone or combination with FJAT91-GFP significantly increased tomato root activity. Moreover, expression levels of the defense-related genes PR-1a, GLUA, and CHI3 in tomato roots were significantly up-regulated by FJAT1458-RFP and co-inoculation of FJAT1458-RFP and FJAT91-GFP at 5 dpi, compared to the control (water, CK) treatment. Noteworthy, expression levels of GLUA in the treatments of FJAT1458-RFP and FJAT1458-RFP + FJAT91-GFP were 12.22- and 12.05-fold higher than that in the CK at 5 dpi, respectively.

CONCLUSIONS

The results suggested that the avirulent strain FJAT1458-RFP could suppress colonization of the virulent strain in tomato roots, and induce tomato plant resistance against BW. © 2022 Society of Chemical Industry.

Diagnosis of Induced Resistance State in Tomato Using Artificial Neural Network Models Based on Supervised Self-Organizing Maps and Fluorescence Kinetics

The aim of this study was to develop three supervised self-organizing map (SOM) models for the automatic recognition of a systemic resistance state in plants after application of a resistance inducer. The pathosystem Fusarium oxysporum f. sp. radicis-lycopersici (FORL) + tomato was used. The inorganic, defense inducer, Acibenzolar-S-methyl (benzo-[1,2,3]-thiadiazole-7-carbothioic acid-S-methyl ester, ASM), reported to induce expression of defense genes in tomato, was applied to activate the defense mechanisms in the plant. A handheld fluorometer, FluorPen FP 100-MAX-LM by SCI, was used to assess the fluorescence kinetics response of the induced resistance in tomato plants. To achieve recognition of resistance induction, three models of supervised SOMs, namely SKN, XY-F, and CPANN, were used to classify fluorescence kinetics data, in order to determine the induced resistance condition in tomato plants. To achieve this, a parameterization of fluorescence kinetics curves was developed corresponding to fluorometer variables of the Kautsky Curves. SKN was the best supervised SOM, achieving 97.22% to 100% accuracy. Gene expression data were used to confirm the accuracy of the supervised SOMs.

Burkholderia ambifaria XN08: A plant growth-promoting endophytic bacterium with biocontrol potential against sharp eyespot in wheat

Plant growth-promoting bacteria (PGPB) have been considered promising biological agents to increase crop yields for years. However, the successful application of PGPB for biocontrol of sharp eyespot in wheat has been limited, partly by the lack of knowledge of the ecological/environmental factors affecting the colonization, prevalence, and activity of beneficial bacteria on the crop. In this study, an endophytic bacterium XN08 with antagonistic activity against Rhizoctonia cerealis (wheat sharp eyespot pathogenic fungus), isolated from healthy wheat plants, was identified as Burkholderia ambifaria according to the sequence analysis of 16S rRNA. The antibiotic synthesis gene amplification and ultra-performance liquid chromatography-quadrupole time-of-flight mass spectrometry (UPLC-QTOF-MS) analyses were used to characterize the secondary metabolites. The results showed that the known powerful antifungal compound named pyrrolnitrin was produced by the strain XN08. In addition, B. ambifaria XN08 also showed the capacity for phosphate solubilization, indole-3-acetic acid (IAA), protease, and siderophore production in vitro. In the pot experiments, a derivate strain carrying the green fluorescent protein (GFP) gene was used to observe its colonization in wheat plants. The results showed that GFP-tagged B. ambifaria could colonize wheat tissues effectively. This significant colonization was accompanied by an enhancement of wheat plants' growth and an induction of immune resistance for wheat seedlings, which was revealed by the higher activities of polyphenol oxidase (PPO), peroxidase (POD), and phenylalanine ammonia-lyase (PAL). As far as we know, this is the first report describing the colonization traits of B. ambifaria in wheat plants. In addition, our results indicated that B. ambifaria XN08 might serve as a new effective biocontrol agent against wheat sharp eyespot disease caused by R. cerealis.

Interplay between Arabidopsis thaliana Genotype, Plant Growth and Rhizosphere Colonization by Phytobeneficial Phenazine-Producing Pseudomonas chlororaphis

Rhizosphere colonization by phytobeneficial Pseudomonas spp. is pivotal in triggering their positive effects on plant health. Many Pseudomonas spp. Determinants, involved in rhizosphere colonization, have already been deciphered. However, few studies have explored the role played by specific plant genes in rhizosphere colonization by these bacteria. Using isogenic Arabidopsis thaliana mutants, we studied the effect of 20 distinct plant genes on rhizosphere colonization by two phenazine-producing P. chlororaphis strains of biocontrol interest, differing in their colonization abilities: DTR133, a strong rhizosphere colonizer and ToZa7, which displays lower rhizocompetence. The investigated plant mutations were related to root exudation, immunity, and root system architecture. Mutations in smb and shv3, both involved in root architecture, were shown to positively affect rhizosphere colonization by ToZa7, but not DTR133. While these strains were not promoting plant growth in wild-type plants, increased plant biomass was measured in inoculated plants lacking fez, wrky70, cbp60g, pft1 and rlp30, genes mostly involved in plant immunity. These results point to an interplay between plant genotype, plant growth and rhizosphere colonization by phytobeneficial Pseudomonas spp. Some of the studied genes could become targets for plant breeding programs to improve plant-beneficial Pseudomonas rhizocompetence and biocontrol efficiency in the field.

Activity of bacteria isolated from bats against Pseudogymnoascus destructans in China

White-nose syndrome, a disease that is caused by the psychrophilic fungus Pseudogymnoascus destructans, has threatened several North America bat species with extinction. Recent studies have shown that East Asian bats are infected with P. destructans but show greatly reduced infections. While several factors have been found to contribute to these reduced infections, the role of specific microbes in limiting P. destructans growth remains unexplored. We isolated three bacterial strains with the ability to inhibit P. destructans, namely, Pseudomonas yamanorum GZD14026, Pseudomonas brenneri XRD11711 and Pseudomonas fragi GZD14479, from bats in China. Pseudomonas yamanorum, with the highest inhibition score, was selected to extract antifungal active substance. Combining mass spectrometry (MS) and nuclear magnetic resonance (NMR) spectroscopy analyses, we identified the active compound inhibiting P. destructans as phenazine-1-carboxylic acid (PCA), and the minimal inhibitory concentration (MIC) was 50.12 μg ml-1 . Whole genome sequencing also revealed the existence of PCA biosynthesis gene clusters. Gas chromatography-mass spectrometry (GC-MS) analysis identified volatile organic compounds. The results indicated that 10 ppm octanoic acid, 100 ppm 3-tert-butyl-4-hydroxyanisole (isoprenol) and 100 ppm 3-methyl-3-buten-1-ol (BHA) inhibited the growth of P. destructans. These results support that bacteria may play a role in limiting the growth of P. destructans on bats.

Biosolid-Amended Soil Enhances Defense Responses in Tomato Based on Metagenomic Profile and Expression of Pathogenesis-Related Genes

Biosolid application is an effective strategy, alternative to synthetic chemicals, for enhancing plant growth and performance and improving soil properties. In previous research, biosolid application has shown promising results with respect to tomato resistance against Fusarium oxysporum f. sp. radicis-lycopersici (Forl). Herein, we aimed at elucidating the effect of biosolid application on the plant-microbiome response mechanisms for tomato resistance against Forl at a molecular level. More specifically, plant-microbiome interactions in the presence of biosolid application and the biocontrol mechanism against Forl in tomato were investigated. We examined whether biosolids application in vitro could act as an inhibitor of growth and sporulation of Forl. The effect of biosolid application on the biocontrol of Forl was investigated based on the enhanced plant resistance, measured as expression of pathogen-response genes, and pathogen suppression in the context of soil microbiome diversity, abundance, and predicted functions. The expression of the pathogen-response genes was variably induced in tomato plants in different time points between 12 and 72 h post inoculation in the biosolid-enriched treatments, in the presence or absence of pathogens, indicating activation of defense responses in the plant. This further suggests that biosolid application resulted in a successful priming of tomato plants inducing resistance mechanisms against Forl. Our results have also demonstrated that biosolid application alters microbial diversity and the predicted soil functioning, along with the relative abundance of specific phyla and classes, as a proxy for disease suppression. Overall, the use of biosolid as a sustainable soil amendment had positive effects not only on plant health and protection, but also on growth of non-pathogenic antagonistic microorganisms against Forl in the tomato rhizosphere and thus, on plant-soil microbiome interactions, toward biocontrol of Forl.

Gene expression and metabolite analysis in barley inoculated with net blotch fungus and plant growth-promoting rhizobacteria

Net blotch, caused by the ascomycete Drechslera teres, can compromise barley production. Beneficial bacteria strains are of substantial interest as biological agents for plant protection in agriculture. Belonging to the genus Paraburkholderia, a bacterium, referred to as strain B25, has been identified as protective for barley against net blotch. The strain Paraburkholderia phytofirmans (strain PsJN), which has no effect on the pathogen's growth, has been used as control. In this study, the expression of target genes involved in cell wall-related processes, defense responses, carbohydrate and phenylpropanoid pathways was studied under various conditions (with or without pathogen and/or with or without bacterial strains) at different time-points (0-6-12-48 h). The results show that specific genes were subjected to a circadian regulation and that the expression of most of them increased in barley infected with D. teres and/or bacterized with the strain PsJN. On the contrary, a decreased gene expression was observed in the presence of strain B25. To complement and enrich the gene expression analysis, untargeted metabolomics was carried out on the same samples. The data obtained show an increase in the production of lipid compounds in barley in the presence of the pathogen. In addition, the presence of strain B25 leads to a decrease in the production of defense compounds in this crop. The results contribute to advance the knowledge on the mechanisms occurring at the onset of D. teres infection and in the presence of a biocontrol agent limiting the severity of net blotch in barley.

Host GRXC6 restricts Tomato yellow leaf curl virus infection by inhibiting the nuclear export of the V2 protein

Geminiviruses cause serious symptoms and devastating losses in crop plants. With a circular, single-stranded DNA genome, geminiviruses multiply their genomic DNA in the nucleus, requiring the nuclear shuttling of viral proteins and viral genomic DNAs. Many host factors, acting as proviral or antiviral factors, play key roles in geminivirus infections. Here, we report the roles of a tomato glutaredoxin (GRX), SlGRXC6, in the infection of Tomato yellow leaf curl virus (TYLCV), a single-component geminivirus. The V2 protein of TYLCV specifically and preferentially interacts with SlGRXC6 among the 55-member tomato GRX family that are broadly involved in oxidative stress responses, plant development, and pathogen responses. We show that overexpressed SlGRXC6 increases the nuclear accumulation of V2 by inhibiting its nuclear export and, in turn, inhibits trafficking of the V1 protein and viral genomic DNA. Conversely, the silenced expression of SlGRXC6 leads to an enhanced susceptibility to TYLCV. SlGRXC6 is also involved in symptom development as we observed a positive correlation where overexpression of SlGRXC6 promotes while knockdown of SlGRXC6 expression inhibits plant growth. We further showed that SlGRXC6 works with SlNTRC80, a tomato NADPH-dependent thioredoxin reductase, to regulate plant growth. V2 didn’t interact with SlNTRC80 but competed with SlNTR80 for binding to SlGRXC6, suggesting that the V2-disrupted SlGRXC6-SlNTRC80 interaction is partially responsible for the virus-caused symptoms. These results suggest that SlGRXC6 functions as a host restriction factor that inhibits the nuclear trafficking of viral components and point out a new way to control TYLCV infection by targeting the V2-SlGRXC6 interaction.

Geminiviruses infect numerous crops, induce a wide range of symptoms, and cause tremendous crop losses annually. Tomato yellow leaf curl virus (TYLCV), a single-component geminivirus, is a causative agent leading to one of the most devastating tomato diseases in the world. As a single-stranded DNA virus, genomic replication occurs in the nucleus and therefore, the nuclear shuttling is a critical step of viral infection. The V2 protein of TYLCV is involved in symptom development and viral trafficking, among other steps, and hijacks host proteins for executing its functions. Nevertheless, host factors involved in the V2-mediated functions are not well addressed. We show that tomato GRXC6 (SlGRXC6) functions as a restriction factor of TYLCV infection by interacting with and preventing V2 from moving out of the nucleus, leading to the inhibited V2-mediated nuclear export of V1 and the V1-viral DNA complex. SlGRXC6 also contributes to symptom development via its interaction with SINTRC80. V2 sequesters SlGRXC6 from forming the SlGRXC6-SlNTRC80 complex and regulates plant growth. Our work, therefore, identified a new host partner of V2 and revealed the mechanisms whereby V2 functions as a pathogenicity determinant and can be targeted for virus control.

Biological Control of Take-All and Growth Promotion in Wheat by Pseudomonas chlororaphis YB-10

Wheat is a worldwide staple food crop, and take-all caused by Gaeumannomyces graminis var. tritici can lead to a tremendous decrease in wheat yield and quality. In this study, strain YB-10 was isolated from wheat rhizospheric soil and identified as Pseudomonas chlororaphis by morphology and 16S rRNA gene sequencing. Pseudomonas chlororaphis YB-10 had extracellular protease and cellulase activities and strongly inhibited the mycelium growth of Gaeumannomyces graminis var. tritici in dual cultures. Up to 87% efficacy of Pseudomonas chlororaphis YB-10 in controlling the take-all of seedlings was observed in pot experiments when wheat seed was coated with the bacterium. Pseudomonas chlororaphis YB-10 was also positive for indole acetic acid (IAA) and siderophore production, and coating wheat seed with the bacterium significantly promoted the growth of seedlings at 107 and 108 CFU/mL. Furthermore, treatment with Pseudomonas chlororaphis YB-10 increased activities of the wheat defense-related enzymes POD, SOD, CAT, PAL and PPO in seedlings, indicating induced resistance against pathogens. Overall, Pseudomonas chlororaphis YB-10 is a promising new seed-coating agent to both promote wheat growth and suppress take-all.

Bacillus subtilis MBI600 Promotes Growth of Tomato Plants and Induces Systemic Resistance Contributing to the Control of Soilborne Pathogens

Bacillus subtilis MBI600 (Bs MBI600) is a recently commercialized plant-growth-promoting rhizobacterium (PGPR). In this study, we investigated the effects of Bs MBI600 on the growth of tomato and its biocontrol efficacy against three main soilborne tomato pathogens (Rhizoctonia solani, Pythium ultimum, and Fusarium oxysporum f.sp. radicis-lycopersici-Forl). Furthermore, the root colonization ability of the Bs MBI600 strain on tomato roots was analyzed in vivo with a yellow fluorescence protein (yfp)-labeled strain, revealing strong colonization ability, which was affected by the root growth substrate. The application of Bs MBI600 on tomato plants resulted in significant increases in shoot and root lengths. Transcriptional activation of two auxin-related genes (SiPin6 and SiLax4) was observed. Single applications of Bs MBI600 on inoculated tomato plants with pathogens revealed satisfactory control efficacy compared to chemical treatment. Transcriptomic analysis of defense-related genes used as markers of the salicylic acid (SA) signaling pathway (PR-1A and GLUA) or jasmonic acid/ethylene (JA/ET) signaling pathway (CHI3, LOXD, and PAL) showed increased transcription patterns in tomato plants treated with Bs MBI600 or Forl. These results indicate the biochemical and molecular mechanisms that are activated after the application of Bs MBI600 on tomato plants and suggest that induction of systemic resistance (ISR) occurred.

Microbial Inoculation for Productivity Improvements and Potential Biological Control in Sugar Beet Crops

Used mainly for sucrose production, sugar beet is one of the most important crops in Castilla y León (Spain). Several studies have demonstrated the benefits of microorganisms in different crop management programs, among which Plant Growth Promoting Rhizobacteria (PGPR). This research aims to assess the beneficial effects of two PGPRs strains (Pseudomonas fluorescens Pf0-1 and Pseudomonas chlororaphis CECT 462) on sugar beet (Beta vulgaris) production. Three treatments: a PGPRs co-inoculation assay of untreated seeds without any chemical treatment (TB), a conventional treatment with commercial seeds and fungicide application (TT); and a control with seeds without protective coating, bacterial inoculation and chemical treatment (ST). The efficacy of PGPRs inoculation on sugar beet production was determined measuring periodically the photosynthetic status of plants, and the final yield and quality of tubers. Aerial and root plant biomass, maximum beet perimeter, polarization, and sugar values of the sugar beet plants inoculated with PGPRs showed higher values and significant differences to sugar beet subjected to other treatments. We could see that PGPRs inoculation (TB treatment) produced significant differences in the quantum yield of PSII (ΦPSII). TB showed the highest value for ΦPSII and the NPQ (non-photochemical quenching), the lowest value, even though the PSII (maximum quantum yield of photosystem II) was very similar in all treatments. The two assayed PGPR strains triggered a significant increase in sugar beet production yield and quality. PGPRs inoculation techniques could be used in different crops and they could be applied as biofertilizers, improving the agricultural production.

Biocontrol and Plant-Growth-Promoting Traits of Talaromyces apiculatus and Clonostachys rosea Consortium against Ganoderma Basal Stem Rot Disease of Oil Palm

Basal stem rot (BSR) disease caused by Ganoderma boninense basidiomycetous fungus is the most economically important disease in oil palms in South East Asia. Unfortunately, there is no single most effective control measure available. Tremendous efforts have been directed in incorporation of environmentally friendly biocontrol approaches in minimizing BSR disease. This study investigated the performance of two potential biocontrol agents (BCAs), AAT0115 and AAB0114 strains recovered from oil palm on suppression of BSR in planta, and also assessed their plant-growth-promoting (PGP) performance. ITS rRNA-sequence phylogeny discriminated the two ascomycetous Talaromyces apiculatus (Ta) AT0115 and Clonostachys rosea (Cr) AAB0114 biocontrol species with PGP characteristics. In vitro studies have demonstrated both Ta and Cr are capable of reducing linear mycelial growth of G. boninense. Inoculation of individual Cr and Ta—as well as Cr+Ta consortium—induced a significant increment in leaf area and bole girth of oil-palm seedlings five months post-inoculation (MPI) under nursery conditions. At five months post-inoculation, shoot and root biomass, and nutrient contents (nitrogen, phosphorus, potassium, calcium, magnesium and boron) were significantly higher in Ta-inoculated seedlings compared to control treated with non-Ta-inoculated maize. Chlorophyll and carotenoids contents in rapidly growing oil-palm seedlings challenged with Cr, Ta or a combination of both were not negatively affected. Cr, Ta and Cr+Ta consortium treated seedlings had 4.9–60% BSR disease reduction compared to the untreated control. Co-inoculation of Cr and Ta resulted in increased BSR control efficiencies by 18–26% (compared with Cr only) and 48–55% (compared with Ta only). Collectively, Cr and Ta, either individually or in consortium showed potential as BSR biocontrol agents while also possess PGP traits in oil palm.

Biology and applications of Clonostachys rosea

Clonostachys rosea is a promising saprophytic filamentous fungus that belongs to phylum Ascomycota. Clonostachys rosea is widespread around the world and exists in many kinds of habitats, with the highest frequency in soil. As an excellent mycoparasite, C. rosea exhibits strong biological control ability against numerous fungal plant pathogens, nematodes and insects. These behaviours are based on the activation of multiple mechanisms such as secreted cell-wall-degrading enzymes, production of antifungal secondary metabolites and induction of plant defence systems. Besides having significant biocontrol activity, C. rosea also functions in the biodegradation of plastic waste, biotransformation of bioactive compounds, as a bioenergy sources and in fermentation. This mini review summarizes information about the biology and various applications of C. rosea and expands on its possible uses.