Necrotic and Cytolytic Activity on Grapevine Leaves Produced by Nep1-Like Proteins of Diplodia seriata

The secret password: Cell death-inducing proteins in filamentous phytopathogens - As versatile tools to develop disease-resistant crops

Cell death-inducing proteins (CDIPs) are some of the secreted effector proteins manifested by filamentous oomycetes and fungal pathogens to invade the plant tissue and facilitate infection. Along with their involvement in different developmental processes and virulence, CDIPs play a crucial role in plant-pathogen interactions. As the name implies, CDIPs cause necrosis and trigger localised cell death in the infected host tissues by the accumulation of higher concentrations of hydrogen peroxide (H2O2), oxidative burst, accumulation of nitric oxide (NO), and electrolyte leakage. They also stimulate the biosynthesis of defense-related phytohormones such as salicylic acid (SA), jasmonic acid (JA), abscisic acid (ABA), and ethylene (ET), as well as the expression of pathogenesis-related (PR) genes that are important in disease resistance. Altogether, the interactions result in the hypersensitive response (HR) in the host plant, which might confer systemic acquired resistance (SAR) in some cases against a vast array of related and unrelated pathogens. The CDIPs, due to their capability of inducing host resistance, are thus unique among the array of proteins secreted by filamentous plant pathogens. More interestingly, a few transgenic plant lines have also been developed expressing the CDIPs with added resistance. Thus, CDIPs have opened an interesting hot area of research. The present study critically reviews the current knowledge of major types of CDIPs identified across filamentous phytopathogens and their modes of action in the last couple of years. This review also highlights the recent breakthrough technologies in studying plant-pathogen interactions as well as crop improvement by enhancing disease resistance through CDIPs.

Secretome analysis of the phytopathogen Macrophomina phaseolina cultivated in liquid medium supplemented with and without soybean leaf infusion

Macrophomina phaseolina (Tassi) Goid. is a fungal pathogen that causes root and stem rot in several economically important crops. However, most of disease control strategies have shown limited effectiveness. Despite its impact on agriculture, molecular mechanisms involved in the interaction with host plant remains poorly understood. Nevertheless, it has been proven that fungal pathogens secrete a variety of proteins and metabolites to successfully infect their host plants. In this study, a proteomic analysis of proteins secreted by M. phaseolina in culture media supplemented with soybean leaf infusion was performed. A total of 250 proteins were identified with a predominance of hydrolytic enzymes. Plant cell wall degrading enzymes together peptidases were found, probably involved in the infection process. Predicted effector proteins were also found that could induce plant cell death or suppress plant immune response. Some of the putative effectors presented similarities to known fungal virulence factors. Expression analysis of ten selected protein-coding genes showed that these genes are induced during host tissue infection and suggested their participation in the infection process. The identification of secreted proteins of M. phaseolina could be used to improve the understanding of the biology and pathogenesis of this fungus. Although leaf infusion was able to induce changes at the proteome level, it is necessary to study the changes induced under conditions that mimic the natural infection process of the soil-borne pathogen M. phaseolina to identify virulence factors.

Characteristics, Roles and Applications of Proteinaceous Elicitors from Pathogens in Plant Immunity

In interactions between pathogens and plants, pathogens secrete many molecules that facilitate plant infection, and some of these compounds are recognized by plant pattern recognition receptors (PRRs), which induce immune responses. Molecules in both pathogens and plants that trigger immune responses in plants are termed elicitors. On the basis of their chemical content, elicitors can be classified into carbohydrates, lipopeptides, proteinaceous compounds and other types. Although many studies have focused on the involvement of elicitors in plants, especially on pathophysiological changes induced by elicitors in plants and the mechanisms mediating these changes, there is a lack of up-to-date reviews on the characteristics and functions of proteinaceous elicitors. In this mini-review, we provide an overview of the up-to-date knowledge on several important families of pathogenic proteinaceous elicitors (i.e., harpins, necrosis- and ethylene-inducing peptide 1 (nep1)-like proteins (NLPs) and elicitins), focusing mainly on their structures, characteristics and effects on plants, specifically on their roles in plant immune responses. A solid understanding of elicitors may be helpful to decrease the use of agrochemicals in agriculture and gardening, generate more resistant germplasms and increase crop yields.

Characterization and necrosis-inducing activity of necrosis- and ethylene-inducing peptide 1-like proteins from Colletotrichum australisinense, the causative agent of rubber tree anthracnose

Colletotrichum australisinense, a member of the Colletotrichum acutatum species complex, is an important pathogen causing rubber tree anthracnose. Genome-wide comparative analysis showed this species complex contains more genes encoding necrosis- and ethylene-inducing peptide 1-like proteins (NLPs) than other Colletotrichum species complexes, but little is known about their necrosis-inducing roles in host. The aim of this study was to analyze NLPs number and type in C. australisinense, and characterize their necrosis-inducing activity in host or non-host. According to phylogenetic relationship, conserved the cysteine residues and the heptapeptide motif (GHRHDWE), 11 NLPs were identified and classified into three types. Five of the eleven NLPs were evaluated for necrosis-inducing activity. CaNLP4 (type 1) could not induce necrosis in host or non-host plants. By contrast, both CaNLP5 and CaNLP9 (type 1) induced necrosis in host and non-host plants, and necrosis-inducing activity was strongest for CaNLP9. CaNLP10 (type 2) and CaNLP11 (type 3) induced necrosis in host but not non-host plants. Substitution of key amino acid residues essential for necrosis induction activity led to loss of CaNLP4 activity. Structural characterization of CaNLP5 and CaNLP9 may explain differences in necrosis-inducing activity. We evaluated the expression of genes coding CaNLP by reverse transcription polymerase chain reaction (RT-PCR) and quantitative real-time PCR (qRT-PCR) at different time-points after pathogen infection. It was found that genes encoding CaNLPs with different activities exhibited significantly different expression patterns. The results demonstrate that CaNLPs are functionally and spatially distinct, and may play different but important roles in C. australisinense pathogenesis.

Pathogenicity Factors of Botryosphaeriaceae Associated with Grapevine Trunk Diseases: New Developments on Their Action on Grapevine Defense Responses

Botryosphaeriaceae are a family of fungi associated with the decay of a large number of woody plants with economic importance and causing particularly great losses in viticulture due to grapevine trunk diseases. In recent years, major advances in the knowledge of the pathogenicity factors of these pathogens have been made possible by the development of next-generation sequencing. This review highlights the knowledge gained on genes encoding small secreted proteins such as effectors, carbohydrate-associated enzymes, transporters and genes associated with secondary metabolism, their representativeness within the Botryosphaeriaceae family and their expression during grapevine infection. These pathogenicity factors are particularly expressed during host-pathogen interactions, facilitating fungal development and nutrition, wood colonization, as well as manipulating defense pathways and inducing impacts at the cellular level and phytotoxicity. This work highlights the need for further research to continue the effort to elucidate the pathogenicity mechanisms of this family of fungi infecting grapevine in order to improve the development of control methods and varietal resistance and to reduce the development and the effects of the disease on grapevine harvest quality and yield.

Metabolomic analysis of sheath blight disease of rice (Oryza sativa L.) induced by Rhizoctonia solani phytotoxin

AIM

To understand the mechanism of necrosis incited by a host-selective phytotoxin designated as Rhizoctonia solani toxin (RST) identified to be a potential pathogenic factor of Rhizoctonia solani AG1 IA, causing sheath blight (ShB) of rice.

METHODS AND RESULTS

The metabolomic changes induced by the phytotoxic metabolite in a ShB susceptible rice cultivar were elucidated by Gas Chromatography-Mass Spectrometry (GC-MS) analysis and compared with that of the pathogen to identify rice metabolites targeted by the phytotoxin. The profiles of about 29 metabolites with various physiological roles in rice plants have been identified worldwide. Unsupervised and supervised multivariate chemometrics (Principal Component Analysis, PCA and Partial Least Squares-Discriminant Analysis, PLS-DA) and cluster (Heat maps) analyses were used to compare the metabolites obtained from chemical profiles of the treatments with sterile distilled water (SDW) control. The results indicated that the rice plant expressed more metabolites in response to the pathogen than the phytotoxin and was lowest in SDW control. The key metabolites expressed in rice in response to the treatments were investigated by the Variable Importance in Projection (VIP) analysis using P< 0.05 VIP >15. The analysis identified 7 and 11 upregulating metabolites in the phytotoxin and the pathogen treatments, respectively, compared to the untreated control. Among the phytotoxin-treated and the pathogen inoculated samples, the phytotoxin treated sample recorded upregulation of 6 metabolites, whereas 9 metabolites were upregulated in the pathogen inoculated samples. These upregulating metabolites are speculated for the necrotic symptoms characteristic to both the phytotoxin and pathogen. In this analysis, hexadecanoic acid and dotriacontane were highly expressed metabolites specific to the phytotoxin and pathogen-treated samples, respectively. Besides upregulation, the metabolites also have a VIP score of >1.5 and hence fulfilled the criteria of classifying them as reliable potential biomarkers. In the pathway analysis, hexadecanoic acid and dotriacontane were identified to be involved in several important biosynthetic pathways of rice, such as the biosynthesis of saturated fatty acid and unsaturated fatty acids cutin, suberin, and wax.

CONCLUSIONS

The study concludes that though certain metabolites induced by the phytotoxin in the susceptible variety during necrosis shares with that of the pathogen, the identification of metabolites specific to the phytotoxin in comparison to the pathogenic and SDW controls indicated that the phytotoxin modulates the host metabolism differently and hence can be a potential pathogenicity factor of the ShB fungus.

SIGNIFICANCE AND IMPACT OF THE STUDY

Due to lack of knowledge on the pathway genes of RST and in the absence of an ShB resistant variety, understanding differentially expressed metabolic changes induced in the susceptible variety by the phytotoxin in comparison to that of the pathogenic and uninoculated controls enables us to identify the key metabolite changes during the ShB infection. Such metabolomic changes can further be used to infer gene functions for exploitation in ShB control.

Stemphylium lycopersici Nep1-like Protein (NLP) Is a Key Virulence Factor in Tomato Gray Leaf Spot Disease

The fungus Stemphylium lycopersici (S. lycopersici) is an economically important plant pathogen that causes grey leaf spot disease in tomato. However, functional genomic studies in S. lycopersici are lacking, and the factors influencing its pathogenicity remain largely unknown. Here, we present the first example of genetic transformation and targeted gene replacement in S. lycopersici. We functionally analyzed the NLP gene, which encodes a necrosis- and ethylene-inducing peptide 1 (Nep1)-like protein (NLP). We found that targeted disruption of the NLP gene in S. lycopersici significantly compromised its virulence on tomato. Moreover, our data suggest that NLP affects S. lycopersici conidiospore production and weakly affects its adaptation to osmotic and oxidative stress. Interestingly, we found that NLP suppressed the production of reactive oxygen species (ROS) in tomato leaves during S. lycopersici infection. Further, expressing the fungal NLP in tomato resulted in constitutive transcription of immune-responsive genes and inhibited plant growth. Through gene manipulation, we demonstrated the function of NLP in S. lycopersici virulence and development. Our work provides a paradigm for functional genomics studies in a non-model fungal pathogen system.

Cultivar Contributes to the Beneficial Effects of Bacillus subtilis PTA-271 and Trichoderma atroviride SC1 to Protect Grapevine Against Neofusicoccum parvum

Grapevine trunk diseases (GTDs) are a big threat for global viticulture. Without effective chemicals, biocontrol strategies are developed as alternatives to better cope with environmental concerns. A combination of biological control agents (BCAs) could even improve sustainable disease management through complementary ways of protection. In this study, we evaluated the combination of Bacillus subtilis (Bs) PTA-271 and Trichoderma atroviride (Ta) SC1 for the protection of Chardonnay and Tempranillo rootlings against Neofusicoccum parvum Bt67, an aggressive pathogen associated to Botryosphaeria dieback (BD). Indirect benefits offered by each BCA and their combination were then characterized in planta, as well as their direct benefits in vitro. Results provide evidence that (1) the cultivar contributes to the beneficial effects of Bs PTA-271 and Ta SC1 against N. parvum, and that (2) the in vitro BCA mutual antagonism switches to the strongest fungistatic effect toward Np-Bt67 in a three-way confrontation test. We also report for the first time the beneficial potential of a combination of BCA against Np-Bt67 especially in Tempranillo. Our findings highlight a common feature for both cultivars: salicylic acid (SA)-dependent defenses were strongly decreased in plants protected by the BCA, in contrast with symptomatic ones. We thus suggest that (1) the high basal expression of SA-dependent defenses in Tempranillo explains its highest susceptibility to N. parvum, and that (2) the cultivar-specific responses to the beneficial Bs PTA-271 and Ta SC1 remain to be further investigated.

Research Progress on Phytopathogenic Fungi and Their Role as Biocontrol Agents

Phytopathogenic fungi decrease crop yield and quality and cause huge losses in agricultural production. To prevent the occurrence of crop diseases and insect pests, farmers have to use many synthetic chemical pesticides. The extensive use of these pesticides has resulted in a series of environmental and ecological problems, such as the increase in resistant weed populations, soil compaction, and water pollution, which seriously affect the sustainable development of agriculture. This review discusses the main advances in research on plant-pathogenic fungi in terms of their pathogenic factors such as cell wall-degrading enzymes, toxins, growth regulators, effector proteins, and fungal viruses, as well as their application as biocontrol agents for plant pests, diseases, and weeds. Finally, further studies on plant-pathogenic fungal resources with better biocontrol effects can help find new beneficial microbial resources that can control diseases.

Aspergillus flavus Exploits Maize Kernels Using an "Orphan" Secondary Metabolite Cluster

Aspergillus flavus is a saprophytic cosmopolitan fungus, capable of infecting crops both pre- and post-harvest and exploiting different secondary metabolites, including aflatoxins. Aflatoxins are known carcinogens to animals and humans, but display no clear effect in host plants such as maize. In a previous study, we mined the genome of A. flavus to identify secondary metabolite clusters putatively involving the pathogenesis process in maize. We now focus on cluster 32, encoding for fungal effectors such as salicylate hydroxylase (SalOH), and necrosis- and ethylene-inducing proteins (npp1 domain protein) whose expression is triggered upon kernel contact. In order to understand the role of this genetic cluster in maize kernel infection, mutants of A. flavus, impaired or enhanced in specific functions (e.g., cluster 32 overexpression), were studied for their ability to cause disease. Within this frame, we conducted histological and histochemical experiments to verify the expression of specific genes within the cluster (e.g., SalOH, npp1), the production of salicylate, and the presence of its dehydroxylated form. Results suggest that the initial phase of fungal infection (2 days) of the living tissues of maize kernels (e.g., aleuron) coincides with a significant increase of fungal effectors such as SalOH and Npp1 that appear to be instrumental in eluding host defences and colonising the starch-enriched tissues, and therefore suggest a role of cluster 32 to the onset of infection.

Apoplastic Cell Death-Inducing Proteins of Filamentous Plant Pathogens: Roles in Plant-Pathogen Interactions

Filamentous pathogens, such as phytopathogenic oomycetes and fungi, secrete a remarkable diversity of apoplastic effector proteins to facilitate infection, many of which are able to induce cell death in plants. Over the past decades, over 177 apoplastic cell death-inducing proteins (CDIPs) have been identified in filamentous oomycetes and fungi. An emerging number of studies have demonstrated the role of many apoplastic CDIPs as essential virulence factors. At the same time, apoplastic CDIPs have been documented to be recognized by plant cells as pathogen-associated molecular patterns (PAMPs). The recent findings of extracellular recognition of apoplastic CDIPs by plant leucine-rich repeat-receptor-like proteins (LRR-RLPs) have greatly advanced our understanding of how plants detect them and mount a defense response. This review summarizes the latest advances in identifying apoplastic CDIPs of plant pathogenic oomycetes and fungi, and our current understanding of the dual roles of apoplastic CDIPs in plant-filamentous pathogen interactions.

Toxicity of Recombinant Necrosis and Ethylene-Inducing Proteins (NLPs) from Neofusicoccum parvum

Neofusicoccum parvum is a fungal pathogen associated with a wide range of plant hosts. Despite being widely studied, the molecular mechanism of infection of N. parvum is still far from being understood. Analysis of N. parvum genome lead to the identification of six putative genes encoding necrosis and ethylene-inducing proteins (NLPs). The sequence of NLPs genes (NprvNep 1-6) were analyzed and four of the six NLP genes were successfully cloned, expressed in E. coli and purified by affinity chromatography. Pure recombinant proteins were characterized according to their phytotoxic and cytotoxic effects to tomato leaves and to mammalian Vero cells, respectively. These assays revealed that all NprvNeps tested are cytotoxic to Vero cells and also induce cell death in tomato leaves. NprvNep2 was the most toxic to Vero cells, followed by NprvNep1 and 3. NprvNep4 induced weaker, but, nevertheless, still significant toxic effects to Vero cells. A similar trend of toxicity was observed in tomato leaves: the most toxic was NprvNep 2 and the least toxic NprvNep 4. This study describes for the first time an overview of the NLP gene family of N. parvum and provides additional insights into its pathogenicity mechanism.