Native soils with their microbiotas elicit a state of alert in tomato plants

Several studies have investigated soil microbial biodiversity, but understanding of the mechanisms underlying plant responses to soil microbiota remains in its infancy. Here, we focused on tomato (Solanum lycopersicum), testing the hypothesis that plants grown on native soils display different responses to soil microbiotas. Using transcriptomics, proteomics, and biochemistry, we describe the responses of two tomato genotypes (susceptible or resistant to Fusarium oxysporum f. sp. lycopersici) grown on an artificial growth substrate and two native soils (conducive and suppressive to Fusarium). Native soils affected tomato responses by modulating pathways involved in responses to oxidative stress, phenol biosynthesis, lignin deposition, and innate immunity, particularly in the suppressive soil. In tomato plants grown on steam-disinfected soils, total phenols and lignin decreased significantly. The inoculation of a mycorrhizal fungus partly rescued this response locally and systemically. Plants inoculated with the fungal pathogen showed reduced disease symptoms in the resistant genotype in both soils, but the susceptible genotype was partially protected from the pathogen only when grown on the suppressive soil. The 'state of alert' detected in tomatoes reveals novel mechanisms operating in plants in native soils and the soil microbiota appears to be one of the drivers of these plant responses.

Tomato MYB49 enhances resistance to Phytophthora infestans and tolerance to water deficit and salt stress

MYB49-overexpressing tomato plants showed significant resistance to Phytophthora infestans and tolerance to drought and salt stresses. This finding reveals the potential application of tomato MYB49 in future molecular breeding. Biotic and abiotic stresses severely reduce the productivity of tomato worldwide. Therefore, it is necessary to find key genes to simultaneously improve plant resistance to pathogens and tolerance to various abiotic stresses. In this study, based on homologous relationships with Arabidopsis R2R3-MYBs (AtMYBs) involved in responses to biotic and abiotic stresses, we identified a total of 24 R2R3-MYB transcription factors in the tomato genome. Among these tomato R2R3-MYBs, MYB49 (Solyc10g008700.1) was clustered into subgroup 11 by phylogenetic analysis, and its expression level was significantly induced after treatment with P. infestans, NaCl and PEG6000. Overexpression of MYB49 in tomato significantly enhanced the resistance of tomato to P. infestans, as evidenced by decreases in the number of necrotic cells, sizes of lesion, abundance of P. infestans, and disease index. Likewise, MYB49-overexpressing transgenic tomato plants also displayed increased tolerance to drought and salt stresses. Compared to WT plants, the accumulation of reactive oxygen species (ROS), malonaldehyde content, and relative electrolyte leakage was decreased, and peroxidase activity, superoxide dismutase activity, chlorophyll content, and photosynthetic rate were increased in MYB49-overexpressing tomato plants under P. infestans, salt or drought stress. These results suggested that tomato MYB49, as a positive regulator, could enhance the capacity to scavenge ROS, inhibit cell membrane damage and cell death, and protect chloroplasts, resulting in an improvement in resistance to P. infestans and tolerance to salt and drought stresses, and they provide a candidate gene for tomato breeding to enhance biotic stress resistance and abiotic stress tolerance.

Tomato 14-3-3 Proteins Are Required for Xv3 Disease Resistance and Interact with a Subset of Xanthomonas euvesicatoria Effectors

The 14-3-3 phospho-binding proteins with scaffolding activity play central roles in the regulation of enzymes and signaling complexes in eukaryotes. In plants, 14-3-3 isoforms are required for disease resistance and key targets of pathogen effectors. Here, we examined the requirement of the tomato (Solanum lycopersicum) 14-3-3 isoform (TFT) protein family for Xv3 disease resistance in response to the bacterial pathogen Xanthomonas euvesicatoria. In addition, we determined whether TFT proteins interact with the repertoire of X. euvesicatoria type III secretion effector proteins, including AvrXv3, the elicitor of Xv3 resistance. We show that multiple TFT contribute to Xv3 resistance. We also show that one or more TFT proteins physically interact with multiple effectors (AvrXv3, XopE1, XopE2, XopN, XopO, XopQ, and XopAU). Genetic analyses indicate that none of the identified effectors interfere with AvrXv3-elicited resistance into Xv3 tomato leaves; however, XopE1, XopE2, and XopO are required to suppress symptom development in susceptible tomato leaves. Phospho-peptide mapping revealed that XopE2 is phosphorylated at multiple residues in planta and residues T66, T131, and S334 are required for maximal binding to TFT10. Together, our data support the hypothesis that multiple TFT proteins are involved in immune signaling during X. euvesicatoria infection.

Non-Structural Protein NSm of Tomato Spotted Wilt Virus Is an Avirulence Factor Recognized by Resistance Genes of Tobacco and Tomato via Different Elicitor Active Sites

Tomato spotted wilt virus (TSWV) is one of the most destructive viral pathogens of plants. Recently, a single dominant gene conferring complete resistance to TSWV (RTSW) was identified in Nicotina alata and introgressed into cultivated tobacco (N. tabacum). However, whether the TSWV carries an avirulence (Avr) factor directed against RTSW remains obscure. In the present study, we identified the non-structural protein (NSm), the movement protein of TSWV, which is an RTSW-specific Avr factor, by using two different transient expression systems. Using amino acid (aa) substitution mutants, we demonstrated the ability to induce RTSW-mediated hypersensitive response (HR) of NSm is independent of its movement function. Moreover, key substitutions (C118Y and T120N), a 21-aa viral effector epitope, and different truncated versions of NSm, which are responsible for the recognition of the Sw-5b resistance gene of tomato, were tested for their ability to trigger HR to TSWV in tobacco. Together, our results demonstrated that RTSW-mediated resistance is triggered by NSm in the same way as by Sw-5b, however, via different elicitor active sites. Finally, an Avr gene-based diagnostic approach was established and used to determine the presence and effectiveness of resistance genes in tobacco.

Type VI glandular trichome density and their derived volatiles are differently induced by jasmonic acid in developing and fully developed tomato leaves: Implications for thrips resistance

Variation in the induction of plant defenses along the plant canopy can determine distribution and colonization of arthropod herbivores within the plant. In tomato, type VI glandular trichomes, which are epidermal defensive structures, and their derived volatiles are induced by the phytohormone jasmonic acid (JA). How JA-mediated induction of these trichome-associated chemical defenses depends on the leaf developmental stage and correlates with resistance against herbivory is unknown. We showed that application of JA reduced thrips-associated damage, however the amplitude of this response was reduced in the fully developed leaves compared to those still developing. Although JA increased type-VI trichome densities in all leaf developmental stages, as well as JA-inducible defensive proteins, these increases were stronger in developing leaves. Remarkably, the concentration of trichome-derived volatiles was induced by JA to a larger degree in developing leaves than in fully developed leaves. In fully developed leaves, the increase in trichome-derived volatiles was explained by an enhanced production per trichome, while in developing leaves this was mainly caused by increases in type-VI trichome densities. Together, we showed that JA-mediated induction of trichome density and chemistry depends on leaf development stage, and it might explain the degree of thrips-associated leaf damage in tomato.

Elucidating the mechanism of action of copper heptagluconate on the plant immune system against Pseudomonas syringae in tomato (Solanum lycopersicum L)

BACKGROUND

Phytopathogenic problems caused by the bacterial pathogen Pseudomonas syringae in tomato are becoming more serious due to the emergence of strains resistant to classical pesticides. This has led to research into new formulations with lower environmental problems. One of the most promising alternatives to the use of classical pesticides is the induction of natural plant defences. New formulations based on Cu complexed with heptagluconic acid induce plant innate defences and could be an alternative to classical treatments based on inorganic Cu against bacterial speck. To study the efficacy of this compound in tomato against P. syringae, we tested its systemic effect Applying the treatments via radicular.

RESULTS

Treated plants showed less infection development and lower number of viable bacteria in leaves. We also observed better performance of parameters involved in plant resistance such as the antioxidant response and the accumulation of phenolic compounds.

CONCLUSION

Results showed that soil drench applications can be highly effective for the prevention and control of bacterial speck in tomato plants, showing a reduction in symptoms of ∼ 50%. Moreover, application of Cu heptagluconate induced accumulation of the plant polyphenols caffeic and chlorogenic acids, and reduced the amount of reactive oxygen species in infected plants. © 2018 Society of Chemical Industry.

Expression of a maize NBS gene ZmNBS42 enhances disease resistance in Arabidopsis

Expression of the ZmNBS42 in Arabidopsis plants conferred resistance to bacterial pathogens, providing potential resistance enhancement of maize in further genetic breeding. Nucleotide-binding site (NBS) domain proteins play critical roles in disease resistance. In this study, we isolate a novel NBS gene ZmNBS42 from maize and systematically investigate its function on disease resistance. We find that the expression levels of ZmNBS42 in maize leaf were strikingly increased in response to Bipolaris maydis inoculation and SA treatment. The spatial expression pattern analysis reveals that, during development, ZmNBS42 is ubiquitously highly expressed in maize root, leaf, stem, internode and seed, but lowly expressed in pericarp and embryo. To better understand the roles of ZmNBS42, we overexpressed ZmNBS42 in heterologous systems. Transient overexpression of ZmNBS42 in the leaves of Nicotiana benthamiana induces a hypersensitive response. ZmNBS42 overexpression (ZmNBS42-OE) Arabidopsis plants produced more SA content than Col-0 plants, and increased the expression levels of some defense-responsive genes compared to Col-0 plants. Moreover, the ZmNBS42-OE Arabidopsis plants displayed enhanced resistance against Pseudomonas syringae pathovar tomato DC3000 (Pst DC3000). These results together suggest that ZmNBS42 can serve as an important regulator in disease resistance, thus better understanding of ZmNBS42 would benefit the resistance enhancement in maize breeding programs.

Distinct Signatures of Host Defense Suppression by Plant-Feeding Mites

Tomato plants are attacked by diverse herbivorous arthropods, including by cell-content-feeding mites, such as the extreme generalist Tetranychus urticae and specialists like Tetranychus evansi and Aculops lycopersici. Mite feeding induces plant defense responses that reduce mite performance. However, T. evansi and A. lycopersici suppress plant defenses via poorly understood mechanisms and, consequently, maintain a high performance on tomato. On a shared host, T. urticae can be facilitated by either of the specialist mites, likely due to the suppression of plant defenses. To better understand defense suppression and indirect plant-mediated interactions between herbivorous mites, we used gene-expression microarrays to analyze the transcriptomic changes in tomato after attack by either a single mite species (T. urticae, T. evansi, A. lycopersici) or two species simultaneously (T. urticae plus T. evansi or T. urticae plus A. lycopersici). Additionally, we assessed mite-induced changes in defense-associated phytohormones using LC-MS/MS. Compared to non-infested controls, jasmonates (JAs) and salicylate (SA) accumulated to higher amounts upon all mite-infestation treatments, but the response was attenuated after single infestations with defense-suppressors. Strikingly, whereas 8 to 10% of tomato genes were differentially expressed upon single infestations with T. urticae or A. lycopersici, respectively, only 0.1% was altered in T. evansi-infested plants. Transcriptome analysis of dual-infested leaves revealed that A. lycopersici primarily suppressed T. urticae-induced JA defenses, while T. evansi dampened T. urticae-triggered host responses on a transcriptome-wide scale. The latter suggests that T. evansi not solely down-regulates plant gene expression, but rather directs it back towards housekeeping levels. Our results provide valuable new insights into the mechanisms underlying host defense suppression and the plant-mediated facilitation of competing herbivores.

The intracellular nucleotide-binding leucine-rich repeat receptor (SlNRC4a) enhances immune signalling elicited by extracellular perception

Plant recognition and defence against pathogens employs a two-tiered perception system. Surface-localized pattern recognition receptors (PRRs) act to recognize microbial features, whereas intracellular nucleotide-binding leucine-rich repeat receptors (NLRs) directly or indirectly recognize pathogen effectors inside host cells. Employing the tomato PRR LeEIX2/EIX model system, we explored the molecular mechanism of signalling pathways. We identified an NLR that can associate with LeEIX2, termed SlNRC4a (NB-LRR required for hypersensitive response-associated cell death-4). Co-immunoprecipitation demonstrates that SlNRC4a is able to associate with different PRRs. Physiological assays with specific elicitors revealed that SlNRC4a generally alters PRR-mediated responses. SlNRC4a overexpression enhances defence responses, whereas silencing SlNRC4 reduces plant immunity. Moreover, the coiled-coil domain of SlNRC4a is able to associate with LeEIX2 and is sufficient to enhance responses upon EIX perception. On the basis of these findings, we propose that SlNRC4a acts as a noncanonical positive regulator of immunity mediated by diverse PRRs. Thus, SlNRC4a could link both intracellular and extracellular immune perceptions.

SlERF2 Is Associated with Methyl Jasmonate-Mediated Defense Response against Botrytis cinerea in Tomato Fruit

Methyl jasmonate (MeJA) and ethylene play important roles in mediating defense responses against Botrytis cinerea. Ethylene response factors (ERFs) are the final components of ethylene signal transduction; whether SlERF2 participates in disease resistance against Botrytis cinerea is unclear. The objective of this study was to investigate the role of SlERF2 in MeJA-mediated defense response by using both sense and antisense SlERF2 tomato fruit. Our results showed that both MeJA treatment and pathogen infection upregulated SlERF2 expression level. Overexpression of SlERF2 enhanced tomato fruit resistance against Botrytis cinerea. MeJA treatment increased ethylene production, promoted the activities of chitinase, β-1,3-glucanase, phenylalanine ammonia-lyase, and peroxidase, and elevated pathogenesis-related protein content and total phenolic content. Moreover, the effects of MeJA on disease response were reinforced in sense SlERF2 tomato fruit, while they were weakened in antisense SlERF2 tomato fruit. These results indicated that SlERF2 was involved in MeJA-mediated disease resistance against Botrytis cinerea in tomato fruit.

Phytophthora infestans RXLR effector SFI5 requires association with calmodulin for PTI/MTI suppressing activity

Pathogens secrete effector proteins to interfere with plant innate immunity, in which Ca2+ /calmodulin (CaM) signalling plays key roles. Thus far, few effectors have been identified that directly interact with CaM for defence suppression. Here, we report that SFI5, an RXLR effector from Phytophthora infestans, suppresses microbe-associated molecular pattern (MAMP)-triggered immunity (MTI) by interacting with host CaMs. We predicted the CaM-binding site in SFI5 using in silico analysis. The interaction between SFI5 and CaM was tested by both in vitro and in vivo assays. MTI suppression by SFI5 and truncated variants were performed in a tomato protoplast system. We found that both the predicted CaM-binding site and the full-length SFI5 protein interact with CaM in the presence of Ca2+ . MTI responses, such as FRK1 upregulation, reactive oxygen species accumulation, and mitogen-activated protein kinase activation were suppressed by truncated SFI5 proteins containing the C-terminal CaM-binding site but not by those without it. The plasma membrane localization of SFI5 and its ability to enhance infection were also perturbed by loss of the CaM-binding site. We conclude that CaM-binding is required for localization and activity of SFI5. We propose that SFI5 suppresses plant immunity by interfering with immune signalling components after activation by CaMs.

Knockout of SlMAPK3 Reduced Disease Resistance to Botrytis cinerea in Tomato Plants

Mitogen-activated protein kinases (MAPKs) play an important role in defense responses to biotic and abiotic stresses. In order to investigate the role of SlMAPK3 in tomato plant resistance to Botrytis cinerea, two lines of slmapk3 mutants and wild-type (WT) tomato plants were used. The results showed that slmapk3 mutants were more susceptible to B. cinerea and that knockout of SlMAPK3 reduced the activities of defense enzymes and enhanced the accumulation of reactive oxygen species (ROS). Furthermore, we detected the expressions of salicylic acid (SA) and jasmonic acid (JA) signaling-related genes and found that knockout of SlMAPK3 enhanced the expressions of SlPR1, SlPAD4 and SlEDS1, whereas reduced the expressions of SlLoxC, SlPI I and SlPI II and enhanced the expressions of SlJAZ1 and SlMYC2. We postulate that SlMAPK3 plays a positive role in tomato plant resistance to B. cinerea through regulating ROS accumulation and SA and JA defense signaling pathways.

Structure of the Cladosporium fulvum Avr4 effector in complex with (GlcNAc)6 reveals the ligand-binding mechanism and uncouples its intrinsic function from recognition by the Cf-4 resistance protein

Effectors are microbial-derived secreted proteins with an essential function in modulating host immunity during infections. CfAvr4, an effector protein from the tomato pathogen Cladosporium fulvum and the founding member of a fungal effector family, promotes parasitism through binding fungal chitin and protecting it from chitinases. Binding of Avr4 to chitin is mediated by a carbohydrate-binding module of family 14 (CBM14), an abundant CBM across all domains of life. To date, the structural basis of chitin-binding by Avr4 effector proteins and of recognition by the cognate Cf-4 plant immune receptor are still poorly understood. Using X-ray crystallography, we solved the crystal structure of CfAvr4 in complex with chitohexaose [(GlcNAc)₆] at 1.95Å resolution. This is the first co-crystal structure of a CBM14 protein together with its ligand that further reveals the molecular mechanism of (GlcNAc)₆ binding by Avr4 effector proteins and CBM14 family members in general. The structure showed that two molecules of CfAvr4 interact through the ligand and form a three-dimensional molecular sandwich that encapsulates two (GlcNAc)₆ molecules within the dimeric assembly. Contrary to previous assumptions made with other CBM14 members, the chitohexaose-binding domain (ChBD) extends to the entire length of CfAvr4 with the reducing end of (GlcNAc)₆ positioned near the N-terminus and the non-reducing end at the C-terminus. Site-directed mutagenesis of residues interacting with (GlcNAc)₆ enabled the elucidation of the precise topography and amino acid composition of Avr4’s ChBD and further showed that these residues do not individually mediate the recognition of CfAvr4 by the Cf-4 immune receptor. Instead, the studies highlighted the dependency of Cf-4-mediated recognition on CfAvr4’s stability and resistance against proteolysis in the leaf apoplast, and provided the evidence for structurally separating intrinsic function from immune receptor recognition in this effector family.

Microbes mobilize an array of secreted effectors to manipulate their hosts during infections, whereas in response, hosts utilize cognate immune receptors to perceive effectors and mount a defense. To date, the structural basis of effector function and recognition by immune receptors are still poorly understood. Here we present the crystal structure in complex with chitohexaose of CfAvr4, a CBM14 lectin and the founding member of a fungal effector family that binds and protects chitin in fungal cell-walls from chitinases. This is the first structure of a CBM14 protein to be co-crystalized with its ligand that further reveals how Avr4 effectors function. Specifically, by leveraging structural and functional data, we elucidate the molecular basis for ligand-binding by CfAvr4 and show that two effector molecules are brought together through the ligand to form a sandwich structure that laminates two chitohexaose molecules within the dimeric assembly. We further show that recognition of CfAvr4 by the cognate Cf-4 immune receptor is not mediated through residues directly interacting with chitohexaose, thereby structurally uncoupling the ligand-binding function of Avr4 from recognition by Cf-4 and challenging early postulations that the broad recognition of Avr4 effectors by Cf-4 stems from perceiving residues implicated in binding their ligand.

The Ralstonia solanacearum csp22 peptide, but not flagellin-derived peptides, is perceived by plants from the Solanaceae family

Ralstonia solanacearum, the causal agent of bacterial wilt disease, is considered one of the most destructive bacterial pathogens due to its lethality, unusually wide host range, persistence and broad geographical distribution. In spite of the extensive research on plant immunity over the last years, the perception of molecular patterns from R. solanacearum that activate immunity in plants is still poorly understood, which hinders the development of strategies to generate resistance against bacterial wilt disease. The perception of a conserved peptide of bacterial flagellin, flg22, is regarded as paradigm of plant perception of invading bacteria; however, no elicitor activity has been detected for R. solanacearum flg22. Recent reports have shown that other epitopes from flagellin are able to elicit immune responses in specific species from the Solanaceae family, yet our results show that these plants do not perceive any epitope from R. solanacearum flagellin. Searching for elicitor peptides from R. solanacearum, we found several protein sequences similar to the consensus of the elicitor peptide csp22, reported to elicit immunity in specific Solanaceae plants. A R. solanacearum csp22 peptide (csp22Rsol ) was indeed able to trigger immune responses in Nicotiana benthamiana and tomato, but not in Arabidopsis thaliana. Additionally, csp22Rsol treatment conferred increased resistance to R. solanacearum in tomato. Transgenic A. thaliana plants expressing the tomato csp22 receptor (SlCORE) gained the ability to respond to csp22Rsol and became more resistant to R. solanacearum infection. Our results shed light on the mechanisms for perception of R. solanacearum by plants, paving the way for improving current approaches to generate resistance against R. solanacearum.

Application of Whole Genome Resequencing in Mapping of a Tomato Yellow Leaf Curl Virus Resistance Gene

Tomato yellow leaf curl virus (TYLCV) has significantly impacted the tomato industry around the world, and the use of insecticides and insect nets have not effectively controlled the spread of this pathogen. The tomato line AVTO1227 is highly resistant to TYLCV. In this study, F2 and BC1 populations derived from AVTO1227 and the susceptible line Money maker were used to assess the genetic mechanism underlying TYLCV resistance. We have identified a recessive TYLCV resistance gene, hereby designated as ty-5, which is linked to SlNACI. Genomic DNA pools from resistant and susceptible groups were constructed, and their genomes were resequenced. The ty-5 gene was identified on an interval encompassing the genomic positions 2.22 Mb to 3.19 Mb on tomato chromosome 4. Genotyping using linkage markers further mapped ty-5 within the interval between markers ty5-25 and ty5-29, where only the pelota gene is located. Consequently, pelota was considered as the candidate gene corresponding to ty-5. Two nucleotide transversions within the promoter region and one transversion in exon region of the pelota gene were detected in the parental lines. However, the relative transcript levels of pelota did not significantly differ among the three tomato lines, regardless of TYLCV infection. This study will facilitate marker-assisted breeding for resistance to TYLCV and lay a foundation for the research of the resistance mechanism of ty-5 in tomato.

Effect of tomato variety, cultivation, climate and processing on Sola l 4, an allergen from Solanum lycopersicum

Tomatoes (Solanum lycopersicum) are one of the most consumed vegetables worldwide. However, tomato allergies in patients suffering from birch pollen allergy occur frequently. Due to highly similar protein structures of the tomato allergen Sola l 4 and the major birch pollen allergen Bet v 1, patients cross-react with allergenic proteins from tomato as well as other fruits or vegetables. The aim of this study was to quantify Sola l 4 in various tomatoes differing in color, size and shape for identification of varieties with a reduced allergen level. Therefore, an indirect competitive ELISA using a specific polyclonal Sola l 4 antibody was developed. In addition, two varieties, both cultivated either conventionally or organically and furthermore dried with different methods, were analyzed to investigate the influence of the cultivation method and processing techniques on Sola l 4 level. Within 23 varieties, Sola l 4 content varied significantly between 0.24 and 1.71 μg Sola l 4/g FW. The tomato cultivars Rugantino and Rhianna showed the significantly lowest level, whereas in cultivars Farbini and Bambello the significantly highest concentration was determined. Drying of tomatoes in the oven and by sun resulted in a significant decrease. The thermal instability was verified for the recombinant Sola l 4 emphasizing the results for the native protein in dried tomato samples. Overall, the Sola l 4 content is cultivar-dependent and no correlation between color and Sola l 4 amount was found. During the drying process of tomatoes Sola l 4 level was significantly reduced due to thermal instability. Growing conditions have a minor effect whereas seasonal effects show a more pronounced impact. These findings could extend the knowledge about the allergen level of different tomato varieties and may help to improve food safety to potentially increase the life quality of patients suffering from birch pollen allergy.

Lignin metabolism involves Botrytis cinerea BcGs1- induced defense response in tomato

BACKGROUND

BcGs1, a cell wall-degrading enzyme (CWDE), was originally derived from Botrytis cinerea. Our previous study revealed that BcGs1 could trigger defense responses and protect plants against various pathogens. We researched the defense response mechanism underlying this BcGs1 elicitation in tomato.

RESULTS

We revealed that the two domains were required for BcGs1's full necrosis activity. According to analysis and quantitative real-time PCR of the up-regulated proteins and genes filtered by iTRAQ-based quantitative proteome approach, oxidative metabolism and phenylpropanoid metabolism were speculated to be involved in BcGs1-triggered defense response in tomato. Furthermore, experimental evidence showed that BcGs1 triggered reactive oxygen species (ROS) burst and increased the level of phenylalanine-ammonia lyase (PAL) and peroxidase (POD) enzyme activity, as well as lignin accumulation. Moreover, histochemical analysis revealed that infiltration of BcGs1 in tomato leaves exhibited cell wall thickening compared with untreated plants.

CONCLUSIONS

The results suggested that BcGs1 activated the basal defense response included lignin metabolism contributed to BcGs1-induced resistance to Botrytis. cinerea infection in tomato.

Lignin metabolism involves Botrytis cinerea BcGs1- induced defense response in tomato

BACKGROUND

BcGs1, a cell wall-degrading enzyme (CWDE), was originally derived from Botrytis cinerea. Our previous study revealed that BcGs1 could trigger defense responses and protect plants against various pathogens. We researched the defense response mechanism underlying this BcGs1 elicitation in tomato.

RESULTS

We revealed that the two domains were required for BcGs1's full necrosis activity. According to analysis and quantitative real-time PCR of the up-regulated proteins and genes filtered by iTRAQ-based quantitative proteome approach, oxidative metabolism and phenylpropanoid metabolism were speculated to be involved in BcGs1-triggered defense response in tomato. Furthermore, experimental evidence showed that BcGs1 triggered reactive oxygen species (ROS) burst and increased the level of phenylalanine-ammonia lyase (PAL) and peroxidase (POD) enzyme activity, as well as lignin accumulation. Moreover, histochemical analysis revealed that infiltration of BcGs1 in tomato leaves exhibited cell wall thickening compared with untreated plants.

CONCLUSIONS

The results suggested that BcGs1 activated the basal defense response included lignin metabolism contributed to BcGs1-induced resistance to Botrytis. cinerea infection in tomato.

Re-analysis of long non-coding RNAs and prediction of circRNAs reveal their novel roles in susceptible tomato following TYLCV infection

BACKGROUND

Long Noncoding-RNAs (LncRNAs) are known to be involved in some biological processes, but their roles in plant-virus interactions remain largely unexplored. While circular RNAs (circRNAs) have been studied in animals, there has yet to be extensive research on them in a plant system, especially in tomato-tomato yellow leaf curl virus (TYLCV) interaction.

RESULTS

In this study, RNA transcripts from the susceptible tomato line JS-CT-9210 either infected with TYLCV or untreated, were sequenced in a pair-end strand-specific manner using ribo-zero rRNA removal library method. A total of 2056 lncRNAs including 1767 long intergenic non-coding RNA (lincRNAs) and 289 long non-coding natural antisense transcripts (lncNATs) were obtained. The expression patterns in lncRNAs were similar in susceptible tomato plants between control check (CK) and TYLCV infected samples. Our analysis suggested that lncRNAs likely played a role in a variety of functions, including plant hormone signaling, protein processing in the endoplasmic reticulum, RNA transport, ribosome function, photosynthesis, glulathione metabolism, and plant-pathogen interactions. Using virus-induced gene silencing (VIGS) analysis, we found that reduced expression of the lncRNA S-slylnc0957 resulted in enhanced resistance to TYLCV in susceptible tomato plants. Moreover, we identified 184 circRNAs candidates using the CircRNA Identifier (CIRI) software, of which 32 circRNAs were specifically expressed in untreated samples and 83 circRNAs in TYLCV samples. Approximately 62% of these circRNAs were derived from exons. We validated the circRNAs by both PCR and Sanger sequencing using divergent primers, and found that most of circRNAs were derived from the exons of protein coding genes. The silencing of these circRNAs parent genes resulted in decreased TYLCV virus accumulation.

CONCLUSION

In this study, we identified novel lncRNAs and circRNAs using bioinformatic approaches and showed that these RNAs function as negative regulators of TYLCV infection. Moreover, the expression patterns of lncRNAs in susceptible tomato plants were different from that of resistant tomato plants, while exonic circRNAs expression positively associated with their respective protein coding genes. This work provides a foundation for elaborating the novel roles of lncRNAs and circRNAs in susceptible tomatoes following TYLCV infection.

The Fusarium oxysporum Avr2-Six5 Effector Pair Alters Plasmodesmatal Exclusion Selectivity to Facilitate Cell-to-Cell Movement of Avr2

Pathogens use effector proteins to manipulate their hosts. During infection of tomato, the fungus Fusarium oxysporum secretes the effectors Avr2 and Six5. Whereas Avr2 suffices to trigger I-2-mediated cell death in heterologous systems, both effectors are required for I-2-mediated disease resistance in tomato. How Six5 participates in triggering resistance is unknown. Using bimolecular fluorescence complementation assays we found that Avr2 and Six5 interact at plasmodesmata. Single-cell transformation revealed that a 2xRFP marker protein and Avr2-GFP only move to neighboring cells in the presence of Six5. Six5 alone does not alter plasmodesmatal transduction as 2xRFP was only translocated in the presence of both effectors. In SIX5-expressing transgenic plants, the distribution of virally expressed Avr2-GFP, and subsequent onset of I-2-mediated cell death, differed from that in wild-type tomato. Taken together, our data show that in the presence of Six5, Avr2 moves from cell to cell, which in susceptible plants contributes to virulence, but in I-2 containing plants induces resistance.

Brassinosteroids act as a positive regulator for resistance against root-knot nematode involving RESPIRATORY BURST OXIDASE HOMOLOG-dependent activation of MAPKs in tomato

Interplay of hormones with reactive oxygen species (ROS) fine-tunes the response of plants to stress; however, the crosstalk between brassinosteroids (BRs) and ROS in nematode resistance is unclear. In this study, we found that low BR biosynthesis or lack of BR receptor increased, whilst exogenous BR decreased the susceptibility of tomato plants to Meloidogyne incognita. Hormone quantification coupled with hormone mutant complementation experiments revealed that BR did not induce the defence response by triggering salicylic acid (SA), jasmonic acid/ethylene (JA/ET) or abscisic acid (ABA) signalling pathway. Notably, roots of BR-deficient plants had decreased apoplastic ROS accumulation, transcript of RESPIRATORY BURST OXIDASE HOMOLOG1 (RBOH1) and WHITEFLY INDUCED1 (WFI1), and reduced activation of mitogen-activated protein kinase 1/2 (MPK1/2) and MPK3. Silencing of RBOH1, WFI1, MPK1, MPK2 and MPK3 all increased the root susceptibility to nematode and attenuated BR-induced resistance against the nematode. Significantly, suppressed transcript of RBOH1 compromised BR-induced activation of MPK1/2 and MPK3. These results strongly suggest that RBOH-dependent MPK activation is involved in the BR-induced systemic resistance against the nematode.

Tomato photorespiratory glycolate-oxidase-derived H2 O2 production contributes to basal defence against Pseudomonas syringae

Despite being essential for C₃ plants, photorespiration is believed to cause a significant crop yield loss even under future climates. However, how photorespiration affects plant basal defence still remains largely unknown. Here, we studied the involvement of photorespiration in tomato-Pseudomonas syringae pv. tomato DC3000 interaction focusing on three photorespiratory genes. Inoculation with P. syringae increased photorespiration rate (Pr) and expression of glycolate oxidase (GOX2), serine glyoxylate aminotransferase (SGT) and serine hydroxyl methyltransferase (SHMT1); however, inhibition of photorespiration by isonicotinic acid hydrazide decreased tomato basal defence against P. syringae. Furthermore, silencing of GOX2, SGT or SHMT1 genes in tomato decreased Pr but increased susceptibility to P. syringae, whereas transient overexpression of GOX2, SGT or SHMT1 in tobacco increased basal defence. Further study revealed that salicylic acid (SA) signalling is involved in GOX2-mediated, SGT-mediated and SHMT1-mediated defence. Moreover, H₂ O₂ pretreatment remarkably alleviated the GOX2 silencing-induced depression in basal defence and SA signalling, whereas it had no effect on that of SGT-silenced and SHMT1-silenced plants. Taken together, these results suggest that H₂ O₂ is critical for GOX2-modulated but not SGT-modulated or SHMT1-modulated SA signalling and subsequent basal defence against P. syringae. This work deepens the understanding of photorespiration-involved defence responses to bacterial attack in plants.

Whole Root Transcriptomic Analysis Suggests a Role for Auxin Pathways in Resistance to Ralstonia solanacearum in Tomato

The soilborne pathogen Ralstonia solanacearum is the causal agent of bacterial wilt and causes significant crop loss in the Solanaceae family. The pathogen first infects roots, which are a critical source of resistance in tomato (Solanum lycopersicum L.). Roots of both resistant and susceptible plants are colonized by the pathogen, yet rootstocks can provide significant levels of resistance. Currently, mechanisms of this 'root-mediated resistance' remain largely unknown. To identify the molecular basis of this resistance, we analyzed the genome-wide transcriptional response of roots of resistant 'Hawaii 7996' and susceptible 'West Virginia 700' (WV) tomatoes at multiple timepoints after inoculation with R. solanacearum. We found that defense pathways in roots of the resistant Hawaii 7996 are activated earlier and more strongly than roots of susceptible WV. Further, auxin signaling and transport pathways are suppressed in roots of the resistant variety. Functional analysis of an auxin transport mutant in tomato revealed a role for auxin pathways in bacterial wilt. Together, our results suggest that roots mediate resistance to R. solanacearum through genome-wide transcriptomic changes that result in strong activation of defense genes and alteration of auxin pathways.

Autophagy contributes to regulate the ROS levels and PCD progress in TMV-infected tomatoes

Programmed cell death (PCD) and autophagy are both important means for plants to resist pathogen. It is also the main biological reaction of plant immunity. In previous studies, we found that TMV local-infection on tomato leaves not only caused the PCD process in the distal root tissues, but also induced autophagy in root-tip cells. However, the reasons for these biological phenomena are unclear. In order to get deeper insight, the role of a putative inducible factor reactive oxidative species (ROS) was investigated. The situ staining and subcellular localization analysis showed that the ROS level in the root tissue of TMV infected plants was significantly promoted. TEM observation showed that the intracellular ROS was excreted into the cell wall and intercellular layer. At the same time, the results of western blot and qRT-PCR showed that the expression of autophagy related protein Atg8 and genes (Atg5, Atg7 and Atg10) were increased. However, in the subsequent DPI inhibition experiments we found that the accumulation of ROS in infected plant root-tip tissues was inhibited and the autophagy in the root-tip cells also decreased. When 3-methyladenine (3-MA) was used to inhibit autophagy, there was no significant change in the ROS level in the apical tissue, while the systemic PCD process of the root-tip cells was elevated. Taken together, these results indicate that local TMV inoculation on the leaves induced the root-tip cells producing and releasing a lot of ROS into the extracellular matrix for defense against pathogen invasion. Meanwhile, ROS acted as a signaling substance and triggered autophagy in root-tip cells, in order to eliminate excessive intracellular ROS oxidative damage and maintain cell survival.

Jasmonic acid-induced tolerance to root-knot nematodes in tomato plants through altered photosynthetic and antioxidative defense mechanisms

Plant parasitic nematodes cause severe damage to cultivated crops globally. Management of nematode population is a major concern as chemicals used as nematicides have negative impact on the environment. Natural plant products can be safely used for the control of nematodes. Among various plant metabolites, plant hormones play an essential role in developmental and physiological processes and also assist the plants to encounter stressful conditions. Keeping this in mind, the present study was designed to evaluate the effect of jasmonic acid (JA) on the growth, pigments, polyphenols, antioxidants, osmolytes, and organic acids under nematode infection in tomato seedlings. It was observed that nematode inoculation reduced the growth of seedlings. Treatment with JA improved root growth (32.79%), total chlorophylls (71.51%), xanthophylls (94.63%), anthocyanins (37.5%), and flavonoids content (21.11%) when compared to inoculated seedlings alone. The JA application enhanced the total antioxidant capacity (lipid- and water-soluble antioxidants) by 38.23 and 34.37%, respectively, in comparison to infected seedlings. Confocal studies revealed that there was higher accumulation of glutathione in hormone-treated seedlings under nematode infection. Treatment with JA increased total polyphenols content (74.56%) in comparison to nematode-infested seedlings. JA-treated seedlings also enhanced osmolyte and organic acid contents under nematode stress. Overall, treatment with JA improved growth, enhanced pigment levels, modulated antioxidant content, and enhanced osmolyte and organic acid content in nematode-infected seedlings.