Botrytis cinerea manipulates the antagonistic effects between immune pathways to promote disease development in tomato

Transcriptome dynamics of Arabidopsis during sequential biotic and abiotic stresses

In nature, plants have to cope with a wide range of stress conditions that often occur simultaneously or in sequence. To investigate how plants cope with multi-stress conditions, we analyzed the dynamics of whole-transcriptome profiles of Arabidopsis thaliana exposed to six sequential double stresses inflicted by combinations of: (i) infection by the necrotrophic fungus Botrytis cinerea, (ii) herbivory by chewing larvae of Pieris rapae, and (iii) drought stress. Each of these stresses induced specific expression profiles over time, in which one-third of all differentially expressed genes was shared by at least two single stresses. Of these, 394 genes were differentially expressed during all three stress conditions, albeit often in opposite directions. When two stresses were applied in sequence, plants displayed transcriptome profiles that were very similar to the second stress, irrespective of the nature of the first stress. Nevertheless, significant first-stress signatures could be identified in the sequential stress profiles. Bioinformatic analysis of the dynamics of co-expressed gene clusters highlighted specific clusters and biological processes of which the timing of activation or repression was altered by a prior stress. The first-stress signatures in second stress transcriptional profiles were remarkably often related to responses to phytohormones, strengthening the notion that hormones are global modulators of interactions between different types of stress. Because prior stresses can affect the level of tolerance against a subsequent stress (e.g. prior herbivory strongly affected resistance to B. cinerea), the first-stress signatures can provide important leads for the identification of molecular players that are decisive in the interactions between stress response pathways.

Aphid performance changes with plant defense mediated by Cucumber mosaic virus titer

BACKGROUND

Cucumber mosaic virus (CMV) causes appreciable losses in vegetables, ornamentals and agricultural crops. The green peach aphid, Myzus persicae Sulzer (Aphididae) is one of the most efficient vectors for CMV. The transmission ecology of aphid-vectored CMV has been well investigated. However, the detailed description of the dynamic change in the plant-CMV-aphid interaction associated with plant defense and virus epidemics is not well known.

RESULTS

In this report, we investigated the relationship of virus titer with plant defense of salicylic acid (SA) and jasmonic acid (JA) during the different infection time and their interaction with aphids in CMV-infected tobacco plants. Our results showed that aphid performance changed with virus titer and plant defense on CMV-inoculated plants. At first, plant defense was low and aphid number increased gradually. The plant defense of SA signaling pathway was induced when virus titer was at a high level, and aphid performance was correspondingly reduced. Additionally, the winged aphids were increased.

CONCLUSION

Our results showed that aphid performance was reduced due to the induced plant defense mediated by Cucumber mosaic virus titer. Additionally, some wingless aphids became to winged aphids. In this way CMV could be transmitted with the migration of winged aphids. We should take measures to prevent aphids in the early stage of their occurrence in the field to prevent virus outbreak.

Cotton S-adenosylmethionine decarboxylase-mediated spermine biosynthesis is required for salicylic acid- and leucine-correlated signaling in the defense response to Verticillium dahliae

Cotton S-adenosylmethionine decarboxylase-, rather than spermine synthase-, mediated spermine biosynthesis is required for salicylic acid- and leucine-correlated signaling in the defense response to Verticillium dahliae. Spermine (Spm) signaling is correlated with plant resistance to the fungal pathogen Verticillium dahliae. We identified genes for key rate-limiting enzymes in the biosynthesis of Spm, namely S-adenosylmethionine decarboxylase (GhSAMDC) and Spm synthase (GhSPMS). These were found by screening suppression subtractive hybridization and cDNA libraries of cotton (Gossypium) species tolerant to Verticillium wilt. Both were induced early and strongly by inoculation with V. dahliae and application of plant hormones. Silencing of GhSPMS or GhSAMDC in cotton leaves led to a significant accumulation of upstream substrates and, ultimately, enhanced plant susceptibility to Verticillium infection. Exogenous supplementation of Spm to the silenced cotton plants improved resistance. When compared with the wild type (WT), constitutive expression of GhSAMDC in Arabidopsis thaliana was associated with greater Verticillium wilt resistance and higher accumulations of Spm, salicylic acid, and leucine during the infection period. By contrast, transgenic Arabidopsis plants that over-expressed GhSPMS were unexpectedly more susceptible than the WT to V. dahliae and they also had impaired levels of putrescine (Put) and salicylic acid (SA). The susceptibility exhibited in GhSPMS-overexpressing Arabidopsis plants was partially reversed by the exogenous supply of Put or SA. In addition, the responsiveness of those two transgenic Arabidopsis lines to V. dahliae was associated with an alteration in transcripts of genes involved in plant resistance to epidermal penetrations and amino acid signaling. Together, these results suggest that GhSAMDC-, rather than GhSPMS-, mediated spermine biosynthesis contributes to plant resistance against V. dahliae through SA- and leucine-correlated signaling.

Optimizing Chemically Induced Resistance in Tomato Against Botrytis cinerea

Resistance-inducing chemicals can offer broad-spectrum disease protection in crops, but can also affect plant growth and interactions with plant-beneficial microbes. We have evaluated different application methods of β-aminobutyric acid (BABA) and jasmonic acid (JA) for long-lasting induced resistance in tomato against Botrytis cinerea. In addition, we have studied nontarget effects on plant growth and root colonization by arbuscular mycorrhizal fungi (AMF). Germinating seeds for 1 week in BABA- or JA-containing solutions promoted seed germination efficiency, did not affect plant growth, and induced resistance in 4-week-old plants. When formulating BABA and JA in carboxymethyl cellulose seed coating, only BABA was able to induce resistance in 4-week-old plants. Root treatment of 1-week-old seedlings with BABA or JA also induced resistance in 4-week-old plants. However, this seedling treatment repressed plant growth at higher concentrations of the chemicals, which was particularly pronounced in hydroponically grown plants after BABA treatment. Both seed coating with BABA, and seedling treatments with BABA or JA, did not affect AMF root colonization in soil-grown tomato. Our study has identified commercially feasible application methods of BABA and JA, which induce durable disease resistance in tomato without concurrent impacts on plant growth or colonization by plant-beneficial AMF.

A Small Secreted Virulence-Related Protein Is Essential for the Necrotrophic Interactions of Sclerotinia sclerotiorum with Its Host Plants

Small, secreted proteins have been found to play crucial roles in interactions between biotrophic/hemi-biotrophic pathogens and plants. However, little is known about the roles of these proteins produced by broad host-range necrotrophic phytopathogens during infection. Here, we report that a cysteine-rich, small protein SsSSVP1 in the necrotrophic phytopathogen Sclerotinia sclerotiorum was experimentally confirmed to be a secreted protein, and the secretion of SsSSVP1 from hyphae was followed by internalization and cell-to-cell movement independent of a pathogen in host cells. SsSSVP1∆SP could induce significant plant cell death and targeted silencing of SsSSVP1 resulted in a significant reduction in virulence. Through yeast two-hybrid (Y2H), coimmunoprecipitation (co-IP) and bimolecular fluorescence complementation (BiFC) assays, we demonstrated that SsSSVP1∆SP interacted with QCR8, a subunit of the cytochrome b-c1 complex of mitochondrial respiratory chain in plants. Double site-directed mutagenesis of two cysteine residues (C38 and C44) in SsSSVP1∆SP had significant effects on its homo-dimer formation, SsSSVP1∆SP-QCR8 interaction and plant cell death induction, indicating that partial cysteine residues surely play crucial roles in maintaining the structure and function of SsSSVP1. Co-localization and BiFC assays showed that SsSSVP1∆SP might hijack QCR8 to cytoplasm before QCR8 targeting into mitochondria, thereby disturbing its subcellular localization in plant cells. Furthermore, virus induced gene silencing (VIGS) of QCR8 in tobacco caused plant abnormal development and cell death, indicating the cell death induced by SsSSVP1∆SP might be caused by the SsSSVP1∆SP-QCR8 interaction, which had disturbed the QCR8 subcellular localization and hence disabled its biological functions. These results suggest that SsSSVP1 is a potential effector which may manipulate plant energy metabolism to facilitate the infection of S. sclerotiorum. Our findings indicate novel roles of small secreted proteins in the interactions between host-non-specific necrotrophic fungi and plants, and highlight the significance to illuminate the pathogenic mechanisms of this type of interaction.

How Microbes Twist Jasmonate Signaling around Their Little Fingers

Plant immunity relies on a complex network of hormone signaling pathways in which jasmonic acid (JA) plays a central role. Successful microbial pathogens or symbionts have developed strategies to manipulate plant hormone signaling pathways to cause hormonal imbalances for their own benefit. These strategies include the production of plant hormones, phytohormone mimics, or effector proteins that target host components to disrupt hormonal signaling pathways and enhance virulence. Here, we describe the molecular details of the most recent and best-characterized examples of specific JA hormonal manipulation by microbes, which exemplify the ingenious ways by which pathogens can take control over the plant’s hormone signaling network to suppress host immunity.

Tomato SlERF.A1, SlERF.B4, SlERF.C3 and SlERF.A3, Members of B3 Group of ERF Family, Are Required for Resistance to Botrytis cinerea

The Ethylene-Responsive Factors (ERFs) comprise a large family of transcriptional factors that play critical roles in plant immunity. Gray mold disease caused by Botrytis cinerea, a typical necrotrophic fungal pathogen, is the serious disease that threatens tomato production worldwide. However, littler is known about the molecular mechanism regulating the immunity to B. cinerea in tomato. In the present study, virus-induced gene silencing (VIGS)-based functional analyses of 18 members of B3 group (also called Group IX) in tomato ERF family were performed to identify putative ERFs that are involved in disease resistance against B. cinerea. VIGS-based silencing of either SlERF.B1 or SlERF.C2 had lethal effect while silencing of SlERF.A3 (Pit4) significantly suppressed vegetative growth of tomato plants. Importantly, silencing of SlERF.A1, SlERF.A3, SlERF.B4, or SlERF.C3 resulted in increased susceptibility to B. cinerea, attenuated the B. cinerea-induced expression of jasmonic acid/ethylene-mediated signaling responsive defense genes and promoted the B. cinerea-induced H₂O₂ accumulation. However, silencing of SlERF.A3 also decreased the resistance against Pseudomonas syringae pv. tomato (Pst) DC3000 but silencing of SlERF.A1, SlERF.B4 or SlERF.C3 did not affect the resistance to this bacterial pathogen. Expression of SlERF.A1, SlERF.A3, SlERF.B4, or SlERF.C3 was induced by B. cinerea and by defense signaling hormones such as salicylic acid, methyl jasmonate, and 1-aminocyclopropane-1-carboxylic acid (an ethylene precursor). SlERF.A1, SlERF.B4, SlERF.C3, and SlERF.A3 proteins were found to localize in nucleus of cells and possess transactivation activity in yeasts. These data suggest that SlERF.A1, SlERF.B4, and SlERF.C3, three previously uncharacterized ERFs in B3 group, and SlERF.A3, a previously identified ERF with function in immunity to Pst DC3000, play important roles in resistance against B. cinerea in tomato.

Emerging Trends in Molecular Interactions between Plants and the Broad Host Range Fungal Pathogens Botrytis cinerea and Sclerotinia sclerotiorum

Fungal plant pathogens are major threats to food security worldwide. Sclerotinia sclerotiorum and Botrytis cinerea are closely related Ascomycete plant pathogens causing mold diseases on hundreds of plant species. There is no genetic source of complete plant resistance to these broad host range pathogens known to date. Instead, natural plant populations show a continuum of resistance levels controlled by multiple genes, a phenotype designated as quantitative disease resistance. Little is known about the molecular mechanisms controlling the interaction between plants and S. sclerotiorum and B. cinerea but significant advances were made on this topic in the last years. This minireview highlights a selection of nine themes that emerged in recent research reports on the molecular bases of plant-S. sclerotiorum and plant-B. cinerea interactions. On the fungal side, this includes progress on understanding the role of oxalic acid, on the study of fungal small secreted proteins. Next, we discuss the exchanges of small RNA between organisms and the control of cell death in plant and fungi during pathogenic interactions. Finally on the plant side, we highlight defense priming by mechanical signals, the characterization of plant Receptor-like proteins and the hormone abscisic acid in the response to B. cinerea and S. sclerotiorum, the role of plant general transcription machinery and plant small bioactive peptides. These represent nine trends we selected as remarkable in our understanding of fungal molecules causing disease and plant mechanisms associated with disease resistance to two devastating broad host range fungi.

Extracellular polysaccharides from Ascomycota and Basidiomycota: production conditions, biochemical characteristics, and biological properties

Fungal polysaccharides (PSs) are the subject of research in many fields of science and industry. Many properties of PSs have already been confirmed and the list of postulated functions continues to grow. Fungal PSs are classified into different groups according to systematic affinity, structure (linear and branched), sugar composition (homo- and heteropolysaccharides), type of bonds between the monomers (β-(1 → 3), β-(1 → 6), and α-(1 → 3)) and their location in the cell (cell wall PSs, exoPSs, and endoPSs). Exopolysaccharides (EPSs) are most frequently studied fungal PSs but their definition, classification, and origin are still not clear and should be explained. Ascomycota and Basidiomycota fungi producing EPS have different ecological positions (saprotrophic and endophytic, pathogenic or symbiotic-mycorrhizae fungi); therefore, EPSs play different biological functions, for example in the protection against environmental stress factors and in interactions with other organisms. EPSs obtained from Ascomycota and Basidiomycota fungal cultures are known for their antioxidant, immunostimulating, antitumor, and antimicrobial properties. The major objective of the presented review article was to provide a detailed description of the state-of-the-art knowledge of the effectiveness of EPS production by filamentous and yeast Ascomycota and Basidiomycota fungi and techniques of derivation of EPSs, their biochemical characteristics, and biological properties allowing comprehensive analysis as well as indication of similarities and differences between these fungal groups. Understanding the role of EPSs in a variety of processes and their application in food or pharmaceutical industries requires improvement of the techniques of their derivation, purification, and characterization. The detailed analyses of data concerning the derivation and application of Ascomycota and Basidiomycota EPSs can facilitate development and trace the direction of application of these EPSs in different branches of industry, agriculture, and medicine.

Phytophthora infestans RXLR Effector AVR1 Interacts with Exocyst Component Sec5 to Manipulate Plant Immunity

Phytophthora infestans secretes numerous RXLR effectors that modulate host defense and thereby pave the way for successful invasion. Here, we show that the RXLR effector AVR1 is a virulence factor that promotes colonization and suppresses callose deposition, a hallmark of basal defense. To identify host targets of AVR1, we performed yeast two-hybrid screens and selected Sec5 as a candidate. Sec5 is a subunit of the exocyst, a protein complex that is involved in vesicle trafficking. AVR1-like (A-L), a close homolog of AVR1, also acts as a virulence factor, but unlike AVR1, A-L does not suppress CRINKLER2 (CRN2)-induced cell death or interact with Sec5. Compared with AVR1, A-L is shorter and lacks the carboxyl-terminal tail, the T-region that is crucial for CRN2-induced cell death suppression and Sec5 interaction. In planta analyses revealed that AVR1 and Sec5 are in close proximity, and coimmunoprecipitation confirmed the interaction. Sec5 is required for secretion of the pathogenesis-related protein PR-1 and callose deposition and also plays a role in CRN2-induced cell death. Our findings show that P. infestans manipulates an exocyst subunit and thereby potentially disturbs vesicle trafficking, a cellular process that is important for basal defense. This is a novel strategy that oomycete pathogens exploit to modulate host defense.

Modifications of Sphingolipid Content Affect Tolerance to Hemibiotrophic and Necrotrophic Pathogens by Modulating Plant Defense Responses in Arabidopsis

Sphingolipids are emerging as second messengers in programmed cell death and plant defense mechanisms. However, their role in plant defense is far from being understood, especially against necrotrophic pathogens. Sphingolipidomics and plant defense responses during pathogenic infection were evaluated in the mutant of long-chain base phosphate (LCB-P) lyase, encoded by the dihydrosphingosine-1-phosphate lyase1 (AtDPL1) gene and regulating long-chain base/LCB-P homeostasis. Atdpl1 mutants exhibit tolerance to the necrotrophic fungus Botrytis cinerea but susceptibility to the hemibiotrophic bacterium Pseudomonas syringae pv tomato (Pst). Here, a direct comparison of sphingolipid profiles in Arabidopsis (Arabidopsis thaliana) during infection with pathogens differing in lifestyles is described. In contrast to long-chain bases (dihydrosphingosine [d18:0] and 4,8-sphingadienine [d18:2]), hydroxyceramide and LCB-P (phytosphingosine-1-phosphate [t18:0-P] and 4-hydroxy-8-sphingenine-1-phosphate [t18:1-P]) levels are higher in Atdpl1-1 than in wild-type plants in response to B. cinerea. Following Pst infection, t18:0-P accumulates more strongly in Atdpl1-1 than in wild-type plants. Moreover, d18:0 and t18:0-P appear as key players in Pst- and B. cinerea-induced cell death and reactive oxygen species accumulation. Salicylic acid levels are similar in both types of plants, independent of the pathogen. In addition, salicylic acid-dependent gene expression is similar in both types of B. cinerea-infected plants but is repressed in Atdpl1-1 after treatment with Pst. Infection with both pathogens triggers higher jasmonic acid, jasmonoyl-isoleucine accumulation, and jasmonic acid-dependent gene expression in Atdpl1-1 mutants. Our results demonstrate that sphingolipids play an important role in plant defense, especially toward necrotrophic pathogens, and highlight a novel connection between the jasmonate signaling pathway, cell death, and sphingolipids.

Tomato histone H2B monoubiquitination enzymes SlHUB1 and SlHUB2 contribute to disease resistance against Botrytis cinerea through modulating the balance between SA- and JA/ET-mediated signaling pathways

BACKGROUND

Histone H2B monoubiquitination pathway has been shown to play critical roles in regulating growth/development and stress response in Arabidopsis. In the present study, we explored the involvement of the tomato histone H2B monoubiquitination pathway in defense response against Botrytis cinerea by functional analysis of SlHUB1 and SlHUB2, orthologues of the Arabidopsis AtHUB1/AtHUB2.

METHODS

We used the TRV-based gene silencing system to knockdown the expression levels of SlHUB1 or SlHUB2 in tomato plants and compared the phenotype between the silenced and the control plants after infection with B. cinerea and Pseudomonas syringae pv. tomato (Pst) DC3000. Biochemical and interaction properties of proteins were examined using in vitro histone monoubiquitination and yeast two-hybrid assays, respectively. The transcript levels of genes were analyzed by quantitative real time PCR (qRT-PCR).

RESULTS

The tomato SlHUB1 and SlHUB2 had H2B monoubiquitination E3 ligases activity in vitro and expression of SlHUB1 and SlHUB2 was induced by infection of B. cinerea and Pst DC3000 and by treatment with salicylic acid (SA) and 1-amino cyclopropane-1-carboxylic acid (ACC). Silencing of either SlHUB1 or SlHUB2 in tomato plants showed increased susceptibility to B. cinerea, whereas silencing of SlHUB1 resulted in increased resistance against Pst DC3000. SlMED21, a Mediator complex subunit, interacted with SlHUB1 but silencing of SlMED21 did not affect the disease resistance to B. cinerea and Pst DC3000. The SlHUB1- and SlHUB2-silenced plants had thinner cell wall but increased accumulation of reactive oxygen species (ROS), increased callose deposition and exhibited altered expression of the genes involved in phenylpropanoid pathway and in ROS generation and scavenging system. Expression of genes in the SA-mediated signaling pathway was significantly upregulated, whereas expression of genes in the jasmonic acid (JA)/ethylene (ET)-mediated signaling pathway were markedly decreased in SlHUB1- and SlHUB2-silenced plants after infection of B. cinerea.

CONCLUSION

VIGS-based functional analyses demonstrate that both SlHUB1 and SlHUB2 contribute to resistance against B. cinerea most likely through modulating the balance between the SA- and JA/ET-mediated signaling pathways.

Beyond plant defense: insights on the potential of salicylic and methylsalicylic acid to contain growth of the phytopathogen Botrytis cinerea

Using Botrytis cinerea we confirmed in the present work several previous studies showing that salicylic acid, a main plant hormone, inhibits fungal growth in vitro. Such an inhibitory effect was also observed for the two salicylic acid derivatives, methylsalicylic and acetylsalicylic acid. In marked contrast, 5-sulfosalicylic acid was totally inactive. Comparative proteomics from treated vs. control mycelia showed that both the intracellular and extracellular proteomes were affected in the presence of salicylic acid or methylsalicylic acid. These data suggest several mechanisms that could potentially account for the observed fungal growth inhibition, notably pH regulation, metal homeostasis, mitochondrial respiration, ROS accumulation and cell wall remodeling. The present observations support a role played by the phytohormone SA and derivatives in directly containing the pathogen. Data are available via ProteomeXchange with identifier PXD002873.

Salicylic acid is required for Mi-1-mediated resistance of tomato to whitefly Bemisia tabaci, but not for basal defense to this insect pest

Plant defense to pests or pathogens involves global changes in gene expression mediated by multiple signaling pathways. A role for the salicylic acid (SA) signaling pathway in Mi-1-mediated resistance of tomato (Solanum lycopersicum) to aphids was previously identified and its implication in the resistance to root-knot nematodes is controversial, but the importance of SA in basal and Mi-1-mediated resistance of tomato to whitefly Bemisia tabaci had not been determined. SA levels were measured before and after B. tabaci infestation in susceptible and resistant Mi-1-containing tomatoes, and in plants with the NahG bacterial transgene. Tomato plants of the same genotypes were also screened with B. tabaci (MEAM1 and MED species, before known as B and Q biotypes, respectively). The SA content in all tomato genotypes transiently increased after infestation with B. tabaci albeit at variable levels. Whitefly fecundity or infestation rates on susceptible Moneymaker were not significantly affected by the expression of NahG gene, but the Mi-1-mediated resistance to B. tabaci was lost in VFN NahG plants. Results indicated that whiteflies induce both SA and jasmonic acid accumulation in tomato. However, SA has no role in basal defense of tomato against B. tabaci. In contrast, SA is an important component of the Mi-1-mediated resistance to B. tabaci in tomato.

Loss of bcbrn1 and bcpks13 in Botrytis cinerea Not Only Blocks Melanization But Also Increases Vegetative Growth and Virulence

Botrytis cinerea is a necrotrophic pathogen that causes gray mold disease in a broad range of plants. Dihydroxynaphthalene (DHN) melanin is a major component of the extracellular matrix of B. cinerea, but knowledge of the exact role of melanin biosynthesis in this pathogen is unclear. In this study, we characterize two genes in B. cinerea, bcpks13 and bcbrn1, encoding polyketide synthase and tetrahydroxynaphthalene (THN) reductases, respectively, and both have predicted roles in DHN melanin biosynthesis. The ∆bcpks13 and ∆bcbrn1 mutants show white and orange pigmentation, respectively, and the mutants are also deficient in conidiation in vitro but show enhanced growth rates and virulence on hosts. Moreover, the mutants display elevated acidification of the complete medium (CM), probably due to oxalic acid secretion and secretion of cell wall-degrading enzymes, and preferably utilize plant cell-wall components as carbon sources for mycelium growth in vitro. In contrast, overexpression of bcbrn1 (OE::bcbrn1 strain) results in attenuated hydrolytic enzyme secretion, acidification ability, and virulence. Taken together, these results indicate that bcpks13 and bcbrn1 participate in diverse cellular and developmental processes, such as melanization and conidiation in B. cinerea in vitro, but they negatively regulate the virulence of this pathogen.

Prosystemin Overexpression in Tomato Enhances Resistance to Different Biotic Stresses by Activating Genes of Multiple Signaling Pathways

Systemin is a signal peptide that promotes the response to wounding and herbivore attack in tomato. This 18-amino acid peptide is released from a larger precursor, prosystemin. To study the role of systemin as a modulator of defense signaling, we generated tomato (Solanum lycopersicum) transgenic plants that overexpress the prosystemin cDNA. We carried out a transcriptomic analysis comparing two different transgenic events with the untransformed control. The Gene Ontology categories of the 503 differentially expressed genes indicated that several biological functions were affected. Systemin promotes the expression of an array of defense genes that are dependent on different signaling pathways and it downregulates genes connected with carbon fixation and carbohydrate metabolism. These alterations present a degree of overlap with the response programs that are classically associated to pathogen defense or abiotic stress protection, implying that end products of the systemin signaling pathway may be more diverse than expected. We show also that the observed transcriptional modifications have a relevant functional outcome, since transgenic lines were more resistant against very different biotic stressors such as aphids (Macrosiphum euphorbiae), phytopathogenic fungi (Botrytis cinerea and Alternaria alternata) and phytophagous larvae (Spodoptera littoralis). Our work demonstrated that in tomato the modulation of a single gene is sufficient to provide a wide resistance against stress by boosting endogenous defense pathways. Overall, the data provided evidence that the systemin peptide might serve as DAMP signal in tomato, acting as a broad indicator of tissue integrity.

Induced systemic resistance against Botrytis cinerea by Micromonospora strains isolated from root nodules

Micromonospora is a Gram positive bacterium that can be isolated from nitrogen fixing nodules from healthy leguminous plants, where they could be beneficial to the plant. Their plant growth promoting activity in legume and non-legume plants has been previously demonstrated. The present study explores the ability of Micromonospora strains to control fungal pathogens and to stimulate plant immunity. Micromonospora strains isolated from surface sterilized nodules of alfalfa showed in vitro antifungal activity against several pathogenic fungi. Moreover, root inoculation of tomato plants with these Micromonospora strains effectively reduced leaf infection by the fungal pathogen Botrytis cinerea, despite spatial separation between both microorganisms. This induced systemic resistance, confirmed in different tomato cultivars, is long lasting. Gene expression analyses evidenced that Micromonospora stimulates the plant capacity to activate defense mechanisms upon pathogen attack. The defensive response of tomato plants inoculated with Micromonospora spp. differs from that of non-inoculated plants, showing a stronger induction of jasmonate-regulated defenses when the plant is challenged with a pathogen. The hypothesis of jasmonates playing a key role in this defense priming effect was confirmed using defense-impaired tomato mutants, since the JA-deficient line def1 was unable to display a long term induced resistance upon Micromonospora spp. inoculation. In conclusion, nodule isolated Micromonospora strains should be considered excellent candidates as biocontrol agents as they combine both direct antifungal activity against plant pathogens and the ability to prime plant immunity.

The Plant Peptidome: An Expanding Repertoire of Structural Features and Biological Functions

Peptides fulfill a plethora of functions in plant growth, development, and stress responses. They act as key components of cell-to-cell communication, interfere with signaling and response pathways, or display antimicrobial activity. Strikingly, both the diversity and amount of plant peptides have been largely underestimated. Most characterized plant peptides to date acting as small signaling peptides or antimicrobial peptides are derived from nonfunctional precursor proteins. However, evidence is emerging on peptides derived from a functional protein, directly translated from small open reading frames (without the involvement of a precursor) or even encoded by primary transcripts of microRNAs. These novel types of peptides further add to the complexity of the plant peptidome, even though their number is still limited and functional characterization as well as translational evidence are often controversial. Here, we provide a comprehensive overview of the reported types of plant peptides, including their described functional and structural properties. We propose a novel, unifying peptide classification system to emphasize the enormous diversity in peptide synthesis and consequent complexity of the still expanding knowledge on the plant peptidome.

Keeping Control: The Role of Senescence and Development in Plant Pathogenesis and Defense

Many plant pathogens show interactions with host development. Pathogens may modify plant development according to their nutritional demands. Conversely, plant development influences pathogen growth. Biotrophic pathogens often delay senescence to keep host cells alive, and resistance is achieved by senescence-like processes in the host. Necrotrophic pathogens promote senescence in the host, and preventing early senescence is a resistance strategy of plants. For hemibiotrophic pathogens both patterns may apply. Most signaling pathways are involved in both developmental and defense reactions. Increasing knowledge about the molecular components allows to distinguish signaling branches, cross-talk and regulatory nodes that may influence the outcome of an infection. In this review, recent reports on major molecular players and their role in senescence and in pathogen response are reviewed. Examples of pathosystems with strong developmental implications illustrate the molecular basis of selected control strategies. A study of gene expression in the interaction between the hemibiotrophic vascular pathogen Verticillium longisporum and its cruciferous hosts shows processes that are fine-tuned to counteract early senescence and to achieve resistance. The complexity of the processes involved reflects the complex genetic control of quantitative disease resistance, and understanding the relationship between disease, development and resistance will support resistance breeding.

CYP94-mediated jasmonoyl-isoleucine hormone oxidation shapes jasmonate profiles and attenuates defence responses to Botrytis cinerea infection

Induced resistance to the necrotrophic pathogen Botrytis cinerea depends on jasmonate metabolism and signalling in Arabidopsis. We have presented here extensive jasmonate profiling in this pathosystem and investigated the impact of the recently reported jasmonoyl-isoleucine (JA-Ile) catabolic pathway mediated by cytochrome P450 (CYP94) enzymes. Using a series of mutant and overexpressing (OE) plant lines, we showed that CYP94B3 and CYP94C1 are integral components of the fungus-induced jasmonate metabolic pathway and control the abundance of oxidized conjugated but also some unconjugated derivatives, such as sulfated 12-HSO4-JA. Despite causing JA-Ile overaccumulation due to impaired oxidation, CYP94 deficiency had negligible impacts on resistance, associated with enhanced JAZ repressor transcript levels. In contrast, plants overexpressing (OE) CYP94B3 or CYP94C1 were enriched in 12-OH-JA-Ile or 12-COOH-JA-Ile respectively. This shift towards oxidized JA-Ile derivatives was concomitant with strongly impaired defence gene induction and reduced disease resistance. CYP94B3-OE, but unexpectedly not CYP94C1-OE, plants displayed reduced JA-Ile levels compared with the wild type, suggesting that increased susceptibility in CYP94C1-OE plants may result from changes in the hormone oxidation ratio rather than absolute changes in JA-Ile levels. Consistently, while feeding JA-Ile to seedlings triggered strong induction of JA pathway genes, induction was largely reduced or abolished after feeding with the CYP94 products 12-OH-JA-Ile and 12-COOH-JA-Ile, respectively. This trend paralleled in vitro pull-down assays where 12-COOH-JA-Ile was unable to promote COI1-JAZ9 co-receptor assembly. Our results highlight the dual function of CYP94B3/C1 in antimicrobial defence: by controlling hormone oxidation status for signal attenuation, these enzymes also define JA-Ile as a metabolic hub directing jasmonate profile complexity.

CYP94-mediated jasmonoyl-isoleucine hormone oxidation shapes jasmonate profiles and attenuates defence responses to Botrytis cinerea infection

Induced resistance to the necrotrophic pathogen Botrytis cinerea depends on jasmonate metabolism and signalling in Arabidopsis. We have presented here extensive jasmonate profiling in this pathosystem and investigated the impact of the recently reported jasmonoyl-isoleucine (JA-Ile) catabolic pathway mediated by cytochrome P450 (CYP94) enzymes. Using a series of mutant and overexpressing (OE) plant lines, we showed that CYP94B3 and CYP94C1 are integral components of the fungus-induced jasmonate metabolic pathway and control the abundance of oxidized conjugated but also some unconjugated derivatives, such as sulfated 12-HSO4-JA. Despite causing JA-Ile overaccumulation due to impaired oxidation, CYP94 deficiency had negligible impacts on resistance, associated with enhanced JAZ repressor transcript levels. In contrast, plants overexpressing (OE) CYP94B3 or CYP94C1 were enriched in 12-OH-JA-Ile or 12-COOH-JA-Ile respectively. This shift towards oxidized JA-Ile derivatives was concomitant with strongly impaired defence gene induction and reduced disease resistance. CYP94B3-OE, but unexpectedly not CYP94C1-OE, plants displayed reduced JA-Ile levels compared with the wild type, suggesting that increased susceptibility in CYP94C1-OE plants may result from changes in the hormone oxidation ratio rather than absolute changes in JA-Ile levels. Consistently, while feeding JA-Ile to seedlings triggered strong induction of JA pathway genes, induction was largely reduced or abolished after feeding with the CYP94 products 12-OH-JA-Ile and 12-COOH-JA-Ile, respectively. This trend paralleled in vitro pull-down assays where 12-COOH-JA-Ile was unable to promote COI1-JAZ9 co-receptor assembly. Our results highlight the dual function of CYP94B3/C1 in antimicrobial defence: by controlling hormone oxidation status for signal attenuation, these enzymes also define JA-Ile as a metabolic hub directing jasmonate profile complexity.

A cupin domain-containing protein with a quercetinase activity (VdQase) regulates Verticillium dahliae's pathogenicity and contributes to counteracting host defenses

We previously identified rutin as part of potato root responses to its pathogen Verticillium dahliae. Rutin was directly toxic to the pathogen at doses greater than 160 μM, a threshold below which many V. dahliae pathogenicity-related genes were up-regulated. We identified and characterized a cupin domain-containing protein (VdQase) with a dioxygenase activity and a potential role in V. dahliae-potato interactions. The pathogenicity of VdQase knock-out mutants generated through Agrobacterium tumefasciens-mediated transformation was significantly reduced on susceptible potato cultivar Kennebec compared to wild type isolates. Fluorescence microscopy revealed a higher accumulation of flavonols in the stems of infected potatoes and a higher concentration of rutin in the leaves in response to the VdQase mutants as compared to wild type isolates. This, along with the HPLC characterization of high residual and non-utilized quercetin in presence of the knockout mutants, indicates the involvement of VdQase in the catabolism of quercetin and possibly other flavonols in planta. Quantification of Salicylic and Jasmonic Acids (SA, JA) in response to the mutants vs. wild type isolates revealed involvement of VdQase in the interference with signaling, suggesting a role in pathogenicity. It is hypothesized that the by-product of dioxygenation 2-protocatechuoylphloroglucinolcarboxylic acid, after dissociating into phloroglucinol and protocatechuoyl moieties, becomes a starting point for benzoic acid and SA, thereby interfering with the JA pathway and affecting the interaction outcome. These events may be key factors for V. dahliae in countering potato defenses and becoming notorious in the rhizosphere.

The toolbox of Trichoderma spp. in the biocontrol of Botrytis cinerea disease

Botrytis cinerea is a necrotrophic fungal pathogen causing disease in many plant species, leading to economically important crop losses. So far, fungicides have been widely used to control this pathogen. However, in addition to their detrimental effects on the environment and potential risks for human health, increasing fungicide resistance has been observed in the B. cinerea population. Biological control, that is the application of microbial organisms to reduce disease, has gained importance as an alternative or complementary approach to fungicides. In this respect, the genus Trichoderma constitutes a promising pool of organisms with potential for B. cinerea control. In the first part of this article, we review the specific mechanisms involved in the direct interaction between the two fungi, including mycoparasitism, the production of antimicrobial compounds and enzymes (collectively called antagonism), and competition for nutrients and space. In addition, biocontrol has also been observed when Trichoderma is physically separated from the pathogen, thus implying an indirect systemic plant defence response. Therefore, in the second part, we describe the consecutive steps leading to induced systemic resistance (ISR), starting with the initial Trichoderma-plant interaction and followed by the activation of downstream signal transduction pathways and, ultimately, the defence response resulting in ISR (ISR-prime phase). Finally, we discuss the ISR-boost phase, representing the effect of ISR priming by Trichoderma spp. on plant responses after additional challenge with B. cinerea.

Phylogenetic and expression analysis of the NPR1-like gene family from Persea americana (Mill.)

The NONEXPRESSOR OF PATHOGENESIS-RELATED GENES1 (NPR1) forms an integral part of the salicylic acid (SA) pathway in plants and is involved in cross-talk between the SA and jasmonic acid/ethylene (JA/ET) pathways. Therefore, NPR1 is essential to the effective response of plants to pathogens. Avocado (Persea americana) is a commercially important crop worldwide. Significant losses in production result from Phytophthora root rot, caused by the hemibiotroph, Phytophthora cinnamomi. This oomycete infects the feeder roots of avocado trees leading to an overall decline in health and eventual death. The interaction between avocado and P. cinnamomi is poorly understood and as such limited control strategies exist. Thus uncovering the role of NPR1 in avocado could provide novel insights into the avocado - P. cinnamomi interaction. A total of five NPR1-like sequences were identified. These sequences were annotated using FGENESH and a maximum-likelihood tree was constructed using 34 NPR1-like protein sequences from other plant species. The conserved protein domains and functional motifs of these sequences were predicted. Reverse transcription quantitative PCR was used to analyze the expression of the five NPR1-like sequences in the roots of avocado after treatment with salicylic and jasmonic acid, P. cinnamomi infection, across different tissues and in P. cinnamomi infected tolerant and susceptible rootstocks. Of the five NPR1-like sequences three have strong support for a defensive role while two are most likely involved in development. Significant differences in the expression profiles of these five NPR1-like genes were observed, assisting in functional classification. Understanding the interaction of avocado and P. cinnamomi is essential to developing new control strategies. This work enables further classification of these genes by means of functional annotation and is a crucial step in understanding the role of NPR1 during P. cinnamomi infection.

Involvement of nitric oxide in the jasmonate-dependent basal defense against root-knot nematode in tomato plants

Jasmonic acid (JA) and nitric oxide (NO) are well-characterized signaling molecules in plant defense responses. However, their roles in plant defense against root-knot nematode (RKN, Meloidogyne incognita) infection are largely unknown. In this study, we found that the transcript levels of the JA- and NO-related biosynthetic and signaling component genes were induced after RKN infection. Application of exogenous JA and sodium nitroprusside (SNP; a NO donor) significantly decreased the number of egg masses in tomato roots after RKN infection and partially alleviated RKN-induced decreases in plant fresh weight and net photosynthetic rate. These molecules also alleviated RKN-induced increases in root electrolyte leakage and membrane peroxidation. Importantly, NO scavenger partially inhibited JA-induced RKN defense. The pharmacological inhibition of JA biosynthesis significantly increased the plants' susceptibility to RKNs, which was effectively alleviated by SNP application, showing that NO may be involved in the JA-dependent RKN defense pathway. Furthermore, both JA and SNP induced increases in protease inhibitor 2 (PI2) gene expression after RKN infestation. Silencing of PI2 compromised both JA- and SNP-induced RKN defense responses, suggesting that the PI2 gene mediates JA- and NO-induced defense against RKNs. This work will be important for deepening the understanding of the mechanisms involved in basal defense against RKN attack in plants.

Antagonism between phytohormone signalling underlies the variation in disease susceptibility of tomato plants under elevated CO2

Increasing CO2 concentrations ([CO2]) have the potential to disrupt plant-pathogen interactions in natural and agricultural ecosystems, but the research in this area has often produced conflicting results. Variations in phytohormone salicylic acid (SA) and jasmonic acid (JA) signalling could be associated with variations in the responses of pathogens to plants grown under elevated [CO2]. In this study, interactions between tomato plants and three pathogens with different infection strategies were compared. Elevated [CO2] generally favoured SA biosynthesis and signalling but repressed the JA pathway. The exposure of plants to elevated [CO2] revealed a lower incidence and severity of disease caused by tobacco mosaic virus (TMV) and by Pseudomonas syringae, whereas plant susceptibility to necrotrophic Botrytis cinerea increased. The elevated [CO2]-induced and basal resistance to TMV and P. syringae were completely abolished in plants in which the SA signalling pathway nonexpressor of pathogenesis-related genes 1 (NPR1) had been silenced or in transgenic plants defective in SA biosynthesis. In contrast, under both ambient and elevated [CO2], the susceptibility to B. cinerea highly increased in plants in which the JA signalling pathway proteinase inhibitors (PI) gene had been silenced or in a mutant affected in JA biosynthesis. However, plants affected in SA signalling remained less susceptible to this disease. These findings highlight the modulated antagonistic relationship between SA and JA that contributes to the variation in disease susceptibility under elevated [CO2]. This information will be critical for investigating how elevated CO2 may affect plant defence and the dynamics between plants and pathogens in both agricultural and natural ecosystems.

Tomato Whole Genome Transcriptional Response to Tetranychus urticae Identifies Divergence of Spider Mite-Induced Responses Between Tomato and Arabidopsis

The two-spotted spider mite Tetranychus urticae is one of the most significant mite pests in agriculture, feeding on more than 1,100 plant hosts, including model plants Arabidopsis thaliana and tomato, Solanum lycopersicum. Here, we describe timecourse tomato transcriptional responses to spider mite feeding and compare them with Arabidopsis in order to determine conserved and divergent defense responses to this pest. To refine the involvement of jasmonic acid (JA) in mite-induced responses and to improve tomato Gene Ontology annotations, we analyzed transcriptional changes in the tomato JA-signaling mutant defenseless1 (def-1) upon JA treatment and spider mite herbivory. Overlay of differentially expressed genes (DEG) identified in def-1 onto those from the timecourse experiment established that JA controls expression of the majority of genes differentially regulated by herbivory. Comparison of defense responses between tomato and Arabidopsis highlighted 96 orthologous genes (of 2,133 DEG) that were recruited for defense against spider mites in both species. These genes, involved in biosynthesis of JA, phenylpropanoids, flavonoids, and terpenoids, represent the conserved core of induced defenses. The remaining tomato DEG support the establishment of tomato-specific defenses, indicating profound divergence of spider mite-induced responses between tomato and Arabidopsis.

Role of dioxygenase α-DOX2 and SA in basal response and in hexanoic acid-induced resistance of tomato (Solanum lycopersicum) plants against Botrytis cinerea

Resistance of tomato (Solanum Lycopersicum) to the fungal pathogen Botrytis cinerea requires complex interplay between hormonal signalling. In this study, we explored the involvement of new oxylipins in the tomato basal and induced response to this necrotroph through the functional analysis of the tomato α-dioxygenase2 (α-DOX2)-deficient mutant divaricata. We also investigated the role of SA in the defence response against this necrotrophic fungus using SA-deficient tomato nahG plants. The plants lacking dioxigenase α-DOX2, which catalyses oxylipins production from fatty acids, were more susceptible to Botrytis, and hexanoic acid-induced resistance (Hx-IR) was impaired; hence α-DOX2 is required for both tomato defence and the enhanced protection conferred by natural inducer hexanoic acid (Hx) against B. cinerea. The divaricata plants accumulated less pathogen-induced callose and presented lower levels of jasmonic acid (JA) and 12-oxo-phytodienoic acid (OPDA) upon infection if compared to the wild type. Glutathion-S-transferase (GST) gene expression decreased and ROS production significantly increased in Botrytis-infected divaricata plants. These results indicate that absence of α-DOX2 influences the hormonal changes, oxidative burst and callose deposition that occur upon Botrytis infection in tomato. The study of SA-deficient nahG tomato plants showed that the plants with low SA levels displayed increased resistance to Botrytis, but were unable to display Hx-IR. This supports the involvement of SA in Hx-IR. NaghG plants displayed reduced callose and ROS accumulation upon infection and an increased GST expression. This reflects a positive relationship between SA and these defensive mechanisms in tomato. Finally, Hx boosted the pathogen-induced callose in nahG plants, suggesting that this priming mechanism is SA-independent. Our results support the involvement of the oxylipins pathway and SA in tomato response to Botrytis, probably through complex crosstalk of the hormonal balance with callose and ROS accumulation, and reinforce the role of the oxidative stress in the outcome of the plant-Botrytis interaction.

BcGs1, a glycoprotein from Botrytis cinerea, elicits defence response and improves disease resistance in host plants

In this study, a necrosis-inducing protein was purified from the culture filtrate of the necrotrophic fungus Botrytis cinerea BC-98 strain. Secreted proteins were collected and fractionated by liquid chromatography. The fraction with the highest necrosis-inducing activity was further purified. A glycoprotein named BcGs1 was identified by 2D electrophoresis and mass spectrometry. The BcGs1 protein consisted of 672 amino acids with a theoretical molecular weight of 70.487 kDa. Functional domain analysis indicated that BcGs1 was a glucan 1,4-alpha-glucosidase, a cell wall-degrading enzyme, with a Glyco_hydro_15 domain and a CBM20_glucoamylase domain. The BcGs1 protein caused necrotic lesions that mimicked a typical hypersensitive response and H2O2 production in tomato and tobacco leaves. BcGs1-treated plants exhibited resistance to B. cinerea, Pseudomonas syringae pv. tomato DC3000 and tobacco mosaic virus in systemic leaves. In addition, BcGs1 triggered elevation of the transcript levels of the defence-related genes PR-1a, TPK1b and Prosystemin. This is the first report of a Botrytis glucan 1,4-alpha-glucosidase triggering host plant immunity as an elicitor. These results lay a foundation for further study of the comprehensive interaction between plants and necrotrophic fungi.

Reactive oxygen and nitrogen (ROS and RNS) species generation and cell death in tomato suspension cultures--Botrytis cinerea interaction

This article reports events connected to cell survival and Botrytis cinerea infection development in cell suspension cultures of two tomato cultivars which show different levels of susceptibility to the pathogen: cv. Corindo (more susceptible) and cv. Perkoz (less susceptible). In parallel changes in reactive oxygen (ROS) and nitrogen (RNS) species generation and in S-nitrosoglutathione reductase (GSNOR) activity were studied. In vivo staining methods with acridine orange (AO) and ethidium bromide (EB) as well as fluorescent microscopy were used to assess tomato and B. cinerea cells death. The biochemical studies of ROS and RNS concentrations in plant cell extract were complemented by in vivo ROS and nitric oxide (NO) imaging using nitro blue tetrazolium (NBT), diaminobenzidine (DAB) and diaminofluorescein diacetate (DAF-DA) staining methods, and confocal microscope technique. B. cinerea infection proceeded slower in Perkoz cell cultures. It was evidenced by measuring the pathogen conidia germination and germination tube development in which nuclei revealing cell death dominated. Two different types of tomato cell death were observed: cells with necrotic nuclei dominated in Corindo whereas in Perkoz cells with characteristic of vacuolar death type prevailed. In Perkoz cells, constitutive levels of NO and S-nitrosothiols (SNO) were significantly higher and hydrogen peroxide (H₂O₂) and superoxide anion (O₂(-)) concentrations were slightly higher as compared with Corindo cells. Moreover, increases in these molecule concentrations as a result of B. cinerea inoculation were observed in both, Perkoz and Corindo cell cultures. The enzymatic GSNOR activity seems to be an important player in controlling the SNO level in tomato cells. Involvements of the studied compounds in molecular mechanisms of tomato resistance to B. cinerea are discussed in the paper.

The Xanthomonas effector XopJ triggers a conditional hypersensitive response upon treatment of N. benthamiana leaves with salicylic acid

XopJ is a Xanthomonas type III effector protein that promotes bacterial virulence on susceptible pepper plants through the inhibition of the host cell proteasome and a resultant suppression of salicylic acid (SA) - dependent defense responses. We show here that Nicotiana benthamiana leaves transiently expressing XopJ display hypersensitive response (HR) -like symptoms when exogenously treated with SA. This apparent avirulence function of XopJ was further dependent on effector myristoylation as well as on an intact catalytic triad, suggesting a requirement of its enzymatic activity for HR-like symptom elicitation. The ability of XopJ to cause a HR-like symptom development upon SA treatment was lost upon silencing of SGT1 and NDR1, respectively, but was independent of EDS1 silencing, suggesting that XopJ is recognized by an R protein of the CC-NBS-LRR class. Furthermore, silencing of NPR1 abolished the elicitation of HR-like symptoms in XopJ expressing leaves after SA application. Measurement of the proteasome activity indicated that proteasome inhibition by XopJ was alleviated in the presence of SA, an effect that was not observed in NPR1 silenced plants. Our results suggest that XopJ - triggered HR-like symptoms are closely related to the virulence function of the effector and that XopJ follows a two-signal model in order to elicit a response in the non-host plant N. benthamiana.

Microbial effectors target multiple steps in the salicylic acid production and signaling pathway

Microbes attempting to colonize plants are recognized through the plant immune surveillance system. This leads to a complex array of global as well as specific defense responses, which are often associated with plant cell death and subsequent arrest of the invader. The responses also entail complex changes in phytohormone signaling pathways. Among these, salicylic acid (SA) signaling is an important pathway because of its ability to trigger plant cell death. As biotrophic and hemibiotrophic pathogens need to invade living plant tissue to cause disease, they have evolved efficient strategies to downregulate SA signaling by virulence effectors, which can be proteins or secondary metabolites. Here we review the strategies prokaryotic pathogens have developed to target SA biosynthesis and signaling, and contrast this with recent insights into how plant pathogenic eukaryotic fungi and oomycetes accomplish the same goal.

Impact of hormonal crosstalk on plant resistance and fitness under multi-attacker conditions

The hormone salicylic acid (SA) generally induces plant defenses against biotrophic pathogens. Jasmonic acid (JA) and its oxylipin derivatives together with ethylene (ET) are generally important hormonal regulators of induced plant defenses against necrotrophic pathogens, whereas JAs together with abscisic acid (ABA) are implicated in induced plant defenses against herbivorous insects. Hormonal crosstalk between the different plant defense pathways has often been hypothesized to be a cost-saving strategy that has evolved as a means of the plant to reduce allocation costs by repression of unnecessary defenses, thereby minimizing trade-offs between plant defense and growth. However, proof for this hypothesis has not been demonstrated yet. In this study the impact of hormonal crosstalk on disease resistance and fitness of Arabidopsis thaliana when under multi-species attack was investigated. Induction of SA- or JA/ABA-dependent defense responses by the biotrophic pathogen Hyaloperonospora arabidopsidis or the herbivorous insect Pieris rapae, respectively, was shown to reduce the level of induced JA/ET-dependent defense against subsequent infection with the necrotrophic pathogen Botrytis cinerea. However, despite the enhanced susceptibility to this second attacker, no additional long-term negative effects were observed on plant fitness when plants had been challenged by multiple attackers. Similarly, when plants were grown in dense competition stands to enlarge fitness effects of induced defenses, treatment with a combination of SA and MeJA did not cause additional negative effects on plant fitness in comparison to the single MeJA treatment. Together, these data support the notion that hormonal crosstalk in plants during multi-attacker interactions allows plants to prioritize their defenses, while limiting the fitness costs associated with induction of defenses.

Jasmonate signalling in Arabidopsis involves SGT1b-HSP70-HSP90 chaperone complexes

Plant hormones play pivotal roles in growth, development and stress responses. Although it is essential to our understanding of hormone signalling, how plants maintain a steady state level of hormone receptors is poorly understood. We show that mutation of the Arabidopsis thaliana co-chaperone SGT1b impairs responses to the plant hormones jasmonate, auxin and gibberellic acid, but not brassinolide and abscisic acid, and that SGT1b and its homologue SGT1a are involved in maintaining the steady state levels of the F-box proteins COI1 and TIR1, receptors for jasmonate and auxin, respectively. The association of SGT1b with COI1 is direct and is independent of the Arabidopsis SKP1 protein, ASK1. We further show that COI1 is a client protein of SGT1b-HSP70-HSP90 chaperone complexes and that the complexes function in hormone signalling by stabilizing the COI1 protein. This study extends the SGT1b-HSP90 client protein list and broadens the functional scope of SGT1b-HSP70-HSP90 chaperone complexes.

Transcriptome analysis of the compatible interaction of tomato with Verticillium dahliae using RNA-sequencing

Tomato Verticillium wilt is a soil-borne vascular disease caused by the necrotrophic fungus Verticillium dahliae. Although some understanding of plant defense mechanisms against V. dahliae infection has been gained for incompatible interactions, including identification of inducible resistant genes and defense signaling pathways, the genes and signaling pathways involved in the compatible interaction remain unclear. To investigate the molecular basis of the compatible interaction between tomato and V. dahliae, transcriptomes of V. dahliae infected tomatoes were compared to those of a control group. A total of approximately 25 million high-quality reads were generated by means of the RNA sequencing (RNA-seq) method. The sequence reads were aligned to the tomato reference genome and analyzed to measure gene expression levels, and to identify alternative splicing events. Comparative analysis between the two samples revealed 1,953 significantly differentially expressed genes (DEGs), including 1,281 up-regulated and 672 down-regulated genes. The RNA-Seq output was confirmed using RT-qPCR for 10 selected genes. The Nr, Swiss-Prot, Gene Ontology (GO), Clusters of Orthologous Groups (COG), and Kyoto Encyclopedia of Genes and Genomes (KEGG) databases were used to annotate DEG functions. Of the 1,953 DEGs identified, 1,953, 1,579, 1,739, 862, and 380 were assigned by Nr, Swiss-Prot, GO, COG, and KEGG, respectively. The important functional groups identified via GO and COG enrichment were those responsible for fundamental biological regulation, secondary metabolism, and signal transduction. Of DEGs assigned to 87 KEGG pathways, most were associated with phenylpropanoid metabolism and plant-pathogen interaction pathways. Most of the DEGs involved in these two pathways were up-regulated, and may be involved in regulating the tomato-V. dahliae compatible interaction. The results will help to identify key susceptible genes and contribute to a better understanding of the mechanisms of tomato susceptible response to V. dahliae.

Improving crop disease resistance: lessons from research on Arabidopsis and tomato

One of the great challenges for food security in the 21st century is to improve yield stability through the development of disease-resistant crops. Crop research is often hindered by the lack of molecular tools, growth logistics, generation time and detailed genetic annotations, hence the power of model plant species. Our knowledge of plant immunity today has been largely shaped by the use of models, specifically through the use of mutants. We examine the importance of Arabidopsis and tomato as models in the study of plant immunity and how they help us in revealing a detailed and deep understanding of the various layers contributing to the immune system. Here we describe examples of how knowledge from models can be transferred to economically important crops resulting in new tools to enable and accelerate classical plant breeding. We will also discuss how models, and specifically transcriptomics and effectoromics approaches, have contributed to the identification of core components of the defense response which will be key to future engineering of durable and sustainable disease resistance in plants.

The phytotoxin coronatine is a multifunctional component of the virulence armament of Pseudomonas syringae

Plant pathogens deploy an array of virulence factors to suppress host defense and promote pathogenicity. Numerous strains of Pseudomonas syringae produce the phytotoxin coronatine (COR). A major aspect of COR function is its ability to mimic a bioactive jasmonic acid (JA) conjugate and thus target the JA-receptor COR-insensitive 1 (COI1). Biological activities of COR include stimulation of JA-signaling and consequent suppression of SA-dependent defense through antagonistic crosstalk, antagonism of stomatal closure to allow bacterial entry into the interior of plant leaves, contribution to chlorotic symptoms in infected plants, and suppression of plant cell wall defense through perturbation of secondary metabolism. Here, we review the virulence function of COR, including updates on these established activities as well as more recent findings revealing COI1-independent activity of COR and shedding light on cooperative or redundant defense suppression between COR and type III effector proteins.

Defense suppression benefits herbivores that have a monopoly on their feeding site but can backfire within natural communities

BACKGROUND

Plants have inducible defenses to combat attacking organisms. Hence, some herbivores have adapted to suppress these defenses. Suppression of plant defenses has been shown to benefit herbivores by boosting their growth and reproductive performance.

RESULTS

We observed in field-grown tomatoes that spider mites (Tetranychus urticae) establish larger colonies on plants already infested with the tomato russet mite (Aculops lycopersici). Using laboratory assays, we observed that spider mites have a much higher reproductive performance on russet mite-infested plants, similar to their performance on the jasmonic acid (JA)-biosynthesis mutant def-1. Hence, we tested if russet mites suppress JA-responses thereby facilitating spider mites. We found that russet mites manipulate defenses: they induce those mediated by salicylic acid (SA) but suppress those mediated by JA which would otherwise hinder growth. This suppression of JA-defenses occurs downstream of JA-accumulation and is independent from its natural antagonist SA. In contrast, spider mites induced both JA- and SA-responses while plants infested with the two mite species together display strongly reduced JA-responses, yet a doubled SA-response. The spider mite-induced JA-response in the presence of russet mites was restored on transgenic tomatoes unable to accumulate SA (nahG), but russet mites alone still did not induce JA-responses on nahG plants. Thus, indirect facilitation of spider mites by russet mites depends on the antagonistic action of SA on JA while suppression of JA-defenses by russet mites does not. Furthermore, russet mite-induced SA-responses inhibited secondary infection by Pseudomonas syringae (Pst) while not affecting the mite itself. Finally, while facilitating spider mites, russet mites experience reduced population growth.

CONCLUSIONS

Our results show that the benefits of suppressing plant defenses may diminish within communities with natural competitors. We show that suppression of defenses via the JA-SA antagonism can be a consequence, rather than the cause, of a primary suppression event and that its overall effect is determined by the presence of competing herbivores and the distinct palette of defenses these induce. Thus, whether or not host-defense manipulation improves an herbivore's fitness depends on interactions with other herbivores via induced-host defenses, implicating bidirectional causation of community structure of herbivores sharing a plant.

Absence of endo-1,4-β-glucanase KOR1 alters the jasmonate-dependent defence response to Pseudomonas syringae in Arabidopsis

During plant-pathogen interactions, the plant cell wall forms part of active defence against invaders. In recent years, cell wall-editing enzymes, associated with growth and development, have been related to plant susceptibility or resistance. Our previous work identified a role for several tomato and Arabidopsis endo-1,4-β-glucanases (EGs) in plant-pathogen interactions. Here we studied the response of the Arabidopsis thaliana T-DNA insertion mutant lacking EG Korrigan1 (KOR1) infected with Pseudomonas syringae. KOR1 is predicted to be an EG which is thought to participate in cellulose biosynthesis. We found that kor1-1 plants were more susceptible to P. syringae, and displayed severe disease symptoms and enhanced bacterial growth if compared to Wassilewskija (Ws) wild-type plants. Hormonal and gene expression analyses revealed that the jasmonic acid (JA) pathway was activated more in kor1-1 plants with an increase in the JA-biosynthesis gene LOX3 and a greater accumulation of JA. Upon infection the accumulation of JA and JA-isoleucine (JA-Ile) was higher than in wild-type plants and increased the induction of LOX3 and the JA-responsive PDF1.2 gene. In addition, the increase of salicylic acid (SA) in healthy and infected kor1-1 may reflect the complex interaction between JA and SA, which results in the more susceptible phenotype displayed by the infected mutant plants. Callose deposition was enhanced in infected kor1-1 and an increase in pathogen-induced hydrogen peroxide took place. The susceptible phenotype displayed by KOR1-deficient plants was coronatine-independent. No significant changes were detected in the hormonal profile of the kor1-1 plants infected by coronatine-deficient P. syringae cmaA, which supports that absence of EG KOR1 alters per se the plant response to infection. We previously reported increased resistance of kor1-1 to B. cinerea, hence, the lack of this EG alters cell wall properties and plant responses in such a way that benefits P. syringae colonisation but restricts B. cinerea invasion.

Expression profiling during arabidopsis/downy mildew interaction reveals a highly-expressed effector that attenuates responses to salicylic acid

Plants have evolved strong innate immunity mechanisms, but successful pathogens evade or suppress plant immunity via effectors delivered into the plant cell. Hyaloperonospora arabidopsidis (Hpa) causes downy mildew on Arabidopsis thaliana, and a genome sequence is available for isolate Emoy2. Here, we exploit the availability of genome sequences for Hpa and Arabidopsis to measure gene-expression changes in both Hpa and Arabidopsis simultaneously during infection. Using a high-throughput cDNA tag sequencing method, we reveal expression patterns of Hpa predicted effectors and Arabidopsis genes in compatible and incompatible interactions, and promoter elements associated with Hpa genes expressed during infection. By resequencing Hpa isolate Waco9, we found it evades Arabidopsis resistance gene RPP1 through deletion of the cognate recognized effector ATR1. Arabidopsis salicylic acid (SA)-responsive genes including PR1 were activated not only at early time points in the incompatible interaction but also at late time points in the compatible interaction. By histochemical analysis, we found that Hpa suppresses SA-inducible PR1 expression, specifically in the haustoriated cells into which host-translocated effectors are delivered, but not in non-haustoriated adjacent cells. Finally, we found a highly-expressed Hpa effector candidate that suppresses responsiveness to SA. As this approach can be easily applied to host-pathogen interactions for which both host and pathogen genome sequences are available, this work opens the door towards transcriptome studies in infection biology that should help unravel pathogen infection strategies and the mechanisms by which host defense responses are overcome.

Polyphenols and the human brain: plant “secondary metabolite” ecologic roles and endogenous signaling functions drive benefits

Flavonoids and other polyphenols are ubiquitous plant chemicals that fulfill a range of ecologic roles for their home plant, including protection from a range of biotic and abiotic stressors and a pivotal role in the management of pathogenic and symbiotic soil bacteria and fungi. They form a natural part of the human diet, and evidence suggests that their consumption is associated with the beneficial modulation of a number of health-related variables, including those related to cardiovascular and brain function. Over recent years, the consensus as to the mechanisms responsible for these effects in humans has shifted away from polyphenols having direct antioxidant effects and toward their modulation of cellular signal transduction pathways. To date, little consideration has been given to the question of why, rather than how, these plant-derived chemicals might exert these effects. Therefore, this review summarizes the evidence suggesting that polyphenols beneficially affect human brain function and describes the current mechanistic hypotheses explaining these effects. It then goes on to describe the ecologic roles and potential endogenous signaling functions that these ubiquitous phytochemicals play within their home plant and discusses whether these functions drive their beneficial effects in humans via a process of “cross-kingdom” signaling predicated on the many conserved similarities in plant, microbial, and human cellular signal transduction pathways.

Calmodulin Gene Expression in Response to Mechanical Wounding and Botrytis cinerea Infection in Tomato Fruit

Calmodulin, a ubiquitous calcium sensor, plays an important role in decoding stress-triggered intracellular calcium changes and regulates the functions of numerous target proteins involved in various plant physiological responses. To determine the functions of calmodulin in fleshy fruit, expression studies were performed on a family of six calmodulin genes (SlCaMs) in mature-green stage tomato fruit in response to mechanical injury and Botrytis cinerea infection. Both wounding and pathogen inoculation triggered expression of all those genes, with SlCaM2 being the most responsive one to both treatments. Furthermore, all calmodulin genes were upregulated by salicylic acid and methyl jasmonate, two signaling molecules involved in plant immunity. In addition to SlCaM2, SlCaM1 was highly responsive to salicylic acid and methyl jasmonate. However, SlCaM2 exhibited a more rapid and stronger response than SlCaM1. Overexpression of SlCaM2 in tomato fruit enhanced resistance to Botrytis-induced decay, whereas reducing its expression resulted in increased lesion development. These results indicate that calmodulin is a positive regulator of plant defense in fruit by activating defense pathways including salicylate- and jasmonate-signaling pathways, and SlCaM2 is the major calmodulin gene responsible for this event.

Bemisia tabaci Q carrying tomato yellow leaf curl virus strongly suppresses host plant defenses

The concurrence of tomato yellow leaf curl virus (TYLCV) with the spread of its vector Bemisia tabaci Q rather than B in China suggests a more mutualistic relationship between TYLCV and Q. Here, we investigated the hypothesis that viruliferous B and Q have different effects on plant defenses. We found the fecundity of nonviruliferous B, nonviruliferous Q, viruliferous Q and viruliferous B was 11.080, 12.060, 10.760, and 11.220 respectively on plants previously attacked by the other biotype, however, on their respective noninfested control leaves fecundity was 12.000, 10.880, 9.760, and 8.020 respectively. Only viruliferous B had higher fecundity on viruliferous Q-infested plants than on control plants. The longevity of viruliferous B showed the same phenomenon. At 1 d infestion, the jasmonic acid content in leaves noninfested and in leaves infested with nonviruliferous B, nonviruliferous Q, viruliferous B and viruliferous Q was 407.000, 281.333, 301.333, 266.667 and 134.000 ng/g FW, respectively. The JA content was lowest in viruliferous Q-infested leaves. The proteinase inhibitor activity and expression of JA-related upstream gene LOX and downstream gene PI II showed the same trend. The substantial suppression of host defenses by Q carrying TYLCV probably enhances the spread of Q and TYLCV in China.

Intervention of Phytohormone Pathways by Pathogen Effectors

The constant struggle between plants and microbes has driven the evolution of multiple defense strategies in the host as well as offense strategies in the pathogen. To defend themselves from pathogen attack, plants often rely on elaborate signaling networks regulated by phytohormones. In turn, pathogens have adopted innovative strategies to manipulate phytohormone-regulated defenses. Tactics frequently employed by plant pathogens involve hijacking, evading, or disrupting hormone signaling pathways and/or crosstalk. As reviewed here, this is achieved mechanistically via pathogen-derived molecules known as effectors, which target phytohormone receptors, transcriptional activators and repressors, and other components of phytohormone signaling in the host plant. Herbivores and sap-sucking insects employ obligate pathogens such as viruses, phytoplasma, or symbiotic bacteria to intervene with phytohormone-regulated defenses. Overall, an improved understanding of phytohormone intervention strategies employed by pests and pathogens during their interactions with plants will ultimately lead to the development of new crop protection strategies.

Systemic resistance to gray mold induced in tomato by benzothiadiazole and Trichoderma harzianum T39

Gray mold (Botrytis cinerea) is an important disease of tomato (Solanum lycopersicum). This study examined defense-related gene expression involved in the resistance to B. cinerea that is induced in tomato plants by benzothiadiazole and Trichoderma harzianum T39 soil drench. In whole plants, transcriptional changes related to salicylic acid and ethylene were induced by the application of a 0.01% benzothiadiazole solution, whereas changes related to jasmonic acid were induced by the application of a 0.4% T39 suspension. On detached leaves, soil treatment by T39 led to enhanced resistance to B. cinerea infection that was proportional to the concentration of the T39 suspension. By 5 days after pathogen inoculation, the plants that had received the 0.04% T39 drench exhibited 62% less severe disease than the untreated plants. The 0.4% T39 drench led to an 84% reduction in disease severity. Observations of B. cinerea infection in leaves harvested from plants grown in the treated soils revealed that drenching with a T39 suspension induces systemic resistance against B. cinerea and primes salicylic acid- and ethylene-related gene expression in a manner proportional to the concentration of the biocontrol agent. Benzothiadiazole treatment induced resistance to gray mold independently of salicylic acid and led to strong priming of two genes known to be involved in defense against B. cinerea, Pti5 and PI2.

Manipulation of methyl jasmonate esterase activity renders tomato more susceptible to Sclerotinia sclerotiorum

Jasmonic acid methyl ester has been discussed as a stress signal in plants. To investigate the relevance of reversible methylation of jasmonic acid, stress responses of transgenic tomato lines with altered expression and activity of methyl jasmonate esterase were analysed. No consistent changes in levels of methyl jasmonate, 12-oxo-phytodienoic acid, jasmonic acid, jasmonic acid isoleucine and expression of the jasmonate-responsive genes AOC and PINII between control line and RNAi as well as overexpressing lines were detectable under basal and wound-induced conditions. In contrast, reduction as well as enhancement of methyl jasmonate esterase activity resulted in increased susceptibility to the fungal pathogen Sclerotinia sclerotiorum despite higher levels of the hormonal active jasmonic acid isoleucine conjugate. Results suggest that methyl jasmonate esterase has a function in vivo in plant defence, which appears not to be related to its in vitro capacity to hydrolyse methyl jasmonate.

Do strigolactones contribute to plant defence?

Strigolactones are multifunctional molecules involved in several processes outside and within the plant. As signalling molecules in the rhizosphere, they favour the establishment of arbuscular mycorrhizal symbiosis, but they also act as host detection cues for root parasitic plants. As phytohormones, they are involved in the regulation of plant architecture, adventitious rooting, secondary growth and reproductive development, and novel roles are emerging continuously. In the present study, the possible involvement of strigolactones in plant defence responses was investigated. For this purpose, the resistance/susceptibility of the strigolactone-deficient tomato mutant Slccd8 against the foliar fungal pathogens Botrytis cinerea and Alternaria alternata was assessed. Slccd8 was more susceptible to both pathogens, pointing to a new role for strigolactones in plant defence. A reduction in the content of the defence-related hormones jasmonic acid, salicylic acid and abscisic acid was detected by high-performance liquid chromatography coupled to tandem mass spectrometry in the Slccd8 mutant, suggesting that hormone homeostasis is altered in the mutant. Moreover, the expression level of the jasmonate-dependent gene PinII, involved in the resistance of tomato to B. cinerea, was lower than in the corresponding wild-type. We propose here that strigolactones play a role in the regulation of plant defences through their interaction with other defence-related hormones, especially with the jasmonic acid signalling pathway.

Role of tomato lipoxygenase D in wound-induced jasmonate biosynthesis and plant immunity to insect herbivores

In response to insect attack and mechanical wounding, plants activate the expression of genes involved in various defense-related processes. A fascinating feature of these inducible defenses is their occurrence both locally at the wounding site and systemically in undamaged leaves throughout the plant. Wound-inducible proteinase inhibitors (PIs) in tomato (Solanum lycopersicum) provide an attractive model to understand the signal transduction events leading from localized injury to the systemic expression of defense-related genes. Among the identified intercellular molecules in regulating systemic wound response of tomato are the peptide signal systemin and the oxylipin signal jasmonic acid (JA). The systemin/JA signaling pathway provides a unique opportunity to investigate, in a single experimental system, the mechanism by which peptide and oxylipin signals interact to coordinate plant systemic immunity. Here we describe the characterization of the tomato suppressor of prosystemin-mediated responses8 (spr8) mutant, which was isolated as a suppressor of (pro)systemin-mediated signaling. spr8 plants exhibit a series of JA-dependent immune deficiencies, including the inability to express wound-responsive genes, abnormal development of glandular trichomes, and severely compromised resistance to cotton bollworm (Helicoverpa armigera) and Botrytis cinerea. Map-based cloning studies demonstrate that the spr8 mutant phenotype results from a point mutation in the catalytic domain of TomLoxD, a chloroplast-localized lipoxygenase involved in JA biosynthesis. We present evidence that overexpression of TomLoxD leads to elevated wound-induced JA biosynthesis, increased expression of wound-responsive genes and, therefore, enhanced resistance to insect herbivory attack and necrotrophic pathogen infection. These results indicate that TomLoxD is involved in wound-induced JA biosynthesis and highlight the application potential of this gene for crop protection against insects and pathogens.

The ATF/CREB transcription factor Atf1 is essential for full virulence, deoxynivalenol production, and stress tolerance in the cereal pathogen Fusarium graminearum

Fusarium graminearum is a necrotrophic plant pathogen of cereals that produces mycotoxins such as deoxynivalenol (DON) and zearalenone (ZEA) in grains. The stress-activated mitogen-activated protein kinase FgOS-2 is a central regulator in F. graminearum and controls, among others, virulence and DON and ZEA production. Here, we characterized the ATF/CREB-activating transcription factor FgAtf1, a regulator that functions downstream of FgOS-2. We created deletion and overexpression mutants of Fgatf1, the latter being also in an FgOS-2 deletion mutant. FgAtf1 localizes to the nucleus and appears to interact with FgOS-2 under osmotic stress conditions. Deletion mutants in Fgatf1 (ΔFgatf1) are more sensitive to osmotic stress and less sensitive to oxidative stress compared with the wild type. Furthermore, sexual reproduction is delayed. ΔFgatf1 strains produced higher amounts of DON under in vitro induction conditions than that of the wild type. However, during wheat infection, DON production by ΔFgatf1 is strongly reduced. The ΔFgatf1 strains displayed strongly reduced virulence to wheat and maize. Interestingly, constitutive expression of Fgatf1 in the wild type led to hypervirulence on wheat, maize, and Brachypodium distachyon. Moreover, constitutive expression of Fgatf1 in the ΔFgOS-2 mutant background almost complements ΔFgOS-2-phenotypes. These data suggest that FgAtf1 may be the most important transcription factor regulated by FgOS-2.