Interaction between SGT1 and cytosolic/nuclear HSC70 chaperones regulates Arabidopsis immune responses

The Involvement of Heat Shock Proteins in the Establishment of Tomato Yellow Leaf Curl Virus Infection

Tomato yellow leaf curl virus (TYLCV), a begomovirus, induces protein aggregation in infected tomatoes and in its whitefly vector Bemisia tabaci. The interactions between TYLCV and HSP70 and HSP90 in plants and vectors are necessity for virus infection to proceed. In infected host cells, HSP70 and HSP90 are redistributed from a soluble to an aggregated state. These aggregates contain, together with viral DNA/proteins and virions, HSPs and components of the protein quality control system such as ubiquitin, 26S proteasome subunits, and the autophagy protein ATG8. TYLCV CP can form complexes with HSPs in tomato and whitefly. Nonetheless, HSP70 and HSP90 play different roles in the viral cell cycle in the plant host. In the infected host cell, HSP70, but not HSP90, participates in the translocation of CP from the cytoplasm into the nucleus. Viral amounts decrease when HSP70 is inhibited, but increase when HSP90 is downregulated. In the whitefly vector, HSP70 impairs the circulative transmission of TYLCV; its inhibition increases transmission. Hence, the efficiency of virus acquisition by whiteflies depends on the functionality of both plant chaperones and their cross-talk with other protein mechanisms controlling virus-induced aggregation.

A subclass of HSP70s regulate development and abiotic stress responses in Arabidopsis thaliana

Members of the HSP70 family function as molecular chaperones to maintain cellular homeostasis and help plants cope with environmental stimuli. However, due to functional redundancy and lack of effective chemical inhibitors, our knowledge of functions of individual HSP70s has remained limited. Here, we confirmed a subclass of HSP70s, including HSP70-1, -2, -3, -4, and -5, localized to the cytosol and nucleus in Arabidopsis thaliana. Histochemical analyses of promoter:GUS reporter lines showed that HSP70-1, -2, -3, and -4 genes were widely expressed, but HSP70-5 was not. In addition, individual HSP70 showed not only similar but also distinct transcriptions when treated by different abiotic stresses and phytohormones. No apparent phenotype was observed when individual HSP70 genes were overexpressed or knocked-out/down, but the double mutant hsp70-1 hsp70-4 and triple mutant hsp70-2 hsp70-4 hsp70-5 plants exhibited developmental phenotypes with shortened specific growth periods, curly and round leaves, twisted petioles, thin stems, and short siliques. Moreover, both mutants were hypersensitive to heat, cold, high glucose, salt and osmotic stress, but hyposensitive to abscisic acid. Genes related to flowering, and the cytokinin, brassinosteroid, and abscisic acid signaling pathways were differentially expressed in both mutants. Our studies suggest that, the individual HSP70 possibly performs both redundant and specific functions with the other members in the cytosolic/nuclear HSP70 subclass, and apart from enabling plants to cope with abiotic stresses, this subclass of cytosolic/nuclear HSP70 proteins also participates in diverse developmental processes and signaling pathways.

Molecular functions of Xanthomonas type III effector AvrBsT and its plant interactors in cell death and defense signaling

Xanthomonas effector AvrBsT interacts with plant defense proteins and triggers cell death and defense response. This review highlights our current understanding of the molecular functions of AvrBsT and its host interactor proteins. The AvrBsT protein is a member of a growing family of effector proteins in both plant and animal pathogens. Xanthomonas type III effector AvrBsT, a member of the YopJ/AvrRxv family, suppresses plant defense responses in susceptible hosts, but triggers cell death signaling leading to hypersensitive response (HR) and defense responses in resistant plants. AvrBsT interacts with host defense-related proteins to trigger the HR cell death and defense responses in plants. Here, we review and discuss recent progress in understanding the molecular functions of AvrBsT and its host interactor proteins in pepper (Capsicum annuum). Pepper arginine decarboxylase1 (CaADC1), pepper aldehyde dehydrogenase1 (CaALDH1), pepper heat shock protein 70a (CaHSP70a), pepper suppressor of the G2 allele of skp1 (CaSGT1), pepper SNF1-related kinase1 (SnRK1), and Arabidopsis acetylated interacting protein1 (ACIP1) have been identified as AvrBsT interactors in pepper and Arabidopsis. Gene expression profiling, virus-induced gene silencing, and transient transgenic overexpression approaches have advanced the functional characterization of AvrBsT-interacting proteins in plants. AvrBsT is localized in the cytoplasm and forms protein-protein complexes with host interactors. All identified AvrBsT interactors regulate HR cell death and defense responses in plants. Notably, CaSGT1 physically binds to both AvrBsT and pepper receptor-like cytoplasmic kinase1 (CaPIK1) in the cytoplasm. During infection with Xanthomonas campestris pv. vesicatoria strain Ds1 (avrBsT), AvrBsT is phosphorylated by CaPIK1 and forms the active AvrBsT-CaSGT1-CaPIK1 complex, which ultimately triggers HR cell death and defense responses. Collectively, the AvrBsT interactor proteins are involved in plant cell death and immunity signaling.

The crystal structure of the Sgt1-Skp1 complex: the link between Hsp90 and both SCF E3 ubiquitin ligases and kinetochores

The essential cochaperone Sgt1 recruits Hsp90 chaperone activity to a range of cellular factors including SCF E3 ubiquitin ligases and the kinetochore in eukaryotes. In these pathways Sgt1 interacts with Skp1, a small protein that heterodimerizes with proteins containing the F-box motif. We have determined the crystal structure of the interacting domains of Saccharomyces cerevisiae Sgt1 and Skp1 at 2.8 Å resolution and validated the interface in the context of the full-length proteins in solution. The BTB/POZ domain of Skp1 associates with Sgt1 via the concave surface of its TPR domain using residues that are conserved in humans. Dimerization of yeast Sgt1 occurs via an insertion that is absent from monomeric human Sgt1. We identify point mutations that disrupt dimerization and Skp1 binding in vitro and find that the interaction with Skp1 is an essential function of Sgt1 in yeast. Our data provide a structural rationale for understanding the phenotypes of temperature-sensitive Sgt1 mutants and for linking Skp1-associated proteins to Hsp90-dependent pathways.

Membrane Proteomics of Arabidopsis Glucosinolate Mutants cyp79B2/B3 and myb28/29

Glucosinolates (Gls) constitute a major group of natural metabolites represented by three major classes (aliphatic, indolic and aromatic) of more than 120 chemical structures. In our previous work, soluble proteins and metabolites in Arabidopsis mutants deficient of aliphatic (myb28/29) and indolic Gls (cyp79B2B3) were analyzed. Here we focus on investigating the changes at the level of membrane proteins in these mutants. Our LC/MS-MS analyses of tandem mass tag (TMT) labeled peptides derived from the cyp79B2/B3 and myb28/29 relative to wild type resulted in the identification of 4,673 proteins, from which 2,171 are membrane proteins. Fold changes and statistical analysis showed 64 increased and 74 decreased in cyp79B2/B3, while 28 increased and 17 decreased in myb28/29. As to the shared protein changes between the mutants, one protein was increased and eight were decreased. Bioinformatics analysis of the changed proteins led to the discovery of three cytochromes in glucosinolate molecular network (GMN): cytochrome P450 86A7 (At1g63710), cytochrome P450 71B26 (At3g26290), and probable cytochrome c (At1g22840). CYP86A7 and CYP71B26 may play a role in hydroxyl-indolic Gls production. In addition, flavone 3'-O-methyltransferase 1 represents an interesting finding as it is likely to participate in the methylation process of the hydroxyl-indolic Gls to form methoxy-indolic Gls. The analysis also revealed additional new nodes in the GMN related to stress and defense activity, transport, photosynthesis, and translation processes. Gene expression and protein levels were found to be correlated in the cyp79B2/B3, but not in the myb28/29.

Endoplasmic reticulum stress responses function in the HRT-mediated hypersensitive response in Nicotiana benthamiana

HRT is a plant coiled-coil, nucleotide-binding and leucine-rich repeat (CC-NB-LRR) disease resistance protein that triggers the hypersensitive response (HR) on recognition of Turnip crinkle virus (TCV) coat protein (CP). The molecular mechanism and significance of HR-mediated cell death for TCV resistance have not been fully elucidated. To identify the genes involved in HRT/TCV CP-mediated HR in Nicotiana benthamiana, we performed virus-induced gene silencing (VIGS) of 459 expressed sequence tags (ESTs) of pathogen-responsive Capsicum annuum genes. VIGS of CaBLP5, which encodes an endoplasmic reticulum (ER)-associated immunoglobulin-binding protein (BiP), silenced NbBiP4 and NbBiP5 and significantly reduced HRT-mediated HR. The induction of ER stress-responsive genes and the accumulation of ER-targeted BiPs in response to HRT-mediated HR suggest that ER is involved in HR in N. benthamiana. BiP4/5 silencing significantly down-regulated HRT at the mRNA and protein levels, and affected SGT1 and HSP90 expression. Co-expression of TCV CP in BiP4/5-silenced plants completely abolished HRT induction. Transient expression of TCV CP alone induced selected ER stress-responsive gene transcripts only in Tobacco rattle virus (TRV)-infected plants, and most of these genes were induced by HRT/TCV CP, except for bZIP60, which was induced specifically in response to HRT/TCV CP. TCV CP-mediated induction of ER stress-responsive genes still occurred in BiP4/5-silenced plants, but HRT/TCV CP-mediated induction of these genes was defective. Tunicamycin, a chemical that inhibits protein N-glycosylation, inhibited HRT-mediated HR, suggesting that ER has a role in HR regulation. These results indicate that BiP and ER, which modulate pattern recognition receptors in innate immunity, also regulate R protein-mediated resistance.

Split-ubiquitin yeast two-hybrid interaction reveals a novel interaction between a natural resistance associated macrophage protein and a membrane bound thioredoxin in Brassica juncea

Natural resistance associated macrophage proteins (NRAMPs) are evolutionarily conserved metal transporters involved in the transport of essential and nonessential metals in plants. Fifty protein interactors of a Brassica juncea NRAMP protein was identified by a Split-Ubiquitin Yeast-Two-Hybrid screen. The interactors were predicted to function as components of stress response, signaling, development, RNA binding and processing. BjNRAMP4.1 interactors were particularly enriched in proteins taking part in photosynthetic or light regulated processes, or proteins predicted to be localized in plastid/chloroplast. Further, many interactors also had a suggested role in cellular redox regulation. Among these, the interaction of a photosynthesis-related thioredoxin, homologous to Arabidopsis HCF164 (High-chlorophyll fluorescence164) was studied in detail. Homology modeling of BjNRAMP4.1 suggested that it could be redox regulated by BjHCF164. In yeast, the interaction between the two proteins was found to increase in response to metal deficiency; Mn excess and exogenous thiol. Excess Mn also increased the interaction in planta and led to greater accumulation of the complex at the root apoplast. Network analysis of Arabidopsis homologs of BjNRAMP4.1 interactors showed enrichment of many protein components, central to chloroplastic/cellular ROS signaling. BjNRAMP4.1 interacted with BjHCF164 at the root membrane and also in the chloroplast in accordance with its proposed function related to photosynthesis, indicating that this interaction occurred at different sub-cellular locations depending on the tissue. This may serve as a link between metal homeostasis and chloroplastic/cellular ROS through protein-protein interaction.

Specific adjustments in grapevine leaf proteome discriminating resistant and susceptible grapevine genotypes to Plasmopara viticola

Grapevine downy mildew is an important disease affecting crop production leading to severe yield losses. This study aims to identify the grapevine cultivar-specific adjustments of leaf proteome that allow the discrimination between resistance and susceptibility towards P. viticola (constitutive (0h) and in after inoculation (6, 12 and 24h). Leaf proteome analysis was performed using 2D difference gel electrophoresis followed by protein identification via mass spectrometry. In addition, we analysed ROS production, antioxidant capacity, lipid peroxidation and gene expression. Proteins related to photosynthesis and metabolism allowed the discrimination of resistant and susceptible grapevine cultivars prior to P. viticola inoculation. Following inoculation increase of hydrogen peroxide levels, cellular redox regulation, establishment of ROS signalling and plant cell death seem to be key points differentiating the resistant genotype. Lipid associated signalling events, particularly related to jasmonates appear also to play a major role in the establishment of resistance. The findings from this study contribute to a better understanding of genotype-specific differences that account for a successful establishment of a defence response to the downy mildew pathogen.

BIOLOGICAL SIGNIFICANCE

Here, we present for the first time grapevine cultivar-specific adjustments of leaf proteome that allow the discrimination between resistance and susceptibility towards P. viticola (constitutive (0h) and in after inoculation (6, 12 and 24h). We have highlighted that, following inoculation, the major factors differentiating the resistant from the susceptible grapevine cultivars are the establishment of effective ROS signalling together with lipid-associated signalling events, particularly related to jasmonates. It is believed that plants infected with biotrophic pathogens suppress JA-mediated responses, however recent evidences shown that jasmonic acid signalling pathway in grapevine resistance against Plasmopara viticola. Our results corroborate those evidences and highlight the importance of lipid- signalling for an effective resistance response against the downy mildew pathogen.

Comparative proteomic study of Arabidopsis mutants mpk4 and mpk6

Arabidopsis MPK4 and MPK6 are implicated in different signalling pathways responding to diverse external stimuli. This was recently correlated with transcriptomic profiles of Arabidopsis mpk4 and mpk6 mutants, and thus it should be reflected also on the level of constitutive proteomes. Therefore, we performed a shot gun comparative proteomic analysis of Arabidopsis mpk4 and mpk6 mutant roots. We have used bioinformatic tools and propose several new proteins as putative MPK4 and MPK6 phosphorylation targets. Among these proteins in the mpk6 mutant were important modulators of development such as CDC48A and phospholipase D alpha 1. In the case of the mpk4 mutant transcriptional reprogramming might be mediated by phosphorylation and change in the abundance of mRNA decapping complex VCS. Further comparison of mpk4 and mpk6 root differential proteomes showed differences in the composition and regulation of defense related proteins. The mpk4 mutant showed altered abundances of antioxidant proteins. The examination of catalase activity in response to oxidative stress revealed that this enzyme might be preferentially regulated by MPK4. Finally, we proposed developmentally important proteins as either directly or indirectly regulated by MPK4 and MPK6. These proteins contribute to known phenotypic defects in the mpk4 and mpk6 mutants.

Characterization and abiotic stress-responsive expression analysis of SGT1 genes in Brassica oleracea

SGT1 genes are involved in enhancing plant responses to various biotic and abiotic stresses. Brassica oleracea is known to contain two types of SGT1 genes, namely suppressor of G2 allele of SKP1 and suppressor of GCR2. In this study, through systematic analysis, four putative SGT1 genes were identified and characterized in B. oleracea. In phylogenetic analysis, the genes clearly formed separate groups, namely BolSGT1a, BolSGT1b (both suppressor of G2 allele of SKP1 types), and BolSGT1 (suppressor of GCR2). Functional domain analysis and organ-specific expression patterns suggested possible roles for BolSGT1 genes during stress conditions. BolSGT1 genes showed significant changes in expression in response to heat, cold, drought, salt, or ABA treatment. Interaction network analysis supported the expression analysis, and showed that the BolSGT1a and BolSGT1b genes are strongly associated with co-regulators during stress conditions. However, the BolSGT1 gene did not show any strong association. Hence, BolSGT1 might be a stress resistance-related gene that functions without a co-regulator. Our results show that BolSGT1 genes are potential target genes to improve B. oleracea resistance to abiotic stresses such as heat, cold, and salt.

New nodes and edges in the glucosinolate molecular network revealed by proteomics and metabolomics of Arabidopsis myb28/29 and cyp79B2/B3 glucosinolate mutants

Glucosinolates present in Brassicales are important for human health and plant defense against insects and pathogens. Here we investigate the proteomes and metabolomes of Arabidopsis myb28/29 and cyp79B2/B3 mutants deficient in aliphatic glucosinolates and indolic glucosinolates, respectively. Quantitative proteomics of the myb28/29 and cyp79B2/B3 mutants led to the identification of 2785 proteins, of which 142 proteins showed significant changes in the two mutants compared to wild type (WT). By mapping the differential proteins using STRING, we detected 59 new edges in the glucosinolate metabolic network. These connections can be classified as primary with direct roles in glucosinolate metabolism, secondary related to plant stress responses, and tertiary involved in other biological processes. Gene Ontology analysis of the differential proteins showed high level of enrichment in the nodes belonging to metabolic process including glucosinolate biosynthesis and response to stimulus. Using metabolomics, we quantified 292 metabolites covering a broad spectrum of metabolic pathways, and 89 exhibited differential accumulation patterns between the mutants and WT. The changing metabolites (e.g., γ-glutamyl amino acids, auxins and glucosinolate hydrolysis products) complement our proteomics findings. This study contributes toward engineering and breeding of glucosinolate profiles in plants in efforts to improve human health, crop quality and productivity.

BIOLOGICAL SIGNIFICANCE

Glucosinolates in Brassicales constitute an important group of natural metabolites important for plant defense and human health. Its biosynthetic pathways and transcriptional regulation have been well-studied. Using Arabidopsis mutants of important genes in glucosinolate biosynthesis, quantitative proteomics and metabolomics led to identification of many proteins and metabolites that are potentially related to glucosinolate metabolism. This study provides a comprehensive insight into the molecular networks of glucosinolate metabolism, and will facilitate efforts toward engineering and breeding of glucosinolate profiles for enhanced crop defense, and nutritional value.

SGT1 is required in PcINF1/SRC2-1 induced pepper defense response by interacting with SRC2-1

PcINF1 was previously found to induce pepper defense response by interacting with SRC2-1, but the underlying mechanism remains uninvestigated. Herein, we describe the involvement of SGT1 in the PcINF1/SRC2-1-induced immunity. SGT1 was observed to be up-regulated by Phytophthora capsici inoculation and synergistically transient overexpression of PcINF1/SRC2-1 in pepper plants. SGT1-silencing compromised HR cell death, blocked H₂O₂ accumulation, and downregulated HR-associated and hormones-dependent marker genes’ expression triggered by PcINF1/SRC2-1 co-overexpression. The interaction between SRC2-1 and SGT1 was found by the yeast two hybrid system and was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation analyses. The SGT1/SRC2-1 interaction was enhanced by transient overexpression of PcINF1 and Phytophthora capsici inoculation, and SGT1-silencing attenuated PcINF1/SRC2-1 interaction. Additionally, by modulating subcellular localizations of SRC2-1, SGT1, and the interacting complex of SGT1/SRC2-1, it was revealed that exclusive nuclear targeting of the SGT1/SRC2-1 complex blocks immunity triggered by formation of SGT1/SRC2-1, and a translocation of the SGT1/SRC2-1 complex from the plasma membrane and cytoplasm to the nuclei upon the inoculation of P. capsici. Our data demonstrate that the SGT1/SRC2-1 interaction, and its nucleocytoplasmic partitioning, is involved in pepper’s immunity against P. capsici, thus providing a molecular link between Ca²⁺ signaling associated SRC2-1 and SGT1-mediated defense signaling.

Transcriptome Profiling of Resistance to Fusarium oxysporum f. sp. conglutinans in Cabbage (Brassica oleracea) Roots

Fusarium wilt caused by Fusarium oxysporum f. sp. conglutinans (FOC) is a destructive disease of Brassica crops, which results in severe yield losses. There is little information available about the mechanism of disease resistance. To obtain an overview of the transcriptome profiles in roots of R4P1, a Brassica oleracea variety that is highly resistant to fusarium wilt, we compared the transcriptomes of samples inoculated with FOC and samples inoculated with distilled water. RNA-seq analysis generated more than 136 million 100-bp clean reads, which were assembled into 62,506 unigenes (mean size = 741 bp). Among them, 49,959 (79.92%) genes were identified based on sequence similarity searches, including SwissProt (29,050, 46.47%), Gene Ontology (GO) (33,767, 54.02%), Clusters of Orthologous Groups (KOG) (14,721, 23.55%) and Kyoto Encyclopedia of Genes and Genomes Pathway database (KEGG) (12,974, 20.76%) searches; digital gene expression analysis revealed 885 differentially expressed genes (DEGs) between infected and control samples at 4, 12, 24 and 48 hours after inoculation. The DEGs were assigned to 31 KEGG pathways. Early defense systems, including the MAPK signaling pathway, calcium signaling and salicylic acid-mediated hypersensitive response (SA-mediated HR) were activated after pathogen infection. SA-dependent systemic acquired resistance (SAR), ethylene (ET)- and jasmonic (JA)-mediated pathways and the lignin biosynthesis pathway play important roles in plant resistance. We also analyzed the expression of defense-related genes, such as genes encoding pathogenesis-related (PR) proteins, UDP-glycosyltransferase (UDPG), pleiotropic drug resistance, ATP-binding cassette transporters (PDR-ABC transporters), myrosinase, transcription factors and kinases, which were differentially expressed. The results of this study may contribute to efforts to identify and clone candidate genes associated with disease resistance and to uncover the molecular mechanism underlying FOC resistance in cabbage.

HSP90 regulates temperature-dependent seedling growth in Arabidopsis by stabilizing the auxin co-receptor F-box protein TIR1

Recent studies have revealed that a mild increase in environmental temperature stimulates the growth of Arabidopsis seedlings by promoting biosynthesis of the plant hormone auxin. However, little is known about the role of other factors in this process. In this report, we show that increased temperature promotes rapid accumulation of the TIR1 auxin co-receptor, an effect that is dependent on the molecular chaperone HSP90. In addition, we show that HSP90 and the co-chaperone SGT1 each interact with TIR1, confirming that TIR1 is an HSP90 client. Inhibition of HSP90 activity results in degradation of TIR1 and interestingly, defects in a range of auxin-mediated growth processes at lower as well as higher temperatures. Our results indicate that HSP90 and SGT1 integrate temperature and auxin signalling in order to regulate plant growth in a changing environment.

A moderate increase in temperature promotes hypocotyl elongation in Arabidopsis. Here, Wang et al. show that elevated temperature not only increases auxin biosynthesis but also acts via the co-chaperones HSP90 and SGT1 to stabilize the TIR1 auxin receptor.

Cucumber Necrosis Virus Recruits Cellular Heat Shock Protein 70 Homologs at Several Stages of Infection

RNA viruses often depend on host factors for multiplication inside cells due to the constraints of their small genome size and limited coding capacity. One such factor that has been exploited by several plant and animal viruses is heat shock protein 70 (HSP70) family homologs which have been shown to play roles for different viruses in viral RNA replication, viral assembly, disassembly, and cell-to-cell movement. Using next generation sequence analysis, we reveal that several isoforms of Hsp70 and Hsc70 transcripts are induced to very high levels during cucumber necrosis virus (CNV) infection of Nicotiana benthamiana and that HSP70 proteins are also induced by at least 10-fold. We show that HSP70 family protein homologs are co-opted by CNV at several stages of infection. We have found that overexpression of Hsp70 or Hsc70 leads to enhanced CNV genomic RNA, coat protein (CP), and virion accumulation, whereas downregulation leads to a corresponding decrease. Hsc70-2 was found to increase solubility of CNV CP in vitro and to increase accumulation of CNV CP independently of viral RNA replication during coagroinfiltration in N. benthamiana. In addition, virus particle assembly into virus-like particles in CP agroinfiltrated plants was increased in the presence of Hsc70-2. HSP70 was found to increase the targeting of CNV CP to chloroplasts during infection, reinforcing the role of HSP70 in chloroplast targeting of host proteins. Hence, our findings have led to the discovery of a highly induced host factor that has been co-opted to play multiple roles during several stages of the CNV infection cycle.

IMPORTANCE

Because of the small size of its RNA genome, CNV is dependent on interaction with host cellular components to successfully complete its multiplication cycle. We have found that CNV induces HSP70 family homologs to a high level during infection, possibly as a result of the host response to the high levels of CNV proteins that accumulate during infection. Moreover, we have found that CNV co-opts HSP70 family homologs to facilitate several aspects of the infection process such as viral RNA, coat protein and virus accumulation. Chloroplast targeting of the CNV CP is also facilitated, which may aid in CNV suppression of host defense responses. Several viruses have been shown to induce HSP70 during infection and others to utilize HSP70 for specific aspects of infection such as replication, assembly, and disassembly. We speculate that HSP70 may play multiple roles in the infection processes of many viruses.

Opposing Effects on Two Phases of Defense Responses from Concerted Actions of HEAT SHOCK COGNATE70 and BONZAI1 in Arabidopsis

The plant immune system consists of multiple layers of responses targeting various phases of pathogen infection. Here, we provide evidence showing that two responses, one controlling stomatal closure and the other mediated by intracellular receptor proteins, can be regulated by the same proteins but in an antagonistic manner. The HEAT SHOCK COGNATE70 (HSC70), while previously known as a negative regulator of stomatal closure, is a positive regulator of immune responses mediated by the immune receptor protein SUPPRESSOR OF NPR1-1, CONSTITUTIVE1 (SNC1) as well as basal defense responses. In contrast to HSC70, a calcium-binding protein, BONZAI1 (BON1), promotes abscisic acid- and pathogen-triggered stomatal closure in addition to and independent of its previously known negative role in SNC1 regulation. BON1 likely regulates stomatal closure through activating SUPPESSOR OF THE G2 ALLELE OF SKP1 VARIANT B and inhibiting HSC70. New functions of BON1 and HSC70 identified in this study thus reveal opposite effects of each of them on immunity. The opposing roles of these regulators at different phases of plant immune responses exemplify the complexity in immunity regulation and suggest that immune receptors may guard positive regulators functioning at stomatal closure control.

A Secreted Effector Protein of Ustilago maydis Guides Maize Leaf Cells to Form Tumors

The biotrophic smut fungus Ustilago maydis infects all aerial organs of maize (Zea mays) and induces tumors in the plant tissues. U. maydis deploys many effector proteins to manipulate its host. Previously, deletion analysis demonstrated that several effectors have important functions in inducing tumor expansion specifically in maize leaves. Here, we present the functional characterization of the effector See1 (Seedling efficient effector1). See1 is required for the reactivation of plant DNA synthesis, which is crucial for tumor progression in leaf cells. By contrast, See1 does not affect tumor formation in immature tassel floral tissues, where maize cell proliferation occurs independent of fungal infection. See1 interacts with a maize homolog of SGT1 (Suppressor of G2 allele of skp1), a factor acting in cell cycle progression in yeast (Saccharomyces cerevisiae) and an important component of plant and human innate immunity. See1 interferes with the MAPK-triggered phosphorylation of maize SGT1 at a monocot-specific phosphorylation site. We propose that See1 interferes with SGT1 activity, resulting in both modulation of immune responses and reactivation of DNA synthesis in leaf cells. This identifies See1 as a fungal effector that directly and specifically contributes to the formation of leaf tumors in maize.

C-Terminally fused affinity Strep-tag II is removed by proteolysis from recombinant human erythropoietin expressed in transgenic tobacco plants

KEY MESSAGE

C -terminally fused Strep -tag II is removed from rhuEPO expressed in tobacco plants. The finding suggests that direct fusion of purification tags at the C -terminus of rhuEPO should be avoided. Asialo-erythropoietin (asialo-EPO), a desialylated form of EPO, is a potent tissue-protective agent. Recently, we and others have exploited a low-cost plant-based expression system to produce recombinant human asialo-EPO (asialo-rhuEPO(P)). To facilitate purification from plant extracts, Strep-tag II was engineered at the C-terminus of EPO. Although asialo-rhuEPO(P) was efficiently expressed in transgenic tobacco plants, affinity purification based on Strep -tag II did not result in the recovery of the protein. In this study, we investigated the stability of Strep-tag II tagged asialo-rhuEPO(P) expressed in tobacco plants to understand whether this fused tag is cleaved or inaccessible. Sequencing RT-PCR products confirmed that fused DNA sequences encoding Strep-tag II were properly transcribed, and three-dimensional protein structure model revealed that the tag must be fully accessible. However, Western blot analysis of leaf extracts and purified asialo-rhuEPO(P) revealed that the Strep-tag II was absent on the protein. Additionally, no peptide fragment containing Strep-tag II was identified in the LC-MS/MS analysis of purified protein further supporting that the affinity tag was absent on asialo-rhuEPO(P). However, Strep-tag II was detected on asialo-rhuEPO(P) that was retained in the endoplasmic reticulum, suggesting that the Strep-tag II is removed during protein secretion or extraction. These findings together with recent reports that C-terminally fused Strep-tag II or IgG Fc domain are also removed from EPO in tobacco plants, suggest that its C-terminus may be highly susceptible to proteolysis in tobacco plants. Therefore, direct fusion of purification tags at the C-terminus of EPO should be avoided while expressing it in tobacco plants.

Transcriptomic and proteomic analyses of resistant host responses in Arachis diogoi challenged with late leaf spot pathogen, Phaeoisariopsis personata

Late leaf spot is a serious disease of peanut caused by the imperfect fungus, Phaeoisariopsis personata. Wild diploid species, Arachis diogoi. is reported to be highly resistant to this disease and asymptomatic. The objective of this study is to investigate the molecular responses of the wild peanut challenged with the late leaf spot pathogen using cDNA-AFLP and 2D proteomic study. A total of 233 reliable, differentially expressed genes were identified in Arachis diogoi. About one third of the TDFs exhibit no significant similarity with the known sequences in the data bases. Expressed sequence tag data showed that the characterized genes are involved in conferring resistance in the wild peanut to the pathogen challenge. Several genes for proteins involved in cell wall strengthening, hypersensitive cell death and resistance related proteins have been identified. Genes identified for other proteins appear to function in metabolism, signal transduction and defence. Nineteen TDFs based on the homology analysis of genes associated with defence, signal transduction and metabolism were further validated by quantitative real time PCR (qRT-PCR) analyses in resistant wild species in comparison with a susceptible peanut genotype in time course experiments. The proteins corresponding to six TDFs were differentially expressed at protein level also. Differentially expressed TDFs and proteins in wild peanut indicate its defence mechanism upon pathogen challenge and provide initial breakthrough of genes possibly involved in recognition events and early signalling responses to combat the pathogen through subsequent development of resistivity. This is the first attempt to elucidate the molecular basis of the response of the resistant genotype to the late leaf spot pathogen, and its defence mechanism.

Pepper heat shock protein 70a interacts with the type III effector AvrBsT and triggers plant cell death and immunity

Heat shock proteins (HSPs) function as molecular chaperones and are essential for the maintenance and/or restoration of protein homeostasis. The genus Xanthomonas type III effector protein AvrBsT induces hypersensitive cell death in pepper (Capsicum annuum). Here, we report the identification of the pepper CaHSP70a as an AvrBsT-interacting protein. Bimolecular fluorescence complementation and coimmunoprecipitation assays confirm the specific interaction between CaHSP70a and AvrBsT in planta. The CaHSP70a peptide-binding domain is essential for its interaction with AvrBsT. Heat stress (37°C) and Xanthomonas campestris pv vesicatoria (Xcv) infection distinctly induce CaHSP70a in pepper leaves. Cytoplasmic CaHSP70a proteins significantly accumulate in pepper leaves to induce the hypersensitive cell death response by Xcv (avrBsT) infection. Transient CaHSP70a overexpression induces hypersensitive cell death under heat stress, which is accompanied by strong induction of defense- and cell death-related genes. The CaHSP70a peptide-binding domain and ATPase-binding domain are required to trigger cell death under heat stress. Transient coexpression of CaHSP70a and avrBsT leads to cytoplasmic localization of the CaHSP70a-AvrBsT complex and significantly enhances avrBsT-triggered cell death in Nicotiana benthamiana. CaHSP70a silencing in pepper enhances Xcv growth but disrupts the reactive oxygen species burst and cell death response during Xcv infection. Expression of some defense marker genes is significantly reduced in CaHSP70a-silenced leaves, with lower levels of the defense hormones salicylic acid and jasmonic acid. Together, these results suggest that CaHSP70a interacts with the type III effector AvrBsT and is required for cell death and immunity in plants.

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.

Pathogen-induced SGT1 of Arachis diogoi induces cell death and enhanced disease resistance in tobacco and peanut

We have identified a transcript derived fragment (TDF) corresponding to SGT1 in a study of differential gene expression on the resistant wild peanut, Arachis diogoi, upon challenge from the late leaf spot pathogen, Phaeoisariopsis personata, and cloned its full-length cDNA followed by subsequent validation through q-PCR. Sodium nitroprusside, salicylic acid, ethephon and methyl jasmonate induced the expression of AdSGT1, while the treatment with abscisic acid did not elicit its up-regulation. AdSGT1 is localized to both nucleus and cytoplasm. Its overexpression induced hypersensitive-like cell death in tobacco under transient conditional expression using the estradiol system, and this conditional expression of AdSGT1 was also associated with the up-regulation of NtHSR203J, HMGR and HIN1, which have been shown to be associated with hypersensitive response in tobacco in earlier studies. Expression of the cDNA in a susceptible cultivated peanut variety enhanced its resistance against the late leaf spot pathogen, Phaeoisariopsis personata, while the heterologous expression in tobacco enhanced its resistance against Phytophthora parasitica var. nicotianae, Alternaria alternata var. nicotianae and Rhizoctonia solani. Constitutive expression in peanut was associated with the co-expression of resistance-related genes, CC-NB-LRR and some protein kinases.

G Protein signaling modulator-3 inhibits the inflammasome activity of NLRP3

Inflammasomes are multi-protein complexes that regulate maturation of the interleukin 1β-related cytokines IL-1β and IL-18 through activation of the cysteine proteinase caspase-1. NOD-like receptor family, pyrin domain containing 3 (NLRP3) protein is a key component of inflammasomes that assemble in response to a wide variety of endogenous and pathogen-derived danger signals. Activation of the NLRP3-inflammasome and subsequent secretion of IL-1β is highly regulated by at least three processes: transcriptional activation of both NLRP3 and pro-IL-1β genes, non-transcriptional priming of NLRP3, and final activation of NLRP3. NLRP3 is predominantly expressed in cells of the hematopoietic lineage. Using a yeast two-hybrid screen, we identified the hematopoietic-restricted protein, G protein signaling modulator-3 (GPSM3), as a NLRP3-interacting protein and a negative regulator of IL-1β production triggered by NLRP3-dependent inflammasome activators. In monocytes, GPSM3 associates with the C-terminal leucine-rich repeat domain of NLRP3. Bone marrow-derived macrophages lacking GPSM3 expression exhibit an increase in NLRP3-dependent IL-1β, but not TNF-α, secretion. Furthermore, GPSM3-null mice have enhanced serum and peritoneal IL-1β production following Alum-induced peritonitis. Our findings suggest that GPSM3 acts as a direct negative regulator of NLRP3 function.

A heavy metal-associated protein (AcHMA1) from the halophyte, Atriplex canescens (Pursh) Nutt., confers tolerance to iron and other abiotic stresses when expressed in Saccharomyces cerevisiae

Many heavy metals are essential for metabolic processes, but are toxic at elevated levels. Metal tolerance proteins provide resistance to this toxicity. In this study, we identified and characterized a heavy metal-associated protein, AcHMA1, from the halophyte, Atriplex canescens. Sequence analysis has revealed that AcHMA1 contains two heavy metal binding domains. Treatments with metals (Fe, Cu, Ni, Cd or Pb), PEG6000 and NaHCO3 highly induced AcHMA1 expression in A. canescens, whereas NaCl and low temperature decreased its expression. The role of AcHMA1 in metal stress tolerance was examined using a yeast expression system. Expression of the AcHMA1 gene significantly increased the ability of yeast cells to adapt to and recover from exposure to excess iron. AcHMA1 expression also provided salt, alkaline, osmotic and oxidant stress tolerance in yeast cells. Finally, subcellular localization of an AcHMA1/GFP fusion protein expressed in tobacco cells showed that AcHMA1 was localized in the plasma membrane. Thus, our results suggest that AcHMA1 encodes a membrane-localized metal tolerance protein that mediates the detoxification of iron in eukaryotes. Furthermore, AcHMA1 also participates in the response to abiotic stress.

GmSGT1 is differently required for soybean Rps genes-mediated and basal resistance to Phytophthora sojae

KEY MESSAGE

Using RNAi approach, we demonstrate that GmSGT1 is an essential component in soybean against Phytophthora sojae, but not required for Rps 2 or Rps 3a-mediated resistance. Utilization of disease resistance in soybean is a major approach to combat root and stem rot disease, which is caused by Phytophthora sojae and poses a growing threat to soybean safety production. The SGT1 protein is essential for disease resistance in many plant species. Here, we analyzed and characterized functions of GmSGT1 gene family in R protein-mediated resistance and basal defense in this important crop. Five candidate genes of GmSGT1 were identified and they were grouped into three clades. Transcriptional levels of all the tested genes were highly induced upon P. sojae infection in four soybean cultivars that confer different resistant levels. Using a gene silencing system in soybean cotyledons, we demonstrated that silencing GmSGT1 genes comprised race-specific resistance in soybean lines carrying genes at the following loci for race-specific resistance to P. sojae: Rps1a, Rps1c, Rps1d, Rps1k, and Rps8. In contrast, the resistance mediated by Rps2 or Rps3a was not affected. Silencing GmSGT1 genes in cotyledons also reduced resistance to this pathogen in a moderately partial resistant cultivar. We further showed that transient overexpression of GmSGT1-1 in Nicotiana benthamiana could enhance the resistance to P. capsici. These results suggest that GmSGT1 is an essential component for soybean in resisting the pathogen and pathways of Rps-mediated disease resistance are diverse in soybean.

Pepper suppressor of the G2 allele of skp1 interacts with the receptor-like cytoplasmic kinase1 and type III effector AvrBsT and promotes the hypersensitive cell death response in a phosphorylation-dependent manner

Xanthomonas campestris pv vesicatoria type III effector protein, AvrBsT, triggers hypersensitive cell death in pepper (Capsicum annuum). Here, we have identified the pepper SGT1 (for suppressor of the G2 allele of skp1) as a host interactor of AvrBsT and also the pepper PIK1 (for receptor-like cytoplasmic kinase1). PIK1 specifically phosphorylates SGT1 and AvrBsT in vitro. AvrBsT specifically binds to the CHORD-containing protein and SGT1 domain of SGT1, resulting in the inhibition of PIK1-mediated SGT1 phosphorylation and subsequent nuclear transport of the SGT1-PIK1 complex. Liquid chromatography-tandem mass spectrometry of the proteolytic peptides of SGT1 identified the residues serine-98 and serine-279 of SGT1 as the major PIK1-mediated phosphorylation sites. Site-directed mutagenesis of SGT1 revealed that the identified SGT1 phosphorylation sites are responsible for the activation of AvrBsT-triggered cell death in planta. SGT1 forms a heterotrimeric complex with both AvrBsT and PIK1 exclusively in the cytoplasm. Agrobacterium tumefaciens-mediated coexpression of SGT1 and PIK1 with avrBsT promotes avrBsT-triggered cell death in Nicotiana benthamiana, dependent on PIK1. Virus-induced silencing of SGT1 and/or PIK1 compromises avrBsT-triggered cell death, hydrogen peroxide production, defense gene induction, and salicylic acid accumulation, leading to the enhanced bacterial pathogen growth in pepper. Together, these results suggest that SGT1 interacts with PIK1 and the bacterial effector protein AvrBsT and promotes the hypersensitive cell death associated with PIK1-mediated phosphorylation in plants.

Nematode Sgt1-homologue D1054.3 binds open and closed conformations of Hsp90 via distinct binding sites

Heat shock protein 90 (Hsp90) is a highly conserved ATP-driven machine involved in client protein maturation, folding, and activation. The chaperone is supported by a set of cochaperones that confer client specificities. One of those proteins is the suppressor of G2 allele of skp1 (Sgt1), which participates together with Hsp90 in the immune responses of plants. Sgt1 consists of three domains: a TPR-, CS-, and SGS-domain, conserved in plants, yeast, and humans. The TPR-domain though is lacking in nematodes and insects. We observe that the Caenorhabditis elegans Sgt1 homologue D1054.3 binds to Hsp90 in the absence of nucleotides but much stronger in the presence of ATP and ATPγS. The latter binding mode is similar to p23, another CS-domain containing Hsp90 cofactor, even though binding is not observable for p23 in the absence of nucleotides. We use point mutations in Hsp90, which accumulate different conformations in the ATPase cycle, to differentiate between binding to open and closed Hsp90 conformations. These data support a strong contribution of the Hsp90 conformation to Sgt1 binding and highlight the ability of this cofactor to interact with all known Hsp90 conformations albeit with different affinities.

A rapid, highly efficient and economical method of Agrobacterium-mediated in planta transient transformation in living onion epidermis

Transient transformation is simpler, more efficient and economical in analyzing protein subcellular localization than stable transformation. Fluorescent fusion proteins were often used in transient transformation to follow the in vivo behavior of proteins. Onion epidermis, which has large, living and transparent cells in a monolayer, is suitable to visualize fluorescent fusion proteins. The often used transient transformation methods included particle bombardment, protoplast transfection and Agrobacterium-mediated transformation. Particle bombardment in onion epidermis was successfully established, however, it was expensive, biolistic equipment dependent and with low transformation efficiency. We developed a highly efficient in planta transient transformation method in onion epidermis by using a special agroinfiltration method, which could be fulfilled within 5 days from the pretreatment of onion bulb to the best time-point for analyzing gene expression. The transformation conditions were optimized to achieve 43.87% transformation efficiency in living onion epidermis. The developed method has advantages in cost, time-consuming, equipment dependency and transformation efficiency in contrast with those methods of particle bombardment in onion epidermal cells, protoplast transfection and Agrobacterium-mediated transient transformation in leaf epidermal cells of other plants. It will facilitate the analysis of protein subcellular localization on a large scale.

An E4 ligase facilitates polyubiquitination of plant immune receptor resistance proteins in Arabidopsis

Proteins with nucleotide binding and leucine-rich repeat domains (NLRs) serve as immune receptors in animals and plants that recognize pathogens and activate downstream defense responses. As high accumulation of NLRs can result in unwarranted autoimmune responses, their cellular concentrations must be tightly regulated. However, the molecular mechanisms of this process are poorly detailed. The F-box protein Constitutive expressor of PR genes 1 (CPR1) was previously identified as a component of a Skp1, Cullin1, F-box protein E3 complex that targets NLRs, including Suppressor of NPR1, Constitutive 1 (SNC1) and Resistance to Pseudomonas syringae 2 (RPS2), for ubiquitination and further protein degradation. From a forward genetic screen, we identified Mutant, snc1-enhancing 3 (MUSE3), an E4 ubiquitin ligase involved in polyubiquitination of its protein targets. Knocking out MUSE3 in Arabidopsis thaliana results in increased levels of NLRs, including SNC1 and RPS2, whereas overexpressing MUSE3 together with CPR1 enhances polyubiquitination and protein degradation of these immune receptors. This report on the functional role of an E4 ligase in plants provides insight into the scarcely understood NLR degradation pathway.

Cloning, molecular characterization and expression analysis of heat shock cognate 70 (Hsc70) cDNA from turbot (Scophthalmus maximus)

As an essential member of the HSP70 family, heat shock cognate 70 (Hsc70) is a constitutively expressed molecular chaperone involved in protein metabolism. In this paper, turbot Hsc70 was cloned and the expression profile was also analyzed. The full-length cDNA of the turbot Hsc70 was 2,292 bp in length, including a 113-bp 5' UTR, a 223-bp 3' UTR and a 1,956-bp open reading frame coding a protein with 651 amino acid residues. Comparison of amino acid sequence revealed the existence of three classical HSP70 family signature motifs, a signature nonapeptide and one repeat of tetrapeptide in turbot Hsc70. The turbot Hsc70-deduced amino acids sequence exhibited 75.4-96.8 % homology with Hsp70s/Hsc70s of 24 other known sequences. In particular, the strongest homology was found with the cognate members of Hsc70 subfamily and the highest identity was found with Japanese flounder Hsc70. Semi-quantitative RT-PCR revealed that turbot Hsc70 transcripts were stably expressed in all tested tissues under normal physiological condition, while the expression levels also increased (~1.5-fold to ~threefold) after heat shock and bacterial infection. In addition, Hsc70 transcripts were detected throughout embryonic development and in turbot embryonic cell line (TEC) in the absence of any stress. Meanwhile, it was also heat inducible, but not cold inducible in TEC. These results suggest that Hsc70 gene may be involved in embryogenesis and cellular protection events under normal and stress condition.

Viral and nonviral elements in potexvirus replication and movement and in antiviral responses

In Potato virus X, a member of the genus Potexvirus, special sequences and structures at the 5' and 3' ends of the nontranslated region function as cis-acting elements for viral replication. These elements greatly affect interactions between viral RNAs and those between viral RNAs and host factors. The potexvirus genome encodes five open-reading frames. Viral replicase, which is required for the synthesis of viral RNA, binds viral RNA elements and host factors to form a viral replication complex at the host cellular membrane. The coat protein (CP) and three viral movement proteins (TGB1, TGB2, and TGB3) have critical roles in mediating cell-to-cell viral movement through plasmodesmata by virion formation or by nonvirion ribonucleoprotein (RNP) complex formation with viral movement proteins (TGBs). The RNP complex, like TGB1-CP-viral RNA, is associated with viral replicase and used for immediate reinitiation of viral replication in newly invaded cells. Higher plants have defense mechanisms against potexviruses such as Rx-mediated resistance and RNA silencing. The CP acts as an avirulence effector for plant defense mechanisms, while TGB1 functions as a viral suppressor of RNA silencing, which is the mechanism of innate immune resistance. Here, we describe recent findings concerning the involvement of viral and host factors in potexvirus replication and in antiviral responses to potexvirus infection.

Type I J-domain NbMIP1 proteins are required for both Tobacco mosaic virus infection and plant innate immunity

Tm-2² is a coiled coil-nucleotide binding-leucine rich repeat resistance protein that confers durable extreme resistance against Tomato mosaic virus (ToMV) and Tobacco mosaic virus (TMV) by recognizing the viral movement protein (MP). Here we report that the Nicotiana benthamiana J-domain MIP1 proteins (NbMIP1s) associate with tobamovirus MP, Tm-2² and SGT1. Silencing of NbMIP1s reduced TMV movement and compromised Tm-2²-mediated resistance against TMV and ToMV. Furthermore, silencing of NbMIP1s reduced the steady-state protein levels of ToMV MP and Tm-2². Moreover, NbMIP1s are required for plant resistance induced by other R genes and the nonhost pathogen Pseudomonas syringae pv. tomato (Pst) DC3000. In addition, we found that SGT1 associates with Tm-2² and is required for Tm-2²-mediated resistance against TMV. These results suggest that NbMIP1s function as co-chaperones during virus infection and plant immunity.

Nucleocytoplasmic partitioning of tobacco N receptor is modulated by SGT1

SGT1 (Suppressor of G2 allele of SKP1) is required to maintain plant disease Resistance (R) proteins with Nucleotide-Binding (NB) and Leucine-Rich Repeat (LRR) domains in an inactive but signaling-competent state. SGT1 is an integral component of a multi-protein network that includes RACK1, Rac1, RAR1, Rboh, HSP90 and HSP70, and in rice the Mitogen-Activated Protein Kinase (MAPK), OsMAPK6. Tobacco (Nicotiana tabacum) N protein, which belongs to the Toll-Interleukin Receptor (TIR)-NB-LRR class of R proteins, confers resistance to Tobacco Mosaic Virus (TMV). Following transient expression in planta, we analyzed the functional relationship between SGT1, SIPK - a tobacco MAPK6 ortholog - and N, using mass spectrometry, confocal microscopy and pathogen assays. Here, we show that tobacco SGT1 undergoes specific phosphorylation in a canonical MAPK target-motif by SIPK. Mutation of this motif to mimic SIPK phosphorylation leads to an increased proportion of cells displaying SGT1 nuclear accumulation and impairs N-mediated resistance to TMV, as does phospho-null substitution at the same residue. Forced nuclear localization of SGT1 causes N to be confined to nuclei. Our data suggest that one mode of regulating nucleocytoplasmic partitioning of R proteins is by maintaining appropriate levels of SGT1 phosphorylation catalyzed by plant MAPK.

The Hv-SGT1 gene from Haynaldia villosa contributes to resistances towards both biotrophic and hemi-biotrophic pathogens in common wheat (Triticum aestivum L.)

The SGT1 protein is essential for R protein-mediated and PAMPs-triggered resistance in many plant species. Here we reported the isolation and characterization of the Hv-SGT1 gene from Haynaldiavillosa (2n = 14, VV). Analysis of the subcellular location of Hv-SGT1 by transient expression of a fusion to GFP indicated its presence in the cytoplasm and nucleus. Levels of Hv-SGT1 transcripts were increased by inoculation with either the biotrophic pathogen Blumeriagraminis DC. f. Sp. tritici (Bgt) or the hemi-biotrophic pathogen Fusariumgraminearum (Fg). Levels of Hv-SGT1 showed substantial increase following treatment with H₂O₂ and methyl jasmonate (MeJA), only slightly induced following exposure to ethephon or abscisic acid, but not changed following exposure to salicylic acid. The demonstration that silencing of Hv-SGT1 substantially reduced resistance to Bgt indicated that Hv-SGT1 was an essential component of disease resistance in H. villosa. The over-expression of Hv-SGT1 in Yangmai 158 enhanced resistance to powdery mildew, and this correlated with increased levels of whole-cell reactive oxygen intermediates at the sites of penetration by the pathogens. Compared with wild-type plants, the expression levels of genes related to the H₂O₂ and JA signaling pathways were lower in the Hv-SGT1 silenced plants and higher in the Hv-SGT1 over-expressing plants. Therefore, the involvement of Hv-SGT1 in H₂O₂ production correlates with the hypersensitive response and jasmonic acid signaling. Our novel demonstration that wheat with over-expressed Hv-SGT1 showed enhanced resistance to both powdery mildew and FHB suggests that it could served as a transgenic genetic resource in wheat breeding for multiple disease resistance.

Arabidopsis HEAT SHOCK TRANSCRIPTION FACTORA1b overexpression enhances water productivity, resistance to drought, and infection

Heat-stressed crops suffer dehydration, depressed growth, and a consequent decline in water productivity, which is the yield of harvestable product as a function of lifetime water consumption and is a trait associated with plant growth and development. Heat shock transcription factor (HSF) genes have been implicated not only in thermotolerance but also in plant growth and development, and therefore could influence water productivity. Here it is demonstrated that Arabidopsis thaliana plants with increased HSFA1b expression showed increased water productivity and harvest index under water-replete and water-limiting conditions. In non-stressed HSFA1b-overexpressing (HSFA1bOx) plants, 509 genes showed altered expression, and these genes were not over-represented for development-associated genes but were for response to biotic stress. This confirmed an additional role for HSFA1b in maintaining basal disease resistance, which was stress hormone independent but involved H₂O₂ signalling. Fifty-five of the 509 genes harbour a variant of the heat shock element (HSE) in their promoters, here named HSE1b. Chromatin immunoprecipitation-PCR confirmed binding of HSFA1b to HSE1b in vivo, including in seven transcription factor genes. One of these is MULTIPROTEIN BRIDGING FACTOR1c (MBF1c). Plants overexpressing MBF1c showed enhanced basal resistance but not water productivity, thus partially phenocopying HSFA1bOx plants. A comparison of genes responsive to HSFA1b and MBF1c overexpression revealed a common group, none of which harbours a HSE1b motif. From this example, it is suggested that HSFA1b directly regulates 55 HSE1b-containing genes, which control the remaining 454 genes, collectively accounting for the stress defence and developmental phenotypes of HSFA1bOx.

Evaluation of higher plant virus resistance genes in the green alga, Chlorella variabilis NC64A, during the early phase of infection with Paramecium bursaria chlorella virus-1

With growing industrial interest in algae plus their critical roles in aquatic systems, the need to understand the effects of algal pathogens is increasing. We examined a model algal host-virus system, Chlorella variabilis NC64A and virus, PBCV-1. C. variabilis encodes 375 homologs to genes involved in RNA silencing and in response to virus infection in higher plants. Illumina RNA-Seq data showed that 325 of these homologs were expressed in healthy and early PBCV-1 infected (≤60min) cells. For each of the RNA silencing genes to which homologs were found, mRNA transcripts were detected in healthy and infected cells. C. variabilis, like higher plants, may employ certain RNA silencing pathways to defend itself against virus infection. To our knowledge this is the first examination of RNA silencing genes in algae beyond core proteins, and the first analysis of their transcription during virus infection.

Characterization of the LOV1-mediated, victorin-induced, cell-death response with virus-induced gene silencing

Victoria blight, caused by Cochliobolus victoriae, is a disease originally described on oat and recapitulated on Arabidopsis. C. victoriae pathogenesis depends upon production of the toxin victorin. In oat, victorin sensitivity is conferred by the Vb gene, which is genetically inseparable from the Pc2 resistance gene. Concurrently, in Arabidopsis, sensitivity is conferred by the LOCUS ORCHESTRATING VICTORIN EFFECTS1 (LOV1) gene. LOV1 encodes a nucleotide-binding site leucine-rich repeat protein, a type of protein commonly associated with disease resistance, and LOV1 "guards" the defense thioredoxin, TRX-h5. Expression of LOV1 and TRX-h5 in Nicotiana benthamiana is sufficient to confer victorin sensitivity. Virus-induced gene silencing was used to characterize victorin-induced cell death in N. benthamiana. We determined that SGT1 is required for sensitivity and involved in LOV1 protein accumulation. We screened a normalized cDNA library and identified six genes that, when silenced, suppressed LOV1-mediated, victorin-induced cell death and cell death induced by expression of the closely related RPP8 resistance gene: a mitochondrial phosphate transporter, glycolate oxidase, glutamine synthetase, glyceraldehyde 3-phosphate dehydrogenase, and the P- and T-protein of the glycine decarboxylase complex. Silencing the latter four also inhibited cell death and disease resistance mediated by the PTO resistance gene. Together, these results provide evidence that the victorin response mediated by LOV1 is a defense response.

The Salmonella type III effector SspH2 specifically exploits the NLR co-chaperone activity of SGT1 to subvert immunity

To further its pathogenesis, S. Typhimurium delivers effector proteins into host cells, including the novel E3 ubiquitin ligase (NEL) effector SspH2. Using model systems in a cross-kingdom approach we gained further insight into the molecular function of this effector. Here, we show that SspH2 modulates innate immunity in both mammalian and plant cells. In mammalian cell culture, SspH2 significantly enhanced Nod1-mediated IL-8 secretion when transiently expressed or bacterially delivered. In addition, SspH2 also enhanced an Rx-dependent hypersensitive response in planta. In both of these nucleotide-binding leucine rich repeat receptor (NLR) model systems, SspH2-mediated phenotypes required its catalytic E3 ubiquitin ligase activity and interaction with the conserved host protein SGT1. SGT1 has an essential cell cycle function and an additional function as an NLR co-chaperone in animal and plant cells. Interaction between SspH2 and SGT1 was restricted to SGT1 proteins that have NLR co-chaperone function and accordingly, SspH2 did not affect SGT1 cell cycle functions. Mechanistic studies revealed that SspH2 interacted with, and ubiquitinated Nod1 and could induce Nod1 activity in an agonist-independent manner if catalytically active. Interestingly, SspH2 in vitro ubiquitination activity and protein stability were enhanced by SGT1. Overall, this work adds to our understanding of the sophisticated mechanisms used by bacterial effectors to co-opt host pathways by demonstrating that SspH2 can subvert immune responses by selectively exploiting the functions of a conserved host co-chaperone.

Transcriptome divergence between introduced and native populations of Canada thistle, Cirsium arvense

Introduced plants may quickly evolve new adaptive traits upon their introduction. Canada thistle (Cirsium arvense - Cardueae, Asteraceae) is one of the worst invasive weeds worldwide. The goal of this study is to compare gene expression profiles of native (European) and introduced (North American) populations of this species, to elucidate the genetic mechanisms that may underlie such rapid adaptation. We explored the transcriptome of ten populations (five per range) of C. arvense in response to three treatments (control, nutrient deficiency and shading) using a customized microarray chip containing 63 690 expressed sequence tags (ESTs), and verified the expression level of 13 loci through real-time quantitative PCR. Only 2116 ESTs (3.5%) were found to be differentially expressed between the ranges, and 4458 ESTs (7.1%) exhibited a significant treatment-by-range effect. Among them was an overrepresentation of loci involved in stimulus and stress responses. Cirsium arvense has evolved different life history strategies on each continent. The two ranges notably differ with regard to R-protein mediated defence, sensitivity to abiotic stresses, and developmental timing. The fact that genotypes from the Midwest exhibit different expression kinetics than remaining North American samples further corroborates the hypothesis that the New World has been colonized twice, independently.

Identification of host proteins modulated by the virulence factor AC2 of Tomato chlorotic mottle virus in Nicotiana benthamiana

Tomato, one of the most important crops cultivated worldwide, has been severely affected by begomoviruses such as the Tomato chlorotic mottle virus (ToCMoV). Virulence factor AC2 is considered crucial for a successful virus-plant interaction and is known to act as a transcriptional activator and in some begomoviruses to function as an RNA silencing suppressor factor. However, the exact functions of the AC2 protein of the begomovirus ToCMoV are not yet established. The aim of the present study was to identify differentially expressed proteins of the model plant Nicotiana benthamiana in response to the expression of the AC2 gene, isolated from ToCMoV. N. benthamiana plants were inoculated with Agrobacterium tumefaciens containing the viral vector Potato virus X (PVX) and with the PVX-AC2 construction. 2DE was performed and proteins were identified by MS. The results showed that the expression of ToCMoV AC2 alters the levels of several host proteins, which are important for normal plant development, causing an imbalance in cellular homeostasis. This study highlights the effect of AC2 in the modulation of plant defense processes by increasing the expression of several oxidative stress-related and pathogenesis-related proteins, as well as its role in modulating the proteome of the photosynthesis and energy production systems.

Disease resistance or growth: the role of plant hormones in balancing immune responses and fitness costs

Plant growth and response to environmental cues are largely governed by phytohormones. The plant hormones ethylene, jasmonic acid, and salicylic acid (SA) play a central role in the regulation of plant immune responses. In addition, other plant hormones, such as auxins, abscisic acid (ABA), cytokinins, gibberellins, and brassinosteroids, that have been thoroughly described to regulate plant development and growth, have recently emerged as key regulators of plant immunity. Plant hormones interact in complex networks to balance the response to developmental and environmental cues and thus limiting defense-associated fitness costs. The molecular mechanisms that govern these hormonal networks are largely unknown. Moreover, hormone signaling pathways are targeted by pathogens to disturb and evade plant defense responses. In this review, we address novel insights on the regulatory roles of the ABA, SA, and auxin in plant resistance to pathogens and we describe the complex interactions among their signal transduction pathways. The strategies developed by pathogens to evade hormone-mediated defensive responses are also described. Based on these data we discuss how hormone signaling could be manipulated to improve the resistance of crops to pathogens.

Discovering host genes involved in the infection by the Tomato Yellow Leaf Curl Virus complex and in the establishment of resistance to the virus using Tobacco Rattle Virus-based post transcriptional gene silencing

The development of high-throughput technologies allows for evaluating gene expression at the whole-genome level. Together with proteomic and metabolomic studies, these analyses have resulted in the identification of plant genes whose function or expression is altered as a consequence of pathogen attacks. Members of the Tomato yellow leaf curl virus (TYLCV) complex are among the most important pathogens impairing production of agricultural crops worldwide. To understand how these geminiviruses subjugate plant defenses, and to devise counter-measures, it is essential to identify the host genes affected by infection and to determine their role in susceptible and resistant plants. We have used a reverse genetics approach based on Tobacco rattle virus-induced gene silencing (TRV-VIGS) to uncover genes involved in viral infection of susceptible plants, and to identify genes underlying virus resistance. To identify host genes with a role in geminivirus infection, we have engineered a Nicotiana benthamiana line, coined 2IRGFP, which over-expresses GFP upon virus infection. With this system, we have achieved an accurate description of the dynamics of virus replication in space and time. Upon silencing selected N. benthamiana genes previously shown to be related to host response to geminivirus infection, we have identified eighteen genes involved in a wide array of cellular processes. Plant genes involved in geminivirus resistance were studied by comparing two tomato lines: one resistant (R), the other susceptible (S) to the virus. Sixty-nine genes preferentially expressed in R tomatoes were identified by screening cDNA libraries from infected and uninfected R and S genotypes. Out of the 25 genes studied so far, the silencing of five led to the total collapse of resistance, suggesting their involvement in the resistance gene network. This review of our results indicates that TRV-VIGS is an exquisite reverse genetics tool that may provide new insights into the molecular mechanisms underlying plant infection and resistance to infection by begomoviruses.

Comparative analysis of barley leaf proteome as affected by drought stress

The adaptive response of Egyptian barley land races to drought stress was analyzed using difference gel electrophoresis (DIGE). Physiological measurements and proteome alterations of accession number 15141, drought tolerant, and accession number 15163, drought sensitive, were compared. Differentially expressed proteins were subjected to MALDI-TOF-MS analysis. Alterations in proteins related to the energy balance and chaperons were the most characteristic features to explain the differences between the drought-tolerant and the drought-sensitive accessions. Further alterations in the levels of proteins involved in metabolism, transcription and protein synthesis are also indicated.

Novel aspects of grapevine response to phytoplasma infection investigated by a proteomic and phospho-proteomic approach with data integration into functional networks

BACKGROUND

Translational and post-translational protein modifications play a key role in the response of plants to pathogen infection. Among the latter, phosphorylation is critical in modulating protein structure, localization and interaction with other partners. In this work, we used a multiplex staining approach with 2D gels to study quantitative changes in the proteome and phosphoproteome of Flavescence dorée-affected and recovered 'Barbera' grapevines, compared to healthy plants.

RESULTS

We identified 48 proteins that differentially changed in abundance, phosphorylation, or both in response to Flavescence dorée phytoplasma infection. Most of them did not show any significant difference in recovered plants, which, by contrast, were characterized by changes in abundance, phosphorylation, or both for 17 proteins not detected in infected plants. Some enzymes involved in the antioxidant response that were up-regulated in infected plants, such as isocitrate dehydrogenase and glutathione S-transferase, returned to healthy-state levels in recovered plants. Others belonging to the same functional category were even down-regulated in recovered plants (oxidoreductase GLYR1 and ascorbate peroxidase). Our proteomic approach thus agreed with previously published biochemical and RT-qPCR data which reported down-regulation of scavenging enzymes and accumulation of H2O2 in recovered plants, possibly suggesting a role for this molecule in remission from infection. Fifteen differentially phosphorylated proteins (| ratio | > 2, p < 0.05) were identified in infected compared to healthy plants, including proteins involved in photosynthesis, response to stress and the antioxidant system. Many were not differentially phosphorylated in recovered compared to healthy plants, pointing to their specific role in responding to infection, followed by a return to a steady-state phosphorylation level after remission of symptoms. Gene ontology (GO) enrichment and statistical analysis showed that the general main category "response to stimulus" was over-represented in both infected and recovered plants but, in the latter, the specific child category "response to biotic stimulus" was no longer found, suggesting a return to steady-state levels for those proteins specifically required for defence against pathogens.

CONCLUSIONS

Proteomic data were integrated into biological networks and their interactions were represented through a hypothetical model, showing the effects of protein modulation on primary metabolic ways and related secondary pathways. By following a multiplex-staining approach, we obtained new data on grapevine proteome pathways that specifically change at the phosphorylation level during phytoplasma infection and following recovery, focusing for the first time on phosphoproteome changes during pathogen infection in this host.

Reprogramming of plants during systemic acquired resistance

Genome-wide microarray analyses revealed that during biological activation of systemic acquired resistance (SAR) in Arabidopsis, the transcript levels of several hundred plant genes were consistently up- (SAR(+) genes) or down-regulated (SAR(-) genes) in systemic, non-inoculated leaf tissue. This transcriptional reprogramming fully depended on the SAR regulator FLAVIN-DEPENDENT MONOOXYGENASE1 (FMO1). Functional gene categorization showed that genes associated with salicylic acid (SA)-associated defenses, signal transduction, transport, and the secretory machinery are overrepresented in the group of SAR(+) genes, and that the group of SAR(-) genes is enriched in genes activated via the jasmonate (JA)/ethylene (ET)-defense pathway, as well as in genes associated with cell wall remodeling and biosynthesis of constitutively produced secondary metabolites. This suggests that SAR-induced plants reallocate part of their physiological activity from vegetative growth towards SA-related defense activation. Alignment of the SAR expression data with other microarray information allowed us to define three clusters of SAR(+) genes. Cluster I consists of genes tightly regulated by SA. Cluster II genes can be expressed independently of SA, and this group is moderately enriched in H2O2- and abscisic acid (ABA)-responsive genes. The expression of the cluster III SAR(+) genes is partly SA-dependent. We propose that SA-independent signaling events in early stages of SAR activation enable the biosynthesis of SA and thus initiate SA-dependent SAR signaling. Both SA-independent and SA-dependent events tightly co-operate to realize SAR. SAR(+) genes function in the establishment of diverse resistance layers, in the direct execution of resistance against different (hemi-)biotrophic pathogen types, in suppression of the JA- and ABA-signaling pathways, in redox homeostasis, and in the containment of defense response activation. Our data further indicated that SAR-associated defense priming can be realized by partial pre-activation of particular defense pathways.

Disease resistance or growth: the role of plant hormones in balancing immune responses and fitness costs

Plant growth and response to environmental cues are largely governed by phytohormones. The plant hormones ethylene, jasmonic acid, and salicylic acid (SA) play a central role in the regulation of plant immune responses. In addition, other plant hormones, such as auxins, abscisic acid (ABA), cytokinins, gibberellins, and brassinosteroids, that have been thoroughly described to regulate plant development and growth, have recently emerged as key regulators of plant immunity. Plant hormones interact in complex networks to balance the response to developmental and environmental cues and thus limiting defense-associated fitness costs. The molecular mechanisms that govern these hormonal networks are largely unknown. Moreover, hormone signaling pathways are targeted by pathogens to disturb and evade plant defense responses. In this review, we address novel insights on the regulatory roles of the ABA, SA, and auxin in plant resistance to pathogens and we describe the complex interactions among their signal transduction pathways. The strategies developed by pathogens to evade hormone-mediated defensive responses are also described. Based on these data we discuss how hormone signaling could be manipulated to improve the resistance of crops to pathogens.