Characterization of an Arabidopsis Mutant That Is Nonresponsive to Inducers of Systemic Acquired Resistance

Salicylic Acid Targets Protein Phosphatase 2A to Attenuate Growth in Plants

Plants, like other multicellular organisms, survive through a delicate balance between growth and defense against pathogens. Salicylic acid (SA) is a major defense signal in plants, and the perception mechanism as well as downstream signaling activating the immune response are known. Here, we identify a parallel SA signaling that mediates growth attenuation. SA directly binds to A subunits of protein phosphatase 2A (PP2A), inhibiting activity of this complex. Among PP2A targets, the PIN2 auxin transporter is hyperphosphorylated in response to SA, leading to changed activity of this important growth regulator. Accordingly, auxin transport and auxin-mediated root development, including growth, gravitropic response, and lateral root organogenesis, are inhibited. This study reveals how SA, besides activating immunity, concomitantly attenuates growth through crosstalk with the auxin distribution network. Further analysis of this dual role of SA and characterization of additional SA-regulated PP2A targets will provide further insights into mechanisms maintaining a balance between growth and defense.

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SA modulates root development independently of NPR1-mediated canonical signaling

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SA attenuates growth through crosstalk with the auxin transport network

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SA upregulates the phosphorylation status of PIN auxin efflux carriers through PP2A

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SA directly targets A subunits of PP2A, inhibiting the activity of the complex

Rhizobacterium-derived diacetyl modulates plant immunity in a phosphate-dependent manner

Plants establish mutualistic associations with beneficial microbes while deploying the immune system to defend against pathogenic ones. Little is known about the interplay between mutualism and immunity and the mediator molecules enabling such crosstalk. Here, we show that plants respond differentially to a volatile bacterial compound through integral modulation of the immune system and the phosphate-starvation response (PSR) system, resulting in either mutualism or immunity. We found that exposure of Arabidopsis thaliana to a known plant growth-promoting rhizobacterium can unexpectedly have either beneficial or deleterious effects to plants. The beneficial-to-deleterious transition is dependent on availability of phosphate to the plants and is mediated by diacetyl, a bacterial volatile compound. Under phosphate-sufficient conditions, diacetyl partially suppresses plant production of reactive oxygen species (ROS) and enhances symbiont colonization without compromising disease resistance. Under phosphate-deficient conditions, diacetyl enhances phytohormone-mediated immunity and consequently causes plant hyper-sensitivity to phosphate deficiency. Therefore, diacetyl affects the type of relation between plant hosts and certain rhizobacteria in a way that depends on the plant's phosphate-starvation response system and phytohormone-mediated immunity.

Modulation of (Homo)Glutathione Metabolism and H2O2 Accumulation during Soybean Cyst Nematode Infections in Susceptible and Resistant Soybean Cultivars

In plant immune responses, reactive oxygen species (ROS) act as signaling molecules that activate defense pathways against pathogens, especially following resistance (R) gene-mediated pathogen recognition. Glutathione (GSH), an antioxidant and redox regulator, participates in the removal of hydrogen peroxide (H2O2). However, the mechanism of GSH-mediated H2O2 generation in soybeans (Glycine max (L.) Merr.) that are resistant to the soybean cyst nematode (SCN; Heterodera glycines Ichinohe) remains unclear. To elucidate this underlying relationship, the feeding of race 3 of H. glycines with resistant cultivars, Peking and PI88788, was compared with that on a susceptible soybean cultivar, Williams 82. After 5, 10, and 15 days of SCN infection, we quantified γ-glutamylcysteine (γ-EC) and (homo)glutathione ((h)GSH), and a gene expression analysis showed that GSH metabolism in resistant cultivars differed from that in susceptible soybean roots. ROS accumulation was examined both in resistant and susceptible roots upon SCN infection. The time of intense ROS generation was related to the differences of resistance mechanisms in Peking and PI88788. ROS accumulation that was caused by the (h)GSH depletion-arrested nematode development in susceptible Williams 82. These results suggest that (h)GSH metabolism in resistant soybeans plays a key role in the regulation of ROS-generated signals, leading to resistance against nematodes.

Biosynthesis and Regulation of Salicylic Acid and N-Hydroxypipecolic Acid in Plant Immunity

Salicylic acid (SA) has long been known to be essential for basal defense and systemic acquired resistance (SAR). N-Hydroxypipecolic acid (NHP), a recently discovered plant metabolite, also plays a key role in SAR and to a lesser extent in basal resistance. Following pathogen infection, levels of both compounds are dramatically increased. Analysis of SA- or SAR-deficient mutants has uncovered how SA and NHP are biosynthesized. The completion of the SA and NHP biosynthetic pathways in Arabidopsis allowed better understanding of how they are regulated. In this review, we discuss recent progress on SA and NHP biosynthesis and their regulation in plant immunity.

Differential Quantitative Requirements for NPR1 Between Basal Immunity and Systemic Acquired Resistance in Arabidopsis thaliana

Non-expressor of pathogenesis-related (PR) genes1 (NPR1) is a key transcription coactivator of plant basal immunity and systemic acquired resistance (SAR). Two mutant alleles, npr1-1 and npr1-3, have been extensively used for dissecting the role of NPR1 in various signaling pathways. However, it is unknown whether npr1-1 and npr1-3 are null mutants. Moreover, the NPR1 transcript levels are induced two- to threefold upon pathogen infection or salicylic acid (SA) treatment, but the biological relevance of the induction is unclear. Here, we used molecular and biochemical approaches including quantitative PCR, immunoblot analysis, site-directed mutagenesis, and CRISPR/Cas9-mediated gene editing to address these questions. We show that npr1-3 is a potential null mutant, whereas npr1-1 is not. We also demonstrated that a truncated npr1 protein longer than the hypothesized npr1-3 protein is not active in SA signaling. Furthermore, we revealed that TGACG-binding (TGA) factors are required for NPR1 induction, but the reverse TGA box in the 5'UTR of NPR1 is dispensable for the induction. Finally, we show that full induction of NPR1 is required for basal immunity, but not for SAR, whereas sufficient basal transcription is essential for full-scale establishment of SAR. Our results indicate that induced transcript accumulation may be differentially required for different functions of a specific gene. Moreover, as npr1-1 is not a null mutant, we recommend that future research should use npr1-3 and potential null T-DNA insertion mutants for dissecting NPR1's function in various physiopathological processes.

Colonisation of Oncidium orchid roots by the endophyte Piriformospora indica restricts Erwinia chrysanthemi infection, stimulates accumulation of NBS-LRR resistance gene transcripts and represses their targeting micro-RNAs in leaves

BACKGROUND

Erwinia chrysanthemi (Ec) is a destructive pathogen which causes soft-rot diseases in diverse plant species including orchids. We investigated whether colonization of Oncidium roots by the endophytic fungus Piriformospora indica (Pi) restricts Ec-induced disease development in leaves, and whether this might be related to the regulation of nucleotide binding site-leucine rich repeat (NBS-LRR) Resistance (R) genes.

RESULTS

Root colonization of Oncidium stackings by Pi restricts progression of Ec-induced disease development in the leaves. Since Pi does not inhibit Ec growth on agar plates, we tested whether NBS-LRR R gene transcripts and the levels of their potential target miRNAs in Oncidium leaves might be regulated by Pi. Using bioinformatic tools, we first identified NBS-LRR R gene sequences from Oncidium, which are predicted to be targets of miRNAs. Among them, the expression of two R genes was repressed and the accumulation of several regulatory miRNA stimulated by Ec in the leaves of Oncidium plants. This correlated with the progression of disease development, jasmonic and salicylic acid accumulation, ethylene synthesis and H₂O₂ production after Ec infection of Oncidium leaves. Interestingly, root colonization by Pi restricted disease development in the leaves, and this was accompanied by higher expression levels of several defense-related R genes and lower expression level of their target miRNA.

CONCLUSION

Based on these data we propose that Pi controls the levels of NBS-LRR R mRNAs and their target miRNAs in leaves. This regulatory circuit correlates with the protection of Oncidium plants against Ec infection, and molecular and biochemical investigations will demonstrate in the future whether, and if so, to what extent these two observations are related to each other.

A novel Arabidopsis pathosystem reveals cooperation of multiple hormonal response-pathways in host resistance against the global crop destroyer Macrophomina phaseolina

Dubbed as a "global destroyer of crops", the soil-borne fungus Macrophomina phaseolina (Mp) infects more than 500 plant species including many economically important cash crops. Host defenses against infection by this pathogen are poorly understood. We established interactions between Mp and Arabidopsis thaliana (Arabidopsis) as a model system to quantitatively assess host factors affecting the outcome of Mp infections. Using agar plate-based infection assays with different Arabidopsis genotypes, we found signaling mechanisms dependent on the plant hormones ethylene, jasmonic acid and salicylic acid to control host defense against this pathogen. By profiling host transcripts in Mp-infected roots of the wild-type Arabidopsis accession Col-0 and ein2/jar1, an ethylene/jasmonic acid-signaling deficient mutant that exhibits enhanced susceptibility to this pathogen, we identified hundreds of genes potentially contributing to a diverse array of defense responses, which seem coordinated by complex interplay between multiple hormonal response-pathways. Our results establish Mp/Arabidopsis interactions as a useful model pathosystem, allowing for application of the vast genomics-related resources of this versatile model plant to the systematic investigation of previously understudied host defenses against a major crop plant pathogen.

Bio-control agents activate plant immune response and prime susceptible tomato against root-knot nematodes

Beneficial microorganisms are generally known to activate plant defense against biotic challenges. However, the molecular mechanisms by which activated plants react more rapidly and actively to pests remain still largely unclear. Tomato plants pre-treated with a mixture of beneficial bio-control agents (BCAs), as soil-drenches, were less sensitive to infection of the root-knot nematode (RKN) Meloidogyne incognita. To unravel the molecular mechanisms of this induced resistance against RKNs, we used qRT-PCR to monitor the expression, in tomato roots and leaves, of 6 key defense genes. Gene transcripts were detected until the 12th day after BCA treatment(3, 7, 8, 12 dpt) and3 and 7 days after nematode inoculation of pre-treated plants. Early after BCA treatment, the salicylic acid (SA)-dependent pathogenesis related gene (PR-gene), PR-1b, marker of the systemic acquired resistance (SAR), was systemically over-expressed. Another PR-gene, PR-5, was over-expressed at later stages of BCA-plant interaction, and only in roots. Activation of defense against RKNs was attested by the early up-regulation of 4 genes (PR-1, PR-3, PR-5, ACO) in pre-treated plants after inoculation. Conversely, the expression of the JA/ET-dependent gene JERF3 did not increase after nematode inoculation in primed plants. A catalase gene (CAT)was highly over-expressed by nematode infection, however, this over-expression was annulled at the earliest stages or limited at the later stages of infection toBCA-treated roots. Enzyme activities, such as glucanase and endochitinase, were enhanced in roots of pre-treated inoculated plants with respect to plants left not inoculated as a control. These findings indicate that BCA interaction with roots primes plants against RKNs. BCA-mediated immunity seems to rely on SA-mediated SAR and to be associated with both the activation of chitinase and glucanase enzyme activities and the inhibition of the plant antioxidant enzyme system. Immunity is triggered at the penetration and movements inside the roots of the invading nematode juveniles but probably acts at the feeding site building stage of nematode infection.

Genome-Wide Identification and Analysis of the NPR1-Like Gene Family in Bread Wheat and Its Relatives

NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1), and its paralogues NPR3 and NPR4, are bona fide salicylic acid (SA) receptors and play critical regulatory roles in plant immunity. However, comprehensive identification and analysis of the NPR1-like gene family had not been conducted so far in bread wheat and its relatives. Here, a total of 17 NPR genes in Triticum aestivum, five NPR genes in Triticum urartu, 12 NPR genes in Triticum dicoccoides, and six NPR genes in Aegilops tauschii were identified using bioinformatics approaches. Protein properties of these putative NPR1-like genes were also described. Phylogenetic analysis showed that the 40 NPR1-like proteins, together with 40 NPR1-related proteins from other plant species, were clustered into three major clades. The TaNPR1-like genes belonging to the same Arabidopsis subfamilies shared similar exon-intron patterns and protein domain compositions, as well as conserved motifs and amino acid residues. The cis-regulatory elements related to SA were identified in the promoter regions of TaNPR1-like genes. The TaNPR1-like genes were intensively mapped on the chromosomes of homoeologous groups 3, 4, and 5, except TaNPR2-D. Chromosomal distribution and collinearity analysis of NPR1-like genes among bread wheat and its relatives revealed that the evolution of this gene family was more conservative following formation of hexaploid wheat. Transcriptome data analysis indicated that TaNPR1-like genes exhibited tissue/organ-specific expression patterns and some members were induced under biotic stress. These findings lay the foundation for further functional characterization of NPR1-like proteins in bread wheat and its relatives.

Mild water stress-induced priming enhance tolerance to Rosellinia necatrix in susceptible avocado rootstocks

BACKGROUND

White root rot (WRR) disease caused by Rosellinia necatrix is one of the most important threats affecting avocado orchards in temperate regions. The eradication of WRR is a difficult task and environmentally friendly control methods are needed to lessen its impact. Priming plants with a stressor (biotic or abiotic) can be a strategy to enhance plant defense/tolerance against future stress episodes but, despite the known underlying common mechanisms, few studies use abiotic-priming for improving tolerance to forthcoming biotic-stress and vice versa ('cross-factor priming'). To assess whether cross-factor priming can be a potential method for enhancing avocado tolerance to WRR disease, 'Dusa' avocado rootstocks, susceptible to R. necatrix, were subjected to two levels of water stress (mild-WS and severe-WS) and, after drought-recovery, inoculated with R. necatrix. Physiological response and expression of plant defense related genes after drought-priming as well as the disease progression were evaluated.

RESULTS

Water-stressed avocado plants showed lower water potential and stomatal limitations of photosynthesis compared to control plants. In addition, NPQ and qN values increased, indicating the activation of energy dissipating mechanisms closely related to the relief of oxidative stress. This response was proportional to the severity of the water stress and was accompanied by the deregulation of pathogen defense-related genes in the roots. After re-watering, leaf photosynthesis and plant water status recovered rapidly in both treatments, but roots of mild-WS primed plants showed a higher number of overexpressed genes related with plant defense than severe-WS primed plants. Disease progression after inoculating primed plants with R. necatrix was significantly delayed in mild-WS primed plants.

CONCLUSIONS

These findings demonstrate that mild-WS can induce a primed state in the WRR susceptible avocado rootstock 'Dusa' and reveal that 'cross-factor priming' with water stress (abiotic stressor) is effective for increasing avocado tolerance against R. necatrix (biotic stressor), underpinning that plant responses against biotic and abiotic stress rely on common mechanisms. Potential applications of these results may involve an enhancement of WRR tolerance of current avocado groves and optimization of water use via low frequency deficit irrigation strategies.

Dynamic ubiquitination determines transcriptional activity of the plant immune coactivator NPR1

Activation of systemic acquired resistance in plants is associated with transcriptome reprogramming induced by the unstable coactivator NPR1. Immune-induced ubiquitination and proteasomal degradation of NPR1 are thought to facilitate continuous delivery of active NPR1 to target promoters, thereby maximising gene expression. Because of this potentially costly sacrificial process, we investigated if ubiquitination of NPR1 plays transcriptional roles prior to its proteasomal turnover. Here we show ubiquitination of NPR1 is a progressive event in which initial modification by a Cullin-RING E3 ligase promotes its chromatin association and expression of target genes. Only when polyubiquitination of NPR1 is enhanced by the E4 ligase, UBE4, it is targeted for proteasomal degradation. Conversely, ubiquitin ligase activities are opposed by UBP6/7, two proteasome-associated deubiquitinases that enhance NPR1 longevity. Thus, immune-induced transcriptome reprogramming requires sequential actions of E3 and E4 ligases balanced by opposing deubiquitinases that fine-tune activity of NPR1 without strict requirement for its sacrificial turnover.

Functional Characterization of Resistance to Powdery Mildew of VvTIFY9 from Vitis vinifera

Powdery mildew is a disease caused by fungal pathogens that harms grape leaves and fruits. The TIFY gene family is a plant-specific super-family involved in the process of plants’ development and their biotic and abiotic stress responses. This study aimed to learn the function of the VvTIFY9 gene to investigate molecular mechanisms of grape resistance to powdery mildew. A VvTIFY9 protein encoding a conserved motif (TIF[F/Y]XG) was characterized in grape (Vitis vinifera). Sequence analysis confirmed that VvTIFY9 contained this conserved motif (TIF[F/Y]XG). Quantitative PCR analysis of VvTIFY9 in various grape tissues demonstrated that the expression of VvTIFY9 was higher in grape leaves. VvTIFY9 was induced by salicylic acid (SA) and methyl jasmonate (MeJA) and it also quickly responded to infection with Erysiphe necator in grape. Analysis of the subcellular localization and transcriptional activation activity of VvTIFY9 showed that VvTIFY9 located to the nucleus and had transcriptional activity. Arabidopsis that overexpressed VvTIFY9 were more resistant to Golovinomyces cichoracearum, and quantitative PCR revealed that two defense-related genes, AtPR1 and AtPDF1.2, were up-regulated in the overexpressing lines. These results indicate that VvTIFY9 is intimately involved in SA-mediated resistance to grape powdery mildew. This study provides the basis for exploring the molecular mechanism of grape resistance to disease resistance and candidate genes for transgenic disease resistance breeding of grape plants.

Cold stress activates disease resistance in Arabidopsis thaliana through a salicylic acid dependent pathway

Exposure to short-term cold stress influences disease resistance by mechanisms that remain poorly characterized. The molecular basis of cold-activated immunity was therefore investigated in Arabidopsis thaliana inoculated with the bacterial pathogen Pst DC3000, using a transcriptomic analysis. Exposure to cold stress for 10 hr was sufficient to activate immunity, as well as H₂ O₂ accumulation and callose deposition. Transcriptome changes induced by the 10-hr cold treatment were similar to those caused by pathogen infection, including increased expression of the salicylic acid (SA) pathway marker genes, PR2 and PR5, and genes playing positive roles in defence against (hemi)-biotrophs. In contrast, transcripts encoding jasmonic acid (JA) pathway markers such as PR4 and MYC2 and transcripts with positive roles in defence against necrotrophs were less abundant following the 10-hr cold treatment. Cold-activated immunity was dependent on SA, being partially dependent on NPR1 and ICS1/SID2. In addition, transcripts encoding SA biosynthesis enzymes such as ICS2, PAL1, PAL2, and PAL4 (but not ICS1/SID2) and MES9 were more abundant, whereas GH3.5/WES1 and SOT12 transcripts that encode components involved in SA modification were less abundant following cold stress treatment. These findings show that cold stress cross-activates innate immune responses via a SA-dependent pathway.

Tackling Plant Phosphate Starvation by the Roots

Plant responses to phosphate deprivation encompass a wide range of strategies, varying from altering root system architecture, entering symbiotic interactions to excreting root exudates for phosphorous release, and recycling of internal phosphate. These processes are tightly controlled by a complex network of proteins that are specifically upregulated upon phosphate starvation. Although the different effects of phosphate starvation have been intensely studied, the full extent of its contribution to altered root system architecture remains unclear. In this review, we focus on the effect of phosphate starvation on the developmental processes that shape the plant root system and their underlying molecular pathways.

Bacterial infection systemically suppresses stomatal density

Many plant pathogens gain entry to their host via stomata. On sensing attack, plants close these pores to restrict pathogen entry. Here, we show that plants exhibit a second longer term stomatal response to pathogens. Following infection, the subsequent development of leaves is altered via a systemic signal. This reduces the density of stomata formed, thus providing fewer entry points for pathogens on new leaves. Arabidopsis thaliana leaves produced after infection by a bacterial pathogen that infects through the stomata (Pseudomonas syringae) developed larger epidermal pavement cells and stomata and consequently had up to 20% reductions in stomatal density. The bacterial peptide flg22 or the phytohormone salicylic acid induced similar systemic reductions in stomatal density suggesting that they might mediate this effect. In addition, flagellin receptors, salicylic acid accumulation, and the lipid transfer protein AZI1 were all required for this developmental response. Furthermore, manipulation of stomatal density affected the level of bacterial colonization, and plants with reduced stomatal density showed slower disease progression. We propose that following infection, development of new leaves is altered by a signalling pathway with some commonalities to systemic acquired resistance. This acts to reduce the potential for future infection by providing fewer stomatal openings.

Salicylic acid: biosynthesis, perception, and contributions to plant immunity

Salicylic acid (SA) has emerged as a key plant defense hormone with critical roles in different aspects of plant immunity. Analysis of Arabidopsis mutants revealed complex regulation of pathogen-induced SA biosynthesis. Studies on SA-insensitive mutants led to the identification of the SA receptors and how SA regulates defense gene expression. Consistent with its critical roles in plant immunity, SA is required for the assembly of a normal root microbiome and various pathogen effectors have evolved to target components of SA biosynthesis or signaling. This review discusses recent advances in SA biology, focusing in particular on the regulation of SA biosynthesis and SA perception.

Plant chemical genetics reveals colistin sulphate as a SA and NPR1-independent PR1 inducer functioning via a p38-like kinase pathway

In plants, low-dose of exogenous bacterial cyclic lipopeptides (CLPs) trigger transient membrane changes leading to activation of early and late defence responses. Here, a forward chemical genetics approach identifies colistin sulphate (CS) CLP as a novel plant defence inducer. CS uniquely triggers activation of the PATHOGENESIS-RELATED 1 (PR1) gene and resistance against Pseudomonas syringae pv. tomato DC3000 (Pst DC3000) in Arabidopsis thaliana (Arabidopsis) independently of the PR1 classical inducer, salicylic acid (SA) and the key SA-signalling protein, NON-EXPRESSOR OF PR1 (NPR1). Low bioactive concentration of CS does not trigger activation of early defence markers such as reactive oxygen species (ROS) and mitogen activated protein kinase (MAPK). However, it strongly suppresses primary root length elongation. Structure activity relationship (SAR) assays and mode-of-action (MoA) studies show the acyl chain and activation of a ∼46 kDa p38-like kinase pathway to be crucial for CS' bioactivity. Selective pharmacological inhibition of the active p38-like kinase pathway by SB203580 reverses CS' effects on PR1 activation and root length suppression. Our results with CS as a chemical probe highlight the existence of a novel SA- and NPR1-independent branch of PR1 activation functioning via a membrane-sensitive p38-like kinase pathway.

Role of calreticulin in biotic and abiotic stress signalling and tolerance mechanisms in plants

Calreticulin (CRT) is calcium binding protein of endoplasmic reticulum (ER) which performs plethora of functions besides it's role as molecular chaperone. Among the three different isoforms of this protein, CRT3 is most closely related to primitive CRT gene of higher plants. Based on their distinct structural and functional organisation, the plant CRTs have been known to contain three different domains: N, P and the C domain. The domain organisation and various biochemical characterstics of plant and animal CRTs are common with the exception of some differences. In plant calreticulin, the important N-glycosylation site(s) are replaced by the glycan chain(s) and several consensus sequences for in vitro phosphorylation by protein kinase CK2 (casein kinase-2), are also present unlike the animal calreticulin. Biotic and abiotic stresses play a significant role in bringing down the crop production. The role of various phytohormones in defense against fungal pathogens is well documented. CRT3 has been reported to play important role in protecting the plants against fungal and bacterial pathogens and in maintaining plant innate immunity. There is remarkable crosstalk between CRT mediated signalling and biotic, abiotic stress, and phytohormone mediated signalling pathways The role of CRT mediated pathway in mitigating biotic and abiotic stress can be further explored in plants so as to strategically modify it for development of stress tolerant plants.

Proteo-metabolomic investigation of transgenic rice unravels metabolic alterations and accumulation of novel proteins potentially involved in defence against Rhizoctonia solani

The generation of sheath blight (ShB)-resistant transgenic rice plants through the expression of Arabidopsis NPR1 gene is a significant development for research in the field of biotic stress. However, to our knowledge, regulation of the proteomic and metabolic networks in the ShB-resistant transgenic rice plants has not been studied. In the present investigation, the relative proteome and metabolome profiles of the non-transformed wild-type and the AtNPR1-transgenic rice lines prior to and subsequent to the R. solani infection were investigated. Total proteins from wild type and transgenic plants were investigated using two-dimensional gel electrophoresis (2-DE) followed by mass spectrometry (MS). The metabolomics study indicated an increased abundance of various metabolites, which draws parallels with the proteomic analysis. Furthermore, the proteome data was cross-examined using network analysis which identified modules that were rich in known as well as novel immunity-related prognostic proteins, particularly the mitogen-activated protein kinase 6, probable protein phosphatase 2C1, probable trehalose-phosphate phosphatase 2 and heat shock protein. A novel protein, 14-3-3GF14f was observed to be upregulated in the leaves of the transgenic rice plants after ShB infection, and the possible mechanistic role of this protein in ShB resistance may be investigated further.

A Chimeric IDD4 Repressor Constitutively Induces Immunity in Arabidopsis via the Modulation of Salicylic Acid and Jasmonic Acid Homeostasis

INDETERMINATE DOMAIN (IDD)/BIRD proteins belong to a highly conserved plant-specific group of transcription factors with dedicated functions in plant physiology and development. Here, we took advantage of the chimeric repressor gene-silencing technology (CRES-T, SRDX) to widen our view on the role of IDD4/IMPERIAL EAGLE and IDD family members in plant immunity. The hypomorphic idd4SRDX lines are compromised in growth and show a robust autoimmune phenotype. Hormonal measurements revealed the concomitant accumulation of salicylic acid and jasmonic acid suggesting that IDDs are involved in regulating the metabolism of these biotic stress hormones. The analysis of immunity-pathways showed enhanced activation of immune MAP kinase-signaling pathways, the accumulation of hydrogen peroxide and spontaneous programmed cell death. The transcriptome of nonelicited idd4SRDX lines can be aligned to approximately 40% of differentially expressed genes (DEGs) in flg22-treated wild-type plants. The pattern of DEGs implies IDDs as pivotal repressors of flg22-dependent gene induction. Infection experiments showed the increased resistance of idd4SRDX lines to Pseudomonas syringae and Botrytis cinerea implying a function of IDDs in defense adaptation to hemibiotrophs and necrotrophs. Genome-wide IDD4 DNA-binding studies (DAP-SEQ) combined with DEG analysis of idd4SRDX lines identified IDD4-regulated functional gene clusters that contribute to plant growth and development. In summary, we discovered that the expression of idd4SRDX activates a wide range of defense-related traits opening up the possibility to apply idd4SRDX as a powerful tool to stimulate innate immunity in engineered crops.

Salicylic acid-induced transcriptional reprogramming by the HAC-NPR1-TGA histone acetyltransferase complex in Arabidopsis

Plant immunity depends on massive expression of pathogenesis-related genes (PRs) whose transcription is de-repressed by pathogen-induced signals. Salicylic acid (SA) acts as a major signaling molecule in plant immunity and systemic acquired resistance triggered by bacterial or viral pathogens. SA signal results in the activation of the master immune regulator, Nonexpressor of pathogenesis-related genes 1 (NPR1), which is thought to be recruited by transcription factors such as TGAs to numerous downstream PRs. Despite its key role in SA-triggered immunity, the biochemical nature of the transcriptional coactivator function of NPR1 and the massive transcriptional reprogramming induced by it remain obscure. Here we demonstrate that the CBP/p300-family histone acetyltransferases, HACs and NPR1 are both essential to develop SA-triggered immunity and PR induction. Indeed HACs and NPR1 form a coactivator complex and are recruited to PR chromatin through TGAs upon SA signal, and finally the HAC−NPR1−TGA complex activates PR transcription by histone acetylation-mediated epigenetic reprogramming. Thus, our study reveals a molecular mechanism of NPR1-mediated transcriptional reprogramming and a key epigenetic aspect of the central immune system in plants.

PBS3 Protects EDS1 from Proteasome-Mediated Degradation in Plant Immunity

Plant immunity is controlled by both positive regulators such as PBS3 and EDS1 and negative regulators such as NPR3 and NPR4. However, the relationships among these important immune regulators remain elusive. In this study, we found that PBS3 interacts with EDS1 in both the cytoplasm and the nucleus, and is required for EDS1 protein accumulation. NPR3 and NPR4, which function as salicylic acid receptors and adaptors of Cullin3-based E3 ligase, interact with and mediate the degradation of EDS1 via the 26S proteasome. We further discovered that PBS3 inhibits the polyubiquitination and subsequent degradation of EDS1 by reducing the association of EDS1 with the Cullin3 adaptors NPR3 and NPR4. Furthermore, we showed that PBS3 and EDS1 also contribute to PAMP-triggered immunity in addition to effector-triggered immunity. Collectively, our study reveals a novel mechanism by which plants fine-tune defense responses by inhibiting the degradation of a positive player in plant immunity.

Signaling mechanisms underlying systemic acquired resistance to microbial pathogens

Plants respond to biotic stress by inducing a variety of responses, which not only protect against the immediate diseases but also provide immunity from future infections. One example is systemic acquired resistance (SAR), which provides long-lasting and broad-spectrum protection at the whole plant level. The induction of SAR prepares the plant for a more robust response to subsequent infections from related and unrelated pathogens. SAR involves the rapid generation of signals at the primary site of infection, which are transported to the systemic parts of the plant presumably via the phloem. SAR signal generation and perception requires an intact cuticle, a waxy layer covering all aerial parts of the plant. A chemically diverse set of SAR inducers has already been identified, including hormones (salicylic acid, methyl salicylate), primary/secondary metabolites (nitric oxide, reactive oxygen species, glycerol-3-phosphate, azelaic acid, pipecolic acid, dihyroabetinal), fatty acid/lipid derivatives (18 carbon unsaturated fatty acids, galactolipids), and proteins (DIR1-Defective in Induced Resistance 1, AZI1-Azelaic acid Induced 1). Some of these are demonstrably mobile and the phloem loading routes for three of these SAR inducers is known. Here we discuss the recent findings related to synthesis, transport, and the relationship between these various SAR inducers.

Semi-dominant mutation in the cysteine-rich receptor-like kinase gene, ALS1, conducts constitutive defence response in rice

Plants have evolved a sophisticated two-branch defence system to prevent the growth and spread of pathogen infection. The novel Cys-rich repeat (CRR) containing receptor-like kinases, known as CRKs, were reported to mediate defence resistance in plants. For rice, there are only two reports of CRKs. A semi-dominant lesion mimic mutant als1 (apoptosis leaf and sheath 1) in rice was identified to demonstrate spontaneous lesions on the leaf blade and sheath. A map-based cloning strategy was used for fine mapping and cloning of ALS1, which was confirmed to be a typical CRK in rice. Functional studies of ALS1 were conducted, including phylogenetic analysis, expression analysis, subcellular location and blast resistance identification. Most pathogenesis-related (PR) genes and other defence-related genes were activated and up-regulated to a high degree. ALS1 was expressed mainly in the leaf blade and sheath, in which further study revealed that ALS1 was present in the vascular bundles. ALS1 was located in the cell membrane of rice protoplasts, and its mutation did not change its subcellular location. Jasmonic acid (JA) and salicylic acid (SA) accumulation were observed in als1, and enhanced blast resistance was also observed. The mutation of ALS1 caused a constitutively activated defence response in als1. The results of our study imply that ALS1 participates in a defence response resembling the common SA-, JA- and NH1-mediated defence responses in rice.

Silencing of CrNPR1 and CrNPR3 Alters Plant Susceptibility to Periwinkle Leaf Yellowing Phytoplasma

Phytoplasmas are prokaryotic plant pathogens that cause considerable loss in many economically important crops, and an increasing number of phytoplasma diseases are being reported on new hosts. Knowledge of plant defense mechanisms against such pathogens should help to improve strategies for controlling these diseases. Salicylic acid (SA)-mediated defense may play an important role in defense against phytoplasmas. Here, we report that SA accumulated in Madagascar periwinkle (Catharanthus roseus) infected with periwinkle leaf yellowing (PLY) phytoplasma. CrPR1a expression was induced in both symptomatic and non-symptomatic tissues of plants exhibiting PLY. NPR1 plays a central role in SA signaling, and two NPR1 homologs, CrNPR1 and CrNPR3, were identified from a periwinkle transcriptome database. Similar to CrPR1a, CrNPR1 expression was also induced in both symptomatic and non-symptomatic tissues of plants exhibiting PLY. Silencing of CrNPR1, but not CrNPR3, significantly repressed CrPR1a induction in Tobacco rattle virus-infected periwinkle plants. In addition, symptoms of PLY progressed fastest in CrNPR1-silenced plants and slowest in CrNPR3-silenced plants. Consistently, expression of CrNPR1, but not CrNPR3, was induced by phytoplasma infection as well as SA treatment. This study highlights the importance of NPR1- and SA-mediated defense against phytoplasma in periwinkle and offers insight into plant-phytoplasma interactions to improve disease control strategies.

Arabidopsis UBC13 differentially regulates two programmed cell death pathways in responses to pathogen and low-temperature stress

UBC13 is required for Lys63-linked polyubiquitination and innate immune responses in mammals, but its functions in plant immunity remain to be defined. Here we used genetic and pathological methods to evaluate roles of Arabidopsis UBC13 in response to pathogens and environmental stresses. Loss of UBC13 failed to activate the expression of numerous cold-responsive genes and resulted in hypersensitivity to low-temperature stress, indicating that UBC13 is involved in plant response to low-temperature stress. Furthermore, the ubc13 mutant displayed low-temperature-induced and salicylic acid-dependent lesion mimic phenotypes. Unlike typical lesion mimic mutants, ubc13 did not enhance disease resistance against virulent bacterial and fungal pathogens, but diminished hypersensitive response and compromised effector-triggered immunity against avirulent bacterial pathogens. UBC13 differently regulates two types of programmed cell death in response to low temperature and pathogen. The lesion mimic phenotype in the ubc13 mutant is partially dependent on SNC1. UBC13 interacts with an F-box protein CPR1 that regulates the homeostasis of SNC1. However, the SNC1 protein level was not altered in the ubc13 mutant, implying that UBC13 is not involved in CPR1-regulated SNC1 protein degradation. Taken together, our results revealed that UBC13 is a key regulator in plant response to low temperature and pathogens.

Arabidopsis VQ10 interacts with WRKY8 to modulate basal defense against Botrytis cinerea

Recent studies in Arabidopsis have revealed that some VQ motif-containing proteins physically interact with WRKY transcription factors; however, their specific biological functions are still poorly understood. In this study, we confirmed the interaction between VQ10 and WRKY8, and show that VQ10 and WRKY8 formed a complex in the plant cell nucleus. Yeast two-hybrid analysis showed that the middle region of WRKY8 and the VQ motif of VQ10 are critical for their interaction, and that this interaction promotes the DNA-binding activity of WRKY8. Further investigation revealed that the VQ10 protein was exclusively localized in the nucleus, and VQ10 was predominantly expressed in siliques. VQ10 expression was strongly responsive to the necrotrophic fungal pathogen, Botrytis cinerea and defense-related hormones. Phenotypic analysis showed that disruption of VQ10 increased mutant plants susceptibility to the fungal pathogen B. cinerea, whereas constitutive-expression of VQ10 enhanced resistance to B. cinerea. Consistent with these findings, expression of the defense-related PLANT DEFENSIN1.2 (PDF1.2) gene was decreased in vq10 mutant plants, after B. cinerea infection, but increased in VQ10-overexpressing transgenic plants. Taken together, our findings provide evidence that VQ10 physically interacts with WRKY8 and positively regulates plant basal resistance against the necrotrophic fungal pathogen B. cinerea.

Systemic Acquired Resistance and Salicylic Acid: Past, Present, and Future

This article is part of the Distinguished Review Article Series in Conceptual and Methodological Breakthroughs in Molecular Plant-Microbe Interactions. Salicylic acid (SA) is a critical plant hormone that regulates numerous aspects of plant growth and development as well as the activation of defenses against biotic and abiotic stress. Here, we present a historical overview of the progress that has been made to date in elucidating the role of SA in signaling plant immune responses. The ability of plants to develop acquired immunity after pathogen infection was first proposed in 1933. However, most of our knowledge about plant immune signaling was generated over the last three decades, following the discovery that SA is an endogenous defense signal. During this timeframe, researchers have identified i) two pathways through which SA can be synthesized, ii) numerous proteins that regulate SA synthesis and metabolism, and iii) some of the signaling components that function downstream of SA, including a large number of SA targets or receptors. In addition, it has become increasingly evident that SA does not signal immune responses by itself but, rather, as part of an intricate network that involves many other plant hormones. Future efforts to develop a comprehensive understanding of SA-mediated immune signaling will therefore need to close knowledge gaps that exist within the SA pathway itself as well as clarify how crosstalk among the different hormone signaling pathways leads to an immune response that is both robust and optimized for maximal efficacy, depending on the identity of the attacking pathogen.

The potato transcription factor StbZIP61 regulates dynamic biosynthesis of salicylic acid in defense against Phytophthora infestans infection

Salicylic acid (SA) signalling plays an essential role in plant innate immunity. In this study, we identified a component in the SA signaling pathway in potato (Solanum tuberosum), the transcription factor StbZIP61, and characterized its function in defence against Phytophthora infestans. Expression of StbZIP61 was induced upon P. infestans infection and following exposure to the defense signaling hormones SA, ethylene and jasmonic acid. Overexpression of StbZIP61 increased the tolerance of potato plants to P. infestans while RNA interference (RNAi) increased susceptibility. Yeast two-hybrid and pull down experiments revealed that StbZIP61 could interact with an NPR3-like protein (StNPR3L) that inhibited its DNA-binding and transcriptional activation activities. Moreover, StNPR3L interacted with StbZIP61 in an SA-dependent manner. Among candidate genes involved in SA-regulated defense responses, StbZIP61 had a significant impact on expression of StICS1, which encodes a key enzyme for SA biosynthesis. StICS1 transcription was induced upon P. infestans infection and this responsive expression to the pathogen was reduced in StbZIP61 RNAi plants. Accordingly, StICS1 expression was remarkably enhanced in StbZIP61-overexpressing plants. Together, our data demonstrate that StbZIP61 functions in concert with StNPR3L to regulate the temporal activation of SA biosynthesis, which contributes to SA-mediated immunity against P. infestans infection in potato.

The ubiquitin-proteasome system as a transcriptional regulator of plant immunity

The ubiquitin-proteasome system (UPS) has been shown to play vital roles in diverse plant developmental and stress responses. The UPS post-translationally modifies cellular proteins with the small molecule ubiquitin, resulting in their regulated degradation by the proteasome. Of particular importance is the role of the UPS in regulating hormone-responsive gene expression profiles, including those triggered by the immune hormone salicylic acid (SA). SA utilizes components of the UPS pathway to reprogram the transcriptome for establishment of local and systemic immunity. Emerging evidence has shown that SA induces the activity of Cullin-RING ligases (CRLs) that fuse chains of ubiquitin to downstream transcriptional regulators and consequently target them for degradation by the proteasome. Here we review how CRL-mediated degradation of transcriptional regulators may control SA-responsive immune gene expression programmes and discuss how the UPS can be modulated by both endogenous and foreign exogenous signals. The highlighted research findings paint a clear picture of the UPS as a central hub for immune activation as well as a battle ground for hijacking by pathogens.

Harpin-inducible defense signaling components impair infection by the ascomycete Macrophomina phaseolina

Soybean (Glycine max) infection by the charcoal rot (CR) ascomycete Macrophomina phaseolina is enhanced by the soybean cyst nematode (SCN) Heterodera glycines. We hypothesized that G. max genetic lines impairing infection by M. phaseolina would also limit H. glycines parasitism, leading to resistance. As a part of this M. phaseolina resistance process, the genetic line would express defense genes already proven to impair nematode parasitism. Using G. max[DT97-4290/PI 642055], exhibiting partial resistance to M. phaseolina, experiments show the genetic line also impairs H. glycines parasitism. Furthermore, comparative studies show G. max[DT97-4290/PI 642055] exhibits induced expression of the effector triggered immunity (ETI) gene NON-RACE SPECIFIC DISEASE RESISTANCE 1/HARPIN INDUCED1 (NDR1/HIN1) that functions in defense to H. glycines as compared to the H. glycines and M. phaseolina susceptible line G. max[Williams 82/PI 518671]. Other defense genes that are induced in G. max[DT97-4290/PI 642055] include the pathogen associated molecular pattern (PAMP) triggered immunity (PTI) genes ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), NONEXPRESSOR OF PR1 (NPR1) and TGA2. These observations link G. max defense processes that impede H. glycines parasitism to also potentially function toward impairing M. phaseolina pathogenicity. Testing this hypothesis, G. max[Williams 82/PI 518671] genetically engineered to experimentally induce GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 expression leads to impaired M. phaseolina pathogenicity. In contrast, G. max[DT97-4290/PI 642055] engineered to experimentally suppress the expression of GmNDR1-1, EDS1-2, NPR1-2 and TGA2-1 by RNA interference (RNAi) enhances M. phaseolina pathogenicity. The results show components of PTI and ETI impair both nematode and M. phaseolina pathogenicity.

Pathogenesis-related proteins and peptides as promising tools for engineering plants with multiple stress tolerance

Pathogenesis-related (PR) proteins and antimicrobial peptides (AMPs) are a group of diverse molecules that are induced by phytopathogens as well as defense related signaling molecules. They are the key components of plant innate immune system especially systemic acquired resistance (SAR), and are widely used as diagnostic molecular markers of defense signaling pathways. Although, PR proteins and peptides have been isolated much before but their biological function remains largely enigmatic despite the availability of new scientific tools. The earlier studies have demonstrated that PR genes provide enhanced resistance against both biotic and abiotic stresses, which make them one of the most promising candidates for developing multiple stress tolerant crop varieties. In this regard, plant genetic engineering technology is widely accepted as one of the most fascinating approach to develop the disease resistant transgenic crops using different antimicrobial genes like PR genes. Overexpression of PR genes (chitinase, glucanase, thaumatin, defensin and thionin) individually or in combination have greatly uplifted the level of defense response in plants against a wide range of pathogens. However, the detailed knowledge of signaling pathways that regulates the expression of these versatile proteins is critical for improving crop plants to multiple stresses, which is the future theme of plant stress biology. Hence, this review provides an overall overview on the PR proteins like their classification, role in multiple stresses (biotic and abiotic) as well as in various plant defense signaling cascades. We also highlight the success and snags of transgenic plants expressing PR proteins and peptides.

The Elongator complex-associated protein DRL1 plays a positive role in immune responses against necrotrophic fungal pathogens in Arabidopsis

DEFORMED ROOT AND LEAVES1 (DRL1) is an Arabidopsis homologue of the yeast TOXIN TARGET4 (TOT4)/KILLER TOXIN-INSENSITIVE12 (KTI12) protein that is physically associated with the RNA polymerase II-interacting protein complex named Elongator. Mutations in DRL1 and Elongator lead to similar morphological and molecular phenotypes, suggesting that DRL1 and Elongator may functionally overlap in Arabidopsis. We have shown previously that Elongator plays an important role in both salicylic acid (SA)- and jasmonic acid (JA)/ethylene (ET)-mediated defence responses. Here, we tested whether DRL1 also plays a similar role as Elongator in plant immune responses. Our results show that, although DRL1 partially contributes to SA-induced cytotoxicity, it does not play a significant role in SA-mediated expression of PATHOGENESIS-RELATED genes and resistance to the virulent bacterial pathogen Pseudomonas syringae pv. maculicola ES4326. In contrast, DRL1 is required for JA/ET- and necrotrophic fungal pathogen Botrytis cinerea-induced defence gene expression and for resistance to B. cinerea and Alternaria brassicicola. Furthermore, unlike the TOT4/KTI12 gene which, when overexpressed in yeast, confers zymocin resistance, a phenotype of the tot4/kti12 mutant, overexpression of DRL1 does not change B. cinerea-induced defence gene expression and resistance to this pathogen. Finally, DRL1 contains an N-terminal P-loop and a C-terminal calmodulin (CaM)-binding domain and is a CaM-binding protein. We demonstrate that both the P-loop and the CaM-binding domain are essential for the function of DRL1 in B. cinerea-induced expression of PDF1.2 and ORA59, and in resistance to B. cinerea, suggesting that the function of DRL1 in plant immunity may be regulated by ATP/GTP and CaM binding.

Soil mixture composition alters Arabidopsis susceptibility to Pseudomonas syringae infection

Pseudomonas syringae is a gram-negative bacterial pathogen that causes disease on more than 100 different plant species, including the model plant Arabidopsis thaliana. Dissection of the Arabidopsis thaliana-Pseudomonas syringae pathosystem has identified many factors that contribute to successful infection or immunity, including the genetics of the host, the genetics of the pathogen, and the environment. Environmental factors that contribute to a successful interaction can include temperature, light, and the circadian clock, as well as the soil environment. As silicon-amended Resilience soil is advertised to enhance plant health, we sought to examine the extent to which this soil might affect the behavior of the A. thaliana-P. syringae model pathosystem and to characterize the mechanisms through which these effects may occur. We found that plants grown in Si-amended Resilience soil displayed enhanced resistance to bacteria compared to plants grown in non-Si-amended Sunshine soil, and salicylic acid biosynthesis and signaling were not required for resistance. Although silicon has been shown to contribute to broad-spectrum resistance, our data indicate that silicon is not the direct cause of enhanced resistance and that the Si-amended Resilience soil has additional properties that modulate plant resistance. Our work demonstrates the importance of environmental factors, such as soil in modulating interactions between the plant and foliar pathogens, and highlights the significance of careful annotation of the environmental conditions under which plant-pathogen interactions are studied.

A Localized Pseudomonas syringae Infection Triggers Systemic Clock Responses in Arabidopsis

The circadian clock drives daily rhythms of many plant physiological responses, providing a competitive advantage that improves plant fitness and survival rates [1-5]. Whereas multiple environmental cues are predicted to regulate the plant clock function, most studies focused on understanding the effects of light and temperature [5-8]. Increasing evidence indicates a significant role of plant-pathogen interactions on clock regulation [9, 10], but the underlying mechanisms remain elusive. In Arabidopsis, the clock function largely relies on a transcriptional feedback loop between morning (CCA1 and LHY)- and evening (TOC1)-expressed transcription factors [6-8]. Here, we focused on these core components to investigate the Arabidopsis clock regulation using a unique biotic stress approach. We found that a single-leaf Pseudomonas syringae infection systemically lengthened the period and reduced the amplitude of circadian rhythms in distal uninfected tissues. Remarkably, the low-amplitude phenotype observed upon infection was recapitulated by a transient treatment with the defense-related phytohormone salicylic acid (SA), which also triggered a significant clock phase delay. Strikingly, despite SA-modulated circadian rhythms, we revealed that the master regulator of SA signaling, NPR1 [11, 12], antagonized clock responses triggered by both SA treatment and P. syringae. In contrast, we uncovered that the NADPH oxidase RBOHD [13] largely mediated the aforementioned clock responses after either SA treatment or the bacterial infection. Altogether, we demonstrated novel and unexpected roles for SA, NPR1, and redox signaling in clock regulation by P. syringae and revealed a previously unrecognized layer of systemic clock regulation by locally perceived environmental cues.

A harpin elicitor induces the expression of a coiled-coil nucleotide binding leucine rich repeat (CC-NB-LRR) defense signaling gene and others functioning during defense to parasitic nematodes

The bacterial effector harpin induces the transcription of the Arabidopsis thaliana NON-RACE SPECIFIC DISEASE RESISTANCE 1/HARPIN INDUCED1 (NDR1/HIN1) coiled-coil nucleotide binding leucine rich repeat (CC-NB-LRR) defense signaling gene. In Glycine max, Gm-NDR1-1 transcripts have been detected within root cells undergoing a natural resistant reaction to parasitism by the syncytium-forming nematode Heterodera glycines, functioning in the defense response. Expressing Gm-NDR1-1 in Gossypium hirsutum leads to resistance to Meloidogyne incognita parasitism. In experiments presented here, the heterologous expression of Gm-NDR1-1 in G. hirsutum impairs Rotylenchulus reniformis parasitism. These results are consistent with the hypothesis that Gm-NDR1-1 expression functions broadly in generating a defense response. To examine a possible relationship with harpin, G. max plants topically treated with harpin result in induction of the transcription of Gm-NDR1-1. The result indicates the topical treatment of plants with harpin, itself, may lead to impaired nematode parasitism. Topical harpin treatments are shown to impair G. max parasitism by H. glycines, M. incognita and R. reniformis and G. hirsutum parasitism by M. incognita and R. reniformis. How harpin could function in defense has been examined in experiments showing it also induces transcription of G. max homologs of the proven defense genes ENHANCED DISEASE SUSCEPTIBILITY1 (EDS1), TGA2, galactinol synthase, reticuline oxidase, xyloglucan endotransglycosylase/hydrolase, alpha soluble N-ethylmaleimide-sensitive fusion protein (α-SNAP) and serine hydroxymethyltransferase (SHMT). In contrast, other defense genes are not directly transcriptionally activated by harpin. The results indicate harpin induces pathogen associated molecular pattern (PAMP) triggered immunity (PTI) and effector-triggered immunity (ETI) defense processes in the root, activating defense to parasitic nematodes.

Rhizosphere-associated Pseudomonas induce systemic resistance to herbivores at the cost of susceptibility to bacterial pathogens

Plant-associated soil microbes are important mediators of plant defence responses to diverse above-ground pathogen and insect challengers. For example, closely related strains of beneficial rhizosphere Pseudomonas spp. can induce systemic resistance (ISR), systemic susceptibility (ISS) or neither against the bacterial foliar pathogen Pseudomonas syringae pv. tomato DC3000 (Pto DC3000). Using a model system composed of root-associated Pseudomonas spp. strains, the foliar pathogen Pto DC3000 and the herbivore Trichoplusia ni (cabbage looper), we found that rhizosphere-associated Pseudomonas spp. that induce either ISS and ISR against Pto DC3000 all increased resistance to herbivory by T. ni. We found that resistance to T. ni and resistance to Pto DC3000 are quantitative metrics of the jasmonic acid (JA)/salicylic acid (SA) trade-off and distinct strains of rhizosphere-associated Pseudomonas spp. have distinct effects on the JA/SA trade-off. Using genetic analysis and transcriptional profiling, we provide evidence that treatment of Arabidopsis with Pseudomonas sp. CH267, which induces ISS against bacterial pathogens, tips the JA/SA trade-off towards JA-dependent defences against herbivores at the cost of a subset of SA-mediated defences against bacterial pathogens. In contrast, treatment of Arabidopsis with the ISR strain Pseudomonas sp. WCS417 disrupts JA/SA antagonism and simultaneously primes plants for both JA- and SA-mediated defences. Our findings show that ISS against the bacterial foliar pathogens triggered by Pseudomonas sp. CH267, which is a seemingly deleterious phenotype, may in fact be an adaptive consequence of increased resistance to herbivory. Our work shows that pleiotropic effects of microbiome modulation of plant defences are important to consider when using microbes to modify plant traits in agriculture.

NLR locus-mediated trade-off between abiotic and biotic stress adaptation in Arabidopsis

Osmotic stress caused by drought, salt or cold decreases plant fitness. Acquired stress tolerance defines the ability of plants to withstand stress following an initial exposure¹. We found previously that acquired osmotolerance after salt stress is widespread among Arabidopsis thaliana accessions². Here, we identify ACQOS as the locus responsible for ACQUIRED OSMOTOLERANCE. Of its five haplotypes, only plants carrying group 1 ACQOS are impaired in acquired osmotolerance. ACQOS is identical to VICTR, encoding a nucleotide-binding leucine-rich repeat (NLR) protein³. In the absence of osmotic stress, group 1 ACQOS contributes to bacterial resistance. In its presence, ACQOS causes detrimental autoimmunity, thereby reducing osmotolerance. Analysis of natural variation at the ACQOS locus suggests that functional and non-functional ACQOS alleles are being maintained due to a trade-off between biotic and abiotic stress adaptation. Thus, polymorphism in certain plant NLR genes might be influenced by competing environmental stresses.

Helicoverpa zea gut-associated bacteria indirectly induce defenses in tomato by triggering a salivary elicitor(s)

Insect gut-associated microbes modulating plant defenses have been observed in beetles and piercing-sucking insects, but the role of caterpillar-associated bacteria in regulating plant induced defenses has not been adequately examined. We identified bacteria from the regurgitant of field-collected Helicoverpa zea larvae using 16S ribosomal RNA (rRNA) gene sequencing and matrix-assisted laser desorption/ionization time of flight (MALDI-TOF) mass spectrometry. A combination of biochemical, molecular, and confocal electron microscopy methods were used to determine the role of caterpillar-associated bacteria in mediating defenses in Solanum lycopersicum (tomato). Laboratory-reared H. zea inoculated with one of the bacteria identified in field-collected H. zea, Enterobacter ludwigii, induced expression of the tomato defense-related enzyme polyphenol oxidase and genes regulated by jasmonic acid (JA), whereas the salicylic acid (SA)-responsive pathogenesis-related gene was suppressed. Additionally, saliva and its main component glucose oxidase from inoculated caterpillars played an important role in elevating tomato anti-herbivore defenses. However, there were only low detectable amounts of regurgitant or bacteria on H. zea-damaged tomato leaves. Our results suggest that H. zea gut-associated bacteria indirectly mediate plant-insect interactions by triggering salivary elicitors. These findings provide a proof of concept that introducing gut bacteria to a herbivore may provide a novel approach to pest management through indirect induction of plant resistance.

Transport of chemical signals in systemic acquired resistance

Systemic acquired resistance (SAR) is a form of broad-spectrum resistance induced in response to local infections that protects uninfected parts against subsequent secondary infections by related or unrelated pathogens. SAR signaling requires two parallel branches, one regulated by salicylic acid (SA), and the other by azelaic acid (AzA) and glycerol-3-phosphate (G3P). AzA and G3P function downstream of the free radicals nitric oxide (NO) and reactive oxygen species (ROS). During SAR, SA, AzA and G3P accumulate in the infected leaves, but only a small portion of these is transported to distal uninfected leaves. SA is preferentially transported via the apoplast, whereas phloem loading of AzA and G3P occurs via the symplast. The symplastic transport of AzA and G3P is regulated by gating of the plasmodesmata (PD). The PD localizing proteins, PDLP1 and PDLP5, regulate SAR by regulating PD gating as well as the subcellular partitioning of a SAR-associated protein.

Bismerthiazol Inhibits Xanthomonas citri subsp. citri Growth and Induces Differential Expression of Citrus Defense-Related Genes

Citrus canker, caused by Xanthomonas citri ssp. citri, is a serious disease that causes substantial economic losses to the citrus industry worldwide. The bactericide bismerthiazol has been used to control rice bacterial blight (X. oryzae pv. oryzae). In this paper, we demonstrate that bismerthiazol can effectively control citrus canker by both inhibiting the growth of X. citri ssp. citri and triggering the plant's host defense response through the expression of several pathogenesis-related genes (PR1, PR2, CHI, and RpRd1) and the nonexpresser of PR genes (NPR1, NPR2, and NPR3) in 'Duncan' grapefruit, especially at early treatment times. In addition, we found that bismerthiazol induced the expression of the marker genes CitCHS and CitCHI in the flavonoid pathway and the PAL1 (phenylalanine ammonia lyase 1) gene in the salicylic acid (SA) biosynthesis pathway at different time points. Moreover, bismerthiazol also induced the expression of the priming defense-associated gene AZI1. Taken together, these results indicate that the induction of the defense response in 'Duncan' grapefruit by bismerthiazol may involve the SA signaling pathway and the priming defense and that bismerthiazol may serve as an alternative to copper bactericides for the control of citrus canker.

Nitric oxide and S-nitrosoglutathione function additively during plant immunity

Nitric oxide (NO) is emerging as a key regulator of diverse plant cellular processes. A major route for the transfer of NO bioactivity is S-nitrosylation, the addition of an NO moiety to a protein cysteine thiol forming an S-nitrosothiol (SNO). Total cellular levels of protein S-nitrosylation are controlled predominantly by S-nitrosoglutathione reductase 1 (GSNOR1) which turns over the natural NO donor, S-nitrosoglutathione (GSNO). In the absence of GSNOR1 function, GSNO accumulates, leading to dysregulation of total cellular S-nitrosylation. Here we show that endogenous NO accumulation in Arabidopsis, resulting from loss-of-function mutations in NO Overexpression 1 (NOX1), led to disabled Resistance (R) gene-mediated protection, basal resistance and defence against nonadapted pathogens. In nox1 plants both salicylic acid (SA) synthesis and signalling were suppressed, reducing SA-dependent defence gene expression. Significantly, expression of a GSNOR1 transgene complemented the SNO-dependent phenotypes of paraquat resistant 2-1 (par2-1) plants but not the NO-related characters of the nox1-1 line. Furthermore, atgsnor1-3 nox1-1 double mutants supported greater bacterial titres than either of the corresponding single mutants. Our findings imply that GSNO and NO, two pivotal redox signalling molecules, exhibit additive functions and, by extension, may have distinct or overlapping molecular targets during both immunity and development.

Posttranslational Modifications of the Master Transcriptional Regulator NPR1 Enable Dynamic but Tight Control of Plant Immune Responses

NPR1, a master regulator of basal and systemic acquired resistance in plants, confers immunity through a transcriptional cascade, which includes transcription activators (e.g., TGA3) and repressors (e.g., WRKY70), leading to the massive induction of antimicrobial genes. How this single protein orchestrates genome-wide transcriptional reprogramming in response to immune stimulus remains a major question. Paradoxically, while NPR1 is essential for defense gene induction, its turnover appears to be required for this function, suggesting that NPR1 activity and degradation are dynamically regulated. Here we show that sumoylation of NPR1 by SUMO3 activates defense gene expression by switching NPR1's association with the WRKY transcription repressors to TGA transcription activators. Sumoylation also triggers NPR1 degradation, rendering the immune induction transient. SUMO modification of NPR1 is inhibited by phosphorylation at Ser55/Ser59, which keeps NPR1 stable and quiescent. Thus, posttranslational modifications enable dynamic but tight and precise control of plant immune responses.

Cytokinin production by Pseudomonas fluorescens G20-18 determines biocontrol activity against Pseudomonas syringae in Arabidopsis

Plant beneficial microbes mediate biocontrol of diseases by interfering with pathogens or via strengthening the host. Although phytohormones, including cytokinins, are known to regulate plant development and physiology as well as plant immunity, their production by microorganisms has not been considered as a biocontrol mechanism. Here we identify the ability of Pseudomonas fluorescens G20-18 to efficiently control P. syringae infection in Arabidopsis, allowing maintenance of tissue integrity and ultimately biomass yield. Microbial cytokinin production was identified as a key determinant for this biocontrol effect on the hemibiotrophic bacterial pathogen. While cytokinin-deficient loss-of-function mutants of G20-18 exhibit impaired biocontrol, functional complementation with cytokinin biosynthetic genes restores cytokinin-mediated biocontrol, which is correlated with differential cytokinin levels in planta. Arabidopsis mutant analyses revealed the necessity of functional plant cytokinin perception and salicylic acid-dependent defence signalling for this biocontrol mechanism. These results demonstrate microbial cytokinin production as a novel microbe-based, hormone-mediated concept of biocontrol. This mechanism provides a basis to potentially develop novel, integrated plant protection strategies combining promotion of growth, a favourable physiological status and activation of fine-tuned direct defence and abiotic stress resilience.

50 years of Arabidopsis research: highlights and future directions

922 I. 922 II. 922 III. 925 IV. 925 V. 926 VI. 927 VII. 928 VIII. 929 IX. 930 X. 931 XI. 932 XII. 933 XIII. Natural variation and genome-wide association studies 934 XIV. 934 XV. 935 XVI. 936 XVII. 937 937 References 937 SUMMARY: The year 2014 marked the 25(th) International Conference on Arabidopsis Research. In the 50 yr since the first International Conference on Arabidopsis Research, held in 1965 in Göttingen, Germany, > 54 000 papers that mention Arabidopsis thaliana in the title, abstract or keywords have been published. We present herein a citational network analysis of these papers, and touch on some of the important discoveries in plant biology that have been made in this powerful model system, and highlight how these discoveries have then had an impact in crop species. We also look to the future, highlighting some outstanding questions that can be readily addressed in Arabidopsis. Topics that are discussed include Arabidopsis reverse genetic resources, stock centers, databases and online tools, cell biology, development, hormones, plant immunity, signaling in response to abiotic stress, transporters, biosynthesis of cells walls and macromolecules such as starch and lipids, epigenetics and epigenomics, genome-wide association studies and natural variation, gene regulatory networks, modeling and systems biology, and synthetic biology.

Crosstalk between nitric oxide and glutathione is required for NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1)-dependent defense signaling in Arabidopsis thaliana

Nitric oxide (NO) is a ubiquitous signaling molecule involved in a wide range of physiological and pathophysiological processes in animals and plants. Although its significant influence on plant immunity is well known, information about the exact regulatory mechanisms and signaling pathways involved in the defense response to pathogens is still limited. We used genetic, biochemical, pharmacological approaches in combination with infection experiments to investigate the NO-triggered salicylic acid (SA)-dependent defense response in Arabidopsis thaliana. The NO donor S-nitrosoglutathione (GSNO) promoted the nuclear accumulation of NONEXPRESSOR OF PATHOGENESIS-RELATED GENES 1 (NPR1) protein accompanied by an elevated SA concentration and the activation of pathogenesis-related (PR) genes, leading to induced resistance of A. thaliana against Pseudomonas infection. Moreover, NO induced a rapid change in the glutathione status, resulting in increased concentrations of glutathione, which is required for SA accumulation and activation of the NPR1-dependent defense response. Our data imply crosstalk between NO and glutathione, which is integral to the NPR1-dependent defense signaling pathway, and further demonstrate that glutathione is not only an important cellular redox buffer but also a signaling molecule in the plant defense response.

A transcriptional reference map of defence hormone responses in potato

Phytohormones are involved in diverse aspects of plant life including the regulation of plant growth, development and reproduction, as well as governing biotic and abiotic stress responses. We have generated a comprehensive transcriptional reference map of the early potato responses to exogenous application of the defence hormones abscisic acid, brassinolides (applied as epibrassinolide), ethylene (applied as the ethylene precursor aminocyclopropanecarboxylic acid), salicylic acid and jasmonic acid (applied as methyl jasmonate). Of the 39000 predicted genes on the microarray, a total of 2677 and 2473 genes were significantly differentially expressed at 1 h and 6 h after hormone treatment, respectively. Specific marker genes newly identified for the early hormone responses in potato include: a homeodomain 20 transcription factor (DMG400000248) for abscisic acid; a SAUR gene (DMG400016561) induced in epibrassinolide treated plants; an osmotin gene (DMG400003057) specifically enhanced by aminocyclopropanecarboxylic acid; a gene weakly similar to AtWRKY40 (DMG402007388) that was induced by salicylic acid; and a jasmonate ZIM-domain protein 1 (DMG400002930) which was specifically activated by methyl jasmonate. An online database has been set up to query the expression patterns of potato genes represented on the microarray that can also incorporate future microarray or RNAseq-based expression studies.