Arabidopsis is susceptible to infection by a downy mildew fungus

Rice 7-Hydroxymethyl Chlorophyll a Reductase Is Involved in the Promotion of Chlorophyll Degradation and Modulates Cell Death Signaling

The loss of green coloration via chlorophyll (Chl) degradation typically occurs during leaf senescence. To date, many Chl catabolic enzymes have been identified and shown to interact with light harvesting complex II to form a Chl degradation complex in senescing chloroplasts; this complex might metabolically channel phototoxic Chl catabolic intermediates to prevent oxidative damage to cells. The Chl catabolic enzyme 7-hydroxymethyl Chl a reductase (HCAR) converts 7-hydroxymethyl Chl a (7-HMC a) to Chl a. The rice (Oryza sativa) genome contains a single HCAR homolog (OsHCAR), but its exact role remains unknown. Here, we show that an oshcar knockout mutant exhibits persistent green leaves during both dark-induced and natural senescence, and accumulates 7-HMC a and pheophorbide a (Pheo a) in green leaf blades. Interestingly, both rice and Arabidopsis hcar mutants exhibit severe cell death at the vegetative stage; this cell death largely occurs in a light intensity-dependent manner. In addition, 7-HMC a treatment led to the generation of singlet oxygen (1O2) in Arabidopsis and rice protoplasts in the light. Under herbicide-induced oxidative stress conditions, leaf necrosis was more severe in hcar plants than in wild type, and HCAR-overexpressing plants were more tolerant to reactive oxygen species than wild type. Therefore, in addition to functioning in the conversion of 7-HMC a to Chl a in senescent leaves, HCAR may play a critical role in protecting plants from high light-induced damage by preventing the accumulation of 7-HMC a and Pheo a in developing and mature leaves at the vegetative stage.

Phospholipase C2 Affects MAMP-Triggered Immunity by Modulating ROS Production

The activation of phosphoinositide-specific phospholipase C (PI-PLC) is one of the earliest responses triggered by the recognition of several microbe-associated molecular patterns (MAMPs) in plants. The Arabidopsis (Arabidopsis thaliana) PI-PLC gene family is composed of nine members. Previous studies suggested a role for PLC2 in MAMP-triggered immunity, as it is rapidly phosphorylated in vivo upon treatment with the bacterial MAMP flg22. Here, we analyzed the role of PLC2 in plant immunity using an artificial microRNA to silence PLC2 expression in Arabidopsis. We found that PLC2-silenced plants are more susceptible to the type III secretion system-deficient bacterial strain Pseudomonas syringae pv tomato (Pst) DC3000 hrcC^- and to the nonadapted pea (Pisum sativum) powdery mildew Erysiphe pisi However, PLC2-silenced plants display normal susceptibility to virulent (Pst DC3000) and avirulent (Pst DC3000 AvrRPM1) P. syringae strains, conserving typical hypersensitive response features. In response to flg22, PLC2-silenced plants maintain wild-type mitogen-activated protein kinase activation and PHI1, WRKY33, and FRK1 immune marker gene expression but have reduced reactive oxygen species (ROS)-dependent responses such as callose deposition and stomatal closure. Accordingly, the generation of ROS upon flg22 treatment is compromised in the PLC2-defficient plants, suggesting an effect of PLC2 in a branch of MAMP-triggered immunity and nonhost resistance that involves early ROS-regulated processes. Consistently, PLC2 associates with the NADPH oxidase RBOHD, suggesting its potential regulation by PLC2.

Expression of the Grape VaSTS19 Gene in Arabidopsis Improves Resistance to Powdery Mildew and Botrytis cinerea but Increases Susceptibility to Pseudomonas syringe pv Tomato DC3000

Stilbene synthase (STS) is a key enzyme that catalyzes the biosynthesis of resveratrol compounds and plays an important role in disease resistance. The molecular pathways linking STS with pathogen responses and their regulation are not known. We isolated an STS gene, VaSTS19, from a Chinese wild grape, Vitis amurensis Rupr. cv. "Tonghua-3", and transferred this gene to Arabidopsis. We then generated VaSTS19-expressing Arabidopsis lines and evaluated the functions of VaSTS19 in various pathogen stresses, including powdery mildew, B. cinerea and Pseudomonas syringae pv. tomato DC3000 (PstDC3000). VaSTS19 enhanced resistance to powdery mildew and B. cinerea, but increased susceptibility to PstDC3000. Aniline blue staining revealed that VaSTS19 transgenic lines accumulated more callose compared to nontransgenic control plants, and showed smaller stomatal apertures when exposed to pathogen-associated molecular patterns (flagellin fragment (flg22) or lipopolysaccharides (LPS)). Analysis of the expression of several disease-related genes suggested that VaSTS19 expression enhanced defense responses though salicylic acid (SA) and/or jasmonic acid (JA) signaling pathways. These findings provide a deeper insight into the function of STS genes in defense against pathogens, and a better understanding of the regulatory cross talk between SA and JA pathways.

Transcriptome changes in Arabidopsis thaliana infected with Pseudomonas syringae during drought recovery

Field-grown plants experience cycles of drought stress and recovery due to variation in soil moisture status. Physiological, biochemical and transcriptome responses instigated by recovery are expected to be different from drought stress and non-stressed state. Such responses can further aid or antagonize the plant's interaction with the pathogen. However, at molecular level, not much is known about plant-pathogen interaction during drought recovery. In the present study, we performed a microarray-based global transcriptome profiling and demonstrated the existence of unique transcriptional changes in Arabidopsis thaliana inoculated with Pseudomonas syringae pv. tomato DC3000 at the time of drought recovery (drought recovery pathogen, DRP) when compared to the individual drought (D) or pathogen (P) or drought recovery (DR). Furthermore, the comparison of DRP with D or DR and P transcriptome revealed the presence of a few common genes among three treatments. Notably, a gene encoding proline dehydrogenase (AtProDH1) was found to be commonly up-regulated under drought recovery (DR), DRP and P stresses. We also report an up-regulation of pyrroline-5-carboxylate biosynthesis pathway during recovery. We propose that AtProDH1 influences the defense pathways during DRP. Altogether, this study provides insight into the understanding of defense responses that operate in pathogen-infected plants during drought recovery.

Sugar Accumulation in Leaves of Arabidopsis sweet11/sweet12 Double Mutants Enhances Priming of the Salicylic Acid-Mediated Defense Response

In compatible interactions, biotrophic microbial phytopathogens rely on the supply of assimilates by the colonized host tissue. It has been found in rice that phloem localized SWEET sucrose transporters can be reprogrammed by bacterial effectors to establish compatibility. We observed that sweet11/sweet12 double mutants, but not single mutants, exhibited increased resistance toward the fungal hemibiotroph Colletotrichum higginsianum (Ch), both in the biotrophic and the necrotrophic colonization phase. We therefore investigated if the phloem localized transporters AtSWEET11 and AtSWEET12 represent additive susceptibility factors in the interaction of Arabidopsis with Ch. AtSWEET12-YFP fusion protein driven by the endogenous promoter strongly accumulated at Ch infection sites and in the vasculature upon challenge with Ch. However, susceptibility of sweet12 single mutants to Ch was comparable to wild type, indicating that the accumulation of AtSWEET12 at Ch infection sites does not play a major role for compatibility. AtSWEET12-YFP reporter protein was not detectable at the plant-pathogen interface, suggesting that AtSWEET12 is not targeted by Ch effectors. AtSWEET11-YFP accumulation in pAtSWEET11:AtSWEET11-YFP plants were similar in Ch infected and mock control leaves. A close inspection of major carbohydrate metabolism in non-infected control plants revealed that soluble sugar and starch content were substantially elevated in sweet11/sweet12 double mutants during the entire diurnal cycle, that diurnal soluble sugar turnover was increased more than twofold in sweet11/sweet12, and that accumulation of free hexoses and sucrose was strongly expedited in double mutant leaves compared to wild type and both single mutants during the course of Ch infection. After 2 days of treatment, free and conjugated SA levels were significantly increased in infected and mock control leaves of sweet11/sweet12 relative to all other genotypes, respectively. Induced genes in mock treated sweet11/sweet12 leaves were highly significantly enriched for several GO terms associated with SA signaling and response compared to mock treated wild-type leaves, indicating sugar-mediated priming of the SA pathway in the double mutant. Infection assays with salicylic acid deficient sweet11/sweet12/sid2 triple mutants demonstrated that reduced susceptibility observed in sweet11/sweet12 was entirely dependent on the SA pathway. We suggest a model how defects in phloem loading of sucrose can influence SA priming and hence, compatibility.

Spatiotemporal Monitoring of Pseudomonas syringae Effectors via Type III Secretion Using Split Fluorescent Protein Fragments

Pathogenic gram-negative bacteria cause serious diseases in animals and plants. These bacterial pathogens use the type III secretion system (T3SS) to deliver effector proteins into host cells; these effectors then localize to different subcellular compartments to attenuate immune responses by altering biological processes of the host cells. The fluorescent protein (FP)-based approach to monitor effectors secreted from bacteria into the host cells is not possible because the folded FP prevents effector delivery through the T3SS Therefore, we optimized an improved variant of self-assembling split super-folder green fluorescent protein (sfGFPOPT) system to investigate the spatiotemporal dynamics of effectors delivered through bacterial T3SS into plant cells. In this system, effectors are fused to 11th β-strand of super-folder GFP (sfGFP11), and when delivered into plant cells expressing sfGFP1-10 β-strand (sfGFP1-10OPT), the two proteins reconstitute GFP fluorescence. We generated a number of Arabidopsis thaliana transgenic lines expressing sfGFP1-10OPT targeted to various subcellular compartments to facilitate localization of sfGFP11-tagged effectors delivered from bacteria. We demonstrate the efficacy of this system using Pseudomonas syringae effectors AvrB and AvrRps4 in Nicotiana benthamiana and transgenic Arabidopsis plants. The versatile split sfGFPOPT system described here will facilitate a better understanding of bacterial invasion strategies used to evade plant immune responses.

Overexpression of a thaumatin-like protein gene from Vitis amurensis improves downy mildew resistance in Vitis vinifera grapevine

Downy mildew is a highly destructive disease in grapevine production. A gene encoding pathogenesis-related (PR) thaumatin-like protein was isolated from the downy mildew-resistant grapevine "Zuoshan-1," a clonal selection from wild Vitis amurensis Rupr. The predicted thaumatin-like protein (VaTLP) has 225 amino acids and it is acidic, with a calculated isoelectric point of 4.8. The full length of the VaTLP gene was transformed into somatic embryogenic calli of V. vinifera 'Thompson Seedless' via Agrobacterium tumefaciens. Real-time RT-PCR confirmed that the VaTLP gene was expressed at a high level in the transgenic grapevines. Improved resistance of the transgenic lines against downy mildew was evaluated using leaf disks and whole plants inoculated with Plasmopara viticola, the pathogen causing grapevine downy mildew disease. Bioassay of the pathogen showed that both hyphae growth and asexual reproduction were inhibited significantly among the transgenic plants. Histological analysis also confirmed this disease resistance by demonstrating the inhibition and malformation of hyphae development in leaf tissue of the transgenic plants. These results indicated that the accumulation of VaTLP could enhance resistance to P. viticola in transgenic 'Thompson Seedless' grapevines.

A Rice PECTATE LYASE-LIKE Gene Is Required for Plant Growth and Leaf Senescence

To better understand the molecular mechanisms behind plant growth and leaf senescence in monocot plants, we identified a mutant exhibiting dwarfism and an early-senescence leaf phenotype, termed dwarf and early-senescence leaf1 (del1). Histological analysis showed that the abnormal growth was caused by a reduction in cell number. Further investigation revealed that the decline in cell number in del1 was affected by the cell cycle. Physiological analysis, transmission electron microscopy, and TUNEL assays showed that leaf senescence was triggered by the accumulation of reactive oxygen species. The DEL1 gene was cloned using a map-based approach. It was shown to encode a pectate lyase (PEL) precursor that contains a PelC domain. DEL1 contains all the conserved residues of PEL and has strong similarity with plant PelC. DEL1 is expressed in all tissues but predominantly in elongating tissues. Functional analysis revealed that mutation of DEL1 decreased the total PEL enzymatic activity, increased the degree of methylesterified homogalacturonan, and altered the cell wall composition and structure. In addition, transcriptome assay revealed that a set of cell wall function- and senescence-related gene expression was altered in del1 plants. Our research indicates that DEL1 is involved in both the maintenance of normal cell division and the induction of leaf senescence. These findings reveal a new molecular mechanism for plant growth and leaf senescence mediated by PECTATE LYASE-LIKE genes.

Synthesis and degradation of long-chain base phosphates affect fumonisin B1-induced cell death in Arabidopsis thaliana

Fumonisin B₁ (FB₁), an inducer of cell death, disrupts sphingolipid metabolism; large accumulations of de novo synthesized free long-chain bases (LCBs) are observed. However, it remains unclear whether tolerance to FB₁ toxicity in plants is connected with preventing the accumulation of free LCBs through their phosphorylation. Here a workflow for the extraction, detection and quantification of LCB phosphates (LCBPs) in Arabidopsis thaliana was developed. We studied the effect of expression of genes for three enzymes involved in the synthesis and degradation of LCBPs, LCB kinase (LCBK1), LCBP phosphatase (SPP1) and lyase (DPL1) on FB₁-induced cell death. As expected, large accumulations of saturated free LCBs, dihydrosphingosine and phytosphingosine, were observed in the FB₁-treated leaves. On the other hand, a high level of sphingenine phosphate was found in the FB₁-treated leaves even though free sphingenine was found in low amounts in these leaves. In comparison of WT and spp1 plants, the LCBP/LCB ratio is likely to be correlated with the degree of FB₁-induced cell death determined by trypan blue staining. The FB₁-treated leaves in dpl1 plants showed severe cell death and the elevation of free LCBs and LCBPs. LCBK1-OX and -KD plants showed resistance and sensitivity to FB₁, respectively, whereas free LCB and LCBP levels in FB₁-treated LCBK1-OX and -KD plants were moderately different to those in FB₁-treated WT plants. Overall, the findings described here suggest that LCBP/LCB homeostasis is an important topic that participates in the tolerance of plant cells to FB₁.

Deep sequencing leads to the identification of eukaryotic translation initiation factor 5A as a key element in Rsv1-mediated lethal systemic hypersensitive response to Soybean mosaic virus infection in soybean

Rsv1, a single dominant resistance locus in soybean, confers extreme resistance to the majority of Soybean mosaic virus (SMV) strains, but is susceptible to the G7 strain. In Rsv1-genotype soybean, G7 infection provokes a lethal systemic hypersensitive response (LSHR), a delayed host defence response. The Rsv1-mediated LSHR signalling pathway remains largely unknown. In this study, we employed a genome-wide investigation to gain an insight into the molecular interplay between SMV G7 and Rsv1-genotype soybean. Small RNA (sRNA), degradome and transcriptome sequencing analyses were used to identify differentially expressed genes (DEGs) and microRNAs (DEMs) in response to G7 infection. A number of DEGs, DEMs and microRNA targets, and the interaction network of DEMs and their target mRNAs responsive to G7 infection, were identified. Knock-down of one of the identified DEGs, the eukaryotic translation initiation factor 5A (eIF5A), diminished the LSHR and enhanced viral accumulation, suggesting the essential role of eIF5A in the G7-induced, Rsv1-mediated LSHR signalling pathway. This work provides an in-depth genome-wide analysis of high-throughput sequencing data, and identifies multiple genes and microRNA signatures that are associated with the Rsv1-mediated LSHR.

Constitutive heterologous overexpression of a TIR-NB-ARC-LRR gene encoding a putative disease resistance protein from wild Chinese Vitis pseudoreticulata in Arabidopsis and tobacco enhances resistance to phytopathogenic fungi and bacteria

Plants use resistance (R) proteins to detect pathogen effector proteins and activate their innate immune response against the pathogen. The majority of these proteins contain an NB-ARC (nucleotide-binding adaptor shared by APAF-1, R proteins, and CED-4) domain along with a leucine-rich repeat (LRR), and some also bear a toll interleukin 1 receptor (TIR) domain. In this study, we characterized a gene encoding a TIR-NB-ARC-LRR R protein (VpTNL1) (GenBank accession number KX649890) from wild Chinese grapevine Vitis pseudoreticulata accession "Baihe-35-1", which was identified previously from a transcriptomic analysis of leaves inoculated with powdery mildew (PM; Erysiphe necator (Schw.)). The VpTNL1 transcript was found to be highly induced in V. pseudoreticulata following inoculation with E. necator, as well as treatment with salicylic acid (SA). Sequence analysis demonstrated that the deduced amino acid sequence contained a TIR domain at the N-terminus, along with an NB-ARC and four LRRs domains within the C-terminus. Constitutive expression of VpTNL1 in Arabidopsis thaliana resulted in either a wild-type or dwarf phenotype. Intriguingly, the phenotypically normal transgenic lines displayed enhanced resistance to Arabidopsis PM, Golovinomyces cichoracearum, as well as to the virulent bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Similarly, constitutive expression of VpTNL1 in Nicotiana tabacum was found to confer enhanced resistance to tobacco PM, Erysiphe cichoacearum DC. Subsequent isolation of the VpTNL1 promoter and deletion analysis indicated that TC-rich repeats and TCA elements likely play an important role in its response to E. necator and SA treatment, respectively. Taken together, these results indicate that VpTNL1 contributes to PM resistance in grapevine and provide an interesting gene target for the future amelioration of grape via breeding and/or biotechnology.

The Loricrin-Like Protein (LLP) of Phytophthora infestans Is Required for Oospore Formation and Plant Infection

Loricrin-like protein (LLP) is characterized by a high content of glycine residues and is a major component of plant cell wall. Here, we identified a Phytophthora infestans ortholog of plant LLP, named PiLLP. In P. infestans, PiLLP is strongly expressed in asexual and sexual developmental stages, including in sporangia, zoospores and germinating cysts, and during oospore formation, as well as in the early stages of infection and during hydrogen peroxide stress. Compared with the wild type, the PiLLP-silenced transformants were defective in oospore formation, had slower colony expansion rates, produced less sporangia with lower germination and zoospore-release rates, and were more sensitive to hydrogen peroxide. Moreover, Nile red staining, and PiLLP-red fluorescent protein fusions indicated that PiLLP is involved in oogonia formation. The silenced transformants also had severely diminished virulence levels that could be partially restored with diphenyleneiodium treatments. The analysis of catalase activity showed a decrease of catalase activity in silenced transformants. Thus, PiLLP is important for sexual and asexual reproduction, and is required for oxidative stress tolerance and plant infection.

Entire Photodamaged Chloroplasts Are Transported to the Central Vacuole by Autophagy

Turnover of dysfunctional organelles is vital to maintain homeostasis in eukaryotic cells. As photosynthetic organelles, plant chloroplasts can suffer sunlight-induced damage. However, the process for turnover of entire damaged chloroplasts remains unclear. Here, we demonstrate that autophagy is responsible for the elimination of sunlight-damaged, collapsed chloroplasts in Arabidopsis thaliana We found that vacuolar transport of entire chloroplasts, termed chlorophagy, was induced by UV-B damage to the chloroplast apparatus. This transport did not occur in autophagy-defective atg mutants, which exhibited UV-B-sensitive phenotypes and accumulated collapsed chloroplasts. Use of a fluorescent protein marker of the autophagosomal membrane allowed us to image autophagosome-mediated transport of entire chloroplasts to the central vacuole. In contrast to sugar starvation, which preferentially induced distinct type of chloroplast-targeted autophagy that transports a part of stroma via the Rubisco-containing body (RCB) pathway, photooxidative damage induced chlorophagy without prior activation of RCB production. We further showed that chlorophagy is induced by chloroplast damage caused by either artificial visible light or natural sunlight. Thus, this report establishes that an autophagic process eliminates entire chloroplasts in response to light-induced damage.

Transcriptomics Analysis of Apple Leaves in Response to Alternaria alternata Apple Pathotype Infection

Alternaria blotch disease of apple (Malus × domestica Borkh.), caused by the apple pathotype of Alternaria alternata, is one of the most serious fungal diseases to affect apples. To develop an understanding of how apples respond to A. alternata apple pathotype (AAAP) infection, we examined the host transcript accumulation over the period between 0 and 72 h post AAAP inoculation. Large-scale gene expression analysis was conducted of the compatible interaction between "Starking Delicious" apple cultivar and AAAP using RNA-Seq and digital gene expression (DGE) profiling methods. Our results show that a total of 9080 differentially expressed genes (DEGs) were detected (>two-fold and FDR < 0.001) by RNA-Seq. During the early phase of infection, 12 h post inoculation (HPI), AAAP exhibited limited fungal development and little change in the transcript accumulation status (950 DEGs). During the intermediate phase of infection, the period between 18 and 36 HPI, increased fungal development, active infection, and increased transcript accumulation were detected (4111 and 3838 DEGs detected at each time point, respectively). The majority of DEGs were detected by 72 HPI, suggesting that this is an important time point in the response of apples' AAAP infection. Subsequent gene ontology (GO) and pathway enrichment analyses showed that DEGs are predominately involved in biological processes and metabolic pathways; results showed that almost gene associated with photosynthesis, oxidation-reduction were down-regulated, while transcription factors (i.e., WRKY, MYB, NAC, and Hsf) and DEGs involved in cell wall modification, defense signaling, the synthesis of defense-related metabolites, including pathogenesis-related (PRs) genes and phenylpropanoid/cyanoamino acid /flavonoid biosynthesis, were activated during this process. Our study also suggested that the cell wall defensive vulnerability and the down-regulation of most PRs and HSP70s in "Starking Delicious" following AAAP infection might interpret its susceptible to AAAP.

Cell wall composition and penetration resistance against the fungal pathogen Colletotrichum higginsianum are affected by impaired starch turnover in Arabidopsis mutants

Penetration resistance represents the first level of plant defense against phytopathogenic fungi. Here, we report that the starch-deficient Arabidopsis thaliana phosphoglucomutase (pgm) mutant has impaired penetration resistance against the hemibiotrophic fungus Colletotrichum higginsianum. We could not determine any changes in leaf cutin and epicuticular wax composition or indolic glucosinolate levels, but detected complex alterations in the cell wall monosaccharide composition of pgm. Notably, other mutants deficient in starch biosynthesis (adg1) or mobilization (sex1) had similarly affected cell wall composition and penetration resistance. Glycome profiling analysis showed that both overall cell wall polysaccharide extractability and relative extractability of specific pectin and xylan epitopes were affected in pgm, suggesting extensive structural changes in pgm cell walls. Screening of mutants with alterations in content or modification of specific cell wall monosaccharides indicated an important function of pectic polymers for penetration resistance and hyphal growth of C. higginsianum during the biotrophic interaction phase. While mutants with affected pectic rhamnogalacturonan-I (mur8) were hypersusceptible, penetration frequency and morphology of fungal hyphae were impaired on pmr5 pmr6 mutants with increased pectin levels. Our results reveal a strong impact of starch metabolism on cell wall composition and suggest a link between carbohydrate availability, cell wall pectin and penetration resistance.

RRP42, a Subunit of Exosome, Plays an Important Role in Female Gametophytes Development and Mesophyll Cell Morphogenesis in Arabidopsis

The exosome complex plays a central and essential role in RNA metabolism. However, current research on functions of exosome subunit in plants is limited. Here, we used an egg cell-specific promoter-controlled CRISPR/Cas9 system to knock out RRP42 which encodes a core subunit of the Arabidopsis exosome and presented evidence that RRP42 is essential for the development of female gametophytes. Next, we designed three different amiRNAs targeting RRP42. The rrp42 knock-down mutants mainly displayed variegated and serrated leaves, especially in cauline leaves. The internal anatomy of cauline leaves displayed irregularly shaped palisade cells and a reduced density of mesophyll cells. Interestingly, we detected highly accumulated mRNAs that encode xyloglucan endotransglucosylase/hydrolases (XTHs) and expansins (EXPAs) during later growth stages in rrp42 knock-down mutants. The mRNA decay kinetics analysis for XTH19, EXPA10, and EXPA11 revealed that RRP42 had a role in the decay of these mRNAs in the cytoplasm. RRP42 is localized to both the nucleus and cytoplasm, and RRP42 is preferentially expressed in cauline leaves during later growth stages. Altogether, our results demonstrate that RRP42 is essential for the development of female gametophytes and plays an important role in mesophyll cell morphogenesis.

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.

NORE1/SAUL1 integrates temperature-dependent defense programs involving SGT1b and PAD4 pathways and leaf senescence in Arabidopsis

Leaf senescence is not only primarily governed by developmental age but also influenced by various internal and external factors. Although some genes that control leaf senescence have been identified, the detailed regulatory mechanisms underlying integration of diverse senescence-associated signals into the senescence programs remain to be elucidated. To dissect the regulatory pathways involved in leaf senescence, we isolated the not oresara1-1 (nore1-1) mutant showing accelerated leaf senescence phenotypes from an EMS-mutagenized Arabidopsis thaliana population. We found that altered transcriptional programs in defense response-related processes were associated with the accelerated leaf senescence phenotypes observed in nore1-1 through microarray analysis. The nore1-1 mutation activated defense program, leading to enhanced disease resistance. Intriguingly, high ambient temperature effectively suppresses the early senescence and death phenotypes of nore1-1. The gene responsible for the phenotypes of nore1-1 contains a missense mutation in SENESCENCE-ASSOCIATED E3 UBIQUITIN LIGASE 1 (SAUL1), which was reported as a negative regulator of premature senescence in the light intensity- and PHYTOALEXIN DEFICIENT 4 (PAD4)-dependent manner. Through extensive double mutant analyses, we recently identified suppressor of the G2 Allele of SKP1b (SGT1b), one of the positive regulators for disease resistance conferred by many resistance (R) proteins, as a downstream signaling component in NORE1-mediated senescence and cell death pathways. In conclusion, NORE1/SAUL1 is a key factor integrating signals from temperature-dependent defense programs and leaf senescence in Arabidopsis. These findings provide a new insight that plants might utilize defense response program in regulating leaf senescence process, possibly through recruiting the related genes during the evolution of the leaf senescence program.

Molecular and Biochemical Examination of Spraing Disease in Potato Tuber in Response to Tobacco rattle virus Infection

Field-grown tubers of potato were examined for infection by Tobacco rattle virus (TRV) and consequent production of corky ringspot or spraing symptoms. A microarray study identified genes that are differentially expressed in tuber tissue in response to TRV infection and to spraing production, suggesting that hypersensitive response (HR) pathways are activated in spraing-symptomatic tubers. This was confirmed by quantitative reverse transcription-polymerase chain reaction (qRT-PCR) of a selected group of HR-related genes and by histochemical staining of excised tuber tissue with spraing symptoms. qRT-PCR of TRV in different regions of the same tuber slice showed that nonsymptomatic areas contained higher levels of virus relative to spraing-symptomatic areas. This suggests that spraing formation is associated with an active plant defense that reduces the level of virus in the infected tuber. Expression of two of the same plant defense genes was similarly upregulated in tubers that were infected with Potato mop-top virus, a virus that also induces spraing formation.

The endophytic symbiont Epichloë festucae establishes an epiphyllous net on the surface of Lolium perenne leaves by development of an expressorium, an appressorium-like leaf exit structure

Epichloë festucae forms a mutualistic symbiotic association with Lolium perenne. This biotrophic fungus systemically colonizes the intercellular spaces of aerial tissues to form an endophytic hyphal network. E. festucae also grows as an epiphyte, but the mechanism for leaf surface colonization is not known. Here we identify an appressorium-like structure, which we call an expressorium that allows endophytic hyphae to penetrate the cuticle from the inside of the leaf to establish an epiphytic hyphal net on the surface of the leaf. We used a combination of scanning electron, transmission electron and confocal laser scanning microscopy to characterize this novel fungal structure and determine the composition of the hyphal cell wall using aniline blue and wheat germ agglutinin labelled with Alexafluor-488. Expressoria differentiate immediately below the cuticle in the leaf blade and leaf sheath intercalary cell division zones where the hyphae grow by tip growth. Differentiation of this structure requires components of both the NoxA and NoxB NADPH oxidase complexes. Major remodelling of the hyphal cell wall occurs following exit from the leaf. These results establish that the symbiotic association of E. festucae with L. perenne involves an interconnected hyphal network of both endophytic and epiphytic hyphae.

Understanding the Impact of Drought on Foliar and Xylem Invading Bacterial Pathogen Stress in Chickpea

In field conditions, plants are concurrently exposed to multiple stresses, where one stressor impacts the plant's response to another stressor, and the resultant net effect of these stresses differs from individual stress response. The present study investigated the effect of drought stress on interaction of chickpea with Pseudomonas syringae pv. phaseolicola (Psp; foliar pathogen) and Ralstonia solanacearum (Rs; xylem inhabiting wilt causing pathogen), respectively, and the net-effect of combined stress on chlorophyll content and cell death. Two type of stress treatments were used to study the influence of each stress factor during combined stress, viz., imposition of drought stress followed by pathogen challenge (DP), and pathogen inoculated plants imposed with drought in course of pathogen infection (PD). Drought stress was imposed at different levels with pathogen inoculum to understand the influence of different stress intensities on stress interaction and their net impact. Drought stressed chickpea plants challenged with Psp infection (DPsp) showed reduced in planta bacterial number compared to Psp infection alone. Similarly, Rs infection of chickpea plants showed reduced in planta bacterial number under severe drought stress. Combined drought and Psp (DPsp) infected plants showed decreased cell death compared to plants infected only with Psp but the extent of cell death was similar to drought stressed plants. Similarly, chlorophyll content in plants under combined stress was similar to the individual drought stressed plants; however, the chlorophyll content was more compared to pathogen only infected plants. Under combined drought and Rs infection (DRs), cell death was similar to individual drought stress but significantly less compared to only Rs infected plants. Altogether, the study proposes that both stress interaction and net effect of combined stress could be majorly influenced by first occurring stress, for example, drought stress in DP treatment. In addition, our results indicate that the outcome of the two stress interaction in plant depends on timing of stress occurrence and nature of infecting pathogen.

Drought Stress Predominantly Endures Arabidopsis thaliana to Pseudomonas syringae Infection

Plant responses to a combination of drought and bacterial pathogen infection, an agronomically important and altogether a new stress, are not well-studied. While occurring concurrently, these two stresses can lead to synergistic or antagonistic effects on plants due to stress-interaction. It is reported that plant responses to the stress combinations consist of both strategies, unique to combined stress and those shared between combined and individual stresses. However, the combined stress response mechanisms governing stress interaction and net impact are largely unknown. In order to study these adaptive strategies, an accurate and convenient methodology is lacking even in model plants like Arabidopsis thaliana. The gradual nature of drought stress imposition protocol poses a hindrance in simultaneously applying pathogen infection under laboratory conditions to achieve combined stress. In present study we aimed to establish systematic combined stress protocol and to study physiological responses of the plants to various degrees of combined stress. Here, we have comprehensively studied the impact of combined drought and Pseudomonas syringae pv. tomato DC3000 infection on A. thaliana. Further, by employing different permutations of drought and pathogen stress intensities, an attempt was made to dissect the contribution of each individual stress effects during their concurrence. We hereby present two main aspects of combined stress viz., stress interaction and net impact of the stress on plants. Mainly, this study established a systematic protocol to assess the impact of combined drought and bacterial pathogen stress. It was observed that as a result of net impact, some physiological responses under combined stress are tailored when compared to the plants exposed to individual stresses. We also infer that plant responses under combined stress in this study are predominantly influenced by the drought stress. Our results show that pathogen multiplication was reduced by drought stress in combined stressed plants. Combined stressed plants also displayed reduced ROS generation and declined cell death which could be attributed to activation of effective basal defense responses. We hypothesize a model on ABA mediated gene regulation to partly explain the possible mechanistic basis for reduced in planta bacterial numbers under combined stress over individual pathogen stress.

The LEA protein, ABR, is regulated by ABI5 and involved in dark-induced leaf senescence in Arabidopsis thaliana

The phytohormone abscisic acid (ABA) modulates plant growth and developmental processes such as leaf senescence. In this study, we investigated the role of the Arabidopsis late embryogenesis abundant (LEA) protein ABR (ABA-response protein) in delaying dark-induced leaf senescence. The ABR gene was up-regulated by treatment with ABA, NaCl and mannitol, as well as by light deprivation. In the dark, abr mutant plants displayed a premature leaf senescence phenotype, and various senescence-associated indicators, such as an increase in chlorophyll degradation and membrane leakiness, were enhanced, whereas 35S:ABR/abr transgenic lines showed a marked delay in dark-induced leaf senescence phenotypes. In vitro and in vivo assays showed that ABI5 bind to the ABR promoter, indicating that ABI5 directly regulates the expression of ABR. The disruption of ABI5 function in abr abi5-1 plants abolished the senescence-accelerating phenotype of the abr mutant, demonstrating that ABI5 is epistatic to ABR. In summary, these results highlight the important role that ABR, which is negatively regulated by ABI5, plays in delaying dark-induced leaf senescence.

The N-end rule pathway regulates pathogen responses in plants

To efficiently counteract pathogens, plants rely on a complex set of immune responses that are tightly regulated to allow the timely activation, appropriate duration and adequate amplitude of defense programs. The coordination of the plant immune response is known to require the activity of the ubiquitin/proteasome system, which controls the stability of proteins in eukaryotes. Here, we demonstrate that the N-end rule pathway, a subset of the ubiquitin/proteasome system, regulates the defense against a wide range of bacterial and fungal pathogens in the model plant Arabidopsis thaliana. We show that this pathway positively regulates the biosynthesis of plant-defense metabolites such as glucosinolates, as well as the biosynthesis and response to the phytohormone jasmonic acid, which plays a key role in plant immunity. Our results also suggest that the arginylation branch of the N-end rule pathway regulates the timing and amplitude of the defense program against the model pathogen Pseudomonas syringae AvrRpm1.

Bacillus cereus AR156 Extracellular Polysaccharides Served as a Novel Micro-associated Molecular Pattern to Induced Systemic Immunity to Pst DC3000 in Arabidopsis

Non-host resistance (NHR) is a broad-spectrum plant defense. Upon colonizing on the surface on the root or leaves of non-host species, pathogens initial encounter preform and induce defense response in plant, such as induced hypersensitive response, PAMPs triggered immunity (PTI), and effector triggered immunity (ETI). The ability of plants to develop an induced systemic response (ISR) in reaction to the colonization by non-pathogenic rhizobacterium depends on interactions between host plants and the colonizing rhizobacterium, and the ISR also can be defined as a NHR. However, how the colonization signal is and how systemic resistance to pathogens is developed is still unclear. In this study, we demonstrated that the extracellular polysaccharides (EPSs) of Bacillus cereus AR156 could act as novel microbe-associated molecular patterns (MAMPs) and function in the early perception status of the ISR of B. cereus AR156. The results revealed that B. cereus AR156 EPS could induce systemic resistance to Pst DC3000 in Arabidopsis. Cellular defense response markers such as hydrogen peroxide accumulation, callose deposition, and defense-associated enzyme were induced upon challenge inoculation in the leaves primed by EPS. Moreover, the defense-related genes PR1, PR2, and PR5 and mitogen-activated kinases (MAPK) cascade marker gene MPK6 were concurrently expressed in the leaves of EPS-treated plants and induced higher resistance to Pst DC3000 in Col-0 than that in the jar1 or etr1 mutants. The protection was absent in the NahG transgenic plants and npr1 mutant, suggesting an activation of the salicylic acid (SA)- and the MAPK-dependent signaling pathways with NPR1-dependent by B. cereus AR156 EPS. In conclusion, B. cereus AR156 EPS play an important role in MAMP perception during the process of rhizobacteria-triggered NHR. This study is the first to illustrate how AR156 induces systemic resistance to Pst DC3000 in Arabidopsis. It also provides the first explanation of how plants perceive colonization of non-pathogenic bacteria and how rhizobacteria trigger ISR to plant pathogens.

Impact of disease on diversity and productivity of plant populations

Experiments were conducted on the role of intra- and inter-genotypic competition in ecological processes operating at the population scale in diseased plant populations.Combinations of Arabidopsis thaliana genotypes showing variation for phenotypic traits relating to competitive ability and pathogen compatibility were infected with the oomycete Hyaloperonospora arabidopsidis and Turnip yellows virus in separate experiments. Plant fitness and competitive ability were estimated from phenotypic measurements.Pathogen-induced reduction in competitive ability for susceptible genotypes increased the competitive ability of resistant genotypes, resulting in maintenance of yield via competitive release. The two diseases had different effects on competitive interactions between plants. In experiments involving the oomycete, the highest yields were produced by mixtures of two weakly competing genotypes.The Arabidopsis model system has elucidated the ecological processes by which compensatory competitive interactions can increase the buffering capacity of plant populations under pathogen attack. Highly competitive genotypes may not maximize the productivity of the population as a whole, as they may over-yield at the expense of less competitive, more productive genotypes. The specific outcomes of competitive interactions cannot be generalized because they depend on the disease and the host genotypes.

Ectopic expression of TAPETUM DETERMINANT1 affects ovule development in Arabidopsis

Plants have evolved to extensively employ leucine-rich repeat receptor-like kinases (LRR-RLKs), the largest family of RLKs, to control growth, development, and defense. In Arabidopsis thaliana, the EXCESS MICROSPOROCYTES1 (EMS1) LRR-RLK and its potential small protein ligand TAPETUM DETERMINANT1 (TPD1) are specifically required for anther cell differentiation; however, TPD1 and EMS1 orthologs also control megaspore mother cell proliferation in rice and maize ovules. Here, the molecular function of TPD1 was demonstrated during ovule development in Arabidopsis using a gain-of-function approach. In ovules, the EMS1 gene was primarily expressed in nucellus epidermis and chalaza, whereas the expression of TPD1 was weakly restricted to the distal end of integuments. Ectopic expression of TPD1 caused pleiotropic defects in ovule and seed development. RNA sequencing analysis showed that ectopic expression of TPD1 altered expression of auxin signaling genes and core cell-cycle genes during ovule development. Moreover, ectopic expression of TPD1 not only affected auxin response but also enhanced expression of cyclin genes CYCD3;3 and CYCA2;3 in ovules. Thus, these results provide insight into the molecular mechanism by which TPD1-EMS1 signaling controls plant development possibly via regulation of auxin signaling and cell-cycle genes.

The Arabidopsis microtubule-associated protein MAP65-3 supports infection by filamentous biotrophic pathogens by down-regulating salicylic acid-dependent defenses

The oomycete Hyaloperonospora arabidopsidis and the ascomycete Erysiphe cruciferarum are obligate biotrophic pathogens causing downy mildew and powdery mildew, respectively, on Arabidopsis. Upon infection, the filamentous pathogens induce the formation of intracellular bulbous structures called haustoria, which are required for the biotrophic lifestyle. We previously showed that the microtubule-associated protein AtMAP65-3 plays a critical role in organizing cytoskeleton microtubule arrays during mitosis and cytokinesis. This renders the protein essential for the development of giant cells, which are the feeding sites induced by root knot nematodes. Here, we show that AtMAP65-3 expression is also induced in leaves upon infection by the downy mildew oomycete and the powdery mildew fungus. Loss of AtMAP65-3 function in the map65-3 mutant dramatically reduced infection by both pathogens, predominantly at the stages of leaf penetration. Whole-transcriptome analysis showed an over-represented, constitutive activation of genes involved in salicylic acid (SA) biosynthesis, signaling, and defense execution in map65-3, whereas jasmonic acid (JA)-mediated signaling was down-regulated. Preventing SA synthesis and accumulation in map65-3 rescued plant susceptibility to pathogens, but not the developmental phenotype caused by cytoskeleton defaults. AtMAP65-3 thus has a dual role. It positively regulates cytokinesis, thus plant growth and development, and negatively interferes with plant defense against filamentous biotrophs. Our data suggest that downy mildew and powdery mildew stimulate AtMAP65-3 expression to down-regulate SA signaling for infection.

Root-Root Interactions: Towards A Rhizosphere Framework

Plant scientists have made great progress in understanding molecular mechanisms controlling root responses to nutrients of arabidopsis (Arabidopsis thaliana) plants under controlled conditions. Simultaneously, ecologists and agronomists have demonstrated that root-root interactions involve more than competition for nutrients. Here, we highlight the importance of both root exudates and soil microbes for root-root interactions, ubiquitous in natural and agricultural ecosystems. We argue that it is time to bring together the recent insights from both scientific disciplines to fully understand root functioning in the real world.

Are Bacterial Volatile Compounds Poisonous Odors to a Fungal Pathogen Botrytis cinerea, Alarm Signals to Arabidopsis Seedlings for Eliciting Induced Resistance, or Both?

Biological control (biocontrol) agents act on plants via numerous mechanisms, and can be used to protect plants from pathogens. Biocontrol agents can act directly as pathogen antagonists or competitors or indirectly to promote plant induced systemic resistance (ISR). Whether a biocontrol agent acts directly or indirectly depends on the specific strain and the pathosystem type. We reported previously that bacterial volatile organic compounds (VOCs) are determinants for eliciting plant ISR. Emerging data suggest that bacterial VOCs also can directly inhibit fungal and plant growth. The aim of the current study was to differentiate direct and indirect mechanisms of bacterial VOC effects against Botrytis cinerea infection of Arabidopsis. Volatile emissions from Bacillus subtilis GB03 successfully protected Arabidopsis seedlings against B. cinerea. First, we investigated the direct effects of bacterial VOCs on symptom development and different phenological stages of B. cinerea including spore germination, mycelial attachment to the leaf surface, mycelial growth, and sporulation in vitro and in planta. Volatile emissions inhibited hyphal growth in a dose-dependent manner in vitro, and interfered with fungal attachment on the hydrophobic leaf surface. Second, the optimized bacterial concentration that did not directly inhibit fungal growth successfully protected Arabidopsis from fungal infection, which indicates that bacterial VOC-elicited plant ISR has a more important role in biocontrol than direct inhibition of fungal growth on Arabidopsis. We performed qRT-PCR to investigate the priming of the defense-related genes PR1, PDF1.2, and ChiB at 0, 12, 24, and 36 h post-infection and 14 days after the start of plant exposure to bacterial VOCs. The results indicate that bacterial VOCs potentiate expression of PR1 and PDF1.2 but not ChiB, which stimulates SA- and JA-dependent signaling pathways in plant ISR and protects plants against pathogen colonization. This study provides new evidence for bacterial VOC-elicited plant ISR that protects Arabidopsis plants from infection by the necrotrophic fungus B. cinerea. Our work reveals that bacterial VOCs primarily act via an indirect mechanism to elicit plant ISR, and have a major role in biocontrol against fungal pathogens.

Mutation of the Glucosinolate Biosynthesis Enzyme Cytochrome P450 83A1 Monooxygenase Increases Camalexin Accumulation and Powdery Mildew Resistance

Small secondary metabolites, including glucosinolates and the major phytoalexin camalexin, play important roles in immunity in Arabidopsis thaliana. We isolated an Arabidopsis mutant with increased resistance to the powdery mildew fungus Golovinomyces cichoracearum and identified a mutation in the gene encoding cytochrome P450 83A1 monooxygenase (CYP83A1), which functions in glucosinolate biosynthesis. The cyp83a1-3 mutant exhibited enhanced defense responses to G. cichoracearum and double mutant analysis showed that this enhanced resistance requires NPR1, EDS1, and PAD4, but not SID2 or EDS5. In cyp83a1-3 mutants, the expression of genes related to camalexin synthesis increased upon G. cichoracearum infection. Significantly, the cyp83a1-3 mutant also accumulated higher levels of camalexin. Decreasing camalexin levels by mutation of the camalexin synthetase gene PAD3 or the camalexin synthesis regulator AtWRKY33 compromised the powdery mildew resistance in these mutants. Consistent with these observations, overexpression of PAD3 increased camalexin levels and enhanced resistance to G. cichoracearum. Taken together, our data indicate that accumulation of higher levels of camalexin contributes to increased resistance to powdery mildew.

Burkholderia phytofirmans PsJN Confers Grapevine Resistance against Botrytis cinerea via a Direct Antimicrobial Effect Combined with a Better Resource Mobilization

Plant innate immunity serves as a surveillance system by providing the first line of powerful weapons to fight against pathogen attacks. Beneficial microorganisms and Microbial-Associated Molecular Patterns might act as signals to trigger this immunity. Burkholderia phytofirmans PsJN, a highly efficient plant beneficial endophytic bacterium, promotes growth in a wide variety of plants including grapevine. Further, the bacterium induces plant resistance against abiotic and biotic stresses. However, no study has deciphered triggered-mechanisms during the tripartite interaction between grapevine, B. phytofirmans PsJN and Botrytis cinerea. Herein, we showed that in contrast with classical rhizobacteria, which are restricted in the root system and act through ISR, B. phytofirmans PsJN is able to migrate until aerial part and forms at leaves surface a biofilm around B. cinerea mycelium to restrict the pathogen. Nevertheless, considering the endophytic level of PsJN in leaves, the plant protection efficacy of B. phytofirmans PsJN could not be explained solely by its direct antifungal effect. Deeper investigations showed a callose deposition, H2O2 production and primed expression of PR1, PR2, PR5, and JAZ only in bacterized-plantlets after pathogen challenge. The presence of PsJN modulated changes in leaf carbohydrate metabolism including gene expression, sugar levels, and chlorophyll fluorescence imaging after Botrytis challenge. Our findings indicated that protection induced by B. phytofirmans PsJN was multifaceted and relied on a direct antifungal effect, priming of defense mechanisms as well as the mobilization of carbon sources in grapevine leaf tissues.

Burkholderia phytofirmans PsJN Confers Grapevine Resistance against Botrytis cinerea via a Direct Antimicrobial Effect Combined with a Better Resource Mobilization

Plant innate immunity serves as a surveillance system by providing the first line of powerful weapons to fight against pathogen attacks. Beneficial microorganisms and Microbial-Associated Molecular Patterns might act as signals to trigger this immunity. Burkholderia phytofirmans PsJN, a highly efficient plant beneficial endophytic bacterium, promotes growth in a wide variety of plants including grapevine. Further, the bacterium induces plant resistance against abiotic and biotic stresses. However, no study has deciphered triggered-mechanisms during the tripartite interaction between grapevine, B. phytofirmans PsJN and Botrytis cinerea. Herein, we showed that in contrast with classical rhizobacteria, which are restricted in the root system and act through ISR, B. phytofirmans PsJN is able to migrate until aerial part and forms at leaves surface a biofilm around B. cinerea mycelium to restrict the pathogen. Nevertheless, considering the endophytic level of PsJN in leaves, the plant protection efficacy of B. phytofirmans PsJN could not be explained solely by its direct antifungal effect. Deeper investigations showed a callose deposition, H2O2 production and primed expression of PR1, PR2, PR5, and JAZ only in bacterized-plantlets after pathogen challenge. The presence of PsJN modulated changes in leaf carbohydrate metabolism including gene expression, sugar levels, and chlorophyll fluorescence imaging after Botrytis challenge. Our findings indicated that protection induced by B. phytofirmans PsJN was multifaceted and relied on a direct antifungal effect, priming of defense mechanisms as well as the mobilization of carbon sources in grapevine leaf tissues.

Ectopic Expression in Arabidopsis thaliana of an NB-ARC Encoding Putative Disease Resistance Gene from Wild Chinese Vitis pseudoreticulata Enhances Resistance to Phytopathogenic Fungi and Bacteria

Plant resistance proteins mediate pathogen recognition and activate innate immune responses to restrict pathogen proliferation. One common feature of these proteins is an NB-ARC domain. In this study, we characterized a gene encoding a protein with an NB-ARC domain from wild Chinese grapevine Vitis pseudoreticulata accession "Baihe-35-1," which was identified in a transcriptome analysis of the leaves following inoculation with Erysiphe necator (Schw.), a causal agent of powdery mildew. Transcript levels of this gene, designated VpCN (GenBank accession number KT265084), increased strongly after challenge of grapevine leaves with E. necator. The deduced amino acid sequence was predicted to contain an NB-ARC domain in the C-terminus and an RxCC-like domain similar to CC domain of Rx protein in the N-terminus. Ectopic expression of VpCN in Arabidopsis thaliana resulted in either a wild-type phenotype or a dwarf phenotype. The phenotypically normal transgenic A. thaliana showed enhance resistance to A. thaliana powdery mildew Golovinomyces cichoracearum, as well as to a virulent bacterial pathogen Pseudomonas syringae pv. tomato DC3000. Moreover, promoter::GUS (β-glucuronidase) analysis revealed that powdery mildew infection induced the promoter activity of VpCN in grapevine leaves. Finally, a promoter deletion analysis showed that TC rich repeat elements likely play an important role in the response to E. necator infection. Taken together, our results suggest that VpCN contribute to powdery mildew disease resistant in grapevine.

Jasmonate signalling drives time-of-day differences in susceptibility of Arabidopsis to the fungal pathogen Botrytis cinerea

The circadian clock, an internal time-keeping mechanism, allows plants to anticipate regular changes in the environment, such as light and dark, and biotic challenges such as pathogens and herbivores. Here, we demonstrate that the plant circadian clock influences susceptibility to the necrotrophic fungal pathogen, Botrytis cinerea. Arabidopsis plants show differential susceptibility to B. cinerea depending on the time of day of inoculation. Decreased susceptibility after inoculation at dawn compared with night persists under constant light conditions and is disrupted in dysfunctional clock mutants, demonstrating the role of the plant clock in driving time-of-day susceptibility to B. cinerea. The decreased susceptibility to B. cinerea following inoculation at subjective dawn was associated with faster transcriptional reprogramming of the defence response with gating of infection-responsive genes apparent. Direct target genes of core clock regulators were enriched among the transcription factors that responded more rapidly to infection at subjective dawn than subjective night, suggesting an influence of the clock on the defence-signalling network. In addition, jasmonate signalling plays a crucial role in the rhythmic susceptibility of Arabidopsis to B. cinerea with the enhanced susceptibility to this pathogen at subjective night lost in a jaz6 mutant.

A quick and robust method for quantification of the hypersensitive response in plants

One of the most studied defense reactions of plants against microbial pathogens is the hypersensitive response (HR). The HR is a complex multicellular process that involves programmed cell death at the site of infection. A standard method to quantify plant defense and the HR is to measure the release of cellular electrolytes into water after infiltration with pathogenic bacteria. In this type of experiment, the bacteria are typically delivered into the plant tissue through syringe infiltration. Here we report the development of a vacuum infiltration protocol that allows multiple plant lines to be infiltrated simultaneously and assayed for defense responses. Vacuum infiltration did not induce more wounding response in Arabidopsis leaf tissue than syringe inoculation, whereas throughput and reproducibility were improved. The method was used to study HR-induced electrolyte loss after treatment with the bacterium Pseudomonas syringae pv. tomato DC3000 harboring the effector AvrRpm1, AvrRpt2 or AvrRps4. Specifically, the influence of bacterial titer on AvrRpm1-induced HR was investigated. Not only the amplitude, but also the timing of the maximum rate of the HR reaction was found to be dose-dependent. Finally, using vacuum infiltration, we were able quantify induction of phospholipase D activity after AvrRpm1 recognition in leaves labeled with (33)PO4.

Expression of Caenorhabditis elegans PCS in the AtPCS1-deficient Arabidopsis thaliana cad1-3 mutant separates the metal tolerance and non-host resistance functions of phytochelatin synthases

Phytochelatin synthases (PCS) play key roles in plant metal tolerance. They synthesize small metal-binding peptides, phytochelatins, under conditions of metal excess. Respective mutants are strongly cadmium and arsenic hypersensitive. However, their ubiquitous presence and constitutive expression had long suggested a more general function of PCS besides metal detoxification. Indeed, phytochelatin synthase1 from Arabidopsis thaliana (AtPCS1) was later implicated in non-host resistance. The two different physiological functions may be attributable to the two distinct catalytic activities demonstrated for AtPCS1, that is the dipeptidyl transfer onto an acceptor molecule in phytochelatin synthesis, and the proteolytic deglycylation of glutathione conjugates. In order to test this hypothesis and to possibly separate the two biological roles, we expressed a phylogenetically distant PCS from Caenorhabditis elegans in an AtPCS1 mutant. We confirmed the involvement of AtPCS1 in non-host resistance by showing that plants lacking the functional gene develop a strong cell death phenotype when inoculated with the potato pathogen Phytophthora infestans. Furthermore, we found that the C. elegans gene rescues phytochelatin synthesis and cadmium tolerance, but not the defect in non-host resistance. This strongly suggests that the second enzymatic function of AtPCS1, which remains to be defined in detail, is underlying the plant immunity function.

Xanthomonas campestris cell-cell signalling molecule DSF (diffusible signal factor) elicits innate immunity in plants and is suppressed by the exopolysaccharide xanthan

Several secreted and surface-associated conserved microbial molecules are recognized by the host to mount the defence response. One such evolutionarily well-conserved bacterial process is the production of cell-cell signalling molecules which regulate production of multiple virulence functions by a process known as quorum sensing. Here it is shown that a bacterial fatty acid cell-cell signalling molecule, DSF (diffusible signal factor), elicits innate immunity in plants. The DSF family of signalling molecules are highly conserved among many phytopathogenic bacteria belonging to the genus Xanthomonas as well as in opportunistic animal pathogens. Using Arabidopsis, Nicotiana benthamiana, and rice as model systems, it is shown that DSF induces a hypersensitivity reaction (HR)-like response, programmed cell death, the accumulation of autofluorescent compounds, hydrogen peroxide production, and the expression of the PATHOGENESIS-RELATED1 (PR-1) gene. Furthermore, production of the DSF signalling molecule in Pseudomonas syringae, a non-DSF-producing plant pathogen, induces the innate immune response in the N. benthamiana host plant and also affects pathogen growth. By pre- and co-inoculation of DSF, it was demonstrated that the DSF-induced plant defence reduces disease severity and pathogen growth in the host plant. In this study, it was further demonstrated that wild-type Xanthomonas campestris suppresses the DSF-induced innate immunity by secreting xanthan, the main component of extracellular polysaccharide. The results indicate that plants have evolved to recognize a widely conserved bacterial communication system and may have played a role in the co-evolution of host recognition of the pathogen and the communication machinery.

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

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

Comparative transcriptome analysis reveals defense-related genes and pathways against downy mildew in Vitis amurensis grapevine

Downy mildew (DM), caused by oomycete Plasmopara viticola (Pv), can lead to severe damage to Vitis vinifera grapevines. Vitis amurensis has generally been regarded as a DM resistant species. However, when V. amurensis 'Shuanghong' were inoculated with Pv strains 'ZJ-1-1' and 'JL-7-2', the former led to obvious DM symptoms (compatible), while the latter did not develop any DM symptoms but exhibited necrosis (incompatible). In order to underlie molecular mechanism in DM resistance, mRNA-seq based expression profiling of 'Shuanghong' was compared at 12, 24, 48 and 72 h post inoculation (hpi) with these two strains. Specific genes and their corresponding pathways responsible for incompatible interaction were extracted by comparing with compatible interaction. In the incompatible interaction, 37 resistance (R) genes were more expressed at the early stage of infection (12 hpi). Similarly, genes involved in defense signaling, including MAPK. ROS/NO, SA, JA, ET and ABA pathways, and genes associated with defense-related metabolites synthesis, such as pathogenesis-related genes and phenylpropanoids/stilbenoids/flavonoids biosynthesizing genes, were also activated mainly during the early stages of infection. On the other hand, Ca(2+) signaling and primary metabolism, such as photosynthesis and fatty acid synthesis, were more repressed after 'JL-7-2' challenge. Further quantification of some key defense-related factors, including phytohormones, phytoalexins and ROS, generally showed much more accumulation during the incompatible interaction, indicating their important roles in DM defense. In addition, a total of 43 and 52 RxLR effectors were detected during 'JL-7-2' and 'ZJ-1-1' infection processes, respectively.

A PCR assay for the quantification of growth of the oomycete pathogen Hyaloperonospora arabidopsidis in Arabidopsis thaliana

The accurate quantification of disease severity is important for the assessment of host-pathogen interactions in laboratory or field settings. The interaction between Arabidopsis thaliana and its naturally occurring downy mildew pathogen, Hyaloperonospora arabidopsidis (Hpa), is a widely used reference pathosystem for plant-oomycete interactions. Current methods for the assessment of disease severity in the Arabidopsis-Hpa interaction rely on measurements at the terminal stage of pathogen development; namely, visual counts of spore-producing structures or the quantification of spore production with a haemocytometer. These assays are useful, but do not offer sensitivity for the robust quantification of small changes in virulence or the accurate quantification of pathogen growth prior to the reproductive stage. Here, we describe a quantitative real-time polymerase chain reaction (qPCR) assay for the monitoring of Hpa growth in planta. The protocol is rapid, inexpensive and can robustly distinguish small changes in virulence. We used this assay to investigate the dynamics of early Hpa mycelial growth and to demonstrate the proof of concept that this assay could be used in screens for novel oomycete growth inhibitors.

Two homologous protein S-acyltransferases, PAT13 and PAT14, cooperatively regulate leaf senescence in Arabidopsis

Lipid modification on the cysteine residues of proteins, known as S-palmitoylation or S-acylation, regulates the subcellular localization and the function of proteins. S-acylation is catalysed by a group of protein acyltransferases (PATs) with a conserved Asp-His-His-Cys (DHHC) motif. The molecular function of S-acylation has been studied in details in yeast and mammalian cells, but its role in plant cells remains unclear. Here it is reported that the expression of two homologous protein acyltransferases- PAT13 and PAT14 -was moderately increased in the older leaves of Arabidopsis. The double mutant of PAT13 and PAT14 displayed a severely early leaf senescence phenotype. The phenotype was complemented by PAT13 or PAT14 overexpression in the double mutant, confirming the roles of PAT13 and PAT14 in this process. Furthermore, the levels of reactive oxygen species (ROS) and cell death were dramatically induced in the double mutant. To investigate the molecular functions of PAT13 and PAT14, their potential S-acylation substrates were predicted by bioinformatics methods. The subcellular localization and S-acylation of a candidate substrate NITRIC OXIDE ASSOCIATED 1 (NOA1), which also plays a role in leaf senescence control, were partially disrupted in the protoplasts of the double mutant. Impairment of S-acylation on NOA1 affected its subcellular localization and its function in leaf senescence regulation. Conclusively, protein S-acyltransferases PAT13 and PAT14 are involved in leaf senescence control- possibly via NOA1 S-acylation-, providing a new sight into the regulation mechanism of S-acylation in leaf senescence.

Divergent sorting of a balanced ancestral polymorphism underlies the establishment of gene-flow barriers in Capsella

In the Bateson–Dobzhansky–Muller model of genetic incompatibilities post-zygotic gene-flow barriers arise by fixation of novel alleles at interacting loci in separated populations. Many such incompatibilities are polymorphic in plants, implying an important role for genetic drift or balancing selection in their origin and evolution. Here we show that NPR1 and RPP5 loci cause a genetic incompatibility between the incipient species Capsella grandiflora and C. rubella, and the more distantly related C. rubella and C. orientalis. The incompatible RPP5 allele results from a mutation in C. rubella, while the incompatible NPR1 allele is frequent in the ancestral C. grandiflora. Compatible and incompatible NPR1 haplotypes are maintained by balancing selection in C. grandiflora, and were divergently sorted into the derived C. rubella and C. orientalis. Thus, by maintaining differentiated alleles at high frequencies, balancing selection on ancestral polymorphisms can facilitate establishing gene-flow barriers between derived populations through lineage sorting of the alternative alleles.

A hybrid incompatibility between Capsella plant species is due to an interaction between two immune regulators. Here, the authors show that highly divergent haplotypes result from balancing selection in the ancestral lineage and their sorting into derived lineages facilitated the evolution of the incompatibility.

Strategies for RUN1 Deployment Using RUN2 and REN2 to Manage Grapevine Powdery Mildew Informed by Studies of Race Specificity

The Toll/interleukin-1 receptor nucleotide-binding site leucine-rich repeat gene, "resistance to Uncinula necator 1" (RUN1), from Vitis rotundifolia was recently identified and confirmed to confer resistance to the grapevine powdery mildew fungus Erysiphe necator (syn. U. necator) in transgenic V. vinifera cultivars. However, sporulating powdery mildew colonies and cleistothecia of the heterothallic pathogen have been found on introgression lines containing the RUN1 locus growing in New York (NY). Two E. necator isolates collected from RUN1 vines were designated NY1-131 and NY1-137 and were used in this study to inform a strategy for durable RUN1 deployment. In order to achieve this, fitness parameters of NY1-131 and NY1-137 were quantified relative to powdery mildew isolates collected from V. rotundifolia and V. vinifera on vines containing alleles of the powdery mildew resistance genes RUN1, RUN2, or REN2. The results clearly demonstrate the race specificity of RUN1, RUN2, and REN2 resistance alleles, all of which exhibit programmed cell death (PCD)-mediated resistance. The NY1 isolates investigated were found to have an intermediate virulence on RUN1 vines, although this may be allele specific, while the Musc4 isolate collected from V. rotundifolia was virulent on all RUN1 vines. Another powdery mildew resistance locus, RUN2, was previously mapped in different V. rotundifolia genotypes, and two alleles (RUN2.1 and RUN2.2) were identified. The RUN2.1 allele was found to provide PCD-mediated resistance to both an NY1 isolate and Musc4. Importantly, REN2 vines were resistant to the NY1 isolates and RUN1REN2 vines combining both genes displayed additional resistance. Based on these results, RUN1-mediated resistance in grapevine may be enhanced by pyramiding with RUN2.1 or REN2; however, naturally occurring isolates in North America display some virulence on vines with these resistance genes. The characterization of additional resistance sources is needed to identify resistance gene combinations that will further enhance durability. For the resistance gene combinations currently available, we recommend using complementary management strategies, including fungicide application, to reduce populations of virulent isolates.

Cysteine-rich receptor-like kinase CRK5 as a regulator of growth, development, and ultraviolet radiation responses in Arabidopsis thaliana

In plants, receptor-like protein kinases play essential roles in signal transduction by recognizing extracellular stimuli and activating the downstream signalling pathways. Cysteine-rich receptor-like kinases (CRKs) constitute a large subfamily of receptor-like protein kinases, with 44 members in Arabidopsis thaliana. They are distinguished by the novel C-X8-C-X2-C motif (DUF26) in the extracellular domains. One of them, CRK5, is an important component of the biochemical machinery involved in the regulation of essential physiological processes. Functional characterization of crk5 mutant plants showed their clear phenotype, manifested by impaired stomatal conductance and accelerated senescence. This phenotype correlated with accumulation of reactive oxygen species, higher foliar levels of ethylene and salicylic acid, and increased transcript abundance for genes associated with signalling pathways corresponding to these hormones. Moreover, the crk5 plants displayed enhanced cell death and oxidative damage in response to ultraviolet radiation. Complementation of CRK5 mutation managed to recover the wild-type phenotype, indicating an essential role of this gene in the regulation of growth, development, and acclimatory responses.

Sr36- and Sr5-Mediated Resistance Response to Puccinia graminis f. sp. tritici Is Associated with Callose Deposition in Wheat Guard Cells

Race-specific resistance of wheat to Puccinia graminis f. sp. tritici is primarily posthaustorial and often involves the induction of a hypersensitive response (HR). The aim of this study was to investigate host defense responses induced in interactions between P. graminis f. sp. tritici races and wheat lines carrying different race-specific stem rust resistance (Sr) genes. In incompatible interactions between wheat lines carrying Sr36 in three genetic backgrounds (LMPG, Prelude, or W2691) and avirulent P. graminis f. sp. tritici races MCCFC or RCCDM, callose accumulated within 24 h in wheat guard cells contacted by a P. graminis f. sp. tritici appressorium, and P. graminis f. sp. tritici ingress was inhibited following appressorium formation. Accordingly, the expression of transcripts encoding a callose synthase increased in the incompatible interaction between LMPG-Sr36 and avirulent P. graminis f. sp. tritici race MCCFC. Furthermore, the inhibition of callose synthesis through the infiltration of 2-deoxy-D-glucose (DDG) increased the ability of P. graminis f. sp. tritici race MCCFC to infect LMPG-Sr36. A similar induction of callose deposition in wheat guard cells was also observed within 24 h after inoculation (hai) with avirulent P. graminis f. sp. tritici race HKCJC on LMPG-Sr5 plants. In contrast, this defense response was not induced in incompatible interactions involving Sr6, Sr24, or Sr30. Instead, the induction of an HR and cellular lignification were noted. The manifestation of the HR and cellular lignification was induced earlier (24 hai) and was more extensive in the resistance response mediated by Sr6 compared with those mediated by Sr24 or Sr30. These results indicate that the resistance mediated by Sr36 is similar to that mediated by Sr5 but different from those triggered by Sr6, Sr24, or Sr30. Resistance responses mediated by Sr5 and Sr36 are prehaustorial, and are a result of very rapid recognition of molecules derived from avirulent isolates of P. graminis f. sp. tritici, in contrast to the responses triggered in lines with Sr6, Sr24, and Sr30.

Pepper aldehyde dehydrogenase CaALDH1 interacts with Xanthomonas effector AvrBsT and promotes effector-triggered cell death and defence responses

Xanthomonas type III effector AvrBsT induces hypersensitive cell death and defence responses in pepper (Capsicum annuum) and Nicotiana benthamiana. Little is known about the host factors that interact with AvrBsT. Here, we identified pepper aldehyde dehydrogenase 1 (CaALDH1) as an AvrBsT-interacting protein. Bimolecular fluorescence complementation and co-immunoprecipitation assays confirmed the interaction between CaALDH1 and AvrBsT in planta. CaALDH1:smGFP fluorescence was detected in the cytoplasm. CaALDH1 expression in pepper was rapidly and strongly induced by avirulent Xanthomonas campestris pv. vesicatoria (Xcv) Ds1 (avrBsT) infection. Transient co-expression of CaALDH1 with avrBsT significantly enhanced avrBsT-triggered cell death in N. benthamiana leaves. Aldehyde dehydrogenase activity was higher in leaves transiently expressing CaALDH1, suggesting that CaALDH1 acts as a cell death enhancer, independently of AvrBsT. CaALDH1 silencing disrupted phenolic compound accumulation, H2O2 production, defence response gene expression, and cell death during avirulent Xcv Ds1 (avrBsT) infection. Transgenic Arabidopsis thaliana overexpressing CaALDH1 exhibited enhanced defence response to Pseudomonas syringae pv. tomato and Hyaloperonospora arabidopsidis infection. These results indicate that cytoplasmic CaALDH1 interacts with AvrBsT and promotes plant cell death and defence responses.

Phenotypic and genetic characterization of resistance in Arabidopsis thaliana to the oomycete pathogen Phytophthora parasitica

The interaction between Arabidopsis thaliana and the oomycete pathogen Phytophthora parasitica emerges as a model for exploring the molecular basis and evolution of recognition and host defense. Phenotypic variation and genetic analysis is essential to dissect the underlying mechanisms in plant-oomycete interaction. In this study, the reaction phenotypes of 28 A. thaliana accessions to P. parasitica strain Pp016 were examined using detached leaf infection assay. The results showed the presence of four distinct groups based on host response and disease development. Of all the accessions examined, Zurich (Zu-1) is highly resistant to P. parasitica. Microscopic characterization showed that rapid and severe hypersensitive response at the primary infection epidermal cells is associated with disease resistance. Furthermore, Zu-1 is resistant to a set of 20 diverse P. parasitica strains, which were collected from different host plants and exhibited differential specificities on a set of tobacco cultivars. However, Zu-1 is susceptible to P. parasitica when the root is inoculated, suggesting differential expression of associated resistance genes in the root and foliar tissues. Genetic analysis by crossing Zu-1 and the susceptible accession Landsberg (Ler) showed that the resistance in Zu-1 to P. parasitica is semi-dominant, as shown by infection assays of F1 progenies, and is likely conferred by a single locus, defined as RPPA1 (Zu-1) (for Resistance to P. parasitica 1), as shown by analysis of F2 segregating populations. By employing specific-locus amplified fragment sequencing (SLAF-seq) strategy to identify molecular markers potentially linked to the locus, the strongest associated region was determined to be located between 7.1 and 11.2 Mb in chromosome IV. The future cloning of RPPA1 (Zu-1) locus will facilitate improved understanding of plant broad-spectrum disease resistance to oomycete pathogens.

The Role of Mitogen-Activated Protein (MAP) Kinase Signaling Components in the Fungal Development, Stress Response and Virulence of the Fungal Cereal Pathogen Bipolaris sorokiniana

Mitogen-activated protein kinases (MAPKs) have been demonstrated to be involved in fungal development, sexual reproduction, pathogenicity and/or virulence in many filamentous plant pathogenic fungi, but genes for MAPKs in the fungal cereal pathogen Bipolaris sorokiniana have not been characterized. In this study, orthologues of three MAPK genes (CsSLT2, CsHOG1 and CsFUS3) and one MAPK kinase kinase (MAPKKK) gene (CsSTE11) were identified in the whole genome sequence of the B. sorokiniana isolate ND90Pr, and knockout mutants were generated for each of them. The ∆Csfus3 and ∆Csste11 mutants were defective in conidiation and formation of appressoria-like structures, showed hypersensitivity to oxidative stress and lost pathogenicity on non-wounded leaves of barley cv. Bowman. When inoculated on wounded leaves of Bowman, the ∆Csfus3 and ∆Csste11 mutants were reduced in virulence compared to the wild type. No morphological changes were observed in the ∆Cshog1 mutants in comparison with the wild type; however, they were slightly reduced in growth under oxidative stress and were hypersensitive to hyperosmotic stress. The ∆Cshog1 mutants formed normal appressoria-like structures but were reduced in virulence when inoculated on Bowman leaves. The ∆Csslt2 mutants produced more vegetative hyphae, had lighter pigmentation, were more sensitive to cell wall degrading enzymes, and were reduced in virulence on Bowman leaves, although they formed normal appressoria like the wild type. Root infection assays indicated that the ∆Cshog1 and ∆Csslt2 mutants were able to infect barley roots while the ∆Csfus3 and ∆Csste11 failed to cause any symptoms. However, no significant difference in virulence was observed for ∆Cshog1 mutants while ∆Csslt2 mutants showed significantly reduced virulence on barley roots in comparison with the wild type. Our results indicated that all of these MAPK and MAPKKK genes are involved in the regulation of fungal development under normal and stress conditions and required for full virulence on barley plants.