Plant-Microbe Interactions: Pathogenesis & Symbiosis
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Dissecting the virulence‐related functionality and cellular transcription mechanism of a conserved hypothetical protein in Xanthomonas oryzae pv. oryzae

Hypothetical proteins without defined functions are largely distributed in all sequenced bacterial genomes. Understanding their potent functionalities is a basic demand for bacteriologists. Xanthomonas oryzae pv. oryzae (Xoo), the causal agent of bacterial leaf blight of rice, is one of the model systems for studying molecular plant pathology. One quarter of proteins in the genome of this bacterium were defined as hypothetical proteins, but their roles in Xoo pathogenicity is unknown. Here, we generated in-frame deletions for six hypothetical proteins selected from strain PXO99A and found one (PXO_03177) of them is required for the full virulence of this strain. PXO_03177 is conserved in Xanthomonas, and is predicted to contain two domains relating to polysaccharide synthesis. However, we found that mutation of this gene did not affect the production or modification of extracellular polysaccharides (EPS) and lipopolysaccharides (LPS), two major polysaccharides produced by Xoo relating to its infection. Interestingly, we found that inactivation of PXO_03177 significantly impaired the biofilm formation and tolerance to SDS, all of which are considered to play key roles during the infection of Xoo on rice leaves. These findings thus enable us to define a function for PXO_03177 in the virulence of Xoo. Furthermore, we also found that the global regulator Clp controls the transcription of PXO_03177 by directly binding to its promoter region, presenting the first cellular regulatory pathway for modulating expression of this hypothetical protein gene. Our results provide the respective reference information for PXO_03177 homologues in Xanthomonas. This article is protected by copyright. All rights reserved.
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Establishing Causality: Opportunities of Synthetic Communities for Plant Microbiome Research

Plant microbiome research highlights the importance of indigenous microbial communities for host phenotypes such as growth and health. It aims to discover the molecular basis by which host-microbe and microbe-microbe interactions shape and maintain microbial communities and to understand the role of individual microorganisms, as well as their collective ecosystem function. Here, we discuss reductionist approaches to disentangle the inherent complexity of interactions in situ. Experimentally tractable, synthetic communities enable testing of hypotheses by targeted manipulation in gnotobiotic systems. Modifications of microbial, host, and environmental parameters allow for the quantitative assessment of host and microbe characteristics with dynamic and spatial resolution. We summarize first insights from this emerging field and discuss current challenges and limitations. Using multifaceted approaches to detect interactions and functions will provide new insights into the fundamental biology of plant-microbe interactions and help to harness the power of the microbiome.
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Transcriptional Regulation of the Immune Receptor FLS2 Controls the Ontogeny of Plant Innate Immunity

Innate immunity plays a vital role in protecting plants and animals from pathogen infections. Immunity varies with age in both animals and plants. However, little is known about the ontogeny of plant innate immunity during seedling development. We report here that the Arabidopsis thaliana microRNA miR172b regulates the transcription of the immune receptor gene FLAGELLIN-SENSING 2 (FLS2) through TARGET OF EAT1 (TOE1) and TOE2, which directly bind to the FLS2 promoter and inhibit its activity. The level of miR172b is very low in the early stage of seedling development, but increases over time, which results in decreased TOE½ protein accumulation and, consequently, increased FLS2 transcription and the ontogeny of FLS2-mediated immunity during seedling development. Our study reveals a role for the miR172b-TOE½ module in regulating plant innate immunity, and elucidates a regulatory mechanism underlying the ontogeny of plant innate immunity.
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Identification of an apiosyltransferase in the plant pathogen Xanthomonas pisi

Identification of an apiosyltransferase in the plant pathogen Xanthomonas pisi | Plant-Microbe Interactions: Pathogenesis & Symbiosis | Scoop.it
The rare branched-chain sugar apiose, once thought to only be present in the plant kingdom, was found in two bacterial species: Geminicoccus roseus and Xanthomonas pisi. Glycans with apiose residues were detected in aqueous methanol-soluble fractions as well as in the insoluble pellet fraction of X. pisi. Genes encoding bacterial uridine diphosphate apiose (UDP-apiose) synthases (bUASs) were characterized in these bacterial species, but the enzyme(s) involved in the incorporation of the apiose into glycans remained unknown. In the X. pisi genome two genes flanking the XpUAS were annotated as hypothetical glycosyltransferase (GT) proteins. The first GT (here on named XpApiT) belongs to GT family 90 and has a Leloir type B fold and a putative lipopolysaccharide-modifying (LPS) domain. The second GT (here on XpXylT) belongs to GT family 2 and has a type A fold. The XpXylT and XpApiT genes were cloned and heterologously expressed in E. coli. Analysis of nucleotide sugar extracts from E. coli expressing XpXylT or XpApiT with UAS showed that recombinant XpApiT utilized UDP-apiose and XpXylT utilized UDP-xylose as substrate. Indirect activity assay (UDP-Glo) revealed that XpApiT is an apiosyltransferase (ApiT) able to specifically use UDP-apiose. Further support for the apiosyltransferase activity was demonstrated by in microbe co-expression of UAS and XpApiT in E. coli showing the utilization of UDP-apiose to generate an apioside detectable in the pellet fraction. This work provides evidence that X. pisi developed the ability to synthesize an apioside of indeterminate function; however, the evolution of the bacterial ApiT remains to be determined. From genetic and evolutionary perspectives, the apiose operon may provide a unique opportunity to examine how genomic changes reflect ecological adaptation during the divergence of a bacterial group.
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Pandemonium Breaks Out: Disruption of Salicylic Acid-Mediated Defense by Plant Pathogens

Salicylic acid (SA) or 2-hydroxybenoic acid is a phenolic plant hormone that plays an essential role in plant defense against biotrophic and semi-biotrophic pathogens. In Arabidopsis, SA is synthesized from chorismate in chloroplast through the ICS1 (Isochorismate synthase I) pathway during pathogen infection. The transcription co-activator NPR1 (Non-Expresser of Pathogenesis Related Gene 1), as the master regulator of SA signaling, interacts with transcription factors to induce the expression of anti-microbial PR ( Pathogenesis-Related) genes. To establish successful infections, plant bacterial, oomycete, fungal and viral pathogens have evolved at least three major strategies to disrupt SA-mediated defense. The first strategy is to reduce SA accumulation directly by converting SA to its inactive derivatives. The second strategy is to interrupt SA biosynthesis by targeting the ICS1 pathway. In the third major strategy, plant pathogens deploy different ways to interfere with SA downstream signaling. The wide array of strategies deployed by plant pathogens highlight the crucial role of disruption of SA-mediated plant defense in plant pathogenesis. A deeper understanding of this topic will greatly expand our knowledge of how plant pathogens cause diseases and consequently pave the road for the development of more effective ways to control these diseases.
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Phytophthora infestans RXLR effectors act in concert at diverse subcellular localisations to enhance host colonisation | Journal of Experimental Botany | Oxford Academic

Oomycetes such as the potato blight pathogen Phytophthora infestans deliver RXLR effectors into plant cells to manipulate host processes and promote disease. Knowledge of where they localise inside host cells is important in understanding their function. Fifty two P. infestans RXLR effectors (PiRXLRs) up-regulated during early stages of infection were expressed as fluorescent protein (FP)-fusions inside cells of the model host Nicotiana benthamiana. FP-PiRXLR fusions were predominantly nucleo-cytoplasmic, nuclear, or plasma-membrane-associated. Some also localised to the endoplasmic reticulum, mitochondria, peroxisomes or microtubules, suggesting diverse sites of subcellular activity. Seven of the 25 PiRXLRs examined during infection accumulated at sites of haustorium penetration, likely due to co-localisation with host target processes; Pi16663 (Avr1), for example, localised to Sec5-associated mobile bodies which showed perihaustorial accumulation. Forty-five FP-RXLR fusions enhanced pathogen leaf colonisation when expressed in N. benthamiana, revealing that their presence was beneficial to infection. Co-expression of PiRXLRs that target and suppress different immune pathways resulted in an additive enhancement of colonisation, indicating the potential to study effector combinations using transient expression assays. We provide a broad platform of high confidence P. infestans effector candidates from which to investigate the mechanisms, singly and in combination, by which this pathogen causes disease.
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An Effector from the Cyst Nematode Heterodera schachtii Derepresses Host rRNA Genes by Altering Histone Acetylation

Cyst nematodes are plant-pathogenic animals that secrete effector proteins into plant root cells to alter host gene expression and reprogram these cells to form specialized feeding sites, known as syncytia. The molecular mechanisms of these effectors are mostly unknown. We determined that the sugar beet cyst nematode (Heterodera schachtii) 32E03 effector protein strongly inhibits the activities of Arabidopsis thaliana histone deacetylases including the HDT1 enzyme, which has a known function in the regulation of rRNA gene expression through chromatin modifications. We determined that plants expressing the 32E03 coding sequence exhibited increased acetylation of histone H3 along the ribosomal DNA (rDNA) chromatin. At low 32E03 expression levels, these chromatin changes triggered the derepression of a subset of ribosomal RNA (rRNA) genes, which were conducive to H. schachtii parasitism. By contrast, high levels of 32E03 caused profound bidirectional transcription along the rDNA, which triggered rDNA-specific small RNA production leading to RNA-directed DNA methylation and silencing of rDNA, which inhibited nematode development. Our data show that the 32E03 effector alters plant rRNA gene expression by modulating rDNA chromatin in a dose-dependent manner. Thus, the 32E03 effector epigenetically regulates plant gene expression to promote cyst nematode parasitism.
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Genome-wide transcriptome profiling of transgenic hop (Humulus lupulus L.) constitutively overexpressing HlWRKY1 and HlWDR1 transcription factors | BMC Genomics | Full Text

Background
The hop plant (Humulus lupulus L.) is a valuable source of several secondary metabolites, such as flavonoids, bitter acids, and essential oils. These compounds are widely implicated in the beer brewing industry and are having potential biomedical applications. Several independent breeding programs around the world have been initiated to develop new cultivars with enriched lupulin and secondary metabolite contents but met with limited success due to several constraints. In the present work, a pioneering attempt has been made to overexpress master regulator binary transcription factor complex formed by HlWRKY1 and HlWDR1 using a plant expression vector to enhance the level of prenylflavonoid and bitter acid content in the hop. Subsequently, we performed transcriptional profiling using high-throughput RNA-Seq technology in leaves of resultant transformants and wild-type hop to gain in-depth information about the genome-wide functional changes induced by HlWRKY1 and HlWDR1 overexpression.

Results
The transgenic WW-lines exhibited an elevated expression of structural and regulatory genes involved in prenylflavonoid and bitter acid biosynthesis pathways. In addition, the comparative transcriptome analysis revealed a total of 522 transcripts involved in 30 pathways, including lipids and amino acids biosynthesis, primary carbon metabolism, phytohormone signaling and stress responses were differentially expressed in WW-transformants. It was apparent from the whole transcriptome sequencing that modulation of primary carbon metabolism and other pathways by HlWRKY1 and HlWDR1 overexpression resulted in enhanced substrate flux towards secondary metabolites pathway. The detailed analyses suggested that none of the pathways or genes, which have a detrimental effect on physiology, growth and development processes, were induced on a genome-wide scale in WW-transgenic lines.

Conclusions
Taken together, our results suggest that HlWRKY1 and HlWDR1 simultaneous overexpression positively regulates the prenylflavonoid and bitter acid biosynthesis pathways in the hop and thus these transgenes are presented as prospective candidates for achieving enhanced secondary metabolite content in the hop.
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Contribution of a lectin, LecM, to the quorum sensing signalling pathway of Ralstonia solanacearum strain OE1‐1 - Hayashi - - Molecular Plant Pathology - Wiley Online Library

The soil‐borne bacterium Ralstonia solanacearum invades the roots and colonizes the intercellular species and then the xylem. The expression of lecM, encoding a lectin LecM, is induced by an OmpR family response regulator HrpG in R. solanacearum strain OE1‐1. LecM contributes to the attachment of strain OE1‐1 to host cells of intercellular spaces. OE1‐1 produces methyl 3‐hydroxymyristate (3‐OH MAME) through a methyltransferase (PhcB) and extracellularly secretes the chemical as a quorum‐sensing (QS) signal, which activates QS. The expression of lecM is also induced by PhcA virulence regulator functioning through QS, and the resultant LecM is implicated in the QS‐dependent production of major exopolysaccharide EPS I and the aggregation of OE1‐1 cells. To investigate LecM functions in QS, we analysed the transcriptome of R. solanacearum strains generated by RNA sequencing technology. In the lecM mutant, the expression of positively QS‐regulated genes (by > 90%) and the expression of negatively QS‐regulated genes (by ~60%) was downregulated and upregulated, respectively. However, phcB and phcA in the lecM mutant were expressed at levels similar to those in strain OE1‐1. The lecM mutant produced significantly less ralfuranone and exhibited a significantly greater swimming motility, which are positively and negatively regulated by QS, respectively. In addition, the extracellular 3‐OH MAME content of lecM mutant was significantly lower than that of OE1‐1. The application of 3‐OH MAME increased more EPS I production in the phcB‐deleted mutant and strain OE1‐1 than in the lecM mutant. Thus, QS‐dependent produced LecM contributes to the QS signalling pathway.

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Role of a receptor-like kinase K1 in pea Rhizobium symbiosis development

The ability of the crop legume Pisum sativum L. to perceive the Nod factor rhizobial signals may depend on several receptors that differ in ligand structure specificity. Identification of pea mutants defective in two types of LysM receptor-like kinases (LysM-RLKs), SYM10 and SYM37, featuring different phenotypic manifestations and impaired at various stages of symbiosis development, corresponds well to this assumption. There is evidence that one of the receptor proteins involved in symbiosis initiation, SYM10, has an inactive kinase domain. This implies the presence of an additional component in the receptor complex, together with SYM10, that remains unknown. Here, we describe a new LysM-RLK, K1, which may serve as an additional component of the receptor complex in pea. To verify the function of K1 in symbiosis, several P. sativum non-nodulating mutants in the k1 gene were identified using the TILLING approach. Phenotyping revealed the blocking of symbiosis development at an appropriately early stage, strongly suggesting the importance of LysM-RLK K1 for symbiosis initiation. Moreover, the analysis of pea mutants with weaker phenotypes provides evidence for the additional role of K1 in infection thread distribution in the cortex and rhizobia penetration. The interaction between K1 and SYM10 was detected using transient leaf expression in Nicotiana benthamiana and in the yeast two-hybrid system. Since the possibility of SYM10/SYM37 complex formation was also shown, we tested whether the SYM37 and K1 receptors are functionally interchangeable using a complementation test. The interaction between K1 and other receptors is discussed.
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Subfamily-specific specialization of RGH1/MLA immune receptors in wild barley | Molecular Plant-Microbe Interactions

The barley disease resistance (R) gene locus Mildew Locus A (Mla) provides isolate-specific resistance against the powdery mildew fungus Blumeria graminis hordei (Bgh) and has been introgressed into modern cultivars from diverse germplasms, including the wild relative Hordeum spontaneum. Known Mla disease resistance specificities to Bgh appear to encode allelic variants of the R Gene Homolog 1 (RGH1) family of nucleotide-binding domain and leucine-rich repeat (NLR) proteins. We here sequenced and assembled the transcriptomes of 50 H. spontaneum accessions representing nine populations distributed throughout the Fertile Crescent. The assembled Mla transcripts exhibited rich sequence diversity, linked neither to geographic origin nor population structure and could be grouped into two similar-sized subfamilies based on two major N-terminal coiled-coil signaling domains that are both capable of eliciting cell death. The presence of positively selected sites, located mainly in the C-terminal leucine-rich repeats of both MLA subfamilies, together with the fact that both coiled-coil signaling domains mediate cell death, implies that the two subfamilies are actively maintained in the population. Unexpectedly, known MLA receptor variants that confer Bgh resistance belong exclusively to one subfamily. Thus, signaling domain divergence, potentially as adaptation to distinct pathogen populations, is an evolutionary signature of functional diversification of an immune receptor.
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Regulation of plant immune signaling by calcium-dependent protein kinases | Molecular Plant-Microbe Interactions

Activation of Ca2+ signaling is a universal response to stress that allows cells to quickly respond to environmental cues. Fluctuations in cytosolic calcium are decoded in plants by calcium-sensing proteins such as Ca2+-dependent protein kinases (CDPKs). The perception of microbes results in an influx of calcium that activates numerous CDPKs that propagate immune signals required for resistance against disease-causing pathogens. This review describes our current understanding of CDPK activation and regulation, and provides a comprehensive overview of CDPK-mediated immune signaling through interaction with various substrates.
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Ancient plants with ancient fungi

Arbuscular mycorrhizas are widespread in land plants including liverworts, some of the closest living relatives of the first plants to colonize land 500 million years ago (MYA). Previous investigations reported near-exclusive colonization of liverworts by the most recently evolved arbuscular mycorrhizal fungi, the Glomeraceae, indicating a recent acquisition from flowering plants at odds with the widely held notion that arbuscular mycorrhizal-like associations in liverworts represent the ancestral symbiotic condition in land plants. We performed an analysis of symbiotic fungi in 674 globally collected liverworts using molecular phylogenetics and electron microscopy. Here, we show every order of arbuscular mycorrhizal fungi colonizes early-diverging liverworts, with non-Glomeraceae being at least 10 times more common than in flowering plants. Arbuscular mycorrhizal fungi in liverworts and other ancient plant lineages (hornworts, lycopods, and ferns) were delimited into 58 taxa and 36 singletons, of which at least 43 are novel and specific to liverworts. The discovery that early plant lineages are colonized by early-diverging fungi supports the hypothesis that arbuscular mycorrhizas are an ancestral symbiosis for all land plants
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Frontiers | Seed transmission of Epichloë endophytes in Lolium perenne is heavily influenced by host genetics | Plant Science

Vertical transmission of symbiotic Epichloë endophytes from host grasses into progeny seed is the primary mechanism by which the next generation of plants is colonised. This process is often imperfect, resulting in endophyte-free seedlings which may have poor ecological fitness if the endophyte confers protective benefits to its host. In this study, we investigated the influence of host genetics and environment on the vertical transmission of Epichloë festucae var. lolii strain AR37 in the temperate forage grass Lolium perenne. The efficiency of AR37 transmission into the seed of over 500 plant genotypes from five genetically diverse breeding populations was determined. In Populations I-III, which had undergone previous selection for high seed infection by AR37, mean transmission was 88%, 93% and 92%, respectively. However, in Populations IV and V, which had not undergone previous selection, mean transmission was 69% and 70%, respectively. The transmission values, together with single nucleotide polymorphism data obtained using genotyping by sequencing for each host, was used to develop a genomic prediction model for AR37 seed transmission. The predictive ability of the model was estimated at r = 0.54. While host genotype contributed greatly to differences in AR37 seed transmission, undefined environmental variables also contributed significantly to seed transmission across different years and geographic locations. There was evidence for a small host genotype by environment effect, however this was less pronounced than genotype or environment alone. Analysis of endophyte infection levels in parent plants within Populations I and IV revealed a loss of endophyte infection over time in Population IV only. This population also had lower average tiller infection frequencies than Population I, suggesting that AR37 failed to colonise all the daughter tillers and therefore seeds. However, we also observed that infection of seed by AR37 may fail during or after initiation of floral development from plants where all tillers remained endophyte-infected over time. While the effects of environment and host genotype on fungal endophyte transmission have been evaluated previously, this is the first study that quantifies the relative impacts of host genetics and environment on endophyte vertical transmission.
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Secreted venom allergen-like proteins of helminths: Conserved modulators of host responses in animals and plants

Secreted venom allergen-like proteins of helminths: Conserved modulators of host responses in animals and plants | Plant-Microbe Interactions: Pathogenesis & Symbiosis | Scoop.it
Despite causing considerable damage to host tissue at the onset of parasitism, invasive helminths establish remarkably persistent infections in both animals and plants. Secretions released by these obligate parasites during host invasion are thought to be crucial for their persistence in infection. Helminth secretions are complex mixtures of molecules, most of which have unknown molecular targets and functions in host cells or tissues. Although the habitats of animal- and plant-parasitic helminths are very distinct, their secretions share the presence of a structurally conserved group of proteins called venom allergen-like proteins (VALs). Helminths abundantly secrete VALs during several stages of parasitism while inflicting extensive damage to host tissue. The tight association between the secretion of VALs and the onset of parasitism has triggered a particular interest in this group of proteins, as improved knowledge on their biological functions may assist in designing novel protection strategies against parasites in humans, livestock, and important food crops.
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Frontiers | Symbiotic effectivity of dual and tripartite associations on soybean (Glycine max L. Merr.) cultivars inoculated with Bradyrhizobium japonicum and AM fungi | Plant Science

Soybean (Glycine max L. Merr.) is regarded worldwide as indisputably one of the most important crops for human food and animal feed. The presence of symbiotic bacteria and fungi is essential for soybean breeding, especially in low-input agricultural systems. Research on the cooperation between different microbial symbionts is a key to understanding how the health and productivity of the plant is supported. The symbiotic effectivity of dual and tripartite symbiotic agents was investigated in two pot experiments on different soybean cultivars with special regard to compatibility. In the Selection experiment, two out of sixteen soybean cultivars (Aliz, Emese) were chosen on the basis of their drought tolerance and used in all the other investigations. In the Compatibility experiment, the compatible coupling of symbiotic partners was selected based on the efficiency of single and co-inoculation with two Bradyrhizobium japonicum strains and two commercial arbuscular mycorrhizal fungal (AMF) products. Significant differences were found in the infectivity and effectivity of the microsymbionts. The rhizobial and AMF inoculation generally improved plant production, photosynthetic efficiency and root activity, but this effect depended on the type of symbiotic assotiation. Despite the low infectivity of AMF, inocula containing fungi were more beneficial than those containing only rhizobia.
In the Drought Stress (DS) experiment, co-inoculated and control plants were grown in chernozem soil originating from organic farms.. Emese was more resistant to drought stress than Aliz and produced a bigger root system. Under DS, the growth parameters of both microbially inoculated cultivars were better than that of control, proving that even drought tolerant genotypes can strengthen their endurance due to inoculation with AMF and nitrogen fixing bacteria. Root electrical capacitance (CR) showed a highly significant linear correlation with root and shoot dry mass and leaf area. The same root biomass was associated with higher CR in inoculated hosts. As CR method detects the absorptive surface increasing due to inoculation, it may be used to check the efficiency of the microbial treatment
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Emerging Roles of Sphingolipid Signaling in Plant Response to Biotic and Abiotic Stresses

Plant sphingolipids being structural components of the plasma membrane and other endomembrane systems act as signaling molecules during biotic and abiotic stresses. However, the role of sphingolipids in plant signal transduction in response to environmental cues has yet to be investigated in detail, and much remains to be learned. In this review, we discuss the signaling roles of sphingolipid metabolites based on previous studies. We attempted to concentrate solely on plant sphingolipids. However, due to the lack of literature, we also considered the microbial sphingolipids that initiate signals during their interaction with plants. The equilibrium of nonphosphorylated and phosphorylated sphingolipid species determine the destiny of plant cell. Nonetheless, the molecular connections among the enzymes responsible for this equilibrium in a coordinated signaling network are poorly understood. A mechanistic link between the phytohormone-sphingolipid interplay has also not yet been fully understood, which provides an opportunity to elucidate in future research. In addition, it might help to identify novel key participants involved in this complex interaction operating under stress conditions. We hope that this review will provide mechanistic insights that will help design future research to fill these gaps in our understanding.
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Adult plant resistance in maize to northern leaf spot is a feature of partial loss-of-function alleles of Hm1

Adult plant resistance in maize to northern leaf spot is a feature of partial loss-of-function alleles of Hm1 | Plant-Microbe Interactions: Pathogenesis & Symbiosis | Scoop.it
Adult plant resistance (APR) is an enigmatic phenomenon in which resistance genes are ineffective in protecting seedlings from disease but confer robust resistance at maturity. Maize has multiple cases in which genes confer APR to northern leaf spot, a lethal disease caused by Cochliobolus carbonum race 1 (CCR1). The first identified case of APR in maize is encoded by a hypomorphic allele, Hm1A, at the hm1 locus. In contrast, wild-type alleles of hm1 provide complete protection at all developmental stages and in every part of the maize plant. Hm1 encodes an NADPH-dependent reductase, which inactivates HC-toxin, a key virulence effector of CCR1. Cloning and characterization of Hm1A ruled out differential transcription or translation for its APR phenotype and identified an amino acid substitution that reduced HC-toxin reductase (HCTR) activity. The possibility of a causal relationship between the weak nature of Hm1A and its APR phenotype was confirmed by the generation of two new APR alleles of Hm1 by mutagenesis. The HCTRs encoded by these new APR alleles had undergone relatively conservative missense changes that partially reduced their enzymatic activity similar to HM1A. No difference in accumulation of HCTR was observed between adult and juvenile plants, suggesting that the susceptibility of seedlings derives from a greater need for HCTR activity, not reduced accumulation of the gene product. Conditions and treatments that altered the photosynthetic output of the host had a dramatic effect on resistance imparted by the APR alleles, demonstrating a link between the energetic or metabolic status of the host and disease resistance affected by HC-toxin catabolism by the APR alleles of HCTR.
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Extreme resistance to Potato Virus Y in potato carrying the Rysto gene is mediated by a TIR-NLR immune receptor

Potato virus Y (PVY) is a major potato pathogen that causes annual losses of billions of dollars. Control of its transmission requires extensive use of environmentally damaging insecticides. Rysto confers extreme resistance (ER) to PVY and is a valuable trait in resistance breeding programs. We isolated Rysto using Resistance gene enrichment sequencing (RenSeq) and PacBio SMRT (Pacific Biosciences Single-Molecule Real Time Sequencing). Rysto encodes a nucleotide binding-leucine rich repeat (NLR) protein with an N-terminal TIR domain, and is sufficient for PVY perception and extreme resistance in transgenic potato plants. We investigated the requirements for Rysto-dependent extreme resistance, and showed that Rysto function is temperature-independent and requires EDS1 and NRG1 proteins. Rysto may prove valuable for creating PVY-resistant cultivars of potato and other Solanaceae crops.
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An RXLR effector secreted by Phytophthora parasitica is a virulence factor and triggers cell death in various plants - Huang - - Molecular Plant Pathology - Wiley Online Library

RXLR effectors encoded by Phytophthora species play a central role in pathogen‐plant interactions. Understanding the biological functions of RXLR effectors are conducive to illuminating pathogenic mechanisms and developing disease‐control strategies. However, the virulence function of Phytophthora parasitica RXLR effectors is poorly understood. Here, we describe the identification of a P. parasitica RXLR effector gene, PPTG00121 (PpE4), which is highly transcribed during the early stages of infection and conserved among various Phytophthora species. Live‐cell imaging of P. parasitica transformants expressing a full‐length PpE4 (E4FL)‐mCherry protein indicated that PpE4 is secreted and accumulates around haustoria during plant infection. Silencing of PpE4 in P. parasitica resulted in significantly reduced virulence on Nicotiana benthamiana. Transient expression of PpE4 in N. benthamiana in turn restored pathogenicity of the PpE4‐silenced lines. Furthermore, expression of PpE4 in both N. benthamiana and Arabidopsis thaliana consistently enhanced plant susceptibility to P. parasitica. These results indicate that PpE4 contributes to pathogen infection. Finally, heterologous expression experiments showed that PpE4 triggers nonspecific cell death in a variety of plants including tobacco, tomato, potato, and A. thaliana. Virus‐induced gene silencing assays revealed that PpE4‐induced cell death is HSP90‐, NPK‐, and SGT1‐dependent, suggesting that PpE4 is recognized by the plant immune system. In conclusion, PpE4 is an important virulence RXLR effector of P. parasitica that is conserved across Phytophthora species and recognized by a wide range of host plants.
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Transcriptome analysis reveals the molecular mechanisms of the defense response to gray leaf spot disease in maize | BMC Genomics | Full Text

Background
Gray leaf spot (GLS), which is caused by the necrotrophic fungi Cercospora zeae-maydis and Cercospora zeina, is one of the most impactful diseases in maize worldwide. The aim of the present study is to identify the resistance genes and understand the molecular mechanisms for GLS resistance.

Results
Two cultivars, ‘Yayu889’ and ‘Zhenghong532,’ which are distinguished as resistant and susceptible cultivars, respectively, were challenged with the GLS disease and a RNA-seq experiment was conducted on infected plants at 81, 89, 91, and 93 days post planting (dap). Compared with the beginning stage at 81 dap, 4666, 1733, and 1166 differentially expressed genes (DEGs) were identified at 89, 91, and 93 dap, respectively, in ‘Yayu889,’ while relatively fewer, i.e., 4713, 881, and 722 DEGs, were identified in ‘Zhenghong532.’ Multiple pathways involved in the response of maize to GLS, including ‘response to salicylic acid,’ ‘protein phosphorylation,’ ‘oxidation-reduction process,’ and ‘carotenoid biosynthetic process,’ were enriched by combining differential expression analysis and Weighted Gene Co-expression Network Analysis (WGCNA). The expression of 12 candidate resistance proteins in these pathways were quantified by the multiple reaction monitoring (MRM) method. This approach identified two candidate resistance proteins, a calmodulin-like protein and a leucine-rich repeat receptor-like protein kinase with SNPs that were located in QTL regions for GLS resistance. Metabolic analysis showed that, compared with ‘Zhenghong532,’ the amount of salicylic acid (SA) and total carotenoids in ‘Yayu889’ increased, while peroxidase activity decreased during the early infection stages, suggesting that increased levels of SA, carotenoids, and reactive oxygen species (ROS) may enhance the defense response of ‘Yayu889’ to GLS.

Conclusion
By combining transcriptome and proteome analyses with comparisons of resistance QTL regions, calmodulin-like protein and leucine-rich repeat receptor-like protein kinase were identified as candidate GLS resistance proteins. Moreover, we found that the metabolic pathways for ROS, SA, and carotenoids are especially active in the resistant cultivar. These findings could lead to a better understanding of the GLS resistance mechanisms and facilitate the breeding of GLS-resistant maize cultivars.
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A Meloidogyne graminicola C‐type lectin, Mg01965 is secreted into the host apoplast to suppress plant defense and promote parasitism - Zhuo - - Molecular Plant Pathology - Wiley Online Library

C‐type lectins (CTLs), a class of multifunctional proteins, are numerous in nematodes. One CTL gene Mg01965 shown to be expressed in the subventral glands especially in the second‐stage juveniles of the root‐knot nematode Meloidogyne graminicola was further analyzed in this study. In vitro RNA interference targeting Mg01965 in the preparasitic juveniles significantly reduced their ability to infect host plant roots. Immunolocalizations showed that Mg01965 is secreted by M. graminicola into the roots during the early parasitic stages and accumulates in the apoplast. Transient expression of Mg01965 in N. benthamiana and targeting it to the apoplast suppressed the burst of reactive oxygen species triggered by flg22. The CTL Mg01965 suppresses plant innate immunity in the host apoplast, promoting nematode parasitism in the early infection stages.
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OsJAZ9 overexpression improves potassium deficiency tolerance in rice by modulating jasmonic acid levels and signaling

Potassium (K) which makes around 2-10% of plants total dry biomass, when become deficient, makes the plants highly susceptible to both abiotic and biotic stresses. Recent evidences suggest overlapping transcriptional responses to K deficiency and Jasmonate (JA) treatment in plants. However, a link between these responses was missing. Notably, K deficiency and JA application produce similar phenotypic and transcriptional responses. Here, we used molecular, physiological and morphological studies to analyze the role of OsJAZ9 in JA homeostasis, K deficiency and sheath blight resistance. We raised OsJAZ9 overexpression, knockdown, translational reporter and C-terminal deleted translational reporter lines in rice to establish the role of JA signaling in K ion homeostasis and OsJAZ9 as a critical component of JA signaling for K deficiency response. OsJAZ9 overexpression and knockdown provide K deficiency tolerance and sensitivity, respectively, by modulating various K transporters and root system architecture. Furthermore, RNA Seq and JA profiling revealed an elevation of JA responsive genes and JA levels in OsJAZ9 OE lines under K deficiency. Our data provide clear evidence on the crucial role of JAZ repressor, OsJAZ9 in improving K deficiency tolerance in rice by altering JA levels and signaling.
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A Dihydroflavonol-4-reductase-like Protein Interacts with NFR5 and Regulates Rhizobial Infection in Lotus japonicus | Molecular Plant-Microbe Interactions

In almost all symbiotic interactions between rhizobia and leguminous plants, host flavonoid-induced synthesis of Nod factors in rhizobia is required to initiate symbiotic response in plants. In this study, we found that Lotus japonicus Nod Factor Receptor 5 (LjNFR5) might directly regulate flavonoid biosynthesis during symbiotic interaction with rhizobia. A yeast two-hybrid analysis revealed that a dihydroflavonol-4-reductase-like protein (LjDFL1) interacts with LjNFR5. The interaction between MtDFL1 and MtNFP, two Medicago truncatula proteins with homology to LjDFL1 and LjNFR5, respectively, was also shown, suggesting that interaction between these two proteins might be conserved in different legumes. LjDFL1 was highly expressed in root hairs and the epidermal cells of root tips. Lotus ljdfl1 mutants and Medicago mtdfl1 mutants produced significantly fewer infection threads (ITs) than the wild-type control plants following rhizobial treatment. Furthermore, the roots of stable transgenic L. japonicus plants overexpressing LjDFL1 formed more ITs than control roots after exposure to rhizobia. These data indicated that LjDFL1 is a positive regulator of symbiotic signaling. However, the expression of LjDFL1 was suppressed by rhizobial treatment, suggesting that a negative feedback loop might be involved in regulation of the symbiotic response in L. japonicus.
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Rpp1 encodes a ULP1-NBS-LRR protein that controls immunity to Phakopsora pachyrhizi in soybean | Molecular Plant-Microbe Interactions

Phakopsora pachyrhizi is the causal agent of Asian soybean rust. Susceptible soybean plants infected by virulent isolates of P. pachyrhizi are characterized by tan-colored lesions and erumpent uredinia on the leaf surface. Germplasm screening and genetic analyses have led to the identification of seven loci, Rpp1 – Rpp7, that provide varying degrees of resistance to P. pachyrhizi (Rpp). Two genes, Rpp1 and Rpp1b, map to the same region on soybean chromosome 18. Rpp1 is unique among the Rpp genes in that it confers an immune response (IR) to avirulent P. pachyrhizi isolates. The IR is characterized by a lack of visible symptoms, whereas resistance provided by Rpp1b – Rpp7 results in red-brown foliar lesions. Rpp1 maps to a region spanning approximately 150 Kb on chromosome 18 between markers Sct_187 and Sat_064 in L85-2378 (Rpp1), an isoline developed from Williams 82 and PI 200492 (Rpp1). To identify Rpp1, we constructed a bacterial artificial chromosome (BAC) library from soybean accession PI 200492. Sequencing of the Rpp1 locus identified three homologous nucleotide binding site-leucine rich repeat (NBS-LRR) candidate resistance genes between Sct_187 and Sat_064. Each candidate gene is also predicted to encode an N-terminal ubiquitin-like protease 1 (ULP1) domain. Co-silencing of the Rpp1 candidates abrogated the immune response in the Rpp1 resistant soybean accession PI 200492, indicating that Rpp1 is a ULP1-NBS-LRR protein and plays a key role in the IR.
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Heterotrimeric G-proteins in unfolded protein response mediate plant growth-defense tradeoffs upstream of steroid and immune signaling

Plants prioritize growth over defense to gain a competitive advantage for limited resources, but change priorities to successfully fight infection and herbivory. Despite the importance of growth-defense tradeoffs in optimizing plant productivity in natural and agricultural populations, the molecular mechanisms that link growth and immunity remain unclear. Here, we demonstrate that growth-defense tradeoffs between pathways activated by BRI1, a steroid receptor, and FLS2, an innate immune receptor, are uncoupled in an Arabidopsis mutant (agg1 agg2) lacking two redundant heterotrimeric G-protein gamma subunits that form stable heterodimers with the Gβ subunit AGB1 to control one arm of the unfolded protein response (UPR) independently of ER stress. Growth inhibition from induced immunity in wild-type plants is likely caused by AGB1-AGG1/2 dimers interacting with nascent BRI1 and FLS2 proteins on the endoplasmic reticulum (ER) membrane and repressing an UPR response that is hardwired to promote BRI1 protein biogenesis and FLS2 protein degradation via autophagy. The ability to unlock and fine-tune growth-defense tradeoffs through UPR signaling provides a novel strategy to increase the natural defenses of crops while maintaining optimal plant productivity.
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