Nicotiana benthamiana calreticulin 3a is required for the ethylene-mediated production of phytoalexins and disease resistance against oomycete pathogen Phytophthora infestans

Phytaspase Is Capable of Detaching the Endoplasmic Reticulum Retrieval Signal from Tobacco Calreticulin-3

Soluble chaperones residing in the endoplasmic reticulum (ER) play vitally important roles in folding and quality control of newly synthesized proteins that transiently pass through the ER en route to their final destinations. These soluble residents of the ER are themselves endowed with an ER retrieval signal that enables the cell to bring the escaped residents back from the Golgi. Here, by using purified proteins, we showed that Nicotiana tabacum phytaspase, a plant aspartate-specific protease, introduces two breaks at the C-terminus of the N. tabacum ER resident calreticulin-3. These cleavages resulted in removal of either a dipeptide or a hexapeptide from the C-terminus of calreticulin-3 encompassing part or all of the ER retrieval signal. Consistently, expression of the calreticulin-3 derivative mimicking the phytaspase cleavage product in Nicotiana benthamiana cells demonstrated loss of the ER accumulation of the protein. Notably, upon its escape from the ER, calreticulin-3 was further processed by an unknown protease(s) to generate the free N-terminal (N) domain of calreticulin-3, which was ultimately secreted into the apoplast. Our study thus identified a specific proteolytic enzyme capable of precise detachment of the ER retrieval signal from a plant ER resident protein, with implications for the further fate of the escaped resident.

Botrytis cinerea tolerates phytoalexins produced by Solanaceae and Fabaceae plants through an efflux transporter BcatrB and metabolizing enzymes

Botrytis cinerea, a plant pathogenic fungus with a wide host range, has reduced sensitivity to fungicides as well as phytoalexins, threatening cultivation of economically important fruits and vegetable crops worldwide. B. cinerea tolerates a wide array of phytoalexins, through efflux and/or enzymatic detoxification. Previously, we provided evidence that a distinctive set of genes were induced in B. cinerea when treated with different phytoalexins such as rishitin (produced by tomato and potato), capsidiol (tobacco and bell pepper) and resveratrol (grape and blueberry). In this study, we focused on the functional analyses of B. cinerea genes implicated in rishitin tolerance. LC/MS profiling revealed that B. cinerea can metabolize/detoxify rishitin into at least 4 oxidized forms. Heterologous expression of Bcin08g04910 and Bcin16g01490, two B. cinerea oxidoreductases upregulated by rishitin, in a plant symbiotic fungus Epichloë festucae revealed that these rishitin-induced enzymes are involved in the oxidation of rishitin. Expression of BcatrB, encoding an exporter of structurally unrelated phytoalexins and fungicides, was significantly upregulated by rishitin but not by capsidiol and was thus expected to be involved in the rishitin tolerance. Conidia of BcatrB KO (ΔbcatrB) showed enhanced sensitivity to rishitin, but not to capsidiol, despite their structural similarity. ΔbcatrB showed reduced virulence on tomato, but maintained full virulence on bell pepper, indicating that B. cinerea activates BcatrB by recognizing appropriate phytoalexins to utilize it in tolerance. Surveying 26 plant species across 13 families revealed that the BcatrB promoter is mainly activated during the infection of B. cinerea in plants belonging to the Solanaceae, Fabaceae and Brassicaceae. The BcatrB promoter was also activated by in vitro treatments of phytoalexins produced by members of these plant families, namely rishitin (Solanaceae), medicarpin and glyceollin (Fabaceae), as well as camalexin and brassinin (Brassicaceae). Consistently, ΔbcatrB showed reduced virulence on red clover, which produces medicarpin. These results suggest that B. cinerea distinguishes phytoalexins and induces differential expression of appropriate genes during the infection. Likewise, BcatrB plays a critical role in the strategy employed by B. cinerea to bypass the plant innate immune responses in a wide variety of important crops belonging to the Solanaceae, Brassicaceae and Fabaceae.

The potato StMKK5-StSIPK module enhances resistance to Phytophthora pathogens through activating the salicylic acid and ethylene signalling pathways

Mitogen-activated protein kinase (MAPK) cascades play pivotal roles in plant responses to both biotic and abiotic stress. A screen of a Nicotiana benthamiana cDNA virus-induced gene silencing (VIGS) library for altered plant responses to inoculation with Phytophthora infestans previously identified an NbMKK gene, encoding a clade D MAPKK that we renamed as NbMKK5, which is involved in immunity to P. infestans. To study the role of the potato orthologous gene, referred to as StMKK5, in the response to P. infestans, we transiently overexpressed StMKK5 in N. benthamiana and observed that cell death occurred at 2 days postinfiltration. Silencing of the highly conserved eukaryotic protein SGT1 delayed the StMKK5-induced cell death, whereas silencing of the MAPK-encoding gene NbSIPK completely abolished the cell death response. Further investigations showed that StMKK5 interacts with, and directly phosphorylates, StSIPK. Furthermore, both StMKK5 and StSIPK trigger salicylic acid (SA)- and ethylene (Eth)-related gene expression, and co-expression of the salicylate hydroxylase NahG with the negative regulator of Eth signalling CTR1 hampers StSIPK-triggered cell death. This observation indicates that the cell death triggered by StMKK5-StSIPK is dependent on the combination of SA- and Eth-signalling. By introducing point mutations, we showed that the kinase activity of both StMKK5 and StSIPK is required for triggering cell death. Genetic analysis showed that StMKK5 depends on StSIPK to trigger plant resistance. Thus, our results define a potato StMKK5-SIPK module that positively regulates immunity to P. infestans via activation of both the SA and Eth signalling pathways.

Identification and genomic characterization of major effect bacterial blight resistance locus (BB-13) in Upland cotton (Gossypium hirsutum L.)

Identification and genomic characterization of major resistance locus against cotton bacterial blight (CBB) using GWAS and linkage mapping to enable genomics-based development of durable CBB resistance and gene discovery in cotton. Cotton bacterial leaf blight (CBB), caused by Xanthomonas citri subsp. malvacearum (Xcm), has periodically been a damaging disease in the USA. Identification and deployment of genetic resistance in cotton cultivars is the most economical and efficient means of reducing crop losses due to CBB. In the current study, genome-wide association study (GWAS) of CBB resistance using an elite diversity panel of 380 accessions, genotyped with the cotton single nucleotide polymorphism (SNP) 63 K array, and phenotyped with race-18 of CBB, localized the CBB resistance to a 2.01-Mb region in the long arm of chromosome D02. Molecular genetic mapping using an F₆ recombinant inbred line (RIL) population showed the CBB resistance in cultivar Arkot 8102 was controlled by a single locus (BB-13). The BB-13 locus was mapped within the 0.95-cM interval near the telomeric region in the long arm of chromosome D02. Flanking SNP markers, i04890Gh and i04907Gh of the BB-13 locus, identified from the combined linkage analysis and GWAS, targeted it to a 371-Kb genomic region. Candidate gene analysis identified thirty putative gene sequences in the targeted genomic region. Nine of these putative genes and two NBS-LRR genes adjacent to the targeted region were putatively involved in plant disease resistance and are possible candidate genes for BB-13 locus. Genetic mapping and genomic targeting of the BB13 locus in the current study will help in cloning the CBB-resistant gene and establishing the molecular genetic architecture of the BB-13 locus towards developing durable resistance to CBB in cotton.

Botrytis cinerea identifies host plants via the recognition of antifungal capsidiol to induce expression of a specific detoxification gene

The gray mold pathogen Botrytis cinerea has a broad host range, causing disease in >400 plant species, but it is not known how this pathogen evolved this polyxenous nature. Botrytis cinerea can metabolize a wide range of phytoalexins, including the stilbenoid resveratrol in grape, and the sesquiterpenoids capsidiol in tobacco and rishitin in potato and tomato. In this study, we analyzed the metabolism of sesquiterpenoid phytoalexins by B. cinerea. Capsidiol was dehydrogenated to capsenone, which was then further oxidized, while rishitin was directly oxidized to epoxy- or hydroxyrishitins, indicating that B. cinerea has separate mechanisms to detoxify structurally similar sesquiterpenoid phytoalexins. RNA-seq analysis revealed that a distinct set of genes were induced in B. cinerea when treated with capsidiol or rishitin, suggesting that B. cinerea can distinguish structurally similar phytoalexins to activate appropriate detoxification mechanisms. The gene most highly upregulated by capsidiol treatment encoded a dehydrogenase, designated Bccpdh. Heterologous expression of Bccpdh in a capsidiol-sensitive plant symbiotic fungus, Epichloë festucae, resulted in an acquired tolerance of capsidiol and the ability to metabolize capsidiol to capsenone, while B. cinerea Δbccpdh mutants became relatively sensitive to capsidiol. The Δbccpdh mutant showed reduced virulence on the capsidiol producing Nicotiana and Capsicum species but remained fully pathogenic on potato and tomato. Homologs of Bccpdh are found in taxonomically distant Ascomycota fungi but not in related Leotiomycetes species, suggesting that B. cinerea acquired the ancestral Bccpdh by horizontal gene transfer, thereby extending the pathogenic host range of this polyxenous pathogen to capsidiol-producing plant species.

RNAi-Mediated Knockdown of Calreticulin3a Impairs Pollen Tube Growth in Petunia

Pollen tube growth depends on several complex processes, including exo/endocytosis, cell wall biogenesis, intracellular transport, and cell signaling. Our previous results provided evidence that calreticulin (CRT)—a prominent calcium (Ca²⁺)-buffering molecular chaperone in the endoplasmic reticulum (ER) lumen—is involved in pollen tube formation and function. We previously cloned and characterized the CRT gene belonging to the CRT1/2 subgroup from Petunia hybrida (PhCRT1/2), and found that post-transcriptional silencing of PhCRT1/2 expression strongly impaired pollen tube growth in vitro. Here, we report cloning of a new PhCRT3a homolog; we identified the full-length cDNA sequence and described its molecular characteristics and phylogenetic relationships to other plant CRT3 genes. Using an RNA interference (RNAi) strategy, we found that knockdown of PhCRT3a gene expression caused numerous defects in the morphology and ultrastructure of cultivated pollen tubes, including disorganization of the actin cytoskeleton and loss of cytoplasmic zonation. Elongation of siPhCRT3a pollen tubes was disrupted, and some of them ruptured. Our present data provide the first evidence that PhCRT3a expression is required for normal pollen tube growth. Thus, we discuss relationships between diverse CRT isoforms in several interdependent processes driving the apical growth of the pollen tube, including actomyosin-dependent cytoplasmic streaming, organelle positioning, vesicle trafficking, and cell wall biogenesis.

AP2/ERF Transcription Factor NbERF-IX-33 Is Involved in the Regulation of Phytoalexin Production for the Resistance of Nicotiana benthamiana to Phytophthora infestans

Plants recognize molecular patterns unique to a certain group of microbes to induce effective resistance mechanisms. Elicitins are secretory proteins produced by plant pathogenic oomycete genera including Phytophthora and Pythium. Treatment of INF1 (an elicitin produced by P. infestans) induces a series of defense responses in Nicotiana species, including reactive oxygen species (ROS) production, transient induction of ethylene production, hypersensitive cell death and accumulation of the sesquiterpenoid phytoalexin capsidiol. In this study, we analyzed the expression profiles of N. benthamiana genes after INF1 treatment by RNAseq analysis. Based on their expression patterns, N. benthamiana genes were categorized into 20 clusters and 4,761 (8.3%) out of 57,140 genes were assigned to the clusters for INF1-induced genes. All genes encoding enzymes dedicated to capsidiol production, 5-epi-aristolochene (EA) synthase (NbEAS, 10 copies) and EA dehydrogenase (NbEAH, 6 copies), and some genes for ethylene production, such as 1-aminocyclopropane 1-carboxylate (ACC) synthase (NbACS) and ACC oxidase (NbACO), were significantly upregulated by INF1 treatment. Analysis of NbEAS1 and NbEAS4 promoters revealed that AGACGCC (GCC box-like motif) is the essential cis-element required for INF1-induced expression of NbEAS genes. Given that the GCC box is known to be targeted by ERF (ethylene-responsive factor) transcription factors, we created a complete list of N. benthamiana genes encoding AP2/ERF family transcription factors, and identified 45 out of 337 AP2/ERF genes in the clusters for INF1-induced genes. Among INF1-induced NbERF genes, silencing of NbERF-IX-33 compromised resistance against P. infestans and INF1-induced production of capsidiol. Recombinant NbERF-IX-33 protein can bind to the promoter sequence of NbEAS4, suggesting that NbERF-IX-33 is a transcription factor directly regulating the expression of genes for phytoalexin production.

HaCRT1 of Heterodera avenae Is Required for the Pathogenicity of the Cereal Cyst Nematode

Cereal cyst nematodes are sedentary biotrophic endoparasites that secrete effector proteins into plant tissues to transit normal cells into specialized feeding sites and suppress plant defenses. To understand the function of nematode effectors in Heterodera avenae, here, we identified a calreticulin protein HaCRT1, which could suppress the cell death induced by Bax when expressed in Nicotiana benthamiana. HaCRT1 is synthetized in the subventral gland cells of pre-parasitic second-stage nematodes. Real-time PCR assays indicated that the expression of HaCRT1 was highest in parasitic second-stage juveniles. The expression of an HaCRT1-RFP fusion in N. benthamiana revealed that it was localized in the endoplasmic reticulum of the plant cell. The ability of H. avenae infecting plants was significantly reduced when HaCRT1 was knocked down by RNA interference in vitro. Arabidopsis thaliana plants expressing HaCRT1 were more susceptible than wild-type plants to Pseudomonas syringae. The induction of defense-related genes, PAD4, WRKY33, FRK1, and WRKY29, after treatment with flg22 was suppressed in HaCRT1-transgenic plants. Also, the ROS accumulation induced by flg22 was reduced in the HaCRT1-transgenic plants compared to wild-type plants. HaCRT1 overexpression increased the cytosolic Ca²⁺ concentration in A. thaliana. These data suggested that HaCRT1 may contribute to the pathogenicity of H. avenae by suppressing host basal defense.

What is the Molecular Basis of Nonhost Resistance?

This article is part of the Top 10 Unanswered Questions in MPMI invited review series.Nonhost resistance is typically considered the ability of a plant species to repel all attempts of a pathogen species to colonize it and reproduce on it. Based on this common definition, nonhost resistance is presumed to be very durable and, thus, of great interest for its potential use in agriculture. Despite considerable research efforts, the molecular basis of this type of plant immunity remains nebulous. We here stress the fact that "nonhost resistance" is a phenomenological rather than a mechanistic concept that comprises more facets than typically considered. We further argue that nonhost resistance essentially relies on the very same genes and pathways as other types of plant immunity, of which some may act as bottlenecks for particular pathogens on a given plant species or under certain conditions. Thus, in our view, the frequently used term "nonhost genes" is misleading and should be avoided. Depending on the plant-pathogen combination, nonhost resistance may involve the recognition of pathogen effectors by host immune sensor proteins, which might give rise to host shifts or host range expansions due to evolutionary-conditioned gains and losses in respective armories. Thus, the extent of nonhost resistance also defines pathogen host ranges. In some instances, immune-related genes can be transferred across plant species to boost defense, resulting in augmented disease resistance. We discuss future routes for deepening our understanding of nonhost resistance and argue that the confusing term "nonhost resistance" should be used more cautiously in the light of a holistic view of plant immunity.

Detoxification of the solanaceous phytoalexins rishitin, lubimin, oxylubimin and solavetivone via a cytochrome P450 oxygenase

Solanaceous plants produce sesquiterpenoid phytoalexins to defend themselves against a variety of pathogens. These toxic compounds are not only harmful to the pathogen but also to the plant, and thus need to be detoxified by the plant after the threat has been eliminated. We report that the detoxification of rishitin, the major phytoalexin in potato tubers and tomato fruits, is mediated by a cytochrome P450 CYP76 family enzyme via the hydroxylation of the isopropenyl group resulting in the formation of 13-hydroxyrishitin, also known as rishitin-M1. We further observed hydroxylation of the potato phytoalexins solavetivone, lubimin and oxylubimin by the same enzyme. Constitutive expression of CYP76 in Nicotiana benthamiana also led to a reduction of the non-potato phytoalexins capsidiol and its derivative capsidiol 3-acetate. We therefore annotated this enzyme as sesquiterpenoid phytoalexins hydroxylase, SPH. This broad range of substrates indicates that SPH functions as a general phytoalexin detoxification enzyme in Solanaceae, and is therefore relevant for a better understanding of plant-pathogen interaction in solanaceous plants, which comprise many economically important crops, such as potato, tomato, eggplant and pepper.

Transcriptional induction of capsidiol synthesis genes by wounding can promote pathogen signal-induced capsidiol synthesis

BACKGROUND

Plants are exposed to various forms of environmental stress. Penetration by pathogens is one of the most serious environmental insults. Wounding caused by tissue damage or herbivory also affects the growth and reproduction of plants. Moreover, wounding disrupts physical barriers present at the plant surface and increases the risk of pathogen invasion. Plants cope with environmental stress by inducing a variety of responses. These stress responses must be tightly controlled, because their unnecessary induction is detrimental to plant growth. In tobacco, WIPK and SIPK, two wound-responsive mitogen-activated protein kinases, have been shown to play important roles in regulating wound responses. However, their contribution to downstream wound responses such as gene expression is not well understood.

RESULTS

To identify genes regulated by WIPK and SIPK, the transcriptome of wounded WIPK/SIPK-suppressed plants was analyzed. Among the genes down-regulated in WIPK/SIPK-suppressed plants, the largest group consisted of those involved in the production of antimicrobial phytoalexins. Almost all genes involved in the biosynthesis of capsidiol, a major phytoalexin in tobacco, were transcriptionally induced by wounding in WIPK/SIPK-dependent and -independent manners. 5-epi-aristolochene synthase (EAS) is the committing enzyme for capsidiol synthesis, and the promoter of EAS4, a member of the EAS family, was analyzed. Reporter gene analysis revealed that at least two regions each 40-50 bp length were involved in activation of the EAS4 promoter by wounding, as well as by artificial activation of WIPK and SIPK. Unlike transcripts of the capsidiol synthesis genes, accumulation of EAS protein and capsidiol itself were not induced by wounding; however, wounding significantly enhanced their subsequent induction by a pathogen-derived elicitor.

CONCLUSIONS

Our results suggest a so-called priming phenomenon since the induction of EAS by wounding is only visible at the transcript level. By inducing transcripts, not the proteins, of EAS and possibly other capsidiol synthesis genes at wound sites, plants can produce large quantities of capsidiol quickly if pathogens invade the wound site, whereas plants can minimize energy loss and avoid the cytotoxic effects of capsidiol where pathogens do not gain entry during wound healing.

The Multifaceted Roles of Plant Hormone Salicylic Acid in Endoplasmic Reticulum Stress and Unfolded Protein Response

Different abiotic and biotic stresses lead to the accumulation of unfolded and misfolded proteins in the endoplasmic reticulum (ER), resulting in ER stress. In response to ER stress, cells activate various cytoprotective responses, enhancing chaperon synthesis, protein folding capacity, and degradation of misfolded proteins. These responses of plants are called the unfolded protein response (UPR). ER stress signaling and UPR can be regulated by salicylic acid (SA), but the mode of its action is not known in full detail. In this review, the current knowledge on the multifaceted role of SA in ER stress and UPR is summarized in model plants and crops to gain a better understanding of SA-regulated processes at the physiological, biochemical, and molecular levels.

Characterization and evolution of gene clusters for terpenoid phytoalexin biosynthesis in tobacco

MAIN CONCLUSION

The study performed genome-wide identification, characterization and evolution analysis of gene clusters for phytoalexin terpenoid biosynthesis in tobacco, and specifically illustrated ones for capsidiol, an efficient defensive specialized metabolite. Terpenoid phytoalexins play an important role in plant self-defense against pest and pathogen attack. Terpenoid biosynthesis involves terpene synthase and cytochrome P450, which always locate and function as cluster(s). In this study, we performed genome-wide investigation of metabolic gene clusters involved in terpenoid production in tobacco (Nicotiana tabacum). Due to the complexity of the tobacco genome, we modified a published prediction pipeline to reduce the influence of the large number of repeats and to improve the annotation of tobacco genes with respect to their metabolic functions. We identified 1181 metabolic gene clusters with 34 of them potentially being involved in terpenoid biosynthesis. Through integration with transcriptome and metabolic pathway annotation analyses, 3 of the 34 terpenoid biosynthesis-related gene clusters were determined to be high-confidence ones, with 2 involved in biosynthesis of capsidiol, a terpenoid recognized as 1 of the effective resistance compounds in the Nicotiana species. The capsidiol-related gene cluster was conserved in N. sylvestris, N. tomentosiformis and N. attenuate. Our findings demonstrate that phytoalexins in tobacco can arise from operon-like gene clusters, a genomic pattern characterized as being beneficial for rapid stress response, gene co-regulation, co-function and co-heredity.

Defense and Counterdefense During Plant-Pathogenic Oomycete Infection

Plant-pathogenic oomycetes include numerous species that are ongoing threats to agriculture and natural ecosystems. Understanding the molecular dialogs between oomycetes and plants is instrumental for sustaining effective disease control. Plants respond to oomycete infection by multiple defense actions including strengthening of physical barriers, production of antimicrobial molecules, and programmed cell death. These responses are tightly controlled and integrated via a three-layered immune system consisting of a multiplex recognition layer, a resilient signal-integration layer, and a diverse defense-action layer. Adapted oomycete pathogens utilize apoplastic and intracellular effector arsenals to counter plant immunity mechanisms within each layer, including by evasion or suppression of recognition, interference with numerous signaling components, and neutralization or suppression of defense actions. A coevolutionary arms race continually drives the emergence of new mechanisms of plant defense and oomycete counterdefense.

RNAi of the sesquiterpene cyclase gene for phytoalexin production impairs pre- and post-invasive resistance to potato blight pathogens

Potato antimicrobial sesquiterpenoid phytoalexins lubimin and rishitin have been implicated in resistance to the late blight pathogen, Phytophthora infestans and early blight pathogen, Alternaria solani. We generated transgenic potato plants in which sesquiterpene cyclase, a key enzyme for production of lubimin and rishitin, is compromised by RNAi to investigate the role of phytoalexins in potato defence. The transgenic tubers were deficient in phytoalexins and exhibited reduced post-invasive resistance to an avirulent isolate of P. infestans, resulting in successful infection of the first attacked cells without induction of cell death. However, cell death was observed in the subsequently penetrated cells. Although we failed to detect phytoalexins and antifungal activity in the extract from wild-type leaves, post-invasive resistance to avirulent P. infestans was reduced in transgenic leaves. On the other hand, A. solani frequently penetrated epidermal cells of transgenic leaves and caused severe disease symptoms presumably from a deficiency in unidentified antifungal compounds. The contribution of antimicrobial components to resistance to penetration and later colonization may vary depending on the pathogen species, suggesting that sesquiterpene cyclase-mediated compounds participate in pre-invasive resistance to necrotrophic pathogen A. solani and post-invasive resistance to hemibiotrophic pathogen P. infestans.

Nicotiana benthamiana RanBP1-1 Is Involved in the Induction of Disease Resistance via Regulation of Nuclear-Cytoplasmic Transport of Small GTPase Ran

Plant cells enhance the tolerances to abiotic and biotic stresses via recognition of the stress, activation and nuclear import of signaling factors, up-regulation of defense genes, nuclear export of mRNA and translation of defense proteins. Nuclear pore-mediated transports should play critical roles in these processes, however, the regulatory mechanisms of nuclear-cytoplasmic transport during stress responses are largely unknown. In this study, a regulator of nuclear export of RNA and proteins, NbRanBP1-1 (Ran-binding protein1-1), was identified as an essential gene for the resistance of Nicotiana benthamiana to potato blight pathogen Phytophthora infestans. NbRanBP1-1-silenced plants showed delayed accumulation of capsidiol, a sesquiterpenoid phytoalexin, in response to elicitor treatment, and reduced resistance to P. infestans. Abnormal accumulation of mRNA was observed in NbRanBP1-1-silenced plants, indicating that NbRanBP1-1 is involved in the nuclear export of mRNA. In NbRanBP1-1-silenced plants, elicitor-induced expression of defense genes, NbEAS and NbWIPK, was not affected in the early stage of defense induction, but the accumulation of NbWIPK protein was reduced. Nuclear export of the small G-protein NbRan1a was activated during the induction of plant defense, whereas this process was compromised in NbRanBP1-1-silenced plants. Silencing of genes encoding the nuclear pore proteins, Nup75 and Nup160, also caused abnormal nuclear accumulation of mRNA, defects in the nuclear export of NbRan1a, and reduced production of capsidiol, resulting in decreased resistance to P. infestans. These results suggest that nuclear export of NbRan is a key event for defense induction in N. benthamiana, and both RanBP1-1 and nucleoporins play important roles in the process.

Measuring Secretion of Capsidiol in Leaf Tissues of Nicotiana benthamiana

Plant species produce a wide variety of antimicrobial metabolites to protect themselves against potential pathogens in natural environments. Phytoalexins are low molecular weight compounds produced by plants in response to attempted attacks of pathogens. Accumulation of phytoalexins in attacked plant tissues can inhibit the growth of penetrating pathogens. Thus phytoalexins play a major role in post-invasion defense against pathogens. Major phytoalexins produced by Solanaceous plants are sesquiterpenoids such as capsidiol produced by Nicotiana and Capsicum species, and rishitin produced by Solanum species, which are synthesized in the cytosol and secreted into the intercellular space of plant tissues. We previously reported that deficiency in capsidiol secretion causes enhanced susceptibility of Nicotiana benthamiana to potato late blight pathogen, Phytophthora infestans. Here, we describe a practical protocol to measure the secreted capsidiol in N. benthamiana.

Expansion of sesquiterpene biosynthetic gene clusters in pepper confers nonhost resistance to the Irish potato famine pathogen

Chemical barriers contribute to nonhost resistance, which is defined as the resistance of an entire plant species to nonadapted pathogen species. However, the molecular basis of metabolic defense in nonhost resistance remains elusive. Here, we report genetic evidence for the essential role of phytoalexin capsidiol in nonhost resistance of pepper (Capsicum spp.) to potato late blight Phytophthora infestans using transcriptome and genome analyses. Two different genes for capsidiol biosynthesis, 5-epi-aristolochene synthase (EAS) and 5-epi-aristolochene-1,3-dihydroxylase (EAH), belong to multigene families. However, only a subset of EAS/EAH gene family members were highly induced upon P. infestans infection, which was associated with parallel accumulation of capsidiol in P. infestans-infected pepper. Silencing of EAS homologs in pepper resulted in a significant decrease in capsidiol accumulation and allowed the growth of nonadapted P. infestans that is highly sensitive to capsidiol. Phylogenetic and genomic analyses of EAS/EAH multigene families revealed that the emergence of pathogen-inducible EAS/EAH genes in Capsicum-specific genomic regions rendered pepper a nonhost of P. infestans. This study provides insights into evolutionary aspects of nonhost resistance based on the combination of a species-specific phytoalexin and sensitivity of nonadapted pathogens.

EIN2-mediated signaling is involved in pre-invasion defense in Nicotiana benthamiana against potato late blight pathogen, Phytophthora infestans

Nicotiana benthamiana ABCG1 and ABCG2 are ABC transporters which are probably involved in the export of capsidiol, the major phytoalexin of Nicotiana species. While capsidiol export by these transporters plays an essential role in post-invasion defense against Phytophthora infestans, they also export unidentified antimicrobial compound(s) involved in pre-invasion defense. In this study, promoter activity of NbABCG2 (Pabcg2a) was analyzed using a GFP marker. Expression of GFP under the control of Pabcg2a was significantly increased by co-expression with the INF1 elicitor from P. infestans. Disruption of the ethylene-responsive GCC box in Pabcg2a compromised INF1-induced activation of Pabcg2a. Consistently, penetration by P. infestans was increased by gene-silencing of NbEIN2, the key ethylene-signaling component, suggesting the involvement of ethylene for pre-invasion defense of N. benthamiana.

Current Understandings of Plant Nonhost Resistance

Nonhost resistance, a resistance of plant species against all nonadapted pathogens, is considered the most durable and efficient immune system of plants but yet remains elusive. The underlying mechanism of nonhost resistance has been investigated at multiple levels of plant defense for several decades. In this review, we have comprehensively surveyed the latest literature on nonhost resistance in terms of preinvasion, metabolic defense, pattern-triggered immunity, effector-triggered immunity, defense signaling, and possible application in crop protection. Overall, we summarize the current understanding of nonhost resistance mechanisms. Pre- and postinvasion is not much deviated from the knowledge on host resistance, except for a few specific cases. Further insights on the roles of the pattern recognition receptor gene family, multiple interactions between effectors from nonadapted pathogen and plant factors, and plant secondary metabolites in host range determination could expand our knowledge on nonhost resistance and provide efficient tools for future crop protection using combinational biotechnology approaches. [Formula: see text] Copyright © 2017 The Author(s). This is an open access article distributed under the CC BY-NC-ND 4.0 International license .

RTP1 encodes a novel endoplasmic reticulum (ER)-localized protein in Arabidopsis and negatively regulates resistance against biotrophic pathogens

Oomycete pathogens cause serious damage to a wide spectrum of plants. Although host pathogen recognition via pathogen effectors and cognate plant resistance proteins is well established, the genetic basis of host factors that mediate plant susceptibility to oomycete pathogens is relatively unexplored. Here, we report on RTP1, a nodulin-related MtN21 family gene in Arabidopsis that mediates susceptibility to Phytophthora parasitica. RTP1 was identified by screening a T-DNA insertion mutant population and encoded an endoplasmic reticulum (ER)-localized protein. Overexpression of RTP1 rendered Arabidopsis more susceptible, whereas RNA silencing of RTP1 led to enhanced resistance to P. parasitica. Moreover, an RTP1 mutant, rtp1-1, displayed localized cell death, increased reactive oxygen species (ROS) production and accelerated PR1 expression, compared to the wild-type Col-0, in response to P. parasitica infection. rtp1-1 showed a similar disease response to the bacterial pathogen Pseudomonas syringae pv. tomato (Pst) DC3000, including increased disease resistance, cell death and ROS production. Furthermore, rpt1-1 exhibited resistance to the fungal pathogen Golovinomyces cichoracearum, but not to the necrotrophic pathogen Botrytis cinerea. Taken together, these results suggest that RTP1 negatively regulates plant resistance to biotrophic pathogens, possibly by regulating ROS production, cell death progression and PR1 expression.

A terpenoid phytoalexin plays a role in basal defense of Nicotiana benthamiana against Potato virus X

Terpenoid phytoalexins function as defense compound against a broad spectrum of pathogens and pests in the plant kingdom. However, the role of phytoalexin in antiviral defense is still elusive. In this study, we identified the biosynthesis pathway of a sesquiterpenoid phytoalexin, capsidiol 3-acetate as an antiviral response against RNA virus Potato Virus X (PVX) in Nicotiana benthamiana. NbTPS1 and NbEAH genes were found strongly induced by PVX-infection. Enzymatic activity and genetic evidence indicated that both genes were involved in the PVX-induced biosynthesis of capsidiol 3-acetate. NbTPS1- or NbEAH-silenced plant was more susceptible to PVX. The accumulation of capsidiol 3-acetate in PVX-infected plant was partially regulated by jasmonic acid signaling receptor COI1. These findings provide an insight into a novel mechanism of how plant uses the basal arsenal machinery to mount a fight against virus attack even in susceptible species.

Insights into animal and plant lectins with antimicrobial activities

Lectins are multivalent proteins with the ability to recognize and bind diverse carbohydrate structures. The glyco -binding and diverse molecular structures observed in these protein classes make them a large and heterogeneous group with a wide range of biological activities in microorganisms, animals and plants. Lectins from plants and animals are commonly used in direct defense against pathogens and in immune regulation. This review focuses on sources of animal and plant lectins, describing their functional classification and tridimensional structures, relating these properties with biotechnological purposes, including antimicrobial activities. In summary, this work focuses on structural-functional elucidation of diverse lectin groups, shedding some light on host-pathogen interactions; it also examines their emergence as biotechnological tools through gene manipulation and development of new drugs.

Nucleoporin 75 is involved in the ethylene-mediated production of phytoalexin for the resistance of Nicotiana benthamiana to Phytophthora infestans

Mature Nicotiana benthamiana shows stable resistance to the oomycete pathogen Phytophthora infestans. Induction of phytoalexin (capsidiol) production is essential for the resistance, which is upregulated via a mitogen-activated protein kinase (MAPK) cascade (NbMEK2-WIPK/SIPK) followed by ethylene signaling. In this study, NbNup75 (encodes a nuclear pore protein Nucleoporin75) was identified as an essential gene for resistance of N. benthamiana to P. infestans. In NbNup75-silenced plants, initial events of elicitor-induced responses such as phosphorylation of MAPK and expression of defense-related genes were not affected, whereas induction of later defense responses such as capsidiol production and cell death induction was suppressed or delayed. Ethylene production induced by either INF1 or NbMEK2 was reduced in NbNup75-silenced plants, whereas the expression of NbEAS (a gene for capsidiol biosynthesis) induced by ethylene was not affected, indicating that Nup75 is required for the induction of ethylene production but not for ethylene signaling. Given that nuclear accumulation of polyA RNA was increased in NbNup75-silenced plants, efficient export of mRNA from nuclei via nuclear pores would be important for the timely upregulation of defense responses. Collectively, Nup75 is involved in the induction of a later stage of defense responses, including the ethylene-mediated production of phytoalexin for the resistance of N. benthamiana to P. infestans.

Variation in capsidiol sensitivity between Phytophthora infestans and Phytophthora capsici is consistent with their host range

Plants protect themselves against a variety of invading pathogenic organisms via sophisticated defence mechanisms. These responses include deployment of specialized antimicrobial compounds, such as phytoalexins, that rapidly accumulate at pathogen infection sites. However, the extent to which these compounds contribute to species-level resistance and their spectrum of action remain poorly understood. Capsidiol, a defense related phytoalexin, is produced by several solanaceous plants including pepper and tobacco during microbial attack. Interestingly, capsidiol differentially affects growth and germination of the oomycete pathogens Phytophthora infestans and Phytophthora capsici, although the underlying molecular mechanisms remain unknown. In this study we revisited the differential effect of capsidiol on P. infestans and P. capsici, using highly pure capsidiol preparations obtained from yeast engineered to express the capsidiol biosynthetic pathway. Taking advantage of transgenic Phytophthora strains expressing fluorescent markers, we developed a fluorescence-based method to determine the differential effect of capsidiol on Phytophtora growth. Using these assays, we confirm major differences in capsidiol sensitivity between P. infestans and P. capsici and demonstrate that capsidiol alters the growth behaviour of both Phytophthora species. Finally, we report intraspecific variation within P. infestans isolates towards capsidiol tolerance pointing to an arms race between the plant and the pathogens in deployment of defence related phytoalexins.

The alternative Medicago truncatula defense proteome of ROS-defective transgenic roots during early microbial infection

ROP-type GTPases of plants function as molecular switches within elementary signal transduction pathways such as the regulation of ROS synthesis via activation of NADPH oxidases (RBOH-respiratory burst oxidase homolog in plants). Previously, we reported that silencing of the Medicago truncatula GTPase MtROP9 led to reduced ROS production and suppressed induction of ROS-related enzymes in transgenic roots (MtROP9i) infected with pathogenic (Aphanomyces euteiches) and symbiotic microorganisms (Glomus intraradices, Sinorhizobium meliloti). While fungal infections were enhanced, S. meliloti infection was drastically impaired. In this study, we investigate the temporal proteome response of M. truncatula MtROP9i transgenic roots during the same microbial interactions under conditions of deprived potential to synthesize ROS. In comparison with control roots (Mtvector), we present a comprehensive proteomic analysis using sensitive MS protein identification. For four early infection time-points (1, 3, 5, 24 hpi), 733 spots were found to be different in abundance: 213 spots comprising 984 proteins (607 unique) were identified after S. meliloti infection, 230 spots comprising 796 proteins (580 unique) after G. intraradices infection, and 290 spots comprising 1240 proteins (828 unique) after A. euteiches infection. Data evaluation by GelMap in combination with a heatmap tool allowed recognition of key proteome changes during microbial interactions under conditions of hampered ROS synthesis. Overall, the number of induced proteins in MtROP9i was low as compared with controls, indicating a dual function of ROS in defense signaling as well as alternative response patterns activated during microbial infection. Qualitative analysis of induced proteins showed that enzymes linked to ROS production and scavenging were highly induced in control roots, while in MtROP9i the majority of proteins were involved in alternative defense pathways such as cell wall and protein degradation.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.

ER-mediated control for abundance, quality, and signaling of transmembrane immune receptors in plants

Plants recognize a wide range of microbes with cell-surface and intracellular immune receptors. Transmembrane pattern recognition receptors (PRRs) initiate immune responses upon recognition of cognate ligands characteristic of microbes or aberrant cellular states, designated microbe-associated molecular patterns or danger-associated molecular patterns (DAMPs), respectively.Pattern-triggered immunity provides a first line of defense that restricts the invasion and propagation of both adapted and non-adapted pathogens. Receptor kinases (RKs) and receptor-like proteins (RLPs) with an extracellular leucine-rich repeat or lysine-motif (LysM) domain are extensively used as PRRs. The correct folding of the extracellular domain of these receptors is under quality control (QC) in the endoplasmic reticulum (ER), which thus provides a critical step in plant immunity. Genetic and structural insight suggests that ERQC regulates not only the abundance and quality of transmembrane receptors but also affects signal sorting between multi-branched pathways downstream of the receptor. However, ERQC dysfunction can also positively stimulate plant immunity, possibly through cell death and DAMP signaling pathways.

The ER quality control and ER associated degradation machineries are vital for viral pathogenesis

The endoplasmic reticulum (ER) is central to protein production and membrane lipid synthesis. The unfolded protein response (UPR) supports cellular metabolism by ensuring protein quality control in the ER. Most positive strand RNA viruses cause extensive remodeling of membranes and require active membrane synthesis to promote infection. How viruses interact with the cellular machinery controlling membrane metabolism is largely unknown. Furthermore, there is mounting data pointing to the importance of the UPR and ER associated degradation (ERAD) machineries in viral pathogenesis in eukaryotes emerging topic. For many viruses, the UPR is an early event that is essential for persistent infection and benefits virus replication. In addition, many viruses are reported to commandeer ER resident chaperones to contribute to virus replication and intercellular movement. In particular, calreticulin, the ubiquitin machinery, and the 26S proteasome are most commonly identified components of the UPR and ERAD machinery that also regulate virus infection. In addition, researchers have noted a link between UPR and autophagy. It is well accepted that positive strand RNA viruses use autophagic membranes as scaffolds to support replication and assembly. However this topic has yet to be explored using plant viruses. The goal of research on this topic is to uncover how viruses interact with this ER-related machinery and to use this information for designing novel strategies to boost immune responses to virus infection.