NbHB1, Nicotiana benthamiana homeobox 1, is a jasmonic acid-dependent positive regulator of pathogen-induced plant cell death

Cell death resulted from loss of fumarylacetoacetate hydrolase in Arabidopsis is related to phytohormone jasmonate but not salicylic acid

Fumarylacetoacetate hydrolase (FAH) catalyzes the final step in Tyr degradation pathway essential to animals but not well understood in plants. Previously, we found that mutation of SSCD1 encoding Arabidopsis FAH causes cell death under short day, which uncovered an important role of Tyr degradation pathway in plants. Since phytohormones salicylic acid (SA) and jasmonate (JA) are involved in programmed cell death, in this study, we investigated whether sscd1 cell death is related to SA and JA, and found that (1) it is accompanied by up-regulation of JA- and SA-inducible genes as well as accumulation of JA but not SA; (2) it is repressed by breakdown of JA signaling but not SA signaling; (3) the up-regulation of reactive oxygen species marker genes in sscd1 is repressed by breakdown of JA signaling; (4) treatment of wild-type Arabidopsis with succinylacetone, an abnormal metabolite caused by loss of FAH, induces expression of JA-inducible genes whereas treatment with JA induces expression of some Tyr degradation genes with dependence of JA signaling. These results demonstrated that cell death resulted from loss of FAH in Arabidopsis is related to JA but not SA, and suggested that JA signaling positively regulates sscd1 cell death by up-regulating Tyr degradation.

GhHB12, a HD-ZIP I Transcription Factor, Negatively Regulates the Cotton Resistance to Verticillium dahliae

The homeodomain-leucine zipper (HD-ZIP) is a plant-specific transcription factor family that plays important roles in plant developmental processes in response to multiple stressors. We previously isolated a cotton HD-ZIP class I transcription factor gene, GhHB12, which is regulated by the circadian clock and photoperiodism. Furthermore, it regulates cotton architecture, phase transition, and photoperiod sensitivity. Here we report that GhHB12 was induced by methyl jasmonate (MeJA) and Verticillium dahliae infection. Additionally, stress-responsive elements were found in the GhHB12 promoter. Promoter fusion analysis showed that GhHB12 was predominantly expressed in primary roots and that it was induced by mechanical damage. Overexpression of GhHB12 increased susceptibility of the cotton plant to the fungal pathogens Botrytis cinerea and V. dahliae, which was coupled with suppression of the jasmonic acid (JA)-response genes GhJAZ2 and GhPR3. Our results suggest that GhHB12, a cotton stress-responsive HD-ZIP I transcription factor, negatively regulates cotton resistance to V. dahliae by suppressing JA-response genes.

The Coiled-Coil and Leucine-Rich Repeat Domain of the Potyvirus Resistance Protein Pvr4 Has a Distinct Role in Signaling and Pathogen Recognition

The pepper Pvr4 protein encoding coiled-coil (CC) nucleotide-binding (NB) leucine-rich repeat (LRR) (NLR) confer hypersensitive response (HR) to potyviruses, including Pepper mottle virus (PepMoV), by recognizing the viral avirulence protein NIb. To figure out the Pvr4-mediated HR mechanism, we analyzed signaling component genes and structure-function relationships of Pvr4, using chimeras and deletion mutants in Nicotiana benthamiana. Molecular chaperone components including HSP90, SGT1, and RAR1 were required, while plant hormones and mitogen-activated protein kinase signaling components had little effect on Pvr4-NIb-mediated HR cell death. Domain swap analyses indicated that the LRR domain of Pvr4 determines recognition of PepMoV-NIb. Our deletion analysis further revealed that the CC domain or CC-NBARC domain alone can trigger autoactive cell death in N. benthamiana. However, the fragments having only an LRR domain could suppress CC-NBARC domain-induced cell death in trans. Further, C-terminal truncation analysis of Pvr4 revealed that a minimum three of five LRR exons showing high similarity was essential for Pvr4 function. The LRR domain may maintain Pvr4 in an inactive state in the absence of NIb. These results provide further insight into the structure and function of NLR protein signaling in plants.

A Meloidogyne incognita effector MiISE5 suppresses programmed cell death to promote parasitism in host plant

Root-knot nematodes (RKNs) are highly specialized parasites that interact with their host plants using a range of strategies. The esophageal glands are the main places where nematodes synthesize effector proteins, which play central roles in successful invasion. The Meloidogyne incognita effector MiISE5 is exclusively expressed within the subventral esophageal cells and is upregulated during early parasitic stages. In this study, we show that MiISE5 can be secreted to barley cells through infectious hyphae of Magnaporthe oryzae. Transgenic Arabidopsis plants expressing MiISE5 became significantly more susceptible to M. incognita. Inversely, the tobacco rattle virus (TRV)-mediated silence of MiISE5 decreased nematode parasitism. Moreover, transient expression of MiISE5 suppressed cell death caused by Burkholderia glumae in Nicotiana benthamiana. Based on transcriptome analysis of MiISE5 transgenic sample and the wild-type (WT) sample, we obtained 261 DEGs, and the results of GO and KEGG enrichment analysis indicate that MiISE5 can interfere with various metabolic and signaling pathways, especially the JA signaling pathway, to facilitate nematode parasitism. Results from the present study suggest that MiISE5 plays an important role during the early stages of parasitism and provides evidence to decipher the molecular mechanisms underlying the manipulation of host immune defense responses by M. incognita.

Overcompensation of herbivore reproduction through hyper-suppression of plant defenses in response to competition

Spider mites are destructive arthropod pests on many crops. The generalist herbivorous mite Tetranychus urticae induces defenses in tomato (Solanum lycopersicum) and this constrains its fitness. By contrast, the Solanaceae-specialist Tetranychus evansi maintains a high reproductive performance by suppressing tomato defenses. Tetranychus evansi outcompetes T. urticae when infesting the same plant, but it is unknown whether this is facilitated by the defenses of the plant. We assessed the extent to which a secondary infestation by a competitor affects local plant defense responses (phytohormones and defense genes), mite gene expression and mite performance. We observed that T. evansi switches to hyper-suppression of defenses after its tomato host is also invaded by its natural competitor T. urticae. Jasmonate (JA) and salicylate (SA) defenses were suppressed more strongly, albeit only locally at the feeding site of T. evansi, upon introduction of T. urticae to the infested leaflet. The hyper-suppression of defenses coincided with increased expression of T. evansi genes coding for salivary defense-suppressing effector proteins and was paralleled by an increased reproductive performance. Together, these observations suggest that T. evansi overcompensates its reproduction through hyper-suppression of plant defenses in response to nearby competitors. We hypothesize that the competitor-induced overcompensation promotes competitive population growth of T. evansi on tomato.

RepA Protein Encoded by Oat dwarf virus Elicits a Temperature-Sensitive Hypersensitive Response-Type Cell Death That Involves Jasmonic Acid-Dependent Signaling

The hypersensitive response (HR) is a component of disease resistance that is often induced by pathogen infection, but essentially no information is available for members of the destructive mastreviruses. We have investigated an HR-type response elicited in Nicotiana species by Oat dwarf virus (ODV) and have found that expression of the ODV RepA protein but not other ODV-encoded proteins elicits the HR-type cell death associated with a burst of H2O2. Deletion mutagenesis indicates that the first nine amino acids (aa) at the N terminus of RepA and the two regions located between aa residues 173 and 195 and between aa residues 241 and 260 near the C terminus are essential for HR-type cell-death elicitation. Confocal and electron microscopy showed that the RepA protein is localized in the nuclei of plant cells and might contain bipartite nuclear localization signals. The HR-like lesions mediated by RepA were inhibited by temperatures above 30°C and involvement of jasmonic acid (JA) in HR was identified by gain- and loss-of-function experiments. To our knowledge, this is the first report of an elicitor of HR-type cell death from mastreviruses.

Hijacking of the jasmonate pathway by the mycotoxin fumonisin B1 (FB1) to initiate programmed cell death in Arabidopsis is modulated by RGLG3 and RGLG4

The mycotoxin fumonisin B1 (FB1) is a strong inducer of programmed cell death (PCD) in plants, but its underlying mechanism remains unclear. Here, we describe two ubiquitin ligases, RING DOMAIN LIGASE3 (RGLG3) and RGLG4, which control FB1-triggered PCD by modulating the jasmonate (JA) signalling pathway in Arabidopsis thaliana. RGLG3 and RGLG4 transcription was sensitive to FB1. Arabidopsis FB1 sensitivity was suppressed by loss of function of RGLG3 and RGLG4 and was increased by their overexpression. Thus RGLG3 and RGLG4 have coordinated and positive roles in FB1-elicited PCD. Mutated JA perception by coi1 disrupted the RGLG3- and RGLG4-related response to FB1 and interfered with their roles in cell death. Although FB1 induced JA-responsive defence genes, it repressed growth-related, as well as JA biosynthesis-related, genes. Consistently, FB1 application reduced JA content in wild-type plants. Furthermore, exogenously applied salicylic acid additively suppressed JA signalling with FB1 treatment, suggesting that FB1-induced salicylic acid inhibits the JA pathway during this process. All of these effects were attenuated in rglg3 rglg4 plants. Altogether, these data suggest that the JA pathway is hijacked by the toxin FB1 to elicit PCD, which is coordinated by Arabidopsis RGLG3 and RGLG4.

Mp10 and Mp42 from the aphid species Myzus persicae trigger plant defenses in Nicotiana benthamiana through different activities

Aphids are phloem-feeding insects that, like other plant parasites, deliver effectors inside their host to manipulate host responses. The Myzus persicae (green peach aphid) candidate effectors Mp10 and Mp42 were previously found to reduce aphid fecundity upon intracellular transient overexpression in Nicotiana benthamiana. We performed functional analyses of these proteins to investigate whether they activate defenses through similar activities. We employed a range of functional characterization experiments based on intracellular transient overexpression in N. benthamiana to determine the subcellular localization of Mp10 and Mp42 and investigate their role in activating plant defense signaling. Mp10 and Mp42 showed distinct subcellular localization in planta, suggesting that they target different host compartments. Also, Mp10 reduced the levels of Agrobacterium-mediated overexpression of proteins. This reduction was not due to an effect on Agrobacterium viability. Transient overexpression of Mp10 but not Mp42 activated jasmonic acid and salicylic acid signaling pathways and decreased susceptibility to the hemibiotrophic plant pathogen Phytophthora capsici. We found that two candidate effectors from the broad-host-range aphid M. persicae can trigger aphid defenses through different mechanisms. Importantly, we found that some (candidate) effectors such as Mp10 interfere with Agrobacterium-based overexpression assays, an important tool to study effector activity and function.

Comparative transcriptome analysis of pepper (Capsicum annuum) revealed common regulons in multiple stress conditions and hormone treatments

Global transcriptome analysis revealed common regulons for biotic/abiotic stresses, and some of these regulons encoding signaling components in both stresses were newly identified in this study. In this study, we aimed to identify plant responses to multiple stress conditions and discover the common regulons activated under a variety of stress conditions. Global transcriptome analysis revealed that salicylic acid (SA) may affect the activation of abiotic stress-responsive genes in pepper. Our data indicate that methyl jasmonate (MeJA) and ethylene (ET)-responsive genes were primarily activated by biotic stress, while abscisic acid (ABA)-responsive genes were activated under both types of stresses. We also identified differentially expressed gene (DEG) responses to specific stress conditions. Biotic stress induces more DEGs than those induced by abiotic and hormone applications. The clustering analysis using DEGs indicates that there are common regulons for biotic or abiotic stress conditions. Although SA and MeJA have an antagonistic effect on gene expression levels, SA and MeJA show a largely common regulation as compared to the regulation at the DEG expression level induced by other hormones. We also monitored the expression profiles of DEG encoding signaling components. Twenty-two percent of these were commonly expressed in both stress conditions. The importance of this study is that several genes commonly regulated by both stress conditions may have future applications for creating broadly stress-tolerant pepper plants. This study revealed that there are complex regulons in pepper plant to both biotic and abiotic stress conditions.

A pepper (Capsicum annuum L.) metacaspase 9 (Camc9) plays a role in pathogen-induced cell death in plants

Metacaspases, which belong to the cysteine-type C14 protease family, are most structurally similar to mammalian caspases than any other caspase-like protease in plants. Atmc9 (Arabidopsis thaliana metacaspase 9) has a unique domain structure, and distinct biochemical characteristics, such as Ca²⁺ binding, pH, redox status, S-nitrosylation and specific protease inhibitors. However, the biological roles of Atmc9 in plant-pathogen interactions remain largely unknown. In this study, a metacaspase gene present as a single copy in the pepper genome, and sharing 54% amino acid sequence identity with Atmc9, was isolated and named Capsicum annuum metacaspase 9 (Camc9). Camc9 encodes a 318-amino-acid polypeptide with an estimated molecular weight of 34.6 kDa, and shares approximately 40% amino acid sequence identity with known type II metacaspases in plants. Quantitative reverse transcription-polymerase chain reaction analyses revealed that the expression of Camc9 was induced by infections of Xanthomonas campestris pv. vesicatoria race 1 and race 3 and treatment with methyl jasmonate. Suppression of Camc9 expression using virus-induced gene silencing enhanced disease resistance and suppressed cell death symptom development following infection with virulent bacterial pathogens. By contrast, overexpression of Camc9 by transient or stable transformation enhanced disease susceptibility and pathogen-induced cell death by regulation of reactive oxygen species production and defence-related gene expression. These results suggest that Camc9 is a possible member of the metacaspase gene family and plays a role as a positive regulator of pathogen-induced cell death in the plant kingdom.

Multiple classes of immune-related proteases associated with the cell death response in pepper plants

Proteases regulate a large number of biological processes in plants, such as metabolism, physiology, growth, and defense. In this study, we carried out virus-induced gene silencing assays with pepper cDNA clones to elucidate the biological roles of protease superfamilies. A total of 153 representative protease genes from pepper cDNA were selected and cloned into a Tobacco rattle virus-ligation independent cloning vector in a loss-of-function study. Silencing of 61 proteases resulted in altered phenotypes, such as the inhibition of shoot growth, abnormal leaf shape, leaf color change, and lethality. Furthermore, the silencing experiments revealed that multiple proteases play a role in cell death and immune response against avirulent and virulent pathogens. Among these 153 proteases, 34 modulated the hypersensitive cell death response caused by infection with an avirulent pathogen, and 16 proteases affected disease symptom development caused by a virulent pathogen. Specifically, we provide experimental evidence for the roles of multiple protease genes in plant development and immune defense following pathogen infection. With these results, we created a broad sketch of each protease function. This information will provide basic information for further understanding the roles of the protease superfamily in plant growth, development, and defense.

Gene expression responses to Rice tungro spherical virus in susceptible and resistant near-isogenic rice plants

Rice cultivar Taichung Native 1 (TN1) is susceptible to Rice tungro spherical virus (RTSV). TW16 is a backcross line developed between TN1 and RTSV-resistant cultivar Utri Merah. RTSV accumulation in TW16 was significantly lower than in TN1, although both TN1 and TW16 remained asymptomatic. We compared the gene expression profiles of TN1 and TW16 infected by RTSV to identify the gene expression patterns accompanying the accumulation and suppression of RTSV. About 11% and 12% of the genes in the entire genome were found differentially expressed by RTSV in TN1 and TW16, respectively. About 30% of the differentially expressed genes (DEGs) were detected commonly in both TN1 and TW16. DEGs related to development and stress response processes were significantly overrepresented in both TN1 and TW16. Evident differences in gene expression between TN1 and TW16 instigated by RTSV included (1) suppression of more genes for development-related transcription factors in TW16; (2) activation of more genes for development-related peptide hormone RALF in TN1; (3) TN1- and TW16-specific regulation of genes for jasmonate synthesis and pathway, and genes for stress-related transcription factors such as WRKY, SNAC, and AP2-EREBP; (4) activation of more genes for glutathione S-transferase in TW16; (5) activation of more heat shock protein genes in TN1; and (6) suppression of more genes for Golden2-like transcription factors involved in plastid development in TN1. The results suggest that a significant number of defense and development-related genes are still regulated in asymptomatic plants even with a very low level of RTSV, and that the TN1- and TW16-specific gene regulations might be associated with regulation of RTSV accumulation in the plants.

Ectopic expression of Capsicum-specific cell wall protein Capsicum annuum senescence-delaying 1 (CaSD1) delays senescence and induces trichome formation in Nicotiana benthamiana

Secreted proteins are known to have multiple roles in plant development, metabolism, and stress response. In a previous study to understand the roles of secreted proteins, Capsicum annuum secreted proteins (CaS) were isolated by yeast secretion trap. Among the secreted proteins, we further characterized Capsicum annuum senescence-delaying 1 (CaSD1), a gene encoding a novel secreted protein that is present only in the genus Capsicum. The deduced CaSD1 contains multiple repeats of the amino acid sequence KPPIHNHKPTDYDRS. Interestingly, the number of repeats varied among cultivars and species in the Capsicum genus. CaSD1 is constitutively expressed in roots, and Agrobacterium-mediated transient overexpression of CaSD1 in Nicotiana benthamiana leaves resulted in delayed senescence with a dramatically increased number of trichomes and enlarged epidermal cells. Furthermore, senescence- and cell division-related genes were differentially regulated by CaSD1-overexpressing plants. These observations imply that the pepper-specific cell wall protein CaSD1 plays roles in plant growth and development by regulating cell division and differentiation.

A common plant cell-wall protein HyPRP1 has dual roles as a positive regulator of cell death and a negative regulator of basal defense against pathogens

Although hybrid proline-rich proteins (HyPRPs) are ubiquitous in plants, little is known about their roles other than as cell-wall structural proteins. We identified the gene HyPRP1 in Capsicum annuum and Nicotiana benthamiana, which encodes a protein containing proline-rich domain and eight-cysteine motif (8CM) that is constitutively expressed in various organs, mostly in the root, but is down-regulated upon inoculation with either incompatible or compatible pathogens. Ectopic expression of HyPRP1 in plants accelerated cell death, showing developmental abnormality with down-regulation of ROS-scavenging genes, and enhanced pathogen susceptibility suppressing expression of defense-related genes. Conversely, silencing of HyPRP1 suppressed pathogen-induced cell death, but enhanced disease resistance, with up-regulation of defense-related genes and inhibition of in planta growth of bacterial pathogens independently of signal molecule-mediated pathways. Furthermore, the secreted 8CM was sufficient for these HyPRP1 functions. Together, our results suggest that a common plant cell-wall structural protein, HyPRP1, performs distinct dual roles in positive regulation of cell death and negative regulation of basal defense against pathogen.

HAHB10, a sunflower HD-Zip II transcription factor, participates in the induction of flowering and in the control of phytohormone-mediated responses to biotic stress

The transcription factor HAHB10 belongs to the sunflower (Helianthus annuus) HD-Zip II subfamily and it has been previously associated with the induction of flowering. In this study it is shown that HAHB10 is expressed in sunflower leaves throughout the vegetative stage and in stamens during the reproductive stage. In short-day inductive conditions the expression of this gene is induced in shoot apexes together with the expression of the flowering genes HAFT and HAAP1. Transgenic Arabidopsis plants expressing HAHB10 cDNA under regulation either by its own promoter or by cauliflower mosaic virus (CaMV) 35S exhibited an early flowering phenotype. This phenotype was completely reverted in a non-inductive light regime, indicating a photoperiod-dependent action for this transcription factor. Gene expression profiling of Arabidopsis plants constitutively expressing HAHB10 indicated that specific flowering transition genes such as FT, FUL, and SEP3 were induced several fold, whereas genes related to biotic stress responses, such as PR1, PR2, ICS1, AOC1, EDS5, and PDF1-2a, were repressed. The expression of HAHB10 and of the flowering genes HASEP3 and HAFT was up-regulated by both salicylic acid (SA) treatment and infection with a virulent strain of Pseudomonas syringae. Basal SA and jasmonic acid (JA) levels in Arabidopsis plants ectopically expressing HAHB10 were similar to those of control plants; however, SA levels differentially increased in the transgenic plants after wounding and infection with P. syringae while JA levels differentially decreased. Taken together, the results indicated that HAHB10 participates in two different processes in plants: the transition from the vegetative to the flowering stage via the induction of specific flowering transition genes and the accumulation of phytohormones upon biotic stresses.