Activation of a mitogen-activated protein kinase pathway is involved in disease resistance in tobacco

Salicylic Acid Alleviated Salt Damage of Populus euphratica: A Physiological and Transcriptomic Analysis

Populus euphratica Oliv. is a model tree for studying abiotic stress, especially salt stress response. Salt stress is one of the most extensive abiotic stresses, which has an adverse effect on plant growth and development. Salicylic acid (SA) is an important signaling molecule that plays an important role in modulating the plant responses to abiotic stresses. To answer whether the endogenous SA can be induced by salt stress, and whether SA effectively alleviates the negative effects of salt on poplar growth is the main purpose of the study. To elucidate the effects of SA and salt stress on the growth of P. euphratica, we examined the morphological and physiological changes of P. euphratica under 300 mM NaCl after treatment with different concentrations of SA. A pretreatment of P. euphratica with 0.4 mM SA for 3 days effectively improved the growth status of plants under subsequent salt stress. These results indicate that appropriate concentrations of exogenous SA can effectively counteract the negative effect of salt stress on growth and development. Subsequently, transcripts involved in salt stress response via SA signaling were captured by RNA sequencing. The results indicated that numerous specific genes encoding mitogen-activated protein kinase, calcium-dependent protein kinase, and antioxidant enzymes were upregulated. Potassium transporters and Na+/H+ antiporters, which maintain K+/Na+ balance, were also upregulated after SA pretreatment. The transcriptome changes show that the ion transport and antioxidant enzymes were the early enhanced systems in response of P. euphratica to salt via SA, expanding our knowledge about SA function in salt stress defense in P. euphratica. This provides a solid foundation for future study of functional genes controlling effective components in metabolic pathways of trees.

The complete chloroplast genome of the threatened Dipentodon sinicus (Dipentodontaceae)

Dipentodon is a monotypic genus of Dipentodontaceae and the only species, Dipentodon sinicus, is scattered in southwest China as well as adjacent Myanmar, northeast India and northern Vietnam. This species was evaluated as vulnerable in 'China Species Red List'. Here, we assembled and characterized the complete chloroplast (cp) genome of D. sinicus using Illumina sequencing data for the first time. The complete cp genome was 158,795 bp in length, consisting of a pair of inverted repeats of 26,587 bp, a large single-copy region of 87,233 bp and a small single-copy region of 18,388 bp. The genome encoded 113 unique genes, including 79 protein-coding genes, 30 tRNA genes and four rRNA genes. Phylogenetic analysis based on 16 complete cp genome sequences indicated that D. sinicus is a member of Huerteales, consistent with its position in the latest classification of flowering plants (AGP IV).

Biochemical properties and in planta effects of NopM, a rhizobial E3 ubiquitin ligase

Nodulation outer protein M (NopM) is an IpaH family type three (T3) effector secreted by the nitrogen-fixing nodule bacterium Sinorhizobium sp. strain NGR234. Previous work indicated that NopM is an E3 ubiquitin ligase required for an optimal symbiosis between NGR234 and the host legume Lablab purpureus Here, we continued to analyze the function of NopM. Recombinant NopM was biochemically characterized using an in vitro ubiquitination system with Arabidopsis thaliana proteins. In this assay, NopM forms unanchored polyubiquitin chains and possesses auto-ubiquitination activity. In a NopM variant lacking any lysine residues, auto-ubiquitination was not completely abolished, indicating noncanonical auto-ubiquitination of the protein. In addition, we could show intermolecular ubiquitin transfer from NopM to C338A (enzymatically inactive NopM form) in vitro Bimolecular fluorescence complementation analysis provided clues about NopM-NopM interactions at plasma membranes in planta NopM, but not C338A, expressed in tobacco cells induced cell death, suggesting that E3 ubiquitin ligase activity of NopM induced effector-triggered immunity responses. Likewise, expression of NopM in Lotus japonicus caused reduced nodule formation, whereas expression of C338A showed no obvious effects on symbiosis. Further experiments indicated that serine residue 26 of NopM is phosphorylated in planta and that NopM can be phosphorylated in vitro by salicylic acid-induced protein kinase (NtSIPK), a mitogen-activated protein kinase (MAPK) of tobacco. Hence, NopM is a phosphorylated T3 effector that can interact with itself, with ubiquitin, and with MAPKs.

MAP kinase signalling: interplays between plant PAMP- and effector-triggered immunity

In plants, mitogen-activated protein kinase (MAPK) cascades are involved in regulating many biological processes including immunity. They relay signals from membrane-residing immune receptors to downstream components for defense activation. Arabidopsis MPK3/6 and MPK4 are activated in two parallel MAPK cascades during PAMP-triggered immunity. MPK3/6 have been implicated in the activation of various immune responses and their inactivation leads to compromised defense against pathogens. On the other hand, the MEKK1-MKK1/2-MPK4 cascade plays critical roles in basal resistance. Disruption of this MAPK cascade results in constitutive defense responses mediated by the NB-LRR protein SUMM2. Interestingly, SUMM2 guards the MEKK1-MKK1/2-MPK4 cascade activity indirectly through monitoring the phosphorylation status of CRCK3, which is a substrate of MPK4. From the pathogens' side, a number of effectors are shown to target various components of MAPK cascades in plants. Inactivation of MPK4 by the Pseudomonas effector HopAI1 triggers SUMM2-mediated immunity. Together, these findings suggest intricate interplays between PAMP-triggered immunity and effector-triggered immunity via MAPK signaling.

The cotton MAPK kinase GhMPK20 negatively regulates resistance to Fusarium oxysporum by mediating the MKK4-MPK20-WRKY40 cascade

Fusarium wilt is one of the most serious diseases affecting cotton. However, the pathogenesis and mechanism by which Fusarium oxysporum overcomes plant defence responses are unclear. Here, a new group D mitogen-activated protein kinase (MAPK) gene, GhMPK20, was identified and functionally analysed in cotton. GhMPK20 expression was significantly induced by F. oxysporum. Virus-induced gene silencing (VIGS) of GhMPK20 in cotton increased the tolerance to F. oxysporum, whereas ectopic GhMPK20 overexpression in Nicotiana benthamiana reduced F. oxysporum resistance via disruption of the salicylic acid (SA)-mediated defence pathway. More importantly, an F. oxysporum-induced MAPK cascade pathway composed of GhMKK4, GhMPK20 and GhWRKY40 was identified. VIGS of GhMKK4 and GhWRKY40 also enhanced F. oxysporum resistance in cotton, and the function of GhMKK4-GhMPK20 was shown to be essential for F. oxysporum-induced GhWRKY40 expression. Together, our results indicate that the GhMKK4-GhMPK20-GhWRKY40 cascade in cotton plays an important role in the pathogenesis of F. oxysporum. This research broadens our knowledge of the negative role of the MAPK cascade in disease resistance in cotton and provides an important scientific basis for the formulation of Fusarium wilt prevention strategies.

Subtle regulation of cotton resistance to Verticillium wilt mediated by MAPKK family members

Verticillium wilt caused by soil-borne fungus of Verticillium dahliae Kleb. is one of the most devastating diseases of cotton. Since the hierarchically organized mitogen-activated protein kinase (MAPK) cascade plays pivotal roles in signaling plant defense against pathogen attack, and the key nodes of MAPKKs (MKKs) may serve as for the convergence and divergence of signals in MAPK cascades, the possible relations between MAPK signaling and cotton Verticillium resistance were examined in this study. A total of 24 MKK genes were identified in the Gossypium hirsutum L. genome and then classified based on phylogenetic analysis. Then the regulation roles of all types of cotton MKKs in activation of cotton disease resistance were tested with the virus-induced gene silencing (VIGS) method. The results showed that three types of MKKs (GhMKK4, GhMKK6 and GhMKK9) positively regulate, while GhMKK10 negatively regulate the cotton resistance to Verticillium wilt. Further, more subtle regulation of cotton resistance mediated by MKK genes were revealed. In GhMKK9, only Gh_A12G2448 and Gh_D12G2574 displayed positive regulation of cotton resistance; whereas only Gh_A12G1883 and Gh_D12G2062 displayed negative regulation of cotton resistance in GhMKK10. All these results show that MKK members in MAPK signal cascades play dual roles in subtly regulating of cotton resistance to Verticillium wilt.

Control of Grain Size and Weight by the OsMKKK10-OsMKK4-OsMAPK6 Signaling Pathway in Rice

Grain size is one of the key agronomic traits that determine grain yield in crops. However, the mechanisms underlying grain size control in crops remain elusive. Here we demonstrate that the OsMKKK10-OsMKK4-OsMAPK6 signaling pathway positively regulates grain size and weight in rice. In rice, loss of OsMKKK10 function results in small and light grains, short panicles, and semi-dwarf plants, while overexpression of constitutively active OsMKKK10 (CA-OsMKKK10) results in large and heavy grains, long panicles, and tall plants. OsMKKK10 interacts with and phosphorylates OsMKK4. We identified an OsMKK4 gain-of-function mutant (large11-1D) that produces large and heavy grains. OsMKK4^A227T encoded by the large11-1D allele has stronger kinase activity than OsMKK4. Plants overexpressing a constitutively active form of OsMKK4 (OsMKK4-DD) also produce large grains. Further biochemical and genetic analyses revealed that OsMKKK10, OsMKK4, and OsMAPK6 function in a common pathway to control grain size. Taken together, our study establishes an important genetic and molecular framework for OsMKKK10-OsMKK4-OsMAPK6 cascade-mediated control of grain size and weight in rice.

Active photosynthetic inhibition mediated by MPK3/MPK6 is critical to effector-triggered immunity

Extensive research revealed tremendous details about how plants sense pathogen effectors during effector-triggered immunity (ETI). However, less is known about downstream signaling events. In this report, we demonstrate that prolonged activation of MPK3 and MPK6, two Arabidopsis pathogen-responsive mitogen-activated protein kinases (MPKs), is essential to ETI mediated by both coiled coil-nucleotide binding site-leucine rich repeats (CNLs) and toll/interleukin-1 receptor nucleotide binding site-leucine rich repeats (TNLs) types of R proteins. MPK3/MPK6 activation rapidly alters the expression of photosynthesis-related genes and inhibits photosynthesis, which promotes the accumulation of superoxide (O2•−) and hydrogen peroxide (H₂O₂), two major reactive oxygen species (ROS), in chloroplasts under light. In the chemical-genetically rescued mpk3 mpk6 double mutants, ETI-induced photosynthetic inhibition and chloroplastic ROS accumulation are compromised, which correlates with delayed hypersensitive response (HR) cell death and compromised resistance. Furthermore, protection of chloroplasts by expressing a plastid-targeted cyanobacterial flavodoxin (pFLD) delays photosynthetic inhibition and compromises ETI. Collectively, this study highlights a critical role of MPK3/MPK6 in manipulating plant photosynthetic activities to promote ROS accumulation in chloroplasts and HR cell death, which contributes to the robustness of ETI. Furthermore, the dual functionality of MPK3/MPK6 cascade in promoting defense and inhibiting photosynthesis potentially allow it to orchestrate the trade-off between plant growth and defense in plant immunity.

Plants follow different strategies to defend themselves against pathogens and activate their immune systems once the pathogens have been detected. One of the responses observed is the inhibition of photosynthesis and the global down-regulation of genes that regulate this process, similar to what is frequently observed in plants under various biotic stress conditions. However, the mechanisms underlying the turning off of the photosynthetic activity and whether this process contributes to plants’ defense against pathogens remain to be determined. In this study, we analyze these mechanisms in Arabidopsis plants and show that prolonged activation of MPK3 and MPK6, two kinases critical for pathogen resistance, results in the inhibition of photosynthesis and accumulation of reactive oxygen species (ROS) in the chloroplasts. We find that this response is similar to that observed during pathogen effector-triggered immunity (ETI). Correspondingly, plants that carry mutant versions of MPK3 and MPK6 result in compromised ETI-induced photosynthetic inhibition and chloroplastic ROS accumulation; thus, these two kinases seem to be essential for ETI. Our results suggest that MPK3/MPK6 activation induces a global down-regulation of photosynthesis along with an up-regulation of defense-related genes, and coordinates the growth and defense trade-off in plants.

The Potato MAP3K StVIK Is Required for the Phytophthora infestans RXLR Effector Pi17316 to Promote Disease

Plant pathogens deliver effectors to manipulate processes in their hosts, creating a suitable environment for invasion and proliferation. Yet, little is known about the host proteins that are targeted by effectors from filamentous pathogens. Here, we show that stable transgenic expression in potato (Solanum tuberosum) and transient expression in Nicotiana benthamiana of the arginine-any amino acid-leucine-arginine effector Pi17316 enhances leaf colonization by the late blight pathogen Phytophthora infestans Expression of Pi17316 also attenuates cell death triggered by the pathogen-associated molecular pattern Infestin1 (INF1), indicating that the effector suppresses pattern-triggered immunity. However, this effector does not attenuate cell death triggered by a range of resistance proteins, showing that it specifically suppresses INF1-triggered cell death (ICD). In yeast two-hybrid assays, Pi17316 interacts directly with the potato ortholog of VASCULAR HIGHWAY1-interacting kinase (StVIK), encoding a predicted MEK kinase (MAP3K). Interaction in planta was confirmed by coimmunoprecipitation and occurs at the plant plasma membrane. Virus-induced gene silencing of VIK in N. benthamiana attenuated P. infestans colonization, whereas transient overexpression of StVIK enhanced colonization, indicating that this host protein acts as a susceptibility factor. Moreover, VIK overexpression specifically attenuated ICD, indicating that it is a negative regulator of immunity. The abilities of Pi17316 to enhance P. infestans colonization or suppress ICD were compromised significantly in NbVIK-silenced plants, demonstrating that the effector activity of Pi17316 is mediated by this MAP3K. Thus, StVIK is exploited by P. infestans as a susceptibility factor to promote late blight disease.

Leptosphaeria maculans Effector Protein AvrLm1 Modulates Plant Immunity by Enhancing MAP Kinase 9 Phosphorylation

Leptosphaeria maculans, the causal agent of blackleg disease in canola (Brassica napus), secretes an array of effectors into the host to overcome host defense. Here we present evidence that the L. maculans effector protein AvrLm1 functions as a virulence factor by interacting with the B. napus mitogen-activated protein (MAP) kinase 9 (BnMPK9), resulting in increased accumulation and enhanced phosphorylation of the host protein. Transient expression of BnMPK9 in Nicotiana benthamiana induces cell death, and this phenotype is enhanced in the presence of AvrLm1, suggesting that induction of cell death due to enhanced accumulation and phosphorylation of BnMPK9 by AvrLm1 supports the initiation of necrotrophic phase of L. maculans infection. Stable expression of BnMPK9 in B. napus perturbs hormone signaling, notably salicylic acid response genes, to facilitate L. maculans infection. Our findings provide evidence that a MAP kinase is directly targeted by a fungal effector to modulate plant immunity.

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Leptosphaeria maculans effector AvrLm1 interacts with the Brassica napus MPK9 (BnMPK9)

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AvrLm1 increases the accumulation and enhances the phosphorylation of BnMPK9

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AvrLm1 enhances BnMPK9-dependent cell death in Nicotiana benthamiana

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Stable expression of BnMPK9 in B. napus facilitates L. maculans infection

GRAIN SIZE AND NUMBER1 Negatively Regulates the OsMKKK10-OsMKK4-OsMPK6 Cascade to Coordinate the Trade-off between Grain Number per Panicle and Grain Size in Rice

Grain number and size are interactive agronomic traits that determine grain yield. However, the molecular mechanisms responsible for coordinating the trade-off between these traits remain elusive. Here, we characterized the rice (Oryza sativa) grain size and number1 (gsn1) mutant, which has larger grains but sparser panicles than the wild type due to disordered localized cell differentiation and proliferation. GSN1 encodes the mitogen-activated protein kinase phosphatase OsMKP1, a dual-specificity phosphatase of unknown function. Reduced expression of GSN1 resulted in larger and fewer grains, whereas increased expression resulted in more grains but reduced grain size. GSN1 directly interacts with and inactivates the mitogen-activated protein kinase OsMPK6 via dephosphorylation. Consistent with this finding, the suppression of mitogen-activated protein kinase genes OsMPK6, OsMKK4, and OsMKKK10 separately resulted in denser panicles and smaller grains, which rescued the mutant gsn1 phenotypes. Therefore, OsMKKK10-OsMKK4-OsMPK6 participates in panicle morphogenesis and acts on a common pathway in rice. We confirmed that GSN1 is a negative regulator of the OsMKKK10-OsMKK4-OsMPK6 cascade that determines panicle architecture. The GSN1-MAPK module coordinates the trade-off between grain number and grain size by integrating localized cell differentiation and proliferation. These findings provide important insights into the developmental plasticity of the panicle and a potential means to improve crop yields.

The Xanthomonas euvesicatoria type III effector XopAU is an active protein kinase that manipulates plant MAP kinase signaling

The Gram-negative bacterium Xanthomonas euvesicatoria (Xe) is the causal agent of bacterial spot disease of pepper and tomato. Xe delivers effector proteins into host cells through the type III secretion system to promote disease. Here, we show that the Xe effector XopAU, which is conserved in numerous Xanthomonas species, is a catalytically active protein kinase and contributes to the development of disease symptoms in pepper plants. Agrobacterium-mediated expression of XopAU in host and non-host plants activated typical defense responses, including MAP kinase phosphorylation, accumulation of pathogenesis-related (PR) proteins and elicitation of cell death, that were dependent on the kinase activity of the effector. XopAU-mediated cell death was not dependent on early signaling components of effector-triggered immunity and was also observed when the effector was delivered into pepper leaves by Xanthomonas campestris pv. campestris, but not by Xe. Protein-protein interaction studies in yeast and in planta revealed that XopAU physically interacts with components of plant immunity-associated MAP kinase cascades. Remarkably, XopAU directly phosphorylated MKK2 in vitro and enhanced its phosphorylation at multiple sites in planta. Consistent with the notion that MKK2 is a target of XopAU, silencing of the MKK2 homolog or overexpression of the catalytically inactive mutant MKK2K99R in N. benthamiana plants reduced XopAU-mediated cell death and MAPK phosphorylation. Furthermore, yeast co-expressing XopAU and MKK2 displayed reduced growth and this phenotype was dependent on the kinase activity of both proteins. Together, our results support the conclusion that XopAU contributes to Xe disease symptoms in pepper plants and manipulates host MAPK signaling through phosphorylation and activation of MKK2.

Mitogen-Activated Protein Kinase Cascades in Plant Hormone Signaling

Mitogen-activated protein kinase (MAPK) modules play key roles in the transduction of environmental and developmental signals through phosphorylation of downstream signaling targets, including other kinases, enzymes, cytoskeletal proteins or transcription factors, in all eukaryotic cells. A typical MAPK cascade consists of at least three sequentially acting serine/threonine kinases, a MAP kinase kinase kinase (MAPKKK), a MAP kinase kinase (MAPKK) and finally, the MAP kinase (MAPK) itself, with each phosphorylating, and hence activating, the next kinase in the cascade. Recent advances in our understanding of hormone signaling pathways have led to the discovery of new regulatory systems. In particular, this research has revealed the emerging role of crosstalk between the protein components of various signaling pathways and the involvement of this crosstalk in multiple cellular processes. Here we provide an overview of current models and mechanisms of hormone signaling with a special emphasis on the role of MAPKs in cell signaling networks. One-sentence summary: In this review we highlight the mechanisms of crosstalk between MAPK cascades and plant hormone signaling pathways and summarize recent findings on MAPK regulation and function in various cellular processes.

The grapevine kinome: annotation, classification and expression patterns in developmental processes and stress responses

Protein kinases (PKs) have evolved as the largest family of molecular switches that regulate protein activities associated with almost all essential cellular functions. Only a fraction of plant PKs, however, have been functionally characterized even in model plant species. In the present study, the entire grapevine kinome was identified and annotated using the most recent version of the grapevine genome. A total of 1168 PK-encoding genes were identified and classified into 20 groups and 121 families, with the RLK-Pelle group being the largest, with 872 members. The 1168 kinase genes were unevenly distributed over all 19 chromosomes, and both tandem and segmental duplications contributed to the expansion of the grapevine kinome, especially of the RLK-Pelle group. Ka/Ks values indicated that most of the tandem and segmental duplication events were under purifying selection. The grapevine kinome families exhibited different expression patterns during plant development and in response to various stress treatments, with many being coexpressed. The comprehensive annotation of grapevine kinase genes, their patterns of expression and coexpression, and the related information facilitate a more complete understanding of the roles of various grapevine kinases in growth and development, responses to abiotic stress, and evolutionary history.

A Mucin-Like Protein of Planthopper Is Required for Feeding and Induces Immunity Response in Plants

The brown planthopper, Nilaparvata lugens, is a pest that threatens rice (Oryza sativa) production worldwide. While feeding on rice plants, planthoppers secrete saliva, which plays crucial roles in nutrient ingestion and modulating plant defense responses, although the specific functions of salivary proteins remain largely unknown. We identified an N. lugens-secreted mucin-like protein (NlMLP) by transcriptome and proteome analyses and characterized its function, both in brown planthopper and in plants. NlMLP is highly expressed in salivary glands and is secreted into rice during feeding. Inhibition of NlMLP expression in planthoppers disturbs the formation of salivary sheaths, thereby reducing their performance. In plants, NlMLP induces cell death, the expression of defense-related genes, and callose deposition. These defense responses are related to Ca2+ mobilization and the MEK2 MAP kinase and jasmonic acid signaling pathways. The active region of NlMLP that elicits plant responses is located in its carboxyl terminus. Our work provides a detailed characterization of a salivary protein from a piercing-sucking insect other than aphids. Our finding that the protein functions in plant immune responses offers new insights into the mechanism underlying interactions between plants and herbivorous insects.

Differential Signaling and Sugar Exchanges in Response to Avirulent Pathogen- and Symbiont-Derived Molecules in Tobacco Cells

Plants interact with microbes whose ultimate aim is to exploit plant carbohydrates for their reproduction. Plant–microbe interactions (PMIs) are classified according to the nature of their trophic exchanges: while mutualistic microbes trade nutrients with plants, pathogens unilaterally divert carbohydrates. The early responses following microbe recognition and the subsequent control of plant sugar distribution are still poorly understood. To further decipher PMI functionality, we used tobacco cells treated with microbial molecules mimicking pathogenic or mutualistic PMIs, namely cryptogein, a defense elicitor, and chitotetrasaccharide (CO4), which is secreted by mycorrhizal fungi. CO4 was perceived by tobacco cells and triggered widespread transient signaling components such as a sharp cytosolic Ca²⁺ elevation, NtrbohD-dependent H₂O₂ production, and MAP kinase activation. These CO4-induced events differed from those induced by cryptogein, i.e., sustained events leading to cell death. Furthermore, cryptogein treatment inhibited glucose and sucrose uptake but not fructose uptake, and promoted the expression of NtSUT and NtSWEET sugar transporters, whereas CO4 had no effect on sugar uptake and only a slight effect on NtSWEET2B expression. Our results suggest that microbial molecules induce different signaling responses that reflect microbial lifestyle and the subsequent outcome of the interaction.

Activation of ZmMKK10, a maize mitogen-activated protein kinase kinase, induces ethylene-dependent cell death

Mitogen-activated protein kinase (MAPK) cascades play important roles in regulating plant growth, development and stress responses. Here, we report that ZmMKK10, a maize MAP kinase kinase, positively regulates cell death. Sequence comparison to Arabidopsis MKKs has led to ZmMKK10 being classified as a group D MKK. Kinase activity analysis of recombinant ZmMKK10 showed that the Mg²⁺ ion was required for its kinase activity. Transient expression of ZmMKK10^WT or ZmMKK10^DD (the active form of ZmMKK10) in maize mesophyll protoplast significantly increased the cell death rate. Inducible expression of ZmMKK10^WT or ZmMKK10^DD in Arabidopsis transgenic plants caused rapid HR-like cell death, whereas induction of ZmMKK10^KR (the inactive form of ZmMKK10) expression in transgenic plants did not yield the same phenotype. Genetic and pharmacological analysis revealed that ZmMKK10-induced cell death in transgenic plants requires the activation of Arabidopsis MPK3 and MPK6 and that it partially depended on ethylene biosynthesis. ZmMPK3 and ZmMPK7, the orthologues of Arabidopsis MPK3 and MPK6, interacted with ZmMKK10 in yeast and ZmMKK10 phosphorylated them both in vitro. Our results demonstrate that ZmMKK10 induces cell death in an ethylene-dependent manner. Furthermore, ZmMPK3 and ZmMPK7 may be the downstream MAPKs in this process.

The Protein Phosphatase 4 and SMEK1 Complex Dephosphorylates HYL1 to Promote miRNA Biogenesis by Antagonizing the MAPK Cascade in Arabidopsis

Phosphorylation plays an essential role in microRNA (miRNA) processing by regulating co-factors of the miRNA biogenesis machinery. HYL1 (Hyponastic Leaves 1), a core co-factor in plant miRNA biogenesis, is a short-lived phosphoprotein. However, the precise balance and regulatory mechanism of the stability and phosphorylation of HYL1 remain unclear. Here, we show that a highly conserved PP4 (Protein Phosphatase 4) and SMEK1 (Suppressor of MEK 1) complex dephosphorylates HYL1 to promote miRNA biogenesis, by antagonizing the MAPK cascade in Arabidopsis. The smek1 mutants exhibit defective miRNA biogenesis due to accelerated degradation of HYL1. SMEK1 stabilizes HYL1 in a dual manner: SMEK1, as a suppressor, inhibits MAPK activation to attenuate HYL1 phosphorylation; SMEK1 assembles a functional PP4 to target HYL1 for dephosphorylation. Moreover, the protein level of SMEK1 is increased in response to abscisic acid. Our results provide insights into the delicate balance between a protein kinase and a phosphatase during miRNA biogenesis.

Identification of MAPKs as signal transduction components required for the cell death response during compatible infection by the synergistic pair Potato virus X-Potato virus Y

Systemic necrosis is one of the most severe symptoms caused in compatible plant-virus interactions and shares common features with the hypersensitive response (HR). Mitogen-activated protein kinase (MAPK) cascades are associated with responses to compatible and incompatible host-virus interactions. Here, we show that virus-induced gene silencing of the Nicotiana benthamiana MAPK genes salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK), and the MAPK kinase (MAPKK) genes MEK1 and MKK1, partially compromised the HR-like response induced by the synergistic interaction of Potato virus X with Potato virus Y (PVX-PVY). Nevertheless, ameliorated cell death induced by PVX-PVY in the MAPK(K)-silenced plants did not facilitate virus accumulation in systemically infected leaves. Dual silencing of SIPK and of the oxylipin biosynthetic gene 9-Lipoxygenase showed that the latter was epistatic to SIPK in response to PVX-PVY infection. These findings demonstrate that SIPK, WIPK, MEK1 and MKK1 function as positive regulators of PVX-PVY-induced cell death.

Two tonoplast MATE proteins function as turgor-regulating chloride channels in Arabidopsis

The central vacuole in a plant cell occupies the majority of the cellular volume and plays a key role in turgor regulation. The vacuolar membrane (tonoplast) contains a large number of transporters that mediate fluxes of solutes and water, thereby adjusting cell turgor in response to developmental and environmental signals. We report that two tonoplast Detoxification efflux carrier (DTX)/Multidrug and Toxic Compound Extrusion (MATE) transporters, DTX33 and DTX35, function as chloride channels essential for turgor regulation in Arabidopsis Ectopic expression of each transporter in Nicotiana benthamiana mesophyll cells elicited a large voltage-dependent inward chloride current across the tonoplast, showing that DTX33 and DTX35 each constitute a functional channel. Both channels are highly expressed in Arabidopsis tissues, including root hairs and guard cells that experience rapid turgor changes during root-hair elongation and stomatal movements. Disruption of these two genes, either in single or double mutants, resulted in shorter root hairs and smaller stomatal aperture, with double mutants showing more severe defects, suggesting that these two channels function additively to facilitate anion influx into the vacuole during cell expansion. In addition, dtx35 single mutant showed lower fertility as a result of a defect in pollen-tube growth. Indeed, patch-clamp recording of isolated vacuoles indicated that the inward chloride channel activity across the tonoplast was impaired in the double mutant. Because MATE proteins are widely known transporters of organic compounds, finding MATE members as chloride channels expands the functional definition of this large family of transporters.

AIK1, A Mitogen-Activated Protein Kinase, Modulates Abscisic Acid Responses through the MKK5-MPK6 Kinase Cascade

The mitogen-activated protein kinase (MAPK) cascade is an evolutionarily conserved signal transduction module involved in transducing extracellular signals to the nucleus for appropriate cellular adjustment. This cascade essentially consists of three components: a MAPK kinase kinase (MAPKKK), a MAPK kinase, and a MAPK, connected to each other by the event of phosphorylation. Here, we report the characterization of a MAPKKK, ABA-INSENSITIVE PROTEIN KINASE1 (AIK1), which regulates abscisic acid (ABA) responses in Arabidopsis (Arabidopsis thaliana). T-DNA insertion mutants of AIK1 showed insensitivity to ABA in terms of both root growth and stomatal response. AIK1 functions in ABA responses via regulation of root cell division and elongation, as well as stomatal responses. The activity of AIK1 is induced by ABA in Arabidopsis and tobacco (Nicotiana benthamiana), and the Arabidopsis protein phosphatase type 2C, ABI1, a negative regulator of ABA signaling, restricts AIK1 activity by dephosphorylation. Bimolecular fluorescence complementation analysis showed that MPK3, MPK6, and AIK1 interact with MKK5. The single mutant seedlings of mpk6 and mkk5 have similar phenotypes to aik1, but mkk4 does not. AIK1 was localized in the cytoplasm and shown to activate MKK5 by protein phosphorylation, which was an ABA-activated process. Constitutively active MKK5 in aik1 mutant seedlings complements the ABA-insensitive root growth phenotype of aik1 The activity of MPK6 was increased by ABA in wild-type seedlings, but its activation by ABA was impaired in aik1 and aik1 mkk5 mutants. These findings clearly suggest that the AIK1-MKK5-MPK6 cascade functions in the ABA regulation of primary root growth and stomatal response.

Convergent Evolution of Pathogen Effectors toward Reactive Oxygen Species Signaling Networks in Plants

Microbial pathogens have evolved protein effectors to promote virulence and cause disease in host plants. Pathogen effectors delivered into plant cells suppress plant immune responses and modulate host metabolism to support the infection processes of pathogens. Reactive oxygen species (ROS) act as cellular signaling molecules to trigger plant immune responses, such as pathogen-associated molecular pattern (PAMP)-triggered immunity (PTI) and effector-triggered immunity. In this review, we discuss recent insights into the molecular functions of pathogen effectors that target multiple steps in the ROS signaling pathway in plants. The perception of PAMPs by pattern recognition receptors leads to the rapid and strong production of ROS through activation of NADPH oxidase Respiratory Burst Oxidase Homologs (RBOHs) as well as peroxidases. Specific pathogen effectors directly or indirectly interact with plant nucleotide-binding leucine-rich repeat receptors to induce ROS production and the hypersensitive response in plant cells. By contrast, virulent pathogens possess effectors capable of suppressing plant ROS bursts in different ways during infection. PAMP-triggered ROS bursts are suppressed by pathogen effectors that target mitogen-activated protein kinase cascades. Moreover, pathogen effectors target vesicle trafficking or metabolic priming, leading to the suppression of ROS production. Secreted pathogen effectors block the metabolic coenzyme NADP-malic enzyme, inhibiting the transfer of electrons to the NADPH oxidases (RBOHs) responsible for ROS generation. Collectively, pathogen effectors may have evolved to converge on a common host protein network to suppress the common plant immune system, including the ROS burst and cell death response in plants.

Vacuolar Chloride Fluxes Impact Ion Content and Distribution during Early Salinity Stress

The ability to control the cytoplasmic environment is a prerequisite for plants to cope with changing environmental conditions. During salt stress, for instance, Na⁺ and Cl^- are sequestered into the vacuole to help maintain cytosolic ion homeostasis and avoid cellular damage. It has been observed that vacuolar ion uptake is tied to fluxes across the plasma membrane. The coordination of both transport processes and relative contribution to plant adaptation, however, is still poorly understood. To investigate the link between vacuolar anion uptake and whole-plant ion distribution during salinity, we used mutants of the only vacuolar Cl^- channel described to date: the Arabidopsis (Arabidopsis thaliana) ALMT9. After 24-h NaCl treatment, almt9 knock-out mutants had reduced shoot accumulation of both Cl^- and Na⁺ In contrast, almt9 plants complemented with a mutant variant of ALMT9 that exhibits enhanced channel activity showed higher Cl^- and Na⁺ accumulation. The altered shoot ion contents were not based on differences in transpiration, pointing to a vacuolar function in regulating xylem loading during salinity. In line with this finding, GUS staining demonstrated that ALMT9 is highly expressed in the vasculature of shoots and roots. RNA-seq analysis of almt9 mutants under salinity revealed specific expression profiles of transporters involved in long-distance ion translocation. Taken together, our study uncovers that the capacity of vacuolar Cl^- loading in vascular cells plays a crucial role in controlling whole-plant ion movement rapidly after onset of salinity.

Transcriptomic profiling of Melon necrotic spot virus-infected melon plants revealed virus strain and plant cultivar-specific alterations

Background

Viruses are among the most destructive and difficult to control plant pathogens. Melon (Cucumis melo L.) has become the model species for the agriculturally important Cucurbitaceae family. Approaches that take advantage of recently developed genomic tools in melon have been extremely useful for understanding viral pathogenesis and can contribute to the identification of target genes for breeding new resistant cultivars. In this work, we have used a recently described melon microarray for transcriptome profiling of two melon cultivars infected with two strains of Melon necrotic spot virus (MNSV) that only differ on their 3′-untranslated regions.

Results

Melon plant tissues from the cultivars Tendral or Planters Jumbo were locally infected with either MNSV-Mα5 or MNSV-Mα5/3’264 and analysed in a time-course experiment. Principal component and hierarchical clustering analyses identified treatment (healthy vs. infected) and sampling date (3 vs. 5 dpi) as the primary and secondary variables, respectively. Out of 7566 and 7074 genes deregulated by MNSV-Mα5 and MNSV-Mα5/3’264, 1851 and 1356, respectively, were strain-specific. Likewise, MNSV-Mα5/3’264 specifically deregulated 2925 and 1618 genes in Tendral and Planters Jumbo, respectively. The GO categories that were significantly affected were clearly different for the different virus/host combinations. Grouping genes according to their patterns of expression allowed for the identification of two groups that were specifically deregulated by MNSV-Mα5/3’264 with respect to MNSV-Mα5 in Tendral, and one group that was antagonistically regulated in Planters Jumbo vs. Tendral after MNSV-Mα5/3’264 infection. Genes in these three groups belonged to diverse functional classes, and no obvious regulatory commonalities were identified. When data on MNSV-Mα5/Tendral infections were compared to equivalent data on cucumber mosaic virus or watermelon mosaic virus infections, cytokinin-O-glucosyltransferase2 was identified as the only gene that was deregulated by all three viruses, with infection dynamics correlating with the amplitude of transcriptome remodeling.

Conclusions

Strain-specific changes, as well as cultivar-specific changes, were identified by profiling the transcriptomes of plants from two melon cultivars infected with two MNSV strains. No obvious regulatory features shared among deregulated genes have been identified, pointing toward regulation through differential functional pathways.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-016-2772-5) contains supplementary material, which is available to authorized users.

Nicotiana benthamiana MAPK-WRKY pathway confers resistance to a necrotrophic pathogen Botrytis cinerea

MEK2-SIPK/WIPK cascade, a Nicotiana benthamiana mitogen-activated protein kinase (MAPK) cascade, is an essential signaling pathway for plant immunity and involved in hypersensitive response (HR) accompanied by cell death. WRKY transcription factors as substrates of SIPK and WIPK have been isolated and implicated in HR cell death. Here, we show virus-induced gene silencing of WRKY genes compromised constitutively active MEK2-triggered cell death in N. benthamiana leaves. In general, HR cell death enhances susceptibility to necrotrophic pathogens such as Botrytis cinerea. However, the WRKY gene silencing elevated susceptibility to B. cinerea. These findings suggest that downstream WRKYs of MEK2-SIPK/WIPK cascade are required for cell death-dependent and -independent immunities in N. benthamiana.

Analysis of crystal structure of Arabidopsis MPK6 and generation of its mutants with higher activity

Mitogen-activated protein kinase (MAPK) cascades, which are the highly conserved signalling modules in eukaryotic organisms, have been shown to play important roles in regulating growth, development, and stress responses. The structures of various MAPKs from yeast and animal have been solved, and structure-based mutants were generated for their function analyses, however, the structures of plant MAPKs remain unsolved. Here, we report the crystal structure of Arabidopsis MPK6 at a 3.0 Å resolution. Although MPK6 is topologically similar to ERK2 and p38, the structures of the glycine-rich loop, MAPK insert, substrate binding sites, and L16 loop in MPK6 show notable differences from those of ERK2 and p38. Based on the structural comparison, we constructed MPK6 mutants and analyzed their kinase activity both in vitro and in planta. MPK6^F364L and MPK6^F368L mutants, in which Phe364 and Phe368 in the L16 loop were changed to Leu, respectively, acquired higher intrinsic kinase activity and retained the normal MAPKK activation property. The expression of MPK6 mutants with basal activity is sufficient to induce camalexin biosynthesis; however, to induce ethylene and leaf senescence, the expression of MPK6 mutants with higher activity is required. The results suggest that these mutants can be used to analyze the specific biological functions of MPK6.

Mitogen-activated protein kinase 4-like carrying an MEY motif instead of a TXY motif is involved in ozone tolerance and regulation of stomatal closure in tobacco

The mitogen-activated protein kinases (MAPKs/MPKs) are important factors in the regulation of signal transduction in response to biotic and abiotic stresses. Previously, we characterized a MAPK from tobacco, Nicotiana tabacum MPK4 (NtMPK4). Here, we found a highly homologous gene, NtMPK4-like (NtMPK4L), in tobacco as well as other species in Solanaceae and Gramineae. Deduced amino acid sequences of their translation products carried MEY motifs instead of conserved TXY motifs of the MAPK family. We isolated the full length NtMPK4L gene and examined the physiological functions of NtMPK4L. We revealed that NtMPK4L was activated by wounding, like NtMPK4. However, a constitutively active salicylic acid-induced protein kinase kinase (SIPKK(EE)), which phosphorylates NtMPK4, did not phosphorylate NtMPK4L. Moreover, a tyrosine residue in the MEY motif was not involved in NtMPK4L activation. We also found that NtMPK4L-silenced plants showed rapid transpiration caused by remarkably open stomata. In addition, NtMPK4L-silenced plants completely lost the ability to close stomata upon ozone treatment and were highly sensitive to ozone, suggesting that this atypical MAPK plays a role in ozone tolerance through stomatal regulation.

Pathogen-Responsive MPK3 and MPK6 Reprogram the Biosynthesis of Indole Glucosinolates and Their Derivatives in Arabidopsis Immunity

Antimicrobial compounds have critical roles in plant immunity; for example, Arabidopsis thaliana and other crucifers deploy phytoalexins and glucosinolate derivatives in defense against pathogens. The pathogen-responsive MITOGEN-ACTIVATED PROTEIN KINASE3 (MPK3) and MPK6 have essential functions in the induction of camalexin, the major phytoalexin in Arabidopsis. In search of cyanide, a coproduct of ethylene and camalexin biosynthesis, we found that MPK3 and MPK6 also affect the accumulation of extracellular thiocyanate ion derived from the indole glucosinolate (IGS) pathway. Botrytis cinerea infection activates MPK3/MPK6, which promote indole-3-yl-methylglucosinolate (I3G) biosynthesis and its conversion to 4-methoxyindole-3-yl-methylglucosinolate (4MI3G). Gain- and loss-of-function analyses demonstrated that MPK3/MPK6 regulate the expression of MYB51 and MYB122, two key regulators of IGS biosynthesis, as well as CYP81F2 and IGMT1/IGMT2, which encode enzymes in the conversion of I3G to 4MI3G, through ETHYLENE RESPONSE FACTOR6 (ERF6), a substrate of MPK3/MPK6. Under the action of PENETRATION2 (PEN2), an atypical myrosinase, and PEN3, an ATP binding cassette transporter, 4MI3G is converted to extracellular unstable antimicrobial compounds, possibly isothiocyanates that can react with nucleophiles and release the stable thiocyanate ion. Recent studies demonstrated the importance of PEN2/PEN3-dependent IGS derivatives in plant immunity. Here, we report that MPK3/MPK6 and their substrate ERF6 promote the biosynthesis of IGSs and the conversion of I3G to 4MI3G, a target of PEN2/PEN3-dependent chemical defenses in plant immunity.

The type 3 effector NopL of Sinorhizobium sp. strain NGR234 is a mitogen-activated protein kinase substrate

Pathogenic bacteria utilize type 3 secretion systems to inject type 3 effectors (T3Es) into host cells, thereby subverting host defense reactions. Similarly, T3Es of symbiotic nitrogen-fixing rhizobia can affect nodule formation on roots of legumes. Previous work showed that NopL (nodulation outer protein L) of Sinorhizobium(Ensifer) sp. strain NGR234 is multiply phosphorylated in eukaryotic cells and that this T3E suppresses responses mediated by mitogen-activated protein (MAP) kinase signaling in yeast (mating pheromone signaling) and plant cells (expression of pathogenesis-related defense proteins). Here, we show that NopL is a MAP kinase substrate. Microscopic observations of fluorescent fusion proteins and bimolecular fluorescence complementation analysis in onion cells indicated that NopL is targeted to the nucleus and forms a complex with SIPK (salicylic acid-induced protein kinase), a MAP kinase of tobacco. In vitro experiments demonstrated that NopL is phosphorylatyed by SIPK. At least nine distinct spots were observed after two-dimensional gel electrophoresis, indicating that NopL can be hyperphosphorylated by MAP kinases. Senescence symptoms in nodules of beans (Phaseolus vulgaris cv. Tendergreen) were analyzed to determine the symbiotic effector activity of different NopL variants with serine to alanine substitutions at identified and predicted phosphorylation sites (serine-proline motif). NopL variants with six or eight serine to alanine substitutions were partially active, whereas NopL forms with 10 or 12 substituted serine residues were inactive. In conclusion, our findings provide evidence that NopL interacts with MAP kinases and reveals the importance of serine-proline motifs for effector activity during symbiosis.

Comparative transcriptome analysis of wheat embryo and endosperm responses to ABA and H2O2 stresses during seed germination

BACKGROUND

Wheat embryo and endosperm play important roles in seed germination, seedling survival, and subsequent vegetative growth. ABA can positively regulate dormancy induction and negatively regulates seed germination at low concentrations, while low H2O2 concentrations promote seed germination of cereal plants. In this report, we performed the first integrative transcriptome analysis of wheat embryo and endosperm responses to ABA and H2O2 stresses.

RESULTS

We used the GeneChip® Wheat Genome Array to conduct a comparative transcriptome microarray analysis of the embryo and endosperm of elite Chinese bread wheat cultivar Zhengmai 9023 in response to ABA and H2O2 treatments during seed germination. Transcriptome profiling showed that after H2O2 and ABA treatments, the 64 differentially expressed genes in the embryo were closely related to DNA synthesis, CHO metabolism, hormone metabolism, and protein degradation, while 121 in the endosperm were involved mainly in storage reserves, transport, biotic and abiotic stresses, hormone metabolism, cell wall metabolism, signaling, and development. Scatter plot analysis showed that ABA treatment increased the similarity of regulated patterns between the two tissues, whereas H2O2 treatment decreased the global expression similarity. MapMan analysis provided a global view of changes in several important metabolism pathways (e.g., energy reserves mobilization, cell wall metabolism, and photosynthesis), as well as related functional groups (e.g., cellular processes, hormones, and signaling and transport) in the embryo and endosperm following exposure of seeds to ABA and H2O2 treatments during germination. Quantitative RT-PCR analysis was used to validate the expression patterns of nine differentially expressed genes.

CONCLUSIONS

Wheat seed germination involves regulation of a large number of genes involved in many functional groups. ABA/H2O2 can repress/promote seed germination by coordinately regulating related gene expression. Our results provide novel insights into the transcriptional regulation mechanisms of embryo and endosperm in response to ABA and H2O2 treatments during seed germination.

Role of brassinosteroid signaling in modulating Tobacco mosaic virus resistance in Nicotiana benthamiana

Plant steroid hormones, brassinosteroids (BRs), play essential roles in plant growth, development and stress responses. However, mechanisms by which BRs interfere with plant resistance to virus remain largely unclear. In this study, we used pharmacological and genetic approaches in combination with infection experiments to investigate the role of BRs in plant defense against Tobacco Mosaic Virus (TMV) in Nicotiana benthamiana. Exogenous applied BRs enhanced plant resistance to virus infection, while application of Bikinin (inhibitor of glycogen synthase kinase-3), which activated BR signaling, increased virus susceptibility. Silencing of NbBRI1 and NbBSK1 blocked BR-induced TMV resistance, and silencing of NbBES1/BZR1 blocked Bikinin-reduced TMV resistance. Silencing of NbMEK2, NbSIPK and NbRBOHB all compromised BR-induced virus resistance and defense-associated genes expression. Furthermore, we found MEK2-SIPK cascade activated while BES1/BZR1 inhibited RBOHB-dependent ROS production, defense gene expression and virus resistance induced by BRs. Thus, our results revealed BR signaling had two opposite effects on viral defense response. On the one hand, BRs enhanced virus resistance through MEK2-SIPK cascade and RBOHB-dependent ROS burst. On the other hand, BES1/BZR1 inhibited RBOHB-dependent ROS production and acted as an important mediator of the trade-off between growth and immunity in BR signaling.

Functional characterization and hormonal regulation of the PHEOPHYTINASE gene LpPPH controlling leaf senescence in perennial ryegrass

Chlorophyll (Chl) degradation occurs naturally during leaf maturation and senescence, and can be induced by stresses, both processes involving the regulation of plant hormones. The objective of this study was to determine the functional roles and hormonal regulation of a gene encoding pheophytin pheophorbide hydrolyase (PPH) that catabolizes Chl degradation during leaf senescence in perennial grass species. A PPH gene, LpPPH, was cloned from perennial ryegrass (Lolium perenne L.). LpPPH was localized in the chloroplast. Overexpressing LpPPH accelerated Chl degradation in wild tobacco, and rescued the stay-green phenotype of the Arabidopsis pph null mutant. The expression level of LpPPH was positively related to the extent of leaf senescence. Exogenous application of abscisic acid (ABA) and ethephon (an ethylene-releasing agent) accelerated the decline in Chl content in leaves of perennial ryegrass, whereas cytokinin (CK) and aminoethoxyvinylglycine (AVG; an ethylene biosynthesis inhibitor) treatments suppressed leaf senescence, corresponding to the up- or down-regulation of LpPPH expression. The promoters of five orthologous PPH genes were predicted to share conserved cis-elements potentially recognized by transcription factors in the ABA and CK pathways. Taken together, the results suggested that LpPPH-mediated Chl breakdown could be regulated positively by ABA and ethylene, and negatively by CK, and LpPPH could be a direct downstream target gene of transcription factors in the ABA and CK signaling pathways.

Comparative transcriptional profiling of orange fruit in response to the biocontrol yeast Kloeckera apiculata and its active compounds

Background

The yeast Kloeckera apiculata strain 34–9 is an antagonist that shows biological control activity against the postharvest fungal pathogens of citrus. An antifungal compound, 2-phenylethanol (PEA), has been identified from the extract of K. apiculata. To better understand the molecular processes underlying the response of citrus fruit tissue to K. apiculata, the extract and PEA, microarray analyses were performed on navel oranges using an Affymetrix Citrus GeneChip.

Results

As many as 801, 339 and 608 differentially expressed genes (DEGs) were identified after the application of K. apiculata, the extract and PEA, respectively. In general, K. apiculata induced the expression of defence-related genes. In addition to chitinase and β-1,3-glucanase, genes involved in ethylene (ET), jasmonic acid (JA), calcium signalling, MAPK signalling and phenylalanine metabolism were induced. In contrast, monodehydroascorbate reductase, superoxide dismutase (SOD), catalase (CAT), peroxidase (POD) and carotenoid biosynthesis genes were down-regulated. The expression profiles for the extract- and PEA-treated samples were similar to that found for yeast (sharing 57.4 % DEGs), with a significant increase in the transcript levels of defence-related genes.

Conclusion

This study provides a global picture of the gene expression changes in navel oranges after the application of the antagonist yeast K. apiculata, its extract and PEA. The interpretation of the DEGs revealed new insight into the molecular processes that regulate the defence responses in orange tissue.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-2333-3) contains supplementary material, which is available to authorized users.

Comparative phospho-proteomics analysis of salt-responsive phosphoproteins regulated by the MKK9-MPK6 cascade in Arabidopsis

Mitogen-activated protein kinase (MAPK) cascades are involved in the salt stress response in plants. However, the identities of specific proteins operating downstream of MAPKs in the salt stress response remain unclear. Our studies showed that mkk9 and mpk6 null mutant seedlings are hyposensitive to salt stress. Moreover, we showed that MPK6 was activated by salt stress, indicating that the MKK9-MPK6 cascade mediated the salt stress response in Arabidopsis. To identify phosphoproteins downstream of the MKK9-MPK6 cascade in the salt stress response pathway, we performed two-dimensional electrophoresis (2-DE) with Pro-Q phosphoprotein staining and matrix-assisted laser desorption ionization time of flight mass spectrometry (MALDI-TOF MS) to identify phosphoproteins induced by salt treatment in mkk9, mpk6, and wild-type seedlings. Phosphorylation of 4 proteins, including Rubisco activase (RCA), plastid ribosomal protein S 1 (PRPS1), plastid division protein (FtsZ2-2), and tortifolia2 (TOR2), was found to be regulated by activation of MKK9-MPK6 cascade. Further Phospho-proteomics analysis of MKK9(DD) mutant seedlings revealed that RCA phosphorylation was up-regulated as a result of MKK9 activation. The finding that the MKK9-MPK6 cascade functions in the salt stress response by regulating phosphorylation of RCA, PRPS1, FtsZ2-2, and TOR2, provides a novel insight into the MAPK-related mechanisms underlying the salt stress response in plants.

A vacuolar phosphate transporter essential for phosphate homeostasis in Arabidopsis

Inorganic phosphate (Pi) is stored in the vacuole, allowing plants to adapt to variable Pi availability in the soil. The transporters that mediate Pi sequestration into vacuole remain unknown, however. Here we report the functional characterization of Vacuolar Phosphate Transporter 1 (VPT1), an SPX domain protein that transports Pi into the vacuole in Arabidopsis. The vpt1 mutant plants were stunted and consistently retained less Pi than wild type plants, especially when grown in medium containing high levels of Pi. In seedlings, VPT1 was expressed primarily in younger tissues under normal conditions, but was strongly induced by high-Pi conditions in older tissues, suggesting that VPT1 functions in Pi storage in young tissues and in detoxification of high Pi in older tissues. As a result, disruption of VPT1 rendered plants hypersensitive to both low-Pi and high-Pi conditions, reducing the adaptability of plants to changing Pi availability. Patch-clamp analysis of isolated vacuoles showed that the Pi influx current was severely reduced in vpt1 compared with wild type plants. When ectopically expressed in Nicotiana benthamiana mesophyll cells, VPT1 mediates vacuolar influx of anions, including Pi, SO4(2-), NO3(-), Cl(-), and malate with Pi as that preferred anion. The VPT1-mediated Pi current amplitude was dependent on cytosolic phosphate concentration. Single-channel analysis showed that the open probability of VPT1 was increased with the increase in transtonoplast potential. We conclude that VPT1 is a transporter responsible for vacuolar Pi storage and is essential for Pi adaptation in Arabidopsis.

A cotton Raf-like MAP3K gene, GhMAP3K40, mediates reduced tolerance to biotic and abiotic stress in Nicotiana benthamiana by negatively regulating growth and development

Mitogen-activated protein kinase (MAPK) cascades mediate various responses in plants. As the top component, MAP3Ks deserve more attention; however, little is known about the role of MAP3Ks, especially in cotton, a worldwide economic crop. In this study, a gene encoding a putative Raf-like MAP3K, GhMAP3K40, was isolated. GhMAP3K40 expression was induced by stress and multiple signal molecules. The plants overexpressing GhMAP3K40 had an enhanced tolerance to drought and salt stress at the germination stage. However, at the seedling stage, the transgenic plants suffered more severe damage after drought, exposure to pathogens and oxidative stress. The defence-related genes and the antioxidant system were activated in transgenic palnts, suggesting that GhMAP3K40 positively regulate the defence response. The transgenic plants were less able to prevent pathogenic invasion, which was due to defects in the cell structure of the leaves. The root system of the control plants were stronger compared with the transgenic plants. These results indicated a negative role of GhMAP3K40 in growth and development and GhMAP3K40 possibly caused the defects by down-regulating the lignin biosynthesis. Overall, these results suggest that GhMAP3K40 may positively regulate defence response but cause reduced tolerance to biotic and abiotic stress by negatively regulating growth and development.

Involvement of NtERF3 in the cell death signalling pathway mediated by SIPK/WIPK and WRKY1 in tobacco plants

We previously reported that one of the ethylene response factors (ERFs), NtERF3, and other members of the subgroup VIII-a ERFs of the AP2/ERF family exhibit cell death-inducing ability in tobacco leaves. In this study, we focused on the involvement of NtERF3 in a cell death signalling pathway in tobacco plants, particularly downstream of NtSIPK/NtWIPK and NtWRKY1, which are mitogen-activated protein kinases and a phosphorylation substrate of NtSIPK, respectively. An ERF-associated amphiphilic repression (EAR) motif-deficient NtERF3b mutant (NtERF3bΔEAR) that lacked cell death-inducing ability suppressed the induction of cell death caused by NtERF3a. The transient co-expression of NtERF3bΔEAR suppressed the hypersensitive reaction (HR)-like cell death induced by NtSIPK and NtWRKY1. The induction of cell death by NtSIPK and NtWRKY1 was also inhibited in transgenic plants expressing NtERF3bΔEAR. Analysis of gene expression, ethylene production and cell death symptoms in salicylic acid-deficient tobacco plants suggested the existence of some feedback regulation in the HR cell death signalling pathway mediated by SIPK/WIPK and WRKY1. Overall, these results suggest that NtERF3 functions downstream of NtSIPK/NtWIPK and NtWRKY1 in a cell death signalling pathway, with some feedback regulation.

WRKY Transcription Factors Phosphorylated by MAPK Regulate a Plant Immune NADPH Oxidase in Nicotiana benthamiana

Pathogen attack sequentially confers pattern-triggered immunity (PTI) and effector-triggered immunity (ETI) after sensing of pathogen patterns and effectors by plant immune receptors, respectively. Reactive oxygen species (ROS) play pivotal roles in PTI and ETI as signaling molecules. Nicotiana benthamiana RBOHB, an NADPH oxidase, is responsible for both the transient PTI ROS burst and the robust ETI ROS burst. Here, we show that RBOHB transactivation mediated by MAPK contributes to R3a/AVR3a-triggered ETI (AVR3a-ETI) ROS burst. RBOHB is markedly induced during the ETI and INF1-triggered PTI (INF1-PTI), but not flg22-tiggered PTI (flg22-PTI). We found that the RBOHB promoter contains a functional W-box in the R3a/AVR3a and INF1 signal-responsive cis-element. Ectopic expression of four phospho-mimicking mutants of WRKY transcription factors, which are MAPK substrates, induced RBOHB, and yeast one-hybrid analysis indicated that these mutants bind to the cis-element. Chromatin immunoprecipitation assays indicated direct binding of the WRKY to the cis-element in plants. Silencing of multiple WRKY genes compromised the upregulation of RBOHB, resulting in impairment of AVR3a-ETI and INF1-PTI ROS bursts, but not the flg22-PTI ROS burst. These results suggest that the MAPK-WRKY pathway is required for AVR3a-ETI and INF1-PTI ROS bursts by activation of RBOHB.

Five Xanthomonas type III effectors suppress cell death induced by components of immunity-associated MAP kinase cascades

Mitogen-activated protein kinase (MAPK) cascades play a fundamental role in signaling of plant immunity and mediate elicitation of cell death. Xanthomonas spp. manipulate plant signaling by using a type III secretion system to deliver effector proteins into host cells. We examined the ability of 33 Xanthomonas effectors to inhibit cell death induced by overexpression of components of MAPK cascades in Nicotiana benthamiana plants. Five effectors inhibited cell death induced by overexpression of MAPKKKα and MEK2, but not of MAP3Kε. In addition, expression of AvrBs1 in yeast suppressed activation of the high osmolarity glycerol MAPK pathway, suggesting that the target of this effector is conserved in eukaryotic organisms. These results indicate that Xanthomonas employs several type III effectors to suppress immunity-associated cell death mediated by MAPK cascades.

MAPK signaling: a key element in plant defense response to insects

Insects have long been the most abundant herbivores, and plants have evolved sophisticated mechanisms to defend against their attack. In particular, plants can perceive specific patterns of tissue damage associated with insect herbivory. Some plant species can perceive certain elicitors in insect oral secretions (OS) that enter wounds during feeding, and rapidly activate a series of intertwined signaling pathways to orchestrate the biosynthesis of various defensive metabolites. Mitogen-activated protein kinases (MAPKs), common to all eukaryotes, are involved in the orchestration of many cellular processes, including development and stress responses. In plants, at least two MAPKs, salicylic acid-induced protein kinase (SIPK) and wound-induced protein kinase (WIPK), are rapidly activated by wounding or insect OS; importantly, genetic studies using transgenic or mutant plants impaired in MAPK signaling indicated that MAPKs play critical roles in regulating the herbivory-induced dynamics of phytohormones, such as jasmonic acid, ethylene and salicylic acid, and MAPKs are also required for transcriptional activation of herbivore defense-related genes and accumulation of defensive metabolites. In this review, we summarize recent developments in understanding the functions of MAPKs in plant resistance to insect herbivores.

A Secreted Effector Protein of Ustilago maydis Guides Maize Leaf Cells to Form Tumors

The biotrophic smut fungus Ustilago maydis infects all aerial organs of maize (Zea mays) and induces tumors in the plant tissues. U. maydis deploys many effector proteins to manipulate its host. Previously, deletion analysis demonstrated that several effectors have important functions in inducing tumor expansion specifically in maize leaves. Here, we present the functional characterization of the effector See1 (Seedling efficient effector1). See1 is required for the reactivation of plant DNA synthesis, which is crucial for tumor progression in leaf cells. By contrast, See1 does not affect tumor formation in immature tassel floral tissues, where maize cell proliferation occurs independent of fungal infection. See1 interacts with a maize homolog of SGT1 (Suppressor of G2 allele of skp1), a factor acting in cell cycle progression in yeast (Saccharomyces cerevisiae) and an important component of plant and human innate immunity. See1 interferes with the MAPK-triggered phosphorylation of maize SGT1 at a monocot-specific phosphorylation site. We propose that See1 interferes with SGT1 activity, resulting in both modulation of immune responses and reactivation of DNA synthesis in leaf cells. This identifies See1 as a fungal effector that directly and specifically contributes to the formation of leaf tumors in maize.

Modes of MAPK substrate recognition and control

Mitogen-activated protein kinase (MAPK) cascades are universal, evolutionary conserved signalling modules, which translate environmental information into appropriate responses via phosphorylation of their substrate proteins. In Arabidopsis, the MAPK MPK3 regulates numerous cellular processes, including the adaptation to abiotic and biotic stresses. The molecular steps immediately downstream of MPK3 induction have, therefore, received abundant attention, and a respectable number of MPK3 targets are known by now. These proteins illustrate the substrate promiscuity of MPK3. They also are evidence for how manifold phosphorylation-regulated functions can be. This review presents the current knowledge about the function and regulation of MPK3-targeted proteins, takes a close look at their primary protein sequences, and proposes a model of how MPK3 recognises, binds, and phosphorylates its targets.

Cellular reprogramming through mitogen-activated protein kinases

Mitogen-activated protein kinase (MAPK) cascades are conserved eukaryote signaling modules where MAPKs, as the final kinases in the cascade, phosphorylate protein substrates to regulate cellular processes. While some progress in the identification of MAPK substrates has been made in plants, the knowledge on the spectrum of substrates and their mechanistic action is still fragmentary. In this focused review, we discuss the biological implications of the data in our original paper (Sustained mitogen-activated protein kinase activation reprograms defense metabolism and phosphoprotein profile in Arabidopsis thaliana; Frontiers in Plant Science 5: 554) in the context of related research. In our work, we mimicked in vivo activation of two stress-activated MAPKs, MPK3 and MPK6, through transgenic manipulation of Arabidopsis thaliana and used phosphoproteomics analysis to identify potential novel MAPK substrates. Here, we plotted the identified putative MAPK substrates (and downstream phosphoproteins) as a global protein clustering network. Based on a highly stringent selection confidence level, the core networks highlighted a MAPK-induced cellular reprogramming at multiple levels of gene and protein expression-including transcriptional, post-transcriptional, translational, post-translational (such as protein modification, folding, and degradation) steps, and also protein re-compartmentalization. Additionally, the increase in putative substrates/phosphoproteins of energy metabolism and various secondary metabolite biosynthesis pathways coincides with the observed accumulation of defense antimicrobial substances as detected by metabolome analysis. Furthermore, detection of protein networks in phospholipid or redox elements suggests activation of downstream signaling events. Taken in context with other studies, MAPKs are key regulators that reprogram cellular events to orchestrate defense signaling in eukaryotes.

Mitogen-activated protein kinase cascades in Vitis vinifera

Protein phosphorylation is one of the most important mechanisms to control cellular functions in response to external and endogenous signals. Mitogen-activated protein kinases (MAPK) are universal signaling molecules in eukaryotes that mediate the intracellular transmission of extracellular signals resulting in the induction of appropriate cellular responses. MAPK cascades are composed of four protein kinase modules: MAPKKK kinases (MAPKKKKs), MAPKK kinases (MAPKKKs), MAPK kinases (MAPKKs), and MAPKs. In plants, MAPKs are activated in response to abiotic stresses, wounding, and hormones, and during plant pathogen interactions and cell division. In this report, we performed a complete inventory of MAPK cascades genes in Vitis vinifera, the whole genome of which has been sequenced. By comparison with MAPK, MAPK kinases, MAPK kinase kinases and MAPK kinase kinase kinase kinase members of Arabidopsis thaliana, we revealed the existence of 14 MAPKs, 5 MAPKKs, 62 MAPKKKs, and 7 MAPKKKKs in Vitis vinifera. We identified orthologs of V. vinifera putative MAPKs in different species, and ESTs corresponding to members of MAPK cascades in various tissues. This work represents the first complete inventory of MAPK cascades in V. vinifera and could help elucidate the biological and physiological functions of these proteins in V. vinifera.

In silico-prediction of downstream MYB interacting partners of MAPK3 in Arabidopsis

Protein-Protein interactions (PPIs) are vital to most biological processes thus the identification of PPIs is of primary importance. Here, we endeavor to identify the downstream interacting partners of (AtMAPK3P) in Arabidopsis thaliana using the docking approach. Out of 131 members of MYB transcription factors 41 members are showing interactions with AtMAPK3P while the rest are showing non-interaction. Using minimal sequence motif search as well as through docking approach several novel MYB interacting proteins were also reported in the present study which need to be confirmed by in vitro kinase assay. Together, the results obtained essentially enhance our knowledge of the MAPK interacting protein network and provide a valuable research resource for developing a nearly important link between pathogen-activated MAPK signaling pathways and downstream transcriptional programming.

Transcriptional analysis of South African cassava mosaic virus-infected susceptible and tolerant landraces of cassava highlights differences in resistance, basal defense and cell wall associated genes during infection

BACKGROUND

Cassava mosaic disease is caused by several distinct geminivirus species, including South African cassava mosaic virus-[South Africa:99] (SACMV). To date, there is limited gene regulation information on viral stress responses in cassava, and global transcriptome profiling in SACMV-infected cassava represents an important step towards understanding natural host responses to plant geminiviruses.

RESULTS

A RNA-seq time course (12, 32 and 67 dpi) study, monitoring gene expression in SACMV-challenged susceptible (T200) and tolerant (TME3) cassava landraces, was performed using the Applied Biosystems (ABI) SOLiD next-generation sequencing platform. The multiplexed paired end sequencing run produced a total of 523 MB and 693 MB of paired-end reads for SACMV-infected susceptible and tolerant cDNA libraries, respectively. Of these, approximately 50.7% of the T200 reads and 55.06% of TME3 reads mapped to the cassava reference genome available in phytozome. Using a log2 fold cut-off (p<0.05), comparative analysis between the six normalized cDNA libraries showed that 4181 and 1008 transcripts in total were differentially expressed in T200 and TME3, respectively, across 12, 32 and 67 days post infection, compared to mock-inoculated. The number of responsive transcripts increased dramatically from 12 to 32 dpi in both cultivars, but in contrast, in T200 the levels did not change significantly at 67 dpi, while in TME3 they declined. GOslim functional groups illustrated that differentially expressed genes in T200 and TME3 were overrepresented in the cellular component category for stress-related genes, plasma membrane and nucleus. Alterations in the expression of other interesting genes such as transcription factors, resistance (R) genes, and histone/DNA methylation-associated genes, were observed. KEGG pathway analysis uncovered important altered metabolic pathways, including phenylpropanoid biosynthesis, sucrose and starch metabolism, and plant hormone signalling.

CONCLUSIONS

Molecular mechanisms for TME3 tolerance are proposed, and differences in patterns and levels of transcriptome profiling between T200 and TME3 with susceptible and tolerant phenotypes, respectively, support the hypothesis that viruses rearrange their molecular interactions in adapting to hosts with different genetic backgrounds.

Carbon monoxide regulates the expression of the wound-inducible gene ipomoelin through antioxidation and MAPK phosphorylation in sweet potato

Carbon monoxide (CO), one of the haem oxygenase (HO) products, plays important roles in plant development and stress adaptation. However, the function of CO involved in wounding responses is seldom studied. A wound-inducible gene, ipomoelin (IPO), of sweet potato (Ipomoea batatas cv. Tainung 57) was used as a target to study the regulation of CO in wounding responses. After wounding for 1h, the endogenous CO content and IbHO expression level were significantly reduced in leaves. IPO expression upon wounding was prohibited by the HO activator hemin, whereas the HO inhibitor zinc protoporphyrin IX elevated IPO expression. The IPO expression induced by wounding, H2O2, or methyl jasmonate was inhibited by CO. CO also affected the activities of ascorbate peroxidase, catalase, and peroxidase, and largely decreased H2O2 content in leaves. CO inhibited the extracellular signal-regulated kinase (ERK) phosphorylation induced by wounding. IbMAPK, the ERK of sweet potato, was identified by immunoblotting, and the interaction with its upstream activator, IbMEK1, was further confirmed by bimolecular fluorescence complementation and co-immunoprecipitation. Conclusively, wounding in leaves repressed IbHO expression and CO production, induced H2O2 generation and ERK phosphorylation, and then stimulated IPO expression.