The LCB2 subunit of the sphingolip biosynthesis enzyme serine palmitoyltransferase can function as an attenuator of the hypersensitive response and Bax-induced cell death

Cotton sphingosine kinase GhLCBK1 participates in fiber cell elongation by affecting sphingosine-1-phophate and auxin synthesis

Sphingolipids serve as essential components of biomembrane and possess significant bioactive properties. Sphingosine-1-phophate (S1P) plays a key role in plant resistance to stress, but its specific impact on plant growth and development remains to be fully elucidated. Cotton fiber cells are an ideal material for investigating the growth and maturation of plant cells. In this study, we examined the content and composition of sphingosine (Sph) and S1P throughout the progression of fiber cell development. The content of S1P elevated gradually during fiber elongation but declined during the transition stage. Exogenous application of S1P promoted fiber elongation while using of FTY720 (an antagonist of S1P), and DMS (an inhibitor of LCBK) hindered fiber elongation. Cotton Long Chain Base Kinase 1 (GhLCBK1) was notably expressed during the fiber elongation stage, containing all conserved domains of LCBK protein and localized in the endoplasmic reticulum. Overexpression GhLCBK1 increased the S1P content and promoted fiber elongation while retarded secondary cell wall (SCW) deposition. Conversely, downregulation of GhLCBK1 reduced the S1P levels, and suppressed fiber elongation, and accelerated SCW deposition. Transcriptome analysis revealed that upregulating GhLCBK1 or applying S1P induced the expression of GhEXPANSIN and auxin related genes. Furthermore, the levels of IAA were elevated and reduced in the fibers when up-regulating or down-regulating GhLCBK1, respectively. Our investigation demonstrated that GhLCBK1 and its product S1P facilitated the elongation of fiber cells by affecting auxin biosynthesis. This study contributes novel insights into the intricate regulatory pathways involved in fiber cell elongation, identifying GhLCBK1 as a potential target gene and laying the groundwork for enhancing fiber quality via genetic manipulation.

Effect of extremely low frequency electromagnetic field on the pathogenicity of Magnaporthe oryzae

Numerous works have reported that extremely low frequency electromagnetic fields (ELF-EMFs) were associated with human health; however, little is known about their effects on the occurrence of agricultural diseases. In this study, Magnaporthe oryzae was used as a model organism, and its pathogenicity under 50 Hz, 3 mT ELF-EMF was studied. Our results showed that the pathogenicity, growth rate, and conidia generation of M. oryzae were enhanced under ELF-EMF exposure. In addition, M. oryzae exposed to ELF-EMF showed enhanced tolerance to cell wall-perturbing agents sodium lauryl sulphate, and increased expression of cell wall integrity-related genes, including RAC1, CDC42, RHO2, and NOX2. In addition, the level of reactive oxygen species (ROS) and the expression level of ROS scavenger system-related gene MoAP1 increased in ELF-EMF-exposed samples, whereas the total antioxidant capacity and the activities of superoxide dismutase and catalase did not change. Results of our study demonstrated that exposure to 50 Hz, 3 mT ELF-EMF enhanced the infection ability of M. oryzae, which present new important challenges for understanding the effect of ELF-EMF exposure on farmland ecology, especially on agricultural diseases.

The function of the phytoplasma effector SWP12 depends on the properties of two key amino acids

Phytoplasmas are insect-borne bacterial pathogens capable of secreting effectors into host cells and interfering with host plant defense response processes. Previous studies have found that the Candidatus Phytoplasma tritici effector SWP12 binds to and destabilizes the wheat transcription factor TaWRKY74, increasing wheat susceptibility to phytoplasmas. Here, we used a Nicotiana benthamiana transient expression system to identify two key functional sites of SWP12 and screened a series of truncated mutants and amino acid substitution mutants to determine whether they inhibit Bax-induced cell death. Using a subcellular localization assay and online structure analysis websites, we found that structure rather than intracellular localization probably affects the function of SWP12. D33A and P85H are two inactive substitution mutants, neither of which interacts with TaWRKY74, and P85H does not inhibit Bax-induced cell death, suppress flg22-triggered reactive oxygen species (ROS) bursts, degrade TaWRKY74, or promote phytoplasma accumulation. D33A can weakly suppress Bax-induced cell death and flg22-triggered ROS bursts and degrade a portion of TaWRKY74 and weakly promote phytoplasma accumulation. S53L, CPP, and EPWB are three SWP12 homolog proteins from other phytoplasmas. Sequence analysis revealed that D33 was conserved in these proteins, and they exhibited the same polarity at P85. Transient expression in N. benthamiana showed that these proteins could inhibit Bax-induced cell death and suppress ROS bursts. Our findings clarified that P85 and D33 of SWP12 play critical and minor roles, respectively, in suppressing the plant defense response and that they play a preliminary role in determining the functions of homologous proteins.

Comprehensive Analysis and Functional Verification of the Pinus massoniana NBS-LRR Gene Family Involved in the Resistance to Bursaphelenchus xylophilus

Pinus massoniana Lamb. is a crucial timber and resin conifer in China, but its plantation industry is threatened by outbreaks of pine wilt disease (PWD) caused by Bursaphelenchus xylophilus (pinewood nematode; PWN). However, as of yet, there is no comprehensive analysis of NBS-LRR genes in P. massoniana involved in its defense against PWN. In this study, 507 NBS genes were identified in the transcriptome of resistant and susceptible P. masoniana inoculated with the PWN. The phylogenetic analysis and expression profiles of resistant and susceptible P. massoniana revealed that the up-regulated PmNBS-LRR97 gene was involved in conferring resistance to PWN. The results of real-time quantitative PCR (qRT-PCR) showed that PmNBS-LRR97 was significantly up-regulated after PWN infection, especially in the stems. Subcellular localization indicated that PmNBS-LRR97 located to the cell membrane. PmNBS-LRR97 significantly activated the expression of reactive oxygen species (ROS)-related genes in P. massoniana. In addition, the overexpression of PmNBS-LRR97 was capable of promoting the production of ROS, aiding in plant growth and development. In summary, PmNBS-LRR97 participates in the defense response to PWN and plays an active role in conferring resistance in P. massoniana. This finding provides new insight into the regulatory mechanism of the R gene in P. massoniana.

Detailed sphingolipid profile responded to salt stress in cotton root and the GhIPCS1 is involved in the regulation of plant salt tolerance

Sphingolipids are major structural components of membrane and active signaling molecules and play an important role in plant developmental processes and stress responses. As land salinization has increased globally, salinity has compromised the growth and productivity of crops such as cotton. Understanding the mechanisms of plant adaptation to salt stress is essential for breeding salt-tolerant crops. In this study, we explored the comprehensive metabolic profile of sphingolipids in cotton root under salt stress using lipidomics. 118 sphingolipid molecular species were identified, of which PhytoSph, PhytoCer, PhytoCer-OHFA, IPC, and GIPC were relatively high in content, and PhytoSph, PhytoCer, PhytoCer-OHFA, Phyto-GluCer, and IPC showed significant changes after salt stress, especially inositol phosphatidyl ceramide (IPC), which was significantly upregulated after salt treatment. Subsequently, we identified the genes encoding IPC synthase (IPCS), and ectopic expression of GhIPCS1 enhanced salt sensitivity in Arabidopsis, which might result from the disruption on the balance between various sphingolipid classes and/or molecular species. Overall, this study reveals key lipids and genes response to salt stress in cotton and provides a theoretical basis for the use of genetic engineering to improve cotton stress resistance.

Lipidomic Remodeling in Begonia grandis Under Heat Stress

Characterization of the alterations in leaf lipidome in Begonia (Begonia grandis Dry subsp. sinensis) under heat stress will aid in understanding the mechanisms of stress adaptation to high-temperature stress often occurring during hot seasons at southern areas in China. The comparative lipidomic analysis was performed using leaves taken from Begonia plants exposed to ambient temperature or heat stress. The amounts of total lipids and major lipid classes, including monoacylglycerol (MG), diacylglycerol (DG), triacylglycerols (TG), and ethanolamine-, choline-, serine-, inositol glycerophospholipids (PE, PC, PS, PI) and the variations in the content of lipid molecular species, were analyzed and identified by tandem high-resolution mass spectrometry. Upon exposure to heat stress, a substantial increase in three different types of TG, including 18:0/16:0/16:0, 16:0/16:0/18:1, and 18:3/18:3/18:3, was detected, which marked the first stage of adaptation processes. Notably, the reduced accumulation of some phospholipids, including PI, PC, and phosphatidylglycerol (PG) was accompanied by an increased accumulation of PS, PE, and phosphatidic acid (PA) under heat stress. In contrast to the significant increase in the abundance of TG, all of the detected lysophospholipids and sphingolipids were dramatically reduced in the Begonia leaves exposed to heat stress, suggesting that a very dynamic and specified lipid remodeling process is highly coordinated and synchronized in adaptation to heat stress in Begonia plants.

Comparative Transcriptomics and Gene Knockout Reveal Virulence Factors of Arthrinium phaeospermum in Bambusa pervariabilis × Dendrocalamopsis grandis

Arthrinium phaeospermum can cause branch wilting of Bambusa pervariabilis × Dendrocalamopsis grandis, causing great economic losses and ecological damage. A. phaeospermum was sequenced in sterile deionized water (CK), rice tissue (T1) and B. pervariabilis × D. grandis (T2) fluid by RNA-Seq, and the function of Ctf1β 1 and Ctf1β 2 was verified by gene knockout. There were 424, 471 and 396 differentially expressed genes between the T2 and CK, T2 and T1, and CK and T1 groups, respectively. Thirty DEGs had verified the change in expression by fluorescent quantitative PCR. Twenty-nine DEGs were the same as the expression level in RNA-Seq. In addition, ΔApCtf1β 1 and ΔApCtf1β 2 showed weaker virulence by gene knockout, and the complementary strains Ctf1β 1 and Ctf1β 2 showed the same virulence as the wild-type strains. Relative growth inhibition of ΔApCtf1β 1 and ΔApCtf1β was significantly decreased by 21.4% and 19.2%, respectively, by adding H₂O₂ compared to the estimates from the wild-type strain and decreased by 25% and 19.4%, respectively, by adding Congo red. The disease index of B. pervariabilis × D. grandis infected by two mutants was significantly lower than that of wild type. This suggested that Ctf1β genes are required for the stress response and virulence of A. phaeospermum.

A Sge1 homolog in Cytospora chrysosperma governs conidiation, virulence and the expression of putative effectors

SIX Gene Expression 1 (Sge1) is an important and well-recognized fungal-specific transcription regulator from the Gti1/Pac2 family that exhibits a conserved function in the vegetative growth, regulating the expression of effector genes and pathogenicity in plant pathogenic fungi. However, its functions in Cytospora chrysosperma, a notorious phytopathogenic fungus in forestry, remain poorly understood. Here, we characterized a Sge1 orthologue, CcSge1, in C. chrysosperma and deleted its Gti1/Pac2 domain for functional analysis. The CcSge1 deletion mutants showed obvious defects in hyphal growth, conidial production and response to hydrogen peroxide. Correspondingly, significantly lower expression of conidiation related genes were found in deletion mutants compared to that of the wild type. Importantly, the CcSge1 deletion mutants totally lost their pathogenicity to the host. Further analysis demonstrated that CcSge1 was responsible for the expression of putative effector genes and the transcription of CcSge1 was under tight control by pathogenicity-related MAP Kinase 1 (CcPmk1). What's more, one of the putative effector gene CCG_07874 was positively regulated by both CcSge1 and CcPmk1. Taken together, these data indicate that CcSge1is indispensable for hyphal radial growth, conidiation, the expression of effector genes and fungal virulence.

Fumonisin B1-Induced Changes in Cotton Fiber Elongation Revealed by Sphingolipidomics and Proteomics

Sphingolipids are essential biomolecules and membrane components, but their regulatory role in cotton fiber development is poorly understood. Here, we found that fumonisin B1 (FB1)—a sphingolipid synthesis inhibitor—could block fiber elongation severely. Using liquid chromatography tandem mass spectrometry (LC-MS/MS), we detected 95 sphingolipids that were altered by FB1 treatment; of these, 29 (mainly simple sphingolipids) were significantly increased, while 33 (mostly complex sphingolipids) were significantly decreased. A quantitative analysis of the global proteome, using an integrated quantitative approach with tandem mass tag (TMT) labeling and LC-MS/MS, indicated the upregulation of 633 and the downregulation of 672 proteins after FB1 treatment. Most differentially expressed proteins (DEPs) were involved in processes related to phenylpropanoid and flavonoid biosynthesis. In addition, up to 20 peroxidases (POD) were found to be upregulated, and POD activity was also increased by the inhibitor. To our knowledge, this is the first report on the effects of FB1 treatment on cotton fiber and ovule sphingolipidomics and proteomics. Our findings provide target metabolites and biological pathways for cotton fiber improvement.

The Basic Leucine Zipper Transcription Factor PlBZP32 Associated with the Oxidative Stress Response Is Critical for Pathogenicity of the Lychee Downy Blight Oomycete Peronophythora litchii

In this study, we utilized the RNAi technique to investigate the functions of PlBZP32, which possesses a basic leucine zipper (bZIP)-PAS structure, and provided insights into the contributions of bZIP transcription factors to oxidative stress, the production of sporangia, the germination of cysts, and the pathogenicity of Peronophythora litchii. This study also revealed the role of PlBZP32 in regulating the enzymatic activities of extracellular peroxidases and laccases in the plant-pathogenic oomycete.

Basic leucine zipper (bZIP) transcription factors are widespread in eukaryotes, including plants, animals, fungi, and oomycetes. However, the functions of bZIPs in oomycetes are rarely known. In this study, we identified a bZIP protein possessing a special bZIP-PAS structure in Peronophythora litchii, named PlBZP32. We found that PlBZP32 is upregulated in zoospores, in cysts, and during invasive hyphal growth. We studied the functions of PlBZP32 using the RNAi technique to suppress the expression of this gene. PlBZP32-silenced mutants were more sensitive to oxidative stress, showed a lower cyst germination rate, and produced more sporangia than the wild-type strain SHS3. The PlBZP32-silenced mutants were also less invasive on the host plant. Furthermore, we analyzed the activities of extracellular peroxidases and laccases and found that silencing PlBZP32 decreased the activities of P. litchii peroxidase and laccase. To our knowledge, this is the first report that the functions of a bZIP-PAS protein are associated with oxidative stress, asexual development, and pathogenicity in oomycetes.

IMPORTANCE In this study, we utilized the RNAi technique to investigate the functions of PlBZP32, which possesses a basic leucine zipper (bZIP)-PAS structure, and provided insights into the contributions of bZIP transcription factors to oxidative stress, the production of sporangia, the germination of cysts, and the pathogenicity of Peronophythora litchii. This study also revealed the role of PlBZP32 in regulating the enzymatic activities of extracellular peroxidases and laccases in the plant-pathogenic oomycete.

Effects of a Sudden Drop in Salinity on Scapharca subcrenata Antioxidant Defenses and Metabolism Determined Using LC-MS Non-targeted Metabolomics

In this experiment, the effects of a sudden drop in salinity on the antioxidant defense system and related gene expression of the ark shell Scapharca subcrenata were examined. The sudden drop in seawater salinity after a rainstorm was simulated, and subsequently differentially expressed metabolic markers were identified by LC-MS non-targeted metabolomics. When the salinity dropped to 14‰ (S14), the total anti-oxidant content, activity of Na⁺/K⁺-ATPase, superoxide dismutase (SOD), and catalase (CAT), content of malondialdehyde, and expression levels of Mn-SOD, CAT, and C-type lectin of S. subcrenata were significantly higher than in groups with salinity of 22‰ (S22) or 30‰ (S30) (P < 0.05). The activity of glutathione peroxidase (GPx), the content of reduced glutathione, and the expression levels of GP_x were not significantly different between S14 and S22, but the values in each group were significantly higher than those in S30 (P < 0.05). Using the metabolomics technique, 361, 271, and 264 metabolites with significant differences were identified from S22 vs. S14, S30 vs. S14, and S30 vs. S22, respectively. The drop in salinity was accompanied by up-regulation of phosphatidylcholine (PC) (20:4 (5Z, 8Z, 11Z, 14Z)/P-18: 1 (11Z)), PC (16:0/22: 6 (4Z, 7Z, 10Z, 13Z, 16Z, 19Z)), phosphatidylethanolamine (PE) (18:4 (6Z, 9Z, 12Z, 15Z)/24:1 (15Z)), phosphatidylinositol (PI) (20:1 (11Z)/0:0), phalluside-1, C16 sphinganine, and LacCer (d18:0/14:0) and by significant down-regulation of PI-Cer (d18:1/14:0) and PE (14:0/16:1(9Z). The results of this study illustrate how these nine metabolites can be used as metabolic markers for the response of S. subcrenata to a sudden drop in salinity. They also provide the theoretical groundwork for selection of bottom areas with salinity that is optimal for release and proliferation of S. subcrenata, which is needed to restore the declining populations of this species.

Host Lipids in Positive-Strand RNA Virus Genome Replication

Membrane association is a hallmark of the genome replication of positive-strand RNA viruses [(+)RNA viruses]. All well-studied (+)RNA viruses remodel host membranes and lipid metabolism through orchestrated virus-host interactions to create a suitable microenvironment to survive and thrive in host cells. Recent research has shown that host lipids, as major components of cellular membranes, play key roles in the replication of multiple (+)RNA viruses. This review focuses on how (+)RNA viruses manipulate host lipid synthesis and metabolism to facilitate their genomic RNA replication, and how interference with the cellular lipid metabolism affects viral replication.

PsMPK7, a stress-associated mitogen-activated protein kinase (MAPK) in Phytophthora sojae, is required for stress tolerance, reactive oxygenated species detoxification, cyst germination, sexual reproduction and infection of soybean

The sensing of stress signals and their transduction into appropriate responses are crucial for the adaptation, survival and infection of phytopathogenic fungi and oomycetes. Amongst evolutionarily conserved pathways, mitogen-activated protein kinase (MAPK) cascades function as key signal transducers that use phosphorylation to convey information. In this study, we identified a gene, designated PsMPK7, one of 14 predicted genes encoding MAPKs in Phytophthora sojae. PsMPK7 was highly transcribed in each tested stage, but was up-regulated in the zoospore, cyst and cyst germination stages. Silencing of PsMPK7 affected the growth of germinated cysts, oospore production and the pathogenicity of soybean. PsMPK7 transcription was induced by stresses from sorbitol, NaCl and hydrogen peroxide. Transformants in which PsMPK7 expression was silenced (PsMPK7-silenced) were significantly more sensitive to osmotic and oxidative stress. Aniline blue and diaminobenzidine staining revealed that the silenced lines did not suppress the host reactive oxygen species (ROS) burst, indicating that either the inoculated plants activated stronger defence responses to the transformants and/or the PsMPK7-silenced transformants failed to overcome plant defences. In addition, extracellular secretion of laccase decreased in the silenced lines. Overall, our results indicate that the PsMPK7 gene encodes a stress-associated MAPK in P. sojae that is important not only for responses to various stresses, but also for ROS detoxification, cyst germination, sexual oospore production and infection of soybean.

Molecular characterization of rice sphingosine-1-phosphate lyase gene OsSPL1 and functional analysis of its role in disease resistance response

Our results indicate that overexpression of OsSPL1 in transgenic tobacco plants attenuated disease resistance and facilitated programmed cell death. Long-chain base phosphates including sphingosine-1-phosphate have been shown to act as signaling mediators in regulating programmed cell death (PCD) and stress responses in mammals. In the present study, we characterized a rice gene OsSPL1, encoding a putative sphingosine-1-phosphate lyase that is involved in metabolism of sphingosine-1-phosphate. Expression of OsSPL1 was down-regulated in rice plants after treatments with salicylic acid, benzothiadiazole and 1-amino cyclopropane-1-carboxylic acid, but was induced by infection with a virulent strain of Magnaporthe oryzae, the causal agent of rice blast disease. Transgenic tobacco lines with overexpression of OsSPL1 were generated and analyzed for the possible role of OsSPL1 in disease resistance response and PCD. The OsSPL1-overexpressing tobacco plants displayed increased susceptibility to infection of Pseudomonas syringae pv. tabaci (Pst), the causal agent of wildfire disease, showing severity of disease symptom and bacterial titers in inoculated leaves, and attenuated pathogen-induced expression of PR genes after infection of Pst as compared to the wild-type and vector-transformed plants. Higher level of cell death, as revealed by dead cell staining, leakage of electrolyte and expression of hypersensitive response indicator genes, was observed in the OsSPL1-overexpressing plants after treatment with fumonisin B1, a fungal toxin that induces PCD in plants. Our results suggest that OsSPL1 has different functions in regulating disease resistance response and PCD in plants.

Roles of lipids as signaling molecules and mitigators during stress response in plants

Lipids are the major constituents of biological membranes that can sense extracellular conditions. Lipid-mediated signaling occurs in response to various environmental stresses, such as temperature change, salinity, drought and pathogen attack. Lysophospholipid, fatty acid, phosphatidic acid, diacylglycerol, inositol phosphate, oxylipins, sphingolipid, and N-acylethanolamine have all been proposed to function as signaling lipids. Studies on these stress-inducible lipid species have demonstrated that each lipid class has specific biological relevance, biosynthetic mechanisms and signaling cascades, which activate defense reactions at the transcriptional level. In addition to their roles in signaling, lipids also function as stress mitigators to reduce the intensity of stressors. To mitigate particular stresses, enhanced syntheses of unique lipids that accumulate in trace quantities under normal growth conditions are often observed under stressed conditions. The accumulation of oligogalactolipids and glucuronosyldiacylglycerol has recently been found to mitigate freezing and nutrition-depletion stresses, respectively, during lipid remodeling. In addition, wax, cutin and suberin, which are not constituents of the lipid bilayer, but are components derived from lipids, contribute to the reduction of drought stress and tissue injury. These features indicate that lipid-mediated defenses against environmental stress contributes to plant survival.

Plant sphingolipids: function follows form

Plant sphingolipids are structurally diverse molecules that are important as membrane components and bioactive molecules. An appreciation of the relationship between structural diversity and functional significance of plant sphingolipids is emerging through characterization of Arabidopsis mutants coupled with advanced analytical methods. It is increasingly apparent that modifications such as hydroxylation and desaturation of the sphingolipid nonpolar long-chain bases and fatty acids influence their metabolic routing to particular complex sphingolipid classes and their functions in signaling pathways and other cellular processes, such as membrane protein targeting. Here, we review recent reports investigating some of the more prevalent sphingolipid structural modifications and their functional importance in plants.

Disruption of sphingolipid biosynthesis in Nicotiana benthamiana activates salicylic acid-dependent responses and compromises resistance to Alternaria alternata f. sp. lycopersici

Sphingolipids play an important role in signal transduction pathways that regulate physiological functions and stress responses in eukaryotes. In plants, recent evidence suggests that their metabolic precursors, the long-chain bases (LCBs) act as bioactive molecules in the immune response. Interestingly, the virulence of two unrelated necrotrophic fungi, Fusarium verticillioides and Alternaria alternata, which are pathogens of maize and tomato plants, respectively, depends on the production of sphinganine-analog mycotoxins (SAMs). These metabolites inhibit de novo synthesis of sphingolipids in their hosts causing accumulation of LCBs, which are key regulators of programmed cell death. Therefore, to gain more insight into the role of sphingolipids in plant immunity against SAM-producing necrotrophic fungi, we disrupted sphingolipid metabolism in Nicotiana benthamiana through virus-induced gene silencing (VIGS) of the serine palmitoyltransfersase (SPT). This enzyme catalyzes the first reaction in LCB synthesis. VIGS of SPT profoundly affected N. benthamiana development as well as LCB composition of sphingolipids. While total levels of phytosphingosine decreased, sphinganine and sphingosine levels increased in SPT-silenced plants, compared with control plants. Plant immunity was also affected as silenced plants accumulated salicylic acid (SA), constitutively expressed the SA-inducible NbPR-1 gene and showed increased susceptibility to the necrotroph A. alternata f. sp. lycopersici. In contrast, expression of NbPR-2 and NbPR-3 genes was delayed in silenced plants upon fungal infection. Our results strongly suggest that LCBs modulate the SA-dependent responses and provide a working model of the potential role of SAMs from necrotrophic fungi to disrupt the plant host response to foster colonization.

The Nicotiana benthamiana mitogen-activated protein kinase cascade and WRKY transcription factor participate in Nep1(Mo)-triggered plant responses

Many bacterial, fungal, and oomycete species secrete necrosis and ethylene-inducing peptide 1 (Nep1)-like proteins (NLP) that trigger programmed cell death (PCD) and innate immune responses in dicotyledonous plants. However, how NLP induce such immune responses is not understood. Here, we show that silencing of the MAPKKKα-MEK2-WIPK mitogen-activated protein kinase (MAPK) cascade through virus-induced gene silencing compromises hydrogen peroxide accumulation and PCD induced by Nep1(Mo) from Magnaporthe oryzae. WIPK interacts with NbWRKY2, a transcription factor in Nicotiana benthamiana, in vitro and in vivo, suggesting an effector pathway that mediates Nep1(Mo)-induced cell death. Unexpectedly, salicylic acid-induced protein kinase (SIPK)- and NbWRKY2-silenced plants showed impaired Nep1(Mo)-induced stomatal closure, decreased Nep1(Mo)-promoted nitric oxide (NO) production in guard cells, and a reduction in Nep1(Mo)-induced resistance against Phytophthora nicotianae. Expression studies by real-time polymerase chain reaction suggested that the MEK2-WIPK-NbWRKY2 pathway regulated Nep1(Mo)triggered NO accumulation could be partly dependent on nitrate reductase, which was implicated in NO synthesis. Taken together, these studies demonstrate that the MAPK cascade is involved in Nep1(Mo)-triggered plant responses and MAPK signaling associated with PCD exhibits shared and distinct components with that for stomatal closure.

Detection of a hypersensitive reaction in the chestnut hybrid 'Bouche de Bétizac' infested by Dryocosmus kuriphilus Yasumatsu

The purpose of the study was the identification of the mechanisms of resistance to Dryocosmus kuriphilus Yasumatsu in the hybrid-resistant cultivar 'Bouche de Bétizac' (Castanea sativa × Castanea crenata). Larvae and eggs of the insect are found in the buds of this cultivar at the end of winter, but there is no gall development after budburst. The hypothesis of the presence of a hypersensitive reaction (HR) in the buds was tested using diaminobenzidine (DAB) to detect H(2)O(2) and by Real Time PCR (RT-PCR) to evaluate the expression of a germin-like protein gene. HR in plants is elicited by the production of reactive oxygen compounds, such as H(2)O(2), and results in the programmed cell death. The DAB test was applied to buds of 'Bouche de Bétizac' and of the susceptible cultivar 'Madonna' (C. sativa) at different stages of budburst. The DAB staining produced brown areas in the swelling buds of 'Bouche de Bétizac', indicating the presence of H(2)O(2). On the contrary, all uninfested buds, as well as the infested buds of 'Madonna', appeared whitish. Papers report that germin and germin-like proteins (GLP) with oxalate oxidase activity are discrete markers of stress-responsive gene products. A strong expression of the chestnut GLP gene was detected by RT-PCR at bud swelling in infested 'Bouche de Bétizac' buds but not in 'Madonna' ones. The results support the hypothesis of the occurrence of an HR in 'Bouche de Bétizac' as response to the cynipid infestation, resulting in cell and larvae death.

Cloning and characterization of two rice long-chain base kinase genes and their function in disease resistance and cell death

Sphingolipid metabolites such as long-chain base 1-phosphates (LCBPs) have been shown to play an important role in plants; however, little is known about their function in plant disease resistance and programmed cell death (PCD). In the present study, we cloned and identified two rice long-chain base kinase (LCBK) genes (OsLCBK1 and OsLCBK2), which are involved in biosynthesis of LCBPs, and performed functional analysis in transgenic tobacco. Expression of OsLCBK1 and OsLCBK2 was induced in rice seedlings after treatments with defense signaling molecules and after infection by Magnaporthe grisea, the causal agent of blast disease. Transgenic tobacco plants overexpressing OsLCBK1 were generated and disease resistance assays indicate that the OsLCBK1-overexpressing plants showed enhanced disease resistance against Pseudmonas syringae pv. tabacci, the causal agent of wildfire disease, and tobacco mosaic virus. Expression levels of some defense-related genes were constitutively up-regulated and further induced after pathogen infection in the OsLCBK1-overexpressing plants. Treatment with fungal toxin fumonisin B1, an effective inducer of PCD in plants, resulted in reduced level of cell death in the OsLCBK1-overexpressing plants, as indicated by cell death staining, leakage of electrolyte and expression of hypersensitive response indicator genes. These data suggest that rice LCBKs, probably through regulation of endogenous LCBP level, play important roles in disease resistance response and PCD in plants.

The role of G-proteins in plant immunity

Heterotrimeric G-proteins play an important regulatory role in multiple physiological processes, including the plant immune response, and substantial progress has been made in elucidating the G-protein-mediated defense-signaling network. This mini-review discusses the importance of G-proteins in plant immunity. We also provide an overview of how G-proteins affect plant cell death and stomatal movement. Our recent studies demonstrated that G-proteins are involved in signal transduction and induction of stomatal closure and defense responses. We also discuss future directions for G-protein signaling studies involving plant immunity.

Sphingolipids and plant defense/disease: the "death" connection and beyond

Sphingolipids comprise a major class of structural materials and lipid signaling molecules in all eukaryotic cells. Over the past two decades, there has been a phenomenal growth in the study of sphingolipids (i.e., sphingobiology) at an average rate of ∼1000 research articles per year. Sphingolipid studies in plants, though accounting for only a small fraction (∼6%) of the total number of publications, have also enjoyed proportionally rapid growth in the past decade. Concomitant with the growth of sphingobiology, there has also been tremendous progress in our understanding of the molecular mechanisms of plant innate immunity. In this review, we (i) cross examine and analyze the major findings that establish and strengthen the intimate connections between sphingolipid metabolism and plant programmed cell death (PCD) associated with plant defense or disease; (ii) highlight and compare key bioactive sphingolipids involved in the regulation of plant PCD and possibly defense; (iii) discuss the potential role of sphingolipids in polarized membrane/protein trafficking and formation of lipid rafts as subdomains of cell membranes in relation to plant defense; and (iv) where possible, attempt to identify potential parallels for immunity-related mechanisms involving sphingolipids across kingdoms.

Silencing of G proteins uncovers diversified plant responses when challenged by three elicitors in Nicotiana benthamiana

Signalling through heterotrimeric G protein composed of α-, β- and γ-subunits is essential in numerous physiological processes. Here we show that this prototypical G protein complex acts mechanistically by controlling elicitor sensitivity towards hypersensitive response (HR) and stomatal closure in Nicotiana benthamiana. Gα-, Gβ1-, and Gβ2-silenced plants were generated using virus-induced gene silencing. All silenced plants were treated with Xanthomonas oryzae harpin, Magnaporthe oryzae Nep1 and Phytophthora boehmeriae boehmerin, respectively. HR was dramatically impaired in Gα- and Gβ2-silenced plants treated with harpin, indicating that harpin-, rather than Nep1- or boehmerin-triggered HR, is Gα- and Gβ2-dependent. Moreover, all Gα-, Gβ1- and Gβ2-silenced plants significantly impaired elicitor-induced stomatal closure, elicitor-promoted nitric oxide (NO) production and active oxygen species accumulation in guard cells. To our knowledge, this is the first report of Gα and Gβ subunits involvement in stomatal closure in response to elicitors. Furthermore, silencing of Gα, Gβ1 and Gβ2 has an effect on the transcription of plant defence-related genes when challenged by three elicitors. In conclusion, silencing of G protein subunits results in many interesting plant cell responses, revealing that plant immunity systems employ both conserved and distinct G protein pathways to sense elicitors from distinct phytopathogens formed during plant-microbe evolution.

The bZIP transcription factor MoAP1 mediates the oxidative stress response and is critical for pathogenicity of the rice blast fungus Magnaporthe oryzae

Saccharomyces cerevisiae Yap1 protein is an AP1-like transcription factor involved in the regulation of the oxidative stress response. An ortholog of Yap1, MoAP1, was recently identified from the rice blast fungus Magnaporthe oryzae genome. We found that MoAP1 is highly expressed in conidia and during invasive hyphal growth. The Moap1 mutant was sensitive to H₂O₂, similar to S. cerevisiae yap1 mutants, and MoAP1 complemented Yap1 function in resistance to H₂O₂, albeit partially. The Moap1 mutant also exhibited various defects in aerial hyphal growth, mycelial branching, conidia formation, the production of extracellular peroxidases and laccases, and melanin pigmentation. Consequently, the Moap1 mutant was unable to infect the host plant. The MoAP1-eGFP fusion protein is localized inside the nucleus upon exposure to H₂O₂, suggesting that MoAP1 also functions as a redox sensor. Moreover, through RNA sequence analysis, many MoAP1-regulated genes were identified, including several novel ones that were also involved in pathogenicity. Disruption of respective MGG_01662 (MoAAT) and MGG_02531 (encoding hypothetical protein) genes did not result in any detectable changes in conidial germination and appressorium formation but reduced pathogenicity, whereas the mutant strains of MGG_01230 (MoSSADH) and MGG_15157 (MoACT) showed marketed reductions in aerial hyphal growth, mycelial branching, and loss of conidiation as well as pathogenicity, similar to the Moap1 mutant. Taken together, our studies identify MoAP1 as a positive transcription factor that regulates transcriptions of MGG_01662, MGG_02531, MGG_01230, and MGG_15157 that are important in the growth, development, and pathogenicity of M. oryzae.

Magnaporthe oryzae is a causal agent of rice blast disease and an important model for understanding of fungal development and pathogenicity. To examine the molecular mechanisms involved in conidium formation and appressorium development of M. oryzae, we identified the transcriptional factor MoAP1 as a regulator of the oxidative stress response. Our results indicated that MoAP1 is a stage-specific regulator for conidium formation, morphology, aerial hyphal growth, and also growth in planta. Additionally, we identified four novel genes whose functions were linked to MoAP1 and pathogenicity. Disruption of MGG_01662 (encoding aminobutyrate aminotransferase, MoAat) and MGG_02531 (hypothetical protein) caused minor phenotypic changes but attenuated virulence, and disruption of MGG_01230 (encoding succinic semialdehyde dehydrogenase, MoSsadh) and MGG_15157 (encoding acetyltransferase, MoAct) resulted in drastic reductions in the growth of aerial hyphae and hyphal branching as well as loss of conidiation and pathogenicity. Our studies extend the current understanding of AP1 functions in fungi and reveal that the MoAP1-mediated regulatory network is associated with the pathogenicity of M. oryzae.

The role of vacuolar processing enzyme (VPE) from Nicotiana benthamiana in the elicitor-triggered hypersensitive response and stomatal closure

Elicitors/pathogen-associated molecular patterns (PAMPs) trigger the plant immune system, leading to rapid programmed cell death (hypersensitive response, HR) and stomatal closure. Previous reports have shown that the vacuolar processing enzyme (VPE), a cysteine proteinase responsible for the maturation of vacuolar proteins, has caspase-1-like activity and mediates TMV- and mycotoxin-induced cell death. The role of VPE from Nicotiana benthamiana in the response to three elicitors: bacterial harpin, fungal Nep1, and oomycete boehmerin, is described here. Single-silenced (NbVPE1a or NbVPE1b) and dual-silenced (NbVPE1a/1b) N. benthamiana plants were produced by virus-induced gene silencing. Although NbVPE silencing does not affect H(2)O(2) accumulation triggered by boehmerin, harpin, or Nep1, the HR is absent in NbVPE1a- and NbVPE1a/1b-silenced plants treated with harpin alone. However, NbVPE-silenced plants develop a normal HR after boehmerin and Nep1 treatment. These results suggest that harpin-triggered HR is VPE-dependent. Surprisingly, all gene-silenced plants show significantly impaired elicitor-induced stomatal closure and elicitor-promoted nitric oxide (NO) production in guard cells. Dual-silenced plants show increased elicitor-triggered AOS production in guard cells. The accumulation of transcripts associated with defence and cell redox is modified by VPE silencing in elicitor signalling. Overall, these results indicate that VPE from N. benthamiana functions not only in elicitor-induced HR, but also in elicitor-induced stomatal closure, suggesting that VPE may be involved in elicitor-triggered immunity.

The role of respiratory burst oxidase homologues in elicitor-induced stomatal closure and hypersensitive response in Nicotiana benthamiana

Active oxygen species (AOS) are central components of the defence reactions of plants against pathogens. Plant respiratory burst oxidase homologues (RBOH) of gp91(phox), a plasma membrane protein of the neutrophil nicotinamide adenine dinucleotide phosphate (NADPH) oxidase, play a prominent role in AOS production. The role of two RBOH from Nicotiana benthamiana, NbrbohA and NbrbohB that encode plant NADPH oxidase in the process of elicitor-induced stomatal closure and hypersensitive cell death is described here. NbrbohA was constitutively expressed at a low level, whereas NbrbohB was induced when protein elicitors exist (such as boehmerin, harpin, or INF1). The virus-induced gene-silencing (VIGS) method was used to produce single-silenced (NbrbohA or NbrbohB) and double-silenced (NbrbohA and NbrbohB) N. benthamiana plants. The hypersensitive response (HR) of cell death and pathogenesis-related (PR) gene expression of these gene-silenced N. benthamiana plants, induced by various elicitors, are examined. The HR cell death and transcript accumulation of genes related to the defence response (PR1) were slightly affected, suggesting that RBOH are not essential for elicitor-induced HR and activation of these genes. Interestingly, gene-silenced plants impaired elicitor-induced stomatal closure and elicitor-promoted nitric oxide (NO) production, but not elicitor-induced cytosolic calcium ion accumulation and elicitor-triggered AOS production in guard cells. These results indicate that RBOH from N. benthamiana function in elicitor-induced stomatal closure, but not in elicitor-induced HR.