Constitutive salicylic acid accumulation in pi4kIIIβ1β2 Arabidopsis plants stunts rosette but not root growth

Multi-omics analyses of sid2 mutant reflect the need of isochorismate synthase ICS1 to cope with sulfur limitation in Arabidopsis thaliana

The SID2 (SA INDUCTION-DEFICIENT2) gene that encodes ICS1 (isochorismate synthase), plays a central role in salicylic acid biosynthesis in Arabidopsis. The sid2 and NahG (encoding a bacterial SA hydroxylase) overexpressing mutants (NahG-OE) have currently been shown to outperform wild type, presenting delayed leaf senescence, higher plant biomass and better seed yield. When grown under sulfate-limited conditions (low-S), sid2 mutants exhibited early leaf yellowing compared to the NahG-OE, the npr1 mutant affected in SA signaling pathway, and WT. This indicated that the hypersensitivity of sid2 to sulfate limitation was independent of the canonical npr1 SA-signaling pathway. Transcriptomic and proteomic analyses revealed that major changes occurred in sid2 when cultivated under low-S, changes that were in good accordance with early senescence phenotype and showed the exacerbation of stress responses. The sid2 mutants displayed a lower sulfate uptake capacity when cultivated under low-S and lower S concentrations in their rosettes. Higher glutathione concentrations in sid2 rosettes under low-S were in good accordance with the higher abundance of proteins involved in glutathione and ascorbate redox metabolism. Amino acid and lipid metabolisms were also strongly modified in sid2 under low-S. Depletion of total fatty acids in sid2 under low-S was consistent with the fact that S-metabolism plays a central role in lipid synthesis. Altogether, our results show that functional ICS1 is important for plants to cope with S limiting conditions.

Phospholipid Signaling in Crop Plants: A Field to Explore

In plant models such as Arabidopsis thaliana, phosphatidic acid (PA), a key molecule of lipid signaling, was shown not only to be involved in stress responses, but also in plant development and nutrition. In this article, we highlight lipid signaling existing in crop species. Based on open access databases, we update the list of sequences encoding phospholipases D, phosphoinositide-dependent phospholipases C, and diacylglycerol-kinases, enzymes that lead to the production of PA. We show that structural features of these enzymes from model plants are conserved in equivalent proteins from selected crop species. We then present an in-depth discussion of the structural characteristics of these proteins before focusing on PA binding proteins. For the purpose of this article, we consider RESPIRATORY BURST OXIDASE HOMOLOGUEs (RBOHs), the most documented PA target proteins. Finally, we present pioneering experiments that show, by different approaches such as monitoring of gene expression, use of pharmacological agents, ectopic over-expression of genes, and the creation of silenced mutants, that lipid signaling plays major roles in crop species. Finally, we present major open questions that require attention since we have only a perception of the peak of the iceberg when it comes to the exciting field of phospholipid signaling in plants.

Genome-wide association study and network analysis of in vitro transformation in Populus trichocarpa support key roles of diverse phytohormone pathways and cross talk

Wide variation in amenability to transformation and regeneration (TR) among many plant species and genotypes presents a challenge to the use of genetic engineering in research and breeding. To help understand the causes of this variation, we performed association mapping and network analysis using a population of 1204 wild trees of Populus trichocarpa (black cottonwood). To enable precise and high-throughput phenotyping of callus and shoot TR, we developed a computer vision system that cross-referenced complementary red, green, and blue (RGB) and fluorescent-hyperspectral images. We performed association mapping using single-marker and combined variant methods, followed by statistical tests for epistasis and integration of published multi-omic datasets to identify likely regulatory hubs. We report 409 candidate genes implicated by associations within 5 kb of coding sequences, and epistasis tests implicated 81 of these candidate genes as regulators of one another. Gene ontology terms related to protein-protein interactions and transcriptional regulation are overrepresented, among others. In addition to auxin and cytokinin pathways long established as critical to TR, our results highlight the importance of stress and wounding pathways. Potential regulatory hubs of signaling within and across these pathways include GROWTH REGULATORY FACTOR 1 (GRF1), PHOSPHATIDYLINOSITOL 4-KINASE β1 (PI-4Kβ1), and OBF-BINDING PROTEIN 1 (OBP1).

Signal Peptide Peptidase and PI4Kβ1/2 play opposite roles in plant ER stress response and immunity

The Arabidopsis pi4kβ1,2 mutant is mutated in the phosphatidylinositol 4-kinase (PI4K) β1 and PI4Kβ2 enzymes which are involved in the biosynthesis of phosphatidylinositol 4-phosphate (PI4P), a minor membrane lipid with important signaling roles. pi4kβ1,2 plants display autoimmunity and shorter roots. Though the pi4kβ1,2 mutant has been extensively characterized, the source of its autoimmunity remains largely unknown. In this study, through a genetic suppressor screen, we identified multiple partial loss-of-function alleles of signal peptide peptidase (spp) that can suppress all the defects of pi4kβ1,2. SPP is an intramembrane cleaving aspartic protease. Interestingly, pi4kβ1,2 plants display enhanced ER stress response and mutations in SPP can suppress such phenotype. Furthermore, reduced ER stress responses were observed in the spp single mutants. Overall, our study reveals a previously unknown function of PI4Kβ and SPP in ER stress and plant immunity.

Deletions within intronic T-DNA lead to reversion of T-DNA mutant phenotypes

Agrobacterium-mediated transformation enables random transfer-DNA (T-DNA) insertion into plant genomes. T-DNA insertion into a gene's exons, introns or untranscribed regions close to the start or stop codon can disrupt gene function. Such T-DNA mutants have been useful for reverse genetics analysis, especially in Arabidopsis thaliana. As T-DNAs are inserted into genomic DNA, they are generally believed to be stably inherited. Here, we report a phenomenon of reversion of intronic T-DNA mutant phenotypes. From a suppressor screen using intronic T-DNA pi4kβ1,2 double mutant, we recovered intragenic mutants of pi4kβ1, which suppressed the autoimmunity of the double mutant. These mutants carried deletions in the intronic T-DNAs, resulting in elevated transcription of normal PI4Kβ1. Such reversion of T-DNA insertional mutant phenotype stresses the need for caution when using intronic T-DNA mutants and reiterates the importance of using irreversible null mutant alleles in genetic analyses.

Arabidopsis Tubby domain-containing F-box proteins positively regulate immunity by modulating PI4Kβ protein levels

The Tubby domain, named after the TUBBY protein in mice, binds to phosphatidylinositol 4,5-bisphosphate. Arabidopsis has 11 Tubby domain-containing proteins referred to as Tubby-Like Proteins (TLPs). Of the 11 TLPs, 10 possess the N-terminal F-box domain, which can interact with SKP-like proteins and form SKP1-Cullin-F-box E3 ligase complexes. Although mice TUBBY has been extensively studied, plant TLPs' functions are scarcely detailed. In this study, we show that the Arabidopsis Tubby-like protein 6 (TLP6) and its redundant homologs, TLP1, TLP2, TLP5, and TLP10, positively regulate Arabidopsis immune responses. Furthermore, in an immunoprecipitation mass spectrometry analysis to search for ubiquitination substrates of the TLPs, we identified two redundant phosphoinositide biosynthesis enzymes, phosphatidylinositol 4-kinase β proteins (PI4Kβs), PI4Kβ1 and PI4Kβ2, as TLP interactors. Importantly, TLP6 overexpression lines fully phenocopy the phenotypes of the pi4kβ1,2 mutant, while TLP6 overexpression also leads to increased PI4Kβ2 ubiquitination and reduction in its protein level in a proteasome-dependent manner. Most significantly, TLP6 overexpression does not further enhance the autoimmunity of the pi4kβ1,2 double mutant, supporting the hypothesis that TLP6 targets the PI4Kβs for ubiquitination and degradation. Thus, our study reveals a novel mechanism where TLPs promote plant immune responses by modulating the PI4Kβs protein levels.

The sorting of cargo proteins in the plant trans-Golgi network

Membrane trafficking contributes to distinct protein compositions of organelles and is essential for proper organellar maintenance and functions. The trans-Golgi network (TGN) acts as a sorting station where various cargo proteins are sorted and directed to post-Golgi compartments, such as the multivesicular body or pre-vacuolar compartment, vacuoles, and plasma membrane. The spatial and temporal segregation of cargo proteins within the TGN, which is mediated with different sets of regulators including small GTPases and cargo adaptors, is a fundamental process in the sorting machinery. Recent studies with powerful imaging technologies have suggested that the TGN possesses spatially distinct subdomains or zones for different trafficking pathways. In this review, we will summarize the spatially and dynamically characteristic features of the plant TGN and their relation to cargo protein trafficking.

Phosphatidylinositol 4-phosphate: a key determinant of plasma membrane identity and function in plants

Phosphatidylinositol 4-phosphate (PI4P) is an anionic phospholipid which has been described as a master regulator of the Golgi apparatus in eukaryotic cells. However, recent evidence suggests that PI4P mainly accumulates at the plasma membrane in all plant cells analyzed so far. In addition, many functions that are typically attributed to phosphatidylinositol 4,5-bisphosphate (PI(4,5)P₂ ) in animal and yeast cells are also supported by PI4P in plants. For example, PI4P is the key anionic lipid that powers the strong electrostatic properties of the plasma membrane. Phosphatidylinositol 4-phosphate is also required for the establishment of stable membrane contacts between the endoplasmic reticulum and the plasma membrane, for exocytosis and to support signaling pathways. Thus, we propose that PI4P has a prominent role in specifying the identity of the plasma membrane and in supporting some of its key functions and should be considered a hallmark lipid of this compartment.

A ménage à trois: salicylic acid, growth inhibition, and immunity

Salicylic acid (SA) is a plant hormone almost exclusively associated with the promotion of immunity. It is also known that SA has a negative impact on plant growth, yet only limited efforts have been dedicated to explain this facet of SA action. In this review, we focus on SA-related reduced growth and discuss whether it is a regulated process and if the role of SA in immunity imperatively comes with growth suppression. We highlight molecular targets of SA that interfere with growth and describe scenarios where SA can improve plant immunity without a growth penalty.

An Arabidopsis mutant deficient in phosphatidylinositol-4-phosphate kinases ß1 and ß2 displays altered auxin-related responses in roots

Phosphatidylinositol 4-kinases (PI4Ks) are the first enzymes that commit phosphatidylinositol into the phosphoinositide pathway. Here, we show that Arabidopsis thaliana seedlings deficient in PI4Kβ1 and β2 have several developmental defects including shorter roots and unfinished cytokinesis. The pi4kβ1β2 double mutant was insensitive to exogenous auxin concerning inhibition of root length and cell elongation; it also responded more slowly to gravistimulation. The pi4kß1ß2 root transcriptome displayed some similarities to a wild type plant response to auxin. Yet, not all the genes displayed such a constitutive auxin-like response. Besides, most assessed genes did not respond to exogenous auxin. This is consistent with data with the transcriptional reporter DR5-GUS. The content of bioactive auxin in the pi4kß1ß2 roots was similar to that in wild-type ones. Yet, an enhanced auxin-conjugating activity was detected and the auxin level reporter DII-VENUS did not respond to exogenous auxin in pi4kß1ß2 mutant. The mutant exhibited altered subcellular trafficking behavior including the trapping of PIN-FORMED 2 protein in rapidly moving vesicles. Bigger and less fragmented vacuoles were observed in pi4kß1ß2 roots when compared to the wild type. Furthermore, the actin filament web of the pi4kß1ß2 double mutant was less dense than in wild-type seedling roots, and less prone to rebuilding after treatment with latrunculin B. A mechanistic model is proposed in which an altered PI4K activity leads to actin filament disorganization, changes in vesicle trafficking, and altered auxin homeostasis and response resulting in a pleiotropic root phenotypes.

Salicylic Acid in Root Growth and Development

In plants, salicylic acid (SA) is a hormone that mediates a plant’s defense against pathogens. SA also takes an active role in a plant’s response to various abiotic stresses, including chilling, drought, salinity, and heavy metals. In addition, in recent years, numerous studies have confirmed the important role of SA in plant morphogenesis. In this review, we summarize data on changes in root morphology following SA treatments under both normal and stress conditions. Finally, we provide evidence for the role of SA in maintaining the balance between stress responses and morphogenesis in plant development, and also for the presence of SA crosstalk with other plant hormones during this process.

A nanodomain-anchored scaffolding complex is required for the function and localization of phosphatidylinositol 4-kinase alpha in plants

Phosphoinositides are low-abundant lipids that participate in the acquisition of membrane identity through their spatiotemporal enrichment in specific compartments. Phosphatidylinositol 4-phosphate (PI4P) accumulates at the plant plasma membrane driving its high electrostatic potential, and thereby facilitating interactions with polybasic regions of proteins. PI4Kα1 has been suggested to produce PI4P at the plasma membrane, but how it is recruited to this compartment is unknown. Here, we pin-point the mechanism that tethers Arabidopsis thaliana phosphatidylinositol 4-kinase alpha1 (PI4Kα1) to the plasma membrane via a nanodomain-anchored scaffolding complex. We established that PI4Kα1 is part of a complex composed of proteins from the NO-POLLEN-GERMINATION, EFR3-OF-PLANTS, and HYCCIN-CONTAINING families. Comprehensive knockout and knockdown strategies revealed that subunits of the PI4Kα1 complex are essential for pollen, embryonic, and post-embryonic development. We further found that the PI4Kα1 complex is immobilized in plasma membrane nanodomains. Using synthetic mis-targeting strategies, we demonstrate that a combination of lipid anchoring and scaffolding localizes PI4Kα1 to the plasma membrane, which is essential for its function. Together, this work opens perspectives on the mechanisms and function of plasma membrane nanopatterning by lipid kinases.

Engineering of benzoxazinoid biosynthesis in Arabidopsis thaliana: Metabolic and physiological challenges

Plant specialised metabolites constitute a layer of chemical defence. Classes of the defence compounds are often restricted to a certain taxon of plants, e.g. benzoxazinoids (BX) are characteristically detected in grasses. BXs confer wide-range defence by controlling herbivores and microbial pathogens and are allelopathic compounds. In the crops maize, wheat and rye high concentrations of BXs are synthesised at an early developmental stage. By transfer of six Bx-genes (Bx1 to Bx5 and Bx8) it was possible to establish the biosynthesis of 2,4-dihydroxy-1,4-benzoxazin-3-one glucoside (GDIBOA) in a concentration of up to 143 nmol/g dry weight in Arabidopsis thaliana. Our results indicate that inefficient channeling of substrates along the pathway and metabolisation of intermediates in host plants might be a general drawback for transgenic establishment of specialised metabolite biosynthesis pathways. As a consequence, BX levels required for defence are not obtained in Arabidopsis. We could show that indolin-2-one (ION), the first specific intermediate, is phytotoxic and is metabolised by hydroxylation and glycosylation by a wide spectrum of plants. In Arabidopsis, metabolic stress due to the enrichment of ION leads to elevated levels of salicylic acid (SA) and in addition to its intrinsic phytotoxicity, ION affects plant morphology indirectly via SA. We could show that Bx3 has a crucial role in the evolution of the pathway, first based on its impact on flux into the pathway and, second by C3-hydroxylation of the phytotoxic ION. Thereby BX3 interferes with a supposedly generic detoxification system towards the non-specific intermediate.

Phospholipids in Salt Stress Response

High salinity threatens crop production by harming plants and interfering with their development. Plant cells respond to salt stress in various ways, all of which involve multiple components such as proteins, peptides, lipids, sugars, and phytohormones. Phospholipids, important components of bio-membranes, are small amphoteric molecular compounds. These have attracted significant attention in recent years due to the regulatory effect they have on cellular activity. Over the past few decades, genetic and biochemical analyses have partly revealed that phospholipids regulate salt stress response by participating in salt stress signal transduction. In this review, we summarize the generation and metabolism of phospholipid phosphatidic acid (PA), phosphoinositides (PIs), phosphatidylserine (PS), phosphatidylcholine (PC), phosphatidylethanolamine (PE) and phosphatidylglycerol (PG), as well as the regulatory role each phospholipid plays in the salt stress response. We also discuss the possible regulatory role based on how they act during other cellular activities.

An Anecdote on Prospective Protein Targets for Developing Novel Plant Growth Regulators

Phytohormones are the main regulatory molecules of core signalling networks associated with plant life cycle regulation. Manipulation of hormone signalling cascade enables the control over physiological traits of plant, which has major applications in field of agriculture and food sustainability. Hence, stable analogues of these hormones are long sought after and many of them are currently known, but the quest for more effective, stable and economically viable analogues is still going on. This search has been further strengthened by the identification of the components of signalling cascade such as receptors, downstream cascade members and transcription factors. Furthermore, many proteins of phytohormone cascades are available in crystallized forms. Such crystallized structures can provide the basis for identification of novel interacting compounds using in silico approach. Plenty of computational tools and bioinformatics software are now available that can aid in this process. Here, the metadata of all the major phytohormone signalling cascades are presented along with discussion on major protein-ligand interactions and protein components that may act as a potential target for manipulation of phytohormone signalling cascade. Furthermore, structural aspects of phytohormones and their known analogues are also discussed that can provide the basis for the synthesis of novel analogues.

The immune components ENHANCED DISEASE SUSCEPTIBILITY 1 and PHYTOALEXIN DEFICIENT 4 are required for cell death caused by overaccumulation of ceramides in Arabidopsis

Sphingolipids have key functions in plant membrane structure and signaling. Perturbations of plant sphingolipid metabolism often induce cell death and salicylic acid (SA) accumulation; SA accumulation, in turn, promotes sphingolipid metabolism and further cell death. However, the underlying molecular mechanisms remain unclear. Here, we show that the Arabidopsis thaliana lipase-like protein ENHANCED DISEASE SUSCEPTIBILITY 1 (EDS1) and its partner PHYTOALEXIN DEFICIENT 4 (PAD4) participate in sphingolipid metabolism and associated cell death. The accelerated cell death 5 (acd5) mutants accumulate ceramides due to a defect in ceramide kinase and show spontaneous cell death. Loss of function of EDS1, PAD4 or SALICYLIC ACID INDUCTION DEFICIENT 2 (SID2) in the acd5 background suppressed the acd5 cell death phenotype and prevented ceramide accumulation. Treatment with the SA analogue benzothiadiazole partially restored sphingolipid accumulation in the acd5 pad4 and acd5 eds1 double mutants, showing that the inhibitory effect of the pad4-1 and eds1-2 mutations on acd5-conferred sphingolipid accumulation partly depends on SA. Moreover, the pad4-1 and eds1-2 mutations substantially rescued the susceptibility of the acd5 mutant to Botrytis cinerea. Consistent with this, B. cinerea-induced ceramide accumulation requires PAD4 or EDS1. Finally, examination of plants overexpressing the ceramide synthase gene LAG1 HOMOLOGUE2 suggested that EDS1, PAD4 and SA are involved in long-chain ceramide metabolism and ceramide-associated cell death. Collectively, our observations reveal that EDS1 and PAD4 mediate ceramide (especially long-chain ceramide) metabolism and associated cell death, by SA-dependent and SA-independent pathways.

Regulation of the microsomal proteome by salicylic acid and deficiency of phosphatidylinositol-4-kinases β1 and β2 in Arabidopsis thaliana

Phosphatidylinositol-4-kinases β1 and β2 (PI4Kβ1/PI4Kβ2), which are responsible for phosphorylation of phosphatidylinositol to phosphatidylinositol-4-phosphate, have important roles in plant vesicular trafficking. Moreover, PI4Kβ1/PI4Kβ2 negatively regulates biosynthesis of phytohormone salicylic acid (SA), a key player in plant immune responses. The study focused on the effect of PI4Kβ1/PI4Kβ2 deficiency and SA level on the proteome of microsomal fraction. For that purpose we used four Arabidopsis thaliana genotypes: wild type; double mutant with impaired function of PI4Kβ1/PI4Kβ2 (pi4kβ1/pi4kβ2) exhibiting high SA level; sid2 mutant with impaired SA biosynthesis depending on the isochorismate synthase 1 and triple mutant sid2/pi4kβ1/pi4kβ2. We identified 1797 proteins whose levels were changed between genotypes. We showed that increased SA concentration affected the levels of 473 proteins. This includes typical SA pathway markers but also points to connections between SA pathway and clathrin-independent endocytosis (flotillins) and exocytosis/protein secretion (syntaxins, tetraspanin) to be investigated in future. In contrast to SA, the absence of PI4Kβ1/PI4Kβ2 itself affected only 27 proteins. Among them we identified CERK1, a receptor for chitin. Although PI4Kβ1/PI4Kβ2 deficiency itself did not have a substantial impact on the proteome of the microsomal fraction, our data clearly show that it enhances proteome changes when SA pathway is modulated in parallel.

Phospholipid Signaling Is a Component of the Salicylic Acid Response in Plant Cell Suspension Cultures

Salicylic acid (SA) is an important signaling molecule involved in plant defense. While many proteins play essential roles in SA signaling, increasing evidence shows that responses to SA appear to involve and require lipid signals. The phospholipid-generated signal transduction involves a family of enzymes that catalyze the hydrolysis or phosphorylation of phospholipids in membranes to generate signaling molecules, which are important in the plant cellular response. In this review, we focus first, the role of SA as a mitigator in biotic/abiotic stress. Later, we describe the experimental evidence supporting the phospholipid–SA connection in plant cells, emphasizing the roles of the secondary lipid messengers (phosphatidylinositol 4,5-bisphosphate (PIP₂) and phosphatidic acid (PA)) and related enzymes (phospholipase D (PLD) and phospholipase C (PLC)). By placing these recent finding in context of phospholipids and SA in plant cells, we highlight the role of phospholipids as modulators in the early steps of SA triggered transduction in plant cells.

Deciphering the Binding of Salicylic Acid to Arabidopsis thaliana Chloroplastic GAPDH-A1

Salicylic acid (SA) has an essential role in the responses of plants to pathogens. SA initiates defence signalling via binding to proteins. NPR1 is a transcriptional co-activator and a key target of SA binding. Many other proteins have recently been shown to bind SA. Amongst these proteins are important enzymes of primary metabolism. This fact could stand behind SA’s ability to control energy fluxes in stressed plants. Nevertheless, only sparse information exists on the role and mechanisms of such binding. Glyceraldehyde 3-phosphate dehydrogenase (GAPDH) was previously demonstrated to bind SA both in human and plants. Here, we detail that the A1 isomer of chloroplastic glyceraldehyde 3-phosphate dehydrogenase (GAPA1) from Arabidopsis thaliana binds SA with a K_D of 16.7 nM, as shown in surface plasmon resonance experiments. Besides, we show that SA inhibits its GAPDH activity in vitro. To gain some insight into the underlying molecular interactions and binding mechanism, we combined in silico molecular docking experiments and molecular dynamics simulations on the free protein and protein–ligand complex. The molecular docking analysis yielded to the identification of two putative binding pockets for SA. A simulation in water of the complex between SA and the protein allowed us to determine that only one pocket—a surface cavity around Asn35—would efficiently bind SA in the presence of solvent. In silico mutagenesis and simulations of the ligand/protein complexes pointed to the importance of Asn35 and Arg81 in the binding of SA to GAPA1. The importance of this is further supported through experimental biochemical assays. Indeed, mutating GAPA1 Asn35 into Gly or Arg81 into Leu strongly diminished the ability of the enzyme to bind SA. The very same cavity is responsible for the NADP⁺ binding to GAPA1. More precisely, modelling suggests that SA binds to the very site where the pyrimidine group of the cofactor fits. NADH inhibited in a dose-response manner the binding of SA to GAPA1, validating our data.

CRISPR/Cas9-targeted mutagenesis of the tomato susceptibility gene PMR4 for resistance against powdery mildew

BACKGROUND

The development of CRISPR/Cas9 technology has facilitated targeted mutagenesis in an efficient and precise way. Previously, RNAi silencing of the susceptibility (S) gene PowderyMildewResistance 4 (PMR4) in tomato has been shown to enhance resistance against the powdery mildew pathogen Oidium neolycopersici (On).

RESULTS

To study whether full knock-out of the tomato PMR4 gene would result in a higher level of resistance than in the RNAi-silenced transgenic plants we generated tomato PMR4 CRISPR mutants. We used a CRISPR/Cas9 construct containing four single-guide RNAs (sgRNAs) targeting the tomato PMR4 gene to increase the possibility of large deletions in the mutants. After PCR-based selection and sequencing of transformants, we identified five different mutation events, including deletions from 4 to 900-bp, a 1-bp insertion and a 892-bp inversion. These mutants all showed reduced susceptibility to On based on visual scoring of disease symptoms and quantification of relative fungal biomass. Histological observations revealed a significantly higher occurrence of hypersensitive response-like cell death at sites of fungal infection in the pmr4 mutants compared to wild-type plants. Both haustorial formation and hyphal growth were diminished but not completely inhibited in the mutants.

CONCLUSION

CRISPR/Cas-9 targeted mutagenesis of the tomato PMR4 gene resulted in mutants with reduced but not complete loss of susceptibility to the PM pathogen On. Our study demonstrates the efficiency and versatility of the CRISPR/Cas9 system as a powerful tool to study and characterize S-genes by generating different types of mutations.

Multiple immunity-related genes control susceptibility of Arabidopsis thaliana to the parasitic weed Phelipanche aegyptiaca

Parasitic weeds represent a major threat to agricultural production across the world. Little is known about which host genetic pathways determine compatibility for any host–parasitic plant interaction. We developed a quantitative assay to characterize the growth of the parasitic weed Phelipanche aegyptiaca on 46 mutant lines of the host plant Arabidopsis thaliana to identify host genes that are essential for susceptibility to the parasite. A. thaliana host plants with mutations in genes involved in jasmonic acid biosynthesis/signaling or the negative regulation of plant immunity were less susceptible to P. aegyptiaca parasitization. In contrast, A. thaliana plants with a mutant allele of the putative immunity hub gene Pfd6 were more susceptible to parasitization. Additionally, quantitative PCR revealed that P. aegyptiaca parasitization leads to transcriptional reprograming of several hormone signaling pathways. While most tested A. thaliana lines were fully susceptible to P. aegyptiaca parasitization, this work revealed several host genes essential for full susceptibility or resistance to parasitism. Altering these pathways may be a viable approach for limiting host plant susceptibility to parasitism.

Identification of salicylic acid-independent responses in an Arabidopsis phosphatidylinositol 4-kinase beta double mutant

BACKGROUND AND AIMS

We have recently shown that an Arabidopsis thaliana double mutant of type III phosphatidylinositol-4-kinases (PI4Ks), pi4kβ1β2, constitutively accumulated a high level of salicylic acid (SA). By crossing this pi4kβ1β2 double mutant with mutants impaired in SA synthesis (such as sid2 impaired in isochorismate synthase) or transduction, we demonstrated that the high SA level was responsible for the dwarfism phenotype of the double mutant. Here we aimed to distinguish between the SA-dependent and SA-independent effects triggered by the deficiency in PI4Kβ1 and PI4Kβ2.

METHODS

To achieve this we used the sid2pi4kβ1β2 triple mutant. High-throughput analyses of phytohormones were performed on this mutant together with pi4kβ1β2 and sid2 mutants and wild-type plants. Responses to pathogens, namely Hyaloperonospora arabidopsidis, Pseudomonas syringae and Botrytis cinerea, and also to the non-host fungus Blumeria graminis, were also determined. Callose accumulation was monitored in response to flagellin.

KEY RESULTS

We show here the prominent role of high SA levels in influencing the concentration of many other tested phytohormones, including abscisic acid and its derivatives, the aspartate-conjugated form of indole-3-acetic acid and some cytokinins such as cis-zeatin. We show that the increased resistance of pi4kβ1β2 plants to the host pathogens H. arabidopsidis, P. syringae pv. tomato DC3000 and Bothrytis cinerea is dependent on accumulation of high SA levels. In contrast, accumulation of callose in pi4kβ1β2 after flagellin treatment was independent of SA. Concerning the response to Blumeria graminis, both callose accumulation and fungal penetration were enhanced in the pi4kβ1β2 double mutant compared to wild-type plants. Both of these processes occurred in an SA-independent manner.

CONCLUSIONS

Our data extensively illustrate the influence of SA on other phytohormone levels. The sid2pi4kβ1β2 triple mutant revealed the role of PI4Kβ1/β2 per se, thus showing the importance of these enzymes in plant defence responses.

Nitric Oxide Enhances Rice Resistance to Rice Black-Streaked Dwarf Virus Infection

BACKGROUND

Rice black-streaked dwarf virus (RBSDV) causes one of the most important rice virus diseases of plants in East Asia. However, molecular mechanism(s)controlling rice resistance to infection is largely unknown.

RESULTS

In this paper, we showed that RBSDV infection in rice significantly induced nitric oxide (NO) production. This finding was further validated through a genetic approach using a RBSDV susceptible (Nipponbare) and a RBSDV resistant (15HPO187) cultivar. The production of endogenous NO was muchhigher in the 15HPO187 plants, leading to a much lower RBSDV disease incidence. Pharmacological studies showed that the applications of NO-releasingcompounds (i.e., sodium nitroprusside [SNP] and nitrosoglutathione [GSNO]) to rice plants reduced RBSDV disease incidence. After RBSDV infection, the levels of OsICS1, OsPR1b and OsWRKY 45 transcripts were significantly up-regulated by NO in Nipponbare. The increased salicylic acid contents were also observed. After the SNP treatment, protein S-nitrosylation in rice plants was also increased, suggesting that the NO-triggered resistance to RBSDV infection was partially mediated at the post-translational level. Although Osnia2 mutant rice produced less endogenous NO after RBSDV inoculation and showed a higher RBSDV disease incidence, its RBSDV susceptibility could be reduced by SNP treatment.

CONCLUSIONS

Collectively, our genetic and molecular evidence revealed that endogenous NO was a vital signal responsible for rice resistance to RBSDV infection.

"Salicylic Acid Mutant Collection" as a Tool to Explore the Role of Salicylic Acid in Regulation of Plant Growth under a Changing Environment

The phytohormone salicylic acid (SA) has a crucial role in plant physiology. Its role is best described in the context of plant response to pathogen attack. During infection, SA is rapidly accumulated throughout the green tissues and is important for both local and systemic defences. However, some genetic/metabolic variations can also result in SA overaccumulation in plants, even in basal conditions. To date, more than forty Arabidopsis thaliana mutants have been described as having enhanced endogenous SA levels or constitutively activated SA signalling pathways. In this study, we established a collection of mutants containing different SA levels due to diverse genetic modifications and distinct gene functions. We chose prototypic SA-overaccumulators (SA-OAs), such as bon1-1, but also “non-typical” ones such as exo70b1-1; the selection of OA is accompanied by their crosses with SA-deficient lines. Here, we extensively studied the plant development and SA level/signalling under various growth conditions in soil and in vitro, and showed a strong negative correlation between rosette size, SA content and PR1/ICS1 transcript signature. SA-OAs (namely cpr5, acd6, bon1-1, fah1/fah2 and pi4kβ1β2) had bigger rosettes under high light conditions, whereas WT plants did not. Our data provide new insights clarifying a link between SA and plant behaviour under environmental stresses. The presented SA mutant collection is thus a suitable tool to shed light on the mechanisms underlying trade-offs between growth and defence in plants.

Salicylic Acid Binding Proteins (SABPs): The Hidden Forefront of Salicylic Acid Signalling

Salicylic acid (SA) is a phytohormone that plays important roles in many aspects of plant life, notably in plant defenses against pathogens. Key mechanisms of SA signal transduction pathways have now been uncovered. Even though details are still missing, we understand how SA production is regulated and which molecular machinery is implicated in the control of downstream transcriptional responses. The NPR1 pathway has been described to play the main role in SA transduction. However, the mode of SA perception is unclear. NPR1 protein has been shown to bind SA. Nevertheless, NPR1 action requires upstream regulatory events (such as a change in cell redox status). Besides, a number of SA-induced responses are independent from NPR1. This shows that there is more than one way for plants to perceive SA. Indeed, multiple SA-binding proteins of contrasting structures and functions have now been identified. Yet, all of these proteins can be considered as candidate SA receptors and might have a role in multinodal (decentralized) SA input. This phenomenon is unprecedented for other plant hormones and is a point of discussion of this review.

Actin depolymerization is able to increase plant resistance against pathogens via activation of salicylic acid signalling pathway

The integrity of the actin cytoskeleton is essential for plant immune signalling. Consequently, it is generally assumed that actin disruption reduces plant resistance to pathogen attack. Here, we demonstrate that actin depolymerization induced a dramatic increase in salicylic acid (SA) levels in Arabidopsis thaliana. Transcriptomic analysis showed that the SA pathway was activated due to the action of isochorismate synthase (ICS). The effect was also confirmed in Brassica napus. This raises the question of whether actin depolymerization could, under particular conditions, lead to increased resistance to pathogens. Thus, we explored the effect of pretreatment with actin-depolymerizing drugs on the resistance of Arabidopsis thaliana to the bacterial pathogen Pseudomonas syringae, and on the resistance of an important crop Brassica napus to its natural fungal pathogen Leptosphaeria maculans. In both pathosystems, actin depolymerization activated the SA pathway, leading to increased plant resistance. To our best knowledge, we herein provide the first direct evidence that disruption of the actin cytoskeleton can actually lead to increased plant resistance to pathogens, and that SA is crucial to this process.

Salicylic Acid Signals Plant Defence against Cadmium Toxicity

Salicylic acid (SA), as an enigmatic signalling molecule in plants, has been intensively studied to elucidate its role in defence against biotic and abiotic stresses. This review focuses on recent research on the role of the SA signalling pathway in regulating cadmium (Cd) tolerance in plants under various SA exposure methods, including pre-soaking, hydroponic exposure, and spraying. Pretreatment with appropriate levels of SA showed a mitigating effect on Cd damage, whereas an excessive dose of exogenous SA aggravated the toxic effects of Cd. SA signalling mechanisms are mainly associated with modification of reactive oxygen species (ROS) levels in plant tissues. Then, ROS, as second messengers, regulate a series of physiological and genetic adaptive responses, including remodelling cell wall construction, balancing the uptake of Cd and other ions, refining the antioxidant defence system, and regulating photosynthesis, glutathione synthesis and senescence. These findings together elucidate the expanding role of SA in phytotoxicology.

A dual role for cell plate-associated PI4Kβ in endocytosis and phragmoplast dynamics during plant somatic cytokinesis

Plant cytokinesis involves membrane trafficking and cytoskeletal rearrangements. Here, we report that the phosphoinositide kinases PI4Kβ1 and PI4Kβ2 integrate these processes in Arabidopsis thaliana (Arabidopsis) roots. Cytokinetic defects of an Arabidopsis pi4kβ1 pi4kβ2 double mutant are accompanied by defects in membrane trafficking. Specifically, we show that trafficking of the proteins KNOLLE and PIN2 at the cell plate, clathrin recruitment, and endocytosis is impaired in pi4kβ1 pi4kβ2 double mutants, accompanied by unfused vesicles at the nascent cell plate and around cell wall stubs. Interestingly, pi4kβ1 pi4kβ2 plants also display ectopic overstabilization of phragmoplast microtubules, which guide membrane trafficking at the cell plate. The overstabilization of phragmoplasts in the double mutant coincides with mislocalization of the microtubule-associated protein 65-3 (MAP65-3), which cross-links microtubules and is a downstream target for inhibition by the MAP kinase MPK4. Based on similar cytokinetic defects of the pi4kβ1 pi4kβ2 and mpk4-2 mutants and genetic and physical interaction of PI4Kβ1 and MPK4, we propose that PI4Kβ and MPK4 influence localization and activity of MAP65-3, respectively, acting synergistically to control phragmoplast dynamics.

Cytidinediphosphate-diacylglycerol synthase 5 is required for phospholipid homeostasis and is negatively involved in hyperosmotic stress tolerance

Cytidinediphosphate diacylglycerol synthase (CDS) uses phosphatidic acid (PA) and cytidinetriphosphate to produce cytidinediphosphate-diacylglycerol, an intermediate for phosphatidylglycerol (PG) and phosphatidylinositol (PI) synthesis. This study shows that CDS5, one of the five CDSs of the Oryza sativa (rice) genome, has multifaceted effects on plant growth and stress responses. The loss of CDS5 resulted in a decrease in PG and PI levels, defective thylakoid membranes, pale leaves in seedlings and growth retardation. In addition, the loss of CDS5 led to an elevated PA level and enhanced hyperosmotic tolerance. The inhibition of phospholipase D (PLD)-derived PA formation in cds5 restored the hyperosmotic stress tolerance of the mutant phenotype to that of the wild type, suggesting that CDS5 functions as a suppressor in PLD-derived PA signaling and negatively affects hyperosmotic stress tolerance.

The Arabidopsis thaliana non-specific phospholipase C2 is involved in the response to Pseudomonas syringae attack

Background and Aims

The non-specific phospholipase C (NPC) is a new member of the plant phospholipase family that reacts to abiotic environmental stresses, such as phosphate deficiency, high salinity, heat and aluminium toxicity, and is involved in root development, silicon distribution and brassinolide signalling. Six NPC genes (NPC1-NPC6) are found in the Arabidopsis genome. The NPC2 isoform has not been experimentally characterized so far.

Methods

The Arabidopsis NPC2 isoform was cloned and heterologously expressed in Escherichia coli. NPC2 enzyme activity was determined using fluorescent phosphatidylcholine as a substrate. Tissue expression and subcellular localization were analysed using GUS- and GFP-tagged NPC2. The expression patterns of NPC2 were analysed via quantitative real-time PCR. Independent homozygous transgenic plant lines overexpressing NPC2 under the control of a 35S promoter were generated, and reactive oxygen species were measured using a luminol-based assay.

Key Results

The heterologously expressed protein possessed phospholipase C activity, being able to hydrolyse phosphatidylcholine to diacylglycerol. NPC2 tagged with GFP was predominantly localized to the Golgi apparatus in Arabidopsis roots. The level of NPC2 transcript is rapidly altered during plant immune responses and correlates with the activation of multiple layers of the plant defence system. Transcription of NPC2 decreased substantially after plant infiltration with Pseudomonas syringae, flagellin peptide flg22 and salicylic acid treatments and expression of the effector molecule AvrRpm1. The decrease in NPC2 transcript levels correlated with a decrease in NPC2 enzyme activity. NPC2-overexpressing mutants showed higher reactive oxygen species production triggered by flg22.

Conclusions

This first experimental characterization of NPC2 provides new insights into the role of the non-specific phospholipase C protein family. The results suggest that NPC2 is involved in the response of Arabidopsis to P. syringae attack.

The chromatin remodeler DDM1 promotes hybrid vigor by regulating salicylic acid metabolism

In plants, hybrid vigor is influenced by genetic and epigenetic mechanisms; however, the molecular pathways are poorly understood. We investigated the potential contributions of epigenetic regulators to heterosis in Arabidposis and found that the chromatin remodeler DECREASED DNA METHYLATION 1 (DDM1) affects early seedling growth heterosis in Col/C24 hybrids. ddm1 mutants showed impaired heterosis and increased expression of non-additively expressed genes related to salicylic acid metabolism. Interestingly, our data suggest that salicylic acid is a hormetic regulator of seedling growth heterosis, and that hybrid vigor arises from crosses that produce optimal salicylic acid levels. Although DNA methylation failed to correlate with differential non-additively expressed gene expression, we uncovered DDM1 as an epigenetic link between salicylic acid metabolism and heterosis, and propose that the endogenous salicylic acid levels of parental plants can be used to predict the heterotic outcome. Salicylic acid protects plants from pathogens and abiotic stress. Thus, our findings suggest that stress-induced hormesis, which has been associated with increased longevity in other organisms, may underlie specific hybrid vigor traits.

Leptosphaeria maculans effector AvrLm4-7 affects salicylic acid (SA) and ethylene (ET) signalling and hydrogen peroxide (H2 O2 ) accumulation in Brassica napus

To achieve host colonization, successful pathogens need to overcome plant basal defences. For this, (hemi)biotrophic pathogens secrete effectors that interfere with a range of physiological processes of the host plant. AvrLm4-7 is one of the cloned effectors from the hemibiotrophic fungus Leptosphaeria maculans 'brassicaceae' infecting mainly oilseed rape (Brassica napus). Although its mode of action is still unknown, AvrLm4-7 is strongly involved in L. maculans virulence. Here, we investigated the effect of AvrLm4-7 on plant defence responses in a susceptible cultivar of B. napus. Using two isogenic L. maculans isolates differing in the presence of a functional AvrLm4-7 allele [absence ('a4a7') and presence ('A4A7') of the allele], the plant hormone concentrations, defence-related gene transcription and reactive oxygen species (ROS) accumulation were analysed in infected B. napus cotyledons. Various components of the plant immune system were affected. Infection with the 'A4A7' isolate caused suppression of salicylic acid- and ethylene-dependent signalling, the pathways regulating an effective defence against L. maculans infection. Furthermore, ROS accumulation was decreased in cotyledons infected with the 'A4A7' isolate. Treatment with an antioxidant agent, ascorbic acid, increased the aggressiveness of the 'a4a7' L. maculans isolate, but not that of the 'A4A7' isolate. Together, our results suggest that the increased aggressiveness of the 'A4A7' L. maculans isolate could be caused by defects in ROS-dependent defence and/or linked to suppressed SA and ET signalling. This is the first study to provide insights into the manipulation of B. napus defence responses by an effector of L. maculans.

Separation and identification of candidate protein elicitors from the cultivation medium of Leptosphaeria maculans inducing resistance in Brassica napus

The Dothideomycete Leptosphaeria maculans, a worldwide fungal pathogen of oilseed rape (Brassica napus), secretes a broad spectrum of molecules into the cultivation medium during growth in vitro. Here, candidate elicitor molecules, which induce resistance in B. napus to L. maculans, were identified in the cultivation medium. The elicitation activity was indicated by increased transcription of pathogenesis-related gene 1 (PR1) and enhanced resistance of B. napus plants to the invasion of L. maculans. The elicitation activity was significantly lowered when the cultivation medium was heated to 80°C. Active components were further characterized by specific cleavage with the proteolytic enzymes trypsin and proteinase K and with glycosidases α-amylase and β-glucanase. The elicitor activity was eliminated by proteolytic digestion while glycosidases had no effect. The filtered medium was fractionated by either ion-exchange chromatography or isoelectric focusing. Mass spectrometry analysis of the most active fractions obtained by both separation procedures revealed predominantly enzymes that can be involved in the degradation of plant cell wall polysaccharides. This is the first study searching for L. maculans-specific secreted elicitors with a potential to be used as defense-activating agents in the protection of B. napus against L. maculans in agriculture. © 2016 American Institute of Chemical Engineers Biotechnol. Prog., 32:918-928, 2016.

Constitutive Negative Regulation of R Proteins in Arabidopsis also via Autophagy Related Pathway?

Even though resistance (R) genes are among the most studied components of the plant immunity, there remain still a lot of aspects to be explained about the regulation of their function. Many gain-of-function mutants of R genes and loss-of-function of their regulators often demonstrate up-regulated defense responses in combination with dwarf stature and/or spontaneous leaf lesions formation. For most of these mutants, phenotypes are a consequence of an ectopic activation of R genes. Based on the compilation and comparison of published results in this field, we have concluded that the constitutively activated defense phenotypes recurrently arise by disruption of tight, constitutive and multilevel negative control of some of R proteins that might involve also their targeting to the autophagy pathway. This mode of R protein regulation is supported also by protein-protein interactions listed in available databases, as well as in silico search for autophagy machinery interacting motifs. The suggested model could resolve some explanatory discrepancies found in the studies of the immunity responses of autophagy mutants.

The Arabidopsis microtubule-associated protein MAP65-3 supports infection by filamentous biotrophic pathogens by down-regulating salicylic acid-dependent defenses

The oomycete Hyaloperonospora arabidopsidis and the ascomycete Erysiphe cruciferarum are obligate biotrophic pathogens causing downy mildew and powdery mildew, respectively, on Arabidopsis. Upon infection, the filamentous pathogens induce the formation of intracellular bulbous structures called haustoria, which are required for the biotrophic lifestyle. We previously showed that the microtubule-associated protein AtMAP65-3 plays a critical role in organizing cytoskeleton microtubule arrays during mitosis and cytokinesis. This renders the protein essential for the development of giant cells, which are the feeding sites induced by root knot nematodes. Here, we show that AtMAP65-3 expression is also induced in leaves upon infection by the downy mildew oomycete and the powdery mildew fungus. Loss of AtMAP65-3 function in the map65-3 mutant dramatically reduced infection by both pathogens, predominantly at the stages of leaf penetration. Whole-transcriptome analysis showed an over-represented, constitutive activation of genes involved in salicylic acid (SA) biosynthesis, signaling, and defense execution in map65-3, whereas jasmonic acid (JA)-mediated signaling was down-regulated. Preventing SA synthesis and accumulation in map65-3 rescued plant susceptibility to pathogens, but not the developmental phenotype caused by cytoskeleton defaults. AtMAP65-3 thus has a dual role. It positively regulates cytokinesis, thus plant growth and development, and negatively interferes with plant defense against filamentous biotrophs. Our data suggest that downy mildew and powdery mildew stimulate AtMAP65-3 expression to down-regulate SA signaling for infection.

Plant Phosphoglycerolipids: The Gatekeepers of Vascular Cell Differentiation

In higher plants, the plant vascular system has evolved as an inter-organ communication network essential to deliver a wide range of signaling factors among distantly separated organs. To become conductive elements, phloem and xylem cells undergo a drastic differentiation program that involves the degradation of the majority of their organelles. While the molecular mechanisms regulating such complex process remain poorly understood, it is nowadays clear that phosphoglycerolipids display a pivotal role in the regulation of vascular tissue formation. In animal cells, this class of lipids is known to mediate acute responses as signal transducers and also act as constitutive signals that help defining organelle identity. Their rapid turnover, asymmetrical distribution across subcellular compartments as well as their ability to rearrange cytoskeleton fibers make phosphoglycerolipids excellent candidates to regulate complex morphogenetic processes such as vascular differentiation. Therefore, in this review we aim to summarize, emphasize and connect our current understanding about the involvement of phosphoglycerolipids in phloem and xylem differentiation.

Arabidopsis PLC2 is involved in auxin-modulated reproductive development

Phospholipase C (PLC) is an enzyme that plays crucial roles in various signal transduction pathways in mammalian cells. However, the role of PLC in plant development is poorly understood. Here we report involvement of PLC2 in auxin-mediated reproductive development in Arabidopsis. Disruption of PLC2 led to sterility, indicating a significant role for PLC2 in reproductive development. Development of both male and female gametophytes was severely perturbed in plc2 mutants. Moreover, elevated auxin levels were observed in plc2 floral tissues, suggesting that the infertility of plc2 plants may be associated with increased auxin concentrations in the reproductive organs. We show that expression levels of the auxin reporters DR5:GUS and DR5:GFP were elevated in plc2 anthers and ovules. In addition, we found that expression of the auxin biosynthetic YUCCA genes was increased in plc2 plants. We conclude that PLC2 is involved in auxin biosynthesis and signaling, thus modulating development of both male and female gametophytes in Arabidopsis.

Growth and stress response in Arabidopsis thaliana, Nicotiana benthamiana, Glycine max, Solanum tuberosum and Brassica napus cultivated under polychromatic LEDs

BACKGROUND

The use of light emitting diodes (LEDs) brings several key advantages over existing illumination technologies for indoor plant cultivation. Among these are that LEDs have predicted lifetimes from 50-100.000 hours without significant drops in efficiency and energy consumption is much lower compared to traditional fluorescent tubes. Recent advances allow LEDs to be used with customized wavelengths for plant growth. However, most of these LED growth systems use mixtures of chips emitting in several narrow wavelengths and frequently they are not compatible with existing infrastructures. This study tested the growth of five different plant species under phosphor coated LED-chips fitted into a tube with a standard G13 base that provide continuous visible light illumination with enhanced blue and red light.

RESULTS

The LED system was characterized and compared with standard fluorescence tubes in the same cultivation room. Significant differences in heat generation between LEDs and fluorescent tubes were clearly demonstrated. Also, LED lights allowed for better control and stability of preset conditions. Physiological properties such as growth characteristics, biomass, and chlorophyll content were measured and the responses to pathogen assessed for five plant species (both the model plants Arabidopsis thaliana, Nicotiana bentamiana and crop species potato, oilseed rape and soybean) under the different illumination sources.

CONCLUSIONS

We showed that polychromatic LEDs provide light of sufficient quality and intensity for plant growth using less than 40% of the electricity required by the standard fluorescent lighting under test. The tested type of LED installation provides a simple upgrade pathway for existing infrastructure for indoor plant growth. Interestingly, individual plant species responded differently to the LED lights so it would be reasonable to test their utility to any particular application.

Importance of phosphoinositide-dependent signaling pathways in the control of gene expression in resting cells and in response to phytohormones

"Phosphoinositide" refers to phosphorylated forms of phosphatidylinositol, including phosphatidylinositol-4-phosphate and phosphatidylinositol-4,5-bisphosphate. Both of these molecules could be in vivo substrates of plant phospholipase C. These phosphoinositides can also be biologically active "per se," by directly binding to proteins and thus altering their location and/or activity. The use of pharmacological agents in Arabidopsis suspension cells allowed us to identify genes whose expression was positively or negatively controlled, in the basal state, by products of phosphoinositide-dependent phospholipase C. In this basal state, it seems that no genes exhibit a phosphoinositide-dependent expression "per se." However, many genes whose expression is altered in the presence of phospholipase C inhibitors appeared to be responsive to salicylic acid. This allowed us to show that salicylic acid acts both by increasing the phosphoinositide pool and by inhibiting the phospholipase C. In response to salicylic acid it is possible to identify genes whose expression is controlled by products of PI-PLC, but also genes whose expression is controlled by phosphoinositides "per se." Our data highlight the importance of phosphoinositide-dependent pathways in gene expression in resting cells and in response to phytohormones.

Lipids in salicylic acid-mediated defense in plants: focusing on the roles of phosphatidic acid and phosphatidylinositol 4-phosphate

Plants have evolved effective defense strategies to protect themselves from various pathogens. Salicylic acid (SA) is an essential signaling molecule that mediates pathogen-triggered signals perceived by different immune receptors to induce downstream defense responses. While many proteins play essential roles in regulating SA signaling, increasing evidence also supports important roles for signaling phospholipids in this process. In this review, we collate the experimental evidence in support of the regulatory roles of two phospholipids, phosphatidic acid (PA), and phosphatidylinositol 4-phosphate (PI4P), and their metabolizing enzymes in plant defense, and examine the possible mechanistic interaction between phospholipid signaling and SA-dependent immunity with a particular focus on the immunity-stimulated biphasic PA production that is reminiscent of and perhaps mechanistically connected to the biphasic reactive oxygen species (ROS) generation and SA accumulation during defense activation.

Phospholipase D affects translocation of NPR1 to the nucleus in Arabidopsis thaliana

Phytohormone salicylic acid (SA) is a crucial component of plant-induced defense against biotrophic pathogens. Although the key players of the SA pathway are known, there are still gaps in the understanding of the molecular mechanism and the regulation of particular steps. In our previous research, we showed in Arabidopsis suspension cells that n-butanol, which specifically modulates phospholipase D activity, significantly suppresses the transcription of the pathogenesis related (PR-1) gene, which is generally accepted as the SA pathway marker. In the presented study, we have investigated the site of n-butanol action in the SA pathway. We were able to show in Arabidopsis plants treated with SA that n-butanol inhibits the transcription of defense genes (PR-1, WRKY38). Fluorescence microscopy of Arabidopsis thaliana mutants expressing 35S::NPR1-GFP (nonexpressor pathogenesis related 1) revealed significantly decreased nuclear localization of NPR1 in the presence of n-butanol. On the other hand, n-butanol did not decrease the nuclear localization of NPR1 in 35S::npr1C82A-GFP and 35S::npr1C216A-GFP mutants constitutively expressing NPR1 monomers. Mass spectrometric analysis of plant extracts showed that n-butanol significantly changes the metabolic fingerprinting while t-butanol had no effect. We found groups of the plant metabolites, influenced differently by SA and n-butanol treatment. Thus, we proposed several metabolites as markers for n-butanol action.

The Arabidopsis pi4kIIIβ1β2 double mutant is salicylic acid-overaccumulating: a new example of salicylic acid influence on plant stature

Growth is the best visible sign of plant comfort. If plants are under stress, a difference in growth with control conditions can indicate that something is going wrong (or better). Phytohormones such as auxin, cytokinins, brassinosteroids or giberellins, are important growth regulators and their role in plant growth was extensively studied. On the other hand the role of salicylic acid (SA), a phytohormone commonly connected with plant defense responses, in plant growth is under-rated. However, studies with SA-overaccumulating mutants directly showed an influence of SA on plant growth. Recently we characterized an Arabidopsis SA-overaccumulating mutant impaired in phosphatidylinositol-4-kinases (pi4kIIIβ1β2). This mutant is dwarf. The crossing with mutants impaired in SA signaling revealed that pi4kIIIβ1β2 stunted rosette is due to high SA, while the short root length is not. This brings into evidence that upper and lower parts of the plants, even though they may share common phenotypes, are differently regulated.