Vitamin B6 contributes to disease resistance against Pseudomonas syringae pv. tomato DC3000 and Botrytis cinerea in Arabidopsis thaliana

Chloride salt enhances plant resistance to biotic stresses

Biotic stresses caused by bacterial and fungal pathogens damage crops; identifying treatments that enhance disease resistance provides important information for understanding plant defenses and sustainable agriculture. Salt stress affects crop yields worldwide; however, studies have focused on the toxic sodium ion, leaving the effects of the chloride ion unclear. In this study, we found that irrigation with a combination of chloride salts (MgCl2, CaCl2, and KCl) suppressed the cell death phenotype of the ceramide kinase mutant acd5. Chloride salt pre-irrigation also significantly limited the cell death caused by Pseudomonas syringae pv maculicola infection and inhibited the multiplication of this bacterial pathogen in a mechanism partially dependent on the salicylic acid pathway. Moreover, chloride salt pre-irrigation improved plant defenses against the fungal pathogen challenge, confining the lesion area caused by Botrytis cinerea infection. Furthermore, the growth of herbivorous larvae of Spodoptera exigua was retarded by feeding on chloride salt irrigated plants. Thus, our data suggest that treatment with Cl- increases broad spectrum resistance to biotic challenges.

Deciphering Aphanomyces euteiches-pea-biocontrol bacterium interactions through untargeted metabolomics

Aphanomyces euteiches causes root rot in pea, leading to significant yield losses. However, the metabolites involved in this pathosystem have not been thoroughly studied. This study aimed to fill this gap and explore mechanisms of bacterial suppression of A. euteiches via untargeted metabolomics using pea grown in a controlled environment. Chemical isotope labeling (CIL), followed by liquid chromatography-mass spectrometry (LC-MS), was used for metabolite separation and detection. Univariate and multivariate analyses showed clear separation of metabolites from pathogen-treated pea roots and roots from other treatments. A three-tier approach positively or putatively identified 5249 peak pairs or metabolites. Of these, 403 were positively identified in tier 1; 940 were putatively identified with high confidence in tier 2. There were substantial changes in amino acid pool, and fatty acid and phenylpropanoid pathway products. More metabolites, including salicylic and jasmonic acids, were upregulated than downregulated in A. euteiches-infected roots. 1-aminocyclopropane-1-carboxylic acid and 12-oxophytodienoic acid were upregulated in A. euteiches + bacterium-treated roots compared to A. euteiches-infected roots. A great number of metabolites were up- or down-regulated in response to A. euteiches infection compared with the control and A. euteiches + bacterium-treated plants. The results of this study could facilitate improved disease management.

Harnessing enzyme cofactors and plant metabolism: an essential partnership

Cofactors are fundamental to the catalytic activity of enzymes. Additionally, because plants are a critical source of several cofactors (i.e., including their vitamin precursors) within the context of human nutrition, there have been several studies aiming to understand the metabolism of coenzymes and vitamins in plants in detail. For example, compelling evidence has been brought forth regarding the role of cofactors in plants; specifically, it is becoming increasingly clear that an adequate supply of cofactors in plants directly affects their development, metabolism, and stress responses. Here, we review the state-of-the-art knowledge on the significance of coenzymes and their precursors with regard to general plant physiology and discuss the emerging functions attributed to them. Furthermore, we discuss how our understanding of the complex relationship between cofactors and plant metabolism can be used for crop improvement.

Untargeted Metabolomics Reveals a Multi-Faceted Resistance Response to Fusarium Head Blight Mediated by the Thinopyrum elongatum Fhb7E Locus Transferred via Chromosome Engineering into Wheat

The Thinopyrum elongatum Fhb7E locus has been proven to confer outstanding resistance to Fusarium Head Blight (FHB) when transferred into wheat, minimizing yield loss and mycotoxin accumulation in grains. Despite their biological relevance and breeding implications, the molecular mechanisms underlying the resistant phenotype associated with Fhb7E have not been fully uncovered. To gain a broader understanding of processes involved in this complex plant-pathogen interaction, we analysed via untargeted metabolomics durum wheat (DW) rachises and grains upon spike inoculation with Fusarium graminearum (Fg) and water. The employment of DW near-isogenic recombinant lines carrying or lacking the Th. elongatum chromosome 7E region including Fhb7E on their 7AL arm, allowed clear-cut distinction between differentially accumulated disease-related metabolites. Besides confirming the rachis as key site of the main metabolic shift in plant response to FHB, and the upregulation of defence pathways (aromatic amino acid, phenylpropanoid, terpenoid) leading to antioxidants and lignin accumulation, novel insights were revealed. Fhb7E conferred constitutive and early-induced defence response, in which specific importance of polyamine biosynthesis, glutathione and vitamin B6 metabolisms, along with presence of multiple routes for deoxynivalenol detoxification, was highlighted. The results suggested Fhb7E to correspond to a compound locus, triggering a multi-faceted plant response to Fg, effectively limiting Fg growth and mycotoxin production.

Variation of vitamin B contents in maize inbred lines: Potential genetic resources for biofortification

Vitamin B and its derivatives possess diverse physiological functions and are essential micronutrients for humans. Their variation in crops is important for the identification of genetic resources used to develop new varieties with enhanced vitamin B. In this research, remarkable variations were observed in kernels of 156 maize inbred lines, ranging from 107.61 to 2654.54 μg per 100 g for vitamin B1, 1.19-37.37 μg per 100 g for B2, 19.60-213.75 μg per 100 g for B3, 43.47-590.86 μg per 100 g for B5, and 138.59-1065.11 μg per 100 g for B6. Growing inbreeds in Hainan and Hebei provinces of China revealed environmental and genotype interactions among these vitamins and the correlations between them in maize grain. Several inbred lines were identified as good sources of vitamin B and promising germplasms for maize breeding, namely By855 and Si273 are overall rich in all the studied vitamins, and GY386B and CML118 are specially enriched with derivatives of vitamin B6. The present study can assist maize breeders with germplasm resources of vitamin B for biofortification to offer people nutritious foods.

The bacterial effector AvrRxo1 inhibits vitamin B6 biosynthesis to promote infection in rice

Xanthomonas oryzae pv. oryzicola (Xoc), which causes rice bacterial leaf streak, invades leaves mainly through stomata, which are often closed as a plant immune response against pathogen invasion. How Xoc overcomes stomatal immunity is unclear. Here, we show that the effector protein AvrRxo1, an ATP-dependent protease, enhances Xoc virulence and inhibits stomatal immunity by targeting and degrading rice OsPDX1 (pyridoxal phosphate synthase), thereby reducing vitamin B6 (VB6) levels in rice. VB6 is required for the activity of aldehyde oxidase, which catalyzes the last step of abscisic acid (ABA) biosynthesis, and ABA positively regulates rice stomatal immunity against Xoc. Thus, we provide evidence supporting a model in which a major bacterial pathogen inhibits plant stomatal immunity by directly targeting VB6 biosynthesis and consequently inhibiting the biosynthesis of ABA in guard cells to open stomata. Moreover, AvrRxo1-mediated VB6 targeting also explains the poor nutritional quality, including low VB6 levels, of Xoc-infected rice grains.

Natural Variation in Vitamin B1 and Vitamin B6 Contents in Rice Germplasm

Insufficient dietary intake of micronutrients contributes to the onset of deficiencies termed hidden hunger-a global health problem affecting approximately 2 billion people. Vitamin B1 (thiamine) and vitamin B6 (pyridoxine) are essential micronutrients because of their roles as enzymatic cofactors in all organisms. Metabolic engineering attempts to biofortify rice endosperm-a poor source of several micronutrients leading to deficiencies when consumed monotonously-have led to only minimal improvements in vitamin B1 and B6 contents. To determine if rice germplasm could be exploited for biofortification of rice endosperm, we screened 59 genetically diverse accessions under greenhouse conditions for variation in vitamin B1 and vitamin B6 contents across three tissue types (leaves, unpolished and polished grain). Accessions from low, intermediate and high vitamin categories that had similar vitamin levels in two greenhouse experiments were chosen for in-depth vitamer profiling and selected biosynthesis gene expression analyses. Vitamin B1 and B6 contents in polished seeds varied almost 4-fold. Genes encoding select vitamin B1 and B6 biosynthesis de novo enzymes (THIC for vitamin B1, PDX1.3a-c and PDX2 for vitamin B6) were differentially expressed in leaves across accessions contrasting in their respective vitamin contents. These expression levels did not correlate with leaf and unpolished seed vitamin contents, except for THIC expression in leaves that was positively correlated with total vitamin B1 contents in polished seeds. This study expands our knowledge of diversity in micronutrient traits in rice germplasm and provides insights into the expression of genes for vitamin B1 and B6 biosynthesis in rice.

Phosphorylated B6 vitamer deficiency in SALT OVERLY SENSITIVE 4 mutants compromises shoot and root development

Stunted growth in saline conditions is a signature phenotype of the Arabidopsis SALT OVERLY SENSITIVE mutants (sos1-5) affected in pathways regulating the salt stress response. One of the mutants isolated, sos4, encodes a kinase that phosphorylates pyridoxal (PL), a B6 vitamer, forming the important coenzyme pyridoxal 5'-phosphate (PLP). Here, we show that sos4-1 and more recently isolated alleles are deficient in phosphorylated B6 vitamers including PLP. This deficit is concomitant with a lowered PL level. Ionomic profiling of plants under standard laboratory conditions (without salt stress) reveals that sos4 mutants are perturbed in mineral nutrient homeostasis, with a hyperaccumulation of transition metal micronutrients particularly in the root, accounting for stress sensitivity. This is coincident with the accumulation of reactive oxygen species, as well as enhanced lignification and suberization of the endodermis, although the Casparian strip is intact and functional. Further, micrografting shows that SOS4 activity in the shoot is necessary for proper root development. Growth under very low light alleviates the impairments, including salt sensitivity, suggesting that SOS4 is important for developmental processes under moderate light intensities. Our study provides a basis for the integration of SOS4 derived B6 vitamers into plant health and fitness.

Crosstalk of DNA Methylation Triggered by Pathogen in Poplars With Different Resistances

DNA methylation plays crucial roles in responses to environmental stimuli. Modification of DNA methylation during development and abiotic stress responses has been confirmed in increasing numbers of plants, mainly annual plants. However, the epigenetic regulation mechanism underlying the immune response to pathogens remains largely unknown in plants, especially trees. To investigate whether DNA methylation is involved in the response to infection process or is related to the resistance differences among poplars, we performed comprehensive whole-genome bisulfite sequencing of the infected stem of the susceptible type Populus × euramerican ‘74/76’ and resistant type Populus tomentosa ‘henan’ upon Lonsdalea populi infection. The results revealed that DNA methylation changed dynamically in poplars during the infection process with a remarkable decrease seen in the DNA methylation ratio. Intriguingly, the resistant P. tomentosa ‘henan’ had a much lower basal DNA methylation ratio than the susceptible P. × euramerican ‘74/76’. Compared to mock-inoculation, both poplar types underwent post-inoculation CHH hypomethylation; however, significant decreases in mC and mCHH proportions were found in resistant poplar. In addition, most differentially CHH-hypomethylated regions were distributed in repeat and promoter regions. Based on comparison of DNA methylation modification with the expression profiles of genes, DNA methylation occurred in resistance genes, pathogenesis-related genes, and phytohormone genes in poplars during pathogen infection. Additionally, transcript levels of genes encoding methylation-related enzymes changed during pathogen infection. Interestingly, small-regulator miRNAs were subject to DNA methylation in poplars experiencing pathogen infection. This investigation highlights the critical role of DNA methylation in the poplar immune response to pathogen infection and provides new insights into epigenetic regulation in perennial plants in response to biotic stress.

Comparative proteomics of Pinus-Fusarium circinatum interactions reveal metabolic clues to biotic stress resistance

Fusarium circinatum, causing pine pitch canker (PPC), affects conifers productivity and health worldwide. Selection and breeding for resistance arises as the most promising approach to fight PPC. Therefore, it is crucial to explore the response of hosts with varying levels of susceptibility to PPC to unveil the genes/pathways behind these phenotypes. We evaluated the dynamics of the needle proteome of a susceptible (Pinus radiata) and a relatively resistant (Pinus pinea) species upon F. circinatum inoculation by GeLC-MS/MS. Integration with physiological data and validation of key genes by qPCR allowed to identify core pathways regulating these contrasting responses. In P. radiata, the pathogen may target both the secondary metabolism to negatively regulate immune response and chloroplast redox proteins to increase energy-producing pathways for amino acid production in its favour. In contrast, chloroplast redox regulation may assure redox homeostasis in P. pinea, as well as nonenzymatic antioxidants. The presence of membrane trafficking-related proteins exclusively in P. pinea likely explains its defence response against F. circinatum. A crosstalk between abscisic acid and epigenetic regulation of gene expression is also proposed in PPC response. These results are useful to support breeding programs aiming to achieve PPC resistance.

Detection of candidate genes and development of KASP markers for Verticillium wilt resistance by combining genome-wide association study, QTL-seq and transcriptome sequencing in cotton

Combining GWAS, QTL-seq and transcriptome sequencing detected basal defense-related genes showing gDNA sequence variation and expression difference in diverse cotton lines, which might be the molecular mechanisms of VW resistance in G. hirsutum. Verticillium wilt (VW), which is caused by the soil-borne fungus Verticillium dahliae, is a major disease in cotton (Gossypim hirsutum) worldwide. To facilitate the understanding of the genetic basis for VW resistance in cotton, a genome-wide association study (GWAS), QTL-seq and transcriptome sequencing were performed. The GWAS of VW resistance in a panel of 120 core elite cotton accessions using the Cotton 63K Illumina Infinium SNP array identified 5 QTL from 18 significant SNPs meeting the 5% false discovery rate threshold on 5 chromosomes. All QTL identified through GWAS were found to be overlapped with previously reported QTL. By combining GWAS, QTL-seq and transcriptome sequencing, we identified eight candidate genes showing both gDNA sequence variation and expression difference between resistant and susceptible lines, most related to transcription factors (TFs), flavonoid biosynthesis and those involving in the plant basal defense and broad-spectrum disease resistance. Ten KASP markers were successfully validated in diverse cotton lines and could be deployed in marker-assisted breeding to enhance VW resistance. These results supported our inference that the gDNA sequence variation or expression difference of those genes involving in the basal defense in diverse cotton lines might be the molecular mechanisms of VW resistance in G. hirsutum.

The Endophytic Fungus Albifimbria verrucaria from Wild Grape as an Antagonist of Botrytis cinerea and Other Grape Pathogens

Gray mold, caused by Botrytis cinerea, is one of the most prevalent fungal diseases in table and wine grapes, affecting grape quality and yields. In this study, we isolated several endophytic fungi, including Alternaria alternata, Bipolaris cynodontis, Phoma sp., and Albifimbria verrucaria, from leaves of Amur grape (Vitis amurensis) cultivar Shuangyou and investigated their biocontrol activity against B. cinerea. In vitro dual assay showed that A. verrucaria isolate SYE-1 inhibited growth of B. cinerea. The isolate also had a wide range of biocontrol activity against Lasiodiplodia theobromae and Elsinoë ampelina. Mycelial growth and conidium germination of B. cinerea were significantly inhibited by metabolites of A. verrucaria in agar plates and culture extracts of A. verrucaria from liquid culture. The isolate produced a total chitinase activity of 0.4 U/ml after incubation for 10 days in Czapek's liquid medium. In addition, application of culture extracts of A. verrucaria prior to B. cinerea inoculation significantly reduced disease severity on grape leaves of the susceptible cultivar Red Globe. Taken together, our results indicate that A. verrucaria has potential as a biocontrol agent to control grape gray mold.

Vitamin B6 Is Under a Tight Balance During Disease Development by Rhizoctonia solani on Different Cultivars of Potato and on Arabidopsis thaliana Mutants

Vitamin B6 is well recognized as an essential antioxidant and plays a role in stress responses. Co-expression of plant and pathogen-derived vitamin B6 genes is critical during disease development of R. solani. However, little is known about the functionality of vitamin B6 vitamers during plant-R. solani interactions and their involvement in disease tolerance. Here, we explored the possible involvement of vitamin B6 during disease progression of potato cultivars of different susceptibility levels to R. solani. A distinct pattern of gene expression, pyridoxine (PN) concentration, and fungal biomass was found in the susceptible cv. Russet Burbank and tolerant cv. Chieftain. Accumulation of reactive oxygen species (ROS) in R. solani mycelia or plant tissues applying non-fluorescence or fluorescence methods was related to up-regulation in the vitamin B6 pathway and is indicative of oxidative stress. Russet Burbank was susceptible to R. solani, which was linked to reduced amounts of VB6 content. Prior to infection, constitutive PN levels were significantly higher in Russet Burbank by 1.6-fold compared to Chieftain. Upon infection with R. solani, PN levels in infected tissues increased more in Chieftain (1.7-fold) compared to Russet Burbank (1.4-fold). R. solani AG3 infection of potato sprouts in both cultivars significantly activates the fungal and plant vitamin B6 and glutathione-S-transferase (GST) genes in a tissue-specific response. Significant fold increases of transcript abundance of the fungal genes ranged from a minimum of 3.60 (RsolSG3GST) to a maximum of 13.91 (RsolAG3PDX2) in the surrounding necrotic lesion tissues (zone 1). On the other hand, PCA showed that the top plant genes STGST and STPDX1.1 were linked to both tissues of necrotic lesions (zone 2) and their surrounding areas of necrotic lesions. Functional characterization of Arabidopsis pdx1.3 mutants challenged with R. solani provided evidence into the role of the vitamin B6 pathway in the maintenance of plant tolerance during disease progression. Overall, we demonstrate that the production of vitamin VB6 is under tight control and is an essential determinant of disease development during the interaction of R. solani with potato cultivars.

Clarification of the dispensability of PDX1.2 for Arabidopsis viability using CRISPR/Cas9

BACKGROUND

PDX1.2 has recently been shown to be a regulator of vitamin B₆ biosynthesis in plants and is implicated in biotic and abiotic stress resistance. PDX1.2 expression is strongly and rapidly induced by heat stress. Interestingly, PDX1.2 is restricted to eudicota, wherein it behaves as a non-catalytic pseudoenzyme and is suggested to provide an adaptive advantage to this clade. A first report on an Arabidopsis insertion mutant claims that PDX1.2 is indispensable for viability, being essential for embryogenesis. However, a later study using an independent insertion allele suggests that knockout mutants of pdx1.2 are viable. Therefore, the essentiality of PDX1.2 for Arabidopsis viability is a matter of debate. Given the important implications of PDX1.2 in stress responses, it is imperative to clarify if it is essential for plant viability.

RESULTS

We have studied the previously reported insertion alleles of PDX1.2, one of which is claimed to be essential for embryogenesis (pdx1.2-1), whereas the other is viable (pdx1.2-2). Our study shows that pdx1.2-1 carries multiple T-DNA insertions, but the T-DNA insertion in PDX1.2 is not responsible for the loss of embryogenesis. By contrast, the pdx1.2-2 allele is an overexpressor of PDX1.2 under standard growth conditions and not a null allele as previously reported. Nonetheless, upregulation of PDX1.2 expression under heat stress is impaired in this mutant line. In wild type Arabidopsis, studies of PDX1.2-YFP fusion proteins show that the protein is enhanced under heat stress conditions. To clarify if PDX1.2 is essential for Arabidopsis viability, we generated several independent mutant lines using the CRISPR-Cas9 gene editing technology. All of these lines are viable and behave similar to wild type under standard growth conditions. Reciprocal crosses of a subset of the CRISPR lines with pdx1.2-1 recovers viability of the latter line and demonstrates that knocking out the functionality of PDX1.2 does not impair embryogenesis.

CONCLUSIONS

Gene editing reveals that PDX1.2 is dispensable for Arabidopsis viability and resolves conflicting reports in the literature on its function.

Enhancement of vitamin B6 levels in rice expressing Arabidopsis vitamin B6 biosynthesis de novo genes

Vitamin B₆ (pyridoxine) is vital for key metabolic reactions and reported to have antioxidant properties in planta. Therefore, enhancement of vitamin B₆ content has been hypothesized to be a route to improve resistance to biotic and abiotic stresses. Most of the current studies on vitamin B₆ in plants are on eudicot species, with monocots remaining largely unexplored. In this study, we investigated vitamin B₆ biosynthesis in rice, with a view to examining the feasibility and impact of enhancing vitamin B₆ levels. Constitutive expression in rice of two Arabidopsis thaliana genes from the vitamin B₆ biosynthesis de novo pathway, AtPDX1.1 and AtPDX2, resulted in a considerable increase in vitamin B₆ in leaves (up to 28.3-fold) and roots (up to 12-fold), with minimal impact on general growth. Rice lines accumulating high levels of vitamin B₆ did not display enhanced tolerance to abiotic stress (salt) or biotic stress (resistance to Xanthomonas oryzae infection). While a significant increase in vitamin B₆ content could also be achieved in rice seeds (up to 3.1-fold), the increase was largely due to its accumulation in seed coat and embryo tissues, with little enhancement observed in the endosperm. However, seed yield was affected in some vitamin B₆ -enhanced lines. Notably, expression of the transgenes did not affect the expression of the endogenous rice PDX genes. Intriguingly, despite transgene expression in leaves and seeds, the corresponding proteins were only detectable in leaves and could not be observed in seeds, possibly pointing to a mode of regulation in this organ.

Transcriptome Sequencing Analysis Provides Insights Into the Response to Fusarium oxysporum in Lilium pumilum

Lily basal rot, caused by Fusarium oxysporum f. sp. lilii, is one of the most serious diseases of lily. Although the lily germplasm which is resistant to F. oxysporum has been used in disease-resistant breeding, few studies on its molecular mechanism of disease resistance have been reported. To comprehensively study the mechanism of resistance to F. oxysporum, transcriptome sequencings of root tissues from Lilium pumilum inoculated with F. oxysporum or sterile water for 6, 12, or 24 h were performed. A total of 50 GB of data were obtained from the transcriptome sequencings of the 6 L. pumilum samples, and 217 098 Unigenes were obtained after the de novo assembly, of which 38.36% Unigenes were annotated. The sequencing results showed that the numbers of differentially expressed genes at 6, 12, and 24 h after inoculation compared with the control were 111, 254, and 2500, respectively. The functional enrichment analysis of the differentially expressed genes showed that several pathways were involved in responses of L. pumilum, mainly including starch and sucrose metabolism, glycolysis/gluconeogenesis, phenylpropanoid biosynthesis, plant hormone signal transduction, flavonoid biosynthesis, vitamin B6 (VB6) biosynthesis, acid biosynthesis, proteasome, and ribosome. Transcription factor analysis revealed that the WRKY and ERF families played important roles in responses of L. pumilum to F. oxysporum. The results of this study elucidate the molecular responses to F. oxysporum in lily and lay a theoretical foundation for improving lily breeding and strategies for lily basal rot resistance.

Tomato Stress-Associated Protein 4 Contributes Positively to Immunity Against Necrotrophic Fungus Botrytis cinerea

Stress-associated proteins (SAPs) are A20 and AN1 domain-containing proteins, some of which play important roles in plant stress signaling. Here, we report the involvement of tomato SlSAP family in immunity. SlSAPs responded with different expression patterns to Botrytis cinerea and defense signaling hormones. Virus-induced gene silencing of each of the SlSAP genes and disease assays revealed that SlSAP4 and SlSAP10 play roles in immunity against B. cinerea. Silencing of SlSAP4 resulted in attenuated immunity to B. cinerea, accompanying increased accumulation of reactive oxygen species and downregulated expression of jasmonate and ethylene (JA/ET) signaling-responsive defense genes. Transient expression of SlSAP4 in Nicotiana benthamiana led to enhanced resistance to B. cinerea. Exogenous application of methyl jasmonate partially restored the resistance of the SlSAP4-silenced plants against B. cinerea. SlSAP4 interacted with three of four SlRAD23 proteins. The A20 domain in SlSAP4 and the Ub-associated domains in SlRAD23d are critical for SlSAP4-SlRAD23d interaction. Silencing of SlRAD23d led to decreased resistance to B. cinerea, but silencing of each of other SlRAD23s did not affect immunity against B. cinerea. Furthermore, silencing of SlSAP4 and each of the SlRAD23s did not affect immunity to Pseudomonas syringae pv. tomato DC3000. These data suggest that SlSAP4 contributes positively to tomato immunity against B. cinereal through affecting JA/ET signaling and may be involved in the substrate ubiquitination process via interacting with SlRAD23d.

Crystal structure of the pseudoenzyme PDX1.2 in complex with its cognate enzyme PDX1.3: a total eclipse

Pseudoenzymes have burst into the limelight recently as they provide another dimension to regulation of cellular protein activity. In the eudicot plant lineage, the pseudoenzyme PDX1.2 and its cognate enzyme PDX1.3 interact to regulate vitamin B₆ biosynthesis. This partnership is important for plant fitness during environmental stress, in particular heat stress. PDX1.2 increases the catalytic activity of PDX1.3, with an overall increase in vitamin B₆ biosynthesis. However, the mechanism by which this is achieved is not known. In this study, the Arabidopsis thaliana PDX1.2-PDX1.3 complex was crystallized in the absence and presence of ligands, and attempts were made to solve the X-ray structures. Three PDX1.2-PDX1.3 complex structures are presented: the PDX1.2-PDX1.3 complex as isolated, PDX1.2-PDX1.3-intermediate (in the presence of substrates) and a catalytically inactive complex, PDX1.2-PDX1.3-K97A. Data were also collected from a crystal of a selenomethionine-substituted complex, PDX1.2-PDX1.3-SeMet. In all cases the protein complexes assemble as dodecamers, similar to the recently reported individual PDX1.3 homomer. Intriguingly, the crystals of the protein complex are statistically disordered owing to the high degree of structural similarity of the individual PDX1 proteins, such that the resulting configuration is a composite of both proteins. Despite the differential methionine content, selenomethionine substitution of the PDX1.2-PDX1.3 complex did not resolve the problem. Furthermore, a comparison of the catalytically competent complex with a noncatalytic complex did not facilitate the resolution of the individual proteins. Interestingly, another catalytic lysine in PDX1.3 (Lys165) that pivots between the two active sites in PDX1 (P1 and P2), and the corresponding glutamine (Gln169) in PDX1.2, point towards P1, which is distinctive to the initial priming for catalytic action. This state was previously only observed upon trapping PDX1.3 in a catalytically operational state, as Lys165 points towards P2 in the resting state. Overall, the study shows that the integration of PDX1.2 into a heteromeric dodecamer assembly with PDX1.3 does not cause a major structural deviation from the overall architecture of the homomeric complex. Nonetheless, the structure of the PDX1.2-PDX1.3 complex highlights enhanced flexibility in key catalytic regions for the initial steps of vitamin B₆ biosynthesis. This report highlights what may be an intrinsic limitation of X-ray crystallography in the structural investigation of pseudoenzymes.

Root metabolic plasticity underlies functional diversity in mycorrhiza-enhanced stress tolerance in tomato

Arbuscular mycorrhizal (AM) symbioses can improve plant tolerance to multiple stresses. We compared three AM fungi (AMF) from different genera, one of them isolated from a dry and saline environment, in terms of their ability to increase tomato tolerance to moderate or severe drought or salt stress. Plant physiological parameters and metabolic profiles were compared in order to find the molecular mechanisms underlying plant protection against stress. Mycorrhizal growth response was determined, and ultrahigh-performance LC-MS was used to compare the metabolic profile of plants under the different treatments. All AMF increased plant tolerance to stress, and the positive effects of the symbiosis were correlated with the severity of the stress. The AMF isolated from the stressful environment was the most effective in improving plant tolerance to salt stress. Differentially accumulated compounds were identified and the antistress properties of some of them were confirmed. We demonstrate that AM symbioses increase plant metabolic plasticity to cope with stress. Some responses were common to all AMF tested, while others were specifically related to particular isolates. Important metabolism reprograming was evidenced upon salt stress, and we identified metabolic pathways and compounds differentially accumulated in mycorrhizas that may underlie their enhanced tolerance to stress.

Selective Determination of Levodopa in the Presence of Vitamin B6, Theophylline and Guaifenesin Using a Glassy Carbon Electrode Modified with a Composite of Hematoxylin and Graphene/ZnO

An electrode has been developed based on using a composite of hematoxylin/graphene/ZnO nanocomposite to modify a glassy carbon electrode (GCE). The electrode (HGGCE) was tested and found to be applicable for the voltammetric analysis of levodopa in the presence of vitamin B₆, theophylline and guaifenesin using a 0.1 M phosphate buffer solution (PBS) pH 7 as the solvent. The HGGCE was used as the working electrode in cyclic voltammetry (CV) and square wave voltammetry (SWV) studies on the electrochemical behavior of levodopa at its surface. The results showed a dramatic enhancement in the oxidation current of levodopa and a shift in its oxidation potential towards more negative potentials as opposed to identical tests using bare GCE as the working electrode. The studies showed that the increase in the oxidation current has two linear profiles in two concentration ranges of 0.05 - 90.0 and 90.0 - 1000.0 μM. The detection limit of SWV analysis using the modified electrode was determined to be 0.03 μM (S/N = 3). Further advantages of the methods based on HGGCE include the simple modification procedure of the electrode, as well as its excellent sensitivity and reproducibility. The modified electrode was eventually found to be applicable to the determination of mixtures of levodopa, vitamin B₆, theophylline and guaifenesin in real samples.

Vitamin B₆ and Its Role in Cell Metabolism and Physiology

Vitamin B₆ is one of the most central molecules in cells of living organisms. It is a critical co-factor for a diverse range of biochemical reactions that regulate basic cellular metabolism, which impact overall physiology. In the last several years, major progress has been accomplished on various aspects of vitamin B₆ biology. Consequently, this review goes beyond the classical role of vitamin B₆ as a cofactor to highlight new structural and regulatory information that further defines how the vitamin is synthesized and controlled in the cell. We also discuss broader applications of the vitamin related to human health, pathogen resistance, and abiotic stress tolerance. Overall, the information assembled shall provide helpful insight on top of what is currently known about the vitamin, along with addressing currently open questions in the field to highlight possible approaches vitamin B₆ research may take in the future.

Vitamin B6 biosynthetic genes expression and antioxidant enzyme properties in tomato against, Erwinia carotovora subsp. carotovora

Vitamin B6 (VitB6) is an essential cofactor for >140 biochemical reactions. Also, VitB6 is a potent antioxidant and helps plants cope with both biotic and abiotic stress conditions. However, the role of VitB6 in plant disease resistance has yet to be confirmed using molecular biology approaches. Here, we analyzed the expression patterns of VitB6 biosynthetic genes, including the de novo (PDX1 [PDX1.2 and 1.3] and PDX2) and the salvage (SOS4) pathways during the response to Erwinia carotovora subsp. carotovora. By quantitative PCR, we found that the most significant upregulation in the transcript profile of PDX2, which showed a 9.2-fold increase in expression at 12 h post inoculation (hpi) compared to 24-48 hpi. We also detected significant upregulation of PDX1.2 and PDX1.3, which were 6.6- and 4.3-fold upregulated at 24 hpi compared to 12 hpi, while SOS4 showed only low-level expression. Also, at 24 hpi, a significant increase in superoxide dismutase, catalase, peroxidase, and polyphenol oxidase activities was observed in plants. Our findings confirm that the expression of de novo and salvage pathway genes is induced by E. carotovora and that this plays an important role in the regulation of defense response by modulating cellular antioxidant capacity.

Expression of Vitis amurensis VaERF20 in Arabidopsis thaliana Improves Resistance to Botrytis cinerea and Pseudomonas syringae pv. Tomato DC3000

Ethylene response factor (ERF) transcription factors play important roles in regulating immune responses in plants. In our study, we characterized a member of the ERF transcription factor family, VaERF20, from the Chinese wild Vitis genotype, V. amurensis Rupr "Shuangyou". Phylogenetic analysis indicated that VaERF20 belongs to group IXc of the ERF family, in which many members are known to contribute to fighting pathogen infection. Consistent with this, expression of VaERF20 was induced by treatment with the necrotrophic fungal pathogen Botrytis cinerea (B. cinerea) in "Shuangyou" and V. vinifera "Red Globe". Arabidopsis thaliana plants over-expressing VaERF20 displayed enhanced resistance to B. cinerea and the bacterium Pseudomonas syringae pv. tomato (Pst) DC3000. Patterns of pathogen-induced reactive oxygen species (ROS) accumulation were entirely distinct in B. cinerea and PstDC3000 inoculated plants. Examples of both salicylic acid (SA) and jasmonic acid/ethylene (JA/ET) responsive defense genes were up-regulated after B. cinerea and PstDC3000 inoculation of the VaERF20-overexpressing transgenic A. thaliana plants. Evidence of pattern-triggered immunity (PTI), callose accumulation and stomatal defense, together with increased expression of PTI genes, was also greater in the transgenic lines. These data indicate that VaERF20 participates in various signal transduction pathways and acts as an inducer of immune responses.

Antioxidant genes of plants and fungal pathogens are distinctly regulated during disease development in different Rhizoctonia solani pathosystems

Biotic stress, as a result of plant-pathogen interactions, induces the accumulation of reactive oxygen species in the cells, causing severe oxidative damage to plants and pathogens. To overcome this damage, both the host and pathogen have developed antioxidant systems to quench excess ROS and keep ROS production and scavenging systems under control. Data on ROS-scavenging systems in the necrotrophic plant pathogen Rhizoctonia solani are just emerging. We formerly identified vitamin B6 biosynthetic machinery of R. solani AG3 as a powerful antioxidant exhibiting a high ability to quench ROS, similar to CATALASE (CAT) and GLUTATHIONE S-TRANSFERASE (GST). Here, we provide evidence on the involvement of R. solani vitamin B6 biosynthetic pathway genes; RsolPDX1 (KF620111.1), RsolPDX2 (KF620112.1), and RsolPLR (KJ395592.1) in vitamin B6 de novo biosynthesis by yeast complementation assays. Since gene expression studies focusing on oxidative stress responses of both the plant and the pathogen following R. solani infection are very limited, this study is the first coexpression analysis of genes encoding vitamin B6, CAT and GST in plant and fungal tissues of three pathosystems during interaction of different AG groups of R. solani with their respective hosts. The findings indicate that distinct expression patterns of fungal and host antioxidant genes were correlated in necrotic tissues and their surrounding areas in each of the three R. solani pathosystems: potato sprout-R. solani AG3; soybean hypocotyl-R. solani AG4 and soybean leaves-R. solani AG1-IA interactions. Levels of ROS increased in all types of potato and soybean tissues, and in fungal hyphae following infection of R. solani AGs as determined by non-fluorescence and fluorescence methods using H2DCF-DA and DAB, respectively. Overall, we demonstrate that the co-expression and accumulation of certain plant and pathogen ROS-antioxidant related genes in each pathosystem are highlighted and might be critical during disease development from the plant's point of view, and in pathogenicity and developing of infection structures from the fungal point of view.

Enhanced Arabidopsis disease resistance against Botrytis cinerea induced by sulfur dioxide

Sulfur dioxide (SO₂) is a common air pollutant that has complex impacts on plants. The effect of prior exposure to 30mgm^-3 SO₂ on defence against Botrytis cinerea (B. cinerea) in Arabidopsis thaliana and the possible mechanisms of action were investigated. The results indicated that pre-exposure to 30mgm^-3 SO₂ resulted in significantly enhanced resistance to B. cinerea infection. SO₂ pre-treatment significantly enhanced the activities of defence-related enzymes including phenylalanine ammonia-lyase (PAL), polyphenol oxidase (PPO), β-1,3-glucanase (BGL) and chitinase (CHI). Transcripts of the defence-related genes PAL, PPO, PR2, and PR3, encoding PAL, PPO, BGL and CHI, respectively, were markedly elevated in Arabidopsis plants pre-exposed to SO₂ and subsequently inoculated with B. cinerea (SO₂+ treatment group) compared with those that were only treated with SO₂ (SO₂) or inoculated with B. cinerea (CK+). Moreover, SO₂ pre-exposure also led to significant increases in the expression levels of MIR393, MIR160 and MIR167 in Arabidopsis. Meanwhile, the expression of known targets involved in the auxin signalling pathway, was negatively correlated with their corresponding miRNAs. Additionally, the transcript levels of the primary auxin-response genes GH3-like, BDL/IAA12, and AXR3/IAA17 were markedly repressed. Our findings indicate that 30mgm^-3 SO₂ pre-exposure enhances disease resistance against B. cinerea in Arabidopsis by priming defence responses through enhancement of defence-related gene expression and enzyme activity, and miRNA-mediated suppression of the auxin signalling pathway.

Analysis of the grape (Vitis vinifera L.) thaumatin-like protein (TLP) gene family and demonstration that TLP29 contributes to disease resistance

Thaumatin-like protein (TLP) is present as a large family in plants, and individual members play different roles in various responses to biotic and abiotic stresses. Here we studied the role of 33 putative grape (Vitis vinifera L.) TLP genes (VvTLP) in grape disease resistance. Heat maps analysis compared the expression profiles of 33 genes in disease resistant and susceptible grape species infected with anthracnose (Elsinoe ampelina), powdery mildew (Erysiphe necator) or Botrytis cinerea. Among these 33 genes, the expression level of TLP29 increased following the three pathogens inoculations, and its homolog from the disease resistant Chinese wild grape V. quinquangularis cv. 'Shang-24', was focused for functional studies. Over-expression of TLP29 from grape 'Shang-24' (VqTLP29) in Arabidopsis thaliana enhanced its resistance to powdery mildew and the bacterium Pseudomonas syringae pv. tomato DC3000, but decreased resistance to B. cinerea. Moreover, the stomatal closure immunity response to pathogen associated molecular patterns was strengthened in the transgenic lines. A comparison of the expression profiles of various resistance-related genes after infection with different pathogens indicated that VqTLP29 may be involved in the salicylic acid and jasmonic acid/ethylene signaling pathways.

The Nitrogen Availability Interferes with Mycorrhiza-Induced Resistance against Botrytis cinerea in Tomato

Mycorrhizal plants are generally quite efficient in coping with environmental challenges. It has been shown that the symbiosis with arbuscular mycorrhizal fungi (AMF) can confer resistance against root and foliar pathogens, although the molecular mechanisms underlying such mycorrhiza-induced resistance (MIR) are poorly understood. Tomato plants colonized with the AMF Rhizophagus irregularis display enhanced resistance against the necrotrophic foliar pathogen Botrytis cinerea. Leaves from arbuscular mycorrhizal (AM) plants develop smaller necrotic lesions, mirrored also by a reduced levels of fungal biomass. A plethora of metabolic changes takes place in AMF colonized plants upon infection. Certain changes located in the oxylipin pathway indicate that several intermediaries are over-accumulated in the AM upon infection. AM plants react by accumulating higher levels of the vitamins folic acid and riboflavin, indolic derivatives and phenolic compounds such as ferulic acid and chlorogenic acid. Transcriptional analysis support the key role played by the LOX pathway in the shoots associated with MIR against B. cinerea. Interestingly, plants that have suffered a short period of nitrogen starvation appear to react by reprogramming their metabolic and genetic responses by prioritizing abiotic stress tolerance. Consequently, plants subjected to a transient nitrogen depletion become more susceptible to B. cinerea. Under these experimental conditions, MIR is severely affected although still functional. Many metabolic and transcriptional responses which are accumulated or activated by MIR such NRT2 transcript induction and OPDA and most Trp and indolic derivatives accumulation during MIR were repressed or reduced when tomato plants were depleted of N for 48 h prior infection. These results highlight the beneficial roles of AMF in crop protection by promoting induced resistance not only under optimal nutritional conditions but also buffering the susceptibility triggered by transient N depletion.

Differential Control Efficacies of Vitamin Treatments against Bacterial Wilt and Grey Mould Diseases in Tomato Plants

Bacterial wilt and grey mould in tomato plants are economically destructive bacterial and fungal diseases caused by Ralstonia solanacearum and Botrytis cinerea, respectively. Various approaches including chemical and biological controls have been attempted to arrest the tomato diseases so far. In this study, in vitro growths of bacterial R. solanacearum and fungal B. cinerea were evaluated using four different vitamins including thiamine (vitamin B1), niacin (vitamin B3), pyridoxine (vitamin B6), and menadione (vitamin K3). In planta efficacies of the four vitamin treatments on tomato protection against both diseases were also demonstrated. All four vitamins showed different in vitro antibacterial activities against R. solanacearum in dose-dependent manners. However, treatment with 2 mM thiamine was only effective in reducing bacterial wilt of detached tomato leaves without phytotoxicity under lower disease pressure (106 colony-forming unit [cfu]/ml). Treatment with the vitamins also differentially reduced in vitro conidial germination and mycelial growth of B. cinerea. The four vitamins slightly reduced the conidial germination, and thiamine, pyridoxine and menadione inhibited the mycelial growth of B. cinerea. Menadione began to drastically suppress the conidial germination and mycelial growth by 5 and 0.5 mM, respectively. Grey mould symptoms on the inoculated tomato leaves were significantly reduced by pyridoxine and menadione pretreatments one day prior to the fungal challenge inoculation. These findings suggest that disease-specific vitamin treatment will be integrated for eco-friendly management of tomato bacterial wilt and grey mould.