Identification and characterization of a pyridoxal reductase involved in the vitamin B6 salvage pathway in Arabidopsis

PDX proteins from Arabidopsis thaliana as novel substrates of cathepsin B: implications for vitamin B6 biosynthesis regulation

Vitamin B6 is a critical molecule for metabolism, development, and stress sensitivity in plants. It is a cofactor for numerous biochemical reactions, can serve as an antioxidant, and has the potential to increase tolerance against both biotic and abiotic stressors. Due to the importance of vitamin B6, its biosynthesis is likely tightly regulated. Plants can synthesize vitamin B6 de novo via the concerted activity of Pyridoxine Biosynthesis Protein 1 (PDX1) and PDX2. Previously, PDX proteins have been identified as targets for ubiquitination, indicating they could be marked for degradation by two highly conserved pathways: the Ubiquitin Proteasome Pathway (UPP) and the autophagy pathway. Initial experiments show that PDXs are in fact degraded, but surprisingly, in a ubiquitin-independent manner. Inhibitor studies pointed toward cathepsin B, a conserved lysosomal cysteine protease, which is implicated in both programed cell death and autophagy in humans and plants. In plants, cathepsin Bs are poorly described, and no confirmed substrates have been identified. Here, we present PDX proteins from Arabidopsis thaliana as interactors and substrates of a plant Cathepsin B. These findings not only describe a novel cathepsin B substrate in plants, but also provide new insights into how plants regulate de novo biosynthesis of vitamin B6.

Advanced Glycation End Products-Induced Alzheimer's Disease and Its Novel Therapeutic Approaches: A Comprehensive Review

Advanced glycation end products (AGEs) accumulate in the brain, leading to neurodegenerative conditions such as Alzheimer's disease (AD). The pathophysiology of AD is influenced by receptors for AGEs and toll-like receptor 4 (TLR4). Protein glycation results in irreversible AGEs through a complicated series of reactions involving the formation of Schiff's base, the Amadori reaction, followed by the Maillard reaction, which causes abnormal brain glucose metabolism, oxidative stress, malfunctioning mitochondria, plaque deposition, and neuronal death. Amyloid plaque and other stimuli activate macrophages, which are crucial immune cells in AD development, triggering the production of inflammatory molecules and contributing to the disease's pathogenesis. The risk of AD is doubled by risk factors for atherosclerosis, dementia, advanced age, and type 2 diabetic mellitus (DM). As individuals age, the prevalence of neurological illnesses such as AD increases due to a decrease in glyoxalase levels and an increase in AGE accumulation. Insulin's role in proteostasis influences hallmarks of AD-like tau phosphorylation and amyloid β peptide clearance, affecting lipid metabolism, inflammation, vasoreactivity, and vascular function. The high-mobility group box 1 (HMGB1) protein, a key initiator and activator of a neuroinflammatory response, has been linked to the development of neurodegenerative diseases such as AD. The TLR4 inhibitor was found to improve memory and learning impairment and decrease Aβ build-up. Therapeutic research into anti-glycation agents, receptor for advanced glycation end products (RAGE) inhibitors, and AGE breakers offers hope for intervention strategies. Dietary and lifestyle modifications can also slow AD progression. Newer therapeutic approaches targeting AGE-related pathways are needed.

PYRIDOX(AM)INE 5'-PHOSPHATE OXIDASE3 of Arabidopsis thaliana maintains carbon/nitrogen balance in distinct environmental conditions

The identification of factors that regulate C/N utilization in plants can make a substantial contribution to optimization of plant health. Here, we explored the contribution of pyridox(am)ine 5'-phosphate oxidase3 (PDX3), which regulates vitamin B6 homeostasis, in Arabidopsis (Arabidopsis thaliana). Firstly, N fertilization regimes showed that ammonium application rescues the leaf morphological phenotype of pdx3 mutant lines but masks the metabolite perturbance resulting from impairment in utilizing soil nitrate as a source of N. Without fertilization, pdx3 lines suffered a C/N imbalance and accumulated nitrogenous compounds. Surprisingly, exploration of photorespiration as a source of endogenous N driving this metabolic imbalance, by incubation under high CO2, further exacerbated the pdx3 growth phenotype. Interestingly, the amino acid serine, critical for growth and N management, alleviated the growth phenotype of pdx3 plants under high CO2, likely due to the requirement of pyridoxal 5'-phosphate for the phosphorylated pathway of serine biosynthesis under this condition. Triggering of thermomorphogenesis by growth of plants at 28 °C (instead of 22 °C) did not appear to require PDX3 function, and we observed that the consequent drive toward C metabolism counters the C/N imbalance in pdx3. Further, pdx3 lines suffered a salicylic acid-induced defense response, probing of which unraveled that it is a protective strategy mediated by nonexpressor of pathogenesis related1 (NPR1) and improves fitness. Overall, the study demonstrates the importance of vitamin B6 homeostasis as managed by the salvage pathway enzyme PDX3 to growth in diverse environments with varying nutrient availability and insight into how plants reprogram their metabolism under such conditions.

B vitamin supply in plants and humans: the importance of vitamer homeostasis

B vitamins are a group of water-soluble micronutrients that are required in all life forms. With the lack of biosynthetic pathways, humans depend on dietary uptake of these compounds, either directly or indirectly, from plant sources. B vitamins are frequently given little consideration beyond their role as enzyme accessory factors and are assumed not to limit metabolism. However, it should be recognized that each individual B vitamin is a family of compounds (vitamers), the regulation of which has dedicated pathways. Moreover, it is becoming increasingly evident that individual family members have physiological relevance and should not be sidelined. Here, we elaborate on the known forms of vitamins B₁ , B₆ and B₉ , their distinct functions and importance to metabolism, in both human and plant health, and highlight the relevance of vitamer homeostasis. Research on B vitamin metabolism over the past several years indicates that not only the total level of vitamins but also the oft-neglected homeostasis of the various vitamers of each B vitamin is essential to human and plant health. We briefly discuss the potential of plant biology studies in supporting human health regarding these B vitamins as essential micronutrients. Based on the findings of the past few years we conclude that research should focus on the significance of vitamer homeostasis - at the organ, tissue and subcellular levels - which could improve the health of not only humans but also plants, benefiting from cross-disciplinary approaches and novel technologies.

Phylogenomic classification and synteny network analyses deciphered the evolutionary landscape of aldo-keto reductase (AKR) gene superfamily in the plant kingdom

Aldo-keto reductase-domain (PF00248) containing proteins (AKRs) are NAD(P)(H)-dependent oxidoreductases of a multigene superfamily that mediate versatile functions in plants ranging from detoxification, metal chelation, potassium ion efflux to specialized metabolism. To uncover the complete repertoire of AKR gene superfamily in plants, a systematic kingdom-wide identification, phylogeny reconstruction, classification and synteny network clustering analyses were performed in this study using 74 diverse plant genomes. Plant AKRs were omnipresent, legitimately classified into 4 groups (based on phylogeny) and 14 subgroups (based on the ≥ 60% of protein sequence identity). Species composition of AKR subgroups highlights their distinct emergence during plant evolution. Loss of AKR subgroups among plants was apparent and that various lineage-, order/family- and species-specific losses were observed. The subgroups IA, IVB and IVF were flourished and diversified well during plant evolution, likely related to the complexity of plant's specialized metabolism and environmental adaptation. About 65% of AKRs were in genomic synteny regions across the plant kingdom and the AKRs relevant to important functions (e.g. vitamin B₆ metabolism) were in profoundly conserved angiosperm-wide synteny communities. This study underscores the evolutionary landscape of plant AKRs and provides a comprehensive resource to facilitate the functional characterization of them.

Influence of Blanching on the Gene Expression Profile of Phenylpropanoid, Flavonoid and Vitamin Biosynthesis, and Their Accumulation in Oenanthe javanica

Field blanching is a process used in agriculture to obtain sweet, delicious, and tender stems of water dropwort by obstructing sunlight. The nutritional and transcriptomic profiling of blanched water dropwort has been investigated in our previous studies. However, the effect of blanching on the production of secondary metabolites and different vitamins in water dropwort has not been investigated at the transcriptomic level. This study explored the transcriptomic variations in the phenylpropanoid biosynthesis, flavonoid biosynthesis, and different vitamin biosynthesis pathways under different blanching periods in the water dropwort stems (pre-blanching, mid-blanching, post-blanching, and control). The results show that polyphenol and flavonoid contents decreased; however, the contents of vitamins (A, B1, B2, and C) and antioxidant activity increased significantly after blanching. Furthermore, the Kyoto Encyclopedia of Genes and Genomes (KEGG) enrichment analysis of blanched water dropwort showed the downregulation of many important genes involved in phenylpropanoid and flavonoid biosynthesis pathways, and the downregulation of these genes might be the reason for the reduction in polyphenol and flavonoid contents. We also examined and highlighted the genes involved in the higher vitamin content, antioxidant activity, pale color, tenderness, and sweetness of the blanched stem of water dropwort. In conclusion, the present study explored the role of phenylpropanoid and vitamin biosynthesis, and it will provide a basis for future investigation and application in the blanch cultivation of water dropwort.

Isotopically Dimethyl Labeling-Based Quantitative Proteomic Analysis of Phosphoproteomes of Soybean Cultivars

Isotopically dimethyl labeling was applied in a quantitative post-translational modification (PTM) proteomic study of phosphoproteomic changes in the drought responses of two contrasting soybean cultivars. A total of 9457 phosphopeptides were identified subsequently, corresponding to 4571 phosphoprotein groups and 3889 leading phosphoproteins, which contained nine kinase families consisting of 279 kinases. These phosphoproteins contained a total of 8087 phosphosites, 6106 of which were newly identified and constituted 54% of the current soybean phosphosite repository. These phosphosites were converted into the highly conserved kinase docking sites by bioinformatics analysis, which predicted six kinase families that matched with those newly found nine kinase families. The overly post-translationally modified proteins (OPP) occupies 2.1% of these leading phosphoproteins. Most of these OPPs are photoreceptors, mRNA-, histone-, and phospholipid-binding proteins, as well as protein kinase/phosphatases. The subgroup population distribution of phosphoproteins over the number of phosphosites of phosphoproteins follows the exponential decay law, Y = 4.13e-0.098X - 0.04. Out of 218 significantly regulated unique phosphopeptide groups, 188 phosphoproteins were regulated by the drought-tolerant cultivar under the water loss condition. These significantly regulated phosphoproteins (SRP) are mainly enriched in the biological functions of water transport and deprivation, methionine metabolic processes, photosynthesis/light reaction, and response to cadmium ion, osmotic stress, and ABA response. Seventeen and 15 SRPs are protein kinases/phosphatases and transcription factors, respectively. Bioinformatics analysis again revealed that three members of the calcium dependent protein kinase family (CAMK family), GmSRK2I, GmCIPK25, and GmAKINβ1 kinases, constitute a phosphor-relay-mediated signal transduction network, regulating ion channel activities and many nuclear events in this drought-tolerant cultivar, which presumably contributes to the development of the soybean drought tolerance under water deprivation process.

Antioxidants in Potatoes: A Functional View on One of the Major Food Crops Worldwide

With a growing world population, accelerating climate changes, and limited arable land, it is critical to focus on plant-based resources for sustainable food production. In addition, plants are a cornucopia for secondary metabolites, of which many have robust antioxidative capacities and are beneficial for human health. Potato is one of the major food crops worldwide, and is recognized by the United Nations as an excellent food source for an increasing world population. Potato tubers are rich in a plethora of antioxidants with an array of health-promoting effects. This review article provides a detailed overview about the biosynthesis, chemical and health-promoting properties of the most abundant antioxidants in potato tubers, including several vitamins, carotenoids and phenylpropanoids. The dietary contribution of diverse commercial and primitive cultivars are detailed and document that potato contributes much more than just complex carbohydrates to the diet. Finally, the review provides insights into the current and future potential of potato-based systems as tools and resources for healthy and sustainable food production.

Of clocks and coenzymes in plants: intimately connected cycles guiding central metabolism?

Plant fitness is a measure of the capacity of a plant to survive and reproduce in its particular environment. It is inherently dependent on plant health. Molecular timekeepers like the circadian clock enhance fitness due to their ability to coordinate biochemical and physiological processes with the environment on a daily basis. Central metabolism underlies these events and it is well established that diel metabolite adjustments are intimately and reciprocally associated with the genetically encoded clock. Thus, metabolic pathway activities are time-of-day regulated. Metabolite rhythms are driven by enzymes, a major proportion of which rely on organic coenzymes to facilitate catalysis. The B vitamin complex is the key provider of coenzymes in all organisms. Emerging evidence suggests that B vitamin levels themselves undergo daily oscillations in animals but has not been studied in any depth in plants. Moreover, it is rarely considered that daily rhythmicity in coenzyme levels may dictate enzyme activity levels and therefore metabolite levels. Here we put forward the proposal that B-vitamin-derived coenzyme rhythmicity is intertwined with metabolic and clock derived rhythmicity to achieve a tripartite homeostasis integrated into plant fitness.

Comparative UHPLC-Q-Orbitrap HRMS-Based Metabolomics Unveils Biochemical Changes of Black Garlic during Aging Process

Aged black garlic (BG) is a functional food in global markets; however, very few studies have ventured into comprehensive profiling of BG metabolomes during the aging process. Herein, we exploited UHPLC-Orbitrap HRMS for a comparative metabolomics analysis. During the heat treatment, organosulfur compounds such as allicin, diallyl disulfide, ajoene, S-allyl-l-cysteine (SAC), and γ-glutamyl-SAC were downregulated. Plenty of glycerophospholipids together with shikimate, aromatic amino acids, and vitamin B6 vitamers were significantly augmented; tryptophan was however consumed to generate downstream products manifested in nicotinate metabolism and aminobenzoate degradation. These secondary metabolites serve as signaling mediators or protectants against extreme thermal exposure. Besides, Heyns compounds and Amadori-rearrangement byproducts with potential mutagenic effects were concentrated. Together, our findings expand the known metabolome space of BG processing and better elucidate the reactivities of the key metabolites. We provide in-depth insights into the biochemical changes of BG that enable further functional or toxicological investigations of this popular food.

Pyridoxal Reductase, PdxI, Is Critical for Salvage of Pyridoxal in Escherichia coli

Pyridoxal 5'-phosphate (PLP) is the biologically active form of vitamin B₆ and an essential cofactor in all organisms. In Escherichia coli, PLP is synthesized via the deoxyxylulose 5-phosphate (DXP)-dependent pathway that includes seven enzymatic steps and generates pyridoxine 5'-phosphate as an intermediate. Additionally, E. coli is able to salvage pyridoxal, pyridoxine, and pyridoxamine B₆ vitamers to produce PLP using kinases PdxK/PdxY and pyridox(am)ine phosphate oxidase (PdxH). We found that E. coli strains blocked in PLP synthesis prior to the formation of pyridoxine 5'-phosphate (PNP) required significantly less exogenous pyridoxal (PL) than strains lacking pdxH and identified the conversion of PL to pyridoxine (PN) during cultivation to be the cause. Our data showed that PdxI, shown to have PL reductase activity in vitro, was required for the efficient salvage of PL in E. coli The pdxI⁺ E. coli strains converted exogenous PL to PN during growth, while pdxI mutants did not. In total, the data herein demonstrated that PdxI is a critical enzyme in the salvage of PL by E. coli IMPORTANCE The biosynthetic pathway of pyridoxal 5'-phosphate (PLP) has extensively been studied in Escherichia coli, yet limited information is available about the vitamin B₆ salvage pathway. We show that the protein PdxI (YdbC) is the primary pyridoxal (PL) reductase in E. coli and is involved in the salvage of PL. The orthologs of PdxI occur in a wide range of bacteria and plants, suggesting that PL reductase in the B₆ salvage pathway is more widely distributed than previously expected.

Analysis of Aldo-Keto Reductase Gene Family and Their Responses to Salt, Drought, and Abscisic Acid Stresses in Medicago truncatula

Salt and drought stresses are two primary abiotic stresses that inhibit growth and reduce the activity of photosynthetic apparatus in plants. Abscisic acid (ABA) plays a key role in abiotic stress regulation in plants. Some aldo-keto reductases (AKRs) can enhance various abiotic stresses resistance by scavenging cytotoxic aldehydes in some plants. However, there are few comprehensive reports of plant AKR genes and their expression patterns in response to abiotic stresses. In this study, we identified 30 putative AKR genes from Medicago truncatula. The gene characteristics, coding protein motifs, and expression patterns of these MtAKRs were analyzed to explore and identify candidate genes in regulation of salt, drought, and ABA stresses. The phylogenetic analysis result indicated that the 52 AKRs in Medicago truncatula and Arabidopsis thaliana can be divided into three groups and six subgroups. Fifteen AKR genes in M. truncatula were randomly selected from each group or subgroup, to investigate their response to salt (200 mM of NaCl), drought (50 g·L^-1 of PEG 6000), and ABA (100 µM) stresses in both leaves and roots. The results suggest that MtAKR1, MtAKR5, MtAKR11, MtAKR14, MtAKR20, and MtAKR29 may play important roles in response to these stresses.

Rice plastidial NAD-dependent malate dehydrogenase 1 negatively regulates salt stress response by reducing the vitamin B6 content

Salinity is an important environmental factor that adversely impacts crop growth and productivity. Malate dehydrogenases (MDHs) catalyse the reversible interconversion of malate and oxaloacetate using NAD(H)/NADP(H) as a cofactor and regulate plant development and abiotic stress tolerance. Vitamin B6 functions as an essential cofactor in enzymatic reactions involved in numerous cellular processes. However, the role of plastidial MDH in rice (Oryza sativa) in salt stress response by altering vitamin B6 content remains unknown. In this study, we identified a new loss-of-function osmdh1 mutant displaying salt stress-tolerant phenotype. The OsMDH1 was expressed in different tissues of rice plants including leaf, leaf sheath, panicle, glume, bud, root and stem and was induced in the presence of NaCl. Transient expression of OsMDH1-GFP in rice protoplasts showed that OsMDH1 localizes to chloroplast. Transgenic rice plants overexpressing OsMDH1 (OsMDH1OX) displayed a salt stress-sensitive phenotype. Liquid chromatography-mass spectrometry (LC-MS) metabolic profiling revealed that the amount of pyridoxine was significantly reduced in OsMDH1OX lines compared with the NIP plants. Moreover, the pyridoxine content was higher in the osmdh1 mutant and lower in OsMDH1OX plants than in the NIP plants under the salt stress, indicating that OsMDH1 negatively regulates salt stress-induced pyridoxine accumulation. Furthermore, genome-wide RNA-sequencing (RNA-seq) analysis indicated that ectopic expression of OsMDH1 altered the expression level of genes encoding key enzymes of the vitamin B6 biosynthesis pathway, possibly reducing the level of pyridoxine. Together, our results establish a novel, negative regulatory role of OsMDH1 in salt stress tolerance by affecting vitamin B6 content of rice tissues.

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.

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.

Bacillus subtilis CBR05 induces Vitamin B6 biosynthesis in tomato through the de novo pathway in contributing disease resistance against Xanthomonas campestris pv. vesicatoria

Expression profiling for genes involved in Vitamin B6 (VitB6) biosynthesis was undertaken to delineate the involvement of de novo and salvage pathway induced by Bacillus subtilis CBR05 against, Xanthomonas campestris pv. vesicatoria in tomato. Pyridoxine biosynthesis (PDX) genes such as PDX1.2 and PDX1.3, were found to be overexpressed significantly at 72 hpi in B. subtilis and pyridoxine inoculated plants. Most significant upregulation was observed in the transcript profile of PDX1.3, which showed more than 12- fold increase in expression. Unfortunately, salt sensitive overlay4 (SOS4) profiling showed irregular expression which corroborates that SOS4 role in VitB6 biosynthesis needs further studies for deciphering a clear notion about their role in tomato. Antioxidant enzymes i.e., superoxide dismutase, catalase, polyphenol oxidase, and peroxidase activities clearly demonstrate escalation till 48 hpi and gets reduced in 72 hpi. Pot trials also confirm that B. subtilis compared to pyridoxine supplementation alone show plant disease resistance and elongated roots. The present study confirms that B. subtilis, as a versatile agent in eliciting induced systemic resistance regulated by de novo pathway as a model for plant defense against X. campestris pv. vesicatoria substantiated by VitB6 biosynthesis. Nevertheless, the study is preliminary and needs further evidence for affirming this phenomenon.

Discovery of pyridoxal reductase activity as part of human vitamin B6 metabolism

BACKGROUND

Pyridoxal 5'-phosphate (PLP) is the active form of vitamin B6. Mammals cannot synthesize vitamin B6, so they rely on dietary uptake of the different B6 forms, and via the B6 salvage pathway they interconvert them into PLP. Humans possess three enzymes in this pathway: pyridoxal kinase, pyridox(am)ine phosphate oxidase and pyridoxal phosphatase. Besides these, a fourth enzyme has been described in plants and yeast but not in humans: pyridoxal reductase.

METHODS

We analysed B6 vitamers in remnant CSF samples of PLP-treated patients and four mammalian cell lines (HepG2, Caco2, HEK293 and Neuro-2a) supplemented with PL as the sole source of vitamin B6.

RESULTS

Strong accumulation of pyridoxine (PN) in CSF of PLP-treated patients was observed, suggesting the existence of a PN-forming enzyme. Our in vitro studies show that all cell lines reduce PL to PN in a time- and dose-dependent manner. We compared the amino acid sequences of known PL reductases to human sequences and found high homology for members of the voltage-gated potassium channel beta subunits and the human aldose reductases. Pharmacological inhibition and knockout of these proteins show that none of the candidates is solely responsible for PL reduction to PN.

CONCLUSIONS

We show evidence for the presence of PL reductase activity in humans. Further studies are needed to identify the responsible protein.

GENERAL SIGNIFICANCE

This study expands the number of enzymes with a role in B6 salvage pathway. We hypothesize a protective role of PL reductase(s) by limiting the intracellular amount of free PL and PLP.

Evolutionary dynamics of the chromatophore genome in three photosynthetic Paulinella species

The thecate amoeba Paulinella is a valuable model for understanding plastid organellogenesis because this lineage has independently gained plastids (termed chromatophores) of alpha-cyanobacterial provenance. Plastid primary endosymbiosis in Paulinella occurred relatively recently (90–140 million years ago, Mya), whereas the origin of the canonical Archaeplastida plastid occurred >1,500 Mya. Therefore, these two events provide independent perspectives on plastid formation on vastly different timescales. Here we generated the complete chromatophore genome sequence from P. longichromatophora (979,356 bp, GC-content = 38.8%, 915 predicted genes) and P. micropora NZ27 (977,190 bp, GC-content = 39.9%, 911 predicted genes) and compared these data to that from existing chromatophore genomes. Our analysis suggests that when a basal split occurred among photosynthetic Paulinella species ca. 60 Mya, only 35% of the ancestral orthologous gene families from the cyanobacterial endosymbiont remained in chromatophore DNA. Following major gene losses during the early stages of endosymbiosis, this process slowed down significantly, resulting in a conserved gene content across extant taxa. Chromatophore genes faced relaxed selection when compared to homologs in free-living alpha-cyanobacteria, likely reflecting the homogeneous intracellular environment of the Paulinella host. Comparison of nucleotide substitution and insertion/deletion events among different P. micropora strains demonstrates that increases in AT-content and genome reduction are ongoing and dynamic processes in chromatophore evolution.

Identification and characterization of a pyridoxal 5'-phosphate phosphatase in tobacco plants

Pyridoxal 5'-phosphate (PLP), the catalytically active form of vitamin B6, is an important cofactor for many biochemical transformations. PLP is also a very reactive molecule, and the most well-established mechanism for maintaining low levels of free PLP is its dephosphorylation by phosphatases. In our previous study, the crude enzyme extract from tobacco leaves rapidly hydrolyzed PLP at a pH optimum of 5.5. Using PLP as a substrate, a novel acid phosphatase was purified from tobacco leaves and characterized. Whether there is a PLP specific phosphatase in plants is still unknown. In this study, a cDNA clone sharing 34.72% homology with human PLP phosphatase sequences was identified from N. tabacum and characterized. The cDNA encodes a polypeptide of 319 amino acid residues, and the recombinant enzyme purified from E. coli exhibited maximum catalytic activity for PLP at pH 7.5. The properties of the purified enzyme, including pH optimum, metal requirement, optimum substrate and inhibitors were similar to those of human PLP phosphatase. Subcellular localization analysis showed that the PLP phosphatase is mainly located in chloroplast. We down-regulated the gene expression with plant RNA interference technology and found that the down-regulation has a greater impact on the transcription of genes encoding vitamin B6 metabolic enzymes. Our study further suggested that the PLP phosphatase plays an important role for maintaining PLP homeostasis within the chloroplast in plants.

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.

Global analysis of threonine metabolism genes unravel key players in rice to improve the abiotic stress tolerance

The diversity in plant metabolites with improved phytonutrients is essential to achieve global food security and sustainable crop yield. Our study using computational metabolomics genome wide association study (cmGWAS) reports on a comprehensive profiling of threonine (Thr) metabolite in rice. Sixteen abiotic stress responsive (AbSR) – Thr metabolite producing genes (ThrMPG), modulate metabolite levels and play a significant role determining both physiological and nutritional importance of rice. These AbSR-ThrMPG were computationally analysed for their protein properties using OryzaCyc through plant metabolic network analyser. A total of 1373 and 1028 SNPs were involved in complex traits and genomic variations. Comparative mapping of AbSR-ThrMPG revealed the chromosomal colinearity with C4 grass species. Further, computational expression pattern of these genes predicted a differential expression profiling in diverse developmental tissues. Protein interaction of protein coding gene sequences revealed that the abiotic stresses (AbS) are multigenic in nature. In silico expression of AbSR-ThrMPG determined the putative involvement in response to individual AbS. This is the first comprehensive genome wide study reporting on AbSR –ThrMPG analysis in rice. The results of this study provide a pivotal resource for further functional investigation of these key genes in the vital areas of manipulating AbS signaling in rice improvement.

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.

Toward Eradication of B-Vitamin Deficiencies: Considerations for Crop Biofortification

'Hidden hunger' involves insufficient intake of micronutrients and is estimated to affect over two billion people on a global scale. Malnutrition of vitamins and minerals is known to cause an alarming number of casualties, even in the developed world. Many staple crops, although serving as the main dietary component for large population groups, deliver inadequate amounts of micronutrients. Biofortification, the augmentation of natural micronutrient levels in crop products through breeding or genetic engineering, is a pivotal tool in the fight against micronutrient malnutrition (MNM). Although these approaches have shown to be successful in several species, a more extensive knowledge of plant metabolism and function of these micronutrients is required to refine and improve biofortification strategies. This review focuses on the relevant B-vitamins (B1, B6, and B9). First, the role of these vitamins in plant physiology is elaborated, as well their biosynthesis. Second, the rationale behind vitamin biofortification is illustrated in view of pathophysiology and epidemiology of the deficiency. Furthermore, advances in biofortification, via metabolic engineering or breeding, are presented. Finally, considerations on B-vitamin multi-biofortified crops are raised, comprising the possible interplay of these vitamins in planta.

Evidence that COG0325 proteins are involved in PLP homeostasis

Pyridoxal 5'-phosphate (PLP) is an essential cofactor for nearly 60 Escherichia coli enzymes but is a highly reactive molecule that is toxic in its free form. How PLP levels are regulated and how PLP is delivered to target enzymes are still open questions. The COG0325 protein family belongs to the fold-type III class of PLP enzymes and binds PLP but has no known biochemical activity although it occurs in all kingdoms of life. Various pleiotropic phenotypes of the E. coli COG0325 (yggS) mutant have been reported, some of which were reproduced and extended in this study. Comparative genomic, genetic and metabolic analyses suggest that these phenotypes reflect an imbalance in PLP homeostasis. The E. coli yggS mutant accumulates the PLP precursor pyridoxine 5'-phosphate (PNP) and is sensitive to an excess of pyridoxine but not of pyridoxal. The pyridoxine toxicity phenotype is complemented by the expression of eukaryotic yggS orthologs. It is also suppressed by the presence of amino acids, specifically isoleucine, threonine and leucine, suggesting the PLP-dependent enzyme transaminase B (IlvE) is affected. These genetic results lay a foundation for future biochemical studies of the role of COG0325 proteins in PLP homeostasis.

Genes of the de novo and Salvage Biosynthesis Pathways of Vitamin B6 are Regulated under Oxidative Stress in the Plant Pathogen Rhizoctonia solani

Vitamin B6 is recognized as an important cofactor required for numerous metabolic enzymes, and has been shown to act as an antioxidant and play a role in stress responses. It can be synthesized through two different routes: salvage and de novo pathways. However, little is known about the possible function of the vitamin B6 pathways in the fungal plant pathogen Rhizoctonia solani. Using genome walking, the de novo biosynthetic pathway genes; RsolPDX1 and RsolPDX2 and the salvage biosynthetic pathway gene, RsolPLR were sequenced. The predicted amino acid sequences of the three genes had high degrees of similarity to other fungal PDX1, PDX2, and PLR proteins and are closely related to other R. solani anastomosis groups. We also examined their regulation when subjected to reactive oxygen species (ROS) stress inducers, the superoxide generator paraquat, or H₂O₂, and compared it to the well-known antioxidant genes, catalase and glutathione-S-transferase (GST). The genes were differentially regulated with transcript levels as high as 33 fold depending on the gene and type of stress reflecting differences in the type of damage induced by ROS. Exogenous addition of the vitamers PN or PLP in culture medium significantly induced the transcription of the vitamin B6 de novo encoding genes as early as 0.5 hour post treatment (HPT). On the other hand, transcription of RsolPLR was vitamer-specific; a down regulation upon supplementation of PN and upregulation with PLP. Our results suggest that accumulation of ROS in R. solani mycelia is linked to transcriptional regulation of the three genes and implicate the vitamin B6 biosynthesis machinery in R. solani, similar to catalases and GST, as an antioxidant stress protector against oxidative stress.

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

Vitamin B6 (VB6) is an important cofactor for numerous enzymatic reactions and plays an important role in abiotic stress tolerance. However, direct molecular evidence supporting a role for VB6 in plant disease resistance remains lacking. In this study, we explored the possible function of VB6 in disease resistance by analyzing disease phenotypes of Arabidopsis mutants with defects in de novo biosynthetic pathway and salvage pathway of VB6 biosynthesis against Pseudomonas syringae pv. tomato (Pst) DC3000 and Botrytis cinerea. Mutations in AtPDX1.2 and AtPDX1.3 genes involved in the de novo pathway, and in AtSOS4 gene involved in the salvage pathway led to increased levels of diseases caused by Pst DC3000 and B. cinerea. The pdx1.2 and pdx1.3 plants had reduced VB6 contents and showed a further reduction in VB6 contents after infection by Pst DC3000 or B. cinerea. Our preliminary results suggest an important role for VB6 in plant disease resistance against different types of pathogens.

Enzymatic conversion from pyridoxal to pyridoxine caused by microorganisms within tobacco phyllosphere

Vitamin B6 (VB6) comprises six interconvertible pyridine compounds (vitamers), among which pyridoxal 5'-phosphate (PLP) is a coenzyme involved in a high diversity of biochemical reactions. In plants, PLP is de novo synthesized, and pyridoxine (PN) is usually maintained as the predominant B6 vitamer. Although the conversion from pyridoxal (PL) to PN catalyzed by PL reductase in plants has been confirmed, the enzyme itself remains largely unknown. We previously found pre-incubation at 35 °C dramatically enhanced PL reductase activity in tobacco leaf homogenate. In this study, we demonstrated that the increase in the reductase activity was a consequence of phyllosphere microbial proliferation. VB6 was detected from tobacco phyllosphere, and PL level was the highest among three non-phosphorylated B6 vitamers. When the sterile tobacco rich in PL were kept in an open, warm and humid environment to promote microorganism proliferation, a significant change from PL to PN was observed. Our results suggest that there may be a plant-microbe interaction in the conversion from PL to PN within tobacco phyllosphere.

The de novo biosynthesis of vitamin B6 is required for disease resistance against Botrytis cinerea in tomato

Vitamin B6 (VB6), an essential cofactor for numerous metabolic enzymes, has recently been shown to act as a potent antioxidant and play important roles in developmental processes and stress responses. However, little is known about the possible function of VB6 in plant disease resistance response against pathogen infection. In the present study, we explored the possible involvement of VB6 in defense response against Botrytis cinerea through functional analysis of tomato VB6 biosynthetic genes. Three de novo VB6 biosynthetic genes (SlPDX1.2, SlPDX1.3, and SlPDX2) and one salvage pathway gene (SlSOS4) were identified and the SlPDX1.2, SlPDX1.3, and SlPDX2 genes were shown to encode functional enzymes involved in de novo biosynthesis of VB6, as revealed by complementation of the VB6 prototrophy in yeast snz1 and sno1 mutants. Expression of SlPDX1.2, SlPDX1.3, and SlSOS4 genes was induced by infection with B. cinerea. Virus-induced gene silencing-mediated knockdown of SlPDX1.2 or SlPDX1.3 but not SlPDX2 and SlSOS4 led to increased severity of disease caused by B. cinerea, indicating that the VB6 de novo biosynthetic pathway but not the salvage pathway is involved in tomato defense response against B. cinerea. Furthermore, the SlPDX1.2- and SlPDX1.3-silenced tomato plants exhibited reduced levels of VB6 contents and reactive oxygen species scavenging capability, increased levels of superoxide anion and H2O2 generation, and increased activity of superoxide dismutase after infection by B. cinerea. Our results suggest that VB6 and its de novo biosynthetic pathway play important roles in regulation of defense response against B. cinerea through modulating cellular antioxidant capacity.

Functional analysis by protein biochemistry

To date a number of cereal genomes are fully sequenced and more are near completion. The information within these genomes will be of most use to scientists when every gene has been functionally characterized leading to the complete annotation of these genomes. This chapter describes how functional characterization of plant proteins can be achieved via in vitro or in vivo methods. The first section of this chapter describes the use of Escherichia coli as a host for expression of plant genes, followed by purification and in vitro characterization of the resultant enzyme. The second section of this chapter details the methods involved in transient gene expression in Zea mays leaf protoplasts for in vivo functional characterization of protein localization.

Genome-wide association analyses provide genetic and biochemical insights into natural variation in rice metabolism

Plant metabolites are important to world food security in terms of maintaining sustainable yield and providing food with enriched phytonutrients. Here we report comprehensive profiling of 840 metabolites and a further metabolic genome-wide association study based on ∼6.4 million SNPs obtained from 529 diverse accessions of Oryza sativa. We identified hundreds of common variants influencing numerous secondary metabolites with large effects at high resolution. We observed substantial heterogeneity in the natural variation of metabolites and their underlying genetic architectures among different subspecies of rice. Data mining identified 36 candidate genes modulating levels of metabolites that are of potential physiological and nutritional importance. As a proof of concept, we functionally identified or annotated five candidate genes influencing metabolic traits. Our study provides insights into the genetic and biochemical bases of rice metabolome variation and can be used as a powerful complementary tool to classical phenotypic trait mapping for rice improvement.

Enzymatic transamination of pyridoxamine in tobacco plants

Vitamin B6 (VB6) comprises a group of pyridine compounds that are involved in a surprisingly high diversity of biochemical reactions. Humans and animals depend largely on plants for their VB6 nutrition. Many studies have focused on biosynthesis of VB6 and comparatively little is known about VB6 metabolic conversion in plants. Recently, we have found that an efficient conversion pathway between pyridoxal (PL) and pyridoxamine (PM) is present in tobacco, but the catalytic enzyme remains an unsolved mystery. In this study, enzymes catalyzing the transamination of PM were purified from tobacco leaves and characterized. Our results suggest that a specific PM-pyruvate aminotranferase dominates the reversible transamination of PM in tobacco, and also show that the apo form of glutamic-oxaloacetic aminotranferase from tobacco, but not the holoenzyme, is able to catalyze the analogous transamination reaction between PM and either oxaloacetate or α-ketoglutarate. PM-pyruvate aminotranferase is involved in a degradation pathway for VB6 compounds in bacteria. Therefore, our study raises questions about whether the degradation pathway of VB6 exists in plants.

Effect of abiotic stress on the abundance of different vitamin B6 vitamers in tobacco plants

There are six different vitamin B6 (VB6) forms, pyridoxal (PL), pyridoxamine (PM), pyridoxine (PN), pyridoxal 5'-phosphate (PLP), pyridoxamine 5'-phosphate (PMP), and pyridoxine 5'-phosphate (PNP), of which PLP is the active form. Although transcriptional regulation of the genes involved in the de novo and salvage pathways of PLP syntheses after stress treatments has been described for Arabidopsis thaliana and tobacco plants, it remains open as to whether this in turn affects VB6 levels. In this study, the effects of chilling, UV radiation, intensity of illumination, osmotic pressure, oxidative and drought stresses on the abundance of different B6 vitamers in tobacco plants were examined by using high performance liquid chromatography (HPLC). The abiotic stressors resulted in significant increase of PLP, and caused some corresponding changes with PL and PN. The highest increase of PLP was 2.5-fold compared to the control plants, followed by a continuous decline back to the control levels. These changes are presumably caused by the regulation and control mechanism on the VB6 metabolism in plants.

The SOS4 pyridoxal kinase is required for maintenance of vitamin B6-mediated processes in chloroplasts

Vitamin B(6) (pyridoxal 5'-phosphate and its vitamers) is an important cofactor in numerous enzymatic reactions. In spite of its importance, the consequences of altering vitamin B(6) content on plant growth and development are not well understood. This study compares two mutants for vitamin B(6)-metabolizing enzymes in Arabidopsis thaliana: a pdx1.3 mutant in the de novo synthesis pathway and a salvage pathway sos4 mutant that accumulates more vitamin B(6). We show that despite a difference in total B(6) content in leaf tissue, both mutants share similar phenotypes, including chlorosis, decreased size, altered chloroplast ultrastructure, and root sensitivity to sucrose. Assay of B(6) vitamer content from isolated chloroplasts showed that, despite differing B(6) vitamer content in whole leaf tissue, both mutants share a common deficiency in total and phosphorylated vitamers in chloroplasts. One of the splice variants of the SOS4 proteins was shown to be located in the chloroplast. Our data indicate that some of the phenotypic consequences shared between the pdx1.3 and sos4 mutants are due to B(6) deficiency in chloroplasts, and show that SOS4 is required for maintenance of phosphorylated B(6) vitamer concentrations in chloroplasts. Further, our data are consistent with a diffusion model for transport of vitamin B(6) into chloroplasts.

Biochemical and proteomic analyses reveal that Populus cathayana males and females have different metabolic activities under chilling stress

Male and female poplars (Populus cathayana Rehd.) respond differently to environmental stresses. However, little is known about sex-dependent responses to chilling at the proteome level. To better understand these differences, a comparative proteomics investigation combined with a biochemical approach was used in the current study. Three-month-old poplar cuttings were treated at 25 or 4 °C for 14 days. Results revealed significant sexual differences in nitrogen metabolic enzymes and free amino acid components in response to chilling. The chilling-treated males showed higher activities of nitrate reductase and glutamine synthetase and higher contents of reduced glutathione, serine, arginine, leucine, glycine, proline and methionine than chilling-treated females. A total of 65 chilling-responsive spots were found, of which 48 showed significant sexual differences. These proteins are involved in photosynthesis, carbon and energy metabolism, metabolic processes of proteins, lipid metabolism, vitamin metabolism, stress defense, and gene expression regulation. The study shows that males have more effective metabolic processes and protective systems to chilling than females.

Plant B vitamin pathways and their compartmentation: a guide for the perplexed

The B vitamins and the cofactors derived from them are essential for life. B vitamin synthesis in plants is consequently as crucial to plants themselves as it is to humans and animals, whose B vitamin nutrition depends largely on plants. The synthesis and salvage pathways for the seven plant B vitamins are now broadly known, but certain enzymes and many transporters have yet to be identified, and the subcellular locations of various reactions are unclear. Although very substantial, what is not known about plant B vitamin pathways is regrettably difficult to discern from the literature or from biochemical pathway databases. Nor do databases accurately represent all that is known about B vitamin pathways-above all their compartmentation-because the facts are scattered throughout the literature, and thus hard to piece together. These problems (i) deter discoveries because newcomers to B vitamins cannot see which mysteries still need solving; and (ii) impede metabolic reconstruction and modelling of B vitamin pathways because genes for reactions or transport steps are missing. This review therefore takes a fresh approach to capture current knowledge of B vitamin pathways in plants. The synthesis pathways, key salvage routes, and their subcellular compartmentation are surveyed in depth, and encoded in the SEED database (http://pubseed.theseed.org/seedviewer.cgi?page=PlantGateway) for Arabidopsis and maize. The review itself and the encoded pathways specifically identify enigmatic or missing reactions, enzymes, and transporters. The SEED-encoded B vitamin pathway collection is a publicly available, expertly curated, one-stop resource for metabolic reconstruction and modeling.

Characterization of enzymes involved in the interconversions of different forms of vitamin B(6) in tobacco leaves

There are six different vitamin B(6) (VB(6)) forms, pyridoxal (PL), pyridoxamine (PM), pyridoxine (PN), pyridoxal 5'-phosphate (PLP), pyridoxamine 5'-phosphate (PMP) and pyridoxine 5'-phosphate (PNP). PLP is a coenzyme required by more than 100 cellular enzymes. In spite of the importance of this vitamin, the understanding of VB(6) metabolic conversion in plants is limited. In this study, we developed a sensitive and reliable method to assay VB(6)-metabolizing enzyme activities by monitoring their products visually using high-performance liquid chromatography. With this method, the reactions catalyzed by PL/PM/PN kinase, PMP/PNP oxidase, PM-pyruvate aminotransferase, PL reductase and PLP phosphatase were all nicely detected using crude protein extracts of tobacco leaves. Under optimal in vitro conditions, specific activities of those enzymes were 0.15 ± 0.03, 0.10 ± 0.03, 0.08 ± 0.02, 0.64 ± 0.13 and 23.08 ± 1.98 nmol product/min/mg protein, respectively. This is the first report on the conversion between PM and PL catalyzed by PM-pyruvate aminotransferase in plants. Furthermore, the PL reductase activity was found to be heat inducible. Our study sheds light on the VB(6) metabolism taking place in plants.