Cytochrome b5 reductase encoded by CBR1 is essential for a functional male gametophyte in Arabidopsis

Mapping yield and yield-related traits using diverse common bean germplasm

Common bean (bean) is one of the most important legume crops, and mapping genes for yield and yield-related traits is essential for its improvement. However, yield is a complex trait that is typically controlled by many loci in crop genomes. The objective of this research was to identify regions in the bean genome associated with yield and a number of yield-related traits using a collection of 121 diverse bean genotypes with different yields. The beans were evaluated in replicated trials at two locations, over two years. Significant variation among genotypes was identified for all traits analyzed in the four environments. The collection was genotyped with the BARCBean6K_3 chip (5,398 SNPs), two yield/antiyield gene-based markers, and seven markers previously associated with resistance to common bacterial blight (CBB), including a Niemann-Pick polymorphism (NPP) gene-based marker. Over 90% of the single-nucleotide polymorphisms (SNPs) were polymorphic and separated the panel into two main groups of small-seeded and large-seeded beans, reflecting their Mesoamerican and Andean origins. Thirty-nine significant marker-trait associations (MTAs) were identified between 31 SNPs and 15 analyzed traits on all 11 bean chromosomes. Some of these MTAs confirmed genome regions previously associated with the yield and yield-related traits in bean, but a number of associations were not reported previously, especially those with derived traits. Over 600 candidate genes with different functional annotations were identified for the analyzed traits in the 200-Kb region centered on significant SNPs. Fourteen SNPs were identified within the gene model sequences, and five additional SNPs significantly associated with five different traits were located at less than 0.6 Kb from the candidate genes. The work confirmed associations between two yield/antiyield gene-based markers (AYD1m and AYD2m) on chromosome Pv09 with yield and identified their association with a number of yield-related traits, including seed weight. The results also confirmed the usefulness of the NPP marker in screening for CBB resistance. Since disease resistance and yield measurements are environmentally dependent and labor-intensive, the three gene-based markers (CBB- and two yield-related) and quantitative trait loci (QTL) that were validated in this work may be useful tools for simplifying and accelerating the selection of high-yielding and CBB-resistant bean cultivars.

A multi-organ maize metabolic model connects temperature stress with energy production and reducing power generation

Climate change has adversely affected maize productivity. Thereby, a holistic understanding of metabolic crosstalk among its organs is important to address this issue. Thus, we reconstructed the first multi-organ maize metabolic model, iZMA6517, and contextualized it with heat and cold stress transcriptomics data using expression distributed reaction flux measurement (EXTREAM) algorithm. Furthermore, implementing metabolic bottleneck analysis on contextualized models revealed differences between these stresses. While both stresses had reducing power bottlenecks, heat stress had additional energy generation bottlenecks. We also performed thermodynamic driving force analysis, revealing thermodynamics-reducing power-energy generation axis dictating the nature of temperature stress responses. Thus, a temperature-tolerant maize ideotype can be engineered by leveraging the proposed thermodynamics-reducing power-energy generation axis. We experimentally inoculated maize root with a beneficial mycorrhizal fungus, Rhizophagus irregularis, and as a proof-of-concept demonstrated its efficacy in alleviating temperature stress. Overall, this study will guide the engineering effort of temperature stress-tolerant maize ideotypes.

Osmotic stress-induced localisation switch of CBR1 from mitochondria to the endoplasmic reticulum triggers ATP production via β-oxidation to respond to osmotic shock

Drought and high salinity are major environmental factors that reduce plant growth and development, leading to loss of plant productivity in agriculture. Under these stress conditions, photosynthesis is greatly suppressed despite the high cellular energy cost of stress response processes. Currently, the process that allows plants to secure the energy required for osmotic stress responses remains elusive. Here, we provide evidence that cytochrome b5 reductase 1 (CBR1), a cytochrome b5 reductase, plays an important role in ATP production in response to NaCl and dehydration stresses. Overexpression and loss of function of CBR1 led to enhanced resistance and sensitivity, respectively, to osmotic stress. Upon exposure to osmotic stress, CBR1 was localised to the endoplasmic reticulum (ER) instead of to mitochondria, where it was localised under normal conditions. Transgenic plants overexpressing ER-targeted CBR1 showed enhanced resistance to osmotic stress. Moreover, CBR1-ER and CBR1-OX plants, had higher levels of ATP and unsaturated fatty acids under osmotic stress. However, these effects were abrogated by thioridazine and 2-deoxy glucose, inhibitors of β-oxidation and glycolysis, respectively. Based on these results, we propose that ER-localised CBR1 triggers ATP production via the production and β-oxidation of polyunsaturated fatty acids under osmotic stress.

Biochemical Alterations in Triticale Seedlings Pretreated with Selective Herbicide and Subjected to Drought or Waterlogging Stress

Waterlogging and drought disrupt crop development and productivity. Triticale is known to be relatively tolerant to different stress factors. In natural conditions, plants are rather subjected to multiple environmental factors. Serrate® (Syngenta) is a systemic selective herbicide suitable for cereal crops such as triticale and wheat to restrain annual grass and broadleaf weeds. Triticale (×Triticosecale Wittm., cv. Rozhen) was grown as soil culture under controlled conditions. Seventeen-day-old plantlets were leaf sprayed with Serrate®. The water stress (drought or waterlogging) was applied after 72 h for 7 days, and then the seedlings were left for recovery. The herbicide does not provoke sharp alterations in the antioxidant state (stress markers level, and antioxidant and xenobiotic-detoxifying enzymes activity). The water stresses and combined treatments enhanced significantly the content of stress markers (malondialdehyde, proline, hydrogen peroxide), non-enzymatic (total phenolics and thiol groups-containing compounds), and enzymatic (activities of superoxide dismutase, catalase, guaiacol peroxidase, glutathione reductase) antioxidants, and xenobiotic-detoxifying enzymes (activities of glutathione S-transferase, NADPH:cytochrome P450 reductase, NADH:cytochrome b5 reductase). These effects were more severely expressed after the drought stress, suggesting that this cultivar is more tolerant to waterlogging than to drought stress.

Tissue-preferential recruitment of electron transfer chains for cytochrome P450-catalyzed phenolic biosynthesis

Cytochrome P450 system consists of P450 monooxygenase and redox pattern(s). While the importance of monooxygenases in plant metabolism is well documented, the metabolic roles of the related redox components have been largely overlooked. Here, we show that distinct electron transfer chains are recruited in phenylpropanoid-monolignol P450 systems to support the synthesis and distribution of different classes of phenolics in different plant tissues. While Arabidopsis cinnamate 4-hydroxylase adopts conventional NADPH-cytochrome P450 oxidoreductase (CPR) electron transfer chain for its para-hydroxylation reaction, ferulate 5-hydroxylase uses both NADPH-CPR-cytochrome b₅ (CB5) and NADH-cytochrome b₅ reductase-CB5 chains to support benzene ring 5-hydroxylation, in which the former route is primarily recruited in the stem for syringyl lignin synthesis, while the latter dominates in the syntheses of 5-hydroxylated phenolics in seeds and seed coat suberin. Our study unveils an additional layer of complexity and versatility of P450 system that the plants evolved for diversifying phenolic repertoires.

Cytochrome b5 reductases: Redox regulators of cell homeostasis

The cytochrome-b5 reductase (CYB5R) family of flavoproteins is known to regulate reduction-oxidation (redox) balance in cells. The five enzyme members are highly compartmentalized at the subcellular level and function as "redox switches" enabling the reduction of several substrates, such as heme and coenzyme Q. Critical insight into the physiological and pathophysiological significance of CYB5R enzymes has been gleaned from several human genetic variants that cause congenital disease and a broad spectrum of chronic human diseases. Among the CYB5R genetic variants, CYB5R3 is well-characterized and deficiency in expression and activity is associated with type II methemoglobinemia, cancer, neurodegenerative disorders, diabetes, and cardiovascular disease. Importantly, pharmacological and genetic-based strategies are underway to target CYB5R3 to circumvent disease onset and mitigate severity. Despite our knowledge of CYB5R3 in human health and disease, the other reductases in the CYB5R family have been understudied, providing an opportunity to unravel critical function(s) for these enzymes in physiology and disease. In this review, we aim to provide the broad scientific community an up-to-date overview of the molecular, cellular, physiological, and pathophysiological roles of CYB5R proteins.

A multigene approach secures hydroxy fatty acid production in Arabidopsis

A central goal of green chemistry is to produce industrially useful fatty acids in oilseed crops. Although genes encoding suitable fatty acid-modifying enzymes are available from more than a dozen wild species, progress has been limited because expression of these enzymes in transgenic plants produces only low yields of the desired products. For example, fatty acid hydroxylase 12 (FAH12) from castor (Ricinus communis) produces only 17% hydroxy fatty acids (HFAs) when expressed in Arabidopsis (Arabidopsis thaliana), compared with 90% HFAs in castor seeds. The transgenic plants also have reduced oil content and seed vigor. Here, we review experiments that have provided for steady increased HFA accumulation and oil content. This research has led to exciting new discoveries of enzymes and regulatory processes in the pathways of both seed oil synthesis and lipid metabolism in other parts of the plant. Recent investigations have revealed that HFA-accumulating seeds are unable to rapidly mobilize HFA-containing triacylglycerol (TAG) storage lipid after germination to provide carbon and energy for seedling development, resulting in reduced seedling establishment. These findings present a new opportunity to investigate a different, key area of lipid metabolism-the pathways of TAG lipolysis and β-oxidation in germinating seedlings.

Biochemical Investigation of Membrane-Bound Cytochrome b5 and the Catalytic Domain of Cytochrome b5 Reductase from Arabidopsis thaliana

The endoplasmic reticulum (ER) membrane of plant cells contains several enzymes responsible for the biosynthesis of a diverse range of molecules essential for plant growth and holds potential for industrial applications. Many of these enzymes are dependent on electron transfer proteins to sustain their catalytic cycles. In plants, two crucial ER-bound electron transfer proteins are cytochrome b5 and cytochrome b5 reductase, which catalyze the stepwise transfer of electrons from NADH to redox enzymes such as fatty acid desaturases, cytochrome P450s, and plant aldehyde decarbonylase. Despite the high significance of plant cytochrome b5 and cytochrome b5 reductase, they have eluded detailed characterization to date. Here, we overexpressed the full-length membrane-bound cytochrome b5 isoform B from the model plant Arabidopsis thaliana in Escherichia coli, purified the protein employing detergents as well as styrene-maleic acid (SMA) copolymers, and biochemically characterized the protein. The SMA-encapsulated cytochrome b5 exhibits a discoidal shape and the characteristic features of the active heme-bound state. We also overexpressed and purified the soluble domain of cytochrome b5 reductase from A. thaliana, establishing its activity, stability, and kinetic parameters. Further, we demonstrated that the plant cytochrome b5, purified in detergents and styrene maleic acid lipid particles (SMALPs), readily accepts electrons from the cognate plant cytochrome b5 reductase and distant electron mediators such as plant NADPH-cytochrome P450 oxidoreductase and cyanobacterial NADPH-ferredoxin reductase. We also measured the kinetic parameters of cytochrome b5 reductase for cytochrome b5. Our studies are the first to report the purification and detailed biochemical characterization of the plant cytochrome b5 and cytochrome b5 reductase from the bacterial overexpression system.

A genome-wide association study reveals that the cytochrome b5 involved in seed reserve mobilization during seed germination in rice

A candidate gene cytochrome b5 for the major QTL qSRMP9 for rice seed reserve mobilization was validated during seed germination using a genome-wide association study approach. Seed reserve mobilization plays important roles in the early seedling growth in rice. However, the genetic basis underlying this process is poorly understood. In this study, the genetic architecture of variation in seed reserve mobilization during seed germination was studied using a genome-wide association study approach in rice. Three quantitative trait loci (QTL) including qSRMP6, qSRMP9, and qSRMP12 for seed reserve mobilization percentage were identified. In which, the candidate gene cytochrome b5 (OsCyb5) for the major QTL qSRMP9 was validated. Disruption of this gene in Oscyb5 mutants reduced the seed reserve mobilization and seedling growth compared with wild-type (WT) in rice. There were no significant differences of grain size, starch, protein and total soluble sugar content in the mature grains between Oscyb5 mutants and WT. However, the α-amylase activity in the germinating seeds of Oscyb5 mutants was significantly decreased compared to that of WT, and then, the starch and sugar mobilization and the glucose accumulation during seed germination were significantly decreased in Oscyb5 mutants. Two elite haplotypes of OsCyb5 associated with the higher seed reserve mobilization percentage and its elite single nucleotide polymorphism variations were mainly existed in the INDICA and AUS accessions. The natural variation of OsCyb5 contributing to seed reserve mobilization might be useful for the future rice breeding.

The scope of flavin-dependent reactions and processes in the model plant Arabidopsis thaliana

Flavin mononucleotide (FMN) and flavin adenine dinucleotide (FAD) are utilized as coenzymes in many biochemical reduction-oxidation reactions owing to the ability of the tricyclic isoalloxazine ring system to employ the oxidized, radical and reduced state. We have analyzed the genome of Arabidopsis thaliana to establish an inventory of genes encoding flavin-dependent enzymes (flavoenzymes) as a basis to explore the range of flavin-dependent biochemical reactions that occur in this model plant. Expectedly, flavoenzymes catalyze many pivotal reactions in primary catabolism, which are connected to the degradation of basic metabolites, such as fatty and amino acids as well as carbohydrates and purines. On the other hand, flavoenzymes play diverse roles in anabolic reactions most notably the biosynthesis of amino acids as well as the biosynthesis of pyrimidines and sterols. Importantly, the role of flavoenzymes goes much beyond these basic reactions and extends into pathways that are equally crucial for plant life, for example the production of natural products. In this context, we outline the participation of flavoenzymes in the biosynthesis and maintenance of cofactors, coenzymes and accessory plant pigments (e. g. carotenoids) as well as phytohormones. Moreover, several multigene families have emerged as important components of plant immunity, for example the family of berberine bridge enzyme-like enzymes, flavin-dependent monooxygenases and NADPH oxidases. Furthermore, the versatility of flavoenzymes is highlighted by their role in reactions leading to tRNA-modifications, chromatin regulation and cellular redox homeostasis. The favorable photochemical properties of the flavin chromophore are exploited by photoreceptors to govern crucial processes of plant adaptation and development. Finally, a sequence- and structure-based approach was undertaken to gain insight into the catalytic role of uncharacterized flavoenzymes indicating their involvement in unknown biochemical reactions and pathways in A. thaliana.

Biosynthesis of alkyne-containing natural products

Alkyne-containing natural products are important molecules that are widely distributed in microbes and plants. Inspired by the advantages of acetylenic products used in the fields of medicinal chemistry, organic synthesis and material science, great efforts have focused on discovering the biosynthetic enzymes and pathways for alkyne formation. Here, we summarize the biosyntheses of alkyne-containing natural products and introduce de novo biosynthetic strategies for alkyne-tagged compound production.

Transcriptional response of Asarum heterotropoides Fr. Schmidt var. mandshuricum (Maxim.) Kitag. leaves grown under full and partial daylight conditions

BACKGROUND

Asarum heterotropides Fr. Schmidt var. mandshuricum (Maxim.) Kitag. is an important medicinal and industrial plant, which is used in the treatment of various diseases. The main bioactive ingredient is the volatile oil having more than 82 identified components of which methyleugenol, safrole, myristicin, and toluene account for about 70% of the total volume. As a sciophyte plant, the amount of light it absorbs through leaves is an important factor for growth and metabolism.

RESULTS

We grew Asarum plants under full, 50, 28, and 12% sunlight conditions to investigate the effect of different light irradiances on the four major volatile oil components. We employed de novo transcriptome sequencing to understand the transcriptional behavior of Asarum leaves regarding the biosynthetic pathways of the four volatile oil components, photosynthesis and biomass accumulation, and hormone signaling. Our results demonstrated that the increasing light conditions promoted higher percent of the four components. Under full sunlight conditions, cinnamyl alcohol dehydrogenase and cytochrome p450719As were upregulated and led the increased methyleugenol, safrole, and myristicin. The transcriptomic data also showed that Asarum leaves, under full sunlight conditions, adjust their photosynthesis-antenna proteins as a photoprotective response with the help of carotenoids. Plant hormone-signaling related genes were also differentially expressed between full sunlight and low light conditions.

CONCLUSIONS

High light induces accumulation of major bioactive ingredients A. heterotropides volatile oil and this is ascribed to upregulation of key genes such as cinnamyl alcohol dehydrogenase and cytochrome p450719As. The transcriptome data presented here lays the foundation of further understanding of light responses in sciophytes and provides guidance for increasing bioactive molecules in Asarum.

Genome-wide identification, characterization and transcriptional profiling of NHX-type (Na+/H+) antiporters under salinity stress in soybean

This study was aimed at the genome-wide identification, a comprehensive in silico characterization of NHX genes from soybean (Glycine max L.) and their tissue-specific expression under varied levels (0-200 mM NaCl) of salinity stress. A total of nine putative NHX genes were identified from soybean. The phylogenetic analysis confirmed a total of five sub-groups and GmNHXs were distributed in three of them. Bioinformatics analyses confirmed all GmNHXs as ion transporters in nature, and all were localized on the vacuolar membrane. Several cis-acting regulatory elements involved in hormonal signal-responsiveness and abiotic stress including salinity responses were identified in the promoter regions of GmNHXs. Amiloride, which is a known Na⁺/H⁺ exchanger activity inhibitor, binding motifs were observed in all the GmNHXs. Furthermore, the identified GmNHXs were predicted-targets of 75 different miRNA candidates. To gain an insight into the functional divergence of GmNHX transporters, qRT-PCR based gene expression analysis was done in control and salt-treated root, stem and leaf tissues of two contrasting Indian soybean varieties MAUS-47 (tolerant) and Gujosoya-2 (sensitive). The gene up-regulation was tissue-specific and varied amongst the soybean varieties, with higher induction in tolerant variety. Maximum induction was observed in GmNHX2 in root tissues of MAUS-47 at 200 mM NaCl stress. Overall, identified GmNHXs may be explored further as potential gene candidates for soybean improvement.

Analysis of QTL DM4.1 for Downy Mildew Resistance in Cucumber Reveals Multiple subQTL: A Novel RLK as Candidate Gene for the Most Important subQTL

One of the biggest problems in cucumber cultivation is cucurbit downy mildew (DM), caused by the obligate biotroph Pseudoperonospora cubensis. Whereas DM in cucumber was previously efficiently controlled by the dm-1 gene from Indian cucumber accession PI 197087, this resistance was broken by new DM strains, prompting the search for novel sources of resistance. A promising source of resistance is the wild cucumber accession PI 197088. It was previously shown that DM resistance in this genotype inherits polygenically. In this paper, we put the focus on one of the QTL, DM4.1 that is located on chromosome 4. QTL DM4.1 was shown to consist of three subQTL: DM4.1.1 affected pathogen-induced necrosis, DM4.1.2 was shown to have an additive effect on sporulation, and DM4.1.3 had a recessive effect on chlorosis as well as an effect on sporulation. Near-isogenic lines (NILs) were produced by introgressing the subQTLs into a susceptible cucumber line (HS279) with good horticultural traits. Transcriptomic analysis revealed that many genes in general, and defense pathway genes in particular, were differentially expressed in NIL DM4.1.1/.2 compared to NIL DM4.1.3 and the susceptible parent HS279. This indicates that the resistance from subQTL DM4.1.1 and/or subQTL DM4.1.2 likely involves defense signaling pathways, whereas resistance due to subQTL DM4.1.3 is more likely to be independent of known defense pathways. Based on fine-mapping data, we identified the RLK gene CsLRK10L2 as a likely candidate for subQTL DM4.1.2, as this gene was found to have a loss-of-function mutation in the susceptible parent HS279, and was strongly upregulated by P. cubensis inoculation in NIL DM4.1.1/.2. Heterologous expression of this gene triggered necrosis, providing further evidence that this gene is indeed causal for subQTL DM4.1.2.

Cytochrome b 5 Is an Obligate Electron Shuttle Protein for Syringyl Lignin Biosynthesis in Arabidopsis

Angiosperms have evolved the metabolic capacity to synthesize p-hydroxyphenyl, guaiacyl (G), and syringyl (S) lignin subunits in their cell walls to better adapt to the harsh terrestrial environment. The structural characteristics of lignin subunits are essentially determined by three cytochrome P450-catalzyed reactions. NADPH-dependent cytochrome P450 oxidoreductase (CPR) is commonly regarded as the electron carrier for P450-catalyzed reactions during monolignol biosynthesis. Here, we show that cytochrome b 5 isoform D (CB5D) is an indispensable electron shuttle protein specific for S-lignin biosynthesis. Arabidopsis (Arabidopsis thaliana) CB5D localizes to the endoplasmic reticulum membrane and physically associates with monolignol P450 enzymes. Disrupting CB5D in Arabidopsis resulted in a >60% reduction in S-lignin subunit levels but no impairment in G-lignin formation compared with the wild type, which sharply contrasts with the impaired G- and S-lignin synthesis observed after disrupting ATR2, encoding Arabidopsis CPR. The defective S-lignin synthesis in cb5d mutants was rescued by the expression of the gene encoding CB5D but not with mutant CB5D devoid of its electron shuttle properties. Disrupting ATR2 suppressed the catalytic activity of both cinnamic acid 4-hydroxylase and ferulate 5-hydroxylase (F5H), but eliminating CB5D specifically depleted the latter's activity. Therefore, CB5D functions as an obligate electron shuttle intermediate that specifically augments F5H-catalyzed reactions, thereby controlling S-lignin biosynthesis.

Genome-Wide Identification of Na+/H+ Antiporter (NHX) Genes in Sugar Beet (Beta vulgaris L.) and Their Regulated Expression under Salt Stress

Salinity is one of the major environment factors that limits the growth of plants and the productivity of crops worldwide. It has been shown that Na+ transporters play a central role in salt tolerance and development of plants. The objective of this study was to identify Na+/H+ antiporter (NHX) genes and investigate their expression patterns in sugar beet (Beta vulgaris L.) subjected to various concentrations of NaCl. A total of five putative NHX genes were identified and distributed on four chromosomes in sugar beet. Phylogenetic analysis revealed that these BvNHX genes are grouped into three major classes, viz Vac- (BvNHX1, -2 and -3), Endo- (BvNHX4), and PM-class NHX (BvNHX5/BvSOS1), and within each class the exon/intron structures are conserved. The amiloride-binding site is found in TM3 at N-terminus of Vac-class NHX proteins. Protein-protein interaction (PPI) prediction suggested that only BvNHX5 putatively interacts with calcineurin B-like proteins (CBL) and CBL-interacting protein kinases (CIPK), implying it might be the primary NHX involved in CBL-CIPK pathway under saline condition. It was also found that BvNHX5 contains one abscisic acid (ABA)-responsive element (ABRE), suggesting that BvNHX5 might be involved in ABA signal responsiveness. Additionally, the qRT-PCR analysis showed that all the BvNHX genes in both roots and leaves are significantly up-regulated by salt, and the transcription levels under high salinity are significantly higher than those under either low or moderate salinity. Taken together, this work gives a detailed overview of the BvNHX genes and their expression patterns under salt stress. Our findings also provide useful information for elucidating the molecular mechanisms of Na+ homeostasis and further functional identification of the BvNHX genes in sugar beet.

Sub genome anchored physical frameworks of the allotetraploid Upland cotton (Gossypium hirsutum L.) genome, and an approach toward reference-grade assemblies of polyploids

Like those of many agricultural crops, the cultivated cotton is an allotetraploid and has a large genome (~2.5 gigabase pairs). The two sub genomes, A and D, are highly similar but unequally sized and repeat-rich, which pose significant challenges for accurate genome reconstruction using standard approaches. Here we report the development of BAC libraries, sub genome specific physical maps, and a new-generation sequencing approach that will lead to a reference-grade genome assembly for Upland cotton. Three BAC libraries were constructed, fingerprinted, and integrated with BAC-end sequences (BES) to produce a de novo whole-genome physical map. The BAC map was partitioned by sub genomes through alignment to the diploid progenitor D-genome reference sequence with densely spaced BES anchor points and computational filtering. The physical maps were validated with FISH and genetic mapping of SNP markers derived from BES. Two pairs of homeologous chromosomes, A11/D11 and A12/D12, were used to assess multiplex sequencing approaches for completeness and scalability. The results represent the first sub genome anchored physical maps of Upland cotton, and a new-generation approach to the whole-genome sequencing, which will lead to the reference-grade assembly of allopolyploid cotton and serve as a general strategy for sequencing other polyploid species.

Glycosylation of inositol phosphorylceramide sphingolipids is required for normal growth and reproduction in Arabidopsis

Sphingolipids are a major component of plant plasma membranes and endomembranes, and mediate a diverse range of biological processes. Study of the highly glycosylated glycosyl inositol phosphorylceramide (GIPC) sphingolipids has been slow as a result of challenges associated with the extractability of GIPCs, and their functions in the plant remain poorly characterized. We recently discovered an Arabidopsis GIPC glucuronosyltransferase, INOSITOL PHOSPHORYLCERAMIDE GLUCURONOSYLTRANSFERASE 1 (IPUT1), which is the first enzyme in the GIPC glycosylation pathway. Plants homozygous for the iput1 loss-of-function mutation were unobtainable, and so the developmental effects of reduced GIPC glucuronosylation could not be analyzed in planta. Using a pollen-specific rescue construct, we have here isolated homozygous iput1 mutants. The iput1 mutants show severe dwarfism, compromised pollen tube guidance, and constitutive activation of salicyclic acid-mediated defense pathways. The mutants also possess reduced GIPCs, increased ceramides, and an increased incorporation of short-chain fatty acids and dihydroxylated bases into inositol phosphorylceramides and GIPCs. The assignment of a direct role for GIPC glycan head groups in the impaired processes in iput1 mutants is complicated by the vast compensatory changes in the sphingolipidome; however, our results reveal that the glycosylation steps of GIPC biosynthesis are important regulated components of sphingolipid metabolism. This study corroborates previously suggested roles for GIPC glycans in plant growth and defense, suggests important roles for them in reproduction and demonstrates that the entire sphingolipidome is sensitive to their status.

MicroRNAs modulate adaption to multiple abiotic stresses in Chlamydomonas reinhardtii

MicroRNAs play an important role in abiotic stress responses in higher plants and animals, but their role in stress adaptation in algae remains unknown. In this study, the expression of identified and putative miRNAs in Chlamydomonas reinhardtii was assessed using quantitative polymerase chain reaction; some of the miRNAs (Cre-miR906-3p) were up-regulated, whereas others (Cre-miR910) were down-regulated when the species was subjected to multiple abiotic stresses. With degradome sequencing data, we also identified ATP4 (the d-subunit of ATP synthase) and NCR2 (NADPH: cytochrome P450 reductase) as one of the several targets of Cre-miR906-3p and Cre-miR910, respectively. Q-PCR data indicated that ATP4, which was expressed inversely in relation to Cre-miR906-3p under stress conditions. Overexpressing of Cre-miR906-3p enhanced resistance to multiple stresses; conversely, overexpressing of ATP4 produced the opposite effect. These data of Q-PCR, degradome sequencing and adaptation of overexpressing lines indicated that Cre-miR906-3p and its target ATP4 were a part of the same pathway for stress adaptation. We found that Cre-miR910 and its target NCR2 were also a part of this pathway. Overexpressing of Cre-miR910 decreased, whereas that of NCR2 increased the adaption to multiple stresses. Our findings suggest that the two classes of miRNAs synergistically mediate stress adaptation in algae.

Cytochrome b5 Reductase 1 Triggers Serial Reactions that Lead to Iron Uptake in Plants

Rhizosphere acidification is essential for iron (Fe) uptake into plant roots. Plasma membrane (PM) H(+)-ATPases play key roles in rhizosphere acidification. However, it is not fully understood how PM H(+)-ATPase activity is regulated to enhance root Fe uptake under Fe-deficient conditions. Here, we present evidence that cytochrome b5 reductase 1 (CBR1) increases the levels of unsaturated fatty acids, which stimulate PM H(+)-ATPase activity and thus lead to rhizosphere acidification. CBR1-overexpressing (CBR1-OX) Arabidopsis thaliana plants had higher levels of unsaturated fatty acids (18:2 and 18:3), higher PM H(+)-ATPase activity, and lower rhizosphere pH than wild-type plants. By contrast, cbr1 loss-of-function mutant plants showed lower levels of unsaturated fatty acids and lower PM H(+)-ATPase activity but higher rhizosphere pH. Reduced PM H(+)-ATPase activity in cbr1 could be restored in vitro by addition of unsaturated fatty acids. Transcript levels of CBR1, fatty acids desaturase2 (FAD2), and fatty acids desaturase3 (FAD3) were increased under Fe-deficient conditions. We propose that CBR1 has a crucial role in increasing the levels of unsaturated fatty acids, which activate the PM H(+)-ATPase and thus reduce rhizosphere pH. This reaction cascade ultimately promotes root Fe uptake.

The cytochrome b5 reductase HPO-19 is required for biosynthesis of polyunsaturated fatty acids in Caenorhabditis elegans

Polyunsaturated fatty acids (PUFAs) are fatty acids with backbones containing more than one double bond, which are introduced by a series of desaturases that insert double bonds at specific carbon atoms in the fatty acid chain. It has been established that desaturases need flavoprotein-NADH-dependent cytochrome b5 reductase (simplified as cytochrome b5 reductase) and cytochrome b5 to pass through electrons for activation. However, it has remained unclear how this multi-enzyme system works for distinct desaturases. The model organism Caenorhabditis elegans contains seven desaturases (FAT-1, -2, -3, -4, -5, -6, -7) for the biosynthesis of PUFAS, providing an excellent model in which to characterize different desaturation reactions. Here, we show that RNAi inactivation of predicted cytochrome b5 reductases hpo-19 and T05H4.4 led to increased levels of C18:1n-9 but decreased levels of PUFAs, small lipid droplets, decreased fat accumulation, reduced brood size and impaired development. Dietary supplementation with different fatty acids showed that HPO-19 and T05H4.4 likely affect the activity of FAT-1, FAT-2, FAT-3, and FAT-4 desaturases, suggesting that these four desaturases use the same cytochrome b5 reductase to function. Collectively, these findings indicate that cytochrome b5 reductase HPO-19/T05H4.4 is required for desaturation to biosynthesize PUFAs in C. elegans.

Analysis of cytochrome b(5) reductase-mediated metabolism in the phytopathogenic fungus Zymoseptoria tritici reveals novel functionalities implicated in virulence

Septoria tritici blotch (STB) caused by the Ascomycete fungus Zymoseptoria tritici is one of the most economically damaging diseases of wheat worldwide. Z. tritici is currently a major target for agricultural fungicides, especially in temperate regions where it is most prevalent. Many fungicides target electron transfer enzymes because these are often important for cell function. Therefore characterisation of genes encoding such enzymes may be important for the development of novel disease intervention strategies. Microsomal cytochrome b5 reductases (CBRs) are an important family of electron transfer proteins which in eukaryotes are involved in the biosynthesis of fatty acids and complex lipids including sphingolipids and sterols. Unlike the model yeast Saccharomyces cerevisiae which possesses only one microsomal CBR, the fully sequenced genome of Z. tritici bears three possible microsomal CBRs. RNA sequencing analysis revealed that ZtCBR1 is the most highly expressed of these genes under all in vitro and in planta conditions tested, therefore ΔZtCBR1 mutant strains were generated through targeted gene disruption. These strains exhibited delayed disease symptoms on wheat leaves and severely limited asexual sporulation. ΔZtCBR1 strains also exhibited aberrant spore morphology and hyphal growth in vitro. These defects coincided with alterations in fatty acid, sphingolipid and sterol biosynthesis observed through GC-MS and HPLC analyses. Data is presented which suggests that Z. tritici may use ZtCBR1 as an additional electron donor for key steps in ergosterol biosynthesis, one of which is targeted by azole fungicides. Our study reports the first functional characterisation of CBR gene family members in a plant pathogenic filamentous fungus. This also represents the first direct observation of CBR functional ablation impacting upon fungal sterol biosynthesis.

Reducing isozyme competition increases target fatty acid accumulation in seed triacylglycerols of transgenic Arabidopsis

One goal of green chemistry is the production of industrially useful fatty acids (FAs) in crop plants. We focus on hydroxy fatty acids (HFAs) and conjugated polyenoic FAs (α-eleostearic acids [ESAs]) using Arabidopsis (Arabidopsis thaliana) as a model. These FAs are found naturally in seed oils of castor (Ricinus communis) and tung tree (Vernicia fordii), respectively, and used for the production of lubricants, nylon, and paints. Transgenic oils typically contain less target FA than that produced in the source species. We hypothesized that competition between endogenous and transgenic isozymes for substrates limits accumulation of unique FAs in Arabidopsis seeds. This hypothesis was tested by introducing a mutation in Arabidopsis diacylglycerol acyltransferase1 (AtDGAT1) in a line expressing castor FA hydroxylase and acyl-Coenzyme A:RcDGAT2 in its seeds. This led to a 17% increase in the proportion of HFA in seed oil. Expression of castor phospholipid:diacylglycerol acyltransferase 1A in this line increased the proportion of HFA by an additional 12%. To determine if our observations are more widely applicable, we investigated if isozyme competition influenced production of ESA. Expression of tung tree FA conjugase/desaturase in Arabidopsis produced approximately 7.5% ESA in seed lipids. Coexpression of VfDGAT2 increased ESA levels to approximately 11%. Overexpression of VfDGAT2 combined with suppression of AtDGAT1 increased ESA accumulation to 14% to 15%. Our results indicate that isozyme competition is a limiting factor in the engineering of unusual FAs in heterologous plant systems and that reduction of competition through mutation and RNA suppression may be a useful component of seed metabolic engineering strategies.

Involvement of Iron-Containing Proteins in Genome Integrity in Arabidopsis Thaliana

The Arabidopsis genome encodes numerous iron-containing proteins such as iron-sulfur (Fe-S) cluster proteins and hemoproteins. These proteins generally utilize iron as a cofactor, and they perform critical roles in photosynthesis, genome stability, electron transfer, and oxidation-reduction reactions. Plants have evolved sophisticated mechanisms to maintain iron homeostasis for the assembly of functional iron-containing proteins, thereby ensuring genome stability, cell development, and plant growth. Over the past few years, our understanding of iron-containing proteins and their functions involved in genome stability has expanded enormously. In this review, I provide the current perspectives on iron homeostasis in Arabidopsis, followed by a summary of iron-containing protein functions involved in genome stability maintenance and a discussion of their possible molecular mechanisms.

Mutation of the inducible ARABIDOPSIS THALIANA CYTOCHROME P450 REDUCTASE2 alters lignin composition and improves saccharification

ARABIDOPSIS THALIANA CYTOCHROME P450 REDUCTASE1 (ATR1) and ATR2 provide electrons from NADPH to a large number of CYTOCHROME P450 (CYP450) enzymes in Arabidopsis (Arabidopsis thaliana). Whereas ATR1 is constitutively expressed, the expression of ATR2 appears to be induced during lignin biosynthesis and upon stresses. Therefore, ATR2 was hypothesized to be preferentially involved in providing electrons to the three CYP450s involved in lignin biosynthesis: CINNAMATE 4-HYDROXYLASE (C4H), p-COUMARATE 3-HYDROXYLASE1 (C3H1), and FERULATE 5-HYDROXYLASE1 (F5H1). Here, we show that the atr2 mutation resulted in a 6% reduction in total lignin amount in the main inflorescence stem and a compositional shift of the remaining lignin to a 10-fold higher fraction of p-hydroxyphenyl units at the expense of syringyl units. Phenolic profiling revealed shifts in lignin-related phenolic metabolites, in particular with the substrates of C4H, C3H1 and F5H1 accumulating in atr2 mutants. Glucosinolate and flavonol glycoside biosynthesis, both of which also rely on CYP450 activities, appeared less affected. The cellulose in the atr2 inflorescence stems was more susceptible to enzymatic hydrolysis after alkaline pretreatment, making ATR2 a potential target for engineering plant cell walls for biofuel production.

Homologous electron transport components fail to increase fatty acid hydroxylation in transgenic Arabidopsis thaliana

Ricinoleic acid, a hydroxylated fatty acid (HFA) present in castor ( Ricinus communis) seeds, is an important industrial commodity used in products ranging from inks and paints to polymers and fuels. However, due to the deadly toxin ricin and allergens also present in castor, it would be advantageous to produce ricinoleic acid in a different agricultural crop. Unfortunately, repeated efforts at heterologous expression of the castor fatty acid hydroxylase (RcFAH12) in the model plant Arabidopsis thaliana have produced only 17-19% HFA in the seed triacylglycerols (TAG), whereas castor seeds accumulate up to 90% ricinoleic acid in the endosperm TAG. RcFAH12 requires an electron supply from NADH:cytochrome b5 reductase (CBR1) and cytochrome b5 (Cb5) to synthesize ricinoleic acid. Previously, our laboratory found a mutation in the Arabidopsis CBR1 gene, cbr1-1, that caused an 85% decrease in HFA levels in the RcFAH12 Arabidopsis line. These results raise the possibility that electron supply to the heterologous RcFAH12 may limit the production of HFA. Therefore, we hypothesized that by heterologously expressing RcCb5, the reductant supply to RcFAH12 would be improved and lead to increased HFA accumulation in Arabidopsis seeds. Contrary to this proposal, heterologous expression of the top three RcCb5 candidates did not increase HFA accumulation. Furthermore, coexpression of RcCBR1 and RcCb5 in RcFAH12 Arabidopsis also did not increase in HFA levels compared to the parental lines. These results demonstrate that the Arabidopsis electron transfer system is supplying sufficient reductant to RcFAH12 and that there must be other bottlenecks limiting the accumulation of HFA.

Homologous electron transport components fail to increase fatty acid hydroxylation in transgenic Arabidopsis thaliana

Ricinoleic acid, a hydroxylated fatty acid (HFA) present in castor ( Ricinus communis) seeds, is an important industrial commodity used in products ranging from inks and paints to polymers and fuels. However, due to the deadly toxin ricin and allergens also present in castor, it would be advantageous to produce ricinoleic acid in a different agricultural crop. Unfortunately, repeated efforts at heterologous expression of the castor fatty acid hydroxylase (RcFAH12) in the model plant Arabidopsis thaliana have produced only 17-19% HFA in the seed triacylglycerols (TAG), whereas castor seeds accumulate up to 90% ricinoleic acid in the endosperm TAG. RcFAH12 requires an electron supply from NADH:cytochrome b5 reductase (CBR1) and cytochrome b5 (Cb5) to synthesize ricinoleic acid. Previously, our laboratory found a mutation in the Arabidopsis CBR1 gene, cbr1-1, that caused an 85% decrease in HFA levels in the RcFAH12 Arabidopsis line. These results raise the possibility that electron supply to the heterologous RcFAH12 may limit the production of HFA. Therefore, we hypothesized that by heterologously expressing RcCb5, the reductant supply to RcFAH12 would be improved and lead to increased HFA accumulation in Arabidopsis seeds. Contrary to this proposal, heterologous expression of the top three RcCb5 candidates did not increase HFA accumulation. Furthermore, coexpression of RcCBR1 and RcCb5 in RcFAH12 Arabidopsis also did not increase in HFA levels compared to the parental lines. These results demonstrate that the Arabidopsis electron transfer system is supplying sufficient reductant to RcFAH12 and that there must be other bottlenecks limiting the accumulation of HFA.