Plant fatty acyl reductases: enzymes generating fatty alcohols for protective layers with potential for industrial applications

Fatty acid de novo biosynthesis in plastids: Key enzymes and their critical roles for male reproduction and other processes in plants

Fatty acid de novo biosynthesis in plant plastids is initiated from acetyl-CoA and catalyzed by a series of enzymes, which is required for the vegetative growth, reproductive growth, seed development, stress response, chloroplast development and other biological processes. In this review, we systematically summarized the fatty acid de novo biosynthesis-related genes/enzymes and their critical roles in various plant developmental processes. Based on bioinformatic analysis, we identified fatty acid synthase encoding genes and predicted their potential functions in maize growth and development, especially in anther and pollen development. Finally, we highlighted the potential applications of these fatty acid synthases in male-sterility hybrid breeding, seed oil content improvement, herbicide and abiotic stress resistance, which provides new insights into future molecular crop breeding.

Characterization of the Fatty Acyl-CoA Reductase (FAR) Gene Family and Its Response to Abiotic Stress in Rice (Oryza sativa L.)

Fatty acyl-CoA reductase (FAR) is an important NADPH-dependent enzyme that can produce primary alcohol from fatty acyl-CoA or fatty acyl-carrier proteins as substrates. It plays a pivotal role in plant growth, development, and stress resistance. Herein, we performed genome-wide identification and expression analysis of FAR members in rice using bioinformatics methods. A total of eight OsFAR genes were identified, and the OsFARs were comprehensively analyzed in terms of phylogenetic relationships, duplication events, protein motifs, etc. The cis-elements of the OsFARs were predicted to respond to growth and development, light, hormones, and abiotic stresses. Gene ontology annotation analysis revealed that OsFAR proteins participate in biological processes as fatty acyl-CoA reductase during lipid metabolism. Numerous microRNA target sites were present in OsFARs mRNAs. The expression analysis showed that OsFARs were expressed at different levels during different developmental periods and in various tissues. Furthermore, the expression levels of OsFARs were altered under abiotic stresses, suggesting that FARs may be involved in abiotic stress tolerance in rice. The findings presented here serve as a solid basis for further exploring the functions of OsFARs.

Divergent evolution of the alcohol-forming pathway of wax biosynthesis among bryophytes

The plant cuticle is a hydrophobic barrier, which seals the epidermal surface of most aboveground organs. While the cuticle biosynthesis of angiosperms has been intensively studied, knowledge about its existence and composition in nonvascular plants is scarce. Here, we identified and characterized homologs of Arabidopsis thaliana fatty acyl-CoA reductase (FAR) ECERIFERUM 4 (AtCER4) and bifunctional wax ester synthase/acyl-CoA:diacylglycerol acyltransferase 1 (AtWSD1) in the liverwort Marchantia polymorpha (MpFAR2 and MpWSD1) and the moss Physcomitrium patens (PpFAR2A, PpFAR2B, and PpWSD1). Although bryophyte harbor similar compound classes as described for angiosperm cuticles, their biosynthesis may not be fully conserved between the bryophytes M. polymorpha and P. patens or between these bryophytes and angiosperms. While PpFAR2A and PpFAR2B contribute to the production of primary alcohols in P. patens, loss of MpFAR2 function does not affect the wax profile of M. polymorpha. By contrast, MpWSD1 acts as the major wax ester-producing enzyme in M. polymorpha, whereas mutations of PpWSD1 do not affect the wax ester levels of P. patens. Our results suggest that the biosynthetic enzymes involved in primary alcohol and wax ester formation in land plants have either evolved multiple times independently or undergone pronounced radiation followed by the formation of lineage-specific toolkits.

Pectate lyase-like lubricates the male gametophyte's path toward its mating partner

The pollen tube is an extension of the male gametophyte in plants and mediates sexual reproduction by delivering the sperm cells to the female gametophyte. To accomplish this task, the elongating pollen tube must break through the thick wall of the pollen grain and penetrate multiple pistillar tissues. Both processes require the loosening of cell wall material-that of the pollen intine and that of the apoplast of the transmitting tract. The enzymatic toolbox for these cell wall modifying processes employed by the invading male gametophyte is elusive. We investigated the role of the pectin-digesting pectate lyase-like (PLL) by combining mutant analysis with microscopy observations, fluorescence recovery after photo-bleaching experiments, and immuno-detection. We show that in Arabidopsis (Arabidopsis thaliana), PLLs are required for intine loosening during the first steps of pollen tube germination. We provide evidence that during pollen tube elongation, PLLs are released by the pollen tube into the extracellular space, suggesting that they may be employed to soften the apoplast of the transmitting tissue. The synergistic enzymatic action of PLLs in the pollen grain, the pollen tube, and the transmitting track contribute to an effective fertilization process.

Genome-wide identification of FAR gene family and functional analysis of NlFAR10 during embryogenesis in the brown planthopper Nilaparvata lugens

Fatty acyl-CoA reductases (FARs) catalyze the synthesis of fatty alcohols from corresponding fatty acid precursors in organisms. However, the function of FARs in insect fecundity and embryogenesis remains largely unclear. Here, a total of 22 putative FAR proteins were identified in the brown planthopper Nilaparvata lugens, a hemipteran insect pest of rice, and most of them were highly expressed in embryonic stages. Among them, NlFAR10 was specifically and highly expressed in the later embryogenesis, but was promiscuously expressed in tissues of adults. The heterologously expressed NlFAR10 was able to produce the intermediate fatty acid alcohols from the corresponding acyl-CoA precursors. When NlFAR10 was silenced through RNAi in vivo, the embryogenesis was obviously inhibited, resulting in low hatching rates. Moreover, the metabolome analyses indicated that loss of NlFAR10 affected lipid metabolism and purine metabolism during embryogenesis. To the best of our knowledge, this is the first report of a FAR member affecting insect embryogenesis, thus providing a new target for future pest management.

The Plant Fatty Acyl Reductases

Fatty acyl reductase (FAR) is a crucial enzyme that catalyzes the NADPH-dependent reduction of fatty acyl-CoA or acyl-ACP substrates to primary fatty alcohols, which in turn acts as intermediate metabolites or metabolic end products to participate in the formation of plant extracellular lipid protective barriers (e.g., cuticular wax, sporopollenin, suberin, and taproot wax). FARs are widely present across plant evolution processes and play conserved roles during lipid synthesis. In this review, we provide a comprehensive view of FAR family enzymes, including phylogenetic analysis, conserved structural domains, substrate specificity, subcellular localization, tissue-specific expression patterns, their varied functions in lipid biosynthesis, and the regulation mechanism of FAR activity. Finally, we pose several questions to be addressed, such as the roles of FARs in tryphine, the interactions between transcription factors (TFs) and FARs in various environments, and the identification of post-transcriptional, translational, and post-translational regulators.

The final piece of the Triangle of U: Evolution of the tetraploid Brassica carinata genome

Ethiopian mustard (Brassica carinata) is an ancient crop with remarkable stress resilience and a desirable seed fatty acid profile for biofuel uses. Brassica carinata is one of six Brassica species that share three major genomes from three diploid species (AA, BB, and CC) that spontaneously hybridized in a pairwise manner to form three allotetraploid species (AABB, AACC, and BBCC). Of the genomes of these species, that of B. carinata is the least understood. Here, we report a chromosome scale 1.31-Gbp genome assembly with 156.9-fold sequencing coverage for B. carinata, completing the reference genomes comprising the classic Triangle of U, a classical theory of the evolutionary relationships among these six species. Our assembly provides insights into the hybridization event that led to the current B. carinata genome and the genomic features that gave rise to the superior agronomic traits of B. carinata. Notably, we identified an expansion of transcription factor networks and agronomically important gene families. Completion of the Triangle of U comparative genomics platform has allowed us to examine the dynamics of polyploid evolution and the role of subgenome dominance in the domestication and continuing agronomic improvement of B. carinata and other Brassica species.

Leaf surfaces and neolithization - the case of Arundo donax L

Arundo donax L. (Arundinoideae subfamily, Poaceae family) is a sub-tropical and temperate climate reed that grows in arid and semi-arid environmental conditions, from eastern China to the Mediterranean basin, suggesting potential adaptations at the epicuticular level. A thorough physical-chemical examination of the adaxial and abaxial surfaces of A. donax leaf was performed herein in an attempt to track such chemophenetic adaptations. This sort of approach is of the utmost importance for the current debate about the hypothetical invasiveness of this species in the Mediterranean basin versus its natural colonization along the Plio-Pleistocene period. We concluded that the leaf surfaces contain, apart from stomata, prickles, and long, thin trichomes, and silicon-rich tetralobate phytolits. Chemically, the dominating elements in the leaf ashes are oxygen and potassium; minor amounts of calcium, silicon, magnesium, phosphorous, sulphur, and chlorine were also detected. In both surfaces the epicuticular waxes (whose density is higher in the adaxial surface than in the abaxial surface) form randomly orientated platelets, with irregular shape and variable size, and aggregated rodlets with variable diameter around the stomata. In the case of green mature leaves, the dominating organic compounds of the epicuticular waxes of both surfaces are triterpenoids. Both surfaces feature identical hydrophobic behaviour, and exhibit the same total transmittance, total reflectance, and absorption of incident light. The above findings suggest easy growth of the plant, remarkable epidermic robustness of the leaf, and control of water loss. These chemophenetic characteristics and human influence support a neolithization process of this species along the Mediterranean basin.

Transcriptome and Physiological Analyses of a Navel Orange Mutant with Improved Drought Tolerance and Water Use Efficiency Caused by Increases of Cuticular Wax Accumulation and ROS Scavenging Capacity

Drought is one of the main abiotic stresses limiting the quality and yield of citrus. Cuticular waxes play an important role in regulating plant drought tolerance and water use efficiency (WUE). However, the contribution of cuticular waxes to drought tolerance, WUE and the underlying molecular mechanism is still largely unknown in citrus. 'Longhuihong' (MT) is a bud mutant of 'Newhall' navel orange with curly and bright leaves. In this study, significant increases in the amounts of total waxes and aliphatic wax compounds, including n-alkanes, n-primary alcohols and n-aldehydes, were overserved in MT leaves, which led to the decrease in cuticular permeability and finally resulted in the improvements in drought tolerance and WUE. Compared to WT leaves, MT leaves possessed much lower contents of malondialdehyde (MDA) and hydrogen peroxide (H₂O₂), significantly higher levels of proline and soluble sugar, and enhanced superoxide dismutase (SOD), catalase (CAT) and peroxidase (POD) activities under drought stress, which might reduce reactive oxygen species (ROS) damage, improve osmotic regulation and cell membrane stability, and finally, enhance MT tolerance to drought stress. Transcriptome sequencing results showed that seven structural genes were involved in wax biosynthesis and export, MAPK cascade, and ROS scavenging, and seven genes encoding transcription factors might play an important role in promoting cuticular wax accumulation, improving drought tolerance and WUE in MT plants. Our results not only confirmed the important role of cuticular waxes in regulating citrus drought resistance and WUE but also provided various candidate genes for improving citrus drought tolerance and WUE.

The production of wax esters in transgenic plants: towards a sustainable source of bio-lubricants

Wax esters are high-value compounds used as feedstocks for the production of lubricants, pharmaceuticals, and cosmetics. Currently, they are produced mostly from fossil reserves using chemical synthesis, but this cannot meet increasing demand and has a negative environmental impact. Natural wax esters are also obtained from Simmondsia chinensis (jojoba) but comparably in very low amounts and expensively. Therefore, metabolic engineering of plants, especially of the seed storage lipid metabolism of oil crops, represents an attractive strategy for renewable, sustainable, and environmentally friendly production of wax esters tailored to industrial applications. Utilization of wax ester-synthesizing enzymes with defined specificities and modulation of the acyl-CoA pools by various genetic engineering approaches can lead to obtaining wax esters with desired compositions and properties. However, obtaining high amounts of wax esters is still challenging due to their negative impact on seed germination and yield. In this review, we describe recent progress in establishing non-food-plant platforms for wax ester production and discuss their advantages and limitations as well as future prospects.

Comparative Investigation of Combined Metabolomics-Flavoromics during the Ripening of Mango (Mangifera indica L.) cv. 'Nam Dok Mai Si Thong' and 'Nam Dok Mai No. 4'

A metabolomics-flavoromics approach was conducted to assess the micromolecules of 'Nam Dok Mai Si Thong' and 'Nam Dok Mai No. 4' mango cultivars from two seasons. During ripening, FAMEs, FFAs, fatty alcohols, sterols, and organic acids were dominant at 0-2 days, whereas amino acids, sugars, and volatile organic compounds, including esters, alcohols, ketones, aldehydes, and terpenes, were at higher levels at 4-8 days. Nine metabolites (palmitic/linoleic/linolenic/citric/malic acids, β-sitosterol, sucrose, glycine, and leucine) and two volatile organic compounds (ethyl octanoate/decanoate) were related to ripening-associated changes within eight days. During ripening, sucrose at 6-8 days, citric/malic acid at 0-2 days, glycine and leucine at 4 days, and ethyl octanoate and ethyl decanoate at 8 days could be used as quality biomarkers for Nam Dok Mai Si Thong; palmitic/linoleic/linolenic acids at 0 days and β-sitosterol at 0-4 days could be used as quality biomarkers for Nam Dok Mai No. 4.

Comparative analysis of the cuticular waxes and related gene expression between 'Newhall' and 'Ganqi 3' navel orange during long-term cold storage

Previously, we obtained a wax-deficient mutant 'Ganqi 3' (MT) from 'Newhall' navel orange (Citrus sinensis [L.] Osbeck cv. Newhall, WT). The weight loss and postharvest decay in MT fruit were much higher than those in WT fruit after long-term cold storage. To understand the underlying mechanism, the changes in the morphology, chemical composition and gene expression of cuticular waxes between WT and MT fruit were compared during 150 days of storage at 4 °C. The density of epicuticular wax crystals and the contents of most of the aliphatic wax fractions in MT fruit were much lower than those in WT fruit over 90 days of storage. Further research revealed that the differences in the morphology and chemical composition of cuticular waxes might be important causes for the differences of postharvest weight loss and decay rates between WT and MT fruit. Notably, the expression profiles of 16 wax-related genes in WT and MT fruit were consistent with the change trends of corresponding cuticular wax components during cold storage. These results suggest that the morphology and chemical composition of cuticular waxes may be regulated by wax-related genes and play an important role in regulating the postharvest weight loss and the tolerances to postharvest decay in navel orange.

Comparative analysis of alfalfa (Medicago sativa L.) seedling transcriptomes reveals genotype-specific drought tolerance mechanisms

Drought is one of the main abiotic factors that affect alfalfa yield. The identification of genes that control this complex trait can provide important insights for alfalfa breeding. However, little is known about how alfalfa responds and adapts to drought stress, particularly in cultivars of differing drought tolerance. In this study, the drought-tolerant cultivar Dryland 'DT' and the drought-sensitive cultivar WL343HQ 'DS' were used to characterize leaf and root physiological responses and transcriptional changes in response to water deficit. Under drought stress, Dryland roots (DTR) showed more differentially expressed genes than WL343HQ roots (DSR), whereas WL343HQ leaves (DSL) showed more differentially expressed genes than Dryland leaves (DTL). Many of these genes were involved in stress-related pathways, carbohydrate metabolism, and lignin and wax biosynthesis, which may have improved the drought tolerance of alfalfa. We also observed that several genes related to ABA metabolism, root elongation, peroxidase activity, cell membrane stability, ubiquitination, and genetic processing responded to drought stress in alfalfa. We highlighted several candidate genes, including sucrose synthase, xylan 1,4-beta-xylosidase, primary-amine oxidase, and alcohol-forming fatty acyl-CoA reductase, for future studies on drought stress resistance in alfalfa and other plant species. In summary, our results reveal the unique drought adaptation and resistance characteristics of two alfalfa genotypes. These findings, which may be valuable for drought resistance breeding, warrant further gene functional analysis to augment currently available information and to clarify the drought stress regulatory mechanisms of alfalfa and other plants.

ZmMs25 encoding a plastid-localized fatty acyl reductase is critical for anther and pollen development in maize

Fatty acyl reductases (FARs) catalyse the reduction of fatty acyl-coenzyme A (CoA) or -acyl carrier protein (ACP) substrates to primary fatty alcohols, which play essential roles in lipid metabolism in plants. However, the mechanism by which FARs are involved in male reproduction is poorly defined. Here, we found that two maize allelic mutants, ms25-6065 and ms25-6057, displayed defective anther cuticles, abnormal Ubisch body formation, impaired pollen exine formation and complete male sterility. Based on map-based cloning and CRISPR/Cas9 mutagenesis, Zm00001d048337 was identified as ZmMs25, encoding a plastid-localized FAR with catalytic activities to multiple acyl-CoA substrates in vitro. Four conserved residues (G101, G104, Y327 and K331) of ZmMs25 were critical for its activity. ZmMs25 was predominantly expressed in anther, and was directly regulated by transcription factor ZmMYB84. Lipidomics analysis revealed that ms25 mutation had significant effects on reducing cutin monomers and internal lipids, and altering the composition of cuticular wax in anthers. Moreover, loss of function of ZmMs25 significantly affected the expression of its four paralogous genes and five cloned lipid metabolic male-sterility genes in maize. These data suggest that ZmMs25 is required for anther development and male fertility, indicating its application potential in maize and other crops.

Silencing GhFAR3.1 reduces wax accumulation in cotton leaves and leads to increased susceptibility to drought stress

Fatty acyl-CoA reductases (FAR) are involved in plant wax synthesis and play important roles in plant growth and development. However, little information is available for cotton FAR genes. In this study, we carried out a genome-wide identification of FAR genes in Upland cotton (Gossypium hirsutum L.) and found 10 GhFARs that form five pairs of homoeologs (GhFAR2A to GhFAR3.4D) distributed on six chromosomes. The 10 GhFARs were separated into two subfamilies. Most GhFARs showed tissue-specific expression patterns, and at least one GhFAR of each pair of homoeologs was relatively highly expressed in at least one of the tissues investigated. GhFAR3.1 was highly expressed in leaves. The function of GhFAR3.1 in wax accumulation and drought tolerance was analyzed using virus-induced gene silencing (VIGS). Silencing GhFAR3.1 reduced the total wax content and relative water content of leaves by over 60% and 13%, respectively, suggesting a role of GhFAR3.1 in wax synthesis and protection against water loss. Compared to the well-watered conditions, drought stress induced significant accumulation of wax in leaves of wild-type plants but not in leaves of GhFAR3.1 silenced plants, leading to less water holding capacity in GhFAR3.1 silenced plants and plant wilting. Silencing GhFAR3.1 had no effect on the expression levels of the wax biosynthesis pathway genes KAS, KCS, and LACS (upstream GhFAR3.1), but reduced the transcript level of its downstream gene WSD. Together, these results suggest that leaf wax content is important for water retention and drought tolerance and that GhFAR3.1 is essential for wax synthesis in cotton leaves. These results also provide the basis for further study on the molecular regulation mechanism of GhFARs in cotton development and surface lipid synthesis.

Biosynthesis of Fatty Alcohols in Engineered Microbial Cell Factories: Advances and Limitations

Concerns about climate change and environmental destruction have led to interest in technologies that can replace fossil fuels and petrochemicals with compounds derived from sustainable sources that have lower environmental impact. Fatty alcohols produced by chemical synthesis from ethylene or by chemical conversion of plant oils have a large range of industrial applications. These chemicals can be synthesized through biological routes but their free forms are produced in trace amounts naturally. This review focuses on how genetic engineering of endogenous fatty acid metabolism and heterologous expression of fatty alcohol producing enzymes have come together resulting in the current state of the field for production of fatty alcohols by microbial cell factories. We provide an overview of endogenous fatty acid synthesis, enzymatic methods of conversion to fatty alcohols and review the research to date on microbial fatty alcohol production. The primary focus is on work performed in the model microorganisms, Escherichia coli and Saccharomyces cerevisiae but advances made with cyanobacteria and oleaginous yeasts are also considered. The limitations to production of fatty alcohols by microbial cell factories are detailed along with consideration to potential research directions that may aid in achieving viable commercial scale production of fatty alcohols from renewable feedstock.

Trichostatin A Affects Developmental Reprogramming of Bread Wheat Microspores towards an Embryogenic Route

Microspores can be developmentally reprogrammed by the application of different stress treatments to initiate an embryogenic pathway leading to the production of doubled haploid (DH) plants. Epigenetic modifications are involved in cell reprogramming and totipotency in response to stress. To increase microspore embryogenesis (ME) efficiency in bread wheat, the effect of the histone deacetylase inhibitor trichostatin A (TSA) has been examined in two cultivars of wheat with different microspore embryogenesis response. Diverse strategies were assayed using 0-0.4 µM TSA as a single induction treatment and after or simultaneously with cold or mannitol stresses. The highest efficiency was achieved when 0.4 µM TSA was applied to anthers for 5 days simultaneously with a 0.7 M mannitol treatment, producing a four times greater number of green DH plants than mannitol. Ultrastructural studies by transmission electron microscopy indicated that mannitol with TSA and mannitol treatments induced similar morphological changes in early stages of microspore reprogramming, although TSA increased the number of microspores with 'star-like' morphology and symmetric divisions. The effect of TSA on the transcript level of four ME marker genes indicated that the early signaling pathways in ME, involving the TaTDP1 and TAA1b genes, may be mediated by changes in acetylation patterns of histones and/or other proteins.

Synthesis of 13 C-labelled cutin and suberin monomeric dicarboxylic acids of the general formula HO2 13 C-(CH2 )n -13 CO2 H (n = 10, 12, 14, 16, 18, 20, 22, 24, 26, 28)

¹³ C-labeled dicarboxylic acids HO₂ ¹³ C-(CH₂ )_n -¹³ CO₂ H (n = 10, 12, 14, 16, 18, 20, 22, 24, 26, 28) have been synthesized as internal standards for LC-MS and GC-MS analysis of cutin and suberin monomer degradation by soil-based microorganisms. Different synthetic strategies had to be applied depending on the chain length of the respective synthetic target and because of economic considerations. ¹³ C-labels were introduced by nucleophilic substitution of a suitable leaving group with labelled potassium cyanide and subsequent hydrolysis of the nitriles to produce the corresponding dicarboxylic acids. All new compounds are characterized by GC/MS, IR, and NMR methods as well as by elemental analysis.

Population sequencing reveals clonal diversity and ancestral inbreeding in the grapevine cultivar Chardonnay

Chardonnay is the basis of some of the world's most iconic wines and its success is underpinned by a historic program of clonal selection. There are numerous clones of Chardonnay available that exhibit differences in key viticultural and oenological traits that have arisen from the accumulation of somatic mutations during centuries of asexual propagation. However, the genetic variation that underlies these differences remains largely unknown. To address this knowledge gap, a high-quality, diploid-phased Chardonnay genome assembly was produced from single-molecule real time sequencing, and combined with re-sequencing data from 15 different Chardonnay clones. There were 1620 markers identified that distinguish the 15 clones. These markers were reliably used for clonal identification of independently sourced genomic material, as well as in identifying a potential genetic basis for some clonal phenotypic differences. The predicted parentage of the Chardonnay haplomes was elucidated by mapping sequence data from the predicted parents of Chardonnay (Gouais blanc and Pinot noir) against the Chardonnay reference genome. This enabled the detection of instances of heterosis, with differentially-expanded gene families being inherited from the parents of Chardonnay. Most surprisingly however, the patterns of nucleotide variation present in the Chardonnay genome indicate that Pinot noir and Gouais blanc share an extremely high degree of kinship that has resulted in the Chardonnay genome displaying characteristics that are indicative of inbreeding.

Phenotypic and transcriptomic characterization of a wheat tall mutant carrying an induced mutation in the C-terminal PFYRE motif of RHT-B1b

BACKGROUND

As central regulators of the gibberellic acid (GA) signaling pathway in plants, DELLA proteins function as growth repressors and affect diverse biological processes. The wheat RHT-B1b and RHT-D1b semi-dwarfing alleles, which encode GA-insensitive DELLA proteins, have been widely adopted in modern wheat varieties to improve lodging tolerance and harvest index. However, the molecular mechanisms by which DELLA modulates these responses in wheat remain largely unknown.

RESULTS

We identified a tall tetraploid wheat mutant line carrying an induced missense mutation (E529K) in the PFYRE motif of RHT-B1b that partially suppressed the semi-dwarf phenotype. The height-increasing effect of RHT-B1bE529K relative to RHT-B1b (19 cm or 21% increase) was significantly smaller than the effect of RHT-B1a (33 cm or 34% increase) relative to RHT-B1b in the same field experiment. The RHT-B1bE529K mutation was also associated with length increases in coleoptiles, seedling shoots, and stem internodes relative to the RHT-B1b allele. We detected no significant differences in germination rate, seedling root length, tiller number, flag leaf size, spike length, or yield components. Using RNA-seq, we compared gene expression profiles of plants encoding RHT-B1b and RHT-B1bE529K in coleoptile, first leaf, and elongating peduncles. We detected limited overlap among tissues of the genes differentially regulated by the two genotypes, and more genes upregulated (77%) than downregulated (23%) in RHT-B1bE529K relative to RHT-B1b. These results suggest that the wheat DELLA protein affects the transcriptome in a tissue-specific manner and that the mutation mainly eliminates or reduces repression functions of the RHT-B1 protein. Our study identified distinct sets of potential DELLA direct or indirect target genes involved in cell wall and carbohydrate metabolisms, cell cycle/division, and hormone pathways.

CONCLUSIONS

We identified the hypomorphic RHT-B1bE529K allele that confers an intermediate plant height and coleoptile elongation. This allele can be useful in rain-fed wheat breeding programs where the strong reduction in height and biomass associated with RHT-B1b has detrimental effects. Transcriptomic characterization of different tissues from the plants encoding RHT-B1bE529K and RHT-B1b provided valuable information for identifying DELLA downstream GA response genes in wheat.

An engineered fatty acid synthase combined with a carboxylic acid reductase enables de novo production of 1-octanol in Saccharomyces cerevisiae

BACKGROUND

The ideal biofuel should not only be a regenerative fuel from renewable feedstocks, but should also be compatible with the existing fuel distribution infrastructure and with normal car engines. As the so-called drop-in biofuel, the fatty alcohol 1-octanol has been described as a valuable substitute for diesel and jet fuels and has already been produced fermentatively from sugars in small amounts with engineered bacteria via reduction of thioesterase-mediated premature release of octanoic acid from fatty acid synthase or via a reversal of the β-oxidation pathway.

RESULTS

The previously engineered short-chain acyl-CoA producing yeast Fas1R1834K/Fas2 fatty acid synthase variant was expressed together with carboxylic acid reductase from Mycobacterium marinum and phosphopantetheinyl transferase Sfp from Bacillus subtilis in a Saccharomyces cerevisiae Δfas1 Δfas2 Δfaa2 mutant strain. With the involvement of endogenous thioesterases, alcohol dehydrogenases, and aldehyde reductases, the synthesized octanoyl-CoA was converted to 1-octanol up to a titer of 26.0 mg L-1 in a 72-h fermentation. The additional accumulation of 90 mg L-1 octanoic acid in the medium indicated a bottleneck in 1-octanol production. When octanoic acid was supplied externally to the yeast cells, it could be efficiently converted to 1-octanol indicating that re-uptake of octanoic acid across the plasma membrane is not limiting. Additional overexpression of aldehyde reductase Ahr from Escherichia coli nearly completely prevented accumulation of octanoic acid and increased 1-octanol titers up to 49.5 mg L-1. However, in growth tests concentrations even lower than 50.0 mg L-1 turned out to be inhibitory to yeast growth. In situ extraction in a two-phase fermentation with dodecane as second phase did not improve growth, indicating that 1-octanol acts inhibitive before secretion. Furthermore, 1-octanol production was even reduced, which results from extraction of the intermediate octanoic acid to the organic phase, preventing its re-uptake.

CONCLUSIONS

By providing chain length control via an engineered octanoyl-CoA producing fatty acid synthase, we were able to specifically produce 1-octanol with S. cerevisiae. Before metabolic engineering can be used to further increase product titers and yields, strategies must be developed that cope with the toxic effects of 1-octanol on the yeast cells.

Identification of Wheat Inflorescence Development-Related Genes Using a Comparative Transcriptomics Approach

Inflorescence represents the highly specialized plant tissue producing the grains. Although key genes regulating flower initiation and development are conserved, the mechanism regulating fertility is still not well explained. To identify genes and gene network underlying inflorescence morphology and fertility of bread wheat, expressed sequence tags (ESTs) from different tissues were analyzed using a comparative transcriptomics approach. Based on statistical comparison of EST frequencies of individual genes in EST pools representing different tissues and verification with RT-PCR and RNA-seq data, 170 genes of 59 gene sets predominantly expressed in the inflorescence were obtained. Nearly one-third of the gene sets displayed differentiated expression profiles in terms of their subgenome orthologs. The identified genes, most of which were predominantly expressed in anthers, encode proteins involved in wheat floral identity determination, anther and pollen development, pollen-pistil interaction, and others. Particularly, 25 annotated gene sets are associated with pollen wall formation, of which 18 encode enzymes or proteins participating in lipid metabolic pathway, including fatty acid ω-hydroxylation, alkane and fatty alcohol biosynthesis, and glycerophospholipid metabolism. We showed that the comparative transcriptomics approach was effective in identifying genes for reproductive development and found that lipid metabolism was particularly active in wheat anthers.

Three endoplasmic reticulum-associated fatty acyl-coenzyme a reductases were involved in the production of primary alcohols in hexaploid wheat (Triticum aestivum L.)

BACKGROUND

The cuticle covers the surface of the polysaccharide cell wall of leaf epidermal cells and forms an essential diffusion barrier between the plant and the environment. The cuticle is composed of cutin and wax. Cuticular wax plays an important role in the survival of plants by serving as the interface between plants and their biotic and abiotic environments, especially restricting nonstomatal water loss. Leaf cuticular waxes of hexaploid wheat at the seedling stage mainly consist of primary alcohols, aldehydes, fatty acids, alkane and esters. Primary alcohols account for more than 80% of the total wax load. Therefore, we cloned several genes encoding fatty acyl-coenzyme A reductases from wheat and analyzed their function in yeast and plants. We propose the potential use of these genes in wheat genetic breeding.

RESULTS

We reported the cloning and characterization of three TaFARs, namely TaFAR6, TaFAR7 and TaFAR8, encoding fatty acyl-coenzyme A reductases (FAR) in wheat leaf cuticle. Expression analysis revealed that TaFAR6, TaFAR7 and TaFAR8 were expressed at the higher levels in the seedling leaf blades, and were expressed moderately or weakly in stamen, glumes, peduncle, flag leaf blade, sheath, spike, and pistil. The heterologous expression of three TaFARs in yeast (Saccharomyces cerevisiae) led to the production of C24:0 and C26:0 primary alcohols. Transgenic expression of the three TaFARs in tomato (Solanum lycopersicum) and rice (Oryza sativa) led to increased accumulation of C24:0-C30:0 primary alcohols. Transient expression of GFP protein-tagged TaFARs revealed that the three TaFAR proteins were localized to the endoplasmic reticulum (ER), the site of wax biosynthesis. The three TaFAR genes were transcriptionally induced by drought, cold, heat, powdery mildew (Blumeria graminis) infection, abscisic acid (ABA) and methyl jasmonate (MeJa) treatments.

CONCLUSIONS

These results indicated that wheat TaFAR6, TaFAR7 and TaFAR8 are involved in biosynthesis of very-long-chain primary alcohols in hexaploid wheat and in response to multiple environmental stresses.

Improved fatty aldehyde and wax ester production by overexpression of fatty acyl-CoA reductases

Background

Fatty aldehydes are industrially relevant compounds, which also represent a common metabolic intermediate in the microbial synthesis of various oleochemicals, including alkanes, fatty alcohols and wax esters. The key enzymes in biological fatty aldehyde production are the fatty acyl-CoA/ACP reductases (FARs) which reduce the activated acyl molecules to fatty aldehydes. Due to the disparity of FARs, identification and in vivo characterization of reductases with different properties are needed for the construction of tailored synthetic pathways for the production of various compounds.

Results

Fatty aldehyde production in Acinetobacter baylyi ADP1 was increased by the overexpression of three different FARs: a native A. baylyi FAR Acr1, a cyanobacterial Aar, and a putative, previously uncharacterized dehydrogenase (Ramo) from Nevskia ramosa. The fatty aldehyde production was followed in real-time inside the cells with a luminescence-based tool, and the highest aldehyde production was achieved with Aar. The fate of the overproduced fatty aldehydes was studied by measuring the production of wax esters by a native downstream pathway of A. baylyi, for which fatty aldehyde is a specific intermediate. The wax ester production was improved with the overexpression of Acr1 or Ramo compared to the wild type A. baylyi by more than two-fold, whereas the expression of Aar led to only subtle wax ester production. The overexpression of FARs did not affect the length of the acyl chains of the wax esters.

Conclusions

The fatty aldehyde production, as well as the wax ester production of A. baylyi, was improved with the overexpression of a key enzyme in the pathway. The wax ester titer (0.45 g/l) achieved with the overexpression of Acr1 is the highest reported without hydrocarbon supplementation to the culture. The contrasting behavior of the different reductases highlight the significance of in vivo characterization of enzymes and emphasizes the possibilities provided by the diversity of FARs for pathway and product modulation.

Electronic supplementary material

The online version of this article (10.1186/s12934-018-0869-z) contains supplementary material, which is available to authorized users.

Occurrence and Biosynthesis of Alkyl Hydroxycinnamates in Plant Lipid Barriers

The plant lipid barriers cuticle and suberin represent one of the largest biological interfaces on the planet. They are comprised of an insoluble polymeric domain with associated organic solvent-soluble waxes. Suberin-associated and plant cuticular waxes contain mixtures of aliphatic components that may include alkyl hydroxycinnamates (AHCs). The canonical alkyl hydroxycinnamates are comprised of phenylpropanoids, typically coumaric, ferulic, or caffeic acids, esterified with long chain to very long chain fatty alcohols. However, many related structures are also present in the plant kingdom. Although their functions remain elusive, much progress has been made on understanding the distribution, biosynthesis, and deposition of AHCs. Herein a summary of the current state of knowledge on plant AHCs is provided.

Metabolic engineering of Saccharomyces cerevisiae for production of very long chain fatty acid-derived chemicals

Production of chemicals and biofuels through microbial fermentation is an economical and sustainable alternative for traditional chemical synthesis. Here we present the construction of a Saccharomyces cerevisiae platform strain for high-level production of very-long-chain fatty acid (VLCFA)-derived chemicals. Through rewiring the native fatty acid elongation system and implementing a heterologous Mycobacteria FAS I system, we establish an increased biosynthesis of VLCFAs in S. cerevisiae. VLCFAs can be selectively modified towards the fatty alcohol docosanol (C₂₂H₄₆O) by expressing a specific fatty acid reductase. Expression of this enzyme is shown to impair cell growth due to consumption of VLCFA-CoAs. We therefore implement a dynamic control strategy for separating cell growth from docosanol production. We successfully establish high-level and selective docosanol production of 83.5 mg l⁻¹ in yeast. This approach will provide a universal strategy towards the production of similar high value chemicals in a more scalable, stable and sustainable manner.

Production of chemicals by microbial fermentation is an economical alternative to chemical synthesis. Here the authors re-engineer the yeast S. cerevisiae to produce the very long chain fatty alcohol docosanol by expressing a heterologous Mycobacteria fatty acid synthase and a specific fatty acid reductase.

Metabolic engineering of fatty alcohol production in transgenic hairy roots of Crambe abyssinica

Biotechnological production of fatty alcohols, important raw materials in the chemical industry, has been receiving considerable attention in recent years. Fatty alcohols are formed by the reduction of fatty acyl-CoAs or fatty acyl-ACPs catalyzed by a fatty acyl reductase (FAR). In this study, we introduced genes encoding FARs from Arabidopsis thaliana (AtFAR5) and Simmondsia chinensis (ScFAR) into Crambe abyssinica hairy roots via Agrobacterium rhizogenes-mediated transformation. The efficiency of the transformation ranged between 30 and 45%. The fatty alcohols were only detected in the transgenic hairy root lines expressing ScFAR gene. In all tested lines stearyl alcohol (18:0-OH), arachidyl alcohol (20:0-OH), and behenyl alcohol (22:0-OH) were produced. The content of 18:0-OH varied from 1 to 3% of total fatty acids and fatty alcohols, while the amount of either 20:0-OH and 22:0-OH did not exceed 2%. The transgenic hairy root lines produced from 0.98 to 2.59 nmol of fatty alcohols per mg of dry weight. Very low activity of ScFAR was detected in the microsomal fractions isolated from the selected hairy root lines. To our knowledge, this is the first report on the fatty alcohol production in the hairy root cultures. Biotechnol. Bioeng. 2017;114: 1275-1282. © 2016 Wiley Periodicals, Inc.

Biochemical characterization and substrate specificity of jojoba fatty acyl-CoA reductase and jojoba wax synthase

Wax esters are used in industry for production of lubricants, pharmaceuticals and cosmetics. The only natural source of wax esters is jojoba oil. A much wider variety of industrial wax esters-containing oils can be generated through genetic engineering. Biotechnological production of tailor-made wax esters requires, however, a detailed substrate specificity of fatty acyl-CoA reductases (FAR) and wax synthases (WS), the two enzymes involved in wax esters synthesis. In this study we have successfully characterized the substrate specificity of jojoba FAR and jojoba WS. The genes encoding both enzymes were expressed heterologously in Saccharomyces cerevisiae and the activity of tested enzymes was confirmed by in vivo studies and in vitro assays using microsomal preparations from transgenic yeast. Jojoba FAR exhibited the highest in vitro activity toward 18:0-CoA followed by 20:1-CoA and 22:1-CoA. The activity toward other 11 tested acyl-CoAs was low or undetectable as with 18:2-CoA and 18:3-CoA. In assays characterizing jojoba WS combinations of 17 fatty alcohols with 14 acyl-CoAs were tested. The enzyme displayed the highest activity toward 14:0-CoA and 16:0-CoA in combination with C16-C20 alcohols as well as toward C18 acyl-CoAs in combination with C12-C16 alcohols. 20:1-CoA was efficiently utilized in combination with most of the tested alcohols.

High production of fatty alcohols in Escherichia coli with fatty acid starvation

Background

Microbial biofuel synthesis attracting increasing attention. Great advances have been made in producing fatty alcohols from fatty acyl-CoAs and fatty acids in Escherichia coli. However, the low titers and limited knowledge regarding the basic characteristics of fatty alcohols, such as location and toxicity, have hampered large-scale industrialization. Further research is still needed.

Results

In this study, we designed a novel and efficient strategy to enhance fatty alcohol production by inducing fatty acid starvation. We report the first use of deletions of acyl-ACP thioesterases to enhance fatty alcohol production. Transcriptional analysis was conducted to investigate the mechanism of the designed strategy. Then, fatty alcohol production was further enhanced by deletion of genes from competing pathways. Fatty alcohols were shown to be extracellular products with low toxicity. The final strain, E. coli MGL2, produced fatty alcohols at the remarkable level of 6.33 g/L under fed-batch fermentation, representing the highest reported titer of fatty alcohols produced by microorganisms.

Conclusions

Deletions of genes responsible for synthesis of fatty acids and competing products are promising strategies for fatty alcohol production. Our investigation of the location and toxicity of fatty alcohols suggest bright future for fatty alcohol production in E. coli.

Electronic supplementary material

The online version of this article (doi:10.1186/s12934-016-0524-5) contains supplementary material, which is available to authorized users.

Microbial production of fatty alcohols

Fatty alcohols have numerous commercial applications, including their use as lubricants, surfactants, solvents, emulsifiers, plasticizers, emollients, thickeners, and even fuels. Fatty alcohols are currently produced by catalytic hydrogenation of fatty acids from plant oils or animal fats. Microbial production of fatty alcohols may be a more direct and environmentally-friendly strategy since production is carried out by heterologous enzymes, called fatty acyl-CoA reductases, able to reduce different acyl-CoA molecules to their corresponding primary alcohols. Successful examples of metabolic engineering have been reported in Saccharomyces cerevisiae and Escherichia coli in which the production of fatty alcohols ranged from 1.2 to 1.9 g/L, respectively. Due to their metabolic advantages, oleaginous yeasts are considered the best hosts for production of fatty acid-derived chemicals. Some of these species can naturally produce, under specific growth conditions, lipids at high titers (>50 g/L) and therefore provide large amounts of fatty acyl-CoAs or fatty acids as precursors. Very recently, taking advantage of such features, over 8 g/L of C16-C18 fatty alcohols have been produced in Rhodosporidium toruloides. In this review we summarize the different metabolic engineering strategies, hosts and cultivation conditions used to date. We also point out some future trends and challenges for the microbial production of fatty alcohols.

Synthesis of oleyl oleate wax esters in Arabidopsis thaliana and Camelina sativa seed oil

Seed oil composed of wax esters with long-chain monoenoic acyl moieties represents a high-value commodity for industry. Such plant-derived sperm oil-like liquid wax esters are biodegradable and can have excellent properties for lubrication. In addition, wax ester oil may represent a superior substrate for biodiesel production. In this study, we demonstrate that the low-input oil seed crop Camelina sativa can serve as a biotechnological platform for environmentally benign wax ester production. Two biosynthetic steps catalysed by a fatty alcohol-forming acyl-CoA reductase (FAR) and a wax ester synthase (WS) are sufficient to achieve wax ester accumulation from acyl-CoA substrates. To produce plant-derived sperm oil-like liquid wax esters, the WS from Mus musculus (MmWS) or Simmondsia chinensis (ScWS) were expressed in combination with the FAR from Mus musculus (MmFAR1) or Marinobacter aquaeolei (MaFAR) in seeds of Arabidopsis thaliana and Camelina sativa. The three analysed enzyme combinations Oleo3:mCherry:MmFAR1∆c/Oleo3:EYFP:MmWS, Oleo3:mCherry:MmFAR1∆c/ScWS and MaFAR/ScWS showed differences in the wax ester molecular species profiles and overall biosynthetic performance. By expressing MaFAR/ScWS in Arabidopsis or Camelina up to 59% or 21% of the seed oil TAGs were replaced by wax esters, respectively. This combination also yielded wax ester molecular species with highest content of monounsaturated acyl moieties. Expression of the enzyme combinations in the Arabidopsis fae1 fad2 mutant background high in oleic acid resulted in wax ester accumulation enriched in oleyl oleate (18:1/18:1 > 60%), suggesting that similar values may be obtained with a Camelina high oleic acid line.

Molecular Characterization of TaFAR1 Involved in Primary Alcohol Biosynthesis of Cuticular Wax in Hexaploid Wheat

Cuticular waxes are complex mixtures of very long chain (VLC) fatty acids and their derivatives in which primary alcohols are the most abundant components in the leaf surface of common wheat (Triticum aestivum L.). However, the genes involved in primary alcohol biosynthesis in wheat are still largely unknown. Here we identified, via a homology-based approach, the TaFAR1 gene belonging to the fatty acyl-CoA reductases (FARs) from wheat. Heterologous expression of TaFAR1 in yeast (Saccharomyces cerevisiae) and in the Arabidopsis (Arabidopsis thaliana) cer4-3 mutant afforded production of C22 primary alcohol and C22-C24 primary alcohols, respectively, and transgenic expression of TaFAR1 in tomato (Solanum lycopersicum) cv MicroTom leaves and fruits resulted in the accumulation of C26-C30 primary alcohols and C30-C34 primary alcohols, respectively. The TaFAR1 protein was localized to the endoplasmic reticulum (ER) in rice (Oryza sativa L.) leaf protoplasts. Moreover, the TaFAR1 expression pattern across various organs correlated with the levels of primary alcohols accumulating in corresponding waxes, and with the presence of platelet-shaped epicuticular wax crystals formed by primary alcohols. A nullisomic-tetrasomic wheat line lacking TaFAR1 had significantly reduced levels of primary alcohols in its leaf blade and anther wax. TaFAR1 was located on chromosome 4AL and appeared to be highly conserved, with only one haplotype among 32 wheat cultivars. Finally, TaFAR1 expression was induced by drought and cold stress in an ABA-dependent manner. Taken together, our results show that TaFAR1 is an active enzyme forming primary alcohols destined for the wheat cuticle.

Analysis of cuticular wax constituents and genes that contribute to the formation of 'glossy Newhall', a spontaneous bud mutant from the wild-type 'Newhall' navel orange

Navel orange (Citrus sinensis [L.] Osbeck) fruit surfaces contain substantial quantities of cuticular waxes, which have important eco-physiological roles, such as water retention and pathogen defense. The wax constituents of ripe navel orange have been studied in various reports, while the wax changes occurring during fruit development and the molecular mechanism underlying their biosynthesis/export have not been investigated. Recently, we reported a spontaneous bud mutant from the wild-type (WT) 'Newhall' Navel orange. This mutant displayed unusual glossy fruit peels and was named 'glossy Newhall' (MT). In this study, we compared the developmental profiles of the epicuticular and intracuticular waxes on the WT and MT fruit surfaces. The formation of epicuticular wax crystals on the navel orange surface was shown to be dependent on the accumulation of high amounts of aliphatic wax components with trace amounts of terpenoids. In sharp contrast, the underlying intracuticular wax layers have relatively low concentrations of aliphatic wax components but high concentrations of cyclic wax compounds, especially terpenoids at the late fruit developmental stages. Our work also showed that many genes that are involved in wax biosynthesis and export pathways were down-regulated in MT fruit peels, leading to a decrease in aliphatic wax component amounts and the loss of epicuticular wax crystals, ultimately causing the glossy phenotype of MT fruits.

Suberin: biosynthesis, regulation, and polymer assembly of a protective extracellular barrier

Suberin is a lipid-phenolic biopolyester deposited in the cell walls of certain boundary tissue layers of plants, such as root endodermis, root and tuber peridermis, and seed coats. Suberin serves as a protective barrier in these tissue layers, controlling, for example, water and ion transport. It is also a stress-induced anti-microbial barrier. The suberin polymer contains a variety of C16-C24 chain-length aliphatics, such as ω-hydroxy fatty acids, α,ω-dicarboxylic fatty acids, and primary fatty alcohols. Suberin also contains high amounts of glycerol and phenolics, especially ferulic acid. In addition, non-covalently linked waxes are likely associated with the suberin polymer. This review focusses on the suberin biosynthetic enzymes identified to date, which include β-ketoacyl-CoA synthases, fatty acyl reductases, long-chain acyl-CoA synthetases, cytochrome P450 monooxygenases, glycerol 3-phosphate acyltransferases, and phenolic acyltransferases. We also discuss recent advances in our understanding of the transport of suberin components intracellularly and to the cell wall, polymer assembly, and the regulation of suberin deposition.

Role of HXXXD-motif/BAHD acyltransferases in the biosynthesis of extracellular lipids

Terrestrial plants have evolved specific adaptations to preserve water and protect themselves from their environment. Such adaptations range from secondary metabolites and specialized structures that conduct water and nutrients, to cell wall modifications (i.e., cuticle and suberin) that prevent dehydration and provide a physical barrier to pathogens. Both the plant cuticle and suberized cell walls contain a lipid polymer framework embedded with waxes, and constitute a promising target for controlled genetic modification to improve desirable agronomic traits. Recent advances in genomic and molecular techniques coupled with the development of robust analytical methods have accelerated progress in comprehending these intractable lipid polymers. Gene products characterized in the wax, cutin and suberin pathways include a subset of HXXXD/BAHD family enzymes that catalyze acyl transfer reactions between CoA-activated hydroxycinnamic acid derivatives and hydroxylated aliphatics. This review highlights our current understanding of HXXXD/BAHD acyltransferases in extracellular lipid biosynthesis and discusses the chemical, ultrastructural and physiological ramifications of impairing the expression of BAHD acyltransferase-encoding genes related to cutin and suberin synthesis.

The bifunctional protein TtFARAT from Tetrahymena thermophila catalyzes the formation of both precursors required to initiate ether lipid biosynthesis

The biosynthesis of ether lipids and wax esters requires as precursors fatty alcohols, which are synthesized by fatty acyl reductases (FARs). The presence of ether glycerolipids as well as branched wax esters has been reported in several free-living ciliate protozoa. In the genome of Tetrahymena thermophila, the only ORF sharing similarities with FARs is fused to an acyltransferase-like domain, whereas, in most other organisms, FARs are monofunctional proteins of similar size and domain structure. Here, we used heterologous expression in plant and yeast to functionally characterize the activities catalyzed by this protozoan protein. Transient expression in tobacco epidermis of a truncated form fused to the green fluorescence protein followed by confocal microscopy analysis suggested peroxisomal localization. In vivo approaches conducted in yeast indicated that the N-terminal FAR-like domain produced both 16:0 and 18:0 fatty alcohols, whereas the C-terminal acyltransferase-like domain was able to rescue the lethal phenotype of the yeast double mutant gat1Δ gat2Δ. Using in vitro approaches, we further demonstrated that this domain is a dihydroxyacetone phosphate acyltransferase that uses preferentially 16:0-coenzyme A as an acyl donor. Finally, coexpression in yeast with the alkyl-dihydroxyacetone phosphate synthase from T. thermophila resulted the detection of various glycerolipids with an ether bond, indicating reconstitution of the ether lipid biosynthetic pathway. Together, these results demonstrate that this FAR-like protein is peroxisomal and bifunctional, providing both substrates required by alkyl-dihydroxyacetone phosphate synthase to initiate ether lipid biosynthesis.

Crystallization and preliminary crystallographic analysis of defective pollen wall (DPW) protein from Oryza sativa

The defective pollen wall (dpw) gene of Oryza sativa encodes a fatty acid reductase (DPW) which plays important roles in primary fatty alcohol synthesis. DPW catalyzes the synthesis of 1-hexadecanol. The enzyme shows a higher specificity for palmitoyl-ACP than for palmitoyl-CoA as the substrate, and can only use NADPH as the cofactor. To gain an understanding of the molecular mechanism underlying the reaction catalyzed by DPW, the gene encoding DPW without the N-terminal 80 amino acids (DPWΔ80) was cloned into pET-28a vector and was overexpressed in Escherichia coli. DPWΔ80 was purified to homogeneity and screened for crystallization. DPWΔ80 in complex with NADPH produced crystals that diffracted X-rays to a resolution of 3.4 Å. The crystals belonged to space group P6₁ or P6₅, with unit-cell parameters a=b=222.8, c=114.0 Å, α=β=90, γ=120°.

Expanding ester biosynthesis in Escherichia coli

To expand the capabilities of whole-cell biocatalysis, we have engineered Escherichia coli to produce various esters. The alcohol O-acyltransferase (ATF) class of enzyme uses acyl-CoA units for ester formation. The release of free CoA upon esterification with an alcohol provides the free energy to facilitate ester formation. The diversity of CoA molecules found in nature in combination with various alcohol biosynthetic pathways allows for the biosynthesis of a multitude of esters. Small to medium volatile esters have extensive applications in the flavor, fragrance, cosmetic, solvent, paint and coating industries. The present work enables the production of these compounds by designing several ester pathways in E. coli. The engineered pathways generated acetate esters of ethyl, propyl, isobutyl, 2-methyl-1-butyl, 3-methyl-1-butyl and 2-phenylethyl alcohols. In particular, we achieved high-level production of isobutyl acetate from glucose (17.2 g l(-1)). This strategy was expanded to realize pathways for tetradecyl acetate and several isobutyrate esters.

Transcriptional regulation of cuticle biosynthesis

Plant cuticle is the hydrophobic protection layer that covers aerial plant organs and plays a pivotal role during plant development and interactions of plants with the environment. The mechanical structure and chemical composition of cuticle lipids and other secondary metabolites vary considerably between plant species, and in response to environmental stimuli and stresses. As the cuticle plays an important role in responses of plants to major abiotic stresses such as drought and high salinity, close attention has been paid to molecular processes underlying the stress-induced biosynthesis of cuticle components. This review addresses the genetic networks responsible for cuticle formation and in particular highlights the role of transcription factors that regulate cuticle formation in response to abiotic stresses.

Production of medium chain length fatty alcohols from glucose in Escherichia coli

Metabolic engineering offers the opportunity to produce a wide range of commodity chemicals that are currently derived from petroleum or other non-renewable resources. Microbial synthesis of fatty alcohols is an attractive process because it can control the distribution of chain lengths and utilize low cost fermentation substrates. Specifically, primary alcohols with chain lengths of 12 to 14 carbons have many uses in the production of detergents, surfactants, and personal care products. The current challenge is to produce these compounds at titers and yields that would make them economically competitive. Here, we demonstrate a metabolic engineering strategy for producing fatty alcohols from glucose. To produce a high level of 1-dodecanol and 1-tetradecanol, an acyl-ACP thioesterase (BTE), an acyl-CoA ligase (FadD), and an acyl-CoA/aldehyde reductase (MAACR) were overexpressed in an engineered strain of Escherichia coli. Yields were improved by balancing expression levels of each gene, using a fed-batch cultivation strategy, and adding a solvent to the culture for extracting the product from cells. Using these strategies, a titer of over 1.6 g/L fatty alcohol with a yield of over 0.13 g fatty alcohol/g carbon source was achieved. These are the highest reported yield of fatty alcohols produced from glucose in E. coli.

Identification of amino acids conferring chain length substrate specificities on fatty alcohol-forming reductases FAR5 and FAR8 from Arabidopsis thaliana

Fatty alcohols play a variety of biological roles in all kingdoms of life. Fatty acyl reductase (FAR) enzymes catalyze the reduction of fatty acyl-coenzyme A (CoA) or fatty acyl-acyl carrier protein substrates to primary fatty alcohols. FAR enzymes have distinct substrate specificities with regard to chain length and degree of saturation. FAR5 (At3g44550) and FAR8 (At3g44560) from Arabidopsis thaliana are 85% identical at the amino acid level and are of equal length, but they possess distinct specificities for 18:0 or 16:0 acyl chain length, respectively. We used Saccharomyces cerevisiae as a heterologous expression system to assess FAR substrate specificity determinants. We identified individual amino acids that affect protein levels or 16:0-CoA versus 18:0-CoA specificity by expressing in yeast FAR5 and FAR8 domain-swap chimeras and site-specific mutants. We found that a threonine at position 347 and a serine at position 363 were important for high FAR5 and FAR8 protein accumulation in yeast and thus are likely important for protein folding and stability. Amino acids at positions 355 and 377 were important for dictating 16:0-CoA versus 18:0-CoA chain length specificity. Simultaneously converting alanine 355 and valine 377 of FAR5 to the corresponding FAR8 residues, leucine and methionine, respectively, almost fully converted FAR5 specificity from 18:0-CoA to 16:0-CoA. The reciprocal amino acid conversions, L355A and M377V, made in the active FAR8-S363P mutant background converted its specificity from 16:0-CoA to 18:0-CoA. This study is an important advancement in the engineering of highly active FAR proteins with desired specificities for the production of fatty alcohols with industrial value.

Wax ester profiling of seed oil by nano-electrospray ionization tandem mass spectrometry

BACKGROUND

Wax esters are highly hydrophobic neutral lipids that are major constituents of the cutin and suberin layer. Moreover they have favorable properties as a commodity for industrial applications. Through transgenic expression of wax ester biosynthetic genes in oilseed crops, it is possible to achieve high level accumulation of defined wax ester compositions within the seed oil to provide a sustainable source for such high value lipids. The fatty alcohol moiety of the wax esters is formed from plant-endogenous acyl-CoAs by the action of fatty acyl reductases (FAR). In a second step the fatty alcohol is condensed with acyl-CoA by a wax synthase (WS) to form a wax ester. In order to evaluate the specificity of wax ester biosynthesis, analytical methods are needed that provide detailed wax ester profiles from complex lipid extracts.

RESULTS

We present a direct infusion ESI-tandem MS method that allows the semi-quantitative determination of wax ester compositions from complex lipid mixtures covering 784 even chain molecular species. The definition of calibration prototype groups that combine wax esters according to their fragmentation behavior enables fast quantitative analysis by applying multiple reaction monitoring. This provides a tool to analyze wax layer composition or determine whether seeds accumulate a desired wax ester profile. Besides the profiling method, we provide general information on wax ester analysis by the systematic definition of wax ester prototypes according to their collision-induced dissociation spectra. We applied the developed method for wax ester profiling of the well characterized jojoba seed oil and compared the profile with wax ester-accumulating Arabidopsis thaliana expressing the wax ester biosynthetic genes MaFAR and ScWS.

CONCLUSIONS

We developed a fast profiling method for wax ester analysis on the molecular species level. This method is suitable to screen large numbers of transgenic plants as well as other wax ester samples like cuticular lipid extracts to gain an overview on the molecular species composition. We confirm previous results from APCI-MS and GC-MS analysis, which showed that fragmentation patterns are highly dependent on the double bond distribution between the fatty alcohol and the fatty acid part of the wax ester.

Metabolic engineering of plant oils and waxes for use as industrial feedstocks

Society has come to rely heavily on mineral oil for both energy and petrochemical needs. Plant lipids are uniquely suited to serve as a renewable source of high-value fatty acids for use as chemical feedstocks and as a substitute for current petrochemicals. Despite the broad variety of acyl structures encountered in nature and the cloning of many genes involved in their biosynthesis, attempts at engineering economic levels of specialty industrial fatty acids in major oilseed crops have so far met with only limited success. Much of the progress has been hampered by an incomplete knowledge of the fatty acid biosynthesis and accumulation pathways. This review covers new insights based on metabolic flux and reverse engineering studies that have changed our view of plant oil synthesis from a mostly linear process to instead an intricate network with acyl fluxes differing between plant species. These insights are leading to new strategies for high-level production of industrial fatty acids and waxes. Furthermore, progress in increasing the levels of oil and wax structures in storage and vegetative tissues has the potential to yield novel lipid production platforms. The challenge and opportunity for the next decade will be to marry these technologies when engineering current and new crops for the sustainable production of oil and wax feedstocks.

Arabidopsis cuticular waxes: advances in synthesis, export and regulation

Cuticular waxes and cutin form the cuticle, a hydrophobic layer covering the aerial surfaces of land plants and acting as a protective barrier against environmental stresses. Very-long-chain fatty acid derived compounds that compose the cuticular waxes are produced in the endoplasmic reticulum of epidermal cells before being exported to the environmental face of the epidermis. Twenty years of genetic studies on Arabidopsis thaliana have led to the molecular characterization of enzymes catalyzing major steps in fatty acid elongation and wax biosynthesis. Although transporters required for wax export from the plasma membrane have been identified, intracellular and extracellular traffic remains largely unknown. In accordance with its major function in producing an active waterproof barrier, wax metabolism is up-regulated at the transcriptional level in response to water deficiency. However its developmental regulation is still poorly described. Here, we discuss the present knowledge of wax functions, biosynthesis and transport as well as the regulation of these processes.