Plastidial wax ester biosynthesis as a tool to synthesize shorter and more saturated wax esters

Background Wax esters (WE) are neutral lipids that consist of a fatty alcohol esterified to a fatty acid. WE are valuable feedstocks in industry for producing lubricants, coatings, and cosmetics. They can be produced chemically from fossil fuel or plant-derived triacylglycerol. As fossil fuel resources are finite, the synthesis of WE in transgenic plants may serve as an alternative source. As chain length and desaturation of the alcohol and acyl moieties determine the physicochemical properties of WE and their field of application, tightly controlled and tailor-made WE synthesis in plants would be a sustainable, beneficial, and valuable commodity. Here, we report the expression of ten combinations of WE producing transgenes in Arabidopsis thaliana. In order to study their suitability for WE production in planta, we analyzed WE amount and composition in the transgenic plants. Results The transgenes consisted of different combinations of a FATTY ACYL-COA/ACP REDUCTASE (FAR) and two WAX SYNTHASES/ACYL-COA:DIACYLGLYCEROL O-ACYLTRANSFERASES (WSD), namely WSD2 and WSD5 from the bacterium Marinobacter aquaeoleoi. We generated constructs with and without plastidial transit peptides to access distinct alcohol and acyl substrate pools within A. thaliana cells. We observed WE formation with plastid and cytosol-localized FAR and WSD in seeds. A comparative WE analysis revealed the production of shorter and more saturated WE by plastid-localized WE biosynthesis compared to cytosolic WE synthesis. Conclusions A shift of WE formation into seed plastids is a suitable approach for tailor-made WE production and can be used to synthesize WE that are mainly derived from mid- and long-chain saturated and monounsaturated substrates.

Effects of 1-MCP on the Physiological Attributes, Volatile Components and Ester-Biosynthesis-Related Gene Expression during Storage of ‘Jinyan’ Kiwifruit

The effects of 1.0 μL/L 1-methylcyclopropene (1-MCP) treatment on aroma quality and ester-biosynthesis-related gene expression of ‘Jinyan’ kiwifruit during room storage were examined, aiming to provide a theoretical basis and technical reference for the postharvest storage of kiwifruit. The results demonstrate that 1-MCP treatment conspicuously inhibited respiration rate, delayed a decrease in fruit firmness and increased soluble solid content (SSC) in ‘Jinyan’ kiwifruit. Compared to the control, the relative content of aroma components markedly changed in 1-MCP treatment kiwifruit during fruit ripening. The characteristic aroma of ‘Jinyan’ kiwifruit included ethyl butanoate, methyl butanoate, E-2-hexanal and hexenal, and 1-MCP treatment significantly reduced the ester content in kiwifruit. During the entire shelf life, the expression levels of AcLOX1, AcLOX5, AcLOX6, AcHPL and AcAAT were significantly inhibited in 1-MCP-treated fruit. However, the transcript level of AcADH was not suppressed by 1-MCP. The lower content of ester volatiles maybe ascribed to the suppression of AcLOXs, AcHPL and AcAAT.

Plastidial Wax Ester Biosynthesis As A Tool To Synthesize Shorter And More Saturated Wax Esters

Background: Wax esters (WE) are neutral lipids that consist of a fatty alcohol esterified to a fatty acid. WE are valuable feedstocks in industry for producing lubricants, coatings and cosmetics. They can be produced chemically from fossil fuel or plant derived triacylglycerol. As fossil fuel resources are finite, the synthesis of WE in transgenic plants may serve as an alternative source. As chain length and desaturation degree of the alcohol and acyl moieties determine the physicochemical properties of WE and their field of application, a tightly controlled tailor-made WE synthesis in plants is aimed for. Here, we report the generation of ten combinations of WE producing transgenes expressed in Arabidopsis thaliana. In order to study their suitability for WE production in planta, we analyzed WE amount and synthesized WE species in the transgenic plants.Results: The transgenes consisted of different combinations of a fatty acyl-CoA/ACP reductase (FAR) and two wax synthases/acyl-CoA:diacylglycerol O-acyltransferases (WSD) namely WSD2 and WSD5 from the bacterium Marinobacter aquaeoleoi. We generated constructs with and without plastidial transit peptides to access diverse alcohol and acyl substrate pools within A. thaliana. We observed WE formation with plastid and cytosol-localized FAR and WSD in seeds. A comparative WE analysis revealed the production of shorter and more saturated WE by plastid-localized WE biosynthesis compared to cytosolic WE synthesis.Conclusions: A shift of WE formation into seed plastids is a suitable approach for tailor-made WE production and can be used to synthesize WE mainly derived from mid and long chain saturated and monounsaturated substrates.

Probing Specificities of Alcohol Acyltransferases for Designer Ester Biosynthesis with a High-Throughput Microbial Screening Platform

Alcohol acyltransferases (AATs) enables microbial biosynthesis of a large space of esters by condensing an alcohol and an acyl CoA. However, substrate promiscuity of AATs prevents microbial biosynthesis of designer esters with high selectivity. Here, we developed a high-throughput microbial screening platform that facilitates rapid identification of AATs for designer ester biosynthesis. First, we established a microplate-based culturing technique with in situ fermentation and extraction of esters. We validated its capability in rapid profiling of the alcohol substrate specificity of 20 chloramphenicol acetyltransferase variants derived from Staphylococcus aureus (CATSa) for microbial biosynthesis of acetate esters with various exogeneous alcohol supply. By coupling the microplate-based culturing technique with a previously established colorimetric assay, we developed a high-throughput microbial screening platform for AATs. We demonstrated that this platform could not only confirm CATSa F97W with enhanced isobutyl acetate synthesis but also identify three ATF1Sc (P348M, P348A, and P348S) variants, derived from Saccharomyces cerevisiae' s AAT and engineered by model-guided protein design, for enhanced butyl acetate production. We anticipate the high-throughput microbial screening platform is a useful tool to identify novel AATs that have important roles in nature and industrial biocatalysis for designer bioester production.

Emission Patterns of Esters and Their Precursors Throughout Ripening and Senescence in ‘Redchief Delicious’ Apple Fruit and Implications Regarding Biosynthesis and Aroma Perception

The volatile profile of ‘Redchief Delicious’ apple (Malus ×domestica Borkh.) fruit was evaluated at 18 time points from 3 weeks before to 8 weeks after onset of autocatalytic ethylene production to capture the dynamics associated with development from mature green to senescent fruit. Minor amounts of ester production began several days before the onset of ethylene production. Ester production rose rapidly as internal ethylene levels increased beyond 22 nmol·L−1 (0.5 µL·L−1). Peak ester production roughly coincided with maximum ethylene synthesis, declining thereafter. Ester production was further evaluated according to the acid- (alkanoate) and alcohol- (alkyl) derived portions of the ester. The maximum rate of production for a given ester tended to occur later in development as the chain length of the alcohol-derived portion declined. The production rate for many esters paralleled the rate of emanation of their respective alcohol substrates, suggesting that availability of the alcohols limits ester production more than availability of the acid substrates. Combining production rates with sensory descriptors and human sensitivity to individual volatiles permitted approximations of aroma sensations likely engendered by the fruit throughout ripening. Overripe and alcoholic sensations are predicted to increase 2 weeks after the initiation of ripening in response to an increase in the production of ethyl esters. Acetate esters predominated, comprising 50% to 80% of esters throughout maturation and ripening, indicating that the substrate acetyl-CoA may be at saturating levels for alcohol acyl transferase (AAT) at the final step of ester formation. Acetate feeding did not enhance ester production, although label from 13C-acetate was extensively incorporated into esters. The data are consistent with the action of multiple AAT isozymes differing in activity and substrate preference. Incorporation of labeled 13C-acetate into precursors of esters, alcohols, and acids, reflected ester biosynthesis via 1- and 2-carbon chain elongation pathways in ripening ‘Redchief Delicious’ apple fruit.

Metabolic, Organoleptic and Transcriptomic Impact of Saccharomyces cerevisiae Genes Involved in the Biosynthesis of Linear and Substituted Esters

Esters constitute a wide family of volatile compounds impacting the organoleptic properties of many beverages including wine and beer. They can be classified according to their chemical structure. Higher Alcohols Acetate differ from Fatty Acids Ethyl Esters whereas a third group, Substituted Ethyl Esters, contributes to the fruitiness of red wines. Derived from yeast metabolism, the biosynthesis of Higher Alcohols Acetates and Fatty Acids Ethyl Esters has been widely in-vestigated at the enzymatic and genetic level. In this work, we confirmed their effective contri-bution in the fruity perception in young red wines by evaluating the effect of their depletion by chemical and sensorial analyses. As previously reported, two pairs of esterases respectively en-coded by the paralogue genes (ATF1, ATF2) and (EEB1 and EHT1) are mostly involved in the bi-osynthesis of Acetate of Higher alcohols and Fatty Acids Ethyl Esters. However, those esterases have a moderate effect on the biosynthesis of Substituted Ethyl Esters that depends to another pair of genes, MGL2 and YJU3 encoding for mono-acyl lipases. These new findings complete our un-derstanding of esters metabolism in the context of wine alcoholic fermentation. In order to evaluate the sensorial impact of esters we attempted to produce a red wine without esters by generating a multiple deletion strain. Surprisingly, we failed to abolish all the esterase activities revealing unsuspected physiological consequences of ester biosynthesis routes. A preliminary RNA-seq analysis depicted the overall impact of the multiple deletion of ATF1, ATF2, EEB1 and EHT1 that triggers the expression shift of 1124 genes involved in nitrogen and lipid metabolism but also chromatin organization and histone acetylation, suggesting an unsuspected regulatory role of ester metabolism