Arabidopsis ECERIFERUM2-LIKEs Are Mediators of Condensing Enzyme Function

3-ketoacyl-CoA synthase 7 from Xanthoceras sorbifolium seeds is a crucial regulatory enzyme for nervonic acid biosynthesis

Nervonic acid (C24:1) is a very-long-chain fatty acid that plays an imperative role in human brain development and other health benefits. In plants, 3-ketoacyl-CoA synthase (KCS) is the key rate-limiting enzyme for C24:1 biosynthesis. Xanthoceras sorbifolium is a valuable oil-producing economic woody species with abundant C24:1 in seed oils, but the key KCS gene responsible for C24:1 accumulation remains unknown. In this work, a correlation analysis between the transcript profiles of KCS and dynamic change of C24:1 content in developing seeds of X. sorbifolium were conducted to screen out three members of KCS, namely XsKCS4, XsKCS7 and XsKCS8, potentially involved in C24:1 biosynthesis. Of which, the XsKCS7 was highly expressed in developing seeds, while XsKCS4 and XsKCS8 displayed the highest expression in fruits and flowers, respectively. Overexpression of XsKCS4, XsKCS7 and XsKCS8 in yeast Saccharomyces cerevisiae and plant Arabidopsis thaliana indicated that only XsKCS7 possessed the ability to facilitate the biosynthesis of C24:1. These findings collectively suggested that XsKCS7 played a crucial role in specific regulation of C24:1 biosynthesis in X. sorbifolium seeds.

The impact of the GLOSSY2 and GLOSSY2-LIKE BAHD-proteins in affecting the product profile of the maize fatty acid elongase

The maize glossy2 and glossy2-like genes are homologs, which encode proteins that belong to the BAHD family of acyltransferases. In planta genetic studies have demonstrated that these genes may be involved in the elongation of very long chain fatty acids (VLCFAs) that are precursors of the cuticular wax fraction of the plant cuticle. VLCFAs are synthesized by a fatty acyl-CoA elongase complex (FAE) that consists of four component enzymes. Previously, we functionally identified the maize FAE component enzymes by their ability to complement haploid Saccharomyces cerevisiae strains that carry lethal deletion alleles for each FAE component enzyme. In this study we used these complemented haploid strains and wild-type diploid strains to evaluate whether the co-expression of either GLOSSY2 or GLOSSY2-LIKE with individual maize FAE component enzymes affects the VLCFA product-profile of the FAE system. Wild-type diploid strains produced VLCFAs of up to 28-carbon chain length. Co-expression of GLOSSY2 or GLOSSY2-LIKE with a combination of maize 3-ketoacyl-CoA synthases stimulated the synthesis of longer VLCFAs, up to 30-carbon chain lengths. However, such results could not be recapitulated when these co-expression experiments were conducted in the yeast haploid mutant strains that lacked individual components of the endogenous FAE system. Specifically, lethal yeast mutant strains that are genetically complemented by the expression of maize FAE-component enzymes produce VLCFAs that range between 20- and 26-carbon chain lengths. However, expressing either GLOSSY2 or GLOSSY2-LIKE in these complemented strains does not enable the synthesis of longer chain VLCFAs. These results indicate that the apparent stimulatory role of GLOSSY2 or GLOSSY2-LIKE to enable the synthesis of longer chain VLCFAs in diploid yeast cells may be associated with mixing plant enzyme components with the endogenous FAE complex.

A chromosome level genome assembly of Pseudoroegneria Libanotica reveals a key Kcs gene involves in the cuticular wax elongation for drought resistance

BACKGROUND

The genus Pseudoroegneria (Nevski) Löve (Triticeae, Poaceae), whose genome symbol was designed as "St", accounts for more than 60% of perennial Triticeae species. The diploid species Psudoroegneria libanotica (2n = 14) contains the most ancient St genome, exhibited strong drought resistance, and was morphologically covered by cuticular wax on the aerial part. Therefore, the St-genome sequencing data could provide fundamental information for studies of genome evolution and reveal its mechanisms of cuticular wax and drought resistance.

RESULTS

In this study, we reported the chromosome-level genome assembly for the St genome of Pse. libanotica, with a total size of 2.99 Gb. 46,369 protein-coding genes annotated and 71.62% was repeat sequences. Comparative analyses revealed that the genus Pseudoroegneria diverged during the middle and late Miocene. During this period, unique genes, gene family expansion, and contraction in Pse. libanotica were enriched in biotic and abiotic stresses, such as fatty acid biosynthesis which may greatly contribute to its drought adaption. Furthermore, we investigated genes associated with the cuticular wax formation and water deficit and found a new Kcs gene evm.TU.CTG175.54. It plays a critical role in the very long chain fatty acid (VLCFA) elongation from C18 to C26 in Pse. libanotica. The function needs more evidence to be verified.

CONCLUSIONS

We sequenced and assembled the St genome in Triticeae and discovered a new KCS gene that plays a role in wax extension to cope with drought. Our study lays a foundation for the genome diversification of Triticeae species and deciphers cuticular wax formation genes involved in drought resistance.

Family characteristics, phylogenetic reconstruction, and potential applications of the plant BAHD acyltransferase family

The BAHD acyltransferase family is a class of proteins in plants that can acylate a variety of primary and specialized secondary metabolites. The typically acylated products have greatly improved stability, lipid solubility, and bioavailability and thus show significant differences in their physicochemical properties and pharmacological activities. Here, we review the protein structure, catalytic mechanism, and phylogenetic reconstruction of plant BAHD acyltransferases to describe their family characteristics, acylation reactions, and the processes of potential functional differentiation. Moreover, the potential applications of the BAHD family in human activities are discussed from the perspectives of improving the quality of economic plants, enhancing the efficacy of medicinal plants, improving plant biomass for use in biofuel, and promoting stress resistance of land plants. This review provides a reference for the research and production of plant BAHD acyltransferases.

Protein interactomes for plant lipid biosynthesis and their biotechnological applications

Plant lipids have essential biological roles in plant development and stress responses through their functions in cell membrane formation, energy storage and signalling. Vegetable oil, which is composed mainly of the storage lipid triacylglycerol, also has important applications in food, biofuel and oleochemical industries. Lipid biosynthesis occurs in multiple subcellular compartments and involves the coordinated action of various pathways. Although biochemical and molecular biology research over the last few decades has identified many proteins associated with lipid metabolism, our current understanding of the dynamic protein interactomes involved in lipid biosynthesis, modification and channelling is limited. This review examines advances in the identification and characterization of protein interactomes involved in plant lipid biosynthesis, with a focus on protein complexes consisting of different subunits for sequential reactions such as those in fatty acid biosynthesis and modification, as well as transient or dynamic interactomes formed from enzymes in cooperative pathways such as assemblies of membrane-bound enzymes for triacylglycerol biosynthesis. We also showcase a selection of representative protein interactome structures predicted using AlphaFold2, and discuss current and prospective strategies involving the use of interactome knowledge in plant lipid biotechnology. Finally, unresolved questions in this research area and possible approaches to address them are also discussed.

Tackling functional redundancy of Arabidopsis fatty acid elongase complexes

Very-long-chain fatty acids (VLCFA) are precursors for various lipids playing important physiological and structural roles in plants. Throughout plant tissues, VLCFA are present in multiple lipid classes essential for membrane homeostasis, and also stored in triacylglycerols. VLCFA and their derivatives are also highly abundant in lipid barriers, such as cuticular waxes in aerial epidermal cells and suberin monomers in roots. VLCFA are produced by the fatty acid elongase (FAE), which is an integral endoplasmic reticulum membrane multi-enzymatic complex consisting of four core enzymes. The 3-ketoacyl-CoA synthase (KCS) catalyzes the first reaction of the elongation and determines the chain-length substrate specificity of each elongation cycle, whereas the other three enzymes have broad substrate specificities and are shared by all FAE complexes. Consistent with the co-existence of multiple FAE complexes, performing sequential and/or parallel reactions to produce the broad chain-length-range of VLCFA found in plants, twenty-one KCS genes have been identified in the genome of Arabidopsis thaliana. Using CRISPR-Cas9 technology, we established an expression platform to reconstitute the different Arabidopsis FAE complexes in yeast. The VLCFA produced in these yeast strains were analyzed in detail to characterize the substrate specificity of all KCS candidates. Additionally, Arabidopsis candidate proteins were transiently expressed in Nicotiana benthamiana leaves to explore their activity and localization in planta. This work sheds light on the genetic and biochemical redundancy of fatty acid elongation in plants.

Exploring and exploiting cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley

Wheat and barley are widely distributed cereal crops whose yields are adversely affected by environmental stresses such as drought, salinity, extreme temperatures, and attacks of pathogens and pests. As the interphase between aerial plant organs and their environments, hydrophobic cuticle largely consists of a cutin matrix impregnated and sealed with cuticular waxes. Increasing evidence supports that the cuticle plays a key role in plant adaptation to abiotic and biotic stresses, which could be harnessed for wheat and barley improvement. In this review, we highlighted recent advances in cuticle biosynthesis and its multifaceted roles in abiotic and biotic stress tolerance of wheat and barley. Current strategies, challenges, and future perspectives on manipulating cuticle biosynthesis for abiotic and biotic stress tolerance in wheat and barley are discussed.

BrWAX3, Encoding a β-ketoacyl-CoA Synthase, Plays an Essential Role in Cuticular Wax Biosynthesis in Chinese Cabbage

In this study, we identified a novel glossy mutant from Chinese cabbage, named SD369, and all wax monomers longer than 26 carbons were significantly decreased. Inheritance analysis revealed that the glossy trait of SD369 was controlled by a single recessive locus, BrWAX3. We fine-mapped the BrWAX3 locus to an interval of 161.82 kb on chromosome A09. According to the annotated genome of Brassica rapa, Bra024749 (BrCER60.A09), encoding a β-ketoacyl-CoA synthase, was identified as the candidate gene. Expression analysis showed that BrCER60.A09 was significantly downregulated in all aerial organs of glossy plants. Subcellular localization indicated that the BrCER60.A09 protein functions in the endoplasmic reticulum. A 5567-bp insertion was identified in exon 1 of BrCER60.A09 in SD369, which lead to a premature stop codon, thus causing a loss of function of the BrCER60.A09 enzyme. Moreover, comparative transcriptome analysis revealed that the ‘cutin, suberine, and wax biosynthesis’ pathway was significantly enriched, and genes involved in this pathway were almost upregulated in glossy plants. Further, two functional markers, BrWAX3-InDel and BrWAX3-KASP1, were developed and validated. Overall, these results provide a new information for the cuticular wax biosynthesis and provide applicable markers for marker-assisted selection (MAS)-based breeding of Brassica rapa.

ECERIFERUM 10 Encoding an Enoyl-CoA Reductase Plays a Crucial Role in Osmotolerance and Cuticular Wax Loading in Arabidopsis

Acquired osmotolerance induced after salt stress is widespread across Arabidopsis thaliana (Arabidopsis) accessions (e.g., Bu-5). However, it remains unclear how this osmotolerance is established. Here, we isolated a mutant showing an acquired osmotolerance-defective phenotype (aod2) from an ion-beam-mutagenized M2 population of Bu-5. aod2 was impaired not only in acquired osmotolerance but also in osmo-shock, salt-shock, and long-term heat tolerances compared with Bu-5, and it displayed abnormal morphology, including small, wrinkled leaves, and zigzag-shaped stems. Genetic analyses of aod2 revealed that a 439-kbp region of chromosome 4 was translocated to chromosome 3 at the causal locus for the osmosensitive phenotype. The causal gene of the aod2 phenotype was identical to ECERIFERUM 10 (CER10), which encodes an enoyl-coenzyme A reductase that is involved in the elongation reactions of very-long-chain fatty acids (VLCFAs) for subsequent derivatization into cuticular waxes, storage lipids, and sphingolipids. The major components of the cuticular wax were accumulated in response to osmotic stress in both Bu-5 WT and aod2. However, less fatty acids, primary alcohols, and aldehydes with chain length ≥ C30 were accumulated in aod2. In addition, aod2 exhibited a dramatic reduction in the number of epicuticular wax crystals on its stems. Endoplasmic reticulum stress mediated by bZIP60 was increased in aod2 under osmotic stress. The only cer10 showed the most pronounced loss of epidermal cuticular wax and most osmosensitive phenotype among four Col-0-background cuticular wax-related mutants. Together, the present findings suggest that CER10/AOD2 plays a crucial role in Arabidopsis osmotolerance through VLCFA metabolism involved in cuticular wax formation and endocytic membrane trafficking.

Regulatory mechanisms underlying cuticular wax biosynthesis

Plants are sessile organisms that have developed hydrophobic cuticles that cover their aerial epidermal cells to protect them from terrestrial stresses. The cuticle layer is mainly composed of cutin, a polyester of hydroxy and epoxy fatty acids, and cuticular wax, a mixture of very-long-chain fatty acids (>20 carbon atoms) and their derivatives, aldehydes, alkanes, ketones, alcohols, and wax esters. During the last 30 years, forward and reverse genetic, transcriptomic, and biochemical approaches have enabled the identification of key enzymes, transporters, and regulators involved in the biosynthesis of cutin and cuticular waxes. In particular, cuticular wax biosynthesis is significantly influenced in an organ-specific manner or by environmental conditions, and is controlled using a variety of regulators. Recent studies on the regulatory mechanisms underlying cuticular wax biosynthesis have enabled us to understand how plants finely control carbon metabolic pathways to balance between optimal growth and development and defense against abiotic and biotic stresses. In this review, we summarize the regulatory mechanisms underlying cuticular wax biosynthesis at the transcriptional, post-transcriptional, post-translational, and epigenetic levels.

Synthesis of C20-38 Fatty Acids in Plant Tissues

Very-long-chain fatty acids (VLCFA) are involved in a number of important plant physiological functions. Disorders in the expression of genes involved in the synthesis of VLCFA lead to a number of phenotypic consequences, ranging from growth retardation to the death of embryos. The elongation of VLCFA in the endoplasmic reticulum (ER) is carried out by multiple elongase complexes with different substrate specificities and adapted to the synthesis of a number of products required for a number of metabolic pathways. The information about the enzymes involved in the synthesis of VLCFA with more than 26 atoms of Carbon is rather poor. Recently, genes encoding enzymes involved in the synthesis of both regular-length fatty acids and VLCFA have been discovered and investigated. Polyunsaturated VLCFA in plants are formed mainly by 20:1 elongation into new monounsaturated acids, which are then imported into chloroplasts, where they are further desaturated. The formation of saturated VLCFA and their further transformation into a number of aliphatic compounds included in cuticular waxes and suberin require the coordinated activity of a large number of different enzymes.

Better together: Protein partnerships for lineage-specific oil accumulation

Plant-derived oils are a major agricultural product that exist in both ubiquitous forms such as common vegetable oils and in specialized forms such as castor oil and coconut oil. These specialized oils are the result of lineage-specific metabolic pathways that create oils rich in unusual fatty acids. Considerable progress has been made toward understanding the enzymes that mediate fatty acid biosynthesis, triacylglycerol assembly, and oil storage. However, efforts to translate this knowledge into renewable bioproducts via engineered oil-producing plants and algae have had limited success. Here, we review recent evidence that protein-protein interactions in each of the three major phases of oil formation appear to have profound effects on specialized oil accumulation. We suggest that furthering our knowledge of the noncatalytic attributes of enzymes and other proteins involved in oil formation will be a critical step toward creating renewable bioproducts derived from high performing, engineered oilseeds.

Understanding the Role of Populus ECERIFERUM2-Likes in the Biosynthesis of Very-Long-Chain Fatty Acids for Cuticular Waxes

Cuticular waxes are derived from very-long-chain fatty acid (VLCFA) precursors made by the concerted action of four enzymes that form the fatty acid (FA) elongation complex. The condensing enzyme of the complex confers specificity to substrates of different chain lengths, yet on its own cannot account for the biosynthesis of VLCFAs longer than 28 carbons (C28). Recent evidence from Arabidopsis thaliana points to a synergistic role of clade II BAHD acyltransferases and condensing enzymes in the elongation of VLCFAs beyond C28. In Populus trichocarpa, clade II is composed of seven uncharacterized paralogous genes (PtCER2-like1-7). In the present study, five of these genes were heterologously expressed in yeast and their respective FA profiles were determined. PtCER2-likes differentially altered the accumulation of C28 and C30 FAs when expressed in the presence of the condensing enzyme AtCER6. Among these, PtCER2-like5 produced the highest levels of C28 FAs in yeast and its expression was localized to the epidermis in β-glucuronidase-reporter poplar lines, consistent with a role in cuticular wax biosynthesis. Complementation of the A. thaliana cer2-5 mutant with PtCER2-like5 increased the levels of C28-derived cuticular waxes at the expense of C30-derived components. Together, these results demonstrate that the role of CER2-likes in cuticular wax biosynthesis is conserved in Populus clade II BAHD acyltransferases.

Biosynthesis and Functions of Very-Long-Chain Fatty Acids in the Responses of Plants to Abiotic and Biotic Stresses

Very-long-chain fatty acids (i.e., fatty acids with more than 18 carbon atoms; VLCFA) are important molecules that play crucial physiological and structural roles in plants. VLCFA are specifically present in several membrane lipids and essential for membrane homeostasis. Their specific accumulation in the sphingolipids of the plasma membrane outer leaflet is of primordial importance for its correct functioning in intercellular communication. VLCFA are found in phospholipids, notably in phosphatidylserine and phosphatidylethanolamine, where they could play a role in membrane domain organization and interleaflet coupling. In epidermal cells, VLCFA are precursors of the cuticular waxes of the plant cuticle, which are of primary importance for many interactions of the plant with its surrounding environment. VLCFA are also major components of the root suberin barrier, which has been shown to be fundamental for nutrient homeostasis and plant adaptation to adverse conditions. Finally, some plants store VLCFA in the triacylglycerols of their seeds so that they later play a pivotal role in seed germination. In this review, taking advantage of the many studies conducted using Arabidopsis thaliana as a model, we present our current knowledge on the biosynthesis and regulation of VLCFA in plants, and on the various functions that VLCFA and their derivatives play in the interactions of plants with their abiotic and biotic environment.