Identification and characterization of an LCAT-like Arabidopsis thaliana gene encoding a novel phospholipase A

Physiological changes and transcriptome profiling in Saccharum spontaneum L. leaf under water stress and re-watering conditions

As the polyploidy progenitor of modern sugarcane, Saccharum spontaneum is considered to be a valuable resistance source to various biotic and abiotic stresses. However, little has been reported on the mechanism of drought tolerance in S. spontaneum. Herein, the physiological changes of S. spontaneum GXS87-16 at three water-deficit levels (mild, moderate, and severe) and after re-watering during the elongation stage were investigated. RNA sequencing was utilized for global transcriptome profiling of GXS87-16 under severe drought and re-watered conditions. There were significant alterations in the physiological parameters of GXS87-16 in response to drought stress and then recovered differently after re-watering. A total of 1569 differentially expressed genes (DEGs) associated with water stress and re-watering were identified. Notably, the majority of the DEGs were induced by stress. GO functional annotations and KEGG pathway analysis assigned the DEGs to 47 GO categories and 93 pathway categories. The pathway categories were involved in various processes, such as RNA transport, mRNA surveillance, plant hormone signal transduction, and plant-pathogen interaction. The reliability of the RNA-seq results was confirmed by qRT-PCR. This study shed light on the regulatory processes of drought tolerance in S. spontaneum and identifies useful genes for genetic improvement of drought tolerance in sugarcane.

Physaria fendleri and Ricinus communis lecithin:cholesterol acyltransferase-like phospholipases selectively cleave hydroxy acyl chains from phosphatidylcholine

Production of hydroxy fatty acids (HFAs) in transgenic crops represents a promising strategy to meet our demands for specialized plant oils with industrial applications. The expression of Ricinus communis (castor) OLEATE 12-HYDROXYLASE (RcFAH12) in Arabidopsis has resulted in only limited accumulation of HFAs in seeds, which probably results from inefficient transfer of HFAs from their site of synthesis (phosphatidylcholine; PC) to triacylglycerol (TAG), especially at the sn-1/3 positions of TAG. Phospholipase As (PLAs) may be directly involved in the liberation of HFAs from PC, but the functions of their over-expression in HFA accumulation and distribution at TAG in transgenic plants have not been well studied. In this work, the functions of lecithin:cholesterol acyltransferase-like PLAs (LCAT-PLAs) in HFA biosynthesis were characterized. The LCAT-PLAs were shown to exhibit homology to LCAT and mammalian lysosomal PLA₂ , and to contain a conserved and functional Ser/His/Asp catalytic triad. In vitro assays revealed that LCAT-PLAs from the HFA-accumulating plant species Physaria fendleri (PfLCAT-PLA) and castor (RcLCAT-PLA) could cleave acyl chains at both the sn-1 and sn-2 positions of PC, and displayed substrate selectivity towards sn-2-ricinoleoyl-PC over sn-2-oleoyl-PC. Furthermore, co-expression of RcFAH12 with PfLCAT-PLA or RcLCAT-PLA, but not Arabidopsis AtLCAT-PLA, resulted in increased occupation of HFA at the sn-1/3 positions of TAG as well as small but insignificant increases in HFA levels in Arabidopsis seeds compared with RcFAH12 expression alone. Therefore, PfLCAT-PLA and RcLCAT-PLA may contribute to HFA turnover on PC, and represent potential candidates for engineering the production of unusual fatty acids in crops.

Castor patatin-like phospholipase A IIIβ facilitates removal of hydroxy fatty acids from phosphatidylcholine in transgenic Arabidopsis seeds

Castor patatin-like phospholipase A IIIβ facilitates the exclusion of hydroxy fatty acids from phosphatidylcholine in developing transgenic Arabidopsis seeds. Hydroxy fatty acids (HFAs) are industrial useful, but their major natural source castor contains toxic components. Although expressing a castor OLEATE 12-HYDROXYLASE in Arabidopsis thaliana leads to the synthesis of HFAs in seeds, a high proportion of the HFAs are retained in phosphatidylcholine (PC). Thus, the liberation of HFA from PC seems to be critical for obtaining HFA-enriched seed oils. Plant phospholipase A (PLA) catalyzes the hydrolysis of PC to release fatty acyl chains that can be subsequently channeled into triacylglycerol (TAG) synthesis or other metabolic pathways. To further our knowledge regarding the function of PLAs from HFA-producing plant species, two class III patatin-like PLA cDNAs (pPLAIIIβ or pPLAIIIδ) from castor or Physaria fendleri were overexpressed in a transgenic line of A. thaliana producing C18-HFA, respectively. Only the overexpression of RcpPLAIIIβ resulted in a significant reduction in seed HFA content with concomitant changes in fatty acid composition. Reductions in HFA content occurred in both PC and TAG indicating that HFAs released from PC were not incorporated into TAG. These results suggest that RcpPLAIIIβ may catalyze the removal of HFAs from PC in the developing seeds synthesizing these unusual fatty acids.

High-performance variants of plant diacylglycerol acyltransferase 1 generated by directed evolution provide insights into structure function

Diacylglycerol acyltransferase 1 (DGAT1) catalyzes the acyl-CoA-dependent biosynthesis of triacylglycerol, the predominant component of seed oil. In some oil crops, including Brassica napus, the level of DGAT1 activity can have a substantial effect on triacylglycerol production. Structure-function insights into DGAT1, however, remain limited because of the lack of a three-dimensional detailed structure for this membrane-bound enzyme. In this study, the amino acid residues governing B. napus DGAT1 (BnaDGAT1) activity were investigated via directed evolution, targeted mutagenesis, in vitro enzymatic assay, topological analysis, and transient expression of cDNA encoding selected enzyme variants in Nicotiana benthamiana. Directed evolution revealed that numerous amino acid residues were associated with increased BnaDGAT1 activity, and 67% of these residues were conserved among plant DGAT1s. The identified amino acid residue substitution sites occur throughout the BnaDGAT1 polypeptide, with 89% of the substitutions located outside the putative substrate binding or active sites. In addition, cDNAs encoding variants I447F or L441P were transiently overexpressed in N. benthamiana leaves, resulting in 33.2 or 70.5% higher triacylglycerol content, respectively, compared with native BnaDGAT1. Overall, the results provide novel insights into amino acid residues underlying plant DGAT1 function and performance-enhanced BnaDGAT1 variants for increasing vegetable oil production.

Acyl-CoA:Lysophosphatidylethanolamine Acyltransferase Activity Regulates Growth of Arabidopsis

Arabidopsis (Arabidopsis thaliana) contains two enzymes (encoded by the At1g80950 and At2g45670 genes) preferentially acylating lysophosphatidylethanolamine (LPE) with acyl-coenzyme A (CoA), designated LYSOPHOSPHATIDYLETHANOLAMINE ACYLTRANSFERASE1 (LPEAT1) and LPEAT2. The transfer DNA insertion mutant lpeat2 and the double mutant lpeat1 lpeat2 showed impaired growth, smaller leaves, shorter roots, less seed setting, and reduced lipid content per fresh weight in roots and seeds and large increases in LPE and lysophosphatidylcholine (LPC) contents in leaves. Microsomal preparations from leaves of these mutants showed around 70% decrease in acylation activity of LPE with 16:0-CoA compared with wild-type membranes, whereas the acylation with 18:1-CoA was much less affected, demonstrating that other lysophospholipid acyltransferases than the two LPEATs could acylate LPE The above-mentioned effects were less pronounced in the single lpeat1 mutant. Overexpression of either LPEAT1 or LPEAT2 under the control of the 35S promotor led to morphological changes opposite to what was seen in the transfer DNA mutants. Acyl specificity studies showed that LPEAT1 utilized 16:0-CoA at the highest rate of 11 tested acyl-CoAs, whereas LPEAT2 utilized 20:0-CoA as the best acyl donor. Both LPEATs could acylate either sn position of ether analogs of LPC The data show that the activities of LPEAT1 and LPEAT2 are, in a complementary way, involved in growth regulation in Arabidopsis. It is shown that LPEAT activity (especially LPEAT2) is essential for maintaining adequate levels of phosphatidylethanolamine, LPE, and LPC in the cells.

Analysis of triglyceride synthesis unveils a green algal soluble diacylglycerol acyltransferase and provides clues to potential enzymatic components of the chloroplast pathway

Background

Microalgal triglyceride (TAG) synthesis has attracted considerable attention. Particular emphasis has been put towards characterizing the algal homologs of the canonical rate-limiting enzymes, diacylglycerol acyltransferase (DGAT) and phospholipid:diacylglycerol acyltransferase (PDAT). Less work has been done to analyze homologs from a phylogenetic perspective. In this work, we used HMMER iterative profiling and phylogenetic and functional analyses to determine the number and sequence characteristics of algal DGAT and PDAT, as well as related sequences that constitute their corresponding superfamilies. We included most algae with available genomes, as well as representative eukaryotic and prokaryotic species.

Results

Amongst our main findings, we identified a novel clade of DGAT1-like proteins exclusive to red algae and glaucophyta and a previously uncharacterized subclade of DGAT2 proteins with an unusual number of transmembrane segments. Our analysis also revealed the existence of a novel DGAT exclusive to green algae with moderate similarity to plant soluble DGAT3. The DGAT3 clade shares a most recent ancestor with a group of uncharacterized proteins from cyanobacteria. Subcellular targeting prediction suggests that most green algal DGAT3 proteins are imported to the chloroplast, evidencing that the green algal chloroplast might have a soluble pathway for the de novo synthesis of TAGs. Heterologous expression of C. reinhardtii DGAT3 produces an increase in the accumulation of TAG, as evidenced by thin layer chromatography.

Conclusions

Our analysis contributes to advance in the knowledge of complex superfamilies involved in lipid metabolism and provides clues to possible enzymatic players of chloroplast TAG synthesis.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-017-3602-0) contains supplementary material, which is available to authorized users.

Oil is on the agenda: Lipid turnover in higher plants

Lipases hydrolyze ester bonds within lipids. This process is called lipolysis. They are key players in lipid turnover and involved in numerous metabolic pathways, many of which are shared between organisms like the mobilization of neutral or storage lipids or lipase-mediated membrane lipid homeostasis. Some reactions though are predominantly present in certain organisms, such as the production of signaling molecules (endocannabinoids) by diacylglycerol (DAG) and monoacylglycerol (MAG) lipases in mammals and plants or the jasmonate production in flowering plants. This review aims at giving an overview of the different functional classes of lipases and respective well-known activities, with a focus on the most recent findings in plant biology for selected classes. Here we will put an emphasis on the physiological role and contribution of lipases to the turnover of neutral lipids found in seed oil and other vegetative tissue as candidates for increasing the economical values of crop plants. This article is part of a Special Issue entitled: Plant Lipid Biology edited by Kent D. Chapman and Ivo Feussner.

A small phospholipase A2-α from castor catalyzes the removal of hydroxy fatty acids from phosphatidylcholine in transgenic Arabidopsis seeds

Ricinoleic acid, an industrially useful hydroxy fatty acid (HFA), only accumulates to high levels in the triacylglycerol fraction of castor (Ricinus communis) endosperm, even though it is synthesized on the membrane lipid phosphatidylcholine (PC) from an oleoyl ester. The acyl chains of PC undergo intense remodeling through the process of acyl editing. The identities of the proteins involved in this process, however, are unknown. A phospholipase A2 (PLA2) is thought to be involved in the acyl-editing process. We show here a role for RcsPLA2α in the acyl editing of HFA esterified to PC. RcsPLA2α was identified by its high relative expression in the castor endosperm transcriptome. Coexpression in Arabidopsis (Arabidopsis thaliana) seeds of RcsPLA2α with the castor fatty acid hydroxylase RcFAH12 led to a dramatic decrease in seed HFA content when compared with RcFAH12 expression alone in both PC and the neutral lipid fraction. The low-HFA trait was heritable and gene dosage dependent, with hemizygous lines showing intermediate HFA levels. The low seed HFA levels suggested that RcsPLA2α functions in vivo as a PLA2 with HFA specificity. Activity assays with yeast (Saccharomyces cerevisiae) microsomes showed a high specificity of RcsPLA2α for ricinoleic acid, superior to that of the endogenous Arabidopsis PLA2α. These results point to RcsPLA2α as a phospholipase involved in acyl editing, adapted to specifically removing HFA from membrane lipids in seeds.

Current progress towards the metabolic engineering of plant seed oil for hydroxy fatty acids production

Hydroxy fatty acids produced in plant seed oil are important industrial material. This review focuses on the use of metabolic engineering approaches for the production of hydroxy fatty acids in transgenic plants. Vegetable oil is not only edible but can also be used for industrial purposes. The industrial demand for vegetable oil will increase with the continued depletion of fossil fuels and ensuing environmental issues such as climate change, caused by increased carbon dioxide in the air. Some plants accumulate high levels of unusual fatty acids in their seeds, and these fatty acids (FAs) have properties that make them suitable for industrial applications. Hydroxy fatty acids (HFAs) are some of the most important of these industrial FAs. Castor oil is the conventional source of HFA. However, due to the presence of toxin ricin in its seeds, castor is not cultivated on a large scale. Lesquerella is another HFA accumulator and is currently being developed as a new crop for a safe source of HFAs. The mechanisms of HFA synthesis and accumulation have been extensively studied using castor genes and the model plant Arabidopsis. HFAs accumulated to 17% in the seed oil of Arabidopsis expressing a FA hydroxylase gene from castor (RcFAH12), but its seed oil content and plant growth decreased. When RcFAH12 gene was coexpressed with additional castor gene(s) in Arabidopsis, ~30% HFAs were accumulated and the seed oil content and plant growth was almost restored to the wild-type level. Further advancement of our understanding of pathways, genes and regulatory mechanisms underlying synthesis and accumulation of HFAs is essential to developing and implementing effective genetic approaches for enhancing HFA production in oilseeds.

Lipid turnover during senescence

Rapid turnover of stored triacylglycerol occurs after seed germination, releasing fatty acids that provide carbon and energy for seedling establishment. Glycerolipid and fatty acid turnover that occurs at other times in the plant life cycle, including senescence is less studied. Although the entire pathway of β-oxidation is induced during senescence, Arabidopsis leaf fatty acids turnover at rates 50 fold lower than in seedlings. Major unknowns in lipid turnover include the identity of lipases responsible for degradation of the wide diversity of galactolipid, phospholipid, and other lipid class structures. Also unknown is the relative flux of the acetyl-CoA product of β-oxidation into alternative metabolic pathways. We present an overview of senescence-related glycerolipid turnover and discuss its function(s) and speculate about how it might be controlled to increase the energy density and nutritional content of crops. To better understand regulation of lipid turnover, we developed a database that compiles and plots transcript expression of lipid-related genes during natural leaf senescence of Arabidopsis. The database allowed identification of coordinated patterns of down-regulation of lipid biosynthesis genes and the contrasting groups of genes that increase, including 68 putative lipases.

Phospholipid:diacylglycerol acyltransferase is a multifunctional enzyme involved in membrane lipid turnover and degradation while synthesizing triacylglycerol in the unicellular green microalga Chlamydomonas reinhardtii

Many unicellular microalgae produce large amounts (∼20 to 50% of cell dry weight) of triacylglycerols (TAGs) under stress (e.g., nutrient starvation and high light), but the synthesis and physiological role of TAG are poorly understood. We present detailed genetic, biochemical, functional, and physiological analyses of phospholipid:diacylglycerol acyltransferase (PDAT) in the green microalga Chlamydomonas reinhardtii, which catalyzes TAG synthesis via two pathways: transacylation of diacylglycerol (DAG) with acyl groups from phospholipids and galactolipids and DAG:DAG transacylation. We demonstrate that PDAT also possesses acyl hydrolase activities using TAG, phospholipids, galactolipids, and cholesteryl esters as substrates. Artificial microRNA silencing of PDAT in C. reinhardtii alters the membrane lipid composition, reducing the maximum specific growth rate. The data suggest that PDAT-mediated membrane lipid turnover and TAG synthesis is essential for vigorous growth under favorable culture conditions and for membrane lipid degradation with concomitant production of TAG for survival under stress. The strong lipase activity of PDAT with broad substrate specificity suggests that this enzyme could be a potential biocatalyst for industrial lipid hydrolysis and conversion, particularly for biofuel production.