Characterization of an oleate 12-desaturase from Physaria fendleri and identification of 5'UTR introns in divergent FAD2 family genes

Variety of Plant Oils: Species-Specific Lipid Biosynthesis

Plant oils represent a large group of neutral lipids with important applications in food, feed and oleochemical industries. Most plants accumulate oils in the form of triacylglycerol within seeds and their surrounding tissues, which comprises three fatty acids attached to a glycerol backbone. Different plant species accumulate unique fatty acids in their oils, serving a range of applications in pharmaceuticals and oleochemicals. To enable the production of these distinctive oils, select plant species have adapted specialized oil metabolism pathways, involving differential gene co-expression networks and structurally divergent enzymes/proteins. Here, we summarize some of the recent advances in our understanding of oil biosynthesis in plants. We compare expression patterns of oil metabolism genes from representative species, including Arabidopsis thaliana, Ricinus communis (castor bean), Linum usitatissimum L. (flax) and Elaeis guineensis (oil palm) to showcase the co-expression networks of relevant genes for acyl metabolism. We also review several divergent enzymes/proteins associated with key catalytic steps of unique oil accumulation, including fatty acid desaturases, diacylglycerol acyltransferases and oleosins, highlighting their structural features and preference toward unique lipid substrates. Lastly, we briefly discuss protein interactomes and substrate channeling for oil biosynthesis and the complex regulation of these processes.

Identification and Functional Characterization of the RcFAH12 Promoter from Castor Bean in Arabidopsis thaliana

Castor (Ricinus communis L.) seed oil is the commercial source of ricinoleate, a valuable raw material used in many industries. Oleoyl-12-hydroxylase (RcFAH12) is a key enzyme in the biosynthesis of ricinoleate, accumulating nearly 90% of the triacylglycerol in castor seeds. Little is known about the transcriptional regulation of RcFAH12. We used rapid amplification of cDNA 5′ ends (5′RACE) to locate the transcription start site (TSS) of RcFAH12, and the sequence of a 2605 bp region, −2506~+99, surrounding the TSS was cloned. We then investigated these regions to promote β-glucuronidase (GUS) expression in transgenic Arabidopsis by the progressive 5′ and 3′ deletions strategies. The GUS staining showed that the GUS accumulation varied in tissues under the control of different deleted fragments of RcFAH12. In addition, the GUS expression driven by the RcFAH12 promoter markedly accumulated in transgenic seeds, which indicated that RcFAH12 might play an important role in the biosynthesis of ricinoleic acid. This study will lay a potential foundation for developing a tissue-specific promoter in oil-seed crops.

Triacylglycerol biosynthesis in shaded seeds of tung tree (Vernicia fordii) is regulated in part by Homeodomain Leucine Zipper 21

Light quantity and quality affect many aspects of plant growth and development. However, few reports have addressed the molecular connections between seed oil accumulation and light conditions, especially dense shade. Shade-avoiding plants can redirect plant resources into extension growth at the expense of leaf and root expansion in an attempt to reach areas containing richer light. Here, we report that tung tree seed oil accumulation is suppressed by dense shade during the rapid oil accumulation phase. Transcriptome analysis confirmed that oil accumulation suppression due to dense shade was attributed to reduced expression of fatty acid and triacylglycerol biosynthesis-related genes. Through weighted gene co-expression network analysis, we identified 32 core transcription factors (TFs) specifically upregulated in densely shaded seeds during the rapid oil accumulation period. Among these, VfHB21, a class I homeodomain leucine zipper TF, was shown to suppress expression of FAD2 and FADX, two key genes related to α-eleostearic acid, by directly binding to HD-ZIP I/II motifs in their respective promoter regions. VfHB21 also binds to similar motifs in the promoters of VfWRI1 and VfDGAT2, two additional key seed lipid regulatory/biosynthetic genes. Functional conservation of HB21 during plant evolution was demonstrated by the fact that AtWRI1, AtSAD1, and AtFAD2 were downregulated in VfHB21-overexpressor lines of transgenic Arabidopsis, with concomitant seed oil reduction, and the fact that AtHB21 expression also was induced by shade. This study reveals some of the regulatory mechanisms that specifically control tung tree seed oil biosynthesis and more broadly regulate plant storage carbon partitioning in response to dense shade conditions.

Association Study of the 5'UTR Intron of the FAD2-2 Gene With Oleic and Linoleic Acid Content in Olea europaea L

Cultivated olive (Olea europaea L. subsp. europaea var. europaea) is the most ancient and spread tree crop in the Mediterranean basin. An important quality trait for the extra virgin olive oil is the fatty acid composition. In particular, a high content of oleic acid and low of linoleic, linolenic, and palmitic acid is considered very relevant in the health properties of the olive oil. The oleate desaturase enzyme encoding-gene (FAD2-2) is the main responsible for the linoleic acid content in the olive fruit mesocarp and, therefore, in the olive oil revealing to be the most important candidate gene for the linoleic acid biosynthesis. In this study, an in silico and structural analysis of the 5'UTR intron of the FAD2-2 gene was conducted with the aim to explore the natural sequence variability and its role in the gene expression regulation. In order to identify functional allele variants, the 5'UTR intron was isolated and partially sequenced in 97 olive cultivars. The sequence analysis allowed to find a 117-bp insertion including two long duplications never found before in FAD2-2 genes in olive and the existence of many intron-mediated enhancement (IME) elements. The sequence polymorphism analysis led to detect 39 SNPs. The candidate gene association study conducted for oleic and linoleic acids content revealed seven SNPs and one indel significantly associated able to explain a phenotypic variation ranging from 7% to 16% among the years. Our study highlighted new structural variants within the FAD2-2 gene in olive, putatively involved in the regulation mechanisms of gene expression associated with the variation of the content of oleic and linoleic acid.

Characterization of a new GmFAD3A allele in Brazilian CS303TNKCA soybean cultivar

KEY MESSAGE

We molecularly characterized a new mutation in the GmFAD3A gene associated with low linolenic content in the Brazilian soybean cultivar CS303TNKCA and developed a molecular marker to select this mutation. Soybean is one of the most important crops cultivated worldwide. Soybean oil has 13% palmitic acid, 4% stearic acid, 20% oleic acid, 55% linoleic acid and 8% linolenic acid. Breeding programs are developing varieties with high oleic and low polyunsaturated fatty acids (linoleic and linolenic) to improve the oil oxidative stability and make the varieties more attractive for the soy industry. The main goal of this study was to characterize the low linoleic acid trait in CS303TNKCA cultivar. We sequenced CS303TNKCA GmFAD3A, GmFAD3B and GmFAD3C genes and identified an adenine point deletion in the GmFAD3A exon 5 (delA). This alteration creates a premature stop codon, leading to a truncated protein with just 207 residues that result in a non-functional enzyme. Analysis of enzymatic activity by heterologous expression in yeast support delA as the cause of low linolenic acid content in CS303TNKCA. Thus, we developed a TaqMan genotyping assay to associate delA with low linolenic acid content in segregating populations. Lines homozygous for delA had a linolenic acid content of 3.3 to 4.4%, and the variation at this locus accounted for 50.83 to 73.70% of the phenotypic variation. This molecular marker is a new tool to introgress the low linolenic acid trait into elite soybean cultivars and can be used to combine with high oleic trait markers to produce soybean with enhanced economic value. The advantage of using CS303TNKCA compared to other lines available in the literature is that this cultivar has good agronomic characteristics and is adapted to Brazilian conditions.

Expression of Castor LPAT2 Enhances Ricinoleic Acid Content at the sn-2 Position of Triacylglycerols in Lesquerella Seed

Lesquerella is a potential industrial oilseed crop that makes hydroxy fatty acid (HFA). Unlike castor its seeds are not poisonous but accumulate lesquerolic acid mostly at the sn-1 and sn-3 positions of triacylglycerol (TAG), whereas castor contains ricinoleic acid (18:1OH) at all three positions. To investigate whether lesquerella can be engineered to accumulate HFAs in the sn-2 position, multiple transgenic lines were made that express castor lysophosphatidic acid acyltransferase 2 (RcLPAT2) in the seed. RcLPAT2 increased 18:1OH at the sn-2 position of TAGs from 2% to 14%–17%, which resulted in an increase of tri-HFA-TAGs from 5% to 13%–14%. Our result is the first example of using a LPAT to increase ricinoleic acid at the sn-2 position of seed TAG. This work provides insights to the mechanism of HFA-containing TAG assembly in lesquerella and directs future research to optimize this plant for HFA production.

Identification of hydroxy fatty acid and triacylglycerol metabolism-related genes in lesquerella through seed transcriptome analysis

Background

Castor oil is the only commercial source of hydroxy fatty acid that has industrial value. The production of castor oil is hampered by the presence of the toxin ricin in its seed. Lesquerella seed also accumulates hydroxy fatty acid and is free of ricin, and thus it is being developed as a new crop for hydroxy fatty acid production. A high-throughput, large-scale sequencing of transcripts from developing lesquerella seeds was carried out by 454 pyrosequencing to generate a database for quality improvement of seed oil and other agronomic traits. Deep mining and characterization of acyl-lipid genes were conducted to uncover candidate genes for further studies of mechanisms underlying hydroxy fatty acid and seed oil synthesis.

Results

A total of 651 megabases of raw sequences from an mRNA sample of developing seeds was acquired. Bioinformatic analysis of these sequences revealed 59,914 transcripts representing 26,995 unique genes that include nearly all known seed expressed genes. Based on sequence similarity with known plant proteins, about 74% (19,861) genes matched with annotated coding genes. Among them, 95% (18,868) showed highest sequence homology with Arabidopsis genes, which will allow translation of genomics and genetics findings from Arabidopsis to lesquerella. Using Arabidopsis acyl-lipid genes as queries, we searched the transcriptome assembly and identified 615 lesquerella genes involved in all known pathways of acyl-lipid metabolism. Further deep mining the transcriptome assembly led to identification of almost all lesquerella genes involved in fatty acid and triacylglycerol synthesis. Moreover, we characterized the spatial and temporal expression profiles of 15 key genes using the quantitative PCR assay.

Conclusions

We have built a lesquerella seed transcriptome that provides a valuable reference in addition to the castor database for discovering genes involved in the synthesis of triacylglycerols enriched with hydroxy fatty acids. The information obtained from data mining and gene expression profiling will provide a resource not only for the study of hydroxy fatty acid metabolism, but also for the biotechnological production of hydroxy fatty acids in existing oilseed crops.

Electronic supplementary material

The online version of this article (doi:10.1186/s12864-015-1413-8) contains supplementary material, which is available to authorized users.