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Oil biosynthesis in a basal angiosperm: transcriptome analysis of Persea Americana mesocarp.

Kilaru A, Cao X, Dabbs PB, Sung HJ, Rahman MM, Thrower N, Zynda G, Podicheti R, Ibarra-Laclette E, Herrera-Estrella L, Mockaitis K, Ohlrogge JB - BMC Plant Biol. (2015)

Bottom Line: The accumulation of TAG, rich in oleic acid, was associated with higher transcript levels for a putative stearoyl-ACP desaturase and endoplasmic reticulum (ER)-associated acyl-CoA synthetases, during fruit development.The orthologs that are distinctively expressed in oil-rich mesocarp tissues of this basal angiosperm, such as WRI2, ER-associated acyl-CoA synthetases, and lipid-droplet associated proteins were also identified.This study provides a foundation for future investigations to increase oil-content and has implications for metabolic engineering to enhance storage oil content in nonseed tissues of diverse species.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. kilaru@etsu.edu.

ABSTRACT

Background: The mechanism by which plants synthesize and store high amounts of triacylglycerols (TAG) in tissues other than seeds is not well understood. The comprehension of controls for carbon partitioning and oil accumulation in nonseed tissues is essential to generate oil-rich biomass in perennial bioenergy crops. Persea americana (avocado), a basal angiosperm with unique features that are ancestral to most flowering plants, stores ~ 70 % TAG per dry weight in its mesocarp, a nonseed tissue. Transcriptome analyses of select pathways, from generation of pyruvate and leading up to TAG accumulation, in mesocarp tissues of avocado was conducted and compared with that of oil-rich monocot (oil palm) and dicot (rapeseed and castor) tissues to identify tissue- and species-specific regulation and biosynthesis of TAG in plants.

Results: RNA-Seq analyses of select lipid metabolic pathways of avocado mesocarp revealed patterns similar to that of other oil-rich species. However, only some predominant orthologs of the fatty acid biosynthetic pathway genes in this basal angiosperm were similar to those of monocots and dicots. The accumulation of TAG, rich in oleic acid, was associated with higher transcript levels for a putative stearoyl-ACP desaturase and endoplasmic reticulum (ER)-associated acyl-CoA synthetases, during fruit development. Gene expression levels for enzymes involved in terminal steps to TAG biosynthesis in the ER further indicated that both acyl-CoA-dependent and -independent mechanisms might play a role in TAG assembly, depending on the developmental stage of the fruit. Furthermore, in addition to the expression of an ortholog of WRINKLED1 (WRI1), a regulator of fatty acid biosynthesis, high transcript levels for WRI2-like and WRI3-like suggest a role for additional transcription factors in nonseed oil accumulation. Plastid pyruvate necessary for fatty acid synthesis is likely driven by the upregulation of genes involved in glycolysis and transport of its intermediates. Together, a comparative transcriptome analyses for storage oil biosynthesis in diverse plants and tissues suggested that several distinct and conserved features in this basal angiosperm species might contribute towards its rich TAG content.

Conclusions: Our work represents a comprehensive transcriptome resource for a basal angiosperm species and provides insight into their lipid metabolism in mesocarp tissues. Furthermore, comparison of the transcriptome of oil-rich mesocarp of avocado, with oil-rich seed and nonseed tissues of monocot and dicot species, revealed lipid gene orthologs that are highly conserved during evolution. The orthologs that are distinctively expressed in oil-rich mesocarp tissues of this basal angiosperm, such as WRI2, ER-associated acyl-CoA synthetases, and lipid-droplet associated proteins were also identified. This study provides a foundation for future investigations to increase oil-content and has implications for metabolic engineering to enhance storage oil content in nonseed tissues of diverse species.

No MeSH data available.


Expression levels for genes associated with triacylglycerol (TAG) assembly. a Schematic of TAG pathway. b The average transcript levels for each enzyme. c The relative distribution of transcript levels for each protein during mesocarp development. The data are expressed as reads per kilobase per million mapped reads (RPKM). The RPKM values for subunits of a protein and for multiple isoforms were summed. Protein abbreviations are provided in Additional file 1: Table S3
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Fig5: Expression levels for genes associated with triacylglycerol (TAG) assembly. a Schematic of TAG pathway. b The average transcript levels for each enzyme. c The relative distribution of transcript levels for each protein during mesocarp development. The data are expressed as reads per kilobase per million mapped reads (RPKM). The RPKM values for subunits of a protein and for multiple isoforms were summed. Protein abbreviations are provided in Additional file 1: Table S3

Mentions: In avocado mesocarp, the orthologs of TAG biosynthesis genes (Fig. 5a) were represented by an average of 75 RPKM/protein, which is seven-fold less than that of fatty acid synthesis gene expression per protein (Fig. 2b and 2c). Similar low expression levels for TAG synthesis genes, relative to fatty acid synthesis genes, were noted in other oil-rich seed and nonseed tissues [14, 16]. In avocado mesocarp, the orthologs of all the genes involved in sequential acylation of glycerol-3-phosphate (G3P), via the Kennedy pathway to generate TAG, i.e. the glycerol-3-phosphate acyltransferase (GPAT9), lysophosphatidic acid acyltransferase (LPAAT), phospatidate phosphatase (PP/PAH), and diacylglycerol acyltransferase (DGAT) were expressed during mesocarp development (Fig. 5; Additional file 1: Table S3). The expression levels for the ortholog of GPAT9 remained similar during mesocarp development, but declined by two-fold for LPAAT2, the predominant LPAAT isoform in the ER (Additional file 1: Table S3; Fig. 5). Among the various orthologs that encode for PP/PAH, transcripts for PAH1 and PAH2 were abundant and their levels remained moderately similar during the development of mesocarp (Additional file 1: Table S3).Fig. 5


Oil biosynthesis in a basal angiosperm: transcriptome analysis of Persea Americana mesocarp.

Kilaru A, Cao X, Dabbs PB, Sung HJ, Rahman MM, Thrower N, Zynda G, Podicheti R, Ibarra-Laclette E, Herrera-Estrella L, Mockaitis K, Ohlrogge JB - BMC Plant Biol. (2015)

Expression levels for genes associated with triacylglycerol (TAG) assembly. a Schematic of TAG pathway. b The average transcript levels for each enzyme. c The relative distribution of transcript levels for each protein during mesocarp development. The data are expressed as reads per kilobase per million mapped reads (RPKM). The RPKM values for subunits of a protein and for multiple isoforms were summed. Protein abbreviations are provided in Additional file 1: Table S3
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

License 1 - License 2
Show All Figures
getmorefigures.php?uid=PMC4537532&req=5

Fig5: Expression levels for genes associated with triacylglycerol (TAG) assembly. a Schematic of TAG pathway. b The average transcript levels for each enzyme. c The relative distribution of transcript levels for each protein during mesocarp development. The data are expressed as reads per kilobase per million mapped reads (RPKM). The RPKM values for subunits of a protein and for multiple isoforms were summed. Protein abbreviations are provided in Additional file 1: Table S3
Mentions: In avocado mesocarp, the orthologs of TAG biosynthesis genes (Fig. 5a) were represented by an average of 75 RPKM/protein, which is seven-fold less than that of fatty acid synthesis gene expression per protein (Fig. 2b and 2c). Similar low expression levels for TAG synthesis genes, relative to fatty acid synthesis genes, were noted in other oil-rich seed and nonseed tissues [14, 16]. In avocado mesocarp, the orthologs of all the genes involved in sequential acylation of glycerol-3-phosphate (G3P), via the Kennedy pathway to generate TAG, i.e. the glycerol-3-phosphate acyltransferase (GPAT9), lysophosphatidic acid acyltransferase (LPAAT), phospatidate phosphatase (PP/PAH), and diacylglycerol acyltransferase (DGAT) were expressed during mesocarp development (Fig. 5; Additional file 1: Table S3). The expression levels for the ortholog of GPAT9 remained similar during mesocarp development, but declined by two-fold for LPAAT2, the predominant LPAAT isoform in the ER (Additional file 1: Table S3; Fig. 5). Among the various orthologs that encode for PP/PAH, transcripts for PAH1 and PAH2 were abundant and their levels remained moderately similar during the development of mesocarp (Additional file 1: Table S3).Fig. 5

Bottom Line: The accumulation of TAG, rich in oleic acid, was associated with higher transcript levels for a putative stearoyl-ACP desaturase and endoplasmic reticulum (ER)-associated acyl-CoA synthetases, during fruit development.The orthologs that are distinctively expressed in oil-rich mesocarp tissues of this basal angiosperm, such as WRI2, ER-associated acyl-CoA synthetases, and lipid-droplet associated proteins were also identified.This study provides a foundation for future investigations to increase oil-content and has implications for metabolic engineering to enhance storage oil content in nonseed tissues of diverse species.

View Article: PubMed Central - PubMed

Affiliation: Department of Biological Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. kilaru@etsu.edu.

ABSTRACT

Background: The mechanism by which plants synthesize and store high amounts of triacylglycerols (TAG) in tissues other than seeds is not well understood. The comprehension of controls for carbon partitioning and oil accumulation in nonseed tissues is essential to generate oil-rich biomass in perennial bioenergy crops. Persea americana (avocado), a basal angiosperm with unique features that are ancestral to most flowering plants, stores ~ 70 % TAG per dry weight in its mesocarp, a nonseed tissue. Transcriptome analyses of select pathways, from generation of pyruvate and leading up to TAG accumulation, in mesocarp tissues of avocado was conducted and compared with that of oil-rich monocot (oil palm) and dicot (rapeseed and castor) tissues to identify tissue- and species-specific regulation and biosynthesis of TAG in plants.

Results: RNA-Seq analyses of select lipid metabolic pathways of avocado mesocarp revealed patterns similar to that of other oil-rich species. However, only some predominant orthologs of the fatty acid biosynthetic pathway genes in this basal angiosperm were similar to those of monocots and dicots. The accumulation of TAG, rich in oleic acid, was associated with higher transcript levels for a putative stearoyl-ACP desaturase and endoplasmic reticulum (ER)-associated acyl-CoA synthetases, during fruit development. Gene expression levels for enzymes involved in terminal steps to TAG biosynthesis in the ER further indicated that both acyl-CoA-dependent and -independent mechanisms might play a role in TAG assembly, depending on the developmental stage of the fruit. Furthermore, in addition to the expression of an ortholog of WRINKLED1 (WRI1), a regulator of fatty acid biosynthesis, high transcript levels for WRI2-like and WRI3-like suggest a role for additional transcription factors in nonseed oil accumulation. Plastid pyruvate necessary for fatty acid synthesis is likely driven by the upregulation of genes involved in glycolysis and transport of its intermediates. Together, a comparative transcriptome analyses for storage oil biosynthesis in diverse plants and tissues suggested that several distinct and conserved features in this basal angiosperm species might contribute towards its rich TAG content.

Conclusions: Our work represents a comprehensive transcriptome resource for a basal angiosperm species and provides insight into their lipid metabolism in mesocarp tissues. Furthermore, comparison of the transcriptome of oil-rich mesocarp of avocado, with oil-rich seed and nonseed tissues of monocot and dicot species, revealed lipid gene orthologs that are highly conserved during evolution. The orthologs that are distinctively expressed in oil-rich mesocarp tissues of this basal angiosperm, such as WRI2, ER-associated acyl-CoA synthetases, and lipid-droplet associated proteins were also identified. This study provides a foundation for future investigations to increase oil-content and has implications for metabolic engineering to enhance storage oil content in nonseed tissues of diverse species.

No MeSH data available.