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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.


Relative gene expression levels for protein isoforms associated with fatty acid biosynthesis in oil-rich tissues of avocado (Pa), oil palm (Eg), rapeseed (Bn) and castor (Rc). Protein abbreviations are provided in Fig. 3a or Additional file 1: Table S3
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Fig4: Relative gene expression levels for protein isoforms associated with fatty acid biosynthesis in oil-rich tissues of avocado (Pa), oil palm (Eg), rapeseed (Bn) and castor (Rc). Protein abbreviations are provided in Fig. 3a or Additional file 1: Table S3

Mentions: The conversion of pyruvate to fatty acids in the plastid involves at least fourteen enzymes and/or protein complexes (Fig. 3a). Several of these proteins are encoded by more than one gene in Arabidopsis (Additional file 1: Table S3; [40, 41]. Comparison of the transcript levels of the orthologs of the gene family members in oil-rich tissues of avocado, oil palm, rapeseed and castor, while indicating some similarities across diverse species and tissues, also revealed several exceptions for avocado (Fig. 4). For example, among the three enzyme components of the pyruvate dehydrogenase complex (PDHC), while the E1α subunit of a heterodimeric protein (E1α2β2) is encoded by a single gene, the E1β subunit of E1 component, and E2 (dihydrolipoamide acetyltransferase, LTA), and E3 (dihydrolipoamide dehydrogenase, LPD) components are apparently encoded by two genes [42–44]. While the transcripts for orthologs of all the genes that encode for Arabidopsis PDHC components were detectable in oil-rich tissues of B. napus, oil palm and castor, only one ortholog for each of these components was detected in avocado (Fig. 4). The expression of a single ortholog in avocado was also noted for other enzymes that are typically encoded by more than one gene in angiosperms (Fig. 4). In the case of biotin carboxyl carrier protein (BCCP) of heteromeric acetyl-CoA carboxylase (ACCase), only the ortholog for BCCP1 (AT5G16390) was represented in the avocado mesocarp transcriptome. Both BCCP1 and 2 orthologs were detectable in oil palm, rapeseed and castor but BCCP1 was the predominant isoform in oil palm mesocarp, while BCCP2 was predominant in castor and rapeseed (Fig. 4). Similarly a single ortholog was represented in avocado transcriptome for hydroxyacyl-[acyl-carrier-protein (ACP)] dehydratase (HAD), and acyl-ACP thioesterase A (FATA), while both orthologs were at least detectable in the transcriptome of oil palm mesocarp and B. napus and castor seeds (Fig. 4). Furthermore, the ortholog that was expressed in avocado for PDHC-E1β, HAD, and FATA was different from the one that was predominantly expressed in oil-rich tissues of monocots and dicots (Fig. 4; [14, 16]. The absence of the second ortholog for PDHC-E1β, LPD, BCCP, and HAD genes in a basal angiosperm species was also observed at the genome level [17]. These data suggest that perhaps different/additional orthologs may have evolved to participate in fatty acid synthesis in seed and nonseed tissues of monocots or dicots, compared to a basal angiosperm.Fig. 3


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)

Relative gene expression levels for protein isoforms associated with fatty acid biosynthesis in oil-rich tissues of avocado (Pa), oil palm (Eg), rapeseed (Bn) and castor (Rc). Protein abbreviations are provided in Fig. 3a or Additional file 1: Table S3
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig4: Relative gene expression levels for protein isoforms associated with fatty acid biosynthesis in oil-rich tissues of avocado (Pa), oil palm (Eg), rapeseed (Bn) and castor (Rc). Protein abbreviations are provided in Fig. 3a or Additional file 1: Table S3
Mentions: The conversion of pyruvate to fatty acids in the plastid involves at least fourteen enzymes and/or protein complexes (Fig. 3a). Several of these proteins are encoded by more than one gene in Arabidopsis (Additional file 1: Table S3; [40, 41]. Comparison of the transcript levels of the orthologs of the gene family members in oil-rich tissues of avocado, oil palm, rapeseed and castor, while indicating some similarities across diverse species and tissues, also revealed several exceptions for avocado (Fig. 4). For example, among the three enzyme components of the pyruvate dehydrogenase complex (PDHC), while the E1α subunit of a heterodimeric protein (E1α2β2) is encoded by a single gene, the E1β subunit of E1 component, and E2 (dihydrolipoamide acetyltransferase, LTA), and E3 (dihydrolipoamide dehydrogenase, LPD) components are apparently encoded by two genes [42–44]. While the transcripts for orthologs of all the genes that encode for Arabidopsis PDHC components were detectable in oil-rich tissues of B. napus, oil palm and castor, only one ortholog for each of these components was detected in avocado (Fig. 4). The expression of a single ortholog in avocado was also noted for other enzymes that are typically encoded by more than one gene in angiosperms (Fig. 4). In the case of biotin carboxyl carrier protein (BCCP) of heteromeric acetyl-CoA carboxylase (ACCase), only the ortholog for BCCP1 (AT5G16390) was represented in the avocado mesocarp transcriptome. Both BCCP1 and 2 orthologs were detectable in oil palm, rapeseed and castor but BCCP1 was the predominant isoform in oil palm mesocarp, while BCCP2 was predominant in castor and rapeseed (Fig. 4). Similarly a single ortholog was represented in avocado transcriptome for hydroxyacyl-[acyl-carrier-protein (ACP)] dehydratase (HAD), and acyl-ACP thioesterase A (FATA), while both orthologs were at least detectable in the transcriptome of oil palm mesocarp and B. napus and castor seeds (Fig. 4). Furthermore, the ortholog that was expressed in avocado for PDHC-E1β, HAD, and FATA was different from the one that was predominantly expressed in oil-rich tissues of monocots and dicots (Fig. 4; [14, 16]. The absence of the second ortholog for PDHC-E1β, LPD, BCCP, and HAD genes in a basal angiosperm species was also observed at the genome level [17]. These data suggest that perhaps different/additional orthologs may have evolved to participate in fatty acid synthesis in seed and nonseed tissues of monocots or dicots, compared to a basal angiosperm.Fig. 3

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.