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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.


Transcript levels for genes associated with glycolysis. a The expression levels for cytosolic glycolysis genes. b The expression levels for plastidial glycolysis genes. c The relative distribution of glycolysis genes in plastid and cytosol. The data, reads per kilobase per million mapped reads (RPKM), are average transcript levels of five developing stages of mesocarp with error bars representing their standard deviation. 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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Fig7: Transcript levels for genes associated with glycolysis. a The expression levels for cytosolic glycolysis genes. b The expression levels for plastidial glycolysis genes. c The relative distribution of glycolysis genes in plastid and cytosol. The data, reads per kilobase per million mapped reads (RPKM), are average transcript levels of five developing stages of mesocarp with error bars representing their standard deviation. 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: Several studies showed that an increase in oil content in mesocarp during fruit growth is accompanied by a decrease in the concentration of reducing sugars [73–75]. Typically, photosynthates are transported into the mesocarp in the form of sucrose via the sugar transporters between the apoplast, cytosol and vacuole and are hydrolyzed to hexoses that are utilized as the carbon source for glycolysis. The transcripts that likely encode for sugar transporters in the plasma membrane and vacuolar membranes were however, poorly expressed in avocado mesocarp with an average of 26 RPKM across all the developmental stages (Additional file 1: Table S3). The avocado fruit, especially in its early stages of growth, in addition to fixation of CO2via ribulose 1,5 bisphosphate carboxylase (RBC), is also capable of carboxylation of phosphoenolpyruvate (PEP) with bicarbonate that is available in the intercellular space of the fruit by PEP carboxylase (PEPC) to produce oxaloacetate and subsequently to malate [76–79]. Akin to mechanisms found in the leaves of C4 and CAM plants, malic enzyme can recycle CO2 produced in the non-green mesocarp layers for subsequent fixation by RBC in the more green tissues and concurrently release pyruvate for fatty acid synthesis. Consistent with this notion, the transcript levels for the orthologs of RBC, phosphoribulokinase and PEPC were high and on average 183, 195 and 216 RPKM, respectively (Additional file 1: Table S3). The abundance of these transcripts, particularly for the ortholog of PEPC in the mesocarp tissue during its development is consistent with its suggested role in carbon assimilation (Fig. 7a).Fig. 7


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)

Transcript levels for genes associated with glycolysis. a The expression levels for cytosolic glycolysis genes. b The expression levels for plastidial glycolysis genes. c The relative distribution of glycolysis genes in plastid and cytosol. The data, reads per kilobase per million mapped reads (RPKM), are average transcript levels of five developing stages of mesocarp with error bars representing their standard deviation. 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

Fig7: Transcript levels for genes associated with glycolysis. a The expression levels for cytosolic glycolysis genes. b The expression levels for plastidial glycolysis genes. c The relative distribution of glycolysis genes in plastid and cytosol. The data, reads per kilobase per million mapped reads (RPKM), are average transcript levels of five developing stages of mesocarp with error bars representing their standard deviation. 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: Several studies showed that an increase in oil content in mesocarp during fruit growth is accompanied by a decrease in the concentration of reducing sugars [73–75]. Typically, photosynthates are transported into the mesocarp in the form of sucrose via the sugar transporters between the apoplast, cytosol and vacuole and are hydrolyzed to hexoses that are utilized as the carbon source for glycolysis. The transcripts that likely encode for sugar transporters in the plasma membrane and vacuolar membranes were however, poorly expressed in avocado mesocarp with an average of 26 RPKM across all the developmental stages (Additional file 1: Table S3). The avocado fruit, especially in its early stages of growth, in addition to fixation of CO2via ribulose 1,5 bisphosphate carboxylase (RBC), is also capable of carboxylation of phosphoenolpyruvate (PEP) with bicarbonate that is available in the intercellular space of the fruit by PEP carboxylase (PEPC) to produce oxaloacetate and subsequently to malate [76–79]. Akin to mechanisms found in the leaves of C4 and CAM plants, malic enzyme can recycle CO2 produced in the non-green mesocarp layers for subsequent fixation by RBC in the more green tissues and concurrently release pyruvate for fatty acid synthesis. Consistent with this notion, the transcript levels for the orthologs of RBC, phosphoribulokinase and PEPC were high and on average 183, 195 and 216 RPKM, respectively (Additional file 1: Table S3). The abundance of these transcripts, particularly for the ortholog of PEPC in the mesocarp tissue during its development is consistent with its suggested role in carbon assimilation (Fig. 7a).Fig. 7

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.