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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 and phylogenetic analysis of Wrinkled (WRI) isoforms (a) Transcript levels for PaWRI-like isoforms in developing mesocarp of avocado. b Phylogenetic analysis of AtWRI orthologs in Oryza sativa, Physcomitrella patens and Persea americana. An AP2 transcription factor from Chlamydomonas reinhardi was used as outgroup. Bootstrap values for 1,000 replicates are indicated and arrows point to possible duplication events
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Fig6: Expression and phylogenetic analysis of Wrinkled (WRI) isoforms (a) Transcript levels for PaWRI-like isoforms in developing mesocarp of avocado. b Phylogenetic analysis of AtWRI orthologs in Oryza sativa, Physcomitrella patens and Persea americana. An AP2 transcription factor from Chlamydomonas reinhardi was used as outgroup. Bootstrap values for 1,000 replicates are indicated and arrows point to possible duplication events

Mentions: Recent studies in Arabidopsis showed that WRI3 and WRI4 can each compensate for the low fatty acid levels of the wri1-4 mutant; they are non-redundant in function and are required in floral tissues for cutin biosynthesis [31]. Interestingly, in avocado mesocarp, the overall expression pattern of WRI orthologs was similar to that of genes that WRI1 is known to regulate such as ACP, BCCP, KASII, and PDHC (Fig. 3a; [30] and to the pattern of oil accumulation (Figs. 1b and 6a). Although complementation and transcriptional activator studies ruled out the role of WRI2 in Arabidopsis fatty acid biosynthesis [31], the high expression levels of its ortholog in mesocarp tissue of a basal angiosperm, during oil accumulation, suggest that it may have a role in nonseed tissue. A phylogenetic tree generated from WRI homologs, from various plant families including dicots, monocots, and a basal angiosperm, revealed a possible gene duplication event of WRI early in land plant evolution as the WRI2 proteins formed a monophyletic group and separated from all other WRI homologs. Other WRI homologs formed two distinct groups with a clade of WRI genes all belonging to P. patens, a bryophyte, separated from the WRI1, WRI3 and WRI4 genes of higher plants (Fig. 6b). The tree constructed for the WRI genes of various species suggests that the PaWRI2-like and AtWRI2 are older than the other WRI genes and have also diverged a great deal from each other. The high expression levels for WRI2-like in avocado mesocarp that were not previously reported in any other oil-rich tissues, along with WRI1 and WRI3 but not WRI4, suggest that perhaps through divergence, the AtWRI2 may have departed its function in oil biosynthesis while the PaWRI2-like retained its function. Although AtWRI2 did not complement wri1 mutant, complementation studies with PaWRI2-like are underway. Based on the gene expression data, it is predicted that WRI2 homolog of avocado may play an additional role in TAG accumulation in this basal angiosperm species.Fig. 6


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 and phylogenetic analysis of Wrinkled (WRI) isoforms (a) Transcript levels for PaWRI-like isoforms in developing mesocarp of avocado. b Phylogenetic analysis of AtWRI orthologs in Oryza sativa, Physcomitrella patens and Persea americana. An AP2 transcription factor from Chlamydomonas reinhardi was used as outgroup. Bootstrap values for 1,000 replicates are indicated and arrows point to possible duplication events
© Copyright Policy - OpenAccess
Related In: Results  -  Collection

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

Fig6: Expression and phylogenetic analysis of Wrinkled (WRI) isoforms (a) Transcript levels for PaWRI-like isoforms in developing mesocarp of avocado. b Phylogenetic analysis of AtWRI orthologs in Oryza sativa, Physcomitrella patens and Persea americana. An AP2 transcription factor from Chlamydomonas reinhardi was used as outgroup. Bootstrap values for 1,000 replicates are indicated and arrows point to possible duplication events
Mentions: Recent studies in Arabidopsis showed that WRI3 and WRI4 can each compensate for the low fatty acid levels of the wri1-4 mutant; they are non-redundant in function and are required in floral tissues for cutin biosynthesis [31]. Interestingly, in avocado mesocarp, the overall expression pattern of WRI orthologs was similar to that of genes that WRI1 is known to regulate such as ACP, BCCP, KASII, and PDHC (Fig. 3a; [30] and to the pattern of oil accumulation (Figs. 1b and 6a). Although complementation and transcriptional activator studies ruled out the role of WRI2 in Arabidopsis fatty acid biosynthesis [31], the high expression levels of its ortholog in mesocarp tissue of a basal angiosperm, during oil accumulation, suggest that it may have a role in nonseed tissue. A phylogenetic tree generated from WRI homologs, from various plant families including dicots, monocots, and a basal angiosperm, revealed a possible gene duplication event of WRI early in land plant evolution as the WRI2 proteins formed a monophyletic group and separated from all other WRI homologs. Other WRI homologs formed two distinct groups with a clade of WRI genes all belonging to P. patens, a bryophyte, separated from the WRI1, WRI3 and WRI4 genes of higher plants (Fig. 6b). The tree constructed for the WRI genes of various species suggests that the PaWRI2-like and AtWRI2 are older than the other WRI genes and have also diverged a great deal from each other. The high expression levels for WRI2-like in avocado mesocarp that were not previously reported in any other oil-rich tissues, along with WRI1 and WRI3 but not WRI4, suggest that perhaps through divergence, the AtWRI2 may have departed its function in oil biosynthesis while the PaWRI2-like retained its function. Although AtWRI2 did not complement wri1 mutant, complementation studies with PaWRI2-like are underway. Based on the gene expression data, it is predicted that WRI2 homolog of avocado may play an additional role in TAG accumulation in this basal angiosperm species.Fig. 6

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.