Oil biosynthesis in a basal angiosperm: transcriptome analysis of Persea Americana mesocarp.
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.
Department of Biological Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. kilaru@etsu.edu.|Department of Biomedical Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. kilaru@etsu.edu.|Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI, 48824, USA. kilaru@etsu.edu.
Aruna
A
Kilaru
Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI, 48824, USA. xia.cao@gmail.com.|Bayer CropSciences, Morrisville, NC, 27560, USA. xia.cao@gmail.com.
Xia
X
Cao
Department of Biological Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. dabbsp@goldmail.etsu.edu.
Parker B
PB
Dabbs
Department of Biological Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. hajung.sung@gmail.com.
Ha-Jung
HJ
Sung
Department of Biological Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. rahmanm@goldmail.etsu.edu.|Department of Biomedical Sciences, East Tennessee State University, Johnson City, TN, 37614, USA. rahmanm@goldmail.etsu.edu.
Md Mahbubur
MM
Rahman
Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI, 48824, USA. throwern@msu.edu.
Nicholas
N
Thrower
School of Informatics and Computing, Indiana University, Bloomington, IN, 47408, USA. gjzynda@umail.iu.edu.
Greg
G
Zynda
School of Informatics and Computing, Indiana University, Bloomington, IN, 47408, USA. mnrusimh@indiana.edu.
Ram
R
Podicheti
Laboratorio Nacional de Genómica para la Biodiversidad-Langebio/Unidad de Genómica Avanzada UGA, Centro de Investigación y Estudios Avanzados del IPN, 36500, Irapuato, Guanajuato, Mexico. ibarralaclette206@gmail.com.|Red de Estudios Moleculares Avanzados, Instituto de Ecología A.C., 91070, Xalapa, Veracruz, Mexico. ibarralaclette206@gmail.com.
Enrique
E
Ibarra-Laclette
Laboratorio Nacional de Genómica para la Biodiversidad-Langebio/Unidad de Genómica Avanzada UGA, Centro de Investigación y Estudios Avanzados del IPN, 36500, Irapuato, Guanajuato, Mexico. lherrera@langebio.cinvestav.mx.
Luis
L
Herrera-Estrella
Department of Biology, Indiana University, Bloomington, IN, 47405, USA. kmockait@indiana.edu.
Keithanne
K
Mockaitis
Great Lakes Bioenergy Research Center, Michigan State University, East Lansing, MI, 48824, USA. ohlrogge@cns.msu.edu.|Department of Plant Biology, Michigan State University, East Lansing, MI, 48824, USA. ohlrogge@cns.msu.edu.
John B
JB
Ohlrogge
D018506:Gene Expression Regulation, Plant; D008055:Lipids / Q000096:biosynthesis*; D008969:Molecular Sequence Data; D027421:Persea / Q000235:genetics* / Q000378:metabolism; D010940:Plant Proteins / Q000235:genetics* / Q000378:metabolism; D018749:RNA, Plant / Q000235:genetics* / Q000378:metabolism; D012639:Seeds / Q000378:metabolism; D017422:Sequence Analysis, DNA; D059467:Transcriptome
D008055:Lipids; D010940:Plant Proteins; D018749:RNA, Plant