Transcriptome Analysis of Genes Involved in Lipid Biosynthesis in the Developing Embryo of Pecan (Carya illinoinensis)

Pecan (Carya illinoinensis) is an
important woody tree species because of the high content of healthy
oil in its nut. Thus far, the pathways and key genes related to oil
biosynthesis in developing pecan seeds remain largely unclear. Our
analyses revealed that mature pecan embryo accumulated more than 80%
oil, in which 90% was unsaturated fatty acids with abundant oleic
acid. RNA sequencing generated 84,643 unigenes in three cDNA libraries
prepared from pecan embryos collected at 105, 120, and 165 days after
flowering (DAF). We identified 153 unigenes associated with lipid
biosynthesis, including 107 unigenes for fatty acid biosynthesis,
34 for triacylglycerol biosynthesis, 7 for oil bodies, and 5 for transcription
factors involved in oil synthesis. The genes associated with fatty
acid synthesis were the most abundantly expressed genes at 120 DAF.
Additionally, the biosynthesis of oil began to increase while crude
fat contents increased from 16.61 to 74.45% (165 DAF). We identified
four SAD, two FAD2, one FAD6, two FAD7, and two FAD8 unigenes responsible
for unsaturated fatty acid biosynthesis. However, FAD3 homologues
were not detected. Consequently, we inferred that the linolenic acid
in developing pecan embryos is generated by FAD7 and FAD8 in plastids
rather than FAD3 in endoplasmic reticula. During pecan embryo development,
different unigenes are expressed for plastidial and cytosolic glycolysis.
Plastidial glycolysis is more relevant to lipid synthesis than cytosolic
glycolysis. The 18 most important genes associated with lipid biosynthesis
were evaluated in five stages of developing embryos using quantitative
PCR (qPCR). The qPCR data were well consistent with their expression
in transcriptomic analyses. Our data would be important for the metabolic
engineering of pecans to increase oil contents and modify fatty acid
composition.