UNIT 2.4.3

Lipid Synthesis in Oilseeds

From acetyl-CoA to triglycerides

🎯 After this unit, you will be able to:

Describe the pathway of fatty acid synthesis in plastids
Explain how fatty acids are modified (desaturation, elongation)
Understand triglyceride (triacylglycerol) assembly in the ER
Identify factors affecting oil content and quality in oilseed crops

🫒 Why Oilseeds Matter

Oilseed crops—such as sunflower, rapeseed (canola), soybean, peanut, and Ethiopia's native niger seed (Guizotia abyssinica)—store energy as oils (triacylglycerols) in their seeds. Oils are the most energy-dense form of carbon storage, providing more than twice the energy per gram as carbohydrates (9 kcal/g vs. 4 kcal/g) .

Key insight: Oil synthesis is a complex, multi-compartment process. Fatty acids are made in plastids, then exported to the endoplasmic reticulum (ER) for assembly into triglycerides and packaging into oil bodies .

🌍 Did you know? Global vegetable oil production exceeds 200 million metric tons annually, with palm oil, soybean oil, and rapeseed oil being the major sources. Ethiopia's niger seed (nug) is a minor but important oilseed for local consumption .

⚙️ Fatty Acid Synthesis: The Basics

Fatty acids are built from 2-carbon units (acetyl-CoA) in a cyclic process. The key features :

Location: Plastids (in plants—different from animals, where it occurs in cytoplasm)
Primary product: Palmitic acid (16:0) and stearic acid (18:0)
Key enzyme: Fatty acid synthase (FAS)—actually a multi-enzyme complex
Energy requirement: Each 2-carbon addition requires 1 ATP and 2 NADPH

🔄 [Diagram: Fatty acid synthesis cycle showing addition of 2-carbon units — to be inserted]

🧪 Key Enzymes in Fatty Acid Synthesis

Enzyme	Function	Location	Regulation
Acetyl-CoA carboxylase (ACCase)	Converts acetyl-CoA → malonyl-CoA (committed step)	Plastid	Key regulatory enzyme; activated by light, inhibited by ADP .
Fatty acid synthase (FAS) complex	Cyclically adds 2-carbon units using malonyl-ACP	Plastid	Multiple enzymes; substrate availability regulates
Ketoacyl-ACP synthase (KAS)	Three isoforms (KAS I, II, III) catalyze condensation steps	Plastid	Chain-length specific
Acyl-ACP thioesterase	Releases free fatty acids from ACP for export from plastid	Plastid	Specificity determines chain length

🔬 Did you know? The herbicide group called "ACCase inhibitors" (e.g., sethoxydim, clethodim) work by blocking acetyl-CoA carboxylase in grasses. This is why they kill grassy weeds but not broadleaf crops—the ACCase in broadleaf plants has a different structure and isn't affected .

✂️ Fatty Acid Modification: Desaturation and Elongation

After synthesis, fatty acids can be modified to create the diversity of oils we see in different crops :

Desaturation (Adding Double Bonds)

Desaturases introduce double bonds into fatty acid chains. Key enzymes include :

Stearoyl-ACP desaturase (SAD): Converts stearic acid (18:0) to oleic acid (18:1Δ9) in plastids. This is the most common desaturation in plants .
ER desaturases (FAD2, FAD3): Introduce additional double bonds to create linoleic (18:2) and linolenic (18:3) acids after fatty acids are incorporated into lipids .

Stearic acid (18:0) → Oleic acid (18:1) → Linoleic acid (18:2) → α-Linolenic acid (18:3)

Elongation (Making Long Chains)

Very long-chain fatty acids (>18 carbons) are made by elongase complexes in the ER. These are important in some oilseeds (e.g., erucic acid in rapeseed) and for wax synthesis .

🔬 [Diagram: Fatty acid desaturation pathway showing FAD2 and FAD3 — to be inserted]

🧴 Triglyceride (Triacylglycerol) Assembly

Free fatty acids are toxic, so they are rapidly incorporated into triglycerides (triacylglycerols) in the endoplasmic reticulum (ER) via the Kennedy pathway :

Step 1: Acyl-CoA formation

Fatty acids are activated to acyl-CoAs by acyl-CoA synthetase (requires ATP) .

Step 2: Glycerol-3-phosphate acylation

GPAT adds fatty acid to position 1 of glycerol-3-phosphate → lysophosphatidic acid .

Step 3: Second acylation

LPAAT adds second fatty acid to position 2 → phosphatidic acid .

Step 4: Dephosphorylation

Phosphatase removes phosphate → diacylglycerol (DAG) .

Step 5: Final acylation

DGAT adds third fatty acid to DAG → triacylglycerol (TAG) .

DGAT (diacylglycerol acyltransferase) is the only enzyme specific to TAG synthesis and is a key target for increasing oil content .

🧴 [Diagram: Kennedy pathway for TAG synthesis — to be inserted]

💧 Oil Bodies: Storing the Oil

Triglycerides are hydrophobic and must be packaged into specialized organelles called oil bodies (also called lipid droplets or oleosomes) .

Structure: A core of TAG surrounded by a phospholipid monolayer coated with proteins (oleosins, caleosins, steroleosins) .
Oleosins: Small proteins that stabilize oil bodies, prevent coalescence, and provide binding sites for lipases during germination .
Size: Typically 0.5-2 μm in diameter .

💧 [Diagram: Oil body structure showing TAG core, phospholipid monolayer, and oleosins — to be inserted]

🥜 Did you know? Oil bodies are so stable that they can be extracted and used in food products. "Oleosomes" are now commercial ingredients for plant-based milks and other products .

📊 Oil Content of Major Oilseeds

Crop	Oil content (% dry weight)	Major fatty acids	Primary use
Oil palm (mesocarp)	45-55%	Palmitic (44%), oleic (39%)	Cooking oil, processed foods
Coconut (copra)	60-65%	Lauric (48%), myristic (19%)	Lauric oil for soaps, cosmetics
Rapeseed (canola)	40-45%	Oleic (61%), linoleic (21%)	Cooking oil, biodiesel
Sunflower	40-50%	Linoleic (65%), oleic (20%)	Cooking oil
Soybean	18-22%	Linoleic (54%), oleic (24%)	Major global oil source
Peanut	45-50%	Oleic (46%), linoleic (32%)	Peanut butter, oil
Niger seed (nug)	40-50%	Linoleic (75%), oleic (13%)	Cooking oil in Ethiopia

🌡️ Factors Affecting Oil Content and Quality

Temperature

Cooler temperatures during seed development increase unsaturation (more double bonds) because desaturases are more active. This is why sunflowers grown in cooler regions produce more polyunsaturated oils .

Genetics

Breeding has dramatically altered oil composition. Examples :

High-oleic sunflower/rapeseed: Mutations in FAD2 desaturase increase oleic acid to >80% (better for frying)
Low-linolenic soybean: Reduced FAD3 to improve oxidative stability
High-erucic rapeseed: Industrial oil (erucic acid not suitable for food)

Nitrogen and Sulfur

Nitrogen supply affects oil content (often negative correlation with protein). Sulfur is needed for synthesis of sulfur-containing amino acids in storage proteins, affecting oil-protein balance .

Water Stress

Moderate drought during seed filling can increase oil content in some crops by reducing carbohydrate dilution, but severe stress reduces both yield and oil .

🌻 High-Oleic Sunflower

Traditional sunflower oil is high in linoleic acid (polyunsaturated), which is good for cold uses but oxidizes quickly during frying. Breeders developed high-oleic sunflower with >80% oleic acid (monounsaturated) by selecting for mutations that reduce FAD2 activity. This oil is more stable for frying and has better shelf life .

🇪🇹 Ethiopian Focus: Niger Seed (Nug)

Niger seed (Guizotia abyssinica) is an important oilseed crop native to Ethiopia. It's grown mainly in the highlands and provides cooking oil for local consumption .

Oil Characteristics

Oil content: 40-50%
Fatty acid profile: Very high in linoleic acid (up to 75%), with oleic (13%) and palmitic (8%) .
Flavor: Distinctive nutty flavor valued in Ethiopian cuisine

Challenges and Opportunities

Oxidative stability: High linoleic acid makes the oil prone to rancidity. Breeding for higher oleic acid could improve shelf life .
Yield improvement: Average yields are low (0.5-0.8 t/ha) compared to potential (>1.5 t/ha). Improved varieties and agronomy needed .
Value addition: Cold-pressed nug oil commands premium prices in specialty markets .

🌱 Improving Niger Seed Oil Quality

Researchers are exploring niger seed germplasm to identify variants with higher oleic acid content. Introducing mutations in the FAD2 gene (similar to high-oleic sunflower) could create nug oil with improved oxidative stability while maintaining its characteristic flavor .

📌 Unit Summary

Process	Location	Key enzymes	Products
Fatty acid synthesis	Plastid	ACCase, FAS complex, KAS, thioesterase	16:0, 18:0, 18:1
Desaturation	Plastid (SAD), ER (FAD2, FAD3)	Stearoyl-ACP desaturase, FAD2, FAD3	18:1, 18:2, 18:3
TAG assembly	ER	GPAT, LPAAT, PAP, DGAT	Triacylglycerol (TAG)
Oil body formation	ER/cytoplasm	Oleosins, caleosins	Stable oil droplets

Reflection question: A processor in Ethiopia wants to produce high-quality nug oil with longer shelf life for export markets. Based on your understanding of lipid synthesis, what genetic and agronomic approaches could be used to modify the fatty acid composition of niger seed oil?

📌 Key terms introduced

ACCase Fatty acid synthase (FAS) ACP (acyl carrier protein) KAS (ketoacyl-ACP synthase) Thioesterase SAD (stearoyl-ACP desaturase) FAD2, FAD3 Kennedy pathway DGAT Oil body Oleosin Niger seed (nug)

✅ Check your understanding

Where in the plant cell does fatty acid synthesis occur, and what is the key regulatory enzyme?
Explain the roles of SAD, FAD2, and FAD3 in determining oil unsaturation.
Describe the Kennedy pathway for triglyceride assembly. What is the role of DGAT?
How does temperature during seed development affect oil composition? Why?
Niger seed oil is high in linoleic acid (18:2). What enzyme could be modified to create a high-oleic version?

Discuss your answers in the course forum.

Plant Biochemistry for Horticulture · HORT 202 · Dilla University · Last updated March 2026