Plant Storage Lipids

Abstract

Plant storage lipids, normally in the form of intracellular triacylglycerol‐rich droplets, are important sources of nutrition for people and livestock; besides, they supply a vast range of renewable industrial products from oleochemicals and bioplastics to paints and biofuels. Storage lipids are mainly found in plant propagules such as seeds and pollen grains, where they form an energy source for post‐germinative growth. The main commercial sources of plant storage lipids are oilseed crops such as soybean, rapeseed and maize or oil‐rich fruits such as olive or oil palm. Triacylglycerols also have several additional nonstorage functions in processes including host–pathogen interactions and abiotic stress responses. Improved knowledge of storage lipid metabolism is being used to create new oil crop varieties and to domesticate new species to supply the ever‐increasing demand for plant oils.

Key Concepts

  • How plants store energy reserves in the form of lipids.
  • What are the key storage lipids in plants.
  • How are plant storage lipids synthesised.
  • Where are store lipids made in plant cells.
  • What are the biotechnological uses of plant storage lipids.
  • How is storage lipid metabolism regulated and how can it be manipulated.

Keywords: fatty acids; oils; lipid droplets; diet; oilseeds; biofuels

Figure 1. : Import of carbon precursors. Sucrose is transported from photosynthetic tissues into developing seeds where it is converted in the cytosol of embryo and/or endosperm cells into precursors, such as glucose 6‐phosphate and phosphoenolpyruvate, for onward transport into plastids for production of fatty acids. Import into plastids occurs via specific carriers such as GPT, glucose 6‐phosphate transporter; TPT, triose phosphate transporter and PPT, phosphoenolpyruvate transporter. : Acetyl‐CoA and malonyl‐CoA are the precursors for assembly of C8–C18 saturated fatty acyl‐ACPs on a plastidial multienzyme fatty acid synthetase complex. Plastids are also the site of the insertion of the first double bond by a soluble desaturase (SOL‐DES) to produce fatty acid monounsaturates. Both unsaturates and monounsaturates are exported via an acyl‐CoA transporter (ACT) from plastids to the endoplasmic reticulum for further processing. : Plastid‐derived acyl‐CoAs can be modified in the endoplasmic reticulum by a huge variety of enzymes to produce some of the hundreds of different fatty acids found in naturally occurring seed oils. However, as not all of these enzymes are present in any given plant species, nontransgenic oilseeds normally accumulate a relatively restricted range of fatty acids. Most fatty acid modification reactions occur via membrane‐bound phosphatidylcholine (PC) specific ER desaturases or desaturase‐like enzymes (ER‐DES) such as hydroxylases or epoxidases. Acyl‐CoAs are then assembled into complex lipids on the endoplasmic reticulum. Similar ER‐located pathways produce the various membrane lipids, storage lipids and also some signalling lipids although recent evidence suggests that these pathways are spatially separated in discrete ER domains. Storage oil bodies can accumulate virtually any type of fatty acid, whereas the biological functions of membrane and signalling lipids require that they only contain a small range of C16 and C18 fatty acids. One of the challenges to producing oilseeds with novel acyl compositions is therefore to maintain the segregation of exotic fatty acids away from pools of membrane or signalling lipids. : Triacylglycerols are assembled via a complex process involving sequential acylation of a glycerol moiety (the traditional Kennedy pathway) plus extensive acyl editing via phosphatidylcholine‐dependent desaturases or desaturase‐like enzymes (see earlier). The final conversion of DAG into TAG can occur via at least three enzymes: DGAT, acyl‐CoA dependent diacylglycerol acyltransferase; PDAT, phosphatidylcholine‐dependent acyltransferase or DGTA, diacylglycerol transacylase. Nascent TAG droplets are coated with a phospholipid monolayer into which is embedded an annulus of specific proteins, such as oleosins and caleosins, hence forming the mature storage oil bodies that are finally released into the cytosol. DAG, diacylglycerol; G3P, glycerol 3‐phosphate; MAG, monoacylglycerol; PA, phosphatidic acid and TAG, triacylglycerol.
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Further Reading

Murphy DJ (2012b) The dynamic roles of intracellular lipid droplets: from archaea to mammals. Protoplasma 249: 541–585.

Vanhercke T, Wood CC, Stymne S, Singh SP and Green AG (2013) Metabolic engineering of plant oils and waxes for use as industrial feedstocks. Plant Biotechnology Journal 11: 196–210.

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How to Cite close
Murphy, Denis J(Feb 2016) Plant Storage Lipids. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0001918.pub3]