Lipids in Chromoplasts and Plastids: Glycolipid and Phospholipid Composition in Plants

Lipids are an essential component of every membrane-containing structure in the plant cell, including plastids, nuclei, mitochondria, peroxisomes, glyoxysomes and the endoplasmic reticulum. Plastids are the richest in lipids: depending on the species studied, the developmental stage, the growing conditions and the type of plastid, lipids make up anywhere from 20 to 50 percent or more of the organelle's dry weight.

What role do lipids play in chromoplasts?

Lipids are central to the organization and functioning of chromoplasts. The earliest studies of the chemical composition of these organelles, carried out by A. S. Vecher and W. Straus, already revealed a high content of lipid compounds. The principal lipid components of chromoplasts are glycolipids and phospholipids, which together account for the bulk of the organelle's lipid pool.

Which lipids make up the chromoplast lipid fraction?

Glycolipids dominate the chromoplast lipid fraction, contributing more than 80 percent of all chromoplast lipids. This fraction includes two main galactolipids — diacyldigalactosylglycerol (DADGG) and diacylmonogalactosylglycerol (DAMGG) — together with a sulfolipid, diacylsulfoquinovosylglycerol. The lipid composition of isolated chromoplasts from the petals of garden nasturtium (Tropaeolum majus) and from the trumpets of the yellow daffodil, as well as from the fruit of the annual pepper (Capsicum annuum), is set out in Table 1.

Among the phospholipids, phosphatidylcholine and phosphatidylglycerol predominate. Phosphatidylserine and phosphatidylethanolamine are known to be absent (in any appreciable amount) from highly purified chloroplast fractions of higher plants and algae; where they do appear in individual experiments, their presence indicates contamination of the plastid fraction by cytoplasmic structures. In the chromoplasts studied from the yellow daffodil and the annual pepper, phosphatidylserine was not detected and ethanolamine was present only in negligible amounts.

Study of the lipid composition of isolated chromoplasts from the petals of garden nasturtium.

Table 1 — Lipid composition of isolated chromoplasts of higher plants, as a percentage of the total lipid fraction

Note. The original data have been recalculated as a percentage of the total lipid fraction by the authors of the monograph.

What minor lipids occur in chromoplasts?

Highly purified chromoplast preparations contain a range of minor lipid compounds in trace amounts. In daffodil chromoplasts, sterol glycosides, their acylated derivatives, fatty acids and mono-, di- and triacylglycerols were detected at trace levels. B. Camara and co-workers likewise reported that plastid preparations from ripe fruit of the annual pepper contain steryl glycosides. B. Liedvogel and co-authors, analysing the lipids of nasturtium chromoplasts by thin-layer chromatography, found steryl glycosides, acylated steryl glycosides, free fatty acids and triacylglycerols, but their combined amount did not exceed 2 percent of the dry weight — roughly 4 percent of the total lipids present in these organelles.

Figure 1 — Structural formulas of chromoplast lipid molecules.

How important are fatty acids in chromoplasts?

Although fatty acids occur in chromoplasts only as minor compounds, they play an important role in these organelles. The bulk of the fatty acids are not present in the free state but are bound into complex lipids — including the galactolipids and phospholipids — and into esterified carotenoids.

Chromoplast fatty acids are distinguished by a high degree of unsaturation. In yellow daffodil chromoplasts, for example, more than 80 percent of all fatty acids are molecules carrying two or three double bonds (linoleic and linolenic acids). Molecules with different degrees of unsaturation are distributed unevenly among the individual lipid fractions.

Almost 96 percent of the fatty acids in DAMGG and more than 85 percent of those in DADGG are molecules with two or three double bonds, whereas diacylglycerol contains about 38 percent of molecules with no double bonds and no more than 55 percent with two or three. In the free fatty acid fraction, 69.7 percent of the molecules have no double bonds, 7 percent carry one, 18.9 percent carry two and 4.4 percent carry three double bonds.

Lipids and chromoplasts

Lipids serve several essential functions in chromoplasts. As a structural component of membranes, they help maintain the integrity of the plastid, support the compartmentation of metabolic processes, and are largely responsible for the organelle's capacity to exchange metabolites between the stroma and the surrounding cytoplasm. At the same time, lipids in chromoplasts can accumulate as storage substances in the form of globules (plastoglobules) and are incorporated into tubular and other formations.

Characteristically, the lipid-containing structures of chromoplasts, being hydrophobic by nature, are the site where carotenoids accumulate. Different chromoplast structures differ from one another in their lipid composition. B. Liedvogel and co-authors examined the chemical composition of chromoplast subfractions from garden nasturtium.

Highly purified isolated plastids from the petals were disrupted by osmotic shock and separated by centrifugation in a sucrose density gradient. This yielded four fractions: pure lipid bodies; tubular formations together with lipid bodies and a small amount of membrane; membrane vesicles with a small quantity of tubular structures and lipid bodies; and starch grains.

The first three fractions were subjected to biochemical analysis. The lipid bodies consisted almost entirely of carotenoids (47.7 percent of dry weight) and galactolipids (25.5 percent DAMGG and 16 percent DADGG); proteins made up 10.6 percent of the dry weight, and no phospholipids were detected. The tubular fraction contained considerably fewer carotenoids (13 percent) but somewhat more galactolipids (28 percent DAMGG and 21 percent DADGG).

The protein content of the tubular fraction did not differ from that of the lipid bodies, but in this fraction phospholipids accounted for 10 percent of the dry weight. The membrane fraction contained only 5 percent carotenoids and 32 percent galactolipids, with a DAMGG-to-DADGG ratio of 3:2. Here the phospholipid content rose to 16 percent and the protein content to 47 percent of the dry weight.

By contrast, the plastoglobules of chromoplasts in wild pansy, marsh marigold, common broom and tulip consist mainly of nonpolar lipids, whereas neither the lipid bodies, the tubules nor the membranes of nasturtium chromoplasts contained these lipids in significant amounts. Overall, in terms of their main lipid components, the chromoplasts of the higher plants studied resemble other types of plastid.

In particular, chloroplasts, etioplasts and proplastids all contain galactolipid molecules as their principal lipids. It is worth noting that galactolipids can serve as markers for detecting possible contamination of other cytoplasmic structures by plastid particles — especially plastid membranes — since these substances are not found in appreciable amounts in any of the non-plastid structures.