Carotenoid Crystals in Chromoplasts: Carotene and Lycopene Accumulation in Plant Cells

What are carotenoid crystals?

Carotenoid crystals are intra-chromoplast structures composed exclusively of carotenoids, distinguishing them from other pigment-storage bodies inside chromoplasts. While other intraplastid formations — internal membranes, plastoglobules and tubules — can also accumulate significant amounts of carotenoids, their biochemical makeup includes abundant lipids and proteins. Carotenoid crystals, by contrast, contain only pigments of the carotene group, chiefly carotene and lycopene, a fact demonstrated in studies of chromoplasts from carrot storage roots and from the corona of the snow-white daffodil.

Definition and general characteristics of carotenoids

Carotenoids are fat-soluble pigments that give plants, algae, fungi, bacteria and many animals their yellow, orange and red colours. Built on a long backbone of conjugated carbon–carbon double bonds, this chromophore absorbs light in the blue-green region and reflects the warmer wavelengths our eyes perceive as gold to scarlet. The same system of alternating double bonds that produces the colour also underlies the pigments' antioxidant behaviour and their photophysical role in harvesting and dissipating light energy.

More than 750 naturally occurring carotenoids have been catalogued, yet only a handful — β-carotene, α-carotene, lycopene, lutein, zeaxanthin and β-cryptoxanthin — dominate the human diet. Because they are lipophilic, carotenoids localise within cell membranes and lipid-rich sub-organellar compartments, including the crystals, plastoglobules and membrane systems of chromoplasts.

Types and classification of carotenoids

Carotenoids fall into two chemical classes: the carotenes, which are pure hydrocarbons, and the xanthophylls, which carry oxygen. This division governs where each pigment accumulates, how the body absorbs it and what biological function it serves.

• Carotenes — oxygen-free hydrocarbons such as β-carotene, α-carotene, γ-carotene and lycopene. β-carotene and lycopene are the two pigments that build the crystalline inclusions of chromoplasts.
• Xanthophylls — oxygenated derivatives such as lutein, zeaxanthin, β-cryptoxanthin and astaxanthin, generally more polar and often found bound within membranes rather than as crystals.

Carotenes and xanthophylls: differences and sources

Carotenes are concentrated in deeply pigmented roots and fruit — carrots (Daucus carota) and pumpkins for α- and β-carotene, tomatoes, watermelon and gac fruit for lycopene. Xanthophylls predominate in green leaves and yellow flowers: lutein and zeaxanthin are abundant in kale, spinach and marigold flowers, while astaxanthin colours salmon and other aquatic organisms. Provitamin A carotenes such as β-carotene, α-carotene and β-cryptoxanthin can be converted into retinol (vitamin A) in the body, whereas lutein and zeaxanthin cannot and instead act as the eye's macular pigments.

Composition of carotenoid crystals

Carotenoid crystals are chemically the purest carotenoid-bearing structures a plant cell produces, containing carotene-group pigments and essentially nothing else. This purity was established through work on chromoplasts of carrot storage roots and the corona of the snow-white daffodil, where the crystalline material proved to be pigment alone. As the crystals grow, they physically deform the plastid, so that chromoplasts take on needle-like, plate-like or prismatic shapes.

How crystals differ from other intra-chromoplast formations

The defining contrast between carotenoid crystals and the other inclusions of chromoplasts is compositional. Internal plastid membranes, plastoglobules and tubules all store carotenoids, but each is rich in lipids and proteins; the crystal is not. Among carotenoid-holding structures, only the crystals hold pigment in a nearly solvent-free, ordered solid state, which is why they can distort the surrounding organelle.

Pigments of the carotene group: carotene and lycopene

Only two pigments crystallise inside chromoplasts — carotene and lycopene, both members of the hydrocarbon carotene class. Their low polarity and rigid, elongated molecular shape favour tight molecular packing, allowing them to assemble into a true crystalline lattice rather than dissolving into lipid droplets. The well-known examples of crystal-bearing chromoplasts are the plastids of cultivated carrot roots, tomato fruit and watermelon.

Mechanisms of carotenoid crystal formation

Carotenoid crystals form not in the plastid stroma but within the intrathylakoid space, where β-carotene and lycopene precipitate out of solution and pack into an ordered solid. This localisation has practical consequences for how the crystals are seen — or missed — under the electron microscope, and it reflects an upstream biochemical route in which pigment synthesis feeds the growing crystal.

Crystallisation within the intrathylakoid space

β-carotene and lycopene crystallise inside the thylakoid lumen rather than in the stroma of the plastid. Under conventional fixation with glutaraldehyde followed by osmium tetroxide treatment, these dense pigment crystals are poorly fixed and are therefore easily extracted by solvents during the dehydration and embedding of tissue in preparation for electron microscopy. Where a crystal once sat, the thylakoid is left with an empty, hollow space — an artefact that long complicated the study of these structures.

How crystal growth reshapes chromoplasts

Carotenoid crystals do not always grow to macroscopic size, as they do in carrot roots, tomatoes and watermelon. In some chromoplasts — notably the plastids of flowers of the noble clivia (Clivia nobilis) and the fruit of physalis — submicroscopic crystals form instead, sometimes exceeding a hundred per chromoplast. Whatever their size, the crystals impose their geometry on the host organelle, and they display double refraction (birefringence) and dichroism, optical signatures shared with the bundled tubule structures of chromoplasts.

The plastids in the flowers of the noble clivia contain carotene crystals, an example of a bloom whose colour comes from crystalline rather than membrane-bound pigment.

Carotenoid biosynthesis and gene regulation

Carotenoid biosynthesis begins with isoprenoid precursors and converges on the enzyme phytoene synthase (PSY), the rate-limiting gatekeeper of the pathway. In plants the isoprenoid building blocks arrive mainly through the plastid-localised methylerythritol phosphate (MEP) pathway, with a parallel cytosolic mevalonate (MVA) pathway contributing precursors; these condense into geranylgeranyl diphosphate (GGPP), the immediate substrate from which PSY makes phytoene, the first committed carotenoid.

PSY activity is controlled at several levels — through light-regulated gene expression, tissue-specific promotion and post-translational control of the PSY protein. Work by Ralf Welsch, Peter Beyer, Dirk Maass and colleagues at the University of Freiburg, published in PLoS ONE, showed that overexpressing a bacterial PSY gene in Arabidopsis thaliana produced strikingly different responses in green versus non-green tissues, revealing that regulation of carotenoid accumulation is tissue-dependent. Such findings, supported in part by the Bill & Melinda Gates Foundation, underpin biotechnology strategies that use tissue-specific promoters to enrich staple crops with provitamin A carotenoids.

Size and morphology of carotene crystals

Carotene crystals span a wide size range, from macroscopic bodies visible in cut roots and fruit to submicroscopic crystallites requiring the electron microscope. Their morphology — needle, plate or prism — and their optical behaviour give chromoplasts their characteristic appearance and colour.

Macroscopic crystals (carrot, tomato, watermelon)

In cultivated carrot roots, tomato fruit and watermelon, carotene crystals grow large enough to shape entire plastids into needle-like, plate-like or prismatic organelles. The intense orange of a carrot root and the deep red of ripe tomato and watermelon flesh are the visible expression of these crystalline pigment deposits, and the genetics of carrot root pigmentation — the accumulation of α- and β-carotene during root development — is a classic model of crystal-driven coloration.

Submicroscopic crystals (clivia, physalis)

Not every chromoplast builds a large crystal. In the flowers of noble clivia and the fruit of physalis, chromoplasts contain numerous submicroscopic crystallites — occasionally more than a hundred within a single organelle. These tiny crystals still consist of pure carotene-group pigment and still deform their host plastids, but their small scale means the colour emerges from the collective effect of many crystals rather than one dominant body.

Optical properties: birefringence and dichroism

Carotene crystals exhibit birefringence (double refraction) and dichroism, the same optical properties shown by the tubules organised into bundles within chromoplasts. Birefringence splits light into two rays travelling at different speeds, while dichroism causes the crystal to absorb light differently depending on its polarisation — behaviours that arise directly from the ordered, anisotropic packing of the elongated pigment molecules.

Carotenoid crystals in plants

Carotenoid crystals occur across many plant organs — storage roots, fleshy fruit and flower petals — and their study demands care because standard microscopy techniques can destroy them. Recognising where crystals form, and how to preserve them, is essential to understanding plant pigmentation.

Crystals in roots and fruit

The best-known crystal-bearing chromoplasts are the plastids of cultivated carrot roots, tomato fruit and watermelon, where carotene and lycopene accumulate as large crystals. These same organs are among the richest dietary sources of provitamin A carotenoids and lycopene, linking the cell biology of the crystal directly to human nutrition.

Crystals in flowers (daffodil, clivia)

Carotene crystals are also found in the chromoplasts of flowers of several species — for example in the plastids of the snow-white daffodil, the peerless daffodil and the noble clivia. In these blooms the crystalline pigment produces vivid yellow to orange corollas, and the daffodil corona in particular has served as a reference tissue for confirming that the crystals contain nothing but carotene-group pigments.

Preservation challenges in electron microscopy

Fixation is the central obstacle to imaging carotene crystals. Because the dense pigment crystals resist proper fixation by glutaraldehyde and osmium tetroxide, solvents used during dehydration and resin embedding dissolve them away, leaving only a hollow void inside the thylakoid where the crystal had been. Interpreting these empty spaces correctly — as former crystal sites rather than genuine cavities — is a recurring theme in the ultrastructural study of chromoplasts.

The chromoplast stroma and its inclusions

The stroma, or matrix, of chromoplasts is far less developed than that of chloroplasts, because much of the chromoplast volume is occupied by other formations — intraplastid membranes, plastoglobules and tubular structures. Even so, in forming chromoplasts the matrix is more developed than in mature plastids, consistent with evidence that developing chromoplasts are metabolically more active.

Matrix development in forming and mature chromoplasts

Matrix abundance tracks metabolic activity across chromoplast development. Young, forming chromoplasts carry a comparatively rich stroma that supports the intense biosynthetic work of pigment production, whereas mature chromoplasts, having largely completed that work, show a reduced matrix as inclusions and membrane systems come to fill the organelle.

Starch grains in chromoplasts

The largest stromal inclusions are the starch grains, which appear in the electron microscope as dull oval or rounded bodies of reduced electron density. Starch synthesised in chromoplasts of different plants can differ in its properties: the starch of the globular chromoplasts in petals of weeping forsythia is readily digested by amylase and malt diastase, whereas the starch of the tubular chromoplasts in the perianth segments of saffron lily is not broken down by those enzymes. Phenological differences between the starch deposits of weeping forsythia and saffron lily were noted as well.

In the chromoplasts of weeping forsythia there are phenological differences between the starch deposits, illustrating how starch behaviour varies with developmental timing.

Almost all forming chromoplasts contain starch, but during the full establishment of these plastids it gradually disappears. The starch grains vanish even when chromoplasts arise from amyloplasts or amylochloroplasts as the chromoplasts reach their final state. At the same time, the chromoplasts of some plants retain starch regardless of the developmental stage of the organelle.

Ribosome-like particles and DNA fibrils

The chromoplast stroma also contains ribosome-like particles and DNA fibrils, evidence that the organelle retains its own genetic and protein-synthesis machinery. Some studies additionally report phytoferritin, protein crystals and osmiophilic bodies of unknown nature within the stroma. During the conversion of chloroplasts into chromoplasts, a further set of structures appears — the previously mentioned plexuses, prolamellar, provesicular and vesicular bodies, together with large grana, supergrana and magnograna, the last being very large thylakoids stacked in parallel as in the grana of chloroplasts; their functional significance remains unknown. B. T. Matienko and E. M. Lebanu proposed that the presence of magnograna at certain stages of the transition from precursor plastids to chromoplasts signals a deviation from the usual metamorphic pathway caused by the absence of particular enzyme systems.

Beyond these, lipid bodies deserve mention: those isolated in purified form from chromoplasts of nasturtium petals had the lowest buoyant density of the plastid components (ρ = 1.03 g/cm³) and lacked phospholipids, differing only slightly from the tubules in galactolipid and protein content. Their most remarkable feature was an ability to accumulate very high carotenoid levels — 47.7% of dry weight — making them the richest carotenoid-bearing structures apart from the pigment crystals themselves; under certain conditions they are thought to transform into tubules.

Carotenoids in nature: animal coloration

Carotenoids colour not only plants but a great many animals, which acquire these pigments through their diet because they cannot synthesise carotenoids themselves. From the pink of flamingos to the red flesh of salmon, animal carotenoid coloration is dietary in origin and often carries a biological message.

Bird plumage and flamingo feathers

Flamingos owe their pink-to-crimson plumage entirely to carotenoids obtained from the algae and crustaceans they eat; deprived of these dietary pigments, their feathers fade to grey-white. The same principle explains the pink flesh of salmon, which accumulate astaxanthin from their prey. Beyond animals, carotenoids and related microbial pigments contribute to the vivid colour bands of features such as the Grand Prismatic Spring, where pigmented microorganisms tint the water's edges.

Carotenoids and sexual selection

In many birds, the brightness of carotenoid-based colours acts as an honest signal in mate choice. Because carotenoids must be eaten and are also demanded by the immune system and antioxidant defences, only well-fed, healthy individuals can afford to divert large amounts of pigment into flamboyant plumage. A more intensely coloured bird therefore advertises its condition, and sexual selection favours these displays as reliable indicators of quality.

Functions and benefits of carotenoids

Carotenoids protect cells from oxidative damage, filter harmful light, serve as the sole dietary source of provitamin A and are linked to lower risk of several chronic diseases. Their conjugated double-bond structure — the same feature that gives them colour — is what makes them effective antioxidants and light filters.

Antioxidant activity and mechanisms

Carotenoids act as antioxidants by quenching singlet oxygen and neutralising reactive oxygen species, protecting lipids and other cell components from oxidative damage. Beyond directly scavenging radicals, some carotenoids influence gene expression, notably activating the Nrf2-dependent pathway that switches on the cell's own battery of protective, detoxifying enzymes. This dual action — direct quenching plus induction of endogenous defences — underlies much of their reputed health value.

Blue light filtering

Lutein and zeaxanthin concentrate in the macula of the human retina, where they absorb high-energy blue light and act as the eye's built-in optical filter. By intercepting damaging short-wavelength light and quenching the free radicals it generates, these macular pigments are associated with a reduced risk of age-related macular degeneration and support long-term eye health.

Health benefits: cardiovascular system and cancer prevention

Diets rich in carotenoids are associated with lower rates of cardiovascular disease and certain cancers, effects attributed to reduced inflammation and oxidative stress. Lycopene from tomatoes has been linked in observational studies to lower prostate cancer risk, while broad carotenoid intake supports immune function. The relationship is not universally protective, however: high-dose β-carotene supplements were found to increase lung cancer risk among smokers, a landmark finding that highlights the difference between whole-food carotenoids and isolated high-dose supplements.

Absorption, metabolism and bioavailability of carotenoids

Because carotenoids are fat-soluble, their absorption depends heavily on dietary fat, which helps solubilise the pigments and form the mixed micelles taken up by intestinal cells through specific transporters. Cooking and processing — chopping, gentle heating, pureeing — break down plant cell walls and can markedly raise bioavailability, which is why cooked tomatoes deliver more available lycopene than raw ones. Provitamin A carotenes such as β-carotene are then cleaved by the enzyme BCO1 into retinol, with BCO2 handling other carotenoids, and vitamin A activity is expressed in retinol activity equivalents that reflect these bioconversion ratios. Genetic differences in these enzymes, along with the choice between supplements and natural food sources, help explain why carotenoid status varies widely between individuals.