Chromoplast Tubes: Ultrastructure, Filament Diameter, and Electron Microscopy Insights

Chromoplast tubules are fine, straight or slightly curved filaments that store carotenoid pigments inside chromoplasts, the plastids responsible for the yellow, orange and red colours of many flowers and fruits. Under electron microscopy these tubules appear as elongated structures with an outer diameter that averages roughly 15–20 nm and a length that can reach up to 10 µm in the elongated chromoplasts of rosehip fruit.

Chromoplast Tubes: Structure and Ultrastructure

The chromoplast tubule is the pigment-carrying sub-structure that gives fibrillar and tubular chromoplasts their intense colour, and its architecture is best resolved by transmission electron microscopy. Each tubule combines a pigment-rich core with a thin peripheral coat of polar molecules, an arrangement that lets carotenoids accumulate in a stable, water-excluding form within the plastid stroma.

Morphology Under Electron Microscopy

Under electron microscopy, chromoplast tubules present as straight or gently bent filaments that most often lie parallel to one another and gather into bundles. Within each bundle the individual tubules are packed in a hexagonal array. This regular, close packing gives the aggregates their characteristic optical behaviour and makes them readily distinguishable from other stromal inclusions.

Diameter and Length Variation Across Plant Species

Tubule diameter varies markedly between plant species and even along a single tubule. In the chromoplasts of the tulip tree the tubules average about 31 nm across, while in the daily-life/plant-cell-chromoplasts.html of some plants the diameter reaches up to 60 nm. Length is equally variable, extending to roughly 10 µm in elongated fruit chromoplasts. This species-dependent variation is one reason a single descriptive term long proved difficult to apply consistently across the older literature.

Cross-Sectional Appearance and Staining Behavior

In cross-section, chromoplast tubules show a cylindrical outline with a strongly contrasted rim and an electron-transparent central region. The peripheral coat stains heavily with osmium tetroxide or with permanganate, whereas the non-polar interior remains uncontrasted under the same treatment. This contrast pattern is the direct visual signature of a lipid-and-protein shell surrounding a hydrophobic pigment core.

Terminology: Fibrils, Microfibrils, Filaments and Tubes

The chromoplast tubule has appeared in the literature under several names — "fibrils", "microfibrils", "filaments" and "tubular filaments" — that ultimately describe the same object. When examined in ultra-thin sections at high magnification, structures given these different labels proved indistinguishable from the objects called tubules, so the terms are now treated as synonyms for a single structural entity.

Classification of Chromoplast Tubes

Chromoplast tubules fall into three groups: unbranched tubules without globules, unbranched tubules associated with plastoglobules, and the rarer branched-and-anastomosed tubules. The great majority are unbranched and arrange themselves in parallel bundles that display dichroism and birefringence when viewed with a polarizing microscope. The decisive feature separating the two unbranched groups is their relationship to plastoglobules.

Unbranched Tubes Without Globules

Unbranched tubules that have no association with plastoglobules are scattered throughout the stroma, tend to be long and vary widely in length, and run straight without interconnecting bridges. They are very stable structures that persist in fully developed petals. Their formation is inhibited by 2-(4-chlorophenylthio)-triethylammonium chloride, an inhibitor of carotenoid biosynthesis, an effect tested on mutant lines of red pepper.

Tubes Associated With Plastoglobules

Tubules linked to plastoglobules occupy discrete regions of the stroma rather than being dispersed. They are shorter, differ only slightly in length, are gently curved, and are joined by interconnecting bridges. Unlike the globule-free type, these tubules often disintegrate in fully developed petals, and their formation is likewise suppressed by the same carotenoid-biosynthesis inhibitor. The contrast between the two groups is summarised below.

Note. * The effect of this compound (an inhibitor of carotenoid biosynthesis) on chromoplast tubules has been tested only on mutant lines of red pepper.

Branched and Anastomosed Tubes

The third and rarest group consists of branched and anastomosed tubules, found in typical form only in the chromoplasts of Typhonium divaricatum and the tulip tree. Similar structures occur in limited numbers in the plastids of other plants — for example in the fruit of annual pepper, in health/benefits-of-pumpkin.html, and in cucumber petals. In pumpkin fruit these plastids contain only a small population of branched and anastomosed tubules.

Biochemical Composition of Chromoplast Tubes

Chromoplast tubules purified from the chromoplasts of garden nasturtium and rugosa rose share a similar biochemical make-up dominated by polar lipids and carotenoids, with a smaller protein fraction and no chlorophyll at all. This composition explains both the staining behaviour of the tubules and their role as dedicated pigment stores rather than photosynthetic membranes.

Polar Lipids and Galactolipids

About 60% of the dry matter of isolated chromoplast tubules consists of polar agronomy/lipids.html, with galactolipids predominating. Other, non-polar lipid components — acylglycerols, sterols and prenylquinones — are present only in minor amounts. This polar-lipid-rich shell is what interacts strongly with osmium and manganese fixatives and defines the contrasted rim seen in cross-section.

Carotenoid Content and Lutein Diesters

A substantial part of the tubule's mass is carotenoid, the pigment class that produces the yellow-to-red colours of these structures. In garden nasturtium roughly 80% of the carotenoids occur as lutein diester, an esterified form of lutein, a xanthophyll. Carotenoids such as lutein and zeaxanthin belong to the xanthophyll subgroup, while carotenes like lycopene (the red pigment of tomato) and beta-carotene (abundant in carrot) are the oxygen-free members of the same family. The heavy accumulation of esterified carotenoids in tubules is the structural basis of stable colour in petals and fruit tissue.

Protein Composition and the 30–32 kDa Polypeptide

Proteins make up less than 30% of the dry weight of chromoplast tubules. Electrophoretic analysis (in the presence of sodium dodecyl sulfate) of proteins extracted from garden nasturtium and rugosa rose tubules showed that a single polypeptide of 30,000–32,000 Da accounts for about 80% of the total tubule protein. Other polypeptides were also detected, but it remains unclear whether they represent contamination from other plastid — or possibly cytoplasmic — structures or are genuine tubule components.

Structural Model of Chromoplast Tubes

The accepted structural model of the chromoplast tubule places pigment molecules in the central core and a less hydrophobic outer coat of polar lipids and proteins around them. This model rests on electron-microscopic observations that regions of moderate lipophilicity are the most strongly contrasted by osmium and manganese compounds, while the non-polar interior stays uncontrasted under identical processing.

Model experiments demonstrated that the pigment molecules are oriented parallel to the long axis of the tubule and exist inside it as science/ultrastructure.html. To date this is the only known case in living nature of a structure with the properties of a nematic liquid crystal, a striking example of biological self-organisation of carotenoids.

The origin of the tubules is still debated. K. Steffen and F. Walter, working on the chromoplasts of Solanum capsicastrum, concluded that the tubules — which they called fibrils — arise from plastoglobules (local swellings on the fibrils) by stretching and elongation, a view shared by several other researchers. Three lines of evidence argue against this idea.

1. If tubules formed from plastoglobule material, they should show the same evenly distributed density on cross-section. Instead, plastoglobules generally have uniform density across the section, whereas tubules show a dense peripheral coat enclosing an electron-transparent interior.
2. Direct stretching of globular material would give tubules and plastoglobules a similar chemical composition, yet the two differ: tubules are rich in polar compounds and contain about 30% protein, while plastoglobules are made mostly of non-polar substances and contain little or no protein.
3. In some plants — for example in the petals of cucumber, garden nasturtium and greater celandine — no plastoglobules were seen during tubule formation.

Another view holds that chromoplast tubules form from material derived directly from chloroplast thylakoids as those membranes break down. However, comparative analysis of the protein spectrum and lipid composition of tubules from wild daffodil and garden nasturtium against the thylakoid membranes of higher plants shows that tubules cannot be assembled by self-assembly of components released from disintegrating granal and intergranal thylakoids.

The evidence therefore indicates that all intra-chromoplast structures, tubules included, are built not from the breakdown products of pre-existing structures but from newly synthesised lipids, proteins and other molecules. It is now well established that chromoplast formation involves the synthesis of carotenoids distinct from those of the precursor plastid, although some proteins, lipids and other molecules released during the breakdown of earlier structures may still be recruited into the chromoplast architecture without being enzymatically degraded to simple compounds.

Chromoplast Structure and Function Overview

Chromoplasts are non-photosynthetic plastids specialised for the synthesis and storage of carotenoid pigments, and they belong to the same organelle family — the plastids — as chloroplasts and leucoplasts. Their defining job is to accumulate carotenoids that colour plant organs, a function that supports pollinator attraction and seed dispersal.

Function and Location of Chromoplasts in Plant Tissues

Chromoplasts occur in the coloured tissues of flowers, fruits, roots and ageing leaves, wherever a plant needs a durable yellow, orange or red pigment. They are found in the flesh of ripe tomato (Solanum lycopersicum), in the storage root of the carrot, in the segments of Valencia oranges and in the petals of many ornamental flowers. Unlike chloroplasts, they carry no chlorophyll and perform no photosynthesis; their role is purely pigment synthesis and storage.

Role of Chromoplasts in Flowers and Fruits

In flowers and fruits, chromoplasts generate the visual signals that recruit animals for reproduction. Bright petals draw pollinators toward nectar and pollen, while brightly coloured ripe fruit advertises an edible reward to animals that then disperse the seeds. The loss of colour in certain white-flowered mutants — where the carotenoid pathway is disrupted — illustrates how directly pigment biosynthesis governs this ecological signalling; the bee orchid is a well-known example of a flower whose colouring functions as a pollinator lure.

Carotenoid Accumulation in Fruits and Flowers

Carotenoid accumulation in chromoplasts depends on the sub-structure that sequesters the pigment — tubules, plastoglobules, crystals or membranes. In tomato fruit lycopene crystallises to give the red flesh; in carrot beta-carotene accumulates in crystalline form; and in many petals xanthophyll esters build up inside tubules like those described above. The type of carotenoid-storing structure that forms determines both the colour and its stability in the tissue.

Types of Chromoplasts and Their Subtypes

Chromoplasts are classified by the internal structure in which they store carotenoids, and five main categories are recognised. This morphological scheme links directly to the pigment being stored and to the plant organ in which the chromoplast develops.

Globular, Membranous, Crystalline, Fibrillar and Tubular Forms

• Globular — carotenoids held within plastoglobules dispersed in the stroma.
• Membranous — pigments associated with concentric internal membranes, as seen in Valencia oranges and some daffodils.
• Crystalline — carotenoids deposited as solid crystals, typified by lycopene in tomato and beta-carotene in carrot.
• Fibrillar — pigments carried in fibrils sheathed in polar lipid and protein.
• Tubular — the tubule form detailed throughout this page, in which carotenoids form a nematic liquid-crystal core inside a lipid-protein tube.

Chromoplast Development and Origin

Chromoplasts develop from other plastid types, most often from chloroplasts, and the pathways of plastid differentiation are interconvertible. All plastids arise from proplastids in meristematic cells and can transform into one another under developmental or hormonal control.

Chloroplast to Chromoplast Conversion During Ripening

The best-studied chromoplast development is the chloroplast-to-chromoplast transition during fruit ripening, exemplified by tomato. As the fruit ripens, thylakoid membranes are dismantled, chlorophyll is degraded and the internal membrane system is remodelled, while carotenoid biosynthesis is switched on and lycopene accumulates. This transition is driven by nuclear-encoded gene expression and by hormonal signals, with ethylene playing a central role in climacteric fruit. Research on tomato chromoplasts by teams including Cristina Barsan, Alain Latché, Christian Chervin, Eduardo Purgatto, Isabel Egea, Jean-Claude Pech and Mondher Bouzayen has mapped the plastidial proteome changes and transcriptional activity that accompany chromoplast differentiation.

Chromoplast Reversion to Chloroplasts

Chromoplasts can in some cases undergo reverse differentiation, a regreening process in which they redevelop into chloroplasts. This has been observed when ripe fruit or coloured tissue is returned to conditions favouring vegetative growth, such as re-exposure to light or renewed shoot activity in citrus. Regreening demonstrates plastid interconvertibility and shows that the plastid genome remains functional and stable even after the shift to carotenoid storage.

Evolution and Endosymbiotic Origin of Plastids

Chromoplasts, like all plastids, trace their ancestry to a free-living prokaryote engulfed by a eukaryotic host, an event described by the theory of symbiogenesis (endosymbiosis). Over evolutionary time the plastid genome underwent substantial reduction as most genes migrated to the nucleus, so chromoplast development today depends heavily on nuclear-chromoplastic genome communication. The nucleus supplies most plastid proteins, and retrograde signalling from the plastid back to the nucleus coordinates gene expression during differentiation.

Chromoplasts Compared With Other Plastid Types

Chromoplasts are one of several plastid types that share a double membrane and a common proplastid origin but differ in pigment content and function. Comparing chromoplasts with chloroplasts, amyloplasts and other leucoplasts clarifies what makes the carotenoid-storing plastid distinct.

Chloroplasts and Chlorophyll Pigment Location

Chloroplasts are the green, photosynthetic plastids of the leaf mesophyll, where chlorophyll is located in the thylakoid membrane. It is on the thylakoid membrane that light absorption and the light reactions of photosynthesis take place, making chloroplasts the site of plant metabolism that converts light energy into sugars. The seasonal breakdown of chlorophyll during autumn leaf senescence unmasks the underlying carotenoids, which is why leaves turn yellow and orange before falling.

Amyloplasts and Starch Storage

Amyloplasts are colourless leucoplasts specialised for starch storage, prominent in the potato tuber. Unlike chromoplasts, they hold no pigment; their starch grains are the reason a slice of potato stains blue-black when treated with an iodine stain such as Lugol's stain. Leucoplasts as a group include amyloplasts (starch), elaioplasts (oils) and proteinoplasts (proteins), all non-pigmented and typical of storage and non-green tissues.

Double-Membraned Plastid Structure

Every plastid, including the chromoplast, is bounded by a double membrane inherited from its endosymbiotic origin. Proteins encoded in the nucleus are imported across this envelope through the TOC and TIC translocon complexes — the translocon of the outer and inner chloroplast membranes — which govern plastid protein import and translocation. This shared envelope architecture underlies the ability of plastids to interconvert while retaining a common transport machinery.

Microscopy and Staining Methods for Studying Chromoplast Tubes

Chromoplast tubules are studied chiefly by transmission electron microscopy combined with heavy-metal fixation, since their ~15–20 nm diameter lies far below the resolution of a light microscope. Sample preparation and staining choices determine whether the tubule's core-and-coat organisation can be resolved at all. In a teaching laboratory, coloured chromoplast-bearing tissue such as tomato or carrot is examined by preparing a wet mount and observing at high magnification, following standard microscopy safety procedures when handling blades and stains.

Osmium Tetroxide and Permanganate Fixation

Fixation with osmium tetroxide or with permanganate is central to visualising chromoplast tubules because these reagents contrast the polar lipid-and-protein coat far more strongly than the non-polar pigment core. The resulting difference in electron density produces the characteristic dense rim around an electron-transparent centre, and it is this differential staining that first supported the core-and-shell structural model.

Ultra-Thin Sectioning Techniques

Ultra-thin sectioning is required to reveal the true tubular nature of these structures. When cut into ultra-thin sections and examined at high magnification, the objects variously labelled fibrils, microfibrils and filaments all resolve into identical tubules — confirming that the differing names reflected sectioning artefacts and orientation rather than genuinely different structures. Careful sectioning also preserves the hexagonal packing of tubules within a bundle.

Applications in Crop Improvement and Plastid Engineering

Understanding chromoplast differentiation opens practical routes to crop improvement, because engineering the plastid pathway that builds carotenoids can raise the nutritional and visual quality of food crops. Boosting carotenoid content increases provitamin A and antioxidant levels in staple and horticultural crops, a major goal of biofortification.

A key molecular lever is the Or gene, which mediates the conversion of leucoplasts and proplastids into carotenoid-accumulating chromoplasts and triggers the formation of pigment-storing structures; the Or gene has been used to enrich carotenoid content in several crops. Plastid protein homeostasis is also regulated by the SP1 ubiquitin ligase, which acts on the TOC translocon to control protein import — work associated with researchers including R. Paul Jarvis, Qihua Ling, Robert G. Sowden, Najiah M. Sadali and Wanping Bian at institutions such as the University of Oxford and the University of Malaya. Hormonal control of chromoplastogenesis, together with the transcriptional and proteomic reprogramming that reshapes the plastid during ripening, provides further targets for tuning colour, shelf life and nutrient density.

Want to write an article? Then register on this site!