The nature of these compounds was discovered during the 19th century. In 1831, Wachenroder H. proposed the term “carotene” for the hydrocarbon pigment he had cristallized from carrot roots. Berzelius J. called the more polar yellow pigments extracted from autumn leaves “xanthophylls” and Tswett M., who separated many pigments by column chromatography, called the whole group “carotenoids”.
Information on their physical and chemical properties as well as spectroscopic and chromatographic data for the determination of carotenoids in biological samples has been given (Rivera Vélez SM, J Nat Prod 2016, 79, 1473).
Among this important group, the numerous compounds consist of C40 chains (tetraterpenes) with conjugated double bonds, they show strong light absorption and often are brightly colored (red, orange). They occur as pigments in bacteria, algae and higher plants. Carotenoids perform three major functions in plants : accessory pigments for light harvesting, prevention of photooxidative damage and pigmentation attracting insects.
The hydrocarbon carotenoids are known as carotenes, while oxygenated derivatives of these hydrocarbons are known as xanthophylls. To date, 700 carotenoids have been identified, of which only 50 are regularly consumed in the human diet and 24 have so far been detected in human plasma. In Western diets, the most abundant carotenoids are the three oxygenated xanthophylls lutein, zeaxanthin and β-cryptoxanthin and the three major carotenes, α-carotene, β-carotene and lycopene. The human intake of carotenoids may be appreciated using databases such as that establlished for Swiss vegetables (Reif C et al., J Food Comp Anal 2013, 29, 64).
Carotenoids are important components of the light harvesting in plants, expanding the absorption spectra of photosynthesis. They also protect plant tissues by helping to absorb the energy from singlet oxygen, an excited form of the oxygen molecule O2 which is formed during photosynthesis. The major carotenoids in this context are lutein, violaxanthin and neoxanthin. In mammals, carotenoids exhibit immunomodulatory actions, likely related to their anticarcinogenic effects. β–Carotene was thus shown to enhance cell-mediated immune responses (Hughes DA, Proc Nutr Soc 1999, 58, 713). The decrease in prostate cancer risk has been linked to the consumption of tomatoes, vegetable rich in lycopene, as prostatic tissues. While there is yet limited direct evidence linking lycopene and prostate cancer (Kavanagh CJ et al., J Natl Cancer Inst 2007, 99, 1074), several observations, including the ability of the prostate to concentrate lycopene, suggest a special protection of lycopene against that pathology (Stanley JC, Lipid technol 2008, 20, 64).
A comprehensive treatise dedicated specifically to the multiple facets of food carotenoids has been published (Food carotenoids – chemistry, biology, and technology, DB Rodriguez-Amaya, IFT Press, Wiley Blackwell, 2016).
A comprehensive review on the chemistry of carotenoids and their physiological effects on human health may be found (Langi P. et al., Methods Mol Biol. 2018, 1852, 57).
Carotenoids consist of eight isoprenoid units joined in such a manner that the arrangement of isoprenoid units is reversed at the center of the molecule so that the two central methyl groups are in a 1,6-position relationship and the remaining non-terminal methyl groups are in a 1,5-position relationship. They are, by far the predominant class of tetraterpenes. They may be also classified in the terpenoids.
Carotenoids can be considered derivatives of lycopene, found in tomatoes, fruits and flowers. Its long straight chain is highly unsaturated and composed of two identical units joined by a double bond between carbon 15 and 15′. Each of these 20 carbon units may be considered to be derived from 4 isoprene units. Lycopene is a bioactive red colored pigment naturally occurring in plants. Interest in lycopene is increasing due to increasing evidence proving its antioxidant activities and its preventive properties toward numerous diseases. In vitro, in vivo and ex vivo studies have demonstrated that lycopene-rich foods are inversely associated to diseases such as cancers, cardiovascular diseases, diabetes, and others. A review of all these aspects may be consulted (Kong KW et al., Molecules 2010, 15, 959).
Carotenoids may be acyclic (seco-carotenoids) or cyclic (mono- or bi-, alicyclic or aryl). Oxyfunctionalization of various carotenoids leads to a large number of xanthophylls in which the function may be a hydroxyl, methoxyl, carbonyl, oxo, formyl or epoxy group.
Phytoene is one of the first intermediates in the biosynthesis of carotenoids. It is formed by coupling of two molecules of geranylgeranyl pyrophosphate by the action of the phytoene synthase. The diphosphate is removed and proton shift leads to the formation of phytoene. It is a colorless product and absorbs light in the UV range only. Dietary phytoene is accumulated in human skin where it can potentially protect the skin (UV absorber, antioxidant, anti inflammatory) (Aust O et al., Int J vitam Nutr Res 2005, 75, 54).
Phytofluene, another colorless carotenoid, has 10 double bonds, but only 5 conjugated double bonds, much lower as compared to coloured carotenoids (they must have at least 7 of these bonds to be coloured. Thus, unlike coloured carotenoids, phytoene and phytofluene absorb maximally in the ultraviolet region (286 and 348 nm, respectively).
Despite being rarities among carotenoids, phytoene and phytofluene are very important for a series of reasons and are attracting much interest recently from diverse point of views. A number of studies indicate that they may be related to the reduction of the risk of developing certain cancers, such as skin, prostate, leukemia, breast and endometrial cancer, and atherosclerosis. A review has summarized much of the research carried out regarding these carotenoids in the context of food science, nutrition and health; highlighting their potential in the context of functional foods and nutricosmetics (Meléndez-Martínez AJ et al., J Food Comp Anal 2018, 75, 91-103).
Among the represented molecules, β–carotene is probably the most important as a precursor of vitamin A by central cleavage into retinol and some retinoic acid (β-carotene provides about 40% of human dietary retinol equivalents). The majority of carotenoids are still manufactured chemically, but, currently, the microbial production of β-carotene is of commercial importance. Blakeslea trispora is one of the best fungal strain employed for the production of β-carotene, commercially utilized by DSM. B. trispora produces up to 3% carotene per cell dry weight. B. trispora is also employed for the production of natural lycopene, commercialized by Vitatene. The marine algae Dunaliella salina is also employed for manufacturing of b-carotene (Cognis, Proalgen Biotech Ltd.).
Carotene and lycopene are known to improve skin properties when ingested as supplements or in vegetal products.They protect against sunburn, increasing the basal defense against UV light-mediated damage to the skin, although their efficacy is not comparable to the use of a sunscreen (Aust O et al., Int. J Vitam Nutr Res 2005, 75, 54–60).
In human serum several carotenes and xanthophylls have been detected. If α– , β-carotene and lycopene are frequently quoted in specialized papers, some others are now determined with precise HPLC methods (lutein, zeaxanthin, cantaxanthin and β-cryptoxanthin). These compounds originate from ingested fruit, green leaves, berries and yellow corn. Egg yolk usually contains about 175-400 mg of lutein and about 200-300 g of zeaxanthin, but this could be affected by hen’s feed composition (Handelman GJ et al., Am J Clin Nutr 1999, 70, 247). Up to 18% losses of xanthophylls have been observed after various cooking methods (Nimalaratne C et al., J Agric Food Chem 2012, 60, 12547).
Lutein has its maximum absorption at 450 nm, cryptoxanthin at 453 nm and zeaxanthin at 454 nm.
Lutein and zeaxanthin are the only xanthophylls found in human serum that are present in retina, macula (the central region of the eye (they are frequently referred to as macular pigment), and lenses. Thus, they are at the origin of the name of the central part of the retina, macula lutea (yellow spot). By absorbing blue-light, the macular pigments protect the underlying photoreceptor cell layer from light damage, possibly initiated by the formation of reactive oxygen species. Increasing the intake of lutein or zeaxanthin may prove to be protective against the development of age-related macular degeneration (Krinsky NI et al., Annu Rev Nutr 2003, 23, 171).
Several nutritional investigations have suggested that lutein supplements may improve visual function and optical density in aged people (Olmedilla B et al., Nutrition 2003, 19, 21).
Natural aromatic derivative of lutein, 3,3′-dihydroxyisorenieratene and its non-hydroxylated parent compound, isorenieratene, are produced by green sulfur bacteria and byBrevibacterium linens, a bacteria used in the dairy industry for production of smear cheese (Saint Paulin, Münster, St Nectaire) (Rattray FP et al., J Dairy Sci 1999, 82, 891). Their high antioxidant activity against free radicals and their specific ability to prevent oxidatively generated damage to DNA make them interesting compounds for the development of endogenous photoprotective systems (Wagener S et al., Free Rad Biol Med 2012, 53, 457).
3,3′-Dihydroxyisorenieratene (R1, R2 = OH)
Isorenieratene (R1, R2 = H)
Astaxanthin, present in yeast, microalgae, crustaceans and fish, is a natural nutritional component, used as a food supplement for human and animal consumption. Its commercial production comes from both natural and synthetic sources.
Astaxanthin can be synthesized only by some bacteria (such as Agrobacterium aurantiacum), fungi (such as Xanthophyllomyces dendrorhous), and microalgae (such as Haematococcus pluvialis). Haematococcus is considered the most promising source of natural astaxanthin at industrial scale. Animals obtain astaxanthin from their diets either by directly ingesting astaxanthin-rich prey or by converting other ingested carotenoids into astaxanthin.
Thus, astaxanthin can be naturally found in widely consumed seafood such as salmon and marine invertebrates such as krill, lobster, and shrimp. Astaxanthin is also found in birds like ﬂamingo and quail. Free astaxanthin is particularly sensitive to oxidation. Consequently, it is usually found either conjugated to proteins, like in salmon muscle or lobster exoskeleton, or esteriﬁed with one or two fatty acids (monoester and diester forms), which stabilizes the molecule.
Astaxanthin shows very good capability at protecting membranous phospholipids and other lipids against peroxidation. Several studies have shown that astaxanthin has a stronger antioxidant activity than vitamin E (up to 500 times) and other carotenoids. In animal models, its modulates immune response (Bennedsen M et al., Immunol Lett 1999, 70, 185), inhibits cancer cell growth (Chew BP et al., Anticancer Res 1999, 19, 1849), and reduces bacterial load and gastric inflammation in vitro and rodent models. In humans, dietary astaxanthin decreases a DNA damage biomarker and acute phase protein, and enhances immune response (Park JS et al., Nutr Metabol 2010, 7, 18). Several research suggest that astaxanthin has protective effects against light-induced retinal damage via the mechanism of its antioxidative effect (Otsuka T. et al., J Pharmacol Sci. 2013, 123, 209). Furthermore, dietary astaxanthin accumulates in the skin and appears to prevent the effects of UVA irradiation on filaggrin metabolism and desquamation in the epidermis and the extracellular matrix in the dermis (Komatsu T. et al., PLoS One. 2017 Feb 7;12(2):e0171178).
Astaxanthin has two types of applications in animal husbandry: as a source of natural color and as a nutritional supplement because of its physiological effects (survivability, growth, photoprotection, fertility, egg quality, protection from cataracts). In Humans, users reported that astaxanthin was as effective as, or more effective, than anti-inﬂammatory drugs.
In the plant kingdom, Arabidopsis thaliana lines overexpressing the Chlamydomonas reinhardtii β-carotene ketolase accumulated high amounts of astaxanthin in the leaves was shown to have lower sensitivity to abscisic acid, to have increased tolerance to oxidative stress and to exhibit enhanced resistance to the bacterial pathogen Pseudomonas syringae pv. tomato DC3000 (Yu L et al., J Agric Food Chem 2019, 67, 12590). That approach provides insight into new roles of carotenoids in plant response to oxidative stress and pathogen infection.
Siphonaxanthin is a rare marine carotenoid abundant in green algae such as Codium fragile, Umbraulva japonica, and Caulerpa lentillifera (Sugawara T et al., Marine Drugs, 2014, 12, 3660). To date, antiangio- genesis, anti degranulation, and anti obesity effects of SPX have been reported (Li ZS et al., J Nutr 2015, 145, 490). This carotenoid exist also as an ester form (siphonein) on its C-19 hydroxyl group. It has been shown that siphonaxanthin Inhibits lipogenesis in hepatocytes via the suppression of liver X receptor α activity (Zheng J et al., Lipids 2018, 53, 41).
An important review has discussed the place of carotenoids in different market sectors, the methods for commercial production, the evidence supporting the health claims made by different industry sector for the most commercially valuable carotenoids on the market: β-carotene, lycopene, lutein, zeaxanthin and astaxanthin (Berman J. et al., Phytochem Rev 2015, 14, 727).
Ketocarotenoids belong to the xanthophyll group and are quite unique in nature. Among these compounds, echinenone and canthaxantin are abundant in cyanobacteria are characteristic of cyanobacteria (Takaichi S et al., Cell Mol Life Sci 2007, 64, 2607).
Violaxanthin is an epoxidized derivative of antheraxanthin (hydroxylated cryptoxanthin) and forms with zeaxanthin the xanthophyll cycle that is said to protect the photosynthetic system of plants against damage by excess light. The mature endocarp of the sweet orange fruit abundantly contained violaxanthin mono- and diesters (Lux PE et al., J Agric Food Chem 2019, 67, 13164).
Two cycloaddition products of trans-violaxanthin with a-tocopherol have been isolated from seeds of Pittosporum tobira and their structures elucidated (Fujiwara Y et al., Tetrahedron Lett 2001, 42, 2693). These C69 carotenoids were named pittosporumxanthins. Their global structure is given below.
Further investigations have revealed the existence of six other forms based on addition of α-tocopherol with antheraxanthin, neoxanthin or violaxanthin (Maoka T et al., J Nat Prod 2008, 71, 622).
Fucoxanthin, as an allenic carotenoid with a 5,6-monoepoxide group, is one of the most abundant carotenoid in brown algae and diatoms. This compound contributes more than 10% of the estimated total production of carotenoids in nature. Fucoxanthin was first isolated by Willstätter R et al. (Ann 1914, 404, 237).
This carotenoid has shown interesting biological properties, such as anticarcinogenetic effects, anti-inflammatory effects and radical scavenging activity. Furthermore, it has been shown that fucoxanthin has an anti-obesity effect in connection with the expression of the uncoupling protein UCP1 in white adipose tissue (Maeda H et al., Biochem Biophys Res Comm 2005, 332, 392; Miyashita K, Lipid Technol 2009, 21, 186). Its ability to enhance the docosahexaenoic acid concentration in the liver of treated obese mice remains to be examined more thoroughly (Tsukui T et al., J Agric Food Chem 2007, 55, 5025). A great part of the biological properties of fucoxanthin may be related to its ability to take up peroxynitrite through the formation of nitrofucoxanthin and the inhibition of the nitration of tyrosine by peroxynitrite (Tsuboi M et al., J Agric Food Chem 2011, 59, 10572). Therefore, fucoxanthin may have the potential to reduce the risk of cancer induced by reactive nitrogen species. Among 15 carotenoids evaluated, fucoxanthin was reported to cause a remarkable reduction in the growth of prostate cancer cells (Hosokawa M et al., Food Sci Technol Res 1999, 5, 243–246, 1999).
Neoxanthin is another common allenic carotenoid, widely distributed in higher plants and algae, which was first isolated from the green leaves of barley in 1938 by Strain HH.
Neoxanthin is thought to be a part of the light harvesting complexes in thylakoids and as a precursor to the plant growth hormone abscisic acid.
More than 40 allenic carotenoids have been described in vegetals and accumulated in animals (Dembitsky VM et al., Prog Lipid Res 2007, 46, 328).
An unusual carotenoid ester was identified in fresh mango lipids as violaxanthin dibutyrate (Pott I et al., Phytochemistry 2003, 64, 825).
Another unusual acetylenic carotenoid was discovered and identified in a marine sponge (Prianos osiros) (Rogers EW et al., J Nat Prod 2005, 68, 450).
That new molecule was shown to be strongly cytotoxic toward cultured human colon tumor cells.
Specific carotenoids are found in mineral sediments or crude oils and are used as biomarkers. Among them, isorenieratane, found in oils of Devonian age, is derived from the carotenoid isorenieratene (an homologue of the previous one but with an unsaturated isoprenoid chain), which is synthesized by photosynthetic green sulfur bacteria (Chlorobiaceae)(Koopmans MP et al., Geochim Cosmochim Acta 1996, 60, 4467).
Another C40 carotenoid, paleorenieratane, also thought to be derived from the same bacteria has been identified in Devonian aged sediments and crude oils (Hartgers WA et al., Organic Geochem 1994, 22, 703).
These green sulfur bacteria are strict anaerobes that require light and hydrogen sulfide in stratified water columns to carry out photosynthesis and are thus markers for these photic zones (euxinic) in depositional environments.
C50-carotenoids, including bacterioruberin which is present in halophilic Archaea, characterize the colored saltern ponds (Lutnaes BF et al., J Nat prod 2002, 65, 1340).
Although green leaves contain unesterified hydroxy carotenoids, most carotenoids in ripe fruit are esterified with fatty acids. However, those of a few fruits, particularly those that remain green when ripe, such as kiwi, undergo limited or no esterification. Carotenoid mono- and diesters were identified in mandarin essential oil (Giuffrida D et al. Flavour Fragr J 2006, 21, 319), in Haematococcus pluvialis (Breithaupt DE et al., J Agric Food Chem 2004, 52, 3870) and in the Antarctic krill Euphausia superba (Takaichi S et al. Comp Biochem Physiol B 2003, 136, 317).
Carotenoids fatty acid esters were also described in the carapace of the spiny lobster Panulirus japonicus (Maoka T et al., J Oleo Sci 2008, 57, 145). Astaxanthin, adonixanthin, and pectenolone were esterified with fatty acids with a large range of chain length and unsaturation.
Several esterified seco-carotenoids, the tobiraxanthins, have been isolated from the seeds of Pittosporum tobira (Fujiwara Y et al., Tetrahedron Lett 2002, 43, 4385). Two fatty acid molecules (lauric or myristic acid) acylated the carotenoid part. One of these compounds is shown below (Tobiraxanthin A1).
Apocarotenoids are carotenoid derivatives formed by the removal of fragments of the carbon backbone from either or both ends of a C40 precursor such as lycopene or beta,beta-carotene. These modification originate in the oxidative degradation at the level of the terminal rings. They may be the result of nonspecific mechanisms (lipoxygenase, photo-oxidation) as well as of specific mechanisms (dioxygenases). They have significant roles as developmental and environmental response signals. They also make important contributions to flavor and nutritional quality of foods (fruits, tea, wine) and tobacco.
Two well-known apocarotenoids are bixin and crocetin which have economic importance as pigments and aroma in foods.
Bixin (cis- or trans-bixin) is one of several highly colored molecules extracted from annatto seed coats (Bixa orellana) and used as a food coloring and cosmetic compound since pre-Colombian times. It was determined that lycopene is the precursor of bixin (Bouvier F et al., Science 2003, 300, 2089). A review of the molecular characterization of bixin may be consulted (Ramamoorthy S et al., Ind Crops Prod 2010, 32, 48).
Crocetin, occurring as various glycosyl esters, is the coloring principle of saffron, pollen harvested from Crocus sativus and in fruits of Gardenia jasminoides. This apocarotenoid originates in the rupture of zeaxanthin (Bouvier F et al., Plant Cell 2003, 15, 47). This carotenoid may have several physiological properties in decreasing the effects of physical fatigue in healthy men and in improving the quality of sleep. A derivative, sodium transcrocetinate, was shown to improve oxygenation in hypoxic tissues. It has been reported to exert a protective effect against cerebral and heart ischemia/reperfusion injury (Chang G et al., Int Immunopharmacol 2019, 71, 361).
Two other C26 and C30 apocarotenoids have been characterized in the seeds of Ditaxis heterantha (a native Euphorbiaceae of Mexico), heteranthin and ditaxin (Mendez-Robles MD et al., J Nat Prod 2006, 69, 1140). The presence of these products explains the use of the Ditaxis seeds as a natural food flavor and coloring.
Carotenoids are in constant turnover involving continuous biosynthesis and catabolism, and are substrates for the
carotene cleavage dioxygenases that catalyze the cleavage of carotenoids to apocarotenoids. These enzymes cleave a broad range of carotenoids, such as lycopene, ß-carotene, δ-carotene, zeaxanthin, violaxanthin, and neoxanthin, to generate aldehydes and ketones that are volatile aroma compounds (Auldridge M.E. et al., Curr Opin Plant Biol 9, 315).
These secondary metabolites are of special interest because of their possible biological functions including herbivore feeding deterrence.
It was shown that several volatile products are present after enzymatic degradation. Among them, the so-called norisoprenoids, the C13-cleavage products of carotenoids, have extremely low detection threshold values (some ng per liter of water for ionone and damascone).
Various bioassays demonstrated that ß-ionone has a strong repellent effect toward both the flea beetle and the spider mite, and significant oviposition deterrence to whiteflies. In contrast, dihydro-ß-ionone had attractant properties, especially to the crucifer flea beetle, while a-ionone did not show any significant activity (Cáceres L.A. et al., J Chem Ecol 42, 107). It may be expected that modified plants producing natural compounds with repellent or attractive effects will be exploited to reduce reliance on synthetic pesticides.
There is an enormous interest in apocarotenoids in detergent, food and perfume industry (Carotenoid-derived aroma compounds. Winterhalter P and Rouseff RL Eds, ACS Symp Series, v.802, 2002).
A peroxidase from edible fungi was shown to be a key enzyme able to degrade carotenoids into important flavor compounds (ionone, cyclocitral, terpineol….) (Zorn H et al., Biol Chem 2003, 384, 1049). A review of the oxidative remodeling of plastid carotenoids initiated by specific dioxygenases has been released by Camara B et al (Arch Biochem Biophys 2004, 430, 16).
Furthermore, apocarotenoids act as visual or volatile signals to attract pollinating agents, and are also key players in plant defense. Studies have shown that the loss of cleavage enzymes induces the development of axillary branches, indicating that apocarotenoids convey signals that regulate plant architecture (Bouvier F et al., Tr Plant Sci 2005, 10, 187). It appears progressively that, even in man, apocarotenoids may exert biological activities. While the mechanism of action of these compounds is not fully known, they have profound effects on gene expression through nuclear receptors (Eroglu A et al., J Lipid Res 2013, 54, 1719).
It is generally accepted that oxidation of carotenoids begins with epoxidation and cleavage to apocarotenals prior a transformation into other derivatives. Several epoxycarotenoids and apocarotenals were observed in experimental oxidation models but also some were identified in processed foods (Rodriguez EB et al., Food Chem 2007, 101, 563). The two compounds shown below were detected in fruit extracts :
Another apocarotenal, trans-β-apo-8′-carotenal, is found in spinach and citrus fruits. It has a low pro-vitamin A activity and is used in pharmaceuticals and cosmetic products. This apocarotenal is also used as an additive (E160e) legalized by the European commission for human food.
The enzymatic cleavage of β-cryptoxanthin or zeaxanthin by carotenoid cleavage dioxygenase 4 gives rise in the flavedo of citrus fruit to β-citraurin (Ma G et al., Plant Physiol 2013, 163, 682). β-citraurin and its esters were found to be the unique flavedo (the outer colored layer of the mesocarp) constituents of mature fruit (Lux PE et al., J Agric Food Chem 2019, 67, 13164).
Apolycopenals have been reported in laboratory animals consuming high lycopene diets. Apo-8′- and apo-10′-lycopenal were observed in the hepatic tissue of rats consuming a high lycopene diet (Gajic M et al., J Nutr 2006, 136, 1552). These products are not limited to mammalian systems but were also identified in extracts of tomato. Thus, apo-6′-, apo-8′-, apo-10′-, apo-12′-, and apo-14′-lycopenals were detected and quantified in tomato (6.5 mg/100 g) (Kopec RE et al., J Agric Food Chem 2010, 58, 3290), apo-12′-carotenal being the most abundant. The presence of multiple apolycopenals in the plasma of humans consuming tomato juice has been documented. This evidence suggests that these products may in fact be absorbed from the food and not solely a product of metabolism in vivo.
Abscisic acid is an end product of neoxanthin or violaxanthine peroxidation and reduction giving an apocarotenal with a short side chain (5 carbons), followed by a final oxidation into an acid form (Seo M et al., Trends Plant Sci 2002, 7, 41).
Trisporoids, derived from b-carotene, regulate the recognition between mating partners, early sexual morphogenesis and development in zygomycete fungi. They are released from the hyphae (sexual cells), exerting their physiological effects upon compatible mating partners. Trisporic acid and some precursors directly influence the transcription of genes involved in sexual development.
The connection between b-carotene and trisporic acid was early established when trisporic acid was established as a substance produced by mated cultures of strains of Blakeslea trispora and Mucor mucedo (van den Ende H., Nature 1967, 215, 211) and as a metabolite of b-carotene after using 14C labeled molecules (Austin DJ et al., Experientia 1970, 26, 348). As for several other b-carotene derived signal compounds, the first step in trisporoid synthesis is the oxidative cleavage of b-carotene (b-carotene oxygenases).
Strigolactones are signaling molecules that play a double role in the rhizosphere as host detection signals for arbuscular mycorrhizal fungi and root parasitic plants (Bouwmeester HJ et al., Trends Plant Sci 2007, 12, 224). In addition to their important role as rhizosphere signaling compounds, it has recently been demonstrated that strigolactones also act as new hormones inhibiting shoot branching in plants and hence are involved in the regulation of above-ground plant architecture (Gomez-Roldan V et al., Nature 2008, 455, 189; Umehara M et al., Nature 2008, 455, 195; Tsuchiya Y et al., Curr Opin Plant Biol 2009, 12, 556). Since the isolation of strigol as a germination stimulant for Striga lutea from roots of Gossypium hirsutum (Cook CE et al., J Am Chem Soc 1972, 94, 6198), more than ten strigolactones have been identified. The 5-deoxystrigol is considered as the precursor of all other strigolactones (Orobanchol, strigol).
The structural core of the molecules consists of a tricyclic lactone (ABC part) that connects via an enol ether bridge to a butyrolactone group (the D-ring). All other strigolactones possess one or two methyl substituents on the A-ring and various combinations of hydroxyl or acetate substituents around the A- and B-rings. Seven novel strigolactones, named zealactones, have been isolated from maize (Charnikhova TV et al., Phytochemistry 2017, 137, 123).
It has been demonstrated that these terpenoids are derived from the carotenoid pathway (Matusova R et al., Plant Physiol 2005, 139, 920).
The biology of strigolactones has been reviewed (Ruyter-Spira C et al., Trends Plant Sci 2013, 18, 72 and Khosla A et al., Curr Op Plant Biol 2016, 33, 57). Their role in the plant defence system has been reviewed (Marzec M, Tr plant Sci 2016, 21, 900).
Carotenoids may be linked to a sugar by a glycosidic link or an ester link. Such compounds are known for plant and bacteria. As an example, a glucoside of rhodopsin, found in the photosynthetic part of Rhodopseudomonas acidophila, is shown below.
Salinixanthin is a glycosylated and acylated carotenoid associated with the protein xanthorodopsine which was isolated from the extremely halophilic eubacterium Salinibacter ruber living in salt pond in Spain (Lutnaes BF et al., J Nat Prod 2002, 65, 1340). This carotenoid has a keto group in the ring, a glycoside group and an acyl tail, probably immersed in the membrane (Balashov SP et al., Cell Mol Life Sci 2007, 64, 2323).
Astaxanthin diglucoside diesters have been determined in lipid extracts of the snow alga (Chlamydomonas nivalis) (Rezanka T et al., Phytochemistry 2008, 69, 479). The C-3 hydroxyl group of astaxanthin is glucosylated and the C-6 hydroxyl group of the glucosyl moiety is esterified with specific fatty acids. Among these fatty acids (14 species were detected), the most abundant were 16:3, 16:4, 18:1, 18:4 and 18:5.
Carotenoids are said to have antioxidant properties, as tocopherols, and thus may prevent the oxidation of the lipid moieties of LDL (low density lipoproteins) which renders these lipoproteins atherogenic.
To know more about their antioxidant properties, consult the VERIS site
The web site of the International Carotenoid Society may be consulted for further information on carotenoids.
A very informative guide to carotenoid analysis in foods has been released by Rodriguez-Amaya DB.
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