Analysis of light-controlled accumulation of carotenoids in mustard (Sinapis alba L.) seedlings

Regulation of Carotenoid Biosynthesis in Photosynthetic Organs

A substantial proportion of the dazzling diversity of colors displayed by living organisms throughout the tree of life is determined by the presence of carotenoids, which most often provide distinctive yellow, orange and red hues. These metabolites play fundamental roles in nature that extend far beyond their importance as pigments. In photosynthetic lineages, carotenoids are essential to sustain life, since they have been exploited to maximize light harvesting and protect the photosynthetic machinery from photooxidative stress. Consequently, photosynthetic organisms have evolved several mechanisms that adjust the carotenoid metabolism to efficiently cope with constantly fluctuating light environments. This chapter will focus on the current knowledge concerning the regulation of the carotenoid biosynthetic pathway in leaves, which are the primary photosynthetic organs of most land plants.

Interdependence of tetrapyrrole metabolism, the generation of oxidative stress and the mitigative oxidative stress response

Tetrapyrroles are involved in light harvesting and light perception, electron-transfer reactions, and as co-factors for key enzymes and sensory proteins. Under conditions in which cells exhibit stress-induced imbalances of photosynthetic reactions, or light absorption exceeds the ability of the cell to use photoexcitation energy in synthesis reactions, redox imbalance can occur in photosynthetic cells. Such conditions can lead to the generation of reactive oxygen species (ROS) associated with alterations in tetrapyrrole homeostasis. ROS accumulation can result in cellular damage and detrimental effects on organismal fitness, or ROS molecules can serve as signals to induce a protective or damage-mitigating oxidative stress signaling response in cells. Induced oxidative stress responses include tetrapyrrole-dependent and -independent mechanisms for mitigating ROS generation and/or accumulation. Thus, tetrapyrroles can be contributors to oxidative stress, but are also essential in the oxidative stress response to protect cells by contributing to detoxification of ROS. In this review, we highlight the interconnection and interdependence of tetrapyrrole metabolism with the occurrence of oxidative stress and protective oxidative stress signaling responses in photosynthetic organisms.

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Tetrapyrroles are involved in light sensing and oxidative stress mitigation.

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Reactive oxygen species (ROS) can form upon light exposure of free tetrapyrroles.

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Tetrapyrrole homeostasis must be tightly regulated to avoid oxidative stress.

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ROS can result in cellular damage or oxidative stress signaling in cells.

Biosynthesis of carotenoids in carrot: an underground story comes to light

Carrot (Daucus carota) is a biannual plant that accumulates massive amounts of carotenoid pigments in the storage root. Although the root of carrot plants was white before domestication, intensive breeding generated the currently known carotenoid-rich varieties, including the widely popular orange carrots that accumulate very high levels of the pro-vitamin A carotenoids β-carotene and, to a lower extent, α-carotene. Recent studies have shown that the developmental program responsible for the accumulation of these health-promoting carotenes in underground roots can be completely altered when roots are exposed to light. Illuminated root sections do not enlarge as much as dark-grown roots, and they contain chloroplasts with high levels of lutein instead of the β-carotene-rich chromoplasts found in underground roots. Analysis of carotenoid gene expression in roots either exposed or not to light has contributed to better understand the contribution of developmental and environmental cues to the root carotenoid profile. In this review, we summarize the main conclusions of this work in the context of our current knowledge of how carotenoid biosynthesis and accumulation is regulated at transcriptional and post-transcriptional levels in carrot roots and other model systems for the study of plant carotenogenesis such as Arabidopsis de-etiolation and tomato fruit ripening.

Carotenoid biosynthesis in Arabidopsis: a colorful pathway

Plant carotenoids are a family of pigments that participate in light harvesting and are essential for photoprotection against excess light. Furthermore, they act as precursors for the production of apocarotenoid hormones such as abscisic acid and strigolactones. In this review, we summarize the current knowledge on the genes and enzymes of the carotenoid biosynthetic pathway (which is now almost completely elucidated) and on the regulation of carotenoid biosynthesis at both transcriptional and post-transcriptional levels. We also discuss the relevance of Arabidopsis as a model system for the study of carotenogenesis and how metabolic engineering approaches in this plant have taught important lessons for carotenoid biotechnology.

Effects of exogenous glucose on carotenoid accumulation in tomato leaves

To investigate the effect of carbohydrate on carotenoid accumulation in leaves, excised plants of tomato (Lycopersicum esculentum var. cerasiformae, wva 106) were supplied with glucose through the transpiration stream for 48 h. We report here that sugar accumulation in leaves led to a decrease of carotenoid content, which was related to the reduction of Chl. The decrease in carotenoid amount correlated with a sugar-induced repression of genes encoding enzymes of the carotenoid and of the Rohmer pathways. The lower 1-deoxy-D-xylulose-5-phosphate synthase transcript level probably leads to a decreased metabolic flux through the methylerythritol pathway and subsequently to a lower amount of substrate available for plastidic isoprenoid synthesis. Differences between responses of young (sink) and mature (source) leaves to carbohydrate accumulation are discussed.

Biliverdin reductase-induced phytochrome chromophore deficiency in transgenic tobacco

Targeted expression of mammalian biliverdin IXalpha reductase (BVR), an enzyme that metabolically inactivates linear tetrapyrrole precursors of the phytochrome chromophore, was used to examine the physiological functions of phytochromes in the qualitative short-day tobacco (Nicotiana tabacum cv Maryland Mammoth) plant. Comparative phenotypic and photobiological analyses of plastid- and cytosol-targeted BVR lines showed that multiple phytochrome-regulated processes, such as hypocotyl and internode elongation, anthocyanin synthesis, and photoperiodic regulation of flowering, were altered in all lines examined. The phytochrome-mediated processes of carotenoid and chlorophyll accumulation were strongly impaired in plastid-targeted lines, but were relatively unaffected in cytosol-targeted lines. Under certain growth conditions, plastid-targeted BVR expression was found to nearly abolish the qualitative inhibition of flowering by long-day photoperiods. The distinct phenotypes of the plastid-targeted BVR lines implicate a regulatory role for bilins in plastid development or, alternatively, reflect the consequence of altered tetrapyrrole metabolism in plastids due to bilin depletion.

Formation of isopentenyl diphosphate via mevalonate does not occur within etioplasts and etiochloroplasts of mustard (Sinapis alba L.) seedlings

Seedlings from the white mustard, Sinapis alba, grown under continuous far-red light exhibit enhanced plastid enzyme activities when compared with dark-grown seedlings (for review, see Mohr 1981). These activities are even more pronounced upon illumination with white light during the etioplast/chloroplast transformation. Etioplasts and etiochloroplasts from the cotyledons of such seedlings show high prenyl-lipid-synthesizing activities when [1-(14)C]isopentenyl diphosphate is used as the precursor. They lack, however, any enzymatic activities for the formation of isopentenyl diphosphate via the mevalonate pathway, i.e. hydroxymethylglutaryl-CoA reductase, mevalonate kinase, phosphomevalonate kinase and diphosphomevalonate decarboxylase, which are present and easily detectable within the endoplasmic reticulum and cytoplasm. These results corroborate the view that the cytoplasm of the plant cell is the only site of isopentenyl-diphosphate formation via the mevalonate pathway.

Carotenoid and chlorophyll biosynthesis in isolated plastids from mustard seedling cotyledons (Sinapis alba L.) during etioplast-chloroplast conversion

Etioplasts and etiochloroplasts, isolated from seedlings of white mustard (Sinapis alba L.) grown in continuous far-red light, and chloroplasts isolated from cotyledons and primary leaves of white-light-grown seedlings exhibit high prenyl-lipid-forming activities. Only the etioplasts and etiochloroplasts, and to a much lesser extent chloroplasts from cotyledons are capable of forming carotenes from isopentenyl diphosphate as substrate, whereas in chloroplasts from primary leaves no such activities could be detected. By subfractionation experiments, it could be demonstrated that the phytoene-synthase complex in etioplasts and etiochloroplasts is present in a soluble form in the stroma, whereas the subsequent enzymes, i.e. the dehydrogenase, cis-trans isomerase and cyclase are bound to both membrane fractions, the prolamellar bodies/prothylakoids and the envelopes. In good agreement with previous results using isolated chromoplasts and chloroplasts, it is concluded that the phytoene-synthase complex may change its topology from a peripheral membrane protein in non-green plastids to a tightly membrane-associated protein in chloroplasts. This change is apparently paralleled by altered functional properties which render the complex undetectable in isolated chloroplasts. Further experiments concerning the reduction of chlorophyll a containing a geranylgeranyl side chain to chlorophyll a indicate that the light-induced etioplast-chloroplast conversion is accompanied by a certain reorganization of the polyprenoid-forming enzymatic equipment.

Carotenoid composition in milo (Sorghum vulgare) shoots as affected by phytochrome and chlorophyll

The composition of coloured carotenoids in the milo shoot was investigated quantitatively (high performance liquid chromatography) during light-mediated plastidogenesis, including the time span of 'photodelay' as caused by medium and high light fluxes. It was found that as long as only the far-red-absorbing form of phytochrome operates, the carotenoid pattern remains virtually the same as in complete darkness (violaxanthin and lutein as major constituents, traces of β-carotene). On the other hand, the pattern changes dramatically in white or red light with increasing amounts of chlorophyll (lutein and β-carotene dominate, β-carotene showing the strongest relative increase). Photodelay during the early phase of plastidogenesis affects the carotenoid composition strongly. Increase of neoxanthin, violaxanthin and β-carotene contents are diminished while lutein accumulation proves resistant towards chlorophyll-mediated photoinhibition. The photodelay can be diminished by an appropriate light pretreatment. The data indicate that light-mediated control over carotenoid accumulation is exerted at three levels: i) a coarse control through phytochrome, ii) fine tuning in connection with chlorophyll accumulation, iii) stabilization of holocomplexes against photodecomposition.

Prenyl lipid formation in spinach chloroplasts and in a cell-free system of Synechococcus (Cyanobacteria): polyprenols, chlorophylls, and fatty acid prenyl esters

Isolated chloroplasts from spinach leaf cells, chloroplast subfractions, and a cell-free system of the cyanobacterium Synechococcus CCAP 6312 incorporated [1-(14)C]isopentenyl pyrophosphate in high yields into prenyl lipids. Products were polyprenols (C20, C45) chlorophylls, quinoid compounds, and fatty acid prenyl esters; prenyl pyrophosphates occurred in trace amounts, and carotenes were only formed to a limited extent in the Synechococcus system. The formation of fatty acid prenyl esters, which is described here for the first time, was found to occur in two different ways in the chloroplast system; by an acyl-CoA: polyprenol acyltransferase reaction associated with the envelope membranes and by a transesterification reaction from chlorophyll associated with the thylakoids. Endogenous fatty acid prenyl esters made up about 3% by weight of total lipids in spinach chloroplasts and were also found to be natural constituents of the cyanobacterial cells.

Ribulose bisphosphate carboxylase capacity and chlorophyll content in developing seedlings of Chenopodium rubrum L. growing under light of different qualities and fluence rates

In order to evaluate the aclimation of Chenopodium seedlings to different quantum fluence rates of R and BL, kinetics of Rubisco capacity, Chl content and chloroplast structure were studied. Under monochromatic light photoreceptors are stimulated selectively and their influence on biosynthetis capacities during chloroplast development can be studied.R irradiations saturate Rubisco capacity even at the lowest quantum fluence rates applied, whereas Chl a+b synthesis depends strongly upon fluence rate of R. Under BL irradiations, both Rubisco capacity and Chl content are fluence rate dependent. R irradiations favour Chl b synthesis relative to Chl a, whereas under BL Chl a content is high relative to Chl b. Under R irradiation Pfr is the main photoreceptor involved in regulation of Rubisco capacity whereas under BL a specific BL absorbing photoreceptor may control the response. From the fluence rate dependency under BL irradiations it is concluded that the blue region of the day light spectrum may be the sensor for monitoring fluence rate and causing the characteristic changes in shade and high/low WL adaptation with respect to Rubisco levels in Chenopodium.

Photooxidative destruction of chloroplasts and its consequences for cytosolic enzyme levels and plant development

Mustard (Sinapis alba L.) seedlings were grown in the presence of herbicides (Difunon, Norflurazon) which inhibit carotenoid synthesis without affecting development, in darkness or in continuous far-red light. In strong white light (12,000 lx) the cotyledons of the herbicide-treated seedlings did not contain normal chloroplasts, but only small chlorophyll-free rudiments whose internal structure had almost disappeared. The plastid marker enzyme NADP-dependent glyceraldehyde-3-phosphate dehydrogenase was almost lacking. Plastid ribosomes and ribosomal RNAs were no longer detectable nor could synthesis of mature plastidal ribosomal RNAs be detected. Cytosolic ribosomes and rRNAs were not affected. Plastid DNA was apparently still intact as shown by restriction analysis. The appearance of marker enzymes of glyoxisomes, mitochondria and cytosol was not impaired while the level of marker enzymes of peroxisomes was drastically lowered. Accumulation of anthocyanin in mustard cotyledons was normal after a short, transient delay. Levels of representative enzymes of flavonoid biogenesis (phenylalanine ammonia-lyase, chalcone synthase) were somewhat increased rather than inhibited in the cotyledons of herbicide-treated, white-light-grown seedlings. The growth rate of hypocotyl and cotyledons was inhibited to the same extent in the herbicide-treated, white-light-grown seedling, although light inhibits growth of hypocotyls and promotes growth of cotyledons. Analysis of the data shows that photomorphogenesis of a herbicide-treated, white-light-grown seedling is normal, and is thus independent of plastid gene expression However, a 'factor' which coacts multiplicatively with phytochrome in determining the growth rate of the organs seems to originate from the plastids. Biogenesis of anthocyanin and synthesis of major enzymes of the flavonoid pathway are not affected adversely by a photooxidative elimination of plastid gene expression.

Coaction of light and cytokinin in photomorphogenesis

Intact mustard seedlings were treated with zeatin and photomorphogenetically active light in different ways: (1) hormone treatment preceding light treatment, (2) light treatment preceding hormone treatment, (3) hormone and light applied simultaneously. Under all experimental conditions the effect of the hormone treatment is multiplicative to the light effect with regard to the increase of cotyledon area. However, the hormone effect is additive to the light effect with regard to increases of the level of NADPH-dependent glyceraldehyde-3-phosphate dehydrogenase (EC 1.2.1.13) and carotenoid contents. Anthocyanin synthesis is inhibited by exogenous zeatin whereby the concentration response curves are similar, irrespective of the extent of anthocyanin formation mediated by light. However, an interaction was found in the sense that the responsiveness toward zeatin is decreased somewhat by the action of phytochrome. Our results show that the responsiveness to light (via the far-red-absorbing form of phytochrome; P fr) is not changed by a preceding or simultaneous hormone treatment. Moreover, the responsiveness of the plant to exogenously applied zeatin is not affected - except in anthocyanin synthesis - by a preceding or simultaneous light treatment. We conclude from our results that the action of phytochrome on the developmental processes is not related to cytokinin levels.

Phytochrome-mediated delay of plastid senescence in mustard cotyledons: changes in pigment contents and ultrastructure

Changes in pigment contents and ultrastructure have followed in cotyledons of mustard (Sinapis alba L.) seedlings during dark-mediated senescence. The seedlings were kept in white light for 7 d, treated with 5 min long wavelength far-red light and then kept in darkness up to 14 d after sowing. Under these conditions the chloroplasts remain stable for 2 d before a sequential plastidal disintegration commences. The data indicate a selective breakdown of the light-harvesting chlorophyll a/b protein. Phytochrome retards the differential loss of chlorophyll a, b and carotenoids and preserves the fine structure of chloroplasts.

Action of light on accumulation of carotenoids and chlorophylls in the milo shoot (Sorghum vulgare Pers.)

We have performed a comprehensive study on the mechanism of regulation of carotenogenesis by light in the shoot of Sorghum vulgare. Our work shows that carotenoid accumulation is simultaneously controlled by phytochrome (Pfr) and by the availability of chlorophyll. Throughout plastidogenesis light dependent chlorophyll and carotenoid accumulation are interdependent processes: Accumulation of chlorophyll in natural light requires the presence of carotenoids; likewise, accumulation of considerable amount of carotenoids depends on the availability of chlorophyll. However, in both cases the efficiency of the biosynthetic pathway, the potential biosynthetic rates ("capacities") are determined by phytochrome. A "push and pull" model of carotenogenesis advanced previously (Frosch and Mohr 1980, Planta 148, 279) to explain carotenogenesis in the mustard (Sinapis alba) seedling also applies to the monocotyledonous milo (Sorghum vulgare) seedling. Therefore, we suggest that the model applies to carotenogenesis in higher plants in general.