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Terpenoid - Wikipedia
Photosynthesis Questions: Paper - 03 - GELI Question …
It is a little known fact that plastoquinone-9, a vital redox cofactor of photosynthesis, doubles as a precursor for the biosynthesis of a vitamin E analog called plastochromanol-8, the physiological significance of which has remained elusive. Gene network reconstruction, GFP fusion experiments, and targeted metabolite profiling of insertion mutants indicated that Arabidopsis possesses two paralogous solanesyl-diphosphate synthases, AtSPS1 (At1g78510) and AtSPS2 (At1g17050), that assemble the side chain of plastoquinone-9 in plastids. Similar paralogous pairs were detected throughout terrestrial plant lineages but were not distinguished in the literature and genomic databases from mitochondrial homologs involvedin the biosynthesis of ubiquinone. The leaves of the atsps2 knock-out were devoidofplastochromanol-8 and displayed severe lossesofboth non-photoactive and photoactive plastoquinone-9, resulting in near complete photoinhibition at high light intensity. Such a photoinhibition was paralleled by significant damage to photosystem II but not to photosystem I. In contrast, in the atsps1 knock-out, a small loss of plastoquinone-9, restricted to the non-photoactive pool, was sufficient to eliminate half of the plastochromanol-8 content of the leaves. Taken together, these results demonstrate that plastochromanol-8 originates from a subfraction of the non-photoactive pool of plastoquinone-9. In contrast to other plastochromanol-8 biosynthetic mutants, neither the single atsps knock-outs nor the atsps1 atsps2 double knock-out displayed any defectsintocopherols accumulation or germination.
AB - In Photosystem 1 (PS1), phylloquinone (PhQ) acts as a secondary electron acceptor from chlorophyll ec 3 and also as an electron donor to the iron-sulfur cluster F X. PS1 possesses two virtually equivalent branches of electron transfer (ET) cofactors from P 700 to F X, and the lifetime of the semiquinone intermediate displays biphasic kinetics, reflecting ET along the two different branches. PhQ in PS1 serves only as an intermediate in ET and is not normally fully reduced to the quinol form. This is in contrast to PS2, in which plastoquinone (PQ) is doubly reduced to plastoquinol (PQH 2) as the terminal electron acceptor. We purified PS1 particles from the menD1 mutant of Chlamydomonas reinhardtii that cannot synthesize PhQ, resulting in replacement of PhQ by PQ in the quinone-binding pocket. The magnitude of the stable flash-induced P 700 + signal of menD1 PS1, but not wild-type PS1, decreased during a train of laser flashes, as it was replaced by a ∼30 ns back-reaction from the preceding radical pair (P 700 +A 0 -). We show that this process of photoinactivation is due to double reduction of PQ in the menD1 PS1 and have characterized the process. It is accelerated at lower pH, consistent with a rate-limiting protonation step. Moreover, a point mutation (PsaA-L722T) in the PhQ A site that accelerates ET to F X ∼2-fold, likely by weakening the sole H-bond to PhQ A, also accelerates the photoinactivation process. The addition of exogenous PhQ can restore activity to photoinactivated PS1 and confer resistance to further photoinactivation. This process also occurs with PS1 purified from the menB PhQ biosynthesis mutant of Synechocystis PCC 6803, demonstrating that it is a general phenomenon in both prokaryotic and eukaryotic PS1.
fatty alcohols and aldehydes - Lipid
1O2 is produced within the photosystems (PS) from excited chlorophyll molecules in the triplet state,. 1O2 is thought to be the major ROS produced in plant cells at high light intensities and to be instrumental in the execution of ROS-induced cell death in leaves. This ROS has a short lifetime (ca. 100 ns in biological tissues), suggesting a small diffusion path in cells. Consequently, 1O2 reacts primarily in the close vicinity of its production site, and efficient 1O2 detoxification mechanisms must function close to the sites of 1O2 production. Accordingly, thylakoid membranes contain various lipid-soluble compounds that can quench 1O2 within the photosystems and around. Carotenoids are considered to be the first line of defense against 1O2 toxicity because of their high efficiency of 1O2 quenching and their localization in close proximity with the chlorophyll molecules in the light-harvesting complexes and the reaction centers of the photosystems,. However, prenyl lipids of the tocopherol family have also been shown to participate in the protection against 1O2. Tocopherols can quench 1O2, thus protecting PSII from photoinhibition, and can terminate lipid peroxidation chain-reactions, thus protecting the thylakoid membranes,,,,. However, chloroplasts contain other prenyl lipids, such as plastoquinone-9, which could provide additional protection against photooxidative stress. Plastoquinones are viewed essentially as mobile electron carriers involved in electron transfer between PSII and PSI. Through their redox state, there are also recognized as regulators of gene expression and enzyme activities,. However, it has been shown in vitro that plastoquinone-9 also has protective and antioxidant properties, being able to dissipate energy in the chlorophyll antennae, to quench 1O2 and to inhibit oxidation of lipid membranes,,. If this function does occur in vivo, it could be of great physiological importance because plastoquinones are diffusible molecules present in relatively high amounts in the thylakoids (estimated to be ~7 molecules per PSII reaction center, ref. ). Moreover, both the head group and the isoprenoid side chain of plastoquinol are able to quench 1O2, likely making this molecule a better antioxidant than tocopherols. The possible role of plastoquinone-9 in planta as an antioxidant and photoprotector is analyzed here in leaves of the model plant Arabidopsis thaliana. To this end, the plastoquinone biosynthesis pathway was manipulated to generate plants that contain noticeably more plastoquinone-9 than the wild type (WT), and the behavior of those plastoquinone-accumulating plants under high light stress conditions was compared to that of wild-type (WT) plants.
Plastoquinone-9 is known as a photosynthetic electron carrier to which has also been attributed a role in the regulation of gene expression and enzyme activities via its redox state. Here, we show that it acts also as an antioxidant in plant leaves, playing a central photoprotective role. When Arabidopsis plants were suddenly exposed to excess light energy, a rapid consumption of plastoquinone-9 occurred, followed by a progressive increase in concentration during the acclimation phase. By overexpressing the plastoquinone-9 biosynthesis gene SPS1 (SOLANESYL DIPHOSPHATE SYNTHASE 1) in Arabidopsis, we succeeded in generating plants that specifically accumulate plastoquinone-9 and its derivative plastochromanol-8. The SPS1-overexpressing lines were much more resistant to photooxidative stress than the wild type, showing marked decreases in leaf bleaching, lipid peroxidation and PSII photoinhibition under excess light. Comparison of the SPS1 overexpressors with other prenyl quinone mutants indicated that the enhanced phototolerance of the former plants is directly related to their increased capacities for plastoquinone-9 biosynthesis.
Lead toxicity in plants - SciELO
HGA synthesis from hydroxyphenylpyruvate (HPP) is catalyzed by HPP dioxygenase (HPPD) while solanesyl diphosphate is synthesized from geranylgeranyl diphosphate (GGDP) and isopentenyl phosphate (IPP) by a reaction catalyzed by solanesyl diphosphate synthase (SPS). Three SPS enzymes are present in Arabidopsis, SPS1, SPS2 and SPS3,. While SPS3 is a ubiquinone biosynthetic enzyme localized in mitochondria, SPS1 and SPS2 have been recently demonstrated to be targeted to plastids and to be involved in plastoquinone biosynthesis. Previously, SPS1 was supposed to be involved in the synthesis of the side chain of ubiquinone while plastoquinone biosynthesis was believed to be dependent on SPS2 only. However, this view has been challenged by recent observations revealing that a mitochondria-targeted gene (SPS3) different from SPS1 and SPS2 is the main, if not sole, contributor of solanesyl diphosphate synthase activity required for ubiquinone biosynthesis. Moreover, the Arabidopsis mutants AtSPS1 and AtSPS2 were found to be affected in plastoquinone-9 and plastochromanol-8 biosynthesis, not in ubiquinone-9 synthesis. Thus, the current view is that both SPS1 and SPS2 catalyze the elongation of the prenyl side chain of plastoquinone. Plastochromanol-8 has been demonstrated to originate from reduced plastoquinone-9 through the action of VTE1,, a tocopherol cyclase enzyme also involved in the biosynthesis of α-tocopherol from its direct precursor, γ-tocopherol ().
In , we examined the effect of excess light energy on the expression of several genes of the plastoquinone-9 and plastochromanol-8 biosynthesis pathway. One can see that the expression of both SPS1 and SPS2 genes was rapidly induced after transfer of plants aged 4 weeks from low light to high light, with the accumulation of SPS1 transcripts being noticeably more pronounced than that of SPS2. The expression pattern of HPPD was close to that of SPS2, with an induction in high light. In striking contrast, HST expression was not affected by light. The VTE1 and VTE3 genes were also activated by high light but this effect was more progressive and continuous than the up-regulation of SPS1, SPS2 and HPPD. So, the plastoquinone-9 biosynthesis pathway is globally up-regulated by high light, with a marked effect on the SPS1 gene in less than 3h after the transfer from low light to high light. Light induction of the plastoquinone pathway is consistent with early data on the incorporation of radiolabelled tyrosine into prenyl lipids. Upon illumination of Xanthium leaves, incorporation of radioactivity into plastoquinone was observed to be much more pronounced and to occur more rapidly than incorporation into tocopherols.
The Carotenoid Biosynthesis Inhibiting ..
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Plastid terminal oxidase - Wikipedia
PSII photoinhibition is related to 1O2 formation resulting from the interaction between molecular oxygen and the triplet excited state of the reaction center chlorophyll molecule P680 and is linked to the degradation of the D1 protein triggered by the formed 1O2,. The involvement of 1O2 in the damage of the PSII reaction centers can also be indirect by inhibiting synthesis of D1 and impairing the repair processes. Elimination of 1O2 produced in the PSII centers is believed to be fulfilled by the β-carotene molecules located in the PSII reaction center as well as by α-tocopherol,,. However, plastoquinone-9 has been demonstrated to be another potent antioxidant, which is able to quench 1O2in vitro,, and to inhibit lipid peroxidation in model systems. Addition of plastoquinone homologues to Chlamydomonas cultures grown in the presence of plastoquinone biosynthesis inhibitors prevented degradation of D1. Moreover, plastoquinone-9 and plastochromanol-8 incorporated into liposomes were found to be more active than α-tocopherol in the inhibition of 1O2-induced lipid peroxidation, and plastoquinone-9 was observed to be a better quencher of 1O2 than α-tocopherol in solvents. Thus, the dramatic loss of plastoquinone-9 observed in our study in Arabidopsis leaves suddenly exposed to excess light energy () is likely to be related to the 1O2-scavenging activity of this compound which involves oxidation and consumption of the prenyl-lipidic molecule. Accordingly, increased production of 1O2 from the PSII centers in the ch1 mutant was associated with an accelerated loss of plastoquinone-9. It is clear that exhaustion of the available pool of plastoquinones under excess light energy can have important consequences for the PSII repair cycle by precluding PSII reassembly. This phenomenon can exacerbate the inhibition of PSII photochemical activity. However, as shown here, exposure of Arabidopsis plants to high PFDs triggered up-regulation of the plastoquinone biosynthesis pathway, thus enhancing the capacity for plastoquinone-9 synthesis during plant acclimation to high light and compensating for the initial loss of plastoquinone-9 (). This phenomenon led in fine to a strong accumulation of plastoquinone-9 in photoacclimated Arabidopsis plants.
Plastid terminal oxidase or plastoquinol ..
A previous work has shown that constitutive overexpression of HST in Arabidopsis has little effect on the plastoquinone-9 concentration in leaves, suggesting that HST activity is not the limiting step for plastoquinone-9 biosynthesis. Our observation that HST gene expression is not responsive to a condition associated with plastoquinone-9 accumulation could be seen as a fact in line with this suggestion. Considering the strong and rapid expression of SPS1 under conditions that induced plastoquinone-9 accumulation in leaves (,), we decided to overexpress this gene in Arabidopsis. Arabidopsis SPS1 cDNA was inserted under the control of the 35S promoter in a plant binary vector. This vector was used to generate transgenic Arabidopsis plants, and a number of stable lines derived from independent transformation events were obtained. shows a selection of homozygous lines (SPS1oex) exhibiting a strong accumulation of SPS1 transcripts. This transformation had marked effects on plastoquinone-9 and its derivative plastochromanol-8 which accumulated in all lines (). The effect was particularly marked for plastochromanol-8 with an accumulation factor of 2 to 3. The plastoquinone-9 accumulation was less marked, but nevertheless reached approximately 150-170% of the WT level. The reduction state of plastoquinone-9 increased very slightly in the SPS1oex leaves (). In contrast, the α-tocopherol concentration did not change significantly in any of the transformed lines. Similarly, the levels of minor forms of tocopherol (δ- and γ-tocopherol) were not modified by the SPS1 overexpression (data not shown). Thus, constitutive overexpression of SPS1 selectively boosted the plastoquinone/plastochromanol pathway, as expected. Importantly, plastoquinone-9 and plastochromanol-8 accumulation in SPS1oex plants had no effect on growth (), did not modify the quantum yield of photosynthetic electron transport () and did not affect the chlorophyll levels (). This could suggest that the extra plastoquinone-9 molecules that accumulate in the SPS1oex lines are not stored in the thylakoid membranes and are not connected to the photosynthetic electron transport chain. The pool of photoactive plastoquinone-9 (connected to the electron transport chain) was estimated in . The size of this pool did not differ significantly between SPS1oex leaves and WT leaves. In contrast, the pool of non-active plastoquinones was noticeably enlarged in the SPS1-overexpressing leaves, suggesting plastoquinone-9 storage in a compartment different from the thylakoid membranes.
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