Strigolactones' ability to regulate root development may be executed by induction of the ethylene pathway

The newly defined phytohormones strigolactones (SLs) were recently shown to act as regulators of root development. Their positive effect on root-hair (RH) elongation enabled examination of their cross talk with auxin and ethylene. Analysis of wild-type plants and hormone-signaling mutants combined with hormonal treatments suggested that SLs and ethylene regulate RH elongation via a common regulatory pathway, in which ethylene is epistatic to SLs. The SL and auxin hormonal pathways were suggested to converge for regulation of RH elongation; this convergence was suggested to be mediated via the ethylene pathway, and to include regulation of auxin transport.Key words: strigolactone, auxin, ethylene, root, root hair, lateral rootStrigolactones (SLs) are newly identified phytohormones that act as long-distance shoot-branching inhibitors (reviewed in ref. ¹). In Arabidopsis, SLs have been shown to be regulators of root development and architecture, by modulating primary root elongation and lateral root formation.^2,3 In addition, they were shown to have a positive effect on root-hair (RH) elongation.² All of these effects are mediated via the MAX2 F-box.^2,3In addition to SLs, two other plant hormones, auxin and ethylene, have been shown to affect root development, including lateral root formation and RH elongation.^4–6 Since all three phytohormones (SLs, auxin and ethylene) were shown to have a positive effect on RH elongation, we examined the epistatic relations between them by examining RH length.⁷ Our results led to the conclusion that SLs and ethylene are in the same pathway regulating RH elongation, where ethylene may be epistatic to SLs.⁷ Moreover, auxin signaling was shown to be needed to some extent for the RH response to SLs: the auxin-insensitive mutant tir1-1,⁸ was less sensitive to SLs than the wild type under low SL concentrations.⁷On the one hand, ethylene has been shown to induce the auxin response,^9–12 auxin synthesis in the root apex,^11,12 and acropetal and basipetal auxin transport in the root.^4,13 On the other, ethylene has been shown to be epistatic to SLs in the SL-induced RH-elongation response.⁷ Therefore, it might be that at least for RH elongation, SLs are in direct cross talk with ethylene, whereas the cross talk between SL and auxin pathways may converge through that of ethylene.⁷ The reduced response to SLs in tir1-1 may be derived from its reduced ethylene sensitivity;^7,14 this is in line with the notion of the ethylene pathway being a mediator in the cross talk between the SL and auxin pathways.The suggested ethylene-mediated convergence of auxin and SLs may be extended also to lateral root formation, and may involve regulation of auxin transport. In the root, SLs have been suggested to affect auxin efflux,^3,15 whereas ethylene has been shown to have a positive effect on auxin transport.^4,13 Hence, it might be that in the root, the SLs'' effect on auxin flux is mediated, at least in part, via the ethylene pathway. Ethylene''s ability to increase auxin transport in roots was associated with its negative effect on lateral root formation: ethylene was suggested to enhance polar IAA transport, leading to alterations in the quantity of auxin that unloads into the tissues to drive lateral root formation.⁴ Under conditions of sufficient phosphate, SL''s effect was similar to that of ethylene: SLs reduced the appearance of lateral roots; this was explained by their ability to change auxin flux.³ Taken together, one possibility is that the SLs'' ability to affect auxin flux and thereby lateral root formation in the roots is mediated by induction of ethylene synthesis.To conclude, root development may be regulated by a network of auxin, SL and ethylene cross talk.⁷ The possibility that similar networks exist elsewhere in the SLs'' regulation of plant development, including shoot architecture, cannot be excluded.     

Timing the switch to phototrophic growth: A possible role of GUN1

In young Arabidopsis seedlings, retrograde signaling from plastids regulates the expression of photosynthesis-associated nuclear genes in response to the developmental and functional state of the chloroplasts. The chloroplast-located PPR protein GUN1 is required for signalling following disruption of plastid protein synthesis early in seedling development before full photosynthetic competence has been achieved. Recently we showed that sucrose repression and the correct temporal expression of LHCB1, encoding a light-harvesting chlorophyll protein associated with photosystem II, are perturbed in gun1 mutant seedlings.¹ Additionally, we demonstrated that in gun1 seedlings anthocyanin accumulation and the expression of the “early” anthocyanin-biosynthesis genes is perturbed. Early seedling development, predominantly at the stage of hypocotyl elongation and cotyledon expansion, is also affected in gun1 seedlings in response to sucrose, ABA and disruption of plastid protein synthesis by lincomycin. These findings indicate a central role for GUN1 in plastid, sucrose and ABA signalling in early seedling development.Key words: ABA, ABI4, anthocyanin, chloroplast, GUN1, retrograde signalling, sucroseArabidopsis seedlings develop in response to light and other environmental cues. In young seedlings, development is fuelled by mobilization of lipid reserves until chloroplast biogenesis is complete and the seedlings can make the transition to phototrophic growth. The majority of proteins with functions related to photosynthesis are encoded by the nuclear genome, and their expression is coordinated with the expression of genes in the chloroplast genome. In developing seedlings, retrograde signaling from chloroplasts to the nucleus regulates the expression of these nuclear genes and is dependent on the developmental and functional status of the chloroplast. Two classes of gun (genomes uncoupled) mutants defective in retrograde signalling have been identified in Arabidopsis: the first, which comprises gun2–gun5, involves mutations in genes encoding components of tetrapyrrole biosynthesis.^2,3 The other comprises gun1, which has mutations in a nuclear gene encoding a plastid-located pentatricopeptide repeat (PPR) protein with an SMR (small MutS-related) domain near the C-terminus.^4,5 PPR proteins are known to have roles in RNA processing⁶ and the SMR domain of GUN1 has been shown to bind DNA,⁴ but the specific functions of these domains in GUN1 are not yet established. However, GUN1 has been shown to be involved in plastid gene expression-dependent,⁷ redox,⁴ ABA1,⁴ and sucrose signaling,^1,4,8 as well as light quality and intensity sensing pathways.^9–11 In addition, GUN1 has been shown to influence anthocyanin biosynthesis, hypocotyl extension and cotyledon expansion.^1,11     

Ethylene-Stimulated Nutations Do Not Require ETR1 Receptor Histidine Kinase Activity

Ethylene influences the growth and development of plants through the action of receptors that have homology to bacterial two-component receptors. In bacteria these receptors function via autophosphorylation of a His residue in the kinase domain followed by phosphotransfer to a conserved Asp residue in a response regulator protein. In Arabidopsis, two of the five receptor isoforms are capable of His kinase activity. However, the role of His kinase activity and phosphotransfer is unclear in ethylene signaling. A previous study showed that ethylene stimulates nutations of the hypocotyl in etiolated Arabidopsis seedlings that are dependent on the ETR1 receptor isoform. The ETR1 receptor is the only isoform in Arabidopsis that contains both a functional His kinase domain and a receiver domain for phosphotransfer. Therefore, we examined the role that ETR1 His kinase activity and phosphotransfer plays in ethylene-stimulated nutations.Key Words: ethylene, nutations, signal transduction, receptors, histidine kinase, phosphotransfer, two component signallingThe gaseous plant hormone ethylene has a role in a variety of physiological events in higher plants such as seed germination, abscission, senescence, fruit ripening, and growth regulation.¹ In etiolated Arabidopsis seedlings, ethylene causes reduced growth of the hypocotyl and root, increased radial expansion of the hypocotyl, and increased tightening of the apical hook.^2,3Previous studies have identified components in the ethylene signaling pathway and led to an inverse-agonist model for signal transduction.^4,5 According to this model, responses to ethylene are mediated by a family of five receptors (ETR1, ERS1, ETR2, EIN4, ERS2) in Arabidopsis that have homology to bacterial two-component receptors.^6–9 In bacterial systems, two-component receptors transduce signal via the autophosphorylation of a His residue in the kinase domain, followed by the transfer of phosphate to a conserved Asp residue in the receiver domain of a response regulator protein.¹⁰ The ethylene receptors of plants can be divided into two subfamilies based on sequence homology in the ethylene-binding domains.¹¹ ETR1 and ERS1 belong to subfamily I, contain all amino acid residues needed for His kinase activity,^6,12 and show His kinase activity in vitro.^13,14 ETR2, EIN4, and ERS2 belong to subfamily II, contain degenerate His kinase domains^7,9 and have Ser/Thr kinase activity in vitro.¹⁴ ERS1 shows both His and Ser/Thr kinase activities in vitro depending on the assay conditions used.¹⁴ While the kinase domain of ETR1 appears to be required for signaling,¹⁵ kinase activity is not.^15–17 It is unclear whether or not histidine kinase activity is involved in ethylene signaling, although, this activity might be involved in growth recovery after ethylene removal.¹⁷Recently, high-resolution, time-lapse imaging revealed that prolonged treatment with ethylene stimulates nutational bending of etiolated Arabidopsis hypocotyls.¹⁸ Nutations are oscillatory bending movements caused by localized differential growth¹⁹ that were originally termed “circumnutations”.²⁰ Nutations have been posited to be important for seedlings to penetrate through the soil²⁰ and thus could be critical for seedling survival. In support of this hypothesis, nutations of rice roots have been reported to increase soil penetration.²¹Mutational analysis revealed that many of the known ethylene signaling components including CTR1, EIN2, EIN3 and EIL1 are involved in signaling leading to ethylene-stimulated nutations.¹⁸ Surprisingly, loss-of-function mutations in ETR1 eliminated ethylene-stimulated nutations while combinatorial loss-of-function mutations in the other four receptor isoforms led to constitutive nutations in air.¹⁸ These results support a model where all the receptors are involved in ethylene-stimulated nutations but the ETR1 receptor is required for and has a contrasting role from the other receptor isoforms in this nutation phenotype. Since the ETR1 receptor is the only receptor isoform that contains both a functional His-kinase domain and a receiver domain,^6,13,14 the roles of His kinase activity and phosphorelay in the nutation phenotype were examined in the current study.Previous work showed that the nutation phenotype in etr1-7 loss-of-function mutants could be rescued with a wild-type, genomic ETR1 transgene.¹⁸ Etr1-7 mutants transformed with a kinase-inactivated genomic ETR1 transgene (gETR1 (G₂)) where the two conserved glycines in the G₂ box of the histidine kinase domain (G545, G547) were changed to alanines were examined to determine if ETR1 His kinase activity is required for ethylene-stimulated nutations. This construct lacks histidine autophosphorylation in vitro.²² Figure 1 shows that ethylene stimulates nutations in etr1-7 gETR1(G₂) seedlings. The period of these nutations was 4.7 ± 1.5 h which is similar to values obtained previously for wild-type seedlings (4.7 ± 1h) and somewhat longer than etr1-7 seedlings transformed with wild-type, genomic ETR1 (3.2 ± 0.6 h). However, the amplitude of these nutations (3.7 ± 1.0°) was approximately half that of nutations previously observed in wild-type seedlings (9.1 ± 6.0°) as well as etr1-7 seedlings transformed with wild-type, genomic ETR1 (8.2 ± 3.6°). This suggests that ETR1 histidine kinase activity is not required for ethylene-stimulated nutations but might have a role in modulating nutation amplitudes.Open in a separate windowFigure 1Ethylene stimulates nutations of etr1-7 seedlings transformed with a kinase-inactivated ETR1 transgene. The hypocotyl angles for four etr1-7 mutants transformed with a kinase-inactivated genomic ETR1 transgene (gETR1(G₂)) are shown. Transformants were obtained from Eric Schaller and have been described previously.²² In this and the following figure, etiolated Arabidopsis seedlings were imaged from the side at 15 min intervals while growing along a vertically orientated agar plate and the hypocotyl angle measured as described previously.¹⁸ Black and gray lines are used to help distinguish the movements of individual seedlings. All seedlings were grown in the presence of 5 µM AVG to block biosynthesis of ethylene by the seedlings. Seedlings were grown in air for 2 h prior to treatment with 10 µL L⁻¹ ethylene (Open in a separate window).To determine whether phosphotransfer through the receiver domain of ETR1 is required for the nutation phenotype, seedlings deficient in ethylene receptor isoforms containing a receiver domain (ETR1, ETR2, EIN4) were transformed with a mutant ETR1 transgene lacking the conserved Asp₆₅₉ required for phosphotransfer (getr1-[D]). Previous work showed that etr1-6 etr2-3 ein4-4 triple loss-of-function mutant seedlings failed to nutate and this nutation phenotype could be rescued when these mutants were transformed with wild-type, genomic ETR1 transgene.¹⁸ Similarly, transformation of the etr1-6 etr2-3 ein4-4 triple mutants with getr1-[D] rescued the nutation phenotype in most seedlings observed (Fig. 2). However, some seedlings (four of the eleven observed) failed to nutate. The reason for this variable rescue is unclear but could reflect differences in expression levels of the mutant transgene in individual plants. Alternatively, this variable rescue could reflect functional differences between the mutant and wild-type transgene suggesting a modulating role for phosphotransfer through the receiver domain of ETR1. Two independent lines were observed with similar results. Of those that did nutate, the period of nutations was 5.0 ± 1.2 h and the amplitude 7.6 ± 3.8° which is similar to values obtained previously for wild-type plants as well as plants transformed with a wild-type, genomic ETR1 transgene.¹⁸Open in a separate windowFigure 2Ethylene stimulates nutations of etr1-6 etr2-3 ein4-4 seedlings transformed with an ETR1 transgene mutated at Asp₆₅₉. The hypocotyl angles from seven etr1-6 etr2-3 ein4-4 triple mutants transformed with an ETR1 transgene mutated at Asp₆₅₉ (getr1[D]) are shown in two panels. One seedling in (A) (black) had no measurable nutations while one in (B) (black) had very small nutations.Conclusions from this and the previous study are that the ETR1 receptor has a unique role in ethylene-stimulated nutations. However, this role does not require either histidine kinase activity or phosphotransfer through the receiver domain of ETR1.     

Cell Migration from Baby to Mother

Fetal cells migrate into the mother during pregnancy. Fetomaternal transfer probably occurs in all pregnancies and in humans the fetal cells can persist for decades. Microchimeric fetal cells are found in various maternal tissues and organs including blood, bone marrow, skin and liver. In mice, fetal cells have also been found in the brain. The fetal cells also appear to target sites of injury. Fetomaternal microchimerism may have important implications for the immune status of women, influencing autoimmunity and tolerance to transplants. Further understanding of the ability of fetal cells to cross both the placental and blood-brain barriers, to migrate into diverse tissues, and to differentiate into multiple cell types may also advance strategies for intravenous transplantation of stem cells for cytotherapeutic repair. Here we discuss hypotheses for how fetal cells cross the placental and blood-brain barriers and the persistence and distribution of fetal cells in the mother.Key Words: fetomaternal microchimerism, stem cells, progenitor cells, placental barrier, blood-brain barrier, adhesion, migrationMicrochimerism is the presence of a small population of genetically distinct and separately derived cells within an individual. This commonly occurs following transfusion or transplantation.^1–3 Microchimerism can also occur between mother and fetus. Small numbers of cells traffic across the placenta during pregnancy. This exchange occurs both from the fetus to the mother (fetomaternal)^4–7 and from the mother to the fetus.^8–10 Similar exchange may also occur between monochorionic twins in utero.^11–13 There is increasing evidence that fetomaternal microchimerism persists lifelong in many child-bearing women.^7,14 The significance of fetomaternal microchimerism remains unclear. It could be that fetomaternal microchimerism is an epiphenomenon of pregnancy. Alternatively, it could be a mechanism by which the fetus ensures maternal fitness in order to enhance its own chances of survival. In either case, the occurrence of pregnancy-acquired microchimerism in women may have implications for graft survival and autoimmunity. More detailed understanding of the biology of microchimeric fetal cells may also advance progress towards cytotherapeutic repair via intravenous transplantation of stem or progenitor cells.Trophoblasts were the first zygote-derived cell type found to cross into the mother. In 1893, Schmorl reported the appearance of trophoblasts in the maternal pulmonary vasculature.¹⁵ Later, trophoblasts were also observed in the maternal circulation.^16–20 Subsequently various other fetal cell types derived from fetal blood were also found in the maternal circulation.^21,22 These fetal cell types included lymphocytes,²³ erythroblasts or nucleated red blood cells,^24,25 haematopoietic progenitors^7,26,27 and putative mesenchymal progenitors.^14,28 While it has been suggested that small numbers of fetal cells traffic across the placenta in every human pregnancy,^29–31 trophoblast release does not appear to occur in all pregnancies.³² Likewise, in mice, fetal cells have also been reported in maternal blood.^33,34 In the mouse, fetomaternal transfer also appears to occur during all pregnancies.³⁵     

Hypoxia: A novel function for VIN3

VERNALIZATION INSENSITIVE 3 (VIN3) encodes a PHD domain chromatin remodelling protein that is induced in response to cold and is required for the establishment of the vernalization response in Arabidopsis thaliana.¹ Vernalization is the acquisition of the competence to flower after exposure to prolonged low temperatures, which in Arabidopsis is associated with the epigenetic repression of the floral repressor FLOWERING LOCUS C (FLC).^2,3 During vernalization VIN3 binds to the chromatin of the FLC locus,¹ and interacts with conserved components of Polycomb-group Repressive Complex 2 (PRC2).^4,5 This complex catalyses the tri-methylation of histone H3 lysine 27 (H3K27me3),^4,6,7 a repressive chromatin mark that increases at the FLC locus as a result of vernalization.^4,7–10 In our recent paper¹¹ we found that VIN3 is also induced by hypoxic conditions, and as is the case with low temperatures, induction occurs in a quantitative manner. Our experiments indicated that VIN3 is required for the survival of Arabidopsis seedlings exposed to low oxygen conditions. We suggested that the function of VIN3 during low oxygen conditions is likely to involve the mediation of chromatin modifications at certain loci that help the survival of Arabidopsis in response to prolonged hypoxia. Here we discuss the implications of our observations and hypotheses in terms of epigenetic mechanisms controlling gene regulation in response to hypoxia.Key words: arabidopsis, VIN3, FLC, hypoxia, vernalization, chromatin remodelling, survival     

RALFs: Peptide regulators of plant growth

Peptide signaling regulates a variety of developmental processes and environmental responses in plants.^1–6 For example, the peptide systemin induces the systemic defense response in tomato⁷ and defensins are small cysteine-rich proteins that are involved in the innate immune system of plants.^8,9 The CLAVATA3 peptide regulates meristem size¹⁰ and the SCR peptide is the pollen self-incompatibility recognition factor in the Brassicaceae.^11,12 LURE peptides produced by synergid cells attract pollen tubes to the embryo sac.⁹ RALFs are a recently discovered family of plant peptides that play a role in plant cell growth.Key words: peptide, growth factor, alkalinization     

The galactolipid monogalactosyldiacylglycerol (MGDG) contributes to photosynthesis-related processes in Arabidopsis thaliana

Processes putatively dependent on the galactolipid monogalactosyldiacylglycerol (MGDG) were recently studied using the knockdown monogalactosyldiacylglycerol synthase 1 (mgd1-1) mutant (∼40% reduction in MGDG). Surprisingly, targeting of chloroplast proteins was not affected in mgd1-1 mutants, suggesting they retain sufficient MGDG to maintain efficient targeting. However, in dark-grown mgd1-1 plants the photoactive to photoinactive protochlorophyllide (Pchlide) ratio was increased, suggesting that photoprotective responses are induced in them. Nevertheless, mgd1-1 could not withstand high light intensities, apparently due to impairment of another photoprotective mechanism, the xanthophyll cycle (and hence thermal dissipation). This was mediated by increased conductivity of the thylakoid membrane leading to a higher pH in the thylakoid interior, which impaired the pH-dependent activation of violaxanthin de-epoxidase (VDE) and PsbS. These findings suggest that MGDG contribute directly to the regulation of photosynthesis-related processes.Key words: conductivity, galactolipid, light stress, photosynthesis, plastid, xanthophyllThe galactolipid monogalactosyldiacylglycerol (MGDG), the major lipid in plastids,¹ is mainly synthesised in inner plastid envelopes,² where monogalactosyldiacylglycerol synthase 1 (MGD1) catalyses the last step of its production.³ Two MGDG-deficient mutants are known: the knockdown mgd1-1 mutant, which accumulates ∼40% less MGDG than wild type,⁴ and the null mutant mgd1-2, which displays extremely severe defects in chloroplast and plant development.⁵ Thus, the mgd1-1 mutant is more suitable for assessing putative roles of MGDG in processes such as protein targeting and photoprotection.There are conflicting indications regarding the involvement of galactolipids in chloroplast protein targeting: some suggest they play an important role,^6–10 but not all.^11,12 The data recently collected for mgd1-1 do not support MGDG''s involvement in protein targeting, since (inter alia) the level of MGDG in mgd1-1 mutants is clearly sufficient for efficient targeting.¹³ Further, the galactolipid associated with the TOC complex¹² is digalactosyldiacylglycerol (DGDG) and the digalactosyldiacylglycerol synthase 1 (dgd1) mutant,¹⁴ which has ∼10% of wild-type levels of DGDG, has impaired import efficiency.^15,16 Hence, this may indicate that DGDG is relatively more important for chloroplast import than MGDG.The prolamellar bodies (PLBs) of etioplasts have high lipid-to-protein ratios compared to thylakoids. Their major lipid and protein are MGDG and NADPH:Pchlide oxidoreductase (POR), respectively,¹⁷ and MGDG putatively plays an important role, interactively with POR, in the formation of PLBs.^18–20 The transformation of PLBs into thylakoids involves phototransformation of photoactive Pchlide (F656), a precursor of chlorophyll. Non-photoactive Pchlide (F631) is susceptible to photooxidative damage, but POR is believed to suppress this.^21,22 After excitation at 440 nm, mgd1-1 mutants display distinctly higher fluorescence emission peaks corresponding to photoactive Pchlide than wild type counterparts and (hence) higher photoactive:non-photoactive Pchlide ratios.¹³ These changes may be photoprotective responses that favour formation of photoactive Pchlide and optimize the plants'' opportunities to use light for chlorophyll production, enabling the transformation of etioplasts into chloroplasts.Interestingly,the xanthophyll cycle, another photoprotective mechanism, is impaired in mgd1-1.¹³ Normally, the xanthophyll cycle pigment violaxanthin is de-epoxidized into antheraxanthin, and then into zeaxanthin, by the enzyme VDE (Fig. 1), which is dependent on MGDG.²³ MGDG is also an integral component of photosynthetic complexes.^24–26 Thus, since mgd1-1 mutants have reduced total amounts of xanthophyll and chlorophyll pigments, but increased chlorophyll a/b ratios, their photosynthesis capacity is unsurprisingly reduced, even though the organization of their electron transport chains is not strongly affected by the MGDG deficiency.¹³Open in a separate windowFigure 1Reactions of the xanthophyll cycle (adapted from ref. ²⁹). VDE, violaxanthin de-epoxidase; ZE, zeaxanthin epoxidase.During short-term high light stress, antheraxanthin and zeaxanthin are thought to facilitate dissipation of excess light energy in the PSII antenna bed by non-photochemical quenching.^27,28 Upon high light stress the pH decreases, triggering photoprotective mechanisms via changes in the PSII antenna system. The PsbS protein, which is involved in thermal dissipation, is protonated and initiates a conformational change in the PSII antenna bed. This change is further stabilized by the de-epoxidation of violaxanthin to zeaxanthin by the luminal VDE.²⁸ However, the thermal dissipation is impaired in mgd1-1 mutants at high light intensities (>1000 µmol m⁻² s⁻¹) making them more susceptible to light stress. Surprisingly, this is not mediated by direct effects on VDE and PsbS activities, but by changes in the proton conductivity of the thylakoid membrane.¹³The steady-state capacity of the xanthophyll cycle is reduced in mgd1-1 mutants, due to a ∼40% reduction in the proton motive force (pmf) across their thylakoid membranes, indicating that they have impaired capacities to energize these membranes. Nevertheless, the pmf is more or less equal to wild type under light-limited conditions (200 µmol m⁻² s⁻¹ light); it is only the increase in pmf in high light intensities that is impaired in the mutants.¹³ This leads to the thylakoid lumen being less acidic in mgd1-1 than in wild type, hampering full activation of VDE and PsbS. Thus, the thylakoid lumen pH is above the threshold level required for full activation of PsbS and VDE under steady-state conditions and so de-epoxidation rates are retarded and the equilibrium between zeaxanthin and violaxanthin starts to shift slightly towards violaxanthin (Fig. 2).¹³ Thus, increased conductivity of the thylakoid membranes is probably responsible for the diminished non-photochemical quenching in mgd1-1, and the findings strongly indicate that MGDG is required for efficient photosynthesis and photoprotection, in addition to being a physical membrane constituent.Open in a separate windowFigure 2Schematic diagram illustrating the normal mode of action of the xanthophyll cycle. In standard light conditions, V is bound to the photosynthetic complexes and harvests light. In strong light, V is released from the complexes and converted to Z by VDE, which is unable to access V when it is associated with the photosynthetic complexes. The newly formed Z then binds to the photosynthetic complexes (at the PsbS protein), where it dissipates excess energy through NPQ. V, violaxanthin; A, antheraxanthin; Z, zeaxanthin; VDE, violaxanthin de-epoxidase; ZE, zeaxanthin epoxidase. Arrows indicate the directions of reactions.