Biomedical subjects

Wah Soon Chow

Publications and source records attributed to Wah Soon Chow.

17 recordsLinked to original sources

PubMed2007

Quantification of cyclic electron flow around Photosystem I in spinach leaves during photosynthetic induction.

The variation of the rate of cyclic electron transport around Photosystem I (PS I) during photosynthetic induction was investigated by illuminating dark-adapted spinach leaf discs with red + far-red actinic light for a varied duration, followed by abruptly turning off the light. The post-illumination re-reduction kinetics of P700+, the oxidized form of the photoactive chlorophyll of the reaction centre of PS I (normalized to the total P700 content), was well described by the sum of three negative exponential terms. The analysis gave a light-induced total electron flux from which the linear electron flux through PS II and PS I could be subtracted, yielding a cyclic electron flux. Our results show that the cyclic electron flux was small in the very early phase of photosynthetic induction, rose to a maximum at about 30 s of illumination, and declined subsequently to <10% of the total electron flux in the steady state. Further, this cyclic electron flow, largely responsible for the fast and intermediate exponential decays, was sensitive to 3-(3,4-dichlorophenyl)-1,1-dimethyl urea, suggesting an important role of redox poising of the cyclic components for optimal function. Significantly, our results demonstrate that analysis of the post-illumination re-reduction kinetics of P700+ allows the quantification of the cyclic electron flux in intact leaves by a relatively straightforward method.

Chlorophyll↗

PubMed2006

Plasticity in the composition of the light harvesting antenna of higher plants preserves structural integrity and biological function.

Arabidopsis plants in which the major trimeric light harvesting complex (LHCIIb) is eliminated by antisense expression still exhibit the typical macrostructure of photosystem II in the granal membranes. Here the detailed analysis of the composition and the functional state of the light harvesting antennae of both photosystem I and II of these plants is presented. Two new populations of trimers were found, both functional in energy transfer to the PSII reaction center, a homotrimer of CP26 and a heterotrimer of CP26 and Lhcb3. These trimers possess characteristic features thought to be specific for the native LHCIIb trimers they are replacing: the long wavelength form of lutein and at least one extra chlorophyll b, but they were less stable. A new population of loosely bound LHCI was also found, contributing to an increased antenna size for photosystem I, which may in part compensate for the loss of the phosphorylated LHCIIb that can associate with this photosystem. Thus, the loss of LHCIIb has triggered concerted compensatory responses in the composition of antennae of both photosystems. These responses clearly show the importance of LHCIIb in the structure and assembly of the photosynthetic membrane and illustrate the extreme plasticity at the level of the composition of the light harvesting system.

Arabidopsis↗

PubMed2006

A mutation affecting ASCORBATE PEROXIDASE 2 gene expression reveals a link between responses to high light and drought tolerance.

Molecular analyses of plants have revealed a number of genes whose expression changes in response to high light (HL), including the H2O2 scavenger, ASCORBATE PEROXIDASE 2 (APX2). We carried out a screen in Arabidopsis thaliana for lesions that alter HL-induced expression of APX2 to identify components in abiotic stress signalling pathways. High light was used as it can be instantaneously applied or removed and accurately measured. We identified a number of alx mutations causing altered APX2 expression. Here we describe the gain-of-function mutant, alx8, which has constitutively higher APX2 expression and higher levels of foliar abscisic acid (ABA) than wild type. In fact, exogenous ABA increased APX2 expression and the APX2 promoter contains ABA response elements. Furthermore, we have shown that HL stress increases ABA in wild-type plants, implicating ABA in the regulation of HL-inducible genes. The alx8 mutant is drought tolerant, exhibits improved water-use efficiency and a number of drought-tolerance genes are upregulated. Additionally, alx8 demonstrates the complexity of ABA-dependent and ABA-independent transcriptional networks as some components in both pathways are upregulated in alx8. This study provides evidence for common steps in drought and HL stress response pathways.

Abscisic Acid↗

PubMed2005

Granal stacking of thylakoid membranes in higher plant chloroplasts: the physicochemical forces at work and the functional consequences that ensue.

The formation of grana in chloroplasts of higher plants is examined in terms of the subtle interplay of physicochemical forces of attraction and repulsion. The attractive forces between two adjacent membranes comprise (1) van der Waals attraction that depends on the abundance and type of atoms in each membrane, on the distance between the membranes and on the dielectric constant, (2) depletion attraction that generates local order by granal stacking at the expense of greater disorder (i.e. entropy) in the stroma, and (3) an electrostatic attraction of opposite charges located on adjacent membranes. The repulsive forces comprise (1) electrostatic repulsion due to the net negative charge on the outer surface of thylakoid membranes, (2) hydration repulsion that operates at small separations between thylakoid membranes due to layers of bound water molecules, and (3) steric hindrance due to bulky protrusions of Photosystem I (PSI) and ATP synthase into the stroma. In addition, specific interactions may occur, but they await experimental demonstration. Although grana are not essential for photosynthesis, they are ubiquitous in higher plants. Grana may have been selected during evolution for the functional advantages that they confer on higher plants. The functional consequences of grana stacking include (1) enhancement of light capture through a vastly increased area-to-volume ratio and connectivity of several PSIIs with large functional antenna size, (2) the ability to control the lateral separation of PSI from PSII and, therefore, the balanced distribution of excitation energy between two photosystems working in series, (3) the reversible fine-tuning of energy distribution between the photosystems by State 1-State 2 transitions, (4) the ability to regulate light-harvesting via controlled thermal dissipation of excess excitation energy, detected as non-photochemical quenching, (5) dynamic flexibility in the light reactions mediated by a granal structure in response to regulation by a trans-thylakoid pH gradient, (6) delaying the premature degradation of D1 and D2 reaction-centre protein(s) in PSII by harbouring photoinactived PSIIs in appressed granal domains, (7) enhancement of the rate of non-cyclic synthesis of adenosine triphosphate (ATP) as well as the regulation of non-cyclic vs. cyclic ATP synthesis, and (8) the potential increase of photosynthetic capacity for a given composition of chloroplast constituents in full sunlight, concomitantly with enhancement of photochemical efficiency in canopy shade. Hence chloroplast ultrastructure and function are intimately intertwined.

Alocasia↗

PubMed2005

Entropy-assisted stacking of thylakoid membranes.

Chloroplasts in plants and some green algae contain a continuous thylakoid membrane system that is structurally differentiated into stacked granal membranes interconnected by unstacked thylakoids, the stromal lamellae. Experiments were conducted to test the hypothesis that the thermodynamic tendency to increase entropy in chloroplasts contributes to thylakoid stacking to form grana. We show that the addition of bovine serum albumin or dextran, two very different water-soluble macromolecules, to a suspension of envelope-free chloroplasts with initially unstacked thylakoids induced thylakoid stacking. This novel restacking of thylakoids occurred spontaneously, accompanied by lateral segregation of PSII from PSI, thereby mimicking the natural situation. We suggest that such granal formation, induced by the macromolecules, is partly explained as a means of generating more volume for the diffusion of macromolecules in a crowded stromal environment, i.e., greater entropy overall. This mechanism may be relevant in vivo where the stroma has a very high concentration of enzymes of carbon metabolism, and where high metabolic fluxes are required.

Buffers↗

PubMed2005

Faster Rubisco is the key to superior nitrogen-use efficiency in NADP-malic enzyme relative to NAD-malic enzyme C4 grasses.

In 27 C4 grasses grown under adequate or deficient nitrogen (N) supplies, N-use efficiency at the photosynthetic (assimilation rate per unit leaf N) and whole-plant (dry mass per total leaf N) level was greater in NADP-malic enzyme (ME) than NAD-ME species. This was due to lower N content in NADP-ME than NAD-ME leaves because neither assimilation rates nor plant dry mass differed significantly between the two C4 subtypes. Relative to NAD-ME, NADP-ME leaves had greater in vivo (assimilation rate per Rubisco catalytic sites) and in vitro Rubisco turnover rates (k(cat); 3.8 versus 5.7 s(-1) at 25 degrees C). The two parameters were linearly related. In 2 NAD-ME (Panicum miliaceum and Panicum coloratum) and 2 NADP-ME (Sorghum bicolor and Cenchrus ciliaris) grasses, 30% of leaf N was allocated to thylakoids and 5% to 9% to amino acids and nitrate. Soluble protein represented a smaller fraction of leaf N in NADP-ME (41%) than in NAD-ME (53%) leaves, of which Rubisco accounted for one-seventh. Soluble protein averaged 7 and 10 g (mmol chlorophyll)(-1) in NADP-ME and NAD-ME leaves, respectively. The majority (65%) of leaf N and chlorophyll was found in the mesophyll of NADP-ME and bundle sheath of NAD-ME leaves. The mesophyll-bundle sheath distribution of functional thylakoid complexes (photosystems I and II and cytochrome f) varied among species, with a tendency to be mostly located in the mesophyll. In conclusion, superior N-use efficiency of NADP-ME relative to NAD-ME grasses was achieved with less leaf N, soluble protein, and Rubisco having a faster k(cat).

Adaptation, Physiological↗

PubMed2005

A simple chlorophyll fluorescence parameter that correlates with the rate coefficient of photoinactivation of photosystem II.

A method of partitioning the energy in a mixed population of active and photoinactivated Photosystem II (PS II) complexes based on chlorophyll fluorescence measurements is presented. There are four energy fluxes, each with its quantum efficiency: a flux associated with photochemical electron flow in active PS II reaction centres (JPS II), thermal dissipation in photoinactivated, non-functional PS IIs (JNF), light-regulated thermal dissipation in active PS IIs (JNPQ) and a combined flux of fluorescence and constitutive, light-independent thermal dissipation (Jf,D). The four quantum efficiencies add up to 1.0, without the need to introduce an 'excess' term E, which in other studies has been claimed to be linearly correlated with the rate coefficient of photoinactivation of PS II (kpi). We examined the correlation of kpi with various fluxes, and found that the combined flux (JNPQ + Jf,D= Jpi) is as well correlated with kpi as is E. This combined flux arises from Fs/Fm ', the ratio of steady-state to maximum fluorescence during illumination, which represents the quantum efficiency of combined non-photochemical dissipation pathways in active PS IIs. Since Fs/Fm ' or its equivalent, Jpi, is a likely source of events leading to photoinactivation of PS II, we conclude that Fs/Fm ' is a simple predictor of kpi.

Capsicum↗

PubMed2005

Photoinactivation of photosystem II in leaves.

Photoinactivation of Photosystem II (PS II), the light-induced loss of ability to evolve oxygen, inevitably occurs under any light environment in nature, counteracted by repair. Under certain conditions, the extent of photoinactivation of PS II depends on the photon exposure (light dosage, x), rather than the irradiance or duration of illumination per se, thus obeying the law of reciprocity of irradiance and duration of illumination, namely, that equal photon exposure produces an equal effect. If the probability of photoinactivation (p) of PS II is directly proportional to an increment in photon exposure (p = kDeltax, where k is the probability per unit photon exposure), it can be deduced that the number of active PS II complexes decreases exponentially as a function of photon exposure: N = Noexp(-kx). Further, since a photon exposure is usually achieved by varying the illumination time (t) at constant irradiance (I), N = Noexp(-kI t), i.e., N decreases exponentially with time, with a rate coefficient of photoinactivation kI, where the product kI is obviously directly proportional to I. Given that N = Noexp(-kx), the quantum yield of photoinactivation of PS II can be defined as -dN/dx = kN, which varies with the number of active PS II complexes remaining. Typically, the quantum yield of photoinactivation of PS II is ca. 0.1micromol PS II per mol photons at low photon exposure when repair is inhibited. That is, when about 10(7) photons have been received by leaf tissue, one PS II complex is inactivated. Some species such as grapevine have a much lower quantum yield of photoinactivation of PS II, even at a chilling temperature. Examination of the longer-term time course of photoinactivation of PS II in capsicum leaves reveals that the decrease in N deviates from a single-exponential decay when the majority of the PS II complexes are inactivated in the absence of repair. This can be attributed to the formation of strong quenchers in severely-photoinactivated PS II complexes, able to dissipate excitation energy efficiently and to protect the remaining active neighbours against damage by light.

Chloroplasts↗

PubMed2004

Populations of photoinactivated photosystem II reaction centers characterized by chlorophyll a fluorescence lifetime in vivo.

Photosystem (PS) II centers, which split water into oxygen, protons, and electrons during photosynthesis, require light but are paradoxically inactivated by it. Prolonged light exposure concomitantly decreased both the functional fraction of PSII reaction centers and the integral PSII chlorophyll (Chl) a fluorescence lifetime in leaf segments of Capsicum annuum L. Acceleration of photoinactivation of PSII by a pretreatment with the inhibitors/uncoupler lincomycin, DTT, or nigericin further reduced PSII Chl a fluorescence lifetimes. A global analysis of fluorescence lifetime distributions revealed the presence of at least two distinct populations of photoinactivated PSII centers, one at 1.25 ns, and the other at 0.58 ns. Light treatment first increased the 1.25-ns component, a weak quencher, at the expense of a component at 2.22 ns corresponding to functional PSII centers. The 0.58-ns component, a strong quencher, emerged later than the 1.25-ns component. The strongly quenching PSII reaction centers could serve to avoid further damage to themselves and protect their functional neighbors by acting as strong energy sinks.

Anti-Bacterial Agents↗

PubMed2004

Glucose-induced expression of carotenoid biosynthesis genes in the dark is mediated by cytosolic ph in the cyanobacterium Synechocystis sp. PCC 6803.

The expression of carotenoid biosynthesis genes coding for phytoene synthase (crtB), phytoene desaturase (crtP), zeta-carotene desaturase (crtQ), and beta-carotene hydroxylase (crtR) is dependent upon light in the cyanobacterium Synechocystis sp. PCC 6803 (Synechocystis). We have demonstrated that the expression of the above four genes was also elevated in the dark-adapted Synechocystis cells upon glucose treatment as a consequence of transcriptional activation. Treatment with glucose analogs such as l-glucose, 3-O-methylglucose, 2-deoxyglucose, and mannose, or inactivation of glucose uptake and phosphorylation by deletion mutation of glucose transporter (glcP) and glucokinase (gk), respectively, did not induce up-regulation of carotenoid genes. When respiratory electron transport or coupling to oxidative phosphorylation was inhibited, glucose induction was not observed, indicating that respiratory electron transport per se is not critical for the expression of these genes. In agreement with this view, the extent of gene expression showed a saturation curve with increasing acridine yellow fluorescence yield, without having a close correlation with the ATP contents or ATP/ADP ratio. The results indicate that glucose induction of carotenoid gene expressions is mediated by an increase in cytosolic pH rather than either redox or glucose sensing.

Carotenoids↗

PubMed2004

A simple alternative approach to assessing the fate of absorbed light energy using chlorophyll fluorescence.

We propose a simplified alternative method for quantifying the partitioning of excitation energy between photochemistry, fluorescence and thermal dissipation. This alternative technique uses existing well-defined quantum efficiencies such as Phi(PS II), leaving no 'excess' efficiency unaccounted for, effectively separates regulated and constitutive thermal dissipation processes, does not require the use of F(o) and F'(o) measurements and gives very similar results to the method proposed by Kramer et al. [(2004) Photosynth Res 79: 209-218]. We demonstrate the use of the technique using chlorophyll fluorescence measurements in grapevine leaves and observe a high dependence on thermal dissipation processes (up to 75%) at both high light and low temperature.

Journal Article↗

PubMed2004

Processes contributing to photoprotection of grapevine leaves illuminated at low temperature.

Photoinactivation of photosystem II (PSII) and energy dissipation at low leaf temperatures were investigated in leaves of glasshouse-grown grapevine (Vitis vinifera L. cv. Riesling). At low temperatures (< 15 degrees C), photosynthetic rates of CO(2) assimilation were reduced. However, despite a significant increase in the amount of light excessive to that required by photosynthesis, grapevine leaves maintained high intrinsic quantum efficiencies of PSII (F(v)/F(m)) and were highly resistant to photoinactivation compared to other species. Non-photochemical energy dissipation involving xanthophylls and fast D1 repair were the main protective processes reducing the 'gross' rate of photoinactivation and the 'net' rate of photoinactivation, respectively. We developed an improved method of energy dissipation analysis that revealed up to 75% of absorbed light is dissipated thermally via pH- and xanthophyll-mediated non-photochemical quenching at low temperatures (5-15 degrees C) and moderate (800 micro mol quanta m(-2) s(-1)) light. Up to 20% of the energy flux contributing to electron transport was dissipated via photorespiration when taking into account temperature-dependent mesophyll conductance; however, this flux used in photorespiration was only a relatively small amount of the total absorbed light energy. Photoreduction of O(2) at photosystem I (PSI) and subsequent superoxide detoxification (water-water cycle) was more sensitive to inhibition by low temperature than photorespiration. Therefore the water-water cycle represents a negligibly small energy sink below 15 degrees C, irrespective of mesophyll conductance.

Journal Article↗

PubMed2003

Cytochrome c oxidase of the cyanobacterium Synechocystis sp. PCC 6803 protects photosynthesis from salt stress.

We generated cytochrome c oxidase (CtaI)-defective cells of the cyanobacterium Synechocystis sp. PCC 6803 in order to investigate the physiological function of the CtaI-mediated respiratory electron transport pathway. When they were salt stressed, CtaI-defective cells showed a substantial decrease in photosynthesis due to reduction of the photochemical efficiency of Photosystem II and of the chlorophyll in the reaction center of the photo-oxidizable form of Photosystem I. These findings demostrate that CtaI-mediated electron transport is important for resistance to salt stress.

Cyanobacteria↗

PubMed2003

NADPH dehydrogenase-mediated respiratory electron transport in thylakoid membranes of the cyanobacterium Synechocystis sp. PCC 6803 is inactive in the light.

Non-photochemical redox changes of the plastoquinone pools in darkness were investigated in the cyanobacterium Synechocystis sp. PCC 6803 by monitoring changes in Chl fluorescence yield during light-to-dark transitions. The inhibitors rotenone and mercury with or without 1 mM succinate fully suppressed the post-illumination increase in Chl fluorescence in both NADPH dehydrogenase-defective (M55) and deltaCtaI cells. The latter cells lack subunit I of cytochrome aa3-type cytochrome c oxidase. These results strongly suggest that NADPH dehydrogenase plays the major role in electron donation in the non-photo-chemical reduction of plastoquinone. The rising phase of post-illumination Chl fluorescence in both wild type pretreated with KCN, and deltaCtaI cells, was significantly slowed by low light illumination. We detected comparable photochemical levels of both photosystem (PS) II and PSI during steady state illumination in wild type and deltaCtaI cells. From these results, we suggest that respiratory electron flow involved in the non-photochemical redox change of plastoquinone is not likely to occur in the light.

Cyanobacteria↗

PubMed2002

The role of inactive photosystem-II-mediated quenching in a last-ditch community defence against high light stress in vivo.

Photoinactivation of photosystem II (PSII), the light-induced loss of ability to evolve oxygen, is an inevitable event during normal photosynthesis, exacerbated by saturating light but counteracted by repair via new protein synthesis. The photoinactivation of PSII is dependent on the dosage of light: in the absence of repair, typically one PSII is photoinactivated per 10(7) photons, although the exact quantum yield of photoinactivation is modulated by a number of factors, and decreases as fewer active PSII targets are available. PSII complexes initially appear to be photoinactivated independently; however, when less than 30% functional PSII complexes remain, they seem to be protected by strongly dissipative PSII reaction centres in several plant species examined so far, a mechanism which we term 'inactive PSII-mediated quenching'. This mechanism appears to require a pH gradient across the photosynthetic membrane for its optimal operation. The residual fraction of functional PSII complexes may, in turn, aid in the recovery of photoinactivated PSII complexes when conditions become less severe. This mechanism may be important for the photosynthetic apparatus in extreme environments such as those experienced by over-wintering evergreen plants, desert plants exposed to drought and full sunlight and shade plants in sustained sunlight.

Energy Transfer↗

PubMed2002

Chloroplast Cu/Zn-superoxide dismutase is a highly sensitive site in cucumber leaves chilled in the light.

Light-chilling stress, the combination of low-light illumination and low temperature, preferentially inactivated photosystem I (PSI) of cucumber (Cucumis sativus L.) leaves, resulting in the photoinhibition of photosynthesis. The extent of PSI photoinhibition, determined in vivo by monitoring absorption changes around 810 nm (induced by far-red light), was closely correlated with the redox state of the PSII electron acceptor Q(A), measured as the chlorophyll fluorescence parameter, 1-qP, where qP is a photochemical quenching coefficient. In contrast, the decrease in the far-red-induced leaf absorptance signal was not well correlated with the limited fragmentation of the PsaA/B gene products in the PSI reaction center after the light-chilling stress. Amongst various enzymes involved in the photooxidative damage such as superoxide dismutase (SOD), ascorbate peroxidase, and NAD(P)H dehydrogenase, only SOD was inhibited by light-chilling treatment. Further, an approximately 3-fold increase in the leaf content of H(2)O(2), a potent inhibitor of Cu/Zn-SOD, was observed after light-chilling stress. From these results, we suggest that Cu/Zn-SOD is the primary target of the light-chilling stress, followed by subsequent inactivation of PSI by reactive oxygen species.

Adaptation, Physiological↗

PubMed2002

Differential susceptibility of photosynthesis to light-chilling stress in rice (Oryza sativa l.) depends on the capacity for photochemical dissipation of light.

The primary target for light-chilling stress in chilling-sensitive cucumber leaves is the chloroplast Cu,Zn-Superoxide dismutase, followed by subsequent inactivation of the photosystem (PS) I by reactive oxygen species (ROS). To test this hypothesis, two rice cultivars that were different in their ecological origins (a chilling-resistant Stejaree 45 and a chilling-sensitive Milyang 23) were evaluated with respect to photosynthetic properties, the ROS scavenging system, and expression of genes that are involved in sucrose synthesis and allocation upon the light-chilling stress. As expected, when the leaves were exposed to various low temperatures with illumination (150 micromol m(-2)s(-1)) for 6 h, the leaf photosynthesis of Milyang 23 decreased faster than that of Stejaree 45. The light-chilling induced differential photoinhibition of photosynthesis between the two cultivars was caused by the photoin-activation of PSII but not of PSI, since the potential quantum yield of PSII followed a similar trend to the changes in photosynthetic rates. The activities of the two chloroplastic antioxidant enzymes (superoxide dismutase and ascorbate peroxidase) that are known to be sensitive to oxidative stress were barely affected by the light-chilling treatments. Among various genes in sucrose metabolism (such as cytosolic FBPase, SPS, SUT, SuSy, and AGPase), the transcript levels of SuSy in Milyang 23 were significantly decreased by light-chilling stress compared to that of Stejaree 45. Based on these results, we propose that PSII, not PSI, is the sensitive site for light-chilling stress in chilling-sensitive rice. The extent of PSII photoinhibition depends on its capacity for the photochemical utilization of light.

Acclimatization↗