Michito Tsuyama

A qualitative analysis of the regulation of cyclic electron flow around photosystem I from the post-illumination chlorophyll fluorescence transient in Arabidopsis: a new platform for the in vivo investigation of the chloroplast redox state

A transient in chlorophyll fluorescence after cessation of actinic light illumination, which has been ascribed to electron donation from stromal reductants to plastoquinone (PQ) by the NAD(P)H-dehydrogenase (NDH) complex, was investigated in Arabidopsis thaliana. The transient was absent in air in a mutant lacking the NDH complex (ndhM). However, in ndhM, the transient was detected in CO2-free air containing 2% O2. To investigate the reason, ndhM was crossed with a pgr5 mutant impaired in ferredoxin (Fd)-dependent electron donation from NADPH to PQ, which is known to be redundant for NDH-dependent PQ reduction in the cyclic electron flow around photosystem I (PSI). In ndhM pgr5, the transient was absent even in CO2-free air with 2% O2, demonstrating that the post-illumination transient can also be induced by the Fd- (or PGR5)-dependent PQ reduction. On the other hand, the transient increase in chlorophyll fluorescence was found to be enhanced in normal air in a mutant impaired in plastid fructose-1,6-bisphosphate aldolase (FBA) activity. The mutant, termed fba3-1, offers unique opportunities to examine the relative contribution of the two paths, i.e., the NDH- and Fd- (or PGR5)-dependent paths, on the PSI cyclic electron flow. Crossing fba3-1 with either ndhM or pgr5 and assessing the transient suggested that the main route for the PSI cyclic electron flow shifts from the NDH-dependent path to the Fd-dependent path in response to sink limitation of linear electron flow.

Reduction of the primary donor P700 of photosystem I during steady-state photosynthesis under low light in Arabidopsis

During steady-state photosynthesis in low-light, 830-nm absorption (A830) by leaves was close to that in darkness in Arabidopsis, indicating that the primary donor P700 in the reaction center of photosystem I (PSI) was in reduced form. However, P700 was not fully oxidized by a saturating light pulse, suggesting the presence of a population of PSI centers with reduced P700 that remains thermodynamically stable during the application of the saturating light pulse (i.e., reduced-inactive P700). To substantiate this, the effects of methyl viologen (MV) and far-red light on P700 oxidation by the saturating light pulse were analyzed, and the cumulative effects of repetitive application of the saturating light pulse on photosynthesis were analyzed using a mutant crr2-2 with impaired PSI cyclic electron flow. We concluded that the reduced-inactive P700 in low-light as revealed by saturating light pulse indicates limitations of electron flow at the PSI acceptor side.

The post‐illumination chlorophyll fluorescence transient indicates the RuBP regeneration limitation of photosynthesis in low light in Arabidopsis

The mechanism of post‐illumination chlorophyll fluorescence transient (PIFT) was investigated in Arabidopsis. PIFT was detected in the wild type after illumination with low light. In the fba3‐2 (fructose‐1,6‐bisphosphate aldolase) mutant, in which PIFT is enhanced, strong light also induced PIFT. PIFT was suppressed not only in the triose phosphate/phosphate translocator (tpt‐2) mutant, but also in tpt‐2 fba3‐2, suggesting that triose phosphates, such as dihydroxyacetone phosphate (DHAP), are involved in the PIFT mechanism. We concluded that PIFT is associated with ribulose‐1,5‐bisphosphate (RuBP)‐regeneration limitation of photosynthesis in low light.

An Analysis of the Mechanism of the Low-wave Phenomenon of Chlorophyll Fluorescence.

The low-wave phenomenon, i.e., the transient drop of yield of modulated chlorophyll fluorescence shortly after application of a pulse of saturating light, was investigated in intact leaves of tobacco and Camellia by measuring fluorescence, CO2 assimilation and absorption at 830 nm simultaneously. Limitations on linear electron flow, due to low electron acceptor levels that were induced by low CO2, induced the low waves of chlorophyll fluorescence. Low-wave amplitudes obtained under different CO2 concentrations and photon-flux densities yielded single-peak curves when plotted as functions of fluorescence parameters such as ΦPS II (quantum yield of Photosystem II) and qN (coefficient of non-photochemical quenching), suggesting that low-wave formation depends on the redox state of the electron transport chain. Low waves paralleled redox changes of P700, the reaction center of Photosystem I (PS I), and an additional electron flow through PS I was detected during the application of saturating pulses that induced low-waves. It is suggested that low waves of chlorophyll fluorescence are induced by increased non-photochemical quenching, as a result of the formation of a trans-thylakoid proton gradient due to cyclic electron flow around PS I.

Thiobencarb herbicide reduces growth, photosynthetic activity, and amount of rieske iron-sulfur protein in the diatom Thalassiosira pseudonana

We investigated the effects of the herbicide thiobencarb on the growth, photosynthetic activity, and expression profile of photosynthesis‐related proteins in the marine diatom Thalassiosira pseudonana. Growth rate was suppressed by 50% at a thiobencarb concentration of 1.26 mg/L. Growth and photosystem II activity (Fv/Fm ratio) were drastically decreased at 5 mg/L, at which the expression levels of 13 proteins increased significantly and those of 11 proteins decreased significantly. Among these proteins, the level of the Rieske iron‐sulfur protein was decreased to less than half of the control level. This protein is an essential component of the cytochrome b6f complex in the photosynthetic electron transport chain. Although the mechanism by which thiobencarb decreased the Rieske iron‐sulfur protein level is not clear, these results suggest that growth was inhibited by interruption of the photosynthetic electron transport chain by thiobencarb.

Growth-phase dependent variation in photosynthetic activity and cellular protein expression profile in the harmful raphidophyte Chattonella antiqua

This study investigated temporal variations in the potential maximum quantum yield of photosystem II (F(v)/F(m) ratio) and growth-phase dependent cellular protein expressions of Chattonella antiqua under laboratory conditions. Despite the culture conditions, significant positive correlations between the F(v)/F(m) ratio and daily growth rate were observed. Threshold F(v)/F(m) ratios associated with positive cell growth were calculated to be >0.44, >0.44, and >0.37, and those associated with active cell growth (growth rate >0.5 div. d(-1)) were >0.58, >0.60, and >0.49 under control culture, low nutrient and intense light conditions, respectively. Proteome profiles obtained by two-dimensional gel electrophoresis (2-DE) indicated that 42 protein spots were differentially expressed at various growth phases of C. antiqua, which indicates changes in cellular physiological status throughout the growth cycle, and suggests that oxygen evolving enhancer 1 and 2-cysteine peroxiredoxin play roles in maintaining the positive growth of C. antiqua.

Simultaneous measurements of quantum yield and CO2 uptake for the assessment of non-assimilative electron flow in tree leaves

Using attached and detached leaves ofAcer palmatum Thunb. andRhaphiolepsis umbellata Makino, pulse-modulated chlorophyll fluorescence and CO2 exchange were measured. Quantum yield of photosynthesis was determined from the fluorescence parameter(Fm′−Fs)/Fm′, where (Fm′−Fs) was defined as the difference between steady state chlorophyll fluorescence (Fs) and maximum fluorescence (Fm′) elicited by a saturating light pulse. The rate of electron transport through photosystem II (total electron flow) was calculated from the product of quantum yield andA (PFD), whereA is the rate of absorbed photons as given by leaf absorptance, and PFD is the photon flux density at the leaf surface. The rate of electron transport dependant on CO2 uptake (assimilative electron flow) was calculated from the gross photosynthetic rate in a leaf. The difference between the rates of total and assimilative electron transport was denoted as the rate of non-assimilative electron transport which depends on photorespiration and oxygen reduction. Available data provided quantitative information on the rate of non-assimilative electron flow in intact leaves. When leaf photosynthesis ofA. palmatum was measured under sunlight, the rates of total and assimilative electron transport were determined to be approximately 900 and 150 μmol equiv. e/mg Chl·h, respectively. The difference (750 μmol equiv. e/mg Chl·h) was attributed to the activity of non-assimilative electron flow. The ratio of total to assimilative electron flow was found to increase gradually with rising in irradiance. The results suggest that non-assimilative electron flow occurred at much higher rate than assimilative electron flow at high irradiance. Implications of the results are briefly discussed in relation to photosynthesis limitation in tree leaves.

PS II down regulation as affected by the maximal activity of photorespiration in leaves of C3 plants

The relationship between down regulation of PS II (as indicated by the quenching of chlorophyll fluorescence) and the maximal activity of photorespiration was investigated in leaves of N. tabacum. When leaves were illuminated with low actinic light under different CO2 concentrations in air, the electron flux dependent on photosynthesis (JCO2) increased with increasing the CO2 concentration, and concomitant decrease of the electron flux dependent on photorespiration (JPR) was observed. Under the conditions, however, the rate of total electron flow (Je) was constant within the wide range of CO2 concentrations. On the other hand, when leaves were illuminated with high actinic light, down regulation of PS II was induced even at high CO2 conditions. This phenomenon was analyzed in relation to the maximal activity of photorespiration. We found that the down regulation of PS II is induced when the electron flux (Je) became larger than the flux JPR. On the basis of the results, implication of the PS II down regulation is discussed.

Estimation of the Photosynthetic Electron Flow in Intact Leaves: Effect of Modulated Light Intensity on the Fluorescence Parameters Fv/Fm and ΔF/Fm’

Previous studies have confirmed the validity of the measurement of quantum yield in PS II for estimating the rate of linear electron transport in leaves (Je) by simultaneous measurements of chlorophyll fluorescence and gas exchange [1], because the product of the quantum yield of PS II (Φpsii) and the photon incidence absorbed by PS II (Ipsii) permits the calculation of Je from the equation Je=(Φpsii)(Ipsii). Genty et al.[2] showed that a linear relationship is obtained between the quantum yield of PS II (ΔF/Fm’) and the apparent quantum yield of CO2 assimilation in non-photorespiratory conditions. However, the actual values of Φpsii and Ipsii are difficult to determine in intact leaves. At present, two different methods for estimating Je have been proposed. Edward and Baker [3] calculated Je from the equation, Je=1/2(ΔF/Fm’)•(PFDabs), under the assumption that the photon flux density absorbed by a leaf (PFDabs) is equally distributed between PS I and PS II. Another method is based on the linear relationship between the relative measurement of quantum yield of PS II (ΔF/Fin’) and the apparent quantum yield of gross CO2 assimilation under non-photorespiratory conditions [2, 4, 5, 6]. They used the relationship for estimating the electron flow under photorespiratory conditions (Je) in leaves.