The mitochondrial CMSII mutation of Nicotiana sylvestris impairs adjustment of photosynthetic carbon assimilation to higher growth irradiance

The CMSII mutant of Nicotiana sylvestris, which lacks a functional mitochondrial complex I, was used to investigate chloroplast-mitochondria interactions in light acclimation of photosynthetic carbon assimilation. CMSII and wild-type (WT) plants were grown at 80 micromol m(-2) s(-1) photosynthetic active radiation (PAR; 80) and 350 micromol m(-2) s(-1) PAR (350). Carbon assimilation at saturating PFD was markedly higher in WT 350 leaves as compared with WT 80 leaves, but was similar in CMS 80 and CMS 350 leaves, suggesting that the mutant is unable to adjust photosynthesis to higher growth irradiance. WT 350 leaves showed several general characteristic light acclimation responses [increases in leaf specific area (LSA), total chlorophyll content, and chlorophyll a/b ratio, and a higher light compensation point]. In contrast, a similar chlorophyll content and chlorophyll a/b ratio were measured for both CMS 80 and CMS 350 leaves, while LSA and the light compensation point acclimated as in the WT. The failure of CMSII to adjust photosynthesis to growth PFD did not result from lower quantum efficiency of PSII, lower whole-chain electron transport rates (ETRs), or lower ribulose-1,5-bisphosphate carboxylase oxygenase (Rubisco) and sucrose phosphate synthase (SPS) capacities. Excess ETR not used for carbon assimilation was even higher in CMS 350 than in WT 350. Since photochemical fluorescence quenching and the initial activity of NADP malate dehydrogenase (NADP-MDH) were identical in WT 350 and CMS 350 leaves but the activation state of NADP-MDH was different, redox signals from primary ETR are not involved in the signal transduction of light acclimation, while a contribution of stromal redox state cannot be excluded. When mature plants were transferred between 350 and 80 conditions, the mutant showed acclimatory tendencies, although adjustments were not as rapid or as marked as in the WT, and the response of the initial activities of Rubisco and NADP-MDH was impaired or altered. Initial activities of Rubisco and SPS at limiting concentration were also affected in CMS 350 as compared with WT plants when compared at growth irradiance or after in situ activation at 1000 micromol m(-2) s(-1) PAR. The data demonstrate that chloroplast-mitochondria interactions are important in light acclimation, and modulation of the activation state of key photosynthetic enzymes could be an important mechanism in this cross-talk.     

On the developmental dependence of leaf respiration: responses to short- and long-term changes in growth temperature

Using measurements of leaf respiratory O(2) uptake (R), we investigated whether immature and mature Arabidopsis thaliana (ecotype Columbia) leaves differed in their response to temperature. Confocal microscopy (using plants with mitochondrially targeted green fluorescent protein [GFP]) was used to determine whether ontogenetic changes in R are associated with concomitant changes in mitochondrial morphology/abundance. Comparisons were made of warm-grown (25/20°C) leaves, warm-grown leaves shifted to cold (5°C) for 10 days, and cold-developed leaves. Short-term Q(10) values and the ability to cold-acclimate were determined. In warm-grown plants, rates of R per mass were highest in immature leaves, decreasing as leaves developed. Moreover, although mitochondrial size (5.6-6.5 μm(3)) remained constant during development, mitochondrial number per μm(3) declined from 0.01 to 0.003 as leaves expanded (i.e., mitochondrial density decreased). Immature and mature leaves did not differ in Q(10) values but did differ in their ability to cold-acclimate. Whereas mature leaves had clear evidence of cold acclimation (e.g., when measured at 25°C, R was highest in cold-developed leaves), young leaves had none. Collectively, the results highlight the changes in rates of R, mitochondrial density, and biomass allocation associated with leaf development and that changes in respiratory flux associated with acclimation only take place within mature tissues.     

Heterogeneity of plant mitochondrial responses underpinning respiratory acclimation to the cold in Arabidopsis thaliana leaves

In this study, we investigated whether changes in mitochondrial abundance, ultrastructure and activity are involved in the respiratory cold acclimation response in leaves of the cold-hardy plant Arabidopsis thaliana. Confocal microscopy [using plants with green fluorescence protein (GFP) targeted to the mitochondria] and transmission electron microscopy (TEM) were used to visualize changes in mitochondrial morphology, abundance and ultrastructure. Measurements of respiratory flux in isolated mitochondria and intact leaf tissue were also made. Warm-grown (WG, 25/ 20 degrees C day/night), 3-week cold-treated (CT) and cold-developed (CD) leaves were sampled. Although CT leaves exhibited some evidence of acclimation (as evidenced by higher rates of respiration at moderate measurement temperatures), it was only the CD leaves that were able to re-establish respiratory flux within the cold. Associated with the recovery of respiratory flux in the CD leaves were: (1) an increase in the total volume of mitochondria per unit volume of tissue in epidermal cells; (2) an increase in the ratio of cristae to matrix within mesophyll cell mitochondria; and (3) an increase in the capacity of the energy-producing cytochrome pathway in mitochondria isolated from whole leaf homogenates. Regardless of growth temperature, we found that contrasting cell types exhibited distinct differences in mitochondrial ultrastructure, morphology and abundance. Collectively, our data demonstrated the diversity and tissue-specific nature of mitochondrial responses that underpin respiratory acclimation to the cold, and revealed the heterogeneity of mitochondrial structure and abundance that exists within leaves.     

Mitochondrial complex I impairment and differential carbon monoxide sensitivity of cytochrome c oxidase in wild type and CMS II mutants of Nicotiana sylvestris

Plant mitochondria unlike their animal counterpart have some unique features with highly branched respiratory chain. The present work was undertaken in order to investigate the effect of loss/dysfunction of plant mitochondrial complex I on the relative flux of electrons through alternative oxidase (AOX) and cytochrome oxidase. Loss of a major subunit of mitochondrial complex I in cytoplasmic male sterile II (CMS II) mutant of Nicotiana sylvestris caused respiratory redox perturbations, as evident from the differential CO sensitivity of cytochrome oxidase. The leaf segments of CMS II mutant when exposed to CO under dark aerobic condition were insensitive to the inhibition of cytochrome oxidase, as against the wild type (WT). The differential CO response of WT and CMS II mutants appeared to be due to differences in the redox state of cytochrome a3 (cyt a3), the terminal electron acceptor during in situ respiration. Cyt a3 appeared to be more in its oxidized form in CMS II and hence unable to form cyt a3-CO complex. Pre-treatment of CMS II leaves with 2,4-dinitrophenol, an uncoupler of oxidative phosphorylation increased the CO response. The slight increase in rotenone-insensitive respiration of CMS II could be attributed partly to enhanced flux of electrons through cytochrome pathway to compensate for the loss of phosphorylation site and partly through AOX, which was induced by nitrate.     

Assessing the photoprotective effectiveness of non-photochemical chlorophyll fluorescence quenching: A new approach

The photoprotective nature of non-photochemical quenching (NPQ) has not been effectively quantified and the major reason is the inability to quantitatively separate NPQ that acts directly to prevent photoinhibition of photosystem II (PSII). Here we describe a technique in which we use the values of the PSII yield and qP measured in the dark following illumination. We expressed the quantum yield of PSII (Φ(PSII)) via NPQ as: Φ(PSII)=qP×(Fv/Fo)/(1+Fv/Fo+NPQ). We then tested this theoretical relationship using Arabidopsis thaliana plants that had been exposed to gradually increasing irradiance. The values of qP in the dark immediately after the illumination period (here denoted qPd) were determined using a previously described technique for Fo' calculation: Fo'(calc.)=1/(1/Fo-1/Fm-1/Fm'). We found that in every case the actual Φ(PSII) deviated from theoretical values at the same point that qPd deviated from a value of 1.0. In an increasing series of irradiance levels, WT leaves tolerated 1000μmolm(-2)s(-1) of light before qP(d) declined. Leaves treated with the uncoupler nigericin, leaves of the mutant lacking PsbS protein and leaves overexpressing PsbS showed a qP(d) reduction at 100, 600 and 2000μmolm(-2)s(-1) respectively, each at an increasing value of NPQ. Therefore we suggest that this simple and timely technique will be instrumental for identifying photoprotective NPQ (pNPQ) and that it is more appropriate than the qE component. Its applications should be broad: for example it will be useful in physiology-based studies to define the optimal level of nonphotochemical quenching for plant protection and productivity.     

Redox Regulation of Light-Harvesting Complex II and cab mRNA Abundance in Dunaliella salina

We demonstrate that photosynthetic adjustment at the level of the light-harvesting complex associated with photosystem II (LCHII) in Dunaliella salina is a response to changes in the redox state of intersystem electron transport as estimated by photosystem II (PSII) excitation pressure. To elucidate the molecular basis of this phenomenon, LHCII apoprotein accumulation and cab mRNA abundance were examined. Growth regimes that induced low, but equivalent, excitation pressures (either 13[deg]C/20 [mu]mol m-2 s-1 or 30[deg]C/150 ([mu]mol m-2 s-1) resulted in increased LHCII apoprotein and cab mRNA accumulation relative to algal cultures grown under high excitation pressures (either 13[deg]C/150 [mu]mol m-2 s-1 or 30[deg]C/2500 [mu]mol m-2 s-1). Thermodynamic relaxation of high excitation pressures, accomplished by shifting cultures from a 13 to a 30[deg]C growth regime at constant irradiance for 12 h, resulted in a 6- and 8-fold increase in LHCII apoprotein and cab mRNA abundance, respectively. Similarly, photodynamic relaxation of high excitation pressure, accomplished by a shift from a light to a dark growth regime at constant temperature, resulted in a 2.4- to 4-fold increase in LHCII apoprotein and cab mRNA levels, respectively. We conclude that photosynthetic adjustment to temperature mimics adjustment to high irradiance through a common redox sensing/signaling mechanism. Both temperature and light modulate the redox state of the first, stable quinone electron acceptor of PSII, which reflects the redox poise of intersystem electron transport. Changes in redox poise signal the nucleus to regulate cab mRNA abundance, which, in turn, determines the accumulation of light-harvesting apoprotein. This redox mechanism may represent a general acclimation mechanism for photosynthetic adjustment to environmental stimuli.     

Leaf chloroplast ultrastructure and photosynthetic properties of a chlorophyll-deficient mutant of rice

Leaf chloroplast ultrastructure and photosynthetic properties of a natural, yellow-green leaf mutant (ygl1) of rice were characterized. Our results showed that chloroplast development was significantly delayed in the mutant leaves compared with the wild-type rice (WT). As leaves matured, more grana stacks formed concurrently with increasing leaf chlorophyll (Chl) content. Except for the lower intercellular CO₂ concentration, the ygl1 plants had a higher leaf net photosynthetic rate, stomatal conductance, and transpiration rate than those of the WT plants. Under equal amounts of Chl, the excitation energy of PSI and PSII was much stronger in the mutant than that in the WT. The ygl1 plants showed higher nonphotochemical quenching and lower photochemical quenching. They also exhibited higher actual photochemical efficiency of PSII with a higher electron transport rate. Under the light of 200 μmol(photon) m^?2 s^?1, the ygl1 mutant showed lesser deepoxidation of violaxanthin in the xanthophyll cycle than WT, but it increased substantially under strong light conditions. In conclusion, the photosynthetic machinery of the ygl1 remained stable during leaf development. The plants were less sensitive to photoinhibition compared with WT due to the active xanthophyll cycle. The ygl1 plants were efficient in both light harvesting and conversion of solar energy.     

Insights on the development, kinetics, and variation of photoinhibition using chlorophyll fluorescence imaging of a chilled, variegated leaf

The effect of chilling on photosystem II (PSII) efficiency was studied in the variegated leaves of Calathea makoyana, in order to gain insight into the causes of chilling-induced photoinhibition. Additionally, a relationship was revealed between (chilling) stress and variation in photosynthesis. Chilling treatments (5 degrees C and 10 degrees C) were performed for different durations (1-7 d) under a moderate irradiance (120 micromol m-2 s-1). The individual leaves were divided into a shaded zone and two illuminated, chilled zones. The leaf tip and sometimes the leaf base were not chilled. Measurements of the dark-adapted Fv/Fm were made on the different leaf zones at the end of the chilling treatment, and then for several days thereafter to monitor recovery. Chilling up to 7 d in the dark did not affect PSII efficiency and visual appearance, whereas chilling in the light caused severe photoinhibition, sometimes followed by leaf necrosis. Photoinhibition increased with the duration of the chilling period, whereas, remarkably, chilling temperature had no effect. In the unchilled leaf tip, photoinhibition also occurred, whereas in the unchilled leaf base it did not. Whatever the leaf zone, photoinhibition became permanent if the mean value dropped below 0.4, although chlorosis and necrosis were associated solely with chilled illuminated tissue. Starch accumulated in the unchilled leaf tip, in contrast to the adjacent chilled irradiated zone. This suggests that photoinhibition was due to a secondary effect in the unchilled leaf tip (sink limitation), whereas it was a direct effect of chilling and irradiance in the chilled illuminated zones. The PSII efficiency and its coefficient of variation showed a unique negative linearity across all leaf zones and different tissue types. The slope of this curve was steeper for chilled leaves than it was for healthy, non-stressed leaves, suggesting that the coefficient of variation may be an important tool for assessing stress in leaves.     

Respiratory complex I deficiency induces drought tolerance by impacting leaf stomatal and hydraulic conductances

To investigate the role of plant mitochondria in drought tolerance, the response to water deprivation was compared between Nicotiana sylvestris wild type (WT) plants and the CMSII respiratory complex I mutant, which has low-efficient respiration and photosynthesis, high levels of amino acids and pyridine nucleotides, and increased antioxidant capacity. We show that the delayed decrease in relative water content after water withholding in CMSII, as compared to WT leaves, is due to a lower stomatal conductance. The stomatal index and the abscisic acid (ABA) content were unaffected in well-watered mutant leaves, but the ABA/stomatal conductance relation was altered during drought, indicating that specific factors interact with ABA signalling. Leaf hydraulic conductance was lower in mutant leaves when compared to WT leaves and the role of oxidative aquaporin gating in attaining a maximum stomatal conductance is discussed. In addition, differences in leaf metabolic status between the mutant and the WT might contribute to the low stomatal conductance, as reported for TCA cycle-deficient plants. After withholding watering, TCA cycle derived organic acids declined more in CMSII leaves than in the WT, and ATP content decreased only in the CMSII. Moreover, in contrast to the WT, total free amino acid levels declined whilst soluble protein content increased in CMSII leaves, suggesting an accelerated amino acid remobilisation. We propose that oxidative and metabolic disturbances resulting from remodelled respiration in the absence of Complex I activity could be involved in bringing about the lower stomatal and hydraulic conductances.     

Fructan synthesis is inhibited by phosphate in warm-grown, but not in cold-treated, excised barley leaves

The inhibition of fructan accumulation by phosphate was investigated in warm-grown and cold-treated barley (Hordeum vulgare) plants. Detached leaves were incubated in water or phosphate for 24 h under lighting or in darkness. Fructosyltransferase, sucrose phosphate synthase (SPS) and cytosolic fructose-1,6-bisphosphatase (FBPase) activities were subsequently analysed, as well as the content of carbohydrates, hexose-phosphates, phosphate, amino acids and protein. In warm-grown leaves, phosphate decreased fructan accumulation and total carbon in carbohydrates and did not affect protein content. Phosphate increased hexose-phosphates, phosphate and amino acids. Fructosyltransferase and FBPase activities were not affected by phosphate feeding, while SPS activity was inhibited by phosphate in incubations in both light and darkness. In cold-treated leaves, which before incubation had higher SPS activities than warm-grown leaves, phosphate had no inhibitory effect on fructan accumulation, carbohydrate content or total C in carbohydrates. The activities of SPS and FBPase were unaffected by phosphate. The results indicate that phosphate decreases fructan accumulation through an inhibition of SPS whenever this activity is not high before a rise in phosphate content.     

Effects of shading on photosynthetic characteristics and chlorophyll fluorescence parameters in leaves of Hydrangea macrophylla

Aims The objectives were to investigate the effects of different light intensities on photosynthetic characteristics and chlorophyll fluorescence parameters, to clarify the physiological responses and photo-protective mechanisms of Hydrangea macrophylla to changes in light regimes in view of the distribution of energy absorbed and photosynthetic characteristics.Methods Three light regimes including natural and shade (shading rate 50% and 75% of natural light) were applied to plants for 60 days. After the treatment, the gas-exchange, chlorophyll a fluorescence and photosynthesis-light curves were measured by a portable leaf gas exchange system (LI-6400).Important findings The results showed that the weak light intensity treatment reduced dark respiration rate, light compensation point and light saturation point of plant, but increased apparent quantum yield, suggesting that plants had the physiological strategy to utilize the weakening light by reducing respiration. The net photosynthetic rate, intercellular CO₂ concentration, transpiration rate and water use efficiency of plants grown below 50% of natural light showed significant difference compared with natural and shading rate 75% of natural light. There were significant difference between natural and shade treatments in the maximal quantum efficiency of PSII (F_v/F_m), as indicated that it was significantly less at full light than that at 50% of natural light. Initial fluorescence intensity (F_o) of plants was higher at full light than that at 50% of natural light, suggesting that photoinhibition occurred in natural light. The non-photochemical quenching (NQP) decreased with the aggravation of shade stress, indicating that shading decreased the efficiency of photochemical reaction by reducing the fraction of incident light in photochemical energy utilization and decreased thermal dissipation through regulating energy distribution in photosystem II (PSII) in the leaves of Hydrangea macrophylla. In general, the 70% of incident light in photochemical energy utilization was distributed to thermal dissipation, 20% was distributed to non-regulated energy dissipation and 4% was distributed to effective photochemical reaction. In conclusion, responses of plants to increased irradiance are governed by strategy: to utilize a high fraction of incident light in photochemistry and regulate energy dissipation in PSII and weaken the accumulation of excess excitation energy in PSII to protect the photosynthetic apparatus in the leaves of H. macrophylla under saturated radiation.     

Temperature-induced greening of<Emphasis Type="Italic"> Chlorella vulgaris</Emphasis>. The role of the cellular energy balance and zeaxanthin-dependent nonphotochemical quenching

When cells of the green alga Chlorella vulgaris Beij. are transferred from growth at 5 degrees C and an irradiance of 150 micromol photons m(-2) s(-1) to 27 degrees C and the same irradiance, they undergo what is normally considered a high-light to low-light phenotypic change. This involves a 3-fold increase in cellular chlorophyll content with a concomitant increase in light-harvesting complex polypeptide levels. This process appears to occur in response to the cellular capacity to utilize the products of photosynthesis, with the redox state of the plastoquinone pool sensing the cellular energy balance. The phenotypic adjustment can be enhanced or blocked using chemical inhibitors that modulate the redox state of the plastoquinone pool. The functional changes in the photosynthetic apparatus that occurred during the high-light to low-light acclimation were examined with special consideration paid to the paradox that 3-(3,4-dichlorophenyl)-1,1-dimethylurea (DCMU)-treated cells, with non-functional photosystem II (PSII), accumulate light-harvesting polypeptides. At the structural and basic functional levels, the light-harvesting complex of the cells treated with DCMU was indistinguishable from that of the untreated, control cells. To examine how PSII was protected in the DCMU-treated cells, we measured the content of xanthophyll-cycle pigments. It appeared that a zeaxanthin-dependent nonphotochemical quenching process was involved in PSII protection during greening in the presence of DCMU. Metabolic inhibitors of mitochondrial respiration were used to examine how the change in cellular energy balance regulates the greening process. Apparently, the mitochondrion acts to supply energy to the chloroplast during greening, and inhibition of mitochondrial respiration diminishes chlorophyll accumulation apparently through an increase in the redox state of the plastoquinone pool.     

Physiological effects and transport of 24-epibrassinolide in heat-stressed barley

This paper presents a study of the metabolic response (dark respiration intensity, photosystem II efficiency, metabolic activity) and the yield of barley treated with 24-epibrassinolide and subjected to high-temperature stress. Transport of exogenously applied 24-epibrassinolide in barley and changes in the profile of brassinosteroids that may occur in tissues after 24-epibrassinolide application were also studied. The water solution of 24-epibrassinolide (0.005 and 0.25 mg dm⁻³) was applied via infiltration of the first and second leaves of 12-day-old seedlings. Control plants were treated with water solution of hormone solvent (ethanol). Fifteen-day-old plants were subjected to high-temperature stress (42°C for 3 h). The influence of hormone treatment and stress conditions was investigated in the first and second leaves based on measurements of PSII efficiency. The aftereffect of plant treatment was investigated in the seventh leaf (measurements of PS II efficiency, dark respiration intensity, metabolic activity). The transport efficiency of 24-epibrassinolide exogenously applied to the first and second leaves, as well as the profile of other brassinosteroids, was also measured on the seventh leaf. Finally, yield formation was estimated. 24-epibrassinolide showed protective action, which manifested itself in the improved functioning of PSII, but this was observed in case of higher hormone concentration and only for the first, older leaf. The PSII efficiency of the seventh leaf was similar in plants treated with brassinosteroid and in the control plants, whereas the respiration intensity and metabolic activity decreased in plants previously treated with higher concentration of 24-epibrassinolide. The use of a higher hormone concentration at the seedling phase ultimately resulted also in lower crop yield. Brassinosteroids—brassinolide and castasterone—were detected in barley leaves. 24-epibrassinolide was found only in trace amounts in control plants. Its exogenous application directly to the apoplast of the first and second leaves resulted in an increase in the 24-epibrassinolide content in the seventh leaf, but did not depend on whether a high or low concentration had been applied to the plants.     

Photoinactivation and photoprotection of photosystem II in nature

Photosystem II plays a central role not only in energy transduction, but also in monitoring the molecular redox mechanisms involved in signal transduction for acclimation to environmental stresses. Central to the regulation of photosystem II (PSII) function as a light-driven molecular machine in higher plant leaves, is an inevitable photo-inactivation of one PSII after 10⁶–10⁷ photons have been delivered to the leaf, although the act of photoinactivation per se requires only one photon. PSII function in acclimated pea leaves shows a reciprocity between irradiance and the time of illumination, demonstrating that the photoinactivation of PSII is a light dosage effect, depending on the number of photons absorbed rather than the rate of photon absorption. Hence, PSII photoinactivation will occur at low as well as high irradiance. There is a heterogeneity of PSII functional stability, possibly with less stable PSII monomers being located in grana margins and more stable PSII dimers in appressed granal domains. Matching the inevitable photoinactivation of PSII, green plants have an intrinsic capacity for D1 protein synthesis to restore PSII function which is saturated at very low light. Photoinhibition of PSII in vivo is often a photoprotective strategy rather than a damaging process.     

Uncoupling light quality from light irradiance effects in Helianthus annuus shoots: putative roles for plant hormones in leaf and internode growth

An attempt has been made to uncouple the effects of the two primary components of shade light, a reduced red to far-red (R/FR) ratio and low photosynthetically active radiation (PAR), on the elongation of the youngest internode of sunflower (Helianthus annuus) seedlings. Maximal internode growth (length and biomass) was induced by a shade light having a reduced R/FR ratio (0.85) under the low PAR of 157 micromol m(-2) s(-1). Reducing the R/FR ratio under normal PAR (421 micromol m(-2) s(-1)) gave similar growth trends, albeit with a reduced magnitude of the response. Leaf area growth showed a rather different pattern, with maximal growth occurring at the higher (normal) PAR of 421 micromol m(-2) s(-1)), but with variable effects being seen with changes in light quality. Reducing the R/FR ratio (by enrichment with FR) gave significant increases in gibberellin A(1) (GA(1)) and indole-3-acetic acid (IAA) contents in both internodes and leaves. By contrast, a lower PAR irradiance had no significant effect on GA(1) and IAA levels in internodes or leaves, but did increase the levels of other GAs, including two precursors of GA(1). Interestingly, both leaf and internode hormone content (GAs, IAA) are positively and significantly correlated with growth of the internode, as are leaf levels of abscisic acid (ABA). However, changes in these three hormones bear little relationship to leaf growth. By implication, then, the leaf may be the major source of GAs and IAA, at least, for the rapidly elongating internode. Several other hormones were also assessed in leaves for plants grown under varying R/FR ratios and PARs. Leaf ethylene production was not influenced by changes in R/FR ratio, but was significantly reduced under the normal (higher) PAR, the irradiance treatment which increased leaf growth. Levels of the growth-active free base and riboside cytokinins were significantly increased in leaves under a reduced R/FR ratio, but only at the higher (normal) PAR irradiance; other light quality treatments evoked no significant changes. Taken in toto, these results indicate that both components of shade light can influence the levels of a wide range of endogenous hormones in internodes and leaves while evoking increased internode elongation and biomass accumulation. However, it is light quality changes (FR enrichment) which are most closely tied to increased hormone content, and especially with increased GA and IAA levels. Finally, the increases seen in internode and leaf GA content with a reduced R/FR ratio are consistent with FR enrichment inducing an overall increase in sunflower seedling GA biosynthesis.     

Up-regulation of the mitochondrial alternative oxidase pathway enhances photosynthetic electron transport under drought conditions

The aim of this study was to explore the role of the mitochondrial alternative oxidase (AOX) in the protection of photosynthesis during drought in wheat leaves. The relative water contents of water-replete and drought-exposed wheat plants were 97.2+/-0.3 and 75+/-2, respectively. Drought increased the amount of leaf AOX protein and also enhanced the rate of AOX-dependent O(2) uptake by the respiratory electron transport chain. The amount of the reduced, active form of the AOX protein was specifically increased by drought. The AOX inhibitor salicylhydroxamic acid (1 mM; SHAM) inhibited 70% of AOX activity in vivo in both water-replete and drought-exposed plants. Plants treated with SHAM were then exposed to low (100), high (350), or excess light (800 mumol photons m(-2) s(-1)) for 90 min. SHAM did not modify chlorophyll a fluorescence quenching parameters in water-replete controls after any of these treatments. However, while the maximal quantum yield of photosystem II (PSII) electron transport (F(v)/F(m)) was not affected by SHAM, the immediate quantum yield of PSII electron transport (Phi(PSII)) and photochemical quenching (qP) were gradually reduced by increasing irradiance in SHAM-treated drought-exposed plants, the decrease being most pronounced at the highest irradiance. Non-photochemical quenching (NPQ) reached near maximum levels in plants subjected to drought at high irradiance. However, a combination of drought and low light caused an intermediate increase in NPQ, which attained higher values when AOX was inhibited. Taken together, these results show that up-regulation of the respiratory AOX pathway protects the photosynthetic electron transport chain from the harmful effects of excess light.     

Antisense reductions in the PsbO protein of photosystem II leads to decreased quantum yield but similar maximal photosynthetic rates

Photosystem (PS) II is the multisubunit complex which uses light energy to split water, providing the reducing equivalents needed for photosynthesis. The complex is susceptible to damage from environmental stresses such as excess excitation energy and high temperature. This research investigated the in vivo photosynthetic consequences of impairments to PSII in Arabidopsis thaliana (ecotype Columbia) expressing an antisense construct to the PsbO proteins of PSII. Transgenic lines were obtained with between 25 and 60% of wild-type (WT) total PsbO protein content, with the PsbO1 isoform being more strongly reduced than PsbO2. These changes coincided with a decrease in functional PSII content. Low PsbO (less than 50% WT) plants grew more slowly and had lower chlorophyll content per leaf area. There was no change in content per unit area of cytochrome b(6)f, ATP synthase, or Rubisco, whereas PSI decreased in proportion to the reduction in chlorophyll content. The irradiance response of photosynthetic oxygen evolution showed that low PsbO plants had a reduced quantum yield, but matched the oxygen evolution rates of WT plants at saturating irradiance. It is suggested that these plants had a smaller pool of PSII centres, which are inefficiently connected to antenna pigments resulting in reduced photochemical efficiency.     

Functional mitochondrial complex I is required by tobacco leaves for optimal photosynthetic performance in photorespiratory conditions and during transients

The importance of the mitochondrial electron transport chain in photosynthesis was studied using the tobacco (Nicotiana sylvestris) mutant CMSII, which lacks functional complex I. Rubisco activities and oxygen evolution at saturating CO(2) showed that photosynthetic capacity in the mutant was at least as high as in wild-type (WT) leaves. Despite this, steady-state photosynthesis in the mutant was reduced by 20% to 30% at atmospheric CO(2) levels. The inhibition of photosynthesis was alleviated by high CO(2) or low O(2). The mutant showed a prolonged induction of photosynthesis, which was exacerbated in conditions favoring photorespiration and which was accompanied by increased extractable NADP-malate dehydrogenase activity. Feeding experiments with leaf discs demonstrated that CMSII had a lower capacity than the WT for glycine (Gly) oxidation in the dark. Analysis of the postillumination burst in CO(2) evolution showed that this was not because of insufficient Gly decarboxylase capacity. Despite the lower rate of Gly metabolism in CMSII leaves in the dark, the Gly to Ser ratio in the light displayed a similar dependence on photosynthesis to the WT. It is concluded that: (a) Mitochondrial complex I is required for optimal photosynthetic performance, despite the operation of alternative dehydrogenases in CMSII; and (b) complex I is necessary to avoid redox disruption of photosynthesis in conditions where leaf mitochondria must oxidize both respiratory and photorespiratory substrates simultaneously.     

The change of chlorophyll fluorescence parameters in winter barley during recovery after freezing shock and as affected by cold acclimation and irradiance

The change of chlorophyll fluorescence parameters in froze leaves of 3 leaf-age seedlings were examined using two winter barley cultivars (Chumai 1 and Mo 103) differing in cold tolerance to investigate physiological response to low temperature as affected by cold acclimation (under 3/1 degrees C, day/night for 5 days before freezing treatment) and irradiation size (high irradiance: 380+/-25 micromol m(-2)s(-1) and low irradiance: 60+/-25 micromol m(-2)s(-1)) during recovery. The results showed that non-lethal freezing shock (exposed to -8 degrees C for 18 h) did not obviously affect maximum quantum efficiency in photosystem II (PSII), but dramatically increased non-photochemical quenching and reduced effective quantum yield in PSII. Cold acclimation significantly improved stability of photosynthetic function of leaves after freezing stress through buffering excessive energy and alleviating photoinhibition during recovery, indicating it increased recovery ability of barley plants from freezing injury. High irradiance was quite harmful to the stability of PSII in barley plants during recovery from freezing injury. The electron transport rate of PSII varied with cold-acclimation, irradiance and genotype. Cold acclimation caused significant increase in electron transport rate of PSII for relatively tolerant cultivar Mo 103, but not for relatively sensitive cultivar Chumai 1. It can be concluded that some chlorophyll fluorescence parameters during recovery from freezing shock may be used as the indicators in identification and evaluation of cold tolerance in barley.