bluuebunny/crossref_metadata_2025 huggingface dataset:
The structure, function and dynamics of photosystem two
Authors: Barber J. , Nield J. , Morris E. P. , Zheleva D. , Hankamer B.
Published: May 2006

One of the greatest challenges in modern photosynthesis research is to elucidate fully the structural and functional properties of photosystem two (PSII). This water‐plasto‐quinone oxidoreductase is located in a membrane complex composed of more than 25 subunits. The primary and secondary structures of all known subunits which constitute the central core of PSII are reviewed. How these subunits interact with each other to produce the tertiary and quaternary structure of PSII in vivo is not fully understood. However, electron microscopy is helping to fill this gap in our knowledge both by single particle analysis and electron crystallography. These studies suggest that active PSII is dimeric, although the functional significance of this oligomeric state is not yet understood. Moreover, the elucidation of the structure of photosystem one (PSI) by X‐ray crystallography has revealed features which are likely to be relevant to PSII structure. It seems highly likely that the D1 protein with CP43 and D2 protein with CP47 (summing 11 transmembrane helices in each case) will have structural similarities to the organisation of PsaA and PsaB. It is likely that the turnover of the D1 protein is aided by the relatively easy removal of CP43 from this arrangement of the PSII core.
Mechanistic and structural aspects of photosynthetic water oxidation
Authors: Renger G.
Published: May 2006

Conclusions on the functional and structural organisation of photosynthetic water oxidation are gathered from a critical survey of the wealth of data reported in the literature and author's own experimental research: (1) the water oxidising complex (WOC) contains a tetranuclear manganese cluster of dimer of dimers' structure and functional heterogeneity of the metal centers, (2) the four step univalent oxidative pathway leading to water oxidation into molecular oxygen and four protons comprises only manganese, tyrosine YZ of polypeptide Dl and the substrate as redox active species, (3) the redox transitions S0→ S1 and S1→ S2 are manganese centered whereas S2→ S3 is most likely a ligand‐centered reaction, (4) there exist several lines of evidence for a marked structural change that accompanies the redox transition S2→ S3, (5) one Ca2+ is an indispensible constituent of a functionally competent WOC while the role of Cl is much less clear and a direct participation disputable, (6) substrate water is most likely bound in all redox states S0,…,S3 and exchangeable with the bulk phase. The protonation state is determined by the redox state S1 and the protein microenvironment. A mechanism is proposed for water oxidation in the WOC that is based on three key postulates: (1) water oxidation takes place in the first coordination sphere of one manganese dimer [MnaMnb]; (2) the essential O‐O bond is preformed in S3 as part of a rapid redox isomerism S3(I)→S3(II) where in S3(II) a nuclear geometry and electronic configuration is attained that corresponds to a peroxidic‐type species; and (3) S3(II) is an ‘entatic state’ for the formation of complexed dioxygen triggered by YZOX induced electron abstraction from the WOC and electronic redistribution to S0(O2).
Molecular aspects of photosystem I
Authors: Scheller Henrik Vibe , Naver Helle , Møller Birger Lindberg
Published: May 2006

Photosystem I (PSI) in higher plants consists of 17 polypeptide subunits. Cofactors are chlorophyll a and b, β‐carotene, phylloquinone and iron‐sulfur clusters. Eight subunits are specific for higher plants while the remaining ones are also present in cyanobacteria. Two 80‐kDa subunits (PSI‐A and ‐B) constitute the major part of PSI and bind most of the pigments and electron donors and acceptors. The 9‐kDa PSI‐C carries the remaining electron acceptors which are [4Fe‐4S] iron sulfur clusters. PSI‐D, ‐E and ‐H have importance for integrity and function at the stromal face of PSI while PSI‐F has importance for function at the lumenal face. PSI‐N is localized at the lumenal side, but its function is unknown. Four subunits are light‐harvesting chlorophyll a/b‐binding proteins. The remaining subunits are integral membrane proteins with poorly understood function. Subunit interactions have been studied in reconstitution experiments and by cross‐linking studies. Based on these data, it is concluded that iron‐sulfur cluster FB is proximal to FX and that FA is the terminal acceptor in PSI. Similarities between PSI and the reaction center from green sulfur bacteria are discussed.
The cytochrome b6f complex. Novel aspects
Authors: Cramer William A. , Soriano Glenda M. , Zhang Huamin , Ponamarev Michael V. , Smith Janet L.
Published: May 2006

The Cyt b6f complex from plant chloroplasts, the green alga Chlamydomonas reinhardtii, and the thermophilic cyanobacterium, Mastigocladus laminosus, can be isolated in a highly active state, in which it is dimeric and contains one bound chlorophyll a molecule per monomeric unit. The latter feature is a distinguishing trait compared to the b6f complex of bacterial photosynthesis and the respiratory chain. In contrast to the trans‐membrane domains of the b6f complex, and of most other integral membrane proteins, which are characterized by an a‐helical structure, the p‐side peripheral domains, consisting of Cyt f and the Rieske protein, have a predominantly β‐strand secondary structure motif. One consequence of this motif is an extension of these polypeptides from the membrane surface. For example, the length of Cyt f is 75 Å. The heme Fe is 45 Å from the α‐carbon of Arg250 at the membrane bilayer interface and, even though Cyt f may be tilted relative to the membrane plane, the heme electron transfer reactions are carried out far from the membrane surface. The presence of an internal 5 water chain, which has the properties of a proton wire, with one water H‐bonded to the histidine‐25 heme ligand, also suggests that the pathway of long distance H+ translocation traverses the extended p‐side protein domain of the b6f complex. A mechanism of H+ transfer in the chain that is coupled to the redox state of the heme, in which a proton is transferred into the chain to compensate the extra electron in the ferro‐heme, is proposed.
Redox signalling and the structural basis of regulation of photosynthesis by protein phosphorylation
Authors: Allen J. F. , Nilsson A.
Published: May 2006

In photosynthesis in chloroplasts and cyanobacteria, redox control of thylakoid protein phosphorylation regulates distribution of absorbed excitation energy between the two photosystems. When electron transfer through chloroplast photosystem II (PSII) proceeds at a rate higher than that through photosystem I (PSI), chemical reduction of a redox sensor activates a thylakoid protein kinase that catalyses phosphorylation of light‐harvesting complex II (LHCII). Phosphorylation of LHCII increases its affinity for PSI and thus redistributes light‐harvesting chlorophyll to PSI at the expense of PSII. This short‐term redox signalling pathway acts by means of reversible, post‐translational modification of pre‐existing proteins. A long‐term equalisation of the rates of light utilisation by PSI and PSII also occurs: by means of adjustment of the stoichiometry of PSI and PSII. It is likely that the same redox sensor controls both state transitions and photosystem stoichiometry. A specific mechanism for integration of these short‐ and long‐term adaptations is proposed. Recent evidence shows that phosphorylation of LHCII causes a change in its 3‐D structure, which implies that the mechanism of state transitions in chloroplasts involves control of recognition of PSI and PSII by LHCII. The distribution of LHCII between PSII and PSI is therefore determined by the higher relative affinity of phospho‐LHCII for PSI, with lateral movement of the two forms of the LHCII being simply a result of their diffusion within the membrane plane. Phosphorylation‐induced dissociation of LHCII trimers may induce lateral movement of monomeric phospho‐LHCII, which binds preferentially to PSI. After dephosphorylation, monomeric, unphosphorylated LHCII may trimerize at the periphery of PSII.
Redox‐controlled thylakoid protein phosphorylation. News and views
Authors: Gal A. , Zer H. , Ohad I.
Published: May 2006

Thylakoid protein phosphorylation regulates state transition and PSII protein turnover under light‐dependent redox control via a signal transduction system. The redox‐dependent activation/deactivation of the membrane‐bound protein kinase(s), mostly localized in the grana partitions, differs for the various phosphoproteins. Reduction of the plastoquinone pool may be sufficient to activate phosphorylation of few of these proteins. Phosphorylation of LHCII, requires the presence of the cytochromebfcomplex in an ‘activating mode’ characterized by the reduction of its high potential path components and ability to interact with a reduced plastoquinol without oxidizing it. Activation and maintenance of this kinase activity is considered to involve alternate interactions with a cytochromebfin its activating mode and with the substrate PSII(LHCII). The segregation of the thylakoid components into grana and stroma partitions appears to be mandatory for the kinase activation process. The protein substrate specificity and kinetics differs for various kinases. The thylakoid redox‐controlled kinase(s) have not yet been isolated. Preparations highly enriched in kinase activity capable to phosphorylate LHCII and PSII core proteins, contain two kinase active bands, resolved by denaturing electrophoresis and renaturation, and having apparent molecular masses of about 53 and 66 kDa. The roughly estimated abundance of these putative kinase(s) in the grana partitions may be compatible with a ratio of kinase(s): PSII(LHCII) dimers:cytochromebfdimers in the range of 1:60:30 and a ratio of kinase:phosphorylation sites of about 1:2000. Only about 10–20% of these sites are phosphorylated during state transition. The low turnover rate of the LHCII kinase(s) (< 5) may be due to hindrance of the required random lateral migration within the grana domain rich in tightly packed PSII(LHCII) and cytochromebfcomplexes.
Ascorbate synthesis and ascorbate peroxidase activity during the early stage of wheat germination
Authors: De Gara L. , de Pinto M. C. , Arrigoni O.
Published: May 2006

Embryos from dry caryopses of wheat (Triticum durum L. cv. Norba) are completely devoid of ascorbate (ASC) but contain a low amount of dehydroascorbate (DHA). The de novo biosynthesis of ASC starts in the wheat embryos after 8–10 h of germination but before the ASC biosynthetic pathway is completely restored the embryos can provide themselves with ASC by the reduction of the stored DHA. Three different proteins having DHA‐reducing capability are present in the embryos during the early stages of germination. However, when the de novo ASC biosynthesis from sugar is completely restored, the DHA reduction capability largely drops and only one DHA‐reducing protein remains active. The presence of three proteins having DHA‐reducing capability and their behaviour during germination is discussed.

Dry embryos are also devoid of ASC peroxidase (EC 1.11.1.11); this hydrogen peroxide scavenger enzyme appears after the same lag as ASC and increases during germination in parallel with the rise in ASC. When ASC biosynthesis is experimentally induced, the ASC peroxidase also appears earlier; moreover the affinities for ASC of the three ASC peroxidase isoenzymes that progressively appear during germination depend on the ASC available in the embryos: highest in the first isoenzyme, that appears when the ASC content is very low, lowest in the isoenzyme that is expressed last, when the ASC content is 10–11 times higher.
Effects of in vivo copper treatment on the photosynthetic apparatus of two Triticum durum cultivars with different stress sensitivity
Authors: Ciscato Massimo , Valcke Roland , Van Loven Karen , Clijsters Herman , Navari‐Izzo Flavia
Published: May 2006

The effect of copper on the photosynthetic apparatus of two cultivars of durum wheat (Triticum durum cvs Adamello and Ofanto) with different sensitivity to drought and nickel stress were investigated. Plants were grown in nutrient solution or in nutrient solution further supplemented with CuSO4 to achieve final concentrations of 3.6 μM and 20 μM Cu. Several fluorescence analyses were performed, in presence or absence of DCMU, and with varying light intensities. Furthermore, light and electron microscopic investigations were carried out. In vivo treatment using 3.6 μ Cu produced a marked reduction in growth of the Cu‐treated plants, but only mild effects on the fluorescence‐related parameters. The Cu‐induced reduction in the area above the fluorescence induction curve and in the time needed to reach the maximum of chlorophyll fluorescence (Fmax) were more pronounced. These results favour the hypothesis that under such conditions copper affects photosynthesis mainly in an indirect way, causing a slowing down of the electron transport as a consequence of the reduced requirement for photosynthesis products. The morphological analyses corroborate this hypothesis, showing toxic effects on the chloroplast structure due to Cu treatment. The differences between the two cultivars were not as pronounced as reported in the case of nickel or drought stresses; nevertheless, cv. Ofanto seemed to be less sensitive also to Cu stress than cv. Adamello.
Metabolic fate of guanosine in higher plants
Authors: Ashihara Hiroshi , Takasawa Yoko , Suzuki Takeo
Published: May 2006

The aim of the present study was to investigate the metabolic fate of guanine nucleotides in higher plants. The rate of uptake of [8‐14C]guanosine by suspension‐cultured Catharanthus roseus cells was more than 20 times higher than that of [8‐14C]guanine. The rate of uptake of [8‐14C]guanosine increased with the age of the culture. Pulse‐chase experiments with [8‐14C]guanosine revealed that some of the guanosine that had been taken up by the cells was converted to guanine nucleotides and incorporated into nucleic acids. A significant amount of [8‐14C]guanosine was degraded directly to xanthine, allantoin and allantoic acid, with the generation of 14CO2 as the final product. The rate of salvage of [8‐14C]guanosine for the synthesis of nucleic acids was highest in young cells, while the rate of degradation increased with the age of the cells. In segments of roots from Vigna mungo seedlings, nearly 50% of the [8‐14C]guanosine that had been absorbed over the course of 15 min was recovered in guanine nucleotides. A significant amount of the radioactivity in nucleotides became associated with nucleic acids and ureides during ‘chase’ periods. In segments of young leaves of Camellia sinensis, [8‐14C]guanosine was initially incorporated into guanine nucleotides, nucleic acids, theobromine and ureides, and the radioactivity in these compounds was transferred to caffeine and CO2 during a 24‐h incubation. Our results suggest that guanosine is an intermediate in the catabolism of guanine nucleotides and that it is re‐utilised for nucleotide synthesis by ‘salvage’ reactions. Guanosine was catabolised by the conventional degradation pathway via xanthine and allantoin. In some plants, guanosine is also utilised for the formation of ureide or the biosynthesis of caffeine.
Proton and calcium flux oscillations in the elongation region correlate with root nutation
Authors: Shabala Sergey N. , Newman Ian A.
Published: May 2006

The involvement of Ca2+ and H+ flux oscillations in root nutation was studied for decapped roots of corn (Zea mays L. cv. Aussie Gold) placed horizontally. Net ion fluxes were measured around the elongation and meristematic regions using a microelectrode ion flux measuring system. High correlation between H+ flux oscillations and root nutations was found in the elongation region. Two oscillatory components of H+ flux, with periods of about 90 min and 7 min, correlated with root circumnutations and micronutations, respectively. The periods of H+ flux oscillations and rhythmical root movements in this region could be modified similarly by external factors including pH. In the meristematic region no association between ion flux behaviour and nutation was apparent. Ion flux oscillations and nutations both decreased in amplitude as the growth rate at the measured location decreased. Possible involvement of ion flux oscillations in root circumnutation is discussed. It is concluded that a model involving an internal oscillator must be developed to explain the H+ flux involvement in root nutations.
Effects of elevated CO2 concentration on photosynthesis, respiration and carbohydrate status of coppice Populus hybrids
Authors: Will Rodney E. , Ceulemans Reinhart
Published: May 2006

To determine how increased atmospheric CO2 will affect the physiology of coppiced plants, sprouts originating from two hybrid poplar clones (Populus trichocarpa × P. deltoides ‐ Beaupre and P. deltoides × P. nigra ‐ Robusta) were grown in open‐top chambers containing ambient or elevated (ambient + 360 μmol mol−1) CO2 concentration. The effects of elevated CO2 concentration on leaf photosynthesis, stomatal conductance, dark respiration, carbohydrate concentration and nitrogen concentration were measured. Furthermore, dark respiration of leaves was partitioned into growth and maintenance components by regressing specific respiration rate vs specific growth rate. Sprouts of both clones exposed to CO2 enrichment showed no indication of photosynthetic down‐regulation. During reciprocal gas exchange measurements, CO2 enrichment significantly increased photosynthesis of all sprouts by approximately 60% (P < 0.01) on both an early and late season sampling date, decreased stomatal conductance of all sprouts by 10% (P < 0.04) on the early sampling date and nonsignificantly decreased dark respiration by an average of 11%. Growth under elevated CO2 had no consistent effect on foliar sugar concentration but significantly increased foliar starch by 80%. Respiration rate was highly correlated with both specific growth rate and percent nitrogen. Long‐term CO2 enrichment did not significantly affect the maintenance respiration coefficient or the growth respiration coefficient. Carbon dioxide enrichment affected the physiology of the sprouts the same way it affected these plants before they were coppiced.
Maturation of somatic embryos of Pinus strobus is promoted by a high concentration of gellan gum
Authors: Klimaszewska Krystyna , Smith Dale R.
Published: May 2006

Application of somatic embryogenesis to Pinus strobus clonal propagation and genetic improvement was hampered by the difficulty in achieving synchronous maturation of a large number of somatic embryos that would germinate and produce plants. Media containing abscisic acid (80 μM) and osmotic agents such as sucrose, polyethylene glycol and/or dextran did not sustain development of mature somatic embryos from plated embryonal masses. This indicated that factors other than osmotic agents might be involved in sustaining development of Pinus strobus somatic embryos to maturity. It was subsequently found that media lacking osmotica but containing a high concentration of gellan gum (1%) induced significant improvement in the development of mature somatic embryos in the presence of 80 or 120 μM abscisic acid. This positive effect was independent of the genotype and all four tested lines displayed similar responses. Media containing gellan gum at concentrations from 0.4 to 1.2% formed gels that varied in their strength. Gel strength was proportional to the concentration of gellan gum in the specific medium but varied depending on the medium formulation. Gel strength increased with the duration of storage of the culture medium by 46% (SD 14) after 14 days of storage. Preliminary results showed that embryos matured on high gellan gum media displayed improved germination frequencies. These results indicate that in Pinus strobus the water status and possibly other medium characteristics that are influenced by increased concentration of gelling agent have stimulatory effects on maturation of somatic embryos.
In‐vitro‐cultured subclover root can develop Fe‐deficiency stress response
Authors: Wei Liangchou , Ocumpaugh William R. , Loeppert Richard H.
Published: May 2006

The Fe‐deficiency stress response is induced in most plants under Fe‐deficient conditions, but whether the shoot and/or the root control development of the stress response is not known. The objectives of the present study were to determine whether in‐vitro‐cultured subclover roots can develop Fe‐deficiency stress response and to examine this approach as a possible screening technique for Fe‐deficiency resistance. One‐cm long root tips of subclover seedlings were cultured in modified White's medium without (‐Fe) or with (+Fe) 100 μM Fe3+EDTA. Root Fe3+ reduction and H+ release were evaluated. On the first day after transfer to the ‐Fe medium, the Fe‐deficiency‐resistant cultivar Koala (Trifolium brachycalycinum Katzn. and Morley) started to release H+, resulting in a decrease in pH of the culture medium, while the susceptible cultivar Karridale (T. subterraneum L.) did not release H+ until the second day. The H+‐release rate of the ‐Fe Koala was approximately twice as high as that of the ‐Fe Karridale for the first 4 days of ‐Fe treatment. Both Koala and Karridale reached their highest H+‐release rates on the fourth day after ‐Fe treatment initiation. The +Fe Koala released H+ after several days of culture, but the H+ release of the ‐Fe Koala was severalfold greater than that of the +Fe Koala. The implicit correlation between H+ release and Fe‐deficiency resistance was substantiated by using a series of subclover cultivars with a range of susceptibilities to Fe deficiency. The pH of the ‐Fe culture media of the series of cultivars was positively correlated to their Fe‐chlorosis scores reported in previous research. The results of the present study indicate that root itself has the full ability to develop Fe‐deficiency stress response and the response is dependent on the root Fe status. The results also suggest that root culture could be used as a simple and efficient alternative technique for screening germplasm for Fe‐deficiency resistance.
Gas exchange is related to the hormone balance in mycorrhizal or nitrogen‐fixing alfalfa subjected to drought
Authors: Goicoechea N. , Antolín M. C. , Sánchez‐Díaz M.
Published: May 2006

The beneficial effect of mycorrhization on photosynthetic gas exchange of host plants under drought conditions could be related to factors other than changes in phosphorus nutrition and water uptake. Our objective was to study the influence of drought on phytohormones and gas exchange parameters in Medicago sativa L. cv. Aragón associated with or in the absence of arbuscular mycorrhizal (AM) fungi and/or nitrogen‐fixing bacteria. Four treatments were used: (1) plants inoculated with Glomus fasciculatum (Taxter sensu Gerd.) Gerdemann and Trappe and Rhizobium meliloti 102 F51 strain (MR); (2) plants inoculated with only Rhizobium (R); (3) plants inoculated with only mycorrhizae (M); and (4) non‐inoculated plants (N). When endophytes were well established, treatments received different levels of phosphorus and nitrogen in the nutrient solution in order to obtain plants similar in size. Sixty days after planting, plants were subjected to two cycles of drought and recovery. Midday leaf water potential (Ψ), CO2 exchange rate (CER), leaf conductance (gw) and transpiration (T), as well as leaf and root abscisic acid (ABA) and cytokinin concentrations were measured after the second drought period. Gas exchange parameters were determined by infrared gas analysis. Cytokinins and ABA levels in tissues were analysed by ELISA and HPLC, respectively. Nodulated R and MR plants had the lowest ABA concentrations in roots under well‐watered conditions. Water stress increased ABA concentrations in leaves of N, R and MR plants, while ABA concentration in M plants did not change. The highest production of ABA under water deficit was in the roots of non‐mycorrhizal plants. The ratio of ABA to cytokinin concentration strongly increased in leaves and roots of non‐mycorrhizal plants under drought. By contrast, this ratio was lowered in roots of M plants and remained unchanged in leaves and roots of MR plants when stress was imposed. The highest leaf conductances and transpirational fluxes under well‐watered conditions were those of nitrogen‐fixing R and MR plants, but these results were not impaired with increased CO2 exchange rates. Photosynthesis, leaf conductance and transpiration rates decreased in all treatments when stress was imposed, with the strongest decrease occurring in non‐mycorrhizal plants. The relationships found between these gas exchange parameters and the hormone concentrations in stressed alfalfa tissues suggest that microsymbionts have an important role in the control of gas exchange of the host plant through hormone production in roots and the ABA/cytokinin balance in leaves. The most relevant effect of mycorrhizal fungi was observed under drought conditions.
The plant aspartate aminotransferase gene family
Authors: Wadsworth Gregory J.
Published: May 2006

Aspartate aminotransferase exists as multiple isozymes in plants. These AAT isozymes are differentially localized in at least four different subcellular compartments, including the cytosol, plastids, mitochondria, and peroxisomes. The recent characterization of cDNA and genomic sequences encoding AAT from several plants has provided insight into the structure, subcellular targeting, and evolution of the AAT isozymes. The AAT isozymes appear to be targeted to subcellular compartments by the same mechanisms utilized by other targeted proteins. Both the plastid AAT and mitochondrial AAT are synthesized as precursors with appropriate N‐terminal targeting sequences which are proteolytically removed after localization in their respective organelles. The cytosolic AAT isozymes lack apparent targeting sequences, thus restricting it to the cytosol. A peroxisomal AAT has not been cloned and therefore its targeting has not been investigated. Sequence analysis demonstrates that the plant AAT isozymes are in the same protein family as the vertebrate and bacterial AATs. Furthermore, similarity between vertebrate and plant mitochondrial AAT isozymes suggest the evolution of this AAT isozyme may predate the divergence of plants and animals. Analysis of plastid and cytosolic AAT suggests that they have diverged more recently than the mitochondrial gene. The sequence analysis also reveals that the cytosolic AAT is diverging at a rate 1.6 times faster than the plastid AAT. The cDNA and genomic clones have been used to investigate expression of the AAT genes during nodulation in legumes and suggest that the nodule‐enhanced expression of plastid AAT is controlled at the transcriptional level.
Plasmodesmatal frequency in relation to short‐distance transport and phloem loading in leaves of barley (Hordeum vulgare). Phloem is not loaded directly from the symplast
Authors: Botha C. E. J. , Cross R. H. M.
Published: May 2006

We investigated the phloem loading pathway in barley, by determining plasmodesmatal frequencies at the electron microscope level for both intermediate and small blade bundles of mature barley leaves. Lucifer yellow was injected intercellularly into bundle sheath, vascular parenchyma, and thin‐walled sieve tubes. Passage of this symplastically transported dye was monitored with an epifluorescence microscope under blue light. Low plasmodesmatal frequencies endarch to the bundle sheath cells are relatively low for most interfaces terminating at the thin‐ and thick‐walled sieve tubes within this C3 species. Lack of connections between vascular parenchyma and sieve tubes, and low frequencies (0.5% plasmodesmata per μm cell wall interface) of connections between vascular parenchyma and companion cells, as well as the very low frequency of pore‐plasmodesmatal connections between companion cells and sieve tubes in small bundles (0.2% plasmodesmata per μm cell wall interface), suggest that the companion cell‐sieve tube complex is symplastically isolated from other vascular parenchyma cells in small bundles.

The degree of cellular connectivity and the potential isolation of the companion cell‐sieve tube complex was determined electrophysiologically, using an electrometer coupled to microcapillary electrodes. The less negative cell potential (average –52 mV) from mesophyll to the vascular parenchyma cells contrasted sharply with the more negative potential (–122.5 mV) recorded for the companion cell‐thin‐walled sieve tube complex. Although intercellular injection of lucifer yellow clearly demonstrated rapid (0.75 μm s‐1) longitudinal and radial transport in the bundle sheath‐vascular parenchyma complex, as well as from the bundle sheath through transverse veins to adjacent longitudinal veins, we were neither able to detect nor present unequivocal evidence in support of the symplastic connectivity of the sieve tubes to the vascular parenchyma. Injection of the companion cell‐sieve tube complex, did not demonstrate backward connectivity to the bundle sheath.

We conclude that the low plasmodesmatal frequencies, coupled with a two‐domain electropotential zonation configuration, and the negative transport experiments using lucifer yellow, precludes symplastic phloem loading in barley leaves.
The carotenoid‐deficient mutant, C‐6E, of Scenedesmus obliquus is blocked at the site of phytoene synthase
Authors: Sandmann Gerhard , Bishop Norman L , Senger Horst
Published: May 2006

In contrast to the wild type strain of Scenedesmus, mutant C‐6E synthesized only trace amounts of the carotenoids violaxanthin and lutein during prolonged heterotrophic growth. All other carotenoids and carotenoid precursors, such as phytoene, were undetectable. Additionally, only reduced levels of chlorophyll a and no chlorophyll b were formed. To evaluate the potential site of inhibition in the pathway for carotenoid biosynthesis the enzymatic activities of geranylgeranyl pyrophosphate synthase and phytoene synthase were assayed in cell‐free extracts. Both enzymes were highly active in extracts of the wild type but only geranylgeranyl pyrophosphate synthase was active in comparable extracts from mutant C‐6E. This observation strongly indicates that the phenotype of C‐6E results from either a mutation of the phytoene synthase structural gene or of a regulatory gene involved in expression of this enzyme. Other phenotypic effects on composition and structure of the photosynthetic apparatus are discussed as a secondary consequence of the carotenoid deficiency in the thylakoid membranes.
Inclination of sun and shade leaves influences chloroplast light harvesting and utilization
Authors: Myers D. A. , Jordan D. N. , Vogelmann T. C.
Published: May 2006

Light harvesting and utilization by chloroplasts located near the adaxial vs the abaxial surface of sun and shade leaves were examined by fluorometry in two herbaceous perennials that differed in their anatomy and leaf inclination. Leaves of Thermopsis montana had well‐developed palisade and spongy mesophyll whereas the photosynthetic tissue of Smilacina stellata consisted of spongy mesophyll only. Leaf orientation depended upon the irradiance during leaf development. When grown under low‐light levels, leaves of S. stellata and T. montana were nearly horizontal, whereas under high‐light levels, S. stellata leaves and T. montana leaves were inclined 600 and 300, respectively. Leaf inclination increased the amount of light that was intercepted by the lower leaf surfaces and affected the photosynthetic properties of the chloroplasts located near the abaxial leaf surface. The slowest rates of quinone pool reduction and reoxidation were found in chloroplasts located near the adaxial leaf surface of T. montana plants grown under high light, indicating large quinone pools in these chloroplasts. Chloroplasts near the abaxial surface of low‐light leaves had lower light utilization capacities as shown by photochemical quenching measurements. The amount of photosystem II (PSII) down regulation, measured from each leaf surface, was also found to be influenced by irradiance and leaf inclination. The greatest difference between down regulation monitored from the adaxial vs abaxial surfaces was found in plants with horizontal leaves. Different energy dissipation mechanisms may be employed by the two species. Values for down regulation in S. stellata were 2–3 times higher than those in T. montana, while the portion of the PSII population which was found to be QB nonreducing was 4–6 times lower in high light S. stellata leaves than in T. montana. All values of Stern‐Volmer type nonphotochemical quenching (NPQ) from S. stellata leaves were similar when quenching analysis was performed at actinic irradiances that were higher than the irradiance to which the leaf surface was exposed during growth. In contrast, with T. montana, NPQ values from the abaxial leaf surface were up to 45% higher than those from the adaxial leaf surface regardless of growth conditions. The observed differences in chloroplast properties between species and between the adaxial and abaxial leaf surfaces may depend upon a complex interaction among light, leaf anatomy and leaf inclination.
Aluminium induces rapid changes in cytosolic pH and free calcium and potassium concentrations in root protoplasts of wheat (Triticum aestivum)
Authors: Lindberg Sylvia , Strid Håkan
Published: May 2006

Addition of aluminium chloride (50 μM Al) caused different effects on the transmembrane electrical potential (PD) of root cells in Al‐tolerant wheat (Triticum aestivum) cv. Kadett and Al‐sensitive cv. WW 20299. As changes in PD of plant cells may depend on transient fluxes of protons, potassium and/or calcium through cell membranes, the effect of Al was investigated on the cytosolic concentrations of these ions in protoplasts isolated from root tips of the same cultivars. The tetra[acetoxymethyl] esters of the fluorescent dyes bis‐carboxyethyl‐carboxyfluorescein, BCECF, K+‐binding benzofuran isophthalate, PBFI, and the stilbene chromophore Fura 2‐AM were used to determine pH, K+ and Ca2+, respectively. Changes in fluorescence ratios, directly reflecting changes in [H+], [K+] and [Ca2+] in the cytosol, were determined by photometry fluorescence microscopy. Additions and removals of Al to and from both cultivars caused hyperpolarizations and depolarizations, respectively, but only in the sensitive cv. WW 20299 did the resting PD decrease gradually. Addition of Al to the protoplasts caused rapid changes in cytosolic pH, free [K+] and [Ca2+]. In both cultivars Al caused a transient oscillating increase in cytosolic [Ca2+] for 1 or 2 min and a rapid pH‐dependent change in cytosolic [K+]. At pH 5 the presence of K+ in the medium diminished the Al‐induced decrease in cytosolic [K+]. Aluminium (50 μM) induced a transient increase in cytosolic [H+] (pH decreased) in both cultivars, but the cytosolic pH returned to its initial value only in the Al‐tolerant cv. Kadett. In the Alsensitive cv. WW 20299, repeated additions of Al caused a gradual decline in pH. Moreover, in the presence of 1 mM KCl, pH recovered completely in both cultivars. Since only the effect on pH differed in the two cultivars, the more toxic effect of Al on the cv. WW 20299 should be related to the change in pH.
Antifreeze protein accumulation in freezing‐tolerant cereals
Authors: Antikainen Mervi , Griffith Marilyn
Published: May 2006

Freezing‐tolerant plants withstand extracellular ice formation at subzero temperatures. Previous studies have shown that winter rye (Secale cereale L.) accumulates proteins in the leaf apoplast during cold acclimation that have antifreeze properties and are similar to pathogenesis‐related proteins. To determine whether the accumulation of these antifreeze proteins is common among herbaceous plants, we assayed antifreeze activity and total protein content in leaf apoplastic extracts from a number of species grown at low temperature, including both monocotyledons (winter and spring rye, winter and spring wheat, winter barley, spring oats, maize) and dicotyledons (spinach, winter and spring oilseed rape [canola], kale, tobacco). Apoplastic polypeptides were also separated by SDS‐PAGE and immunoblotted to determine whether plants generally respond to low temperature by accumulating pathogenesis‐related proteins. Our results showed that significant levels of antifreeze activity were present only in the apoplast of freezing‐tolerant monocotyledons after cold acclimation at 5/20C. Moreover, only a closely related group of plants, rye, wheat and barley, accumulated antifreeze proteins similar to pathogenesis‐related proteins during cold acclimation. The results indicate that the accumulation of antifreeze proteins is a specific response that may be important in the freezing tolerance of some plants, rather than a general response of all plants to low temperature stress.
Desiccation decreases abscisic acid content in hybrid larch (Larix X leptoeuropaea) somatic embryos
Authors: Dronne Sandrine , Label Philippe , Lelu Marie‐Anne
Published: May 2006

Previous studies indicated that the high endogenous abscisic acid (ABA) content of hybrid larch (Larix X leptoeuropaea) somatic embryos was correlated with low germination frequency. However, when dried, the germination rate of the somatic embryos improved. Therefore, our present objective was to study the effect of desiccation on the levels of ABA and its glucose ester metabolite. Cotyledonary somatic embryos were subjected to drying treatments at 40C under relative humidities of 98 and 59% for one week and the levels of both ABA and abscisic acid glucose ester (ABAGE) were followed by enzyme‐linked immunoassay (ELISA). During desiccation at 98% relative humidity (RH) both ABA and ABAGE levels decreased in an irregular fashion. Regardless of RH, transient increases in ABA were observed that were paralleled by marked decreases in ABAGE. It is concluded that the desiccation of somatic embryos which leads to a decrease in ABA content, could explain the enhanced germination capacity of such embryos.
Sucrose metabolism and fruit growth in parthenocarpic vs seeded blueberry (Vaccinium ashei) fruits
Authors: Cano‐Medrano Raquel , Darnell Rebecca L.
Published: May 2006

Although fruit set and development are induced by applications of gibberellins, final fruit weight of gibberellin‐induced parthenocarpic fruit is often less than that of pollinated fruit. We examined changes in the activities of sucrose‐metabolizing enzymes and sugar accumulation in developing fruits of cultivated blueberry (Vaccinium ashei Reade) and their correlation with fruit growth upon pollination or exogenous applications of gibberellic acid (GA3). The objective was to determine if differences in fruit growth could be attributed to differences in enzyme activities and subsequent sugar accumulation in fruits. The fruit development period of GA3‐treated fruits was 15 days longer than that of pollinated fruits. At maturity, GA3‐treated fruit accumulated an average of 180 mg dry weight while pollinated fruit accumulated 390 mg dry weight. Dry weight accumulation in nonpollinated fruits was negligible and these fruits abscised by 45 days after bloom (DAB). The total carbon (C) cost (dry weight C + respiratory C) for fruit development was 109 and 244 mg C fruit‐1 for GA3‐treated and pollinated fruits, respectively. Hexose concentration increased to 100 mg (g fresh weight)‐1 at ripening in both GA3‐treated and pollinated fruits. Nonpollinated fruits reached a maximum hexose concentration at 45 DAB. Sucrose phosphate synthase (EC 2.4.1.14) and sucrose synthase (EC 2.4.1.13) activities reached a maximum of ≤5.0 μmol (g fresh weight)‐1 h‐1 in both GA3‐treated and pollinated fruits. Soluble acid invertase (EC 3.2.1.26) activity increased to about 60 μmol (g fresh weight)‐1 h‐1 in both GA3‐treated and pollinated fruits at ripening, while in nonpollinated fruits, a maximum soluble acid invertase activity of 0.12 μmol (g fresh weight)‐1 h‐1 was measured at 24 DAB. Insoluble acid invertase activity declined during the early stages of fruit growth and remained relatively low throughout fruit development. Neutral invertase activity was low throughout development, increasing to 5 μmol (g fresh weight)‐1 h‐1 at ripening in GA3‐treated and pollinated fruits. Our studies demonstrate that blueberry fruit development does not appear to be limited by sucrose metabolizing enzyme activity and/or the ability to accumulate sugars in either GA3‐treated or pollinated fruits.
A study of photorespiratory ammonia production in the C4 plant Amaranthus edulis, using mutants with altered photosynthetic capacities
Authors: Lacuesta M. , Dever L. V. , Muñoz‐Rueda A. , Lea P. J.
Published: May 2006

Previous studies have indicated that the rate of photorespiration in C4 plants is low or negligible. In this study, wild‐type and mutant leaves of the C4 plant Amaranthus edulis were treated with the glutamine synthetase inhibitor, phosphinothricin and the glycine decarboxylase inhibitor, aminoacetonitrile, at different concentrations of CO2. The time course of ammonia accumulation in leaves of the wild type was compared with a mutant lacking phosphoenolpyruvate carboxylase activity (EC 4.1.1.31), and with three different mutants that accumulated glycine. The increase in the concentration of ammonia in the leaves, stimulated by the treatments was used as a measurement of the rate of photorespiration in C4 plants.

The application of glutamine and glycine maintained the rate of photorespiratory ammonia production for a longer period in the wild type, and increased the rate in a mutant lacking phosphoenolpyruvate carboxylase suggesting that there was a lack of amino donors in these plants.

The calculated rate of photorespiration in Amaranthus edulis wild‐type leaves when the supply of amino donors was enough to maintain the photorespiratory nitrogen flow, accounted for approximately 6% of the total net photosynthetic CO2 assimilation rate. In a mutant lacking phosphoenolpyruvate carboxylase, however, this rate increased to 48%, when glutamine was fed to the leaf, a value higher than that found in some C3 plants. In mutants of Amaranthus edulis that accumulated glycine, the rate of photorespiration was reduced to 3% of the total net CO2 assimilation rate. The rate of ammonia produced during photorespiration was 60% of the total produced by all metabolic reactions in the leaves. The data suggests that photorespiration is an active process in C4 plants, which can play an important role in photosynthetic metabolism in these plants.
Cytokinins in Rosa hybrida in relation to bud break
Authors: Dieleman J. A. , Verstappen F. W. A. , Nicander B. , Kuiper D. , Tillberg E. , Tromp J.
Published: May 2006

To assess the role of endogenous cytokinins in growth and development of Rosa hybrida, their concentrations in bleeding sap and in roots, stem, leaves, axillary shoots and bottom breaks in three stages of development were quantified. Cytokinins were purified by means of immunoaffinity chromatography and HPLC, and identified by retention time, UV spectrum and GC‐MS. The major translocation form in the xylem was zeatin riboside (ZR). In all mature tissues, cytokinins of the zeatin‐type were predominant, amounting to 80–90% of the total cytokinin concentration. The stems contained high concentrations of cytokinins, probably caused by lateral movement of ZR from the xylem to adjacent stem tissue and the ability of the stem to metabolize cytokinins. In young leaves the contribution of isopentenyl adenine (iP)‐type cytokinins to the total cytokinin pool was about 50%, indicating that these leaves might be capable of de novo synthesis of cytokinins. In older leaves, the concentration of an unidentified cytokinin‐like compound increased to more than 50% of total cytokinins. This compound, which was also found in the roots, might be a storage form of cytokinins. In young axillary shoots, about 50% of the cytokinins are iP‐compounds, suggesting either import of iP‐type cytokinins via the phloem or de novo synthesis of cytokinins. In buds forming bottom breaks, ZR and zeatin riboside monophosphate (ZRMP) are the main cytokinins, indicating that these buds receive their cytokinins from the roots.
Regulation of petB mRNA stability in pea chloroplasts by redox poise
Authors: Alexciev Krassimir , Tullberg Anna
Published: May 2006

The intensity and quality of light affect the expression of many genes which code for proteins of the photosynthetic apparatus. A number of proteins in pea (Pisum sativum L. cv. Sockerärt de grace) chloroplasts undergo changes in in organello protein synthesis under different redox conditions: different subsets of plastid proteins are synthesised in response to changes in the redox poise. Under equal redox conditions, the photosynthesis‐related psbA and petB mRNAs behave differently. In the presence of various chemical redox reagents the amounts of petB mRNA, which codes for cytochrome b6 of the b6/f complex of photosynthetic membranes, change: they drastically decrease under oxidising conditions, whilst reducing conditions do not cause any significant change. In contrast, psbA mRNA levels are not altered by changes in redox poise. Slot‐blot hybridisation assays show that the decay rate of petB mRNA is affected by different redox potentials. Northern blot analysis indicates that the observed effects are not due to processing events but rather to differential degradation of petB containing sequences. We therefore conclude that the level of petB mRNA in pea chloroplasts is post‐transcriptionally regulated through the redox potential. We hypothesize that redox‐regulated RNA‐binding protein(s) is involved in the selective and regulatable degradation of chloroplast mRNA.
Characterization of chitinases and β‐1,3‐glucanases in grapefruit flavedo during fruit development
Authors: McCollum T. G. , Doostdar Hamed , Mayer Richard T. , McDonald Roy E.
Published: May 2006

Chitinase (EC 3.2.1.14) and β‐1,3‐glucanase (EC 3.2.1.39) activities in the flavedo of grapefruit (Citrus paradisi cv. Marsh) were determined at 17 times during the course of fruit development. Chitinase activity is initially high in flavedo, but drops rapidly and is low, although fairly constant throughout the remainder of fruit development. In contrast to chitinase, β‐1,3‐glucanase activity is lowest in young fruit and increases during development. Western blots of crude flavedo extracts following SDS‐PAGE were probed with antibodies raised against purified citrus chitinase and β‐1,3‐glucanase. Results of immunostaining revealed that changes in the activities of chitinase and β‐1,3‐glucanase were reflected in the amount of chitinase and glucanase protein present in the extracts. Only a single chitinase band was detected on western blots of crude flavedo extracts, whereas one glucanase band was present in young fruit and a second one appeared later in older fruit. Partial purification of flavedo chitinases and glucanases was performed using extracts prepared from immature and mature fruit for the two enzymes, respectively. Acidic and basic forms of both enzymes were present in the extracts; acidic and basic forms of chitinase were present in nearly equal amounts whereas basic glucanases predominated (91% of total activity). Acidic and basic chitinases differed in substrate specificity as well as products of degradation indicating the heterogeneous nature of the enzymes. Both acidic and basic glucanases required the presence of β‐1,3 linkages for activity, were active against both soluble and insoluble β‐1,3 glucans and generated similar products.