bluuebunny/crossref_metadata_2025 huggingface dataset:
Elicitation of H2O2 production in cucumber hypocotyl segments by oligo‐1,4‐α‐D‐galacturonides and an oligo‐β‐glucan preparation from cell walls of Phythophthora megasperma f. sp. glycinea
Authors: Svalheim Øystein , Robertsen Børre
Published: May 2006

The production of H2O2 by cucumber hypocotyl segments (Cucumis sativus L. cv. Wisconsin SMR 58) in response to α‐1,4‐linked oligomers of galacturonic acid and oligo‐β‐glucans from the cell walls of Phytophthora megasperma f. sp. glycinea was studied. Oligogalacturonides with degrees of polymerization of 9 to 13 elicited H2O2 production, the most effective being the deca‐, undeca‐ and dodecamers. A similar relationship between size and effect was previously obtained when oligogalacturonides were tested for their ability to elicit lignification in cucumber hypocotyls. The oligogalacturonide‐induced increase in H2O2 concentration was detected after 4 h, reaching a maximum after 10 h of incubation. The glucan elicitor induced lignification at a 100‐fold lower concentration than the oligogalacturonides, but yielded only 10% of the maximum H2O2 accumulation seen with oligogalacturonides. The glucan elicitor‐induced H2O2 production was detectable after 2 h, and reached a maximum after 4 to 6 h. Catalase abolished the elicitation of both phenol red oxidation and lignification in cucumber hypocotyls. At least part of the oligogalacturonide‐induced H2O2 production appeared to be dependent upon de novo protein synthesis.
Expression and occurrence of uracil‐DNA glycosylase in higher plants
Authors: Bones Atle M.
Published: May 2006

Uracil‐DNA glycosylase (UDG) is the first enzyme in the base excision repair pathway for removal of uracil in DNA. DNA repair capacity is likely to be a critical factor in mutagenesis and thereby in the capacity to prevent genetic damage and unwanted variation. We have studied expression of UDG in 9 higher plant species. The highest expression of UDG was measured in Solanum tuberosum. A comparison of 6 Solanum tuberosum cultivars showed that the specific activity ranged from 30 pmol mg1 protein min−1 in the cultivar Laila to 80 pmol mg−1 protein min−1 in the cultivar Ostara. Measurement of UDG in Begonia X cheimantha gave no indications of enzyme activity. The possible effects of no or low UDG activity is discussed.

In vitro cultures of Solanum tuberosum and Thymus vulgaris were used to examine the effect of auxin and cytokinin on the UDG activity. Axillary shoots of Solanum tuberosum were cultured on medium including 20 variations in hormone concentration. Auxin (1‐naphtaleneacetic acid) increased the expression of UDG. Plants cultured on medium supplemented with 3 mg 1−1 1‐naphtaleneacetic acid showed a specific UDG activity which was approximately 3‐fold higher than the activity in controls. The cytokinin benzyladenine reduced the specific UDG activity at concentrations in the range 0.25–10 mg 1−1.

In vitro cultured Saintpaulia ionantha was used to examine UDG activity during initiation, conditioning and multiplication cycles. In general, highest expression of UDG was measured in the conditioning cycle on hormone free medium. Measurement of UDG expression during single subculture periods, clearly showed that UDG expression may vary over one culture period. Expression of UDG was in general highest three weeks after transfer to fresh medium.

Of different seedling organs from 0‐ to 15‐day‐old Brassica napus L., roots and hypocotyls showed the highest UDG activities. In cotyledons a very low and nearly constant specific activity was observed. In 12‐day‐old seedlings the activity in roots was approximately 20 times higher than the activity in cotyledons.
Carbohydrate content and glycosidase activities following cold hardening in two grass species
Authors: Tronsmo A. M. , Kobro G. , Morgenlie S. , Flengsrud R.
Published: May 2006

The freezing resistance of the grass species Phleum pratense L. (timothy) and Phalaris arundinaces L. increases significantly after cold hardening. The content and composition of soluble carbohydrates were determined in the plants after short day treatment, cold hardening and dehardening. The amounts of mono‐, di‐ and trisaccharides were reduced during the short day treatment, increased during cold hardening and decreased again during dehardening. The total amounts of soluble carbohydrates (mono‐, di‐, tri‐ and polysaccharides) were the same in hardened and dehardened plants, indicating that during hardening soluble polysaccharides (fructose polymers, fructans) were converted to mono‐ and oligosaccharides. Sucrose increased most after hardening conditions and, in P. arundinacea, a significant increase in 1‐F‐fructosylsucrose (isokestose) was also observed.

Invertase (β‐fructofuranosidase. EC 3.2.1.26) activity increased following cold hardening and decreased following dehardening, while the α‐galactosidase (EC 3.2.1.22) activity seemed to increase after dehardening. The glycosidases are probably involved in the mobilisation of polysaccharides during cold hardening.
Characterization of the laser‐induced blue, green and red fluorescence signatures of leaves of wheat and soybean grown under different irradiance
Authors: Stober Fred , Lichtenthaler Hartmut K.
Published: May 2006

The blue, green and red fluorescence emission of green wheat (Triticum aestivum L. var. Rector) and soybean leaves (Glycine max L. var. Maple Arrow) as induced by UV light (nitrogen laser: 337 nm) was determined in a phytochamber and in plants grown in the field. The fluorescence emission spectra show a blue maximum near 450 nm, a green shoulder near 530 nm and the two red chlorophyll fluorescence maxima near 690 and 735 nm. The ratio of blue to red fluorescence, F450/F690, exhibited a clear correlation to the irradiance applied during the growth of the plants. In contrast, the chlorophyll fluorescence ratio, F690/F735, and the ratio of blue to green fluorescence, F450/F530, seem not to be or are only slightly influenced by the irradiance applied during plant growth. The blue fluorescence F450 only slightly decreased, whereas the red chlorophyll fluorescence decreased with increasing irradiance applied during growth of the plants. This, in turn, resulted in greatly increased values of the ratio, F450/F690, from 0.5 – 1.5 to 6.4 – 8.0. The decrease in the chlorophyll fluorescence with increasing irradiance seems to be caused by the accumulation of UV light absorbing substances in the epidermal layer which considerably reduces the UV laser light which passes through the epidermis and excites the chlorophyll fluorescence of the chloroplasts in the subepidermal mesophyll cells.
The enigmatic cytochrome b‐559 of oxygenic photosynthesis
Authors: Cramer William A. , Tae Gun‐Sik , Furbacher Paul N. , Böttger Michel
Published: May 2006

The ubiquitous and obligatory association of cytochrome b‐559 with the photosystem II reaction center of oxygenic photosynthesis is a conundrum since it seems not to have a function in the primary electron transport pathway of oxygen evolution. A model for the cytochrome structure that satisfies the cis‐positive rule for membrane protein assembly consists of two short, non‐identical hydrophobic membrane‐spanning polypeptides (α and β), each containing a single histidine residue, as ligands for the bridging heme prosthetic group that is on the side of the membrane opposite to the water splitting apparatus. The ability of the heterodimer, but not the single α‐subunit, to satisfy the cis‐positive rule implies that the cytochrome inserts into the membrane as a heterodimer, with some evidence implicating it as the first membrane inserted unit of the assembling reaction center. The very positive redox potential of the cytochrome can be explained by a position for the heme in a hydrophobic niche near the stromal aqueous interface where it is also influenced by the large positive dipole potential of the parallel α‐helices of the cytochrome. The requirement for the cytochrome in oxygenic photosynthesis may be a consequence of the presence of the strongly oxidizing reaction center needed for H2O‐splitting. This may lead to the need, under conditions of stress or plastid development, for an alternate source of electrons when the H2O‐splitting system is not operative as a source of reductant for the reaction center.
Does the alternative pathway ameliorate chilling injury in sensitive plant tissues?
Authors: Purvis Albert C. , Shewfelt Robert L.
Published: May 2006

Free radical processes have been observed in senescence and several membrane‐associated disorders of plants including chilling, freezing, and desiccation injuries. The mitochondria of plant tissues exposed to low temperatures, and other abiotic and biotic stresses, produce superoxide and/or hydrogen peroxide when electron transport through the cytochrome pathway is impaired due to the energy state of the cell or to stress‐induced physical changes in the membrane components. The superoxide and/or hydrogen peroxide produced can diffuse throughout the cell causing peroxidation of membrane lipids which results in membrane disruption, increased permeability and metabolic disturbances, and eventually the visible symptoms of chilling injury. The alternative pathway of electron transport in the mitochondria, which is induced by low temperatures in some plant tissues, can mediate these degradative processes by reducing the level of superoxide generated by the mitochondria.
The effect of exposure to enhanced UV‐B radiation on the penetration of monochromatic and polychromatic UV‐B radiation in leaves of Brassica napus
Authors: Cen Yan‐Ping , Bornman Janet F.
Published: May 2006

Using quartz optical fibres, penetration of both monochromatic (310 nm) and polychromatic UV‐B (280–320 nm) radiation in leaves of Brassica napus L. (cv. Ceres) was measured. Plants were grown under either visible light (750 μmol m−2 s−1 photosynthetically active radiation) or with the addition of 8. 9 KJ m−2 day−1 biologically effective UV‐B (UV‐BBE) radiation. Results showed that of the 310 nm radiation that penetreated the leaf, 90% was within the intial one third of the leaf with high attenuation in the leaf epidermis, especially in UV‐treated plants. Polychromatic UV‐B radiation, relative to incident radiation, showed a relatively uniform spectral distribution within the leaf, except for collimated radiation. Over 30% of the UV‐screening pigments in the leaf, including flavonoids, were found in the adaxial epidermal layer, making this layer less transparent to UV‐B radiation than the abaxial epidermis, which contained less than 12% of the UV‐screening pigments. UV‐screening pigments increased by 20% in UV‐treated leaves relative to control leaves. Densely arranged epicuticular wax on the adaxial leaf surface of UV‐treated plants may have further decreased penetration of UV‐B radiation by reflectance. An increased leaf thickness, and decreases in leaf area and leaf dry weight were also found for UV‐treated plants.
Photosynthesis in ozone‐exposed duckweed (Lemna gibba)
Authors: Aarnes H. , Baumann C. , Eriksen A. B. , Solås A. , Sundbye A. , Sølvernes K. A.
Published: May 2006

The photosynthetic light saturation curve in duckweed was lowered by 20–25% after ozone exposure (300 nmol mol−1, 1 h). The light flux and oxygen concentration during ozone‐exposure had no effect on reduction of net photosynthesis. Net photosynthesis and photorespiration were both depressed by about 40% after exposure for 1 h to 360 nmol mol−1 ozone. We could not find any change in dark respiration after ozone exposure below 300 nmol mol−1. When the concentration of ozone was doubled from 150 nmol mol−1 to 300 nmol mol−1, the uptake of ozone in duckweed changed from 100 nmol m−2 s−1 to 170 nmol m−2 s−1. We found no differences in fluorescence (pattern) between ozone treated plants and the control plants during a period of 150 min after ozone treatment, but there was an increase in synthesis of the Dl‐protein and a significant reduction in degradation after ozone treatment (300 nmol mol−1, 1 h). These results, together with fluorescence measurements, indicate that photochemical electron transport was not responsible for the ozone‐induced reduction in net photosynthesis.
Differences in Protein Synthesis and Peroxidase isoenzymes between recalcitrant and regenerating ProtoPlasts
Authors: Siminis C. I. , Kanellis A. K. , Roubelakis‐Angelakis K. A.
Published: May 2006

The reasons for the inability of recalcitrant mesophyll protoplasts to divide and re‐enter the cell cycle are unknown. Changes in protein profile, indole‐3‐acetic acid (IAA)‐oxidase and peroxidase activities, and isoenzymes were compared in protoplasts of recalcitrant grapcvine (Vitis vinifera) L. cv. Sultanina) and regenerating tobacco (Nicotiana tabacum) L. cv. Xanthi). Using [35S]‐methionine. SDS‐PAGE and two‐dimensional separation of proteins, differences in protein profile during protoplast culture were assessed. The changes in the de novo synthesized proteins were both qualitative and quantitative between the two species. The number of proteins which changed was double in tobacco compared to grapevine protoplasts. Peroxidase and IAA‐oxidase activities increased significantly in tobacco protoplasts during culture whereas in grapevine they remained low. In tobacco protoplasts. 3 and 7 basic and acidic peroxidases, respectively, were induced during protoplast culture. which were not detected in the intact leaf, whereas in grapevine no new peroxidases were induced during protoplast culture.
Effect of assimilate supply on the growth of individual cucumber fruits
Authors: Marcelis Leo F. M.
Published: May 2006

The effects of assimilate supply on the growth of individual fruits during different stages of fruit development were analysed in cucumber (Cucumis sativus L. cv. Corona). The assimilate supply was varied by maintaining different numbers of fruits per plant or maintaining different irradiances. The growth rate of a cucumber fruit strongly increased with increasing assimilate supply, but its growing period was not noticeably affected. At a low assimilate supply both cell number and cell size were reduced. Increasing the assimilate supply at different stages of fruit development showed that the early development of a cucumber fruit was not crucial for setting its growth potential. A small number of cells, due to a low assimilate supply, during early fruit development, was to a great extent compensated by an increased expansion rate of individual cells. It is concluded that cell number is not an important determinant of fruit size in cucumber, although fruit size is often positively correlated with cell number. In the early stages of fruit development the effect of irradiance on the fruit growth rate depends on the presence of an earlier developed fruit because of dom‐inance between fruits. In later stages of fruit development, a decrease in irradiance reduced the growth rate of all fruits relatively to the same extent independent of age or presence of other fruits.
Variations in seeds of three endemic leguminous species at different altitudes
Authors: Angosto Trinidad , Matilla Angel J.
Published: May 2006

Seeds of three legumes, Adenocarpus decorticans. Astragalus granatensis ssp. granatensis and Cytisus reverchonii, endemic to the Betic Cordillera, were collected from different altitudes. Under laboratory conditions, various factors and endogenous characteristics which may affect their germination potential in the field were studied. The seeds of all three species required chemical scarification in order to germinate, and the maximum germination temperature of all three proved to be correspondingly higher the higher the altitude of their ecological niche. However, each species showed differences in germinability and optimum temperature according to altitude. Variations in free and bound abscisic acid (ABA) and proline, according to altitude, did not follow the same pattern in each species; they increased in Cytisus, fell in Adenocarpus, and remain unchanged in Astragalus. The highest germination rates were obtained when free‐ABA levels were lowest in all species. The polyamines, spermidine, spermine, putrescine and cadaverine, were present in all species, spermi‐dine being the most abundant. Putrescine was the only polyamine whose level varied with altitude, decreasing in Adenocarpus and Cytisus, and increasing in Astragalus.
The interrelationship of growth and frost tolerance in winter rye
Authors: Griffith Marilyn , Mclntyre Heather C. H.
Published: May 2006

The reduction in growth of winter cereals that occurs in the fall is thought to be required for the development of frost resistance. In the present study, the interrelationship of freezing tolerance and growth was examined by raising winter rye (Secale cereale cv. Puma) plants at 20/16°C (day/night) and at 5/3°C under 8‐, 16‐ and 24‐h daylengths to vary growth rates and frost tolerance. Temperature and irradiance were quantified as thermal time, photothermal time and photosynthetic photon flux and examined by multiple linear regression in order to determine their effects on growth and frost tolerance of rye shoots. At low temperature, both growth and frost tolerance were markedly influenced by daylength and irradiance. Plants grown at 5/3°C with a short daylength accumulated shoot dry weight and increased frost tolerance at a greater rate per unit photothermal time or photon flux than plants grown at longer daylengths. Moreover, 5/3°C plants grown with a 16‐h day grew more slowly and were less frost tolerant than plants grown with a 24‐h day. We conclude that the interrelationship between growth and frost tolerance is a quantitative one. Frost tolerance is induced only by low temperature, but the development of forst tolerance is dependent upon both irradiance, which affects the amount of photoassimilate available, and daylength, which may affect the partitioning of photoassimilates between growth and frost tolerance.
Osmotic regulation of assimilate unloading from seed coats of Vicia faba. Assimilate partitioning to and within attached seed coats.
Authors: Patrick J. W.
Published: May 2006

The significance of the osmotic potential of the seed apoplast sap as a regulator of assimilate transfer to and within coats of developing seed of Vicia faba (cv. Coles Prolific) was assessed using attached empty seed coats and intact developing seed. Following surgical removal of the embryos, through windows cut in the pod walls and underlying seed coats, the resulting attached “empty” seed coats were filled with solutions of known osmotic potentials (–0. 02 versus –0. 75 MPa). Sucrose efflux from the coats was elevated at the higher osmotic potential (high osmotic concentration) for the first 190 min of exchange. Thereafter, this efflux was depressed relative to efflux from coats exposed to the low osmotic potential (high osmotic concentration) solution. This subsequent reversal in efflux was attributable to an enhanced diminution of the coat sucrose pools at the high external osmotic potential. Indeed, when expressed as a proportion of the current sucrose pool size, relative efflux remained elevated for coats exposed to the high osmotic potential solution. Measurement of potassium and sucrose fluxes to and from their respective pools in the coat tissues demonstrated that the principal, fluxes, sensitive to variative in the external osmotic potential, were phloem import into and efflux from the “empty” coats. Phloem import, consistent with a pressure‐driven phloem transport mechanism, responded inversely with changes in the external osmotic potential. In contrast, sucrose and potassium efflux from the coats exhibited a positive dependence on the osmotic potential. Growth rates of whole seed were approximately doubled by enclosing selected pods in water jackets held at temperatures of 25°C. compared to 15°C. The osmotic potential of sap collected from the seed apoplast remained constant and independent of the temperature‐induced changes in seed growth rates and hence phloem import. Based on these findings, it is proposed that control of phloem import by changes in the external osmotic potential observed with “empty” seed coats has no significance as a regulator of assimilate import by intact seed. Rather, maintenance of the seed apoplast osmotic potential, independent of seed growth rate, suggests that the observed osmotic regulation of efflux from the coats may play a key role in integrating assimilate demand by the embryo with phloem import.
Cation fluxes in the saps of Sinapis alba during the floral transition
Authors: Havelange A. , Bernier G.
Published: May 2006

Plants of Sinapis alba L. were induced to flower by either a single long day or a single displaced short day. The levels of three cations. Ca2+, Mg2+ and K+, were measured by atomic absorption spectrophotometry in exudates from roots, leaves and apical stem tips. The export of all three cations out of the root system (root exudate) was increased in induced as compared to non‐induced plants. No changes were observed in cation export out of the mature leaves (leaf exudate). The supply of cations to the apical bud (apical exudate) did not originate from the phloem and, so, should mainly be of apoplastic origin. Only the supply of Ca2+ to the apical bud was increased, not the supply of Mg2+ or K+. The increase in Ca2+ supply was transient and occurred at about the same time as a conspicuous stimulation of cell division, previously detected in the apical bud.
Possible mechanisms of afterripening in Xanthium seeds
Authors: Esashi Yohji , Ogasawara Maria , Górecki Ryszard , Leopold A. Carl
Published: May 2006

Breaking dormancy in some seeds requires a period of dry storage. In the seeds of Xanthium pennsylvanicum Wallr., the process of afterripening proceeds optimally at water contents between 7 and 14%: this range of dehydration can be identified with water binding region 2, in which water is bound with low enthalpy. At water contents below 7%. Seeds remained primarily dormant over 3 years. Attempts to alter the afterripening with atmospheres of elevated nitrogen showed no effect. and with oxygen there was no consistent effect. There were no changes is osmotic value of the seed sap, or in its sugar or amino acid contents. We speculate that afterripening in Xanthium may involve some nonenxymatic reactions which remove substances which inhibit germination. Candidates for these reactions include the Amadori and Maillard reactions.
Correlations between the ascorbate‐glutathione pathway and effectiveness in legume root nodules
Authors: Dalton David A. , Langeberg Lorene , Treneman Nancy C.
Published: May 2006

Legume root nodules use the ascorbate‐glutathione pathway to remove harmful H2O2. In the present study. effective and ineffective nodules from soybean and alfalfa were compared with regard to this pathway. Effective nodules had higher activity of all 4 enzymes (ascorbate peroxidase, EC 1. 11. 1. 11: monodehydroascorbate reductase, EC 1. 6. 5. 4: dehydroascorbate reductase, EC 1. 8. 5. 1: and glutathione reductase, EC 1. 6. 4. 2). The concentration of thiol tripeptides (primarily homoglutathione) was about 1 mM in effective nodules – a level 3–4‐fold higher than in ineffective nodules. Effective nodules contained higher levels of NAD+. NADP+ and NADPH. but not of NADH or ascorbate. The increased capacity for peroxide scavenging in effective nodules as compared to ineffective nodules emphasizes the important protective role that this pathway may play in processes related to nitrogen fixation.
Mechanisms of reaction of hematoxylin with aluminium‐treated wheat roots
Authors: Ownby James D.
Published: May 2006

Hematoxylin stain is used for localization of aluminium in plant root tissue and is the basis of a rapid assay of relative At tolerance among wheat cultivars. In the present study, mechanisms by which hematoxylin might selectively stain Al‐sensitive wheat roots have been examined. The results are consistent with the idea that in Al‐sensitive cultivars, hematoxylin forms complexes with Al that precipitate with phosphate as AlPO4 in intercellular spaces: (1) Al and P are co‐localized in the cell wall region of the outer cortex of Al‐stressed roots by using x‐ray microanalysis: (2) the molybdenum blue histochemical stain for extracellular phosphate reveals areas of stain that parallel those observed with hematoxylin; (3) in vitro, the presence of phosphate in an Al‐hematoxylin reaction mixture causes formation of precipitate when the P/Al ratio exceeds 1. 0. I suggest that selective hematoxylin staining of Al‐sensitive wheat cultivars is the result of direct damage by Al to root cells, leading to leakage of phosphorus into the cell wall region. Cultivars whose roots are damaged by Al in this way are likely to be judged Al‐sensitive when other criteria such as growth or crop yield are used.
Gene expression induced by spermine in isolated embryonic axes of chick‐pea seeds
Authors: Bueno Milagros , Garrido Dolores , Matilla Angel
Published: May 2006

Nuclei and mRNA obtained from the embryonic axes of chick‐pea (Cicer arietinum) seeds grown in the presence of spermine (Spm) have been used to study certain characteristics of gene expression induced by this polyamine. In the embryonic axes, free Spm varied between 100 and 300 μg (g fresh weight)−1. whereas Spm bound to macromolecules varied between 6 and 30 μg (g fresh weight)−1. Isolated nuclei of the controls synthesized RNA at a rate of 1. 1 nmol (3H)‐UTP (mg DNA)−1min−1. and RNA polymerase activity increased concomitantly with growth. although this activity was inhibited by α‐amanitin (5 mg I−1). Isolated nuclei from axes grown in a medium with Spm had a lesser transcriptional activity than the controls: this activity was weaker still when abscisic acid (ABA) was present. Nevertheless. Spm stimulated RNA synthesis in vivo [measured as incorporated (3H)‐uridine into trichloroacetic acid‐insoluble material]. These results suggest a cytoplasmic control of transcription by Spm. Gene expression at the mRNA level was studied by analyzing its in vitro translated products (one‐ and two‐dimensional PAGE). Some mRNA species were enhanced by Spm whereas others were suppressed or only weakly expressed. A polypeptide of 26–27 kDa, absent in the control, appeared strongly with Spm or ABA, and very weakly in the presence of Spm plus ABA. In presence of Spm the mRNA encoding this protein was not expressed in vivo. The mode of action of Spm in our system is discussed in view of these findings.
Ammonium requirement for radiation‐induced accumulation of polyamines in suspension‐cultured grape cells
Authors: Triantaphylides Christian , Nespoulous Louis , Chervin Christian
Published: May 2006

The effects of radiation‐mediated free radical production on polyamine metabolism were investigated in grape cells (Vitis vinifera L. cv. Gamay) using a cell suspension culture. Putrescine (Put) synthesis was triggered in irradiated cells (0. 5 kGy) only when ammonium was present in the culture medium. Under these conditions. Put accumulated to a continuously high level. As also described for other kinds of stress, the level of spermidine was slightly enhanced and that of spermine unchanged. The role of ammonium was assessed by studying non‐irradiated cell cultures. In the presence of ammonium, a transient increase of both arginine decarboxylase (ADC) activity and of Put synthesis was observed during the lag phase of growth. This Put enhancement was inhibited by difluoromethyl arginine and not by difluoromethyl ornithine, showing that increased Put synthesis occurs via the ADC pathway. When ammonium was withheld from the culture medium. ADC activity was still triggered though transient Put accumulation was completely suppressed. These results emphasize the importance of ammonium availability in cultured cells as a limiting factor for Put production. Polyamine synthesis, therefore, cannot be stimulated by gamma irradiation in the absence of an ammonium supply. These results support the hypothesis that Put synthesis is a detoxification process of the ammonium produced as a result of nitrogen recycling within stressed plant cells.
The regulation of napin gene expression in secondary embryos of Brassica napus
Authors: Fleming Andrew J. , Hanke David E.
Published: May 2006

Embryos derived from somatic tissues proceed through a series of morphological stages very similar to those observed during zygotic embryo development. However, somatic embryos do show some developmental characteristics distinct from those of zygotic embryos (e. g. curtailed cotyledon filling, precocious germination) which are not observed in zygotic embryos in vivo. Whether these differences reflect the abnormality of the in vitro culture environment or whether they are due to biochemical or developmental peculiarities of the somatic embryos is unclear. Using a system of secondary embryogenesis in Brassica napus cv. Primor, we have attempted to address this problem by analysing the level of napin storage protein transcripts and their regulation by ABA throughout somatic embryo development. We have in addition measured the endogenous levels of ABA in the embryos, and characterised the effect of ABA (and other medium components) on napin transcript level, embryo germination, and embryo survival. Our analysis, in conjunction with the published data on zygotic and microspore‐derived embryo development, allows us to conclude that although somatic and zygotic embryos show the same basic pattern of napin gene expression and regulation, the secondary embryos are characterised by a basal constitutive derepression of the napin gene, in contrast to the tight developmental control reported in zygotic embryos.
Acquisition of desiccation tolerance in soybeans
Authors: Sun Wendell Q. , Leopold A. Carl
Published: May 2006

The entry into a desiccation‐tolerant state is a major developmental component of seed maturation. Development of desiccation tolerance of embryonic axes of soybean [Glycine max (L.) Merrill cv. Chippewa 64] was studied by measuring changes in electrolyte leakage. germination and relative growth rate after axes were rapidly air‐dried to various water contents. Axes acquired the full capacity for germination at 34 days after flowering (DAF). and reached physiological maturity (maximum dry weight) at 48 DAF. When dried to water content h = 0. 08 (g water g−1 dry weight). few axes germinated before 42 DAF. but more than 90% germinated after 48 DAF. However, electrolyte leakage of rehydrated axes showed a linear decline from 30 to 55 DAF. For developing axes there was a critical water content or desiccation threshold. which could be estimated by using the electrolyte leakage method. The threshold of desiccation tolerance decreased gradually from h = 1. 10 to 0. 18 as axes matured from 28 to 55 DAF. The development of desiccation tolerance continued after physiological maturity at 48 DAF. We conclude that the acquisition of desiccation tolerance of soybean axes is a gradual event, rather than an abrupt transition.
Endonuclease activity from tobacco nuclei specific for ultraviolet radiation‐damaged DNA
Authors: Murphy Terence M. , Martin Charles P. , Kami James
Published: May 2006

Endonuclease activity specific for UV damaged DNA was isolated from tobacco leaf nuclei and detected by relaxation of supercoiled pUC 19 plasmid DNA. The activity did not require divalent cations or ATP. It acted on photoproducts induced by as little as 24 J m−2 of UV‐C (primarily 254 nm) radiation. but not on photoproducts produced by UV‐B (290–320 nm) radiation in the presence of acetophenone and a N2 atmosphere or by UV‐A (320–400 nm) radiation in the presence of 4′‐methoxy‐methyltrioxsalen in a N2 atmosphere and not on the products of OsO4 oxidation of the DNA. Using end‐labeled DNA of defined sequence, it was possible to identify sites in UV‐C‐irradiated DNA that were cut by the endonuclease preparation: most sites were assocrated with pyrimidine pairs. Cleavage by the tobacco endonuclease was not eliminated by treatment with Escherichia coli photolyase and light, suggesting that the endonuclease did not recognize cyclobutadipyrimidines.
Role of other plant organs in gibberellic acid‐induced delay of leaf senescence in alstroemeria cut flowers
Authors: Jordi W. , Dekhuijzen H. M. , Stoopen G. M. , Overbeek J. H. M.
Published: May 2006

Chlorophyll loss in leaves of cut flowers of alstroemeria (Alstroemeria pelegrina L. cv. Westland) was rapid in darkness and counteracted by irradiation and treatment of the flowers with gibberellic acid (GA3). The mechanism of the effect of GA3 under dark conditions was investigated. The content of various carbohydrates in the leaves under dark conditions rapidly decreased; this was not influenced by treatment with GA3. indicating that the loss of carbohydrates in the leaves did not induce the loss of chlorophyll. Placing the cut flowers in various solutions of organic and inorganic nutrients exhibited no significant effect on the retention of chlorophyll in leaves of dark‐senescing flowers. The total nitrogen content in leaves of dark‐senescing cut flowers decreased with time. Leaves of GA3‐treated flowers retained more nitrogen. In contrast, the buds of GA3‐treated flowers retained less nitrogen during senescence in the dark than control buds. To investigate whether GA3 affects export of assimilates from the leaf to various parts of control and GA3‐treated flowers, we labelled one leaf with radioactive carbon dioxide. 14C‐assimilates accumulated preferentially in the flowers, in which the relative specific activity of the youngest floral buds was highest. No significant differences were observed in the distribution of 14C‐labelled compounds between the buds of control and GA3‐treated flowers. To establish the importance of source‐sink relations for the loss of leaf chlorophyll we removed the flower buds (i. e. the strongest sink) from the cut flowers. This removal only slightly delayed chlorophyll loss as compared to the large delay caused by GA3‐treatment. In addition, detached leaf tips exhibited chlorophyll loss in the dark, which was delayed by GA3‐treatment in a fashion comparable with that in flowers. Together these data demonstrate that interactions of the leaves with other plant organs are not essential for chlorophyll loss during senescence in the dark. Additionally, we have found no evidence that GA3 delays the loss of chlorophyll by affecting the transport of nutrients within the cut flowers.
The protein import apparatus of chloroplasts
Authors: Soll Jürgen , Alefsen Heike
Published: May 2006

Routing of cytosolically synthesized precursor proteins into chloroplasts is a specific process which involves a multitude of soluble and membrane components. In this review we wil1 focus on early events of the translocation pathway of nuclear coded plastidic precursor proteins and compare import routes for polypeptide of the outer chloroplast envelope to that of internal chloroplast compartments. A number of proteins housed in the chloroplast envelopes have been implied to be involved in the translocation process, but so far a certain function has not been assigned to any of these proteins. The only exception could be an envelope localized hsc 70 homologue which could retain the import competence of a precursor protein in transit into the organelle.
Polarized and specular reflectance variation with leaf surface features
Authors: Grant Lois , Daughtry C.S.T. , Vanderbilt V.C.
Published: May 2006

The linearly polarized reflectance from a leaf depends on the characteristics of the leaf surface. In the present study the leaf reflectance of a number of plant species with varying surface characteristics was measured at the Brewster angle with a polarization photometer having 5 visible and near‐infrared wavelength bands. We found that all leaf surfaces polarized incident light. Differences among species could be explained by variation in surface features. The results support our hypothesis that the polarized light is reflected by the leaf surface, not by its interior. Two mechanisms appeared responsible for the linearly polarized reflectance: (1) specular reflectance and (2) surface particle scattering. In most cases, large values of linearly polarized reflectance could be attributed to specular reflectance from the leaf surface. Attribution required knowledge of the optical dimensions of features on the leaf surface.
The redox levels and subcellular distribution of pyridine nucleotides in illuminated barley leaf protoplasts studied by rapid fractionation
Authors: Wigge Bosse , Krömer Silke , Gardeström Per
Published: May 2006

The redox level and compartmentation of pyridine nucleotides was studied under photorespiratory and non‐photorespiratory conditions using rapid fractionation of barley (Hordeum vulgare L. cv. Gunilla, Svalöv) leaf protoplasts. From comparative measurements of the NADPH/NADP+ ratio and the ATP/ADP ratio one acidic and one alkaline extraction medium was chosen which quenched the metabolism very efficiently. The mitochondrial NADH/NAD+ was higher under photorespiratory conditions than under non‐photorespiratory conditions. Aminoacetonitrile, an inhibitor of the photorespiratory conversion of glycine to serine, lowered the mitochondrial NADH/NAD+ ratio. This supports the hypothesis that glycine oxidation is coupled to oxidative phosphorylation to provide ATP to the cytosol. The chloroplastic NADPH/NADP+ as well as the NADH/NAD+ ratios were quite stable in saturating and limiting CO2 as well as in the presence of aminoacetonitrile, although the triosephosphate/phosphoglycerate ratios changed. Thus, the redox level in the stroma seems to be tightly regulated.
Chlorophyll synthetase activity is relocated from transforming prolamellar bodies to developing thylakoids during irradiation of dark‐grown wheat
Authors: Lindsten Agneta , Wiktorsson Bengt , Ryberg Margareta , Sundqvist Christer
Published: May 2006

Analyses of the esterification of newly formed chlorophyllide in irradiated dark‐grown leaves of wheat (Triticum aestivum L. cv. Kosack) suggest a translocation of chlorophyll synthetase activity from transforming prolamellar bodies to developing thylakoids. We have fractionated plastid inner membranes from dark‐grown leaves and from leaves irradiated for 5, 10, or 20 min and compared the in vitro esterification of chlorophyllide in two fractions, corresponding (in density) to the prolamellar body and the prothylakoid fraction of dark‐grown leaves. The relative amounts of chlorophyllide, and total protein, as well as the specific esterification activity, increased with irradiation time in the prothylakoid fraction. The esterification of chlorophyllide seems to depend on a transformation of the prolamellar body structure. The results are discussed also in relation to other events initiated by irradiation, such as the Shibata‐shift and the altered distribution of NADPH‐protochlorophyllide oxidoreductase (EC 1.3.1.33).
Characterization of wall‐bound invertase isoforms of Picea abies cells and regulation by ectomycorrhizal fungi
Authors: Salzer Peter , Hager Achim
Published: May 2006

In culture, the ectomycorrhiza‐forming fungi Amanita muscaria (Pers. ex Fries) Hock. and Hebeloma crustuliniforme (Bull. ex Fries) Quel. only grow on media with glucose or fructose but not with sucrose as sole carbohydrate source. This is due to their lack of wall‐bound invertase activity. Therefore, utilization of sucrose by the fungi within a mycorrhizal association is believed to depend on the wall‐bound invertase activity of the host. This enzyme activity was studied in the apoplast of suspension cultured cells of Picea abies (L.) Karst. An ionically and a tightly wall‐bound isoform of acid invertase were found that function as β‐d‐fructofuranoside‐fructohydrolases (EC 3.2.1.26). The ionically bound enzyme could be easily released from walls of intact cells with buffer of high ionic strength. In its native form, the ionically bound invertase isoform is a monomeric protein with a molecular mass of 61 kDa, as determined by gel filtration and SDS‐PAGE. Glycoprotein nature of the enzyme was demonstrated with antibodies directed against the digoxigenin‐labeled protein. The Km values of both enzymes for sucrose, their natural substrate, are relatively high (ionically bound invertase Km= 16 mM, tightly bound invertase Km= 8.6 mM). Activity of both wall‐bound invertase isoforms strongly depends on the apoplastic pH. They have a narrow pH‐optimum and exhibit highest activity at pH 4.5. with elevated activity between pH 4.5 and 6.0. Furthermore, fructose acts as competitive inhibitor of both isoforms, whereas glucose is not inhibitory. Unloading of sucrose from host cells to the apoplastic interface of the Hartig net in ectomycorrhizae appears to depend on the rate of hydrolysis by the wall‐bound invertase of the host. Since the activity of the plant invertase depends on the actual pH value and the fructose concentration in the mycorrhizal interface, we suggest that the fungus can actively influence the activity of the plant invertase by acidification of the cell wall and by fructose uptake. Thus, the fungus itself can regulate its own supply of glucose and fructose.
Effects of the embryonic axis and phytohormones on proteolysis of the storage protein in buckwheat seed
Authors: Dunaevsky Yakov E. , Belozersky Mikhail A.
Published: May 2006

The role of the embryonic axis in regulation of proteolysis of the main storage protein was studied in buckwheat (Fagopyrum esculentum) seed. Polyacrylamide gel electrophoresis (PAGE) analysis revealed that removal of the embryonic axis had no effect on the first stage of hydrolysis, that is proteolytic modification, of 13S globulin. This modification took place in the growing seedlings also in the presence of cycloheximide, i.e. it was due to an enzyme present in dry seed. However, in isolated cotyledons the 13S globulin was not degraded completely. Incubation of isolated cotyledons with cytokinins, gibberellic acid and indoleacetic acid could not replace the excised embryonic axis. At the same time, proteolysis of the 13S globulin in the growing seedlings was strongly inhibited by casein hydrolyzate. It is suggested that a complete proteolysis of the modified storage protein is regulated by the concentration of hydrolysis products at the site of hydrolysis. The embryonic axis serves, most probably, as a site of efflux of the products of protein hydrolysis in the cotyledons during seedling growth and thus regulates the course of proteolysis.

Abscisic acid (10–100 μM) was without effect on modification of the 13S globulin, but suppressed the complete proteolysis of the protein by inhibiting, apparently, the synthesis of the cysteine proteinase in the growing seedlings.
Effects of atmospheric SO2 on Azolla and Anabaena symbiosis
Authors: Hur Jae‐Seoun , Wellburn Alan R.
Published: May 2006

The water fern Azolla pinnata R. Br. was fumigated for 1 week with either 25, 50 or 100 nl 1−1 SO2. The symbiosis of Azolla with Anabaena azollae (spp.) was severely damaged by atmospheric SO2 even at concentrations as low as 25 nl 1−1, with significant reductions in growth, reduction of C2H2, NH3 assimilation, protein synthesis, and heterocyst development. These disturbances appear to be mainly responsible for the extreme sensitivity of this fern to atmospheric SO2. Changes in violaxanthin/antheraxanthin and epoxy‐lutein/lutein ratios also indicate that free radical products are induced by atmospheric SO2. These results suggest that the Azolla‐Anabaena symbiotic system is a very responsive and reliable lower plant model to study the detailed effects of total sulphur deposition upon the balances between various important plant metabolic processes.
On the participation of chloride in bicarbonate‐induced reversal of anion inhibition of photosystem II electron transport in spinach thylakoids
Authors: Jajoo Anjana , Bharti Sudhakar
Published: May 2006

Inhibition of electron transport through photosystem II (PS II) by formate (HCO−2) or nitrite (NO−2) in the presence or absence of chloride ions was studied. The inhibition induced by HCO−2 or NO−2 is overcome by HCO−3 more in the presence, than in the absence of Cl−. The data on electron transport are supported by chlorophyll a fluorescence measurements. In experiments. In experiments in which water oxidation was blocked. Cl− was found to facilitate electron transport between bound quinone A (Q−A) and the plastoquinone (PQ) pool. It can thus be concluded that in addition to the well known site of action of Cl− on water oxidation, another site of Cl− action is between Q−A and the PQ pool.
Kinetic parameters of nitrate uptake by different catch crop species: effects of low temperatures or previous nitrate starvation
Authors: Lainé P. , Ourry A. , Macduff J. , Boucaud J. , Salette J.
Published: May 2006

The pollution of aquifers by NO−3 in temperate environments is aggravated by farming practices that leave the ground bare during winter. The use of catch crops during this time may decrease nitrate loss from the soil. Nitrate uptake by several catch crop species (Brassica napus L., Sinapis alba L., Brassica rapa L., Raphanus sativus L., Trifolium alexandrinum L., Trifolium incarnatum L., Phacelia tanacetifolia Benth., Lolium perenne L., Lolium multiflorum Lam. and Secale cereale L.) was here studied in relation to transpiration rate and low temperatures applied to the whole plant or to roots only. The Michaelis constant (Km), maximum uptake rate (Vmax), time of induction and contributions of inducible and constitutive mechanisms were estimated from measurements of NO−3 depletion in the uptake medium. There were large differences between species, with Km (μM) values ranging between 5.12 ± 0.64 (Trifolium incarnatum) and 36.4 ± 1.97 (Lolium perenne). Maximum NO−3 uptake rates expressed per unit root weight were influenced by ageing, temperature and previous NO−3 nutrition. They were also closely correlated with water flow through the roots and with shoot/root ratio of these species. The combined results from all species and treatments showed that Vmax increased with shoot/root ratio, suggesting a regulatory role for the shoots in NO−3 uptake. Overall, the results showed a great diversity in NO−3

uptake characteristics between species in terms of kinetic parameters, contribution of the constitutive system (100% of total uptake in ryegrass, nil in Fabaceae) and time of induction.
Xyloglucan oligosaccharides with at least two α‐d‐xylose residues act as acceptor substrates for xyloglucan endotransglycosylase and promote the depolymerisation of xyloglucan
Authors: Lorences Ester P. , Fry Stephen C.
Published: May 2006

A xyloglucan‐derived pentasaccharide. Xyl2‐Glc3, was shown by viscometry to promote the depolymerisation of xyloglucan by enzyme extracts from bean (Phaseolus vulgaris L. cv. Canadian Wonder) leaves and pea (Pisum sativum L. cv. Alaska) stems. Xyl2‐Glc3 was also shown by a radiochemical assay to act as an acceptor substrate for xyloglucan endotransglycosylase activity (XET: EC 2.4.1.—) present in the same extracts. In both these assays, a heptasaccharide (Xyl3‐Glc4) was more effective than Xyl2‐Glc3 whereas two isomeric tetrasaccharides (Xyl1‐Glc3) were essentially ineffective. The agreement in the structural requirements of the two assays suggests that they share a common basis; we therefore propose that the oligosaccharide‐sensitive enzyme that depolymerises xyloglucan is XET rather than cellulase (EC 3.2.1.4). In the viscometric assay, the penta‐ and heptasaccharides would, according to our interpretation, compete with high molecular weight xyloglucan molecules as acceptor substrates for XET, leading to a decrease in the weight‐average molecular weight of the xyloglucan and, therefore, to a decrease in viscosity.

Our results indicate that oligosaccharides have to possess two α‐d‐xylose residues in order to act as acceptor substrates for XET. The non‐reducing end of a high‐molecular weight xyloglucan can also act as an acceptor substrate. Therefore, it is likely that exo‐hydrolysis by α‐d‐xylosidase would destroy the ability of a polysaccharide to act as an acceptor, even though α‐d‐xylosidase may remove only a single xylose residue from each polysaccharide molecule.