Flooding tolerance of Carex species. II. Root gas-exchange capacity

Root CO₂ and O₂ gas exchange were measured in young Carex extensa Good. (flooding sensitive), C. remota L. and C. pseudocyperus L. (both flooding tolerant) plants, precultured either aerobically or anaerobically. Temperature changes form 21 to 11 °C had small effects on root CO₂ release from respiration. In C. extensa, root respiration rates decreased when plants were precultured anaerobically, while in C. pseudocyperus preculture conditions had no effect on root CO₂ release. In contrast to CO₂, temperature decrease significantly enhanced radial oxygen loss from the roots during the light phase, indicating that at 20 °C the O₂ transported form the shoot to the root met the demand for root respiration quite well, while at 11 °C excess O₂ entered the root and was released into the anaerobic nutrient solution. In C. remota and C. pseudocyperus, the maximal O₂ concentration of a previously anaerobic nutrient solution was attained after several days of equilibration with the atmosphere through the plant body and was approximately one-third of that found in C. extensa, indicating that the diffusion resistance of the root/rhizosphere interface to O₂ is much lower in the C. extensa root than in the flooding-tolerant Carex species. A calculation of the maximal attainable root length that can be sustained by pure O₂ diffusion from CO₂ exchange, and anatomical data obtained earlier, revealed that longitudinal diffusion of O₂ through the root is sufficient for the oxygen supply of the root. It is concluded that the postulate of a gas mass-flow into the root is not necessary for the understanding of flooding tolerance of Carex species. Received: 10 February 1998 / Accepted: 2 July 1998     

Flooding tolerance of Carex species in relation to field distribution and aerenchyma formation

The flooding tolerance of Carex species was studied in relation to their field distribution and their capacity to form root aerenchyma under controlled conditions. In an alpine meadow, six Carex species were selected which were distributed in a clear zonation correlating with water content of the soil. Carex sempervirens and C. ferruginea were only found on nonflooded soil, the latter species preferring moister conditions. Carex davalliana and C. nigra were both associated with water-saturated soil, whereas C. limosa and C. rostrata preferred partially submerged conditions. Carex davalliana and C. limosa were bound to flooded soils with a relatively high redox potential and horizontally flowing groundwater. Carex rostrata and C. nigra grew in stagnant soil-flooded conditions with low soil redox potentials. The amount of aerenchyma in the roots of all species increased when grown in oxygen-deficient stagnant agar. This increase in root porosity, combined with increased root diameter, presumably improved internal aeration of the roots. Although all species survived experimental soil flooding, partial submergence was lethal to C. sempervirens and, surprisingly, also to the wetland species C. davalliana. Carex ferruginea showed a reduced growth rate during partial submergence. The three other species, all wetland plants, reached highest biomass production under soil-flooded and partially submerged conditions, with slower growth on free-draining soil. It is concluded that aerenchyma is not constitutive in the Carex species under study, and is best developed in Carex species from wetlands. Species with less aerenchyma perform poorly when soil-flooded, but conditions of partial submergence could even affect species with a considerable amount of root aerenchyma.     

Nuclear DNA contents of all species of Helleborus (Ranunculaceae) discriminate between species and sectional divisions

 Genome size (C-values) and pollen viability staining were applied as new criteria to investigate the species of the genus Helleborus Linnaeus (Ranunculaceae). All species have the same chromosome number (2n=32). However, the nuclear DNA content, as measured by flow cytometry with propidium iodide, could be demonstrated to range between 19 pg to 35.7 pg. The different genome sizes of the species coincided to a large extent with earlier determined section boundaries based on morphology. Flow cytometry can be a convenient method to discriminate between some species. Received April 17, 2001 Accepted May 7, 2001     

Directionality of auditory nerve fiber responses to pure tone stimuli in the grassfrog, Rana temporaria. I. Spike rate responses

We studied the directionality of spike rate responses of auditory nerve fibers of the grassfrog, Rana temporaria, to pure tone stimuli. All auditory fibers showed spike rate directionality. The strongest directionality was seen at low frequencies (200 – 400 Hz), where the spike rate could change by up to nearly 200␣spikes s⁻¹. with sound direction. At higher frequencies the directional spike rate changes were mostly below 100 spikes s⁻¹. In equivalent dB SPL terms (calculated using the fibers' rate-intensity curves) the maximum directionalities were up to 15 dB at low frequencies and below 10 dB at higher frequencies. Two types of directional patterns were observed. At frequencies below 500 Hz relatively strong responses were evoked by stimuli from the ipsilateral (+90^o) and contralateral (−90^o) directions while the weakest responses were evoked by stimuli from frontal (0^o or +30^o) or posterior (−135^o) directions. At frequencies above 800 Hz the strongest responses were evoked by stimuli from the ipsilateral direction while gradually weaker responses were seen as the sound direction shifted towards the contralateral side. At frequencies between 500 and 800 Hz both directional patterns were seen. The directionality was highly intensity dependent. No special adaptations for localization of conspecific calls were found. Accepted: 23 November 1996     

Isolation and characterization of four type-1 ribosome-inactivating proteins, with polynucleotide:adenosine glycosidase activity, from leaves of Phytolacca dioica L.

Four type-1 (single-chain) ribosome-inactivating proteins (RIPs), with isoelectric points between 9.5 and 9.7, were isolated from leaves of Phytolacca dioica L. The purification procedure furnished the four proteins with an overall yield of about 16% and separated them from a protein of 29 407 ± 2 Da, as determined by electrospray mass spectrometry, whose N-terminal amino acid sequence differed from that of pokeweed (Phytolacca americana L.) leaf chitinase (PLC-B) by only one amino acid (R17I). The four RIPs (PD-L1 to PD-L4) inhibited protein synthesis by a rabbit reticulocyte lysate with 50% inhibition at the picomolar level, and produced the β-fragment, diagnostic of the specific enzymatic action of RIPs, on yeast ribosomes. Comparison of their N-terminal sequences, up to residue 45, showed that PD-L1 is identical to PD-L2 [designated the isoleucine (Ile) form from the N-terminal residue] and PD-L3 is identical to PD-L4 [designated the valine (Val) form from the N-terminal residue] and that there are 35 identical residues between the two forms. Furthermore, the Val form presents the same number of identical residues as PD-S2, an RIP isolated from the seeds of the same plant. With the exception of PD-L4, the purified RIPs gave a positive reaction when stained for sugars on SDS-PAGE gels and, when analyzed by electrospray mass spectrometry, had M_r values of 32 715 ± 1 (PD-L1), 31 542 ± 1 (PD-L2), 30 356 ± 1 (PD-L3) and 29 185 ± 1 Da (PD-L4). The 1171 kDa difference in M_r, within the same RIP form, could be due to glycosylation. Like leaf saporins and many other RIPs, the four RIPs released several adenines from poly(A), herring sperm DNA and rRNA 16S + 23S, thus acting as polynucleotide:adenosine glycosidases. This property was less pronounced in PD-L1 and PD-L3 than in PD-L2 and PD-L4, respectively. The proteins PD-L1 and PD-L4 showed 3.7% reactivity with the antiserum anti-dianthin 32 and no reactivity with antisera to PAP-R saporin-S6, momordin I and even PD-S2, an RIP isolated from the seeds of the same plant. Protein PD-L4 showed 12.5% cross-reactivity with anti-PD-L1, while the opposite cross-reactivity was 100%. Received: 5 August 1998 / Accepted: 28 October 1998     

Arbuscular mycorrhizal symbiosis and nonhydraulic signaling of soil drying in Vigna unguiculata (L.) Walp.

 We examined the influence of Glomus intraradices on nonhydraulic signaling of soil drying, in a drought-avoiding plant having stomates that are extremely sensitive to changes in soil moisture. Cowpea [Vigna un guiculata (L.) Walp. 'White Acre'] seedlings were grown in a greenhouse with root systems split between two pots. The 2×3×2 experimental design included two levels of mycorrhizal colonization (presence or absence of Glomus intraradices Schenck & Smith UT143), three levels of phosphorus fertilization within each mycorrhizal treatment and two levels of water (both pots watered or one pot watered, one pot allowed to dry). Stomatal conductance was mostly similar in fully watered mycorrhizal and nonmycorrhizal controls. However, g _s of half-dried, nonmycorrhizal plants was reduced on fewer days and to a lesser extent than g _s of half-dried, mycorrhizal plants, perhaps related to quicker soil drying in mycorrhizal pots. The partial soil drying treatment had little effect on leaf relative water content or osmotic potential, indicating that declines in g _s and leaf growth were induced by some nonhydraulic factor. Leaf growth was inhibited only in nonmycorrhizal plants, evidently due to a difference in phosphorus nutrition between mycorrhizal and nonmycorrhizal plants. The mycorrhizal effect on g _s was not associated with phosphorus nutrition. Inhibition of g _s was directly related to extent of soil drying, while inhibition of leaf growth was inversely related to extent of soil drying. Accepted: 4 August 1995     

Analysis of phloem protein patterns from different organs of Cucurbita maxima Duch. by matrix-assisted laser desorption/ionization time of flight mass spectroscopy combined with sodium dodecyl sulfate-polyacrylamide gel electrophoresis

Sieve tubes mediate the long-distance transport of nutrients and signals between source and sink organs of plants. To detect mobile phloem proteins that are differentially distributed in source and sink organs of Cucurbita maxima, we used both one-dimensional gel electrophoresis (SDS-PAGE) and matrix-assisted laser desorption/ionization time-of-flight mass spectrometry (MALDI-TOF MS). Both techniques revealed that phloem protein patterns depend on the sampling site: whilst several proteins were consistently observed in all phloem samples studied others appeared to occur in a organ-specific manner. For a characterization and identification of distinct phloem polypeptides, two approaches were chosen. First, protein bands resolved by SDS-PAGE were eluted from the polyacrylamide gel and the masses of the proteins were then determined by MALDI-TOF MS. Second, proteins resolved by SDS-PAGE were subjected to proteolytic degradation and the resulting peptides were analyzed by MALDI-TOF MS; the masses of the proteolytic peptides were used for a database search. By the latter approach, three mobile phloem compounds were identified as the phloem-specific protein PP2 (D.E. Bostwick et al., 1992, The Plant Cell 4, 1539–1548) a chymotrypsin and an aspartic proteinase inhibitor. None of the other polypeptides studied corresponded to any of the protein sequences present in the database. Furthermore, MALDI-TOF MS analyses indicated that some of the mobile phloem proteins occur in a covalently modified form and that the extent of the modification depends upon the plant organ. Received: 25 July 1998 / Accepted: 22 September 1998     

Fourier-transform infrared and Raman spectroscopic evidence for the incorporation of cinnamaldehydes into the lignin of transgenic tobacco (Nicotiana tabacum L.) plants with reduced expression of cinnamyl alcohol dehydrogenase

Xylem from stems of genetically manipulated tobacco plants which had had cinnamyl alcohol dehydrogenase (CAD; EC 1.1.1.195) activity down-regulated to a greater or lesser degree (clones 37 and 49, respectively) by the insertion of antisense CAD cDNA had similar, or slightly higher, lignin contents than xylem from wild-type plants. Fourier-transform infrared (FT-IR) microspectroscopy indicated that down-regulation of CAD had resulted in the incorporation of moieties with conjugated carbonyl groups into lignin and that the overall extent of cross-linking, particularly of guaiacyl (4-hydroxy-3-methoxyphenyl) rings, in the lignin had altered. The FT-Raman spectra of manipulated xylem exhibited maxima consistent with the presence of elevated levels of aldehydic groups conjugated to a carbon-carbon double bond and a guaiacyl ring. These maxima were particularly intense in the spectra of xylem from clone 37, the xylem of which exhibits a uniform red coloration, and their absolute frequencies matched those of coniferaldehyde. Furthermore, xylem from clone 37 was found to have a higher content of carbonyl groups than that of clone 49 or the wild-type (clone 37: clone 49: wild-type; 2.4:1.6:1.0) as measured by a degradative chemical method. This is the first report of the combined use of FT-IR and FT-Raman spectroscopies to study lignin structure in situ. These analyses provide strong evidence for the incorporation of cinnamaldehyde groups into the lignin of transgenic plants with down-regulated CAD expression. In addition, these non-destructive analyses also suggest that the plants transformed with antisense CAD, in particular clone 37, may contain lignin that is less condensed (cross-linked) than that of the wild-type. Received: 27 May 1996 / Accepted: 30 July 1996     

MHC class I genes in a New World primate, the cotton-top tamarin (Saguinus oedipus), have evolved by an active process of loci turnover

 Lymphocytes of a New World primate, the cotton-top tamarin (Saguinus oedipus), express classical G–related major histocompatibility complex (MHC) class I molecules with unusually limited polymorphism and variability. Three G-related loci, an F locus, an E locus, and two pseudogenes (So-N1 and So-N3) have been identified by cDNA library screening and extensive PCR analysis of both cDNA and genomic DNA from the cotton-top tamarin. Furthermore, each genus of the subfamily Callitrichinae (tamarins and marmosets) appears to express its own unique set of MHC class I genes, likely due to a rapid turnover of loci. The rapid emergence of unique MHC class I genes in the Callitrichinae genera, resulting from an active process of duplication and inactivation of loci, may account for the limited diversity of the MHC class I genes in the cotton-top tamarin. To determine the nature of the entire complement of MHC class I genes in the cotton-top tamarin, we synthesized a genomic DNA library and screened it with MHC class I-specific probes. We isolated nine new MHC class I pseudogenes from this library. These newly isolated tamarin G–related MHC class I pseudogenes are not closely related to any of their functional counterparts in the tamarin, suggesting that they do not share a recent common ancestral gene with the tamarin's currently expressed MHC class I loci. In addition, these tamarin sequences display a high rate of nonsynonymous substitutions in their putative peptide binding region. This indicates that the genes from which they have derived were likely subject to positive selection and, therefore, were once functional. Our data support the notion that an extremely high rate of loci turnover is largely responsible for the limited diversity of the MHC class I genes in the cotton-top tamarin. Received: 15 September 1997 / Revised: 2 July 1998     

Cortinarius sarcoflammeus sp. nov, a new species of subgenus Dermocybe (Agaricales) growing in Sphagnum bogs

 Cortinarius sarcoflammeus is proposed as a new species belonging to subgenus Dermocybe, on the basis of its morphological, chemical and ecological characters. The strong red-orange colour of the context and stipe base, large spores, sphagnicolous habitat and high dermorubin content are characteristic for the new species. Holotypes of C. huronensis and C. huronensis var. olivaceus have been examined for comparison, and their differences discussed. Photographs and line drawings of C. sarcoflammeus are added. Received February 13, 2001 Accepted March 23, 2001     

Sucrose metabolism in cyanobacteria: sucrose synthase from Anabaena sp. strain PCC 7119 is remarkably different from the plant enzymes with respect to substrate affinity and amino-terminal sequence

The pathway of sucrose metabolism in cyanobacteria is just starting to be elucidated. The present study describes the first isolation and biochemical characterization of a prokaryotic sucrose synthase (SS, EC 2.4.1.13). Two SS forms (SS-I and SS-II) were detected in Anabaena sp. strain PCC 7119. The isoform SS-II was purified 457-fold and its amino-terminal portion sequenced. Substrate specificity, kinetic constants, native protein and subunit molecular masses, and the effect of different ions and metabolites were studied for SS-II. Anabaena SS was shown to be a tetramer with a 92-kDa polypeptide that was recognized by maize SS polyclonal antibodies. Some striking differences from plant enzymes were demonstrated with respect to substrate affinities, regulation by metal ions and ATP, and the amino-acid sequence of the N-terminal region. Received: 27 April 1999 / Accepted: 20 July 1999     

Pollen vectors and inflorescence morphology in four species of Salix

 Many plant species exhibit inflorescence morphologies intermediate between pollination syndromes and may therefore employ generalist pollination strategies. We studied how wind and insect pollination are related to inflorescence morphology in the floodplain species Salix alba, S. elaeagnos, S. daphnoides and S. triandra. Insect exclusion experiments showed that all four species were primarily pollinated by insects, but were capable of some seed set when wind was the only pollen vector. Such a generalist pollination system may provide reproductive assurance in these pioneer species. High wind pollination success was associated with slender and divided stigmatic lobes and low ovule number per catkin, which may enhance filtering capacity for airborne pollen. In contrast, species that relied more on insect pollination had robust stigmata and many ovules per catkin, which may reduce the number of insect visits necessary for pollination. Received April 18, 2002; accepted July 23, 2002 Published online: November 28, 2002 Addresses of authors: S. Karrenberg (e-mail: karrenberg@bio.indiana.edu), Department of Biology, Indiana University, Jordan Hall, 1001 East Third Street, Bloomington, IN 47405, USA. P. J. Edwards, Geobotanical Institute, ETH, Zürichbergstrasse 38, CH-8044 Zürich, Switzerland. J. Kollmann, Department of Ecology, Royal Veterinary and Agricultural University, Rolighedsvej 21, DK-1958 Frederiksberg, Denmark.     

High levels of antioxidant enzymes correlate with delayed senescence in nonnetted muskmelon fruits

We investigated the senescence process in two nonnetted muskmelon (Cucumis melo L.)␣varieties␣Clipper and Jerac differing in their storage life. Our results indicate that senescence in Jerac (the short-storage-life variety, less than 7 d) is the result of lipid peroxidation by free radicals, membrane phospholipid breakdown, and a drop in the level of antioxidants, resulting in increased membrane leakiness. By contrast, evidence is presented that the high levels of two enzymes implicated in antioxidative defence, superoxide dismutase (SOD) and catalase, combined with changes in the three different classes of SOD during the storage stage, are involved in delaying the senescence process in Clipper and this could explain, at least, to some extent, the long storage life of Clipper (longer than 14 d). Received: 15 April 1997 / Accepted: 17 July 1997     

Nitrification in the rhizosphere of a flooding-resistant and a flooding-non-resistant Rumex species under drained and waterlogged conditions

Abstract The effects of the flooding-resistant plant species Rumex palustris and the non-flooding-resistant plant species Rumex acetosa on nitrification were compared. The plants were grown under drained and waterlogged conditions on a mixture of calcareous riversand and sieved grassland soil with a high potential nitrifying activity. In the shoots of R. acetosa , but not in those of R. palustris , the ratio between the amounts of accumulated carboxylates and organic nitrogen, ((CA-A)/N_org.), appeared to be a useful indicator of ammonium or nitrate consumption by tghe plant. In both plant species, the inorganic nitrogen source had no observed effect on the (C-A)/N_org. ratio in the roots.
The growth of R. acetosa , but not that of R. palustris was inhibited by waterlogging of the soil. Both the activity and the growth of the ammonium-oxidizing bacteria were repressed under drained and waterlogged conditions in soils with R. palustris , a condition that was attributed to a competitive ammonium uptake by its relatively fast growing roots. In the presence of R. acetosa , the activity and growth of the ammonium-oxidizing bacteria were inhibited under waterlogged, but not under drained, conditions. he growth and activity of nitrite-oxidizing bacteria in the absence of actively ammonium-oxidizing, nitrite-producing bacteria was likely due to organotrophic growth.     

Self-pollination in Euphrasia willkommii Freyn (Scrophulariaceae), an endemic species from the alpine of the Sierra Nevada (Spain)

 The reproductive ecology of Euphrasia willkommii (Scrophulariaceae), an endemic species from the Mediterranean alpine environments of the SE Spain, has been experimentally studied during two reproductive seasons. The flowers of this plant species were visited by very few insects belonging only to two generalist taxa, thrips and ants. Nevertheless, reproduction is not pollen limited in E. willkommii. Hand-pollinations demonstrated that this plant species is capable of selfing, reproductive success being similar in autogamous and allogamous crosses. Moreover, pollinator-exclusion experiments also showed that, under natural conditions, this plant relies predominantly on selfing, seed production being similar in presence or absence of pollinators. Selfing in E. willkommii is presumably an ecological mechanism to ensure successful reproduction in a harsh environment where pollinator availability is extremely low. Received May 2, 2001 Accepted December 6, 2001     

Orderly formation of the double ring structures for plastid and mitochondrial division in the unicellular red alga Cyanidioschyzon merolae

The formation of the plastid-dividing ring (PD ring) and mitochondrion-dividing ring (MD ring) was studied in a highly synchronous culture of the unicellular red alga Cyanidioschyzon merolae. The timing and the order of formation of the MD and PD rings were determined by observing organelles around the onset of their division, using transmission electron microscopy. In  C. merolae, there is one chloroplast and one mitochondrion per cell, and the shape of the chloroplast changes sequentially from acorn-like, to round, to trapezoidal, to peanut-shaped, in that order, during the early stage of chloroplast division. None of the cells with acorn-shaped or round chloroplasts contained organelles with PD rings or MD rings, while all of the cells with peanut-shaped chloroplasts contained organelles with both PD rings and MD rings. In cells with peanut-shaped chloroplasts, the PD and MD rings were double ring structures, with an outer ring located on the cytoplasmic face of the outer membrane of the organelle, and an inner ring located in the matrix beneath the inner membrane. These results suggested that the double ring structures of the PD ring and the MD ring form when chloroplasts are trapezoidal in shape. Detailed three-dimensional observation of cells with trapezoidal chloroplasts revealed the following steps in the formation of the double ring structures of the PD and MD rings: (i) the inner ring of the PD ring forms first, followed by the outer ring; (ii) then the MD ring forms and becomes visible; (iii) when the double ring structures of the two rings have formed, the microbody then moves from its remote location to the plane of division of the mitochondrion and contraction of the PD and MD rings commences. These steps were also confirmed by computer-aided three-dimensional reconstruction of the images from serial thin sections. This study reveals the order of formation of the double ring structures of the PD and MD rings, and the behavior of the microbody around the onset of division of plastids and mitochondria. The results also provide the first evidence that the inner PD ring is not a tension element formed by the contractile pressure but a definite structure, independent of the outer ring. Received: 31 March 1998 / Accepted: 14 May 1998     

The effects of extracellular calmodulin on initiation of Hippeastrum rutilum pollen germination and tube growth

The effects of anti-calmodulin (CaM) serum, the CaM antagonist W7-agarose, the Ca²⁺ chelator ethyleneglycol-bis-(β-aminoethyl)-N,N,N′,N′-tetraacetic acid (EGTA) and exogenous pure CaM on pollen germination and tube growth of Hippeastrum rutilum Herb were studied. Pollen germination and tube growth were inhibited or completely stopped by anti-CaM serum in a dose-dependent manner, while the same amount of preimmune serum had no effect on either process. Pollen germination and tube growth were also inhibited or completely stopped by the CaM antagonist W7-agarose and the Ca²⁺ chelator EGTA. The addition of exogenous pure CaM enhanced pollen germination and tube growth, whereas the same amount of bovine serum albumin had no effect. The inhibitory effects caused by anti-CaM serum, W7-agarose and EGTA-washing could be reversed completely by the addition of exogenous pure CaM. These results indicate that extracellular CaM initiates pollen germination and tube growth, whereas exogenous CaM enhances the above processes, and may provide a novel view for understanding the control of pollen germination and tube growth. Received: 12 December 1996 / Accepted: 15 January 1997     

Rubber particles from four different species, examined by transmission electron microscopy and electron-paramagnetic-resonance spin labeling, are found to consist of a homogeneous rubber core enclosed by a contiguous, monolayer biomembrane

The physical characteristics of rubber particles from the four rubber (cis-1,4-polyisoprene) producing species Euphorbia lactiflua Phil., Ficus elastica Roxb., Hevea brasiliensis Müll. Arg., and Parthenium argentatum Gray, were investigated using transmission electron microscopy (TEM) and electron-paramagnetic-resonance (EPR) spin labeling spectroscopy. Transmission electron microscopy showed the rubber particles to be composed of a spherical, homogeneous, core of rubber enclosed by a contiguous, electron-dense, single-track surface layer. The biochemical composition of the surface layer and its single-track TEM suggested that a monolayer biomembrane was the surface structure most compatible with the hydrophobic rubber core. The EPR spectra for a series of positional isomers of doxyl stearic acid, used to label the surface layer of the rubber particles, exhibited flexibility gradients and evidence for lipid-protein interactions for all four rubber particle types that is consistent with a biomembrane-like surface. The EPR spectra confirmed that the surface biomembrane is a monolayer. Thus, rubber particles appear similar to oil bodies in their basic architecture. The EPR spectra also provided information on protein location and degree of biomembrane penetration that correlated with the known properties of the rubber-particle-bound proteins. The monolayer biomembrane serves as an interface between the hydrophobic rubber interior and the aqueous cytosol and prevents aggregation of the particles. An unexpected observation for the probes in pure polyisoprene was evidence of an intrinsic flexibility gradient associated with the stearic acid molecule itself. Received: 22 May 1999 / Accepted: 21 June 1999     

The influence of alterations in ADP-glucose pyrophosphorylase activities on starch structure and composition in potato tubers

In order to examine whether alterations in the supply of precursor molecules into the starch biosynthetic pathway affected various characteristics of the starch, starch was isolated from potato (Solanum tuberosum L.) tubers containing reduced amounts of the enzyme ADP-glucose pyrophosphorylase (AGPase). It was found that although the type of crystalline polymorph in the starch was not altered, the amylose content was severely reduced. In addition, amylopectin from the transgenic plants accumulated more relatively short chains than that from control plants and the sizes of starch granules were reduced. The starch granules from the transgenic plants contained a greater amount of granule-bound starch synthase enzyme, which led to an increase in the maximum activity of the enzyme per unit starch tested. The K _m for ADP-glucose was, at most, only slightly altered in the transgenic lines. Potato plants containing reduced AGPase activity were also transformed with a bacterial gene coding for AGPase to test whether this enzyme can incorporate phosphate monoesters into amylopectin. A slight increase in phosphate contents in the starch in comparison with the untransformed control was found, but not in comparison with starch from the line with reduced AGPase activity into which the bacterial gene was transformed. Received: 2 February 1999 / Accepted: 25 March 1999     

Iron limitation results in induction of ferricyanide reductase and ferric chelate reductase activities in Chlamydomonas reinhardtii

The green alga Chlamydomonas reinhardtii Dangeard CW-15 exhibited very low rates of plasma-membrane Fe(III) reductase activity when grown under Fe-sufficient conditions. After switching the medium to an Fe-free formulation, both ferricyanide reductase and ferric chelate reductase activities rapidly increased, reaching a maximum after 3 d under iron-free conditions. Both of the Fe(III) reductase activities increased in parallel over time, they exhibited similar K _m values (approximately 10 μM) with respect to Fe(III), displayed the same pH profile of activity, and both exhibited the same degree of light stimulation which could be inhibited by 3-(3′,4′-dichlorophenyl)-1,1-dimethylurea (DCMU). Furthermore, ferricyanide competitively inhibited ferric chelate reduction by iron-limited cells. These results indicate that both Fe(III) reductase activities were mediated by the same iron-limitation-induced plasma-membrane reductase. No evidence was found for the presence of Fe(III)-reducing substances in the culture medium, or for the involvement of active oxygen species in the process of Fe(III) reduction. Chlamydomonas reinhardtii appears to respond to iron limitation in a manner similar to Strategy I higher plants. Received: 24 June 1997 / Accepted: 2 August 1997