The role of Euglena gracilis paramylon in modulating xylem hormone levels,photosynthesis and water‐use efficiency in Solanum lycopersicum L

β‐1,3‐glucans such as paramylon act as elicitors in plants, modifying the hormonal levels and the physiological responses. Plant hormones affect all phases of the plant life cycle and their responses to environmental stresses, both biotic and abiotic. The aim of this study was to investigate the effects of a root treatment with Euglena gracilis paramylon on xylem hormonal levels, photosynthetic performance and dehydration stress in tomato (Solanum lycopersicum). Paramylon granules were processed to obtain the linear fibrous structures capable to interact with tomato cell membrane. Modulation of hormone levels (abscisic acid, jasmonic acid and salicylic acid) and related physiological responses such as CO₂ assimilation rate, stomatal and mesophyll conductance, intercellular CO₂ concentration, transpiration rate, water‐use efficiency, quantum yield of photosystem II and leaf water potential were investigated. The results indicate a clear dose‐dependent effect of paramylon on the hormonal content of xylem sap, photosynthetic performance and dehydration tolerance. Paramylon has the capability to enhance plant defense capacity against abiotic stress, such as drought, by modulating the conductance to CO₂ diffusion from air to the carboxylation sites and improving the water‐use efficiency.     

Localization of glyoxylate-cycle marker enzymes in peroxisomes of senescent leaves and green cotyledons

Crude particulate homogenates from leaves of barley (Hordeum vulgare L.), rice (Oryza sativa L.), leaf-beet (Beta vulgaris var.cicla L.) and pumpkin (Cucurbita pepo L.) cotyledons were separated on sucrose density gradients. The peroxisomal fractions appeared at a buoyant density of 1.25 g·cm^–3 and contained most of the activities of catalase (EC 1.11.1.6), and hydroxypyruvate reductase (EC 1.1.1.81) on the gradients. In peroxisomal fractions from detached leaves and green cotyledons incubated in permanent darkness we detected the presence of isocitrate lyase (EC 4.1.3.1) and malate synthase (EC 4.1.3.2), key enzymes of the glyoxylate cycle, and-oxidation activity (except in pumpkin). As proposed by H. Gut and P. Matile (1988, Planta176, 548–550) the glyoxylate cycle may be functional during leaf senescence, and the presence of two key enzymes indicates a transition from leaf peroxisome to glyoxysome; for pumpkin cotyledons in particular a double transition occurs (glyoxysome to leaf peroxisome during greening, and leaf peroxisome to glyoxysome during senescence).We are grateful to Professor P. Matile (Zürich, Switzerland) for his encouragement in pursuing this work.     

Effect of nanomolar concentrations of sodium dodecyl sulfate, a catalytic inductor of alpha-helices, on human calcitonin incorporation and channel formation in planar lipid membranes

Human Calcitonin (hCt) is a peptide hormone which has a regulatory action in calcium-phosphorus metabolism. It is currently used as a therapeutic tool in bone pathologies such as osteoporosis and Paget's disease. However, due to its amphiphilic property tends to form a gelatinous solution in water which consists of fibrils that limits its therapeutic use. Here we show that sodium dodecyl sulfate (SDS), an anionic detergent able to induce and stabilize alpha-helices in polypeptides, at a monomeric concentration ranging between 0.26 mM-5 pM (all concentrations are below the CMC), increases the rate and number of hCt channel formation in planar lipid membranes, at both high and low hCt concentrations, with a maximum increase at a molecular hCt/SDS ratio of 1000:1. This effect could be interpreted as a counteraction to the fibrillation process of hCt molecules by removing molecules available for aggregation from the fluid; furthermore, this action, independently of channel formation in the cell membrane, could improve the peptide-receptor interaction. The action of SDS could be attributable to the strength of the sulfate negative charge and the hydrophobic chain; in fact, a similar effect was obtained with lauryl sarcosine and not with a neutral detergent such as n-dodecyl-beta-D-maltoside. The very low molecular ratio between SDS and peptide is suggestive of a possible catalytic action of SDS that could induce alpha-helices, the appropriate structures for interacting with the membrane. Moreover, in the experimental conditions investigated, the addition of SDS does not modify the membrane's electrical properties and most of the channel properties. This finding may contribute to the knowledge of environment-folding diseases due to protein and peptides.     

Mycorrhizal colonization impacts on phenolic content and antioxidant properties of artichoke leaves and flower heads two years after field transplant

Greenhouse and field experiments were carried out in order to investigate the influence of mycorrhizal inoculation on total phenolic content (TPC) and antioxidant activity, expressed as antiradical power (ARP), of artichoke (Cynara cardunculus L. var. scolymus F.) leaves and flower heads extracts. The establishment of mycorrhizal symbiosis was monitored in pot and field grown plants, and the persistence of the inoculated AMF in roots after 2 years’ growth in the field was assessed by fungal ITS sequencing. Both in the greenhouse and in the field, marked increases in TPC and ARP were detected in leaves and flower heads of artichoke plants inoculated with the AM fungal species Glomus intraradices, either alone or in mixture with Glomus mosseae. In the field, plants inoculated with Glomus mix showed flower heads ARP content increases of 52.7 and 30.0% in the first and second year, respectively, compared with uninoculated plants. After 2 years’ growth in the field ITS rDNA sequences clustering with those of G. mosseae and G. intraradices were retrieved only from inoculated plant roots. Our data show that mycorrhizal inoculation may represent an efficient and sustainable strategy to improve productivity and enhance plant biosynthesis of secondary metabolites with health promoting activities.     

Effect of sterols on beta-amyloid peptide (AbetaP 1-40) channel formation and their properties in planar lipid membranes

We investigate the role played by membrane composition on the interaction and self-assembly of beta-amyloid peptide (AbetaP1-40) during pore formation in planar lipid membranes (PLMs). Incorporation studies showed that AbetaP does not interact with zwitterionic membranes made up of phosphatidylcholine, whereas the addition of cholesterol or ergosterol to the membranes leads to channel formation. Among the PLMs used, a higher propensity of AbetaP to form channels at low applied potential (+/-20 mV) was observed in 7-dehydrocholesterol and in oxidized cholesterol PLMs. These channels present long lifetimes, high-occurrence frequencies, and are voltage dependent. In particular, the AbetaP channel in oxidized cholesterol showed anion selectivity. Thus cholesterol (and sterols in general) could be considered as targets for AbetaP, which prevents the fibrillation process by increasing incorporation into membranes. Furthermore, by switching the channel selectivity versus anions, cholesterol helps to reduce the imbalance of the cellular ions, calcium included, induced by membrane depolarization, which could be one of the factors responsible for cytotoxicity in Alzheimer's disease.     

Gibberellin-like activity in suspensors of Tropaeolum majus L. and Cytisus laburnum L.

Gibberellins in the embryo-suspensor system have been considered so far only in Phaseolus coccineus. We present in this report the localization of gibberellin-like substances in the suspensors of Tropaeolum majus L. and Cytisus laburnum L. The total gibberellin activity (expressed as gibberellic-acid equivalent in the -amylase bioassay) in 2000 suspensors (106 mg fresh weight; FW) of C. laburnum and in 600 suspensors (236 mg FW) of T. majus were 50.9 g g^-1 FW and 8.9 g g^-1 FW respectively.Abbreviation GA gibberellin     

The extreme dwarf phenotype of the GA-sensitive mutant of sunflower, dwarf2, is generated by a deletion in the ent-kaurenoic acid oxidase1 (HaKAO1) gene sequence

A dwarf mutant, dw arf 2 (dw2), was isolated from sunflower (Helianthus annuus). The most obvious alterations of dw2 plants were the lack of stem growth, reduced size of leaves, petioles and flower organs, retarded flower development. Pollen and ovules were produced but the filaments failed to extrude the anthers from the corolla. The dw2 phenotype was mainly because of reduced cell size. In dw2 leaves, the dark-green color was not so much due to higher pigment content, but was correlated with a changed leaf morphology. The mutant responded to the application of bioactive gibberellins (GAs). The levels of ent-7α-hydroxykaurenoic acid, GA(19), GA(20) and GA(1) in dw2?seedlings were severely decreased relative to those in its wild type (WT). ent-Kaurenoic acid was actively converted to ent-7α-hydroxykaurenoic acid in WT plants but quite poorly in dw2 plants. All together these data suggested that the dw2 mutation severely reduced the flux through the biosynthetic pathway leading to active GAs by hampering the conversion of ent-kaurenoic acid to GA(12). Two ent-kaurenoic acid oxidase (KAO) genes were identified. HaKAO1 was expressed everywhere in sunflower organs, while HaKAO2 was mainly expressed in roots. We demonstrated that a DNA deletion in HaKAO1 of dw2 generated aberrant mRNA-splicing, causing a premature stop codon in the amino acid sequence. In dw2 calli, Agrobacterium-mediated transfer of WT HaKAO1 cDNA restored the WT endogenous levels of GAs. In segregating BC(1) progeny, the deletion co-segregated with the dwarf phenotype. The deletion was generated near to a breakpoint of a more complex chromosome rearrangement.     

Mediterranean sea cliff plants: morphological and physiological responses to environmental conditions

Aims The plants of Mediterranean sea cliff ecosystems are resistant to several environmental challenges. In this study, six species typical of the coastal rocky cliffs have been analyzed in order to evaluate their diverse morphological and physiological responses to their environment across the seasons, and to examine the strategy of the ecological group to which each species belongs. Since these species are widespread across the Mediterranean region, our aim was also to highlight their ecophysiological features in habitats where the direct influence of the sea is stronger.Methods The selected species are characteristic of the sea cliffs of Elba island (Tyrrhenian sea, Italy): the halophyte Crithmum maritimum, the semideciduous Helichrysum italicum and Lavandula stoechas and the sclerophylls Myrtus communis, Quercus ilex and Rhamnus alaternus. Four morphological traits—canopy height, leaf area, specific leaf area and leaf dry matter content—and two physiological traits—leaf water potential (LWP) and photosynthetic efficiency (PE), measured before the dawn and at midday—were analyzed. Water potential was measured by a pressure chamber and photosynthetic efficiency was determined by the analysis of chlorophyll fluorescence. Plant performance was also evaluated by calculating chronic (PI chr) and dynamic photoinhibition (PI dyn).Important findings Crithmum maritimum showed high resistance to the recurrent dry periods, because of the high water storage capacity of its leaves and its PE declined markedly only in July, under the harshest climatic conditions. Semideciduous taxa utilize primarily an avoidance strategy, which aims at reducing the overall leaf surface, while sclerophylls mostly show a tolerance strategy towards the prevailing stressors, as demonstrated by LWP and PE, that are lower in the sclerophylls than in the semideciduous taxa during summer, due to osmoregulation and photoinhibition, respectively. Furthermore, variability of physiological parameters was higher in the sclerophylls than in the semideciduous taxa, because the former had to withstand wider oscillations of their LWP and PE. The sclerophyllous taxa underwent a slight loss of PE also in winter, likely owing to the combined action of low temperature and high irradiance. In Mediterranean sea cliff ecosystems, the stressful combination of high irradiance, high temperatures and low rainfall typical of the summer season may have been intensified by the shallow soil which displays a poor water storage capacity. On the other hand, winter stress, caused by high solar radiation and low temperatures, does not seem to seriously affect the performance of the studied species.     

In vitro Culture of Immature Embryos of Cytisus laburnum

Immature embryos of Cytisus laburnum L. were cultivated in vitro and four culture media, different techniques of substrate preparation, sucrose concentration and the effect of suspensor removal were tested. The best results were obtained with N6 medium supplemented with 2 mg dm⁻³ glycine and set up using a double-layer culture system, in which the top layer had a higher osmotic potential than the bottom one. These conditions allowed normal embryogenic development in up to 45 % of early globular embryos, that were able to develop until a complete maturity. Osmotic potential and mineral nutrients of the medium demonstrated to be crucial for the successful culture and their effects were dependent on embryo age at the time of excision. The presence of an intact suspensor showed to be beneficial only for early globular embryos while older developmental stage embryos were not significantly affected. This revised version was published online in July 2006 with corrections to the Cover Date.