Georgios Liakopoulos

The Relationship between Anatomy and Photosynthetic Performance of Heterobaric Leaves

Heterobaric leaves show heterogeneous pigmentation due to the occurrence of a network of transparent areas that are created from the bundle sheaths extensions (BSEs). Image analysis showed that the percentage of photosynthetically active leaf area (Ap) of the heterobaric leaves of 31 plant species was species dependent, ranging from 91% in Malva sylvestris to only 48% inGynerium sp. Although a significant portion of the leaf surface does not correspond to photosynthetic tissue, the photosynthetic capacity of these leaves, expressed per unit of projected area (Pmax), was not considerably affected by the size of their transparent leaf area (At). This means that the photosynthetic capacity expressed per Ap(P*max) should increase with At. Moreover, the expression of P*max could be allowing the interpretation of the photosynthetic performance in relation to some critical anatomical traits. The P*max, irrespective of plant species, correlated with the specific leaf transparent volume (λt), as well as with the transparent leaf area complexity factor (CFAt), parameters indicating the volume per unit leaf area and length/density of the transparent tissues, respectively. Moreover, both parameters increased exponentially with leaf thickness, suggesting an essential functional role of BSEs mainly in thick leaves. The results of the present study suggest that although the Ap of an heterobaric leaf is reduced, the photosynthetic performance of each areole is increased, possibly due to the light transferring capacity of BSEs. This mechanism may allow a significant increase in leaf thickness and a consequent increase of the photosynthetic capacity per unit (projected) area, offering adaptive advantages in xerothermic environments.

New Insights into the Properties of Pubescent Surfaces: Peach Fruit as a Model

The surface of peach (Prunus persica ‘Calrico’) is covered by a dense indumentum, which may serve various protective purposes. With the aim of relating structure to function, the chemical composition, morphology, and hydrophobicity of the peach skin was assessed as a model for a pubescent plant surface. Distinct physicochemical features were observed for trichomes versus isolated cuticles. Peach cuticles were composed of 53% cutan, 27% waxes, 23% cutin, and 1% hydroxycinnamic acid derivatives (mainly ferulic and p-coumaric acids). Trichomes were covered by a thin cuticular layer containing 15% waxes and 19% cutin and were filled by polysaccharide material (63%) containing hydroxycinnamic acid derivatives and flavonoids. The surface free energy, polarity, and work of adhesion of intact and shaved peach surfaces were calculated from contact angle measurements of water, glycerol, and diiodomethane. The removal of the trichomes from the surface increased polarity from 3.8% (intact surface) to 23.6% and decreased the total surface free energy chiefly due to a decrease on its nonpolar component. The extraction of waxes and the removal of trichomes led to higher fruit dehydration rates. However, trichomes were found to have a higher water sorption capacity as compared with isolated cuticles. The results show that the peach surface is composed of two different materials that establish a polarity gradient: the trichome network, which has a higher surface free energy and a higher dispersive component, and the cuticle underneath, which has a lower surface free energy and higher surface polarity. The significance of the data concerning water-plant surface interactions is discussed within a physiological context.

“Carbon gain vs. water saving, growth vs. defence”: Two dilemmas with soluble phenolics as a joker

Despite that phenolics are considered as a major weapon against herbivores and pathogens, the primal reason for their evolution may have been the imperative necessity for their UV-absorbing and antioxidant properties in order for plants to compensate for the adverse terrestrial conditions. In dry climates the choice concerning the first dilemma (carbon gain vs. water saving) needs the appropriate structural and metabolic modulations, which protect against stresses such as high UV and visible radiation or drought, but reduce photosynthesis and increase oxidative pressure. Thus, when water saving is chosen, priority is given to protection (including phenolic synthesis), instead of carbon gain and hence growth. At the global level, the different choices by the individual species are expressed by an interspecific negative relationship between total phenolics and photosynthesis. On the other hand, the accumulation of phenolics in water saving plants offers additional defensive functions because these multifunctional compounds can also act as pro-oxidant, antifeeding or toxic factors. Therefore phenolics, as biochemical jokers, can give the answer to both dilemmas: water saving involves high concentrations of phenolics which also offer high level of defence.

Novel Secoiridoid Glucosides in Olea europaea Leaves Suffering from Boron Deficiency

From the methanol extract of boron deficient Olea europaea leaves, two secoiridoid glycosides, not detected in leaf extracts of untreated plants, 6′-E-p-coumaroyl-secologanoside and 6′-O-[(2E)-2,6-dimethyl-8-hydroxy-2-octenoyloxy]-secologanoside, were isolated together with three known secoiridoid glycosides, oleuropein, oleoside dimethyl ester, and secologanoside. The structures of the isolated compounds were established by means of NMR and MS spectral analyses. The above novel secoiridoids were synthesized by the plant as a physiological response to nutrient stress.

Alarm Photosynthesis: Calcium Oxalate Crystals as an Internal CO2 Source in Plants

A new photosynthetic path named “alarm photosynthesis” uses mesophyll calcium oxalate crystals as the CO2 source when stomata are closed, providing adaptive advantages under drought conditions. Calcium oxalate crystals are widespread among animals and plants. In land plants, crystals often reach high amounts, up to 80% of dry biomass. They are formed within specific cells, and their accumulation constitutes a normal activity rather than a pathological symptom, as occurs in animals. Despite their ubiquity, our knowledge on the formation and the possible role(s) of these crystals remains limited. We show that the mesophyll crystals of pigweed (Amaranthus hybridus) exhibit diurnal volume changes with a gradual decrease during daytime and a total recovery during the night. Moreover, stable carbon isotope composition indicated that crystals are of nonatmospheric origin. Stomatal closure (under drought conditions or exogenous application of abscisic acid) was accompanied by crystal decomposition and by increased activity of oxalate oxidase that converts oxalate into CO2. Similar results were also observed under drought stress in Dianthus chinensis, Pelargonium peltatum, and Portulacaria afra. Moreover, in A. hybridus, despite closed stomata, the leaf metabolic profiles combined with chlorophyll fluorescence measurements indicated active photosynthetic metabolism. In combination, calcium oxalate crystals in leaves can act as a biochemical reservoir that collects nonatmospheric carbon, mainly during the night. During the day, crystal degradation provides subsidiary carbon for photosynthetic assimilation, especially under drought conditions. This new photosynthetic path, with the suggested name “alarm photosynthesis,” seems to provide a number of adaptive advantages, such as water economy, limitation of carbon losses to the atmosphere, and a lower risk of photoinhibition, roles that justify its vast presence in plants.

Gene Expression and Biochemical Characterization of Azotobacter vinelandii Cyclophilins and Protein Interaction Studies of the Cytoplasmic Isoform with dnaK and lpxH

The soil nitrogen-fixing bacterium Azotobacter vinelandii possesses two cyclophilins, comprising putative cytoplasmic and periplasmic isoforms, designated as AvPPIB and AvPPIA, respectively. Both recombinant cyclophilins have been purified and their peptidyl-prolyl cis/trans isomerase activity against Suc-Ala-Xaa-Pro-Phe-pNA synthetic peptides has been characterized. The substrate specificity of both cyclophilins is typical for bacterial cyclophilins, with Suc-Ala-Ala-Pro-Phe-pNA being the most rapidly catalyzed substrate. The cytoplasmic cyclophilin also displays a chaperone function in the citrate synthase thermal aggregation assay. Using real-time quantitative RT-PCR, we demonstrate that AvppiB is expressed under various physiological and growth conditions, mainly upregulated by acetate and downregulated by the stationary growth state, while AvppiA shows a tendency for downregulation under the tested conditions. Further, we identified chaperone protein dnaK and UDP-2, 3-diacylglucosamine hydrolase lpxH as probable interacting partners of AvPPIB and we demonstrate their physical interaction by coexpression studies. An increase in AvPPIB PPIase activity in the presence of AvdnaK and a decrease in the presence of AvlpxH further confirms each interaction. However, the PPIase activity does not seem to be essential for those interactions since AvPPIB active site mutants still interact with dnaK and lpxH, while their minor PPIase activity cannot be modulated by the interaction.

Photosynthetic capacity is negatively correlated with the concentration of leaf phenolic compounds across a range of different species

This study reveals a negative relationship between leaf phenolic compounds and photosynthetic Amax among different plant species. This indicates a functional integration among carbon gain and the concentration of leaf phenolic compounds that reflects the trade-off between growth and defence/protection demands.

Cloning, characterization and transcriptional analysis of two phosphate acetyltransferase isoforms from Azotobacter vinelandii

Acetate is abundant in soil contributing to a great extent on carbon cycling in nature. Phosphate acetyltransferase (Pta, EC 2.3.1.8) catalyzes the reversible transfer of the acetyl group from acetyl-P to CoA forming acetyl-CoA and inorganic phosphate, participating to acetate assimilation/dissimilation reactions. In the present study, we demonstrate that Azotobacter vinelandii, a nitrogen-fixing, free-living, soil bacterium, possesses two class II phosphate acetyltransferase isoforms, AvPTA-1 and AvPTA-2, with different kinetic properties. At the acetyl-CoA forming direction, AvPTA-1 has lower affinity for acetyl-P and higher affinity for CoA than AvPTA-2 while at the acetyl-P forming direction; activity was measured only for AvPTA-1. Quantification of their expression patterns by RT-qPCR indicated that both genes are expressed during exponential growth on glucose or acetate and are down-regulated in the stationary phase. The ammonium availability during acetate growth resulted in up-regulation of Avpta-2 expression only. Further, the gene expression patterns of other related gene transcripts were also investigated, in order to understand the influence of each pathway in the assimilation/dissimilation of acetate.

Leaf anatomy affects the extraction of photosynthetic pigments by DMSO.

Dimethylsulfoxide (DMSO) is a widely used solvent for the extraction of chlorophylls (Chls) from leaves of higher plants. The method is preferred because the time-consuming steps of grinding and centrifuging are not required and the extracts are stable for a long time period. However, the extraction efficiency of this solvent is not comparable among plant species, whereas the particular leaf anatomical characteristics responsible for this unevenness remain unknown. In order to examine the influence of leaf anatomy on the extraction efficiency of DMSO (i.e. the concentration of Chls extracted with DMSO as % of the concentration of Chls extracted with 80% acetone), leaves of 19 plant species with different anatomical characteristics were incubated for 40min in DMSO at 65 degrees C. Under these conditions, heterobaric leaves, which are characterized by the occurrence of bundle sheath extensions in the mesophyll, showed lower extraction efficiency of DMSO compared to homobaric leaves and conifer needles. Microscopical observations of DMSO incubated leaf tissues showed that bundle sheath extensions behave as anatomical barriers which prevent the diffusion of DMSO within heterobaric leaves, even after prolonged incubation with the solvent. The effect was stronger in heterobaric leaves possessing thick bundle sheath extensions. The extraction efficiency of DMSO in these leaves was improved by vacuum infiltration of the samples in the presence of warm (65 degrees C) solvent.

Induced parthenocarpic cherry tomato fruits did not shown significant differences in l -ascorbate content but showed different pattern in GalLDH and GME expression

Parthenocarpy in tomato is often induced by auxins to overcome fertilization problems due to low temperatures. To estimate the effect of this agronomical practice on the physiology and dietary value of cherry tomato fruits we determined l-ascorbic acid, the expression and immunolocalization of galactono 1,4 lactone dehydrogenase and the expression of GDP-mannose 3′,5′-epimerase, key genes in l-ascorbic acid biosynthesis. The levels of l-ascorbic acid did not differ between seeded and parthenocarpic fruits while the relative expression of galactono 1,4 lactone dehydrogenase and GDP-mannose 3′,5′-epimerase gene transcripts showed some significant differences between seeded and parthenocarpic fruits. The galactono 1,4 lactone dehydrogenase immunohistolocalization signal was stronger in the ovules and mature embryos of seed-containing fruits. Our data suggest that although there were differences in the expression of the studied genes and in enzyme localization, these did not cause differences in the l-ascorbic acid content of parthenocarpic fruits produced by auxin application.