Physico-chemical properties of plant cuticles and their functional and ecological significance.

Most aerial plant surfaces are covered with a lipid-rich cuticle, which is a barrier for the bidirectional transport of substances between the plant and the surrounding environment. This review article provides an overview of the significance of the leaf cuticle as a barrier for the deposition and absorption of water and electrolytes. After providing insights into the physico-chemical properties of plant surfaces, the mechanisms of foliar absorption are revised with special emphasis on solutes. Due to the limited information and relative importance of the leaf cuticle of herbaceous and deciduous cultivated plants, an overview of the studies developed with Alpine conifers and treeline species is provided. The significance of foliar water uptake as a phenomenon of ecophysiological relevance in many areas of the world is also highlighted. Given the observed variability in structure and composition among, for example, plant species and organs, it is concluded that it is currently not possible to establish general permeability and wettability models that are valid for predicting liquid-surface interactions and the subsequent transport of water and electrolytes across plant surfaces.

Biofunctional Microgel-Based Fertilizers for Controlled Foliar Delivery of Nutrients to Plants.

Foliar application of micronutrients (e.g. Fe3+ ) onto plants over an extended time is challenging and often not possible due to insufficient rainfastness. Smart delivery systems which enable micronutrient release over several weeks would offer innovative and sustainable options to improve plant health and food production. Herein, we report a novel foliar fertilizer delivery system based on functional pH-responsive biohybrid microgels that have orthogonal functionality as carriers of micronutrients and employ peptides (termed anchor peptides) as foliar adhesion promoters. The anchor peptides bind to hydrophobic surfaces and the waxy "islands" of plant leaves. Our system requires no auxiliaries and is loadable, storable, and applicable from aqueous dispersion. We report the synthesis and functionalization of microgels, their loading with Fe3+ ions, and a proof of concept for the biofunctional microgel-based fertilizer system is demonstrated for iron-deficient cucumber plants.

Limits of active laser triangulation as an instrument for high precision plant imaging.

Laser scanning is a non-invasive method for collecting and parameterizing 3D data of well reflecting objects. These systems have been used for 3D imaging of plant growth and structure analysis. A prerequisite is that the recorded signals originate from the true plant surface. In this paper we studied the effects of species, leaf chlorophyll content and sensor settings on the suitability and accuracy of a commercial 660 nm active laser triangulation scanning device. We found that surface images of Ficus benjamina leaves were inaccurate at low chlorophyll concentrations and a long sensor exposure time. Imaging of the rough waxy leaf surface of leek (Allium porrum) was possible using very low exposure times, whereas at higher exposure times penetration and multiple refraction prevented the correct imaging of the surface. A comparison of scans with varying exposure time enabled the target-oriented analysis to identify chlorotic, necrotic and healthy leaf areas or mildew infestations. We found plant properties and sensor settings to have a strong influence on the accuracy of measurements. These interactions have to be further elucidated before laser imaging of plants is possible with the high accuracy required for e.g., the observation of plant growth or reactions to water stress.

Modelling of Lemna minor L. growth as influenced by nutrient supply, supplemental light, CO2 and harvest intervals for a continuous indoor cultivation.

Given the proper conditions, Lemna spp. rapidly produce a high amount of valuable biomass which is considered as an alternative source for feed and food. For a continuous and long-term indoor production under controlled conditions, environmental and harvest parameters have to be optimized to suppress algal growth and constantly yield a high-quality product. Experimentally assessing the effect of a larger number of parameters on the growth rate ri is impossible due to the theoretically high number of parameter combinations. Thus, a SIMILE® - based model has been developed. This enables production parameters to be assessed individually for its effect on the growth rate r i by a differential equation. Start values for numerical integration were taken from measured data and analytical solutions of the differential growth equation. At 400 ppm CO2, the regrowth rate ri in an optimized laboratory set-up amounted to 216 g FM·m-2d-1, harvesting one third of the biomass at intervals of 5 days. In up-scaled set-ups, lower regrowth rates ri of about 173 g FM·m-2d-1 (Kalkar) and 190 g FM·m-2d-1 (Berlin) were obtained, because temperature and light conditions were below optimum. At 3,500 ppm CO2, the regrowth rate ri in laboratory set-up increased to 323 g FM·m-2d-1 by shortening the harvest interval to three days. Maximum growth rates ri were obtained with an NH4 +/NO3 - ratio of 1/9 at 1.14 mM total N concentration. The results indicate how to optimize culture conditions and harvest intervals. Model runs closely match the experimental data taken from the three different approaches and thus confirm the validity of the model.

Ligands of boron in Pisum sativum nodules are involved in regulation of oxygen concentration and rhizobial infection.

Boron (B) is an essential nutrient for N(2)-fixing legume-rhizobia symbioses, and the capacity of borate ions to bind and stabilize biomolecules is the basis of any B function. We used a borate-binding-specific resin and immunostaining techniques to identify B ligands important for the development of Pisum sativum-Rhizobium leguminosarum 3841 symbiotic nodules. arabinogalactan-extensin (AGPE), recognized by MAC 265 antibody, appeared heavily bound to the resin in extracts derived from B-sufficient, but not from B-deficient nodules. MAC 265 stained the infection threads and the extracellular matrix of cortical cells involved in the oxygen diffusion barrier. In B-deprived nodules, immunolocalization of MAC 265 antigens was significantly reduced. Leghaemoglobin (Lb) concentration largely decreased in B-deficient nodules. The absence of MAC 203 antigens in B-deficient nodules suggests a high internal oxygen concentration, as this antibody detects an epitope on the lipopolysaccharide (LPS) of bacteroids typically expressed in micro-aerobically grown R. leguminosarum 3841. However, B-deprived nodules did not accumulate oxidized lipids and proteins, and revealed a decrease in the activity of the major antioxidant enzyme ascorbate peroxidase (APX). Therefore, B deficiency reduced the stability of nodule macromolecules important for rhizobial infection, and for regulation of oxygen concentration, resulting in non-functional nodules, but did not appear to induce oxidative damage in low-B nodules.

Membrane-associated, boron-interacting proteins isolated by boronate affinity chromatography.

Boron deficiency symptoms point to a role for boron in plant membranes, but the molecular partners interacting with boron have not yet been identified. The objective of the present study was to isolate and identify membrane-associated proteins with an ability to interact with boron. Boron-interacting proteins were isolated from root microsomal preparations of arabidopsis (Arabidopsis thaliana) and maize (Zea mays) using phenylboronate affinity chromatography, subsequently separated by two-dimensional gel electrophoresis and identified using MALDI-TOF (matrix-assisted laser desorption ionization-time of flight) peptide mass fingerprinting. Twenty-six boron-binding membrane-associated proteins were identified in A. thaliana, and nine in Z. mays roots. Additional unidentified proteins were also present. Common to both species were the beta-subunit of mitochondrial ATP synthase, several beta-glucosidases, a luminal-binding protein and fructose bisphosphate aldolase. In A. thaliana, binding of these proteins to boron was significantly reduced after 4 d of boron deprivation. The relatively high number of diverse proteins identified as boron interacting, many of which are usually enriched in membrane microdomains, supports the hypothesis that boron plays a role in plant membranes by cross-linking glycoproteins, and may be involved in their recruitment to membrane microdomains.

Equivalent pore radii of hydrophilic foliar uptake routes in stomatous and astomatous leaf surfaces--further evidence for a stomatal pathway.

Foliar uptake pathways for hydrophilic solutes were studied by the analysis of co-uptake of 15N-labelled urea, NH4+ or NO3- and 13C-labelled sucrose across leaf surfaces of various plant species. Uptake of N (y) and sucrose (x) were strongly correlated. Curvilinear regression revealed significantly positive intercepts with the y-axis indicating the involvement of a sucrose-excluding pathway consisting of small pores with radii <0.5 nm. Depending on plant species, N source, leaf side and aperture of stomata, these small pores accounted for 6-62% of total N uptake. Regression analysis revealed that in stomatous leaf surfaces of Vicia faba L., Coffea arabica L. and Prunus cerasus L., the remaining N uptake occurred via another pathway with an estimated average pore radius (r(P)) greater than 20 nm. This is two orders of magnitude greater than previous estimations of cuticular r(P), indicating that this pathway, which was only found in stomatous leaf surfaces, was probably not located in the cuticle but at the surfaces of the stomatal pores. In astomatous leaf surfaces of C. arabica and Populus x canadensis Moench, average r(P) was 2.0 and 2.4 nm, respectively, which is four to eight times larger than previous estimations of cuticular r(P). These results indicate that for polar solutes, the size exclusion limits of plant surfaces can be considerably larger than previously estimated. The far-reaching implications of these findings are discussed.

Size exclusion limits and lateral heterogeneity of the stomatal foliar uptake pathway for aqueous solutes and water-suspended nanoparticles.

Penetration rates of foliar-applied polar solutes are highly variable and the underlying mechanisms are not yet fully understood. The contribution of stomata especially, is still a matter of debate. Thus, the size exclusion limits of the stomatal foliar uptake pathway, its variability and its transport capacity have been investigated. The size exclusion limits were analyzed by studying the penetration of water-suspended hydrophilic particles of two different sizes (43 nm or 1.1 microm diameter) into leaves of Vicia faba (L.). To avoid agglutination of the particles, plants were kept in water-saturated atmosphere. Penetration of the larger particles was never detected, whereas after 2 to 9 days, the smaller particles occasionally penetrated the leaf interior through stomatal pores. Permeability of stomata to Na(2)-fluorescein along the leaf blade of Allium porrum (L.) was highly variable and not correlated with the position on the leaf. When evaporated residues of the foliar-applied solutions were rewetted repeatedly, approximately 60% of the previously penetrated stomata were penetrated again. The average rate constant of penetration of an individual stoma was in the same order of magnitude as typical rate constants reported for the cuticular pathway. The observed sparseness of stomatal penetration together with its high lateral variability but local and temporal persistency was taken as evidence that stomata contributing to uptake differ from non-penetrated ones in the wettability of their guard cell cuticle. These results show that the stomatal pathway is highly capacitive because of its large size exclusion limit above 10 nm and its high transport velocity, but at the same time the high variability renders this pathway largely unpredictable.

1H-NMR metabolomic profiling reveals a distinct metabolic recovery response in shoots and roots of temporarily drought-stressed sugar beets.

Yield formation in regions with intermittent drought periods depends on the plant's ability to recover after cessation of the stress. The present work assessed differences in metabolic recovery of leaves and roots of drought-stressed sugar beets with high temporal resolution. Plants were subjected to drought for 13 days, and rewatered for 12 days. At one to two-day intervals, plant material was harvested for untargeted 1H-NMR metabolomic profiling, targeted analyses of hexose-phosphates, starch, amino acids, nitrate and proteins, and physiological measurements including relative water content, osmotic potential, electrolyte leakage and malondialdehyde concentrations. Drought triggered changes in primary metabolism, especially increases in amino acids in both organs, but leaves and roots responded with different dynamics to rewatering. After a transient normalization of most metabolites within 8 days, a second accumulation of amino acids in leaves might indicate a stress imprint beneficial in upcoming drought events. Repair mechanisms seemed important during initial recovery and occurred at the expense of growth for at least 12 days. These results indicate that organ specific metabolic recovery responses might be related to distinct functions and concomitant disparate stress levels in above- and belowground organs. With respect to metabolism, recovery was not simply a reversal of the stress responses.

2-D clinorotation alters the uptake of some nutrients in Arabidopsis thaliana.

Future long-term spaceflight missions rely on bioregenerative life support systems (BLSS) in order to provide the required resources for crew survival. Higher plants provide an essential part since they supply food and oxygen and recycle carbon dioxide. There are indications that under space conditions plants might be inefficient regarding the uptake, transport and distribution of nutrients, which in turn affects growth and metabolism. Therefore, Arabidopsis thaliana (Col-0) seeds were germinated and grown for five days under fast clinorotation (2-D clinostat, 60rpm) in order to simulate microgravity. Concentrations of ten different nutrients (potassium, sulfur, phosphorus, calcium, sodium, magnesium, manganese, iron, zinc, and boron) in shoots of plants grown under reduced and normal (1g) gravity conditions were compared. A protocol was developed for the determination of different nutrients by means of inductively coupled plasma optical emission spectrometry (ICPOES), flame emission spectrometry and spectrophotometry. The concentrations of boron and sulfur were significantly decreased in clinorotated shoots, while the concentration of sodium was elevated, suggesting that altered gravity conditions differentially affected nutrient uptake. Possible mechanisms for such effects include reduced transpiration, altered expression of channels or transporters and direct effects on nutrient assimilation. The observed nutrient imbalances might have a negative impact on plant growth and nutritional quality during prolonged space missions.

Early biotic stress detection in tomato (Solanum lycopersicum) by BVOC emissions.

We investigated impacts of early and mild biotic stress on Biogenic Volatile Organic Compounds (BVOC) emissions from tomato in order to test their potential for early (biotic) stress detection. Tomato plants were exposed to two common fungal pathogens, Botrytis cinerea and Oidium neolycopesici and the sap-sucking aphid Myzus persicae. Furthermore, plants were exposed to methyl jasmonate (MeJA) in order to identify BVOC emissions related to activation of jasmonic acid (JA) signalling pathway. These emissions where then used as a reference for identifying active JA signalling pathway in plants at early stages of biotic stress. After infection by the necrotrophic fungus B. cinerea, changes in BVOC emissions indicated that tomato plants had predominantly activated the jasmonic acid (JA) signalling pathway. The plants were able to modify their defence pathways in order to overcome fungal infection. When tomato plants were infected with the biotrophic fungus O. neolycopersici, only minor changes in BVOC emissions were observed with additional emissions of the sesquiterpene α-copaene. α-copaene emissions allowed the identification of general biotic stress in the plants, without pinpointing the actual triggered defence pathway. BVOC emissions during M. persicae attack had changed before the occurrence of visual symptoms. Despite low infestation rates, plants emitted methyl salicylate indicating activation of the SA-mediated defence pathway.

Boron Supply Enhances Aluminum Tolerance in Root Border Cells of Pea (Pisum sativum) by Interacting with Cell Wall Pectins.

Aluminum (Al) toxicity is the primary factor limiting crop growth in acidic soils. Boron (B) alleviates Al toxicity in plants, which is mainly considered to be due to the formation of Rhamnogalacturonan II-B (RGII-B) complexes, which helps to stabilize the cytoskeleton. It is unclear yet whether this is due to the increasing of net negative charges and/or further mechanisms. Kinetics of Al accumulation and adsorption were investigated using entire cells, cell wall and pectin of root border cells (RBCs) of pea (Pisum sativum), to reveal the mechanism of B in interacting with alkali-soluble and chelator-soluble pectin for an increased Al tolerance in RBCs. The results show that B could rescue RBCs from Al-induced cell death by accumulating more Al in the cell wall, predominately in alkali-soluble pectin. Boron also promotes Al3+ adsorption and inhibits Al3+ desorption from alkali-soluble pectin. Thus, more Al3+ is immobilized within the alkali-soluble pectin fraction and less in the chelator-soluble pectin, rendering Al3+ less mobile. Boron induces an increase of RG-II (KDO,2-keto-3-deoxyoctonic acid) content for forming more borate-RGII complexes, and the decrease of pectin methyl-esterification, thus creates more negative charges to immobilize Al3+ in cell wall pectin. The study provides evidence that abundant B supply enhances the immobilization of Al in alkali-soluble pectin, thus most likely reducing the entry of Al3+ into the symplast from the surroundings.

Osmotic adjustment of young sugar beets (Beta vulgaris) under progressive drought stress and subsequent rewatering assessed by metabolite analysis and infrared thermography.

The main objective of this work was to provide the chronology of physiological and metabolic alterations occurring under drought and demonstrate how these relate to a phenotypic approach (infrared thermal imaging, IRT). This should provide tools to tailor phenotyping approaches for drought tolerance and underlying metabolic alterations. In the present study, destructive analysis of growth and cell morphology, water status, osmotic adjustment, metabolic changes and membrane damage were combined with non-destructive determination of leaf temperature using infrared thermography (IRT) in 6-week-old sugar beets subjected to progressive drought stress and subsequent rewatering. Different methods were suitable for the characterisation of the dynamic development of distinct stress phases: although IRT allowed detection of initial impairment of transpiration within 1 day of drought stress, destructive methods allowed us to distinguish a phase of metabolic adjustment including redirection of carbon flow into protective mechanisms and a subsequent phase of membrane destabilisation and cellular damage. Only the combination of invasive and non-invasive methods allowed for the differentiation of the complete sequence of physiological changes induced by drought stress. This could be especially beneficial for the selection of phenotypes that are adapted to early drought. During rewatering, sugar beet shoots rapidly re-established water relations, but membrane damage and partial stomatal closure persisted longer, which could have an impact on subsequent stress events. During the onset of secondary growth, taproots required more time to recover the water status and to readjust primary metabolites than shoots.

Short-term boron deprivation inhibits endocytosis of cell wall pectins in meristematic cells of maize and wheat root apices.

By using immunofluorescence microscopy, we observed rapidly altered distribution patterns of cell wall pectins in meristematic cells of maize (Zea mays) and wheat (Triticum aestivum) root apices. This response was shown for homogalacturonan pectins characterized by a low level (up to 40%) of methylesterification and for rhamnogalacturonan II pectins cross-linked by a borate diol diester. Under boron deprivation, abundance of these pectins rapidly increased in cell walls, whereas their internalization was inhibited, as evidenced by a reduced and even blocked accumulation of these cell wall pectins within brefeldin A-induced compartments. In contrast, root cells of species sensitive to the boron deprivation, like zucchini (Cucurbita pepo) and alfalfa (Medicago sativa), do not internalize cell wall pectins into brefeldin A compartments and do not show accumulation of pectins in their cell walls under boron deprivation. For maize and wheat root apices, we favor an apoplastic target for the primary action of boron deprivation, which signals deeper into the cell via endocytosis-mediated pectin signaling along putative cell wall-plasma membrane-cytoskeleton continuum.