Mechanical properties of epidermal cells of whole living roots of Arabidopsis thaliana: an atomic force microscopy study

The knowledge of mechanical properties of root cell walls is vital to understand how these properties interact with relevant genetic and physiological processes to bring about growth. Expansion of cell walls is an essential component of growth, and the regulation of cell wall expansion is one of the ways in which the mechanics of growth is controlled, managed and directed. In this study, the inherent surface mechanical properties of living Arabidopsis thaliana whole-root epidermal cells were studied at the nanoscale using the technique of atomic force microscopy (AFM). A novel methodology was successfully developed to adapt AFM to live plant roots. Force-Indentation (F-I) experiments were conducted to investigate the mechanical properties along the length of the root. F-I curves for epidermal cells of roots were also generated by varying turgor pressure. The F-I curves displayed a variety of features due to the heterogeneity of the surface. Hysteresis is observed. Application of conventional models to living biological systems such as cell walls in nanometer regimes tends to increase error margins to a large extent. Hence information from the F-I curves were used in a preliminary semiquantitative analysis to infer material properties and calculate two parameters. The work done in the loading and unloading phases (hysteresis) of the force measurements were determined separately and were expressed in terms of "Index of Plasticity" (η), which characterized the elasticity properties of roots as a viscoelastic response. Scaling approaches were used to find the ratio of hardness to reduced modulus (H/E(*)).

Anatomical, biochemical, and photosynthetic responses to recent allopolyploidy in Glycine dolichocarpa (Fabaceae)

PREMISE OF THE STUDY

Previous studies have shown that polyploidy has pronounced effects on photosynthesis. Most of these studies have focused on synthetic or recently formed autopolyploids, and comparatively little is known about the integrated effects of natural allopolyploidy, which involves hybridity and genome doubling and often incorporates multiple genotypes through recurrent origins and lineage recombination.

METHODS

Glycine dolichocarpa (designated T2) is a natural allotetraploid with multiple origins. We quantified 21 anatomical, biochemical, and physiological phenotypes relating to photosynthesis in T2 and its diploid progenitors, G. tomentella (D3) and G. syndetika (D4). To assess how direction of cross affects these phenotypes, we included three T2 accessions having D3-like plastids (T2(D3)) and two accessions having D4-like plastids (T2(D4)).

KEY RESULTS

T2 accessions were transgressive (more extreme than any diploid accession) for 17 of 21 phenotypes, and species means differed significantly in T2 vs. both progenitors for four of 21 phenotypes (higher for guard cell length, electron transport capacity [J(max)] per palisade cell, and J(max) per mesophyll cell; lower for palisade cells per unit leaf area). Within T2, four of 21 parameters differed significantly between T2(D3) and T2(D4) (palisade cell volume; chloroplast number and volume per unit leaf area; and J(max) per unit leaf area).

CONCLUSIONS

T2 is characterized by transgressive photosynthesis-related phenotypes (including an ca. 2-fold increase in J(max) per cell), as well as by significant intraspecies variation correlating with plastid type. These data indicate prominent roles for both nucleotypic effects and cytoplasmic factors in photosynthetic responses to allopolyploidy.

Polyamines interact with hydroxyl radicals in activating Ca(2+) and K(+) transport across the root epidermal plasma membranes

Reactive oxygen species (ROS) are integral components of the plant adaptive responses to environment. Importantly, ROS affect the intracellular Ca(2+) dynamics by activating a range of nonselective Ca(2+)-permeable channels in plasma membrane (PM). Using patch-clamp and noninvasive microelectrode ion flux measuring techniques, we have characterized ionic currents and net K(+) and Ca(2+) fluxes induced by hydroxyl radicals (OH(•)) in pea (Pisum sativum) roots. OH(•), but not hydrogen peroxide, activated a rapid Ca(2+) efflux and a more slowly developing net Ca(2+) influx concurrent with a net K(+) efflux. In isolated protoplasts, OH(•) evoked a nonselective current, with a time course and a steady-state magnitude similar to those for a K(+) efflux in intact roots. This current displayed a low ionic selectivity and was permeable to Ca(2+). Active OH(•)-induced Ca(2+) efflux in roots was suppressed by the PM Ca(2+) pump inhibitors eosine yellow and erythrosine B. The cation channel blockers gadolinium, nifedipine, and verapamil and the anionic channel blockers 5-nitro-2(3-phenylpropylamino)-benzoate and niflumate inhibited OH(•)-induced ionic currents in root protoplasts and K(+) efflux and Ca(2+) influx in roots. Contrary to expectations, polyamines (PAs) did not inhibit the OH(•)-induced cation fluxes. The net OH(•)-induced Ca(2+) efflux was largely prolonged in the presence of spermine, and all PAs tested (spermine, spermidine, and putrescine) accelerated and augmented the OH(•)-induced net K(+) efflux from roots. The latter effect was also observed in patch-clamp experiments on root protoplasts. We conclude that PAs interact with ROS to alter intracellular Ca(2+) homeostasis by modulating both Ca(2+) influx and efflux transport systems at the root cell PM.

Programmed cell death in plants: protective effect of mitochondrial-targeted quinones

Ubiquinone or plastoquinone covalently linked to synthetic decyltriphenylphosphonium (DTPP(+)) or rhodamine cations prevent programmed cell death (PCD) in pea leaf epidermis induced by chitosan or CN(-). PCD was monitored by recording the destruction of cell nuclei. CN(-) induced the destruction of nuclei in both epidermal cells (EC) and guard cells (GC), whereas chitosan destroyed nuclei in EC not in GC. The half-maximum concentrations for the protective effects of the quinone derivatives were within the pico- and nanomolar range. The protective effect of the quinones was removed by a protonophoric uncoupler and reduced by tetraphenylphosphonium cations. CN(-)-Induced PCD was accelerated by the tested quinone derivatives at concentrations above 10(-8)-10(-7) M. Unlike plastoquinone linked to the rhodamine cation (SkQR1), DTPP(+) derivatives of quinones suppressed menadione-induced H(2)O(2) generation in the cells. The CN(-)-induced destruction of GC nuclei was prevented by DTPP(+) derivatives in the dark not in the light. SkQR1 inhibited this process both in the dark and in the light, and its effect in the light was similar to that of rhodamine 6G. The data on the protective effect of cationic quinone derivatives indicate that mitochondria are involved in PCD in plants.

Tissue-autonomous promotion of palisade cell development by phototropin 2 in Arabidopsis

Light is an important environmental information source that plants use to modify their growth and development. Palisade parenchyma cells in leaves develop cylindrical shapes in response to blue light; however, the photosensory mechanism for this response has not been elucidated. In this study, we analyzed the palisade cell response in phototropin-deficient mutants. First, we found that two different light-sensing mechanisms contributed to the response in different proportions depending on the light intensity. One response observed under lower intensities of blue light was mediated exclusively by a blue light photoreceptor, phototropin 2 (PHOT2). Another response was elicited under higher intensities of light in a phototropin-independent manner. To determine the tissue in which PHOT2 perceives the light stimulus to regulate the response, green fluorescent protein (GFP)-tagged PHOT2 (P2G) was expressed under the control of tissue-specific promoters in the phot1 phot2 mutant background. The results revealed that the expression of P2G in the mesophyll, but not in the epidermis, promoted palisade cell development. Furthermore, a constitutively active C-terminal kinase fragment of PHOT2 fused to GFP (P2CG) promoted the development of cylindrical palisade cells in the proper direction without the directional cue provided by light. Hence, in response to blue light, PHOT2 promotes the development of cylindrical palisade cells along a predetermined axis in a tissue-autonomous manner.

Laticiferous canal formation in fruits of Decaisnea fargesii: a programmed cell death process?

Programmed cell death (PCD), a topic of abiding interest, remodels plants at the cell, tissue, and organ levels involving various developmental processes of plants. The aim of this study is to provide a morphological characterization of evidence of PCD involvement in the laticiferous canal formation in fruit of Decaisnea fargesii. Several ultrastructural features of PCD have been observed including disintegration of vacuole and plasma membranes, cell wall degeneration, degenerated cytoplasm, abundant membrane structures and flocculent material, mitochondria and misshapen nuclei coupled with degraded plastids in vacuoles, and nuclei enveloped by rubber granule. In D. fargesii, the nuclei of the secretory epidermal cells become TUNEL-positive from the sunken stage to the late expanding stage, then DAPI-negative during the mature stage, indicating an early event of deoxyribonucleic acid (DNA) cleavage and a late event of complete DNA degeneration. Gel electrophoresis indicates that DNA cleavage is random and does not result in the laddering pattern indicating multiples of internucleosomal units. During the PCD of secretory epidermal cells, the rubber granules continue to be synthesized and accumulated in the secretory epidermal cells despite nuclear degradation. The PCD's role in laticiferous canal formation suggests that PCD may play important roles in gland development of plants.

Lotus japonicus symRK-14 uncouples the cortical and epidermal symbiotic program

SYMRK is a leucine-rich-repeat (LRR)-receptor kinase that mediates intracellular symbioses of legumes with rhizobia and arbuscular mycorrhizal fungi. It participates in signalling events that lead to epidermal calcium spiking, an early cellular response that is typically considered as central for intracellular accommodation and nodule organogenesis. Here, we describe the Lotus japonicus symRK-14 mutation that alters a conserved GDPC amino-acid sequence in the SYMRK extracellular domain. Normal infection of the epidermis by fungal or bacterial symbionts was aborted in symRK-14. Likewise, epidermal responses of symRK-14 to bacterial signalling, including calcium spiking, NIN gene expression and infection thread formation, were significantly reduced. In contrast, no major negative effects on the formation of nodule primordia and cortical infection were detected. Cumulatively, our data show that the symRK-14 mutation uncouples the epidermal and cortical symbiotic program, while indicating that the SYMRK extracellular domain participates in transduction of non-equivalent signalling events. The GDPC sequence was found to be highly conserved in LRR-receptor kinases in legumes and non-legumes, including the evolutionarily distant bryophytes. Conservation of the GDPC sequence in nearly one-fourth of LRR-receptor-like kinases in the genome of Arabidopsis thaliana suggests, however, that this sequence might also play an important non-symbiotic function in this plant.

An overview on plant cuticle biomechanics

Plant biomechanics combines the principles of physics, chemistry and engineering to answer questions about plant growth, development and interaction with the environment. The epidermal-growth-control theory, postulated in 1867 and verified in 2007, states that epidermal cells determine the rate of organ elongation since they are under tension, while inner tissues are under compression. The lipid cuticle layer is deposited on the surface of outer epidermal cell walls and modifies the chemical and mechanical nature of these cell walls. Thus, the plant cuticle plays a key role in plant interaction with the environment and in controlling organ expansion. Rheological analyses indicate that the cuticle is a mostly viscoelastic and strain-hardening material that stiffens the comparatively more elastic epidermal cell walls. Cuticle stiffness can be attributed to polysaccharides and flavonoids present in the cuticle whereas a cutin matrix is mainly responsible for its extensibility. Environmental conditions such as temperature and relative humidity have a plasticizing effect on the mechanical properties of cuticle since they lower cuticle stiffness and strength. The external appearance of agricultural commodities, especially fruits, is of great economic value. Mechanical properties of the cuticle can have a positive or negative effect on disorders like fruit cracking, fungal pathogen penetration and pest infestation. Cuticle rheology has significant variability within a species and thus can be subjected to selection in order to breed cultivars resistant to pests, infestation and disorders.

Induced epidermal permeability modulates resistance and susceptibility of wheat seedlings to herbivory by Hessian fly larvae

Salivary secretions of neonate Hessian fly larvae initiate a two-way exchange of molecules with their wheat host. Changes in properties of the leaf surface allow larval effectors to enter the plant where they trigger plant processes leading to resistance and delivery of defence molecules, or susceptibility and delivery of nutrients. To increase understanding of the host plant's response, the timing and characteristics of the induced epidermal permeability were investigated. Resistant plant permeability was transient and limited in area, persisting just long enough to deliver defence molecules before gene expression and permeability reverted to pre-infestation levels. The abundance of transcripts for GDSL-motif lipase/hydrolase, thought to contribute to cuticle reorganization and increased permeability, followed the same temporal profile as permeability in resistant plants. In contrast, susceptible plants continued to increase in permeability over time until the entire crown of the plant became a nutrient sink. Permeability increased with higher infestation levels in susceptible but not in resistant plants. The ramifications of induced plant permeability on Hessian fly populations are discussed.

A radioactive assay allowing the quantitative measurement of cuticular permeability of intact Arabidopsis thaliana leaves

Cuticular penetration of five different ¹⁴C-labeled chemicals (benzoic acid, bitertanole, carbaryl, epoxiconazole and 4-nitrophenol) into Arabidopsis thaliana leaves was measured and permeances P (ms⁻¹) were calculated. Thus, cuticular barrier properties of A. thaliana leaves have been characterized quantitatively. Epoxiconazole permeance of A. thaliana was 2.79 × 10⁻⁸ ms⁻¹. When compared with cuticular permeances measured with intact stomatous and astomatous leaf sides of Prunus laurocerasus, frequently used in the past as a model species studying cuticular permeability, A. thaliana has a 48- to 66-fold higher permeance. When compared with epoxiconazole permeability of isolated cuticles of different species (Citrus aurantium, Hedera helix and P. laurocerasus) A. thaliana permeability is between 17- to 199-fold higher. Co-permeability experiments, simultaneously measuring ¹⁴C-epoxiconazole and ³H₂O permeability of isolated cuticles of three species (C. aurantium, H. helix and P. laurocerasus) showed that ³H₂O permeability was highly correlated with epoxiconazole permeability. The regression equation of this correlation can be used predicting cuticular transpiration of intact stomatous leaves of A. thaliana, where a direct measurement of cuticular permeation using ³H₂O is impossible. Water permeance estimated for A. thaliana was 4.55 × 10⁻⁸ m⁻¹, which is between 12- and 91-fold higher than water permeances measured with isolated cuticles of C. aurantium, H. helix and P. laurocerasus. This indicates that cuticular water permeability of the intact stomatous leaves of the annual species A. thaliana is fairly high and in the upper range compared with most P values of perennial species published in the past.

A radioactive assay allowing the quantitative measurement of cuticular permeability of intact Arabidopsis thaliana leaves

Cuticular penetration of five different ¹⁴C-labeled chemicals (benzoic acid, bitertanole, carbaryl, epoxiconazole and 4-nitrophenol) into Arabidopsis thaliana leaves was measured and permeances P (ms⁻¹) were calculated. Thus, cuticular barrier properties of A. thaliana leaves have been characterized quantitatively. Epoxiconazole permeance of A. thaliana was 2.79 × 10⁻⁸ ms⁻¹. When compared with cuticular permeances measured with intact stomatous and astomatous leaf sides of Prunus laurocerasus, frequently used in the past as a model species studying cuticular permeability, A. thaliana has a 48- to 66-fold higher permeance. When compared with epoxiconazole permeability of isolated cuticles of different species (Citrus aurantium, Hedera helix and P. laurocerasus) A. thaliana permeability is between 17- to 199-fold higher. Co-permeability experiments, simultaneously measuring ¹⁴C-epoxiconazole and ³H₂O permeability of isolated cuticles of three species (C. aurantium, H. helix and P. laurocerasus) showed that ³H₂O permeability was highly correlated with epoxiconazole permeability. The regression equation of this correlation can be used predicting cuticular transpiration of intact stomatous leaves of A. thaliana, where a direct measurement of cuticular permeation using ³H₂O is impossible. Water permeance estimated for A. thaliana was 4.55 × 10⁻⁸ m⁻¹, which is between 12- and 91-fold higher than water permeances measured with isolated cuticles of C. aurantium, H. helix and P. laurocerasus. This indicates that cuticular water permeability of the intact stomatous leaves of the annual species A. thaliana is fairly high and in the upper range compared with most P values of perennial species published in the past.

Overexpression of Arabidopsis ECERIFERUM1 promotes wax very-long-chain alkane biosynthesis and influences plant response to biotic and abiotic stresses

Land plant aerial organs are covered by a hydrophobic layer called the cuticle that serves as a waterproof barrier protecting plants against desiccation, ultraviolet radiation, and pathogens. Cuticle consists of a cutin matrix as well as cuticular waxes in which very-long-chain (VLC) alkanes are the major components, representing up to 70% of the total wax content in Arabidopsis (Arabidopsis thaliana) leaves. However, despite its major involvement in cuticle formation, the alkane-forming pathway is still largely unknown. To address this deficiency, we report here the characterization of the Arabidopsis ECERIFERUM1 (CER1) gene predicted to encode an enzyme involved in alkane biosynthesis. Analysis of CER1 expression showed that CER1 is specifically expressed in the epidermis of aerial organs and coexpressed with other genes of the alkane-forming pathway. Modification of CER1 expression in transgenic plants specifically affects VLC alkane biosynthesis: waxes of TDNA insertional mutant alleles are devoid of VLC alkanes and derivatives, whereas CER1 overexpression dramatically increases the production of the odd-carbon-numbered alkanes together with a substantial accumulation of iso-branched alkanes. We also showed that CER1 expression is induced by osmotic stresses and regulated by abscisic acid. Furthermore, CER1-overexpressing plants showed reduced cuticle permeability together with reduced soil water deficit susceptibility. However, CER1 overexpression increased susceptibility to bacterial and fungal pathogens. Taken together, these results demonstrate that CER1 controls alkane biosynthesis and is highly linked to responses to biotic and abiotic stresses.

Root hair-specific EXPANSIN A7 is required for root hair elongation in Arabidopsis

Expansins are non-hydrolytic cell wall-loosening proteins that are involved in the cell wall modifications that underlie many plant developmental processes. Root hair growth requires the accumulation of cell wall materials and dynamic cell wall modification at the tip region. Although several lines of indirect evidence support the idea that expansin-mediated wall modification occurs during root hair growth, the involvement of these proteins remains to be demonstrated in vivo. In this study, we used RNA interference (RNAi) to examine the biological function of Arabidopsis thaliana EXPANSIN A7 (AtEXPA7), which is expressed specifically in the root hair cell. The root hairspecific AtEXPA7 promoter was used to drive RNAi expression, which targeted two independent regions in the AtEXPA7 transcript. Quantitative reverse transcriptase-PCR analyses were used to examine AtEXPA7 transcript levels. In four independent RNAi transformant lines, RNAi expression reduced AtEXPA7 transcript levels by 25-58% compared to controls. Accordingly, the root hairs of RNAi transformant lines were 25-48% shorter than control plants and exhibited a broader range of lengths than the controls. Our results provide in vivo evidence that expansins are required for root hair tip growth.

The biophysical design of plant cuticles: an overview

The outer surfaces of epidermal cell walls are impregnated with an extracellular matrix called the cuticle. This composite matrix provides several functions at the interface level that enable plants to thrive in different habitats and withstand adverse environmental conditions. The lipid polymer cutin, which is the main constituent of the plant cuticle, has some unique biophysical properties resulting from its composition and structure. This review summarizes the progress made towards understanding the biophysical significance of this biopolymer with special focus on its structural, thermal, biomechanical, and hydric properties and relationships. The physiological relevance of such biophysical properties is discussed in light of existing knowledge on the plant cuticle.

A new, vapour-phase mechanism for stomatal responses to humidity and temperature

A new mechanism for stomatal responses to humidity and temperature is proposed. Unlike previously-proposed mechanisms, which rely on liquid water transport to create water potential gradients within the leaf, the new mechanism assumes that water transport to the guard cells is primarily through the vapour phase. Under steady-state conditions, guard cells are assumed to be in near-equilibrium with the water vapour in the air near the bottom of the stomatal pore. As the water potential of this air varies with changing air humidity and leaf temperature, the resultant changes in guard cell water potential produce stomatal movements. A simple, closed-form, mathematical model based on this idea is derived. The new model is parameterized for a previously published set of data and is shown to fit the data as well as or better than existing models. The model contains mathematical elements that are consistent with previously-proposed mechanistic models based on liquid flow as well as empirical models based on relative humidity. As such, it provides a mechanistic explanation for the realm of validity for each of these approaches.

The positive effect of skin transpiration in peach fruit growth

The effect of fruit transpiration on the mechanisms driving peach (Prunus persica (L.) Batsch) daily growth was investigated. In peach, fruit water losses increase during the season and might play a key role in determining fruit growth. Skin transpiration was reduced during the cell expansion stage by enclosing fruit in plastic bags fitted with holes. In the first year, diameter changes of bagged and control fruit were precisely monitored for 15 days, and percentage dry matter and soluble solids content were determined during the experiment and at harvest. In the second year, midday fruit water potential, daily patterns of fruit growth and of vascular and transpiration flows were monitored. Bagging reduced fruit daily growth on some days, and negatively affected both fruit dry matter percentage and soluble solids content. Fruit transpiration rate was reduced during the midday hours, thus increasing midday fruit water potential and lowering xylem inflows. In accordance with the Münch hypothesis on traslocation, these conditions likely decreased the necessary gradient needed for the transport of phloem sap to sink organs, as in the afternoon, bagged fruit showed lower phloem inflows. These data suggest that skin transpiration in peach has a positive effect on fruit growth, as it enhances fruit phloem import.

Peroxisome dynamics in Arabidopsis plants under oxidative stress induced by cadmium

Peroxisomes are organelles with an essentially oxidative metabolism that are involved in various metabolic pathways such as fatty acid beta-oxidation, photorespiration, and metabolism of reactive oxygen species (ROS) and reactive nitrogen species. These organelles are highly dynamic but there is little information about the regulation of, and the effects of environment on, peroxisome movement. In this work a stable Arabidopsis line expressing the GFP-SKL peptide targeted to peroxisomes was characterized. Peroxisome-associated fluorescence was observed in all tissues, including leaves (mesophyll and epidermal cells, trichomes, and stomata) and roots. The dynamics of peroxisomes in epidermal cells was examined by confocal laser microscope, and various types of movement were observed. The speed of movement differed depending on the plant age. Treatment of plants with CdCl(2) (100 microM) produced a significant increase in speed, which was dependent on endogenous ROS and Ca(2+), but was not related to actin cytoskeleton modifications. In light of the results obtained, it is proposed that the increase in peroxisomal motility observed in Arabidopsis plants could be a cellular mechanism of protection against the Cd-imposed oxidative stress. Other possible roles for the enhanced peroxisome movement in plant cell physiology are discussed.

Plants on early alert: glandular trichomes as sensors for insect herbivores

The ability of caterpillar or moth 'footsteps' to elicit defenses in the tomato (Solanum lycopersicum) plant was examined. Although touch responses frequently have been observed in plants, the role of herbivore 'touch' in eliciting antiherbivore defenses has not been adequately examined. A combination of methods, including in situ hybridization, reverse transcriptase-polymerase chain reaction, quantitative real-time polymerase chain reaction and gas chromatography-mass spectrometry, was used to determine the role of trichomes in mediating these touch responses. Mutants compromised in jasmonic acid and glandular trichomes were used to test whether both of these were required for these touch responses. We demonstrated that the rupture of foliar glandular trichomes by caterpillar or moth contact induced the expression of defense transcripts (e.g. proteinase inhibitor 2, or PIN2) regulated by jasmonic acid. Neither chewing nor the release of salivary components was required to initiate this induced response. Jasmonic acid and the genes encoding proteins involved in its biosynthesis were identified in the trichomes. Using mutants, we showed that both jasmonic acid and trichomes were required for the contact-induced expression of PIN2. In addition, hydrogen peroxide, formed on the leaf surface, was required for PIN2 expression. Because these defenses would be activated before egg hatch, this early detection system for herbivores may be of considerable ecological significance.

The Arabidopsis RESURRECTION1 gene regulates a novel antagonistic interaction in plant defense to biotrophs and necrotrophs

We report a role for the Arabidopsis (Arabidopsis thaliana) RESURRECTION1 (RST1) gene in plant defense. The rst1 mutant exhibits enhanced susceptibility to the biotrophic fungal pathogen Erysiphe cichoracearum but enhanced resistance to the necrotrophic fungal pathogens Botrytis cinerea and Alternaria brassicicola. RST1 encodes a novel protein that localizes to the plasma membrane and is predicted to contain 11 transmembrane domains. Disease responses in rst1 correlate with higher levels of jasmonic acid (JA) and increased basal and B. cinerea-induced expression of the plant defensin PDF1.2 gene but reduced E. cichoracearum-inducible salicylic acid levels and expression of pathogenesis-related genes PR1 and PR2. These results are consistent with rst1's varied resistance and susceptibility to pathogens of different life styles. Cuticular lipids, both cutin monomers and cuticular waxes, on rst1 leaves were significantly elevated, indicating a role for RST1 in the suppression of leaf cuticle lipid synthesis. The rst1 cuticle exhibits normal permeability, however, indicating that the disease responses of rst1 are not due to changes in this cuticle property. Double mutant analysis revealed that the coi1 mutation (causing defective JA signaling) is completely epistatic to rst1, whereas the ein2 mutation (causing defective ethylene signaling) is partially epistatic to rst1, for resistance to B. cinerea. The rst1 mutation thus defines a unique combination of disease responses to biotrophic and necrotrophic fungi in that it antagonizes salicylic acid-dependent defense and enhances JA-mediated defense through a mechanism that also controls cuticle synthesis.

A mutual support mechanism through intercellular movement of CAPRICE and GLABRA3 can pattern the Arabidopsis root epidermis

The patterning of the Arabidopsis root epidermis depends on a genetic regulatory network that operates both within and between cells. Genetic studies have identified a number of key components of this network, but a clear picture of the functional logic of the network is lacking. Here, we integrate existing genetic and biochemical data in a mathematical model that allows us to explore both the sufficiency of known network interactions and the extent to which additional assumptions about the model can account for wild-type and mutant data. Our model shows that an existing hypothesis concerning the autoregulation of WEREWOLF does not account fully for the expression patterns of components of the network. We confirm the lack of WEREWOLF autoregulation experimentally in transgenic plants. Rather, our modelling suggests that patterning depends on the movement of the CAPRICE and GLABRA3 transcriptional regulators between epidermal cells. Our combined modelling and experimental studies show that WEREWOLF autoregulation does not contribute to the initial patterning of epidermal cell fates in the Arabidopsis seedling root. In contrast to a patterning mechanism relying on local activation, we propose a mechanism based on lateral inhibition with feedback. The active intercellular movements of proteins that are central to our model underlie a mechanism for pattern formation in planar groups of cells that is centred on the mutual support of two cell fates rather than on local activation and lateral inhibition.

Variations in relative stomatal and biochemical limitations to photosynthesis in a young blackbutt (Eucalyptus pilularis) plantation subjected to different weed control regimes

Foliar gas exchange and carbon (delta(13)C) and oxygen (delta(18)O) isotope ratios were measured in a young blackbutt (Eucalyptus pilularis Sm.) plantation subjected to four weed control treatments defined by the width of the weed-free strip maintained for the first 12 months after planting. Treatments were: 2-m-wide weed-free strip (50% of plot area, 2.0MWC), 1.5-m-wide weed-free strip (37.5% of plot area, 1.5MWC), 1-m-wide weed-free strip (25% of plot area, 1.0MWC) and no weed control (NWC). Our objectives were to determine (1) if decreasing the width of the weed control strip (decreasing herbicide use) affected growth and leaf photosynthesis of the plantation, and (2) the effects of the weed control regimes on variations in relative stomatal and biochemical limitations to photosynthesis. Trees in the 1.0MWC treatment had lower foliar light-saturated photosynthetic rate (A(sat)) than trees in the 2.0MWC treatment. An increase in metabolic limitation was responsible for the decrease in A(sat) in the 1.0MWC trees, which was also partly confirmed by the isotopic data. Compared with trees in the 1.0MWC, 1.5MWC and 2.0MWC treatments, A(sat) of NWC trees was significantly lower, a difference that was attributable mainly to stomatal limitation and to a lesser extent to biochemical limitation. The results support the conclusion that different weed control regimes cause differences in relative stomatal and biochemical limitations to plantation photosynthesis. This report contributes to a growing body of literature on competition for soil resources between trees and weeds. Our results highlight the usefulness of the stable isotopic method in supporting analysis of the response of net photosynthesis to varying intercellular CO(2) concentration for determining the relative stomatal and non-stomatal limitations to photosynthesis under experimental conditions in the field.

Rapid and dynamic subcellular reorganization following mechanical stimulation of Arabidopsis epidermal cells mimics responses to fungal and oomycete attack

BACKGROUND

Plant cells respond to the presence of potential fungal or oomycete pathogens by mounting a basal defence response that involves aggregation of cytoplasm, reorganization of cytoskeletal, endomembrane and other cell components and development of cell wall appositions beneath the infection site. This response is induced by non-adapted, avirulent and virulent pathogens alike, and in the majority of cases achieves penetration resistance against the microorganism on the plant surface. To explore the nature of signals that trigger this subcellular response and to determine the timing of its induction, we have monitored the reorganization of GFP-tagged actin, microtubules, endoplasmic reticulum (ER) and peroxisomes in Arabidopsis plants - after touching the epidermal surface with a microneedle.

RESULTS

Within 3 to 5 minutes of touching the surface of Arabidopsis cotyledon epidermal cells with fine glass or tungsten needles, actin microfilaments, ER and peroxisomes began to accumulate beneath the point of contact with the needle. Formation of a dense patch of actin was followed by focusing of actin cables on the site of contact. Touching the cell surface induced localized depolymerization of microtubules to form a microtubule-depleted zone surrounding a dense patch of GFP-tubulin beneath the needle tip. The concentration of actin, GFP-tubulin, ER and peroxisomes remained focused on the contact site as the needle moved across the cell surface and quickly dispersed when the needle was removed.

CONCLUSION

Our results show that plant cells can detect the gentle pressure of a microneedle on the epidermal cell surface and respond by reorganizing subcellular components in a manner similar to that induced during attack by potential fungal or oomycete pathogens. The results of our study indicate that during plant-pathogen interactions, the basal defence response may be induced by the plant's perception of the physical force exerted by the pathogen as it attempts to invade the epidermal cell surface.

Diurnal changes in epidermal UV transmittance of plants in naturally high UV environments

Studies were conducted on three herbaceous plant species growing in naturally high solar UV environments in the subalpine of Mauna Kea, Hawaii, USA, to determine if diurnal changes in epidermal UV transmittance (T(UV)) occur in these species, and to test whether manipulation of the solar radiation regime could alter these diurnal patterns. Additional field studies were conducted at Logan, Utah, USA, to determine if solar UV was causing diurnal T(UV) changes and to evaluate the relationship between diurnal changes in T(UV) and UV-absorbing pigments. Under clear skies, T(UV), as measured with a UV-A-pulse amplitude modulation fluorometer for leaves of Verbascum thapsus and Oenothera stricta growing in native soils and Vicia faba growing in pots, was highest at predawn and sunset and lowest at midday. These patterns in T(UV) closely tracked diurnal changes in solar radiation and were the result of correlated changes in fluorescence induced by UV-A and blue radiation but not photochemical efficiency (F(v)/F(m)) or initial fluorescence yield (F(o)). The magnitude of the midday reduction in T(UV) was greater for young leaves than for older leaves of Verbascum. Imposition of artificial shade eliminated the diurnal changes in T(UV) in Verbascum, but reduction in solar UV had no effect on diurnal T(UV) changes in Vicia. In Vicia, the diurnal changes in T(UV) occurred without detectable changes in the concentration of whole-leaf UV-absorbing compounds. Results suggest that plants actively control diurnal changes in UV shielding, and these changes occur in response to signals other than solar UV; however, the underlying mechanisms responsible for rapid changes in T(UV) remain unclear.

Cell-to-cell pathway dominates xylem-epidermis hydraulic connection in Tradescantia fluminensis (Vell. Conc.) leaves

A steady supply of water is indispensable for leaves to fulfil their photosynthetic function. Understanding water movement in leaves, especially factors that regulate the movement of water flux from xylem to epidermis, requires that the nature of the transport pathway be elucidated. To determine the hydraulic linkage between xylem and epidermis, epidermal cell turgor pressure (P (t)) in leaves of Tradescantia fluminensis was monitored using a cell pressure probe in response to a 0.2 MPa step change in xylem pressure applied at the leaf petiole. Halftime of P (t) changes (T(x)(1/2)) were 10-30 times greater than that of water exchange across an individual cell membrane (T(m)(1/2)) suggesting that cell-to-cell water transport constitutes a significant part of the leaf hydraulic path from xylem to epidermis. Furthermore, perfusion of H(2)O(2) resulted in increases of both T(m)(1/2) and T(x)(1/2) by a factor of 2.5, indicating that aquaporins may play a role in the xylem to epidermis hydraulic link. The halftime for water exchange (T(m)(1/2)) did not differ significantly between cells located at the leaf base (2.5 s), middle (2.6 s) and tip (2.5 s), indicating that epidermal cell hydraulic properties are similar along the length of the leaf. Following the pressure application to the xylem (0.2 MPa), P (t) changed by 0.12, 0.06 and 0.04 MPa for epidermal cells at the base, middle and the tip of the leaf, respectively. This suggests that pressure dissipation between xylem and epidermis is significant, and that the pressure drop along the vein may be due to its structural similarities to a porous pipe, an idea which was further supported by measurements of xylem hydraulic resistance using a perfusion technique.