Biomedical subjects

Hubert H Felle

Publications and source records attributed to Hubert H Felle.

11 recordsLinked to original sources

PubMed2007

Systemic signalling in barley through action potentials.

Using apoplastic voltage- and ion selective microprobes, in barley leaves action potentials (APs) have been measured, which propagate acropetally as well as basipetally from leaf to leaf or from root to leaf following the application of mild salt stress (e.g. 30-50 mM KCl or NH(4)Cl) or amino acids (e.g. 1 mM glutamic acid or 5 mM GABA). Voltage changes were biphasic, followed an 'all-or-none' characteristic, and propagated at 20-30 cm min(-1) irrespective of the direction. With the salt-induced APs, a strong initial depolarization is the main AP-releasing factor that first causes Ca(2+) influx and then anion efflux. Ca(2+) influx coincides with an initial slower depolarization, the rapid anion efflux causes the typical voltage 'break-through'. Subsequently, K(+)-efflux starts after the depolarizing voltage has passed the K(+) equilibrium potential (inversion of the K(+) driving force). Glutamic acid and GABA induce APs not through membrane depolarization, but presumably by binding to a putative receptor or to ligand-gated Ca(2+)-conducting channels, respectively, followed by Ca(2+) induced activation of anion efflux. APs are accompanied by transient apoplastic pH increase (about 1 unit), and by cytoplasmic pH decrease (about 0.5 units). The apoplastic pH change is interpreted as an indicator of stress, the cytoplasmic pH change as a prerequisite for defence related gene activation. Since APs are released by agents added in a moderate concentration range, it is suggested that they may serve as first and fast systemic signals following attack from pathogens.

Action Potentials↗

PubMed2006

Apoplastic pH during low-oxygen stress in Barley.

BACKGROUND AND AIMS: Anoxia leads to an energy crisis, tolerance of which varies from plant to plant. Although the apoplast represents an important storage and reaction space, and engages in the mediation of membrane transport, this extracellular compartment has not yet been granted a role during oxygen shortage. Here, an attempt is made to highlight the importance of the apoplast during oxygen stress and to test whether information about it is transferred systemically in Hordeum vulgare. METHODS: Non-invasive ion-selective microprobes were used which, after being inserted through open stomata, directly contact the apoplastic fluid and continuously measure the apoplastic pH and changes to it. KEY RESULTS: (a) Barley leaves respond to oxygen stress with apoplastic alkalinization and membrane depolarization. These responses are persistent under anoxia (N2; O2 < 3%) but transient under hypoxia. (b) Being applied to the root, the information 'anoxia' is signalled to the leaf as an increase in pH, whereas 'hypoxia' is not: flooding of the roots within the first 2 h has no effect on the leaf apoplastic pH, whereas anoxia (N2) or chemical anoxia (NaCN/salicylic hydroxamic acid) rapidly increase the leaf apoplastic pH. (c) Under anoxia, the proton motive force suffers a decrease by over 70 %, which impairs H(+) -driven transport. CONCLUSIONS: Although anoxia-induced apoplastic alkalinization is a general response to stress, its impact on the proton motive force (reduction) and thus on transport mediation of energy-rich compounds is evident. It is concluded that anoxia tolerance depends on how the plant is able to hold the proton motive force and H(+) turnover at a level that guarantees sufficient energy is harvested to overcome the crisis.

Hordeum↗

PubMed2006

A nucleoporin is required for induction of Ca2+ spiking in legume nodule development and essential for rhizobial and fungal symbiosis.

Nuclear-cytoplasmic partitioning and traffic between cytoplasmic and nuclear compartments are fundamental processes in eukaryotic cells. Nuclear pore complexes mediate transport of proteins, RNAs and ribonucleoprotein particles in and out of the nucleus. Here we present positional cloning of a plant nucleoporin gene, Nup133, essential for a symbiotic signal transduction pathway shared by Rhizobium bacteria and mycorrhizal fungi. Mutation of Nup133 results in a temperature sensitive nodulation deficient phenotype and absence of mycorrhizal colonization. Root nodules developing with reduced frequency at permissive temperatures are ineffective and electron microscopy show that Rhizobium bacteria are not released from infection threads. Measurement of ion fluxes using a calcium-sensitive dye show that Nup133 is required for the Ca2+ spiking normally detectable within minutes after application of purified rhizobial Nod-factor signal molecules to root hairs. Localization of NUP133 in the nuclear envelope of root cells and root hair cells shown with enhanced yellow fluorescent protein fusion proteins suggests a novel role for NUP133 nucleoporins in a rapid nuclear-cytoplasmic communication after host-plant recognition of symbiotic microbes. Our results identify a component of an intriguing signal process requiring interaction at the cell plasma membrane and at intracellular nuclear and plastid organelle-membranes to induce a second messenger.

Alleles↗

PubMed2005

pH regulation in anoxic plants.

BACKGROUND: pH regulation is the result of a complex interaction of ion transport, H+ buffering, H+-consuming and H+-producing reactions. Cells under anoxia experience an energy crisis; an early response thereof (in most tissues) is a rapid cytoplasmic acidification of roughly half a pH unit. Depending on the degree of anoxia tolerance, this pH remains relatively stable for some time, but then drops further due to an energy shortage, which, in concert with a general breakdown of transmembrane gradients, finally leads to cell death unless the plant finds access to an energy source. SCOPE: In this review the much-debated origin of the initial pH change and its regulation under anoxia is discussed, as well as the problem of how tissues deal with the energy crisis and to what extent pH regulation and membrane transport from and into the vacuole and the apoplast is a part thereof. CONCLUSIONS: It is postulated that, because a foremost goal of cells under anoxia must be energy production (having an anaerobic machinery that produces insufficient amounts of ATP), a new pH is set to ensure a proper functioning of the involved enzymes. Thus, the anoxic pH is not experienced as an error signal and is therefore not reversed to the aerobic level. Although acclimated and anoxia-tolerant tissues may display higher cytoplasmic pH than non-acclimated or anoxia-intolerant tissues, evidence for an impeded pH-regulation is missing even in the anoxia-intolerant tissues. For sufficient energy production, residual H+ pumping is vital to cope with anoxia by importing energy-rich compounds; however it is not vital for pH-regulation. Whereas the initial acidification is not due to energy shortage, subsequent uncontrolled acidosis occurring in concert with a general gradient breakdown damages the cell but may not be the primary event.

Acclimatization↗

PubMed2005

Cation fluxes cause plasma membrane depolarization involved in beta-glucan elicitor-signaling in soybean roots.

Inducible and specific ion fluxes on plasma membranes represent very early events during elicitation of plant cells. The hierarchy of such ion fluxes involved is still unknown. The effect of Phytophthora sojae-derived beta-glucan elicitors on the plasma membrane potential as well as on surface K+, Ca2+, and H+ fluxes has been investigated on soybean roots using ion-selective microelectrodes. Beta-Glucans with different degrees of polymerization transiently depolarized the plasma membrane. The elicitor concentration necessary for half-maximal depolarization closely resembled the corresponding binding affinities of soybean root membranes toward the respective beta-glucans. Upon repeated elicitor treatment, the root cells responded partially refractory, suggesting a complex responsiveness of the system. Within the root hair space, characteristic decreasing K(+)- and Ca(2+)-free concentrations were induced by the elicitors, probably causing depolarization through the influx of positive charges. Whereas K+ fluxes were inverted after passing the K+ equilibrium (Nernst-) potential, Ca2+ influx continued. No anion fluxes sufficient to account for charge compensation were observed under the same experimental conditions. K+ and Ca2+ fluxes as well as depolarization were inhibited by 100 microM or less of the Ca2+ antagonist La3+. Contrasting other systems, in soybean the main cause for elicitor-induced plasma membrane depolarization is the activation of cation instead of anion fluxes.

Cations↗

PubMed2004

Apoplastic pH signaling in barley leaves attacked by the powdery mildew fungus Blumeria graminis f. sp. hordei.

To investigate apoplastic responses of barley (Hordeum vulgare L.) to the barley powdery mildew fungus Blumeria graminis f. sp. hordei, noninvasive microprobe techniques were employed. H(+)- and Ca(2+)-selective microprobes were inserted into open stomata of barley leaves inoculated with Blumeria graminis f. sp. hordei race A6 conidia. Resistance gene-mediated responses of barley genotype Ingrid (susceptible parent line) and the near-isogenic resistant Ingrid backcross lines (I-mlo5, I-Mla12, and I-Mlg) were continuously monitored from 20 min to 4 days after inoculation. The main events were categorized as short-term responses around 2 h after inoculation (hai), intermediate responses around 8 and 12 hai, and long-term responses starting between 21 and 24 hai. Short-term responses were rapid transient decreases of apoplastic H(+)- and Ca2+ activities that lasted minutes only. Kinetics were similar for all genotypes tested, and thus, these short-term responses were attributed as nonspecific first encounters of fungal surface material with the host plasma membrane. This is supported by the observation that a microinjected chitin oligomer (GlcNAc)8 yielded similar apoplastic alkalinization. Intermediate responses are trains of H+ (increase) spikes that, being different in susceptible Ingrid and penetration-resistant I-mlo5 (or I-Mlg), were interpreted as accompanying specific events of papillae formation. Long-term events were massive slow and long-lasting alkalinizations up to two pH units above control. Since these latter changes were only observed with near-isogenic hypersensitive reaction (HR)-mounting genotypes I-Mla12 and I-Mlg but not with I-mlo5 or, to a smaller extent, with susceptible Ingrid, both lacking significant rates of HR, they were rated as cell death specific. It is concluded that apoplastic pH changes are important indicators of host-pathogen interactions that correlate with both the different stages of fungal development and the different types of host defense response.

Ascomycota↗

PubMed2003

Plant recognition of symbiotic bacteria requires two LysM receptor-like kinases.

Although most higher plants establish a symbiosis with arbuscular mycorrhizal fungi, symbiotic nitrogen fixation with rhizobia is a salient feature of legumes. Despite this host range difference, mycorrhizal and rhizobial invasion shares a common plant-specified genetic programme controlling the early host interaction. One feature distinguishing legumes is their ability to perceive rhizobial-specific signal molecules. We describe here two LysM-type serine/threonine receptor kinase genes, NFR1 and NFR5, enabling the model legume Lotus japonicus to recognize its bacterial microsymbiont Mesorhizobium loti. The extracellular domains of the two transmembrane kinases resemble LysM domains of peptidoglycan- and chitin-binding proteins, suggesting that they may be involved directly in perception of the rhizobial lipochitin-oligosaccharide signal. We show that NFR1 and NFR5 are required for the earliest physiological and cellular responses to this lipochitin-oligosaccharide signal, and demonstrate their role in the mechanism establishing susceptibility of the legume root for bacterial infection.

Amino Acid Sequence↗

PubMed2002

The mode of action of cell wall-degrading enzymes and their interference with Nod factor signalling in Medicago sativa root hairs.

Medicago sativa L. (alfalfa) root hairs respond to Nod factors [NodRm-IV(C16:2,S)] in a host-specific manner with depolarization and rapid ion fluxes. Protoplasts prepared from these cells using the cell wall-digesting enzymes pectolyase and cellulase do not, or to a rather small extent, respond to Nod factors. In an effort to understand this activity loss we analyzed the mode of action of both enzymes with respect to their effects on the root hairs as well as their interference with the Nod factor response. (i) In the presence of the enzymes, Nod factor at saturating concentrations neither depolarized the plasma membrane of root hairs nor caused ion fluxes. Even after removal of the enzymes, Nod factor responses were strongly refractory. (ii) After a lag-phase of 12-18 s, pectolyase depolarized the plasma membrane, alkalized the external space, acidified the cytosol and increased the cytosolic Ca(2+) activity. (iii) Cellulase, without a lag-phase, depolarized the plasma membrane, acidified the cytosol, but only marginally increased the cytosolic Ca(2+) activity. Unlike pectolyase, the cellulase response was only weakly refractory to a second addition. (iv) Neither enzyme increased the membrane conductance, but pectolyase inhibited the H(+)-pump. (v) Pectolyase shows all the signs of an elicitor, while cellulase yields a mixed response. (vi) Denatured enzymes yielded strong effects similar to those of untreated enzymes. We conclude that the effects shown do not originate from enzymatic activity, but from interactions of the proteins with cell wall or plasma membrane constituents. It is further concluded that these enzymes (pectolyase more so than cellulase) trigger defense-related signal pathways, which makes protoplasts prepared with such enzymes unsuitable for studies of symbiotic or defense-related signalling.

Calcium↗

PubMed2002

CO2 provides an intermediate link in the red light response of guard cells.

Guard cells in intact leafs display light-induced membrane potential changes, which alter the direction of K+-transport across the plasma membrane (Roelfsema et al., 2001). A beam of blue light, but not red light, directed at the impaled guard cell triggers this response, while both light qualities induce opening of stomata. To gain insight into this apparent contradiction, we explored the possible interaction between red light and CO2. Guard cells in the intact plant were impaled with double-barrelled electrodes and illuminated with red light. Cells that were hyperpolarized in CO2-free air, depolarized after a switch to air with 700 micro l l(-1) CO2, in a reversible manner. As a result, K+-fluxes across the plasma membrane changed direction, to favour K+ extrusion and stomatal closure in the presence of CO2. Concurrent with the depolarization, an inward current across the plasma membrane appeared, most likely due to activation of anion channels. Guard cell responses to CO2 could be recorded in darkness as well as in red light. However, in darkness some cells spontaneously depolarized, these cells hyperpolarized again in red light. Here, red light was projected on a large area of the leaf and decreased the intracellular CO2 concentration by about 250 micro l l(-1), as measured with a miniature CO2 sensor placed in the substomatal cavity. We conclude, that in intact leaves the red light response of guard cells is mediated through a decrease of the intercellular CO2 concentration.

Algorithms↗

PubMed2002

CO(2)-triggered chloride release from guard cells in intact fava bean leaves. Kinetics of the onset of stomatal closure.

The influence of CO(2) on Cl(-) release from guard cells was investigated within the intact leaf by monitoring the Cl(-) activity in the apoplastic fluid of guard cells with a Cl(-)-sensitive microelectrode. In illuminated leaves adapted to a CO(2) concentration within the cuvette of 350 microL L(-1), an increase of 250 microL L(-1) CO(2) triggered a transient rise in the apoplastic Cl(-) activity from 3 to 14 mM within 10 min. This Cl(-) response was similar to the Cl(-) efflux evoked by turning off the light, when the substomatal CO(2) was kept constant (CO(2) clamp). Without CO(2) clamp, substomatal CO(2) increased by 120 microL L(-1) upon "light off." The response to an increase in CO(2) within the cuvette from 250 to 500 microL L(-1) in dark-adapted leaves was equivalent to the response to an increase from 350 to 600 microL L(-1) in the light. No Cl(-) efflux was triggered by 2-min CO(2) pulses (150-800 microL L(-1)). After a switch from 350 microL L(-1) to CO(2)-free cuvette air, the guard cells were less sensitive to a rise in CO(2) and to light off, but the sensitivity to both stimuli partially recovered. Changes in CO(2) also caused changes of the guard cell apoplastic voltage, which were generally faster than the observed Cl(-) responses, and which also promptly occurred when CO(2) did not initiate Cl(-) efflux. The comparatively slow activation of Cl(-) efflux by CO(2) indicates that an intermediate effector derived from CO(2) has to accumulate to fully activate plasma membrane anion channels of guard cells.

Adaptation, Physiological↗

PubMed2002

The apoplastic pH of the substomatal cavity of Vicia faba leaves and its regulation responding to different stress factors.

The apoplastic pH of the substomatal cavity is an essential determinant of stomatal movement. In detached leaves of Vicia faba substomatal apoplastic pH and its dependence on external (stress) factors was investigated using a non-invasive approach: pH-microsensors were inserted into open stomata, and upon contact with the apoplastic fluid, pH was measured continuously, as apoplastic pH was challenged by changed conditions of light, atmosphere (NH(3), CO(2)), and xylem sap (abscisic acid, cyanide, fusicoccin, pH, inorganic salts). Apoplastic pH proved extremely sensitive to infiltration and local flooding, which rapidly increased the apoplastic pH by more than 1.5 pH units. Recovery from infiltration took several hours, during which light effects on the apoplastic pH were strongly impeded. This indicates that pH tests carried out under such conditions may not be representative of the undisturbed leaf. NH(3), flushed across the stomata, yielded a rapid apoplastic alkalinization from which an apoplastic buffer capacity of 2-3 mM per pH unit was calculated. Fusicoccin, fed into the xylem sap acidified the apoplast, whereas cyanide alkalized it, thus underscoring the importance of the plasma membrane H(+) pump for apoplastic pH regulation. To address the question to what extent pH was a drought signal, the effect of iso-osmotic pH changes, fed into the xylem through the petiole were tested. It is demonstrated that the apoplastic response remained below 0.1 pH per pH unit imposed, regardless of the buffer capacity. An increase in the osmolarity of the bath solution (harbouring the cut petiole) using KCl, NaCl, CaCl(2) or sorbitol alkalized the substomatal apoplast. It is suggested that pH may only act as drought signal when accompanied by elevated osmolarity.

Abscisic Acid↗