Gravity susception by buoyancy: floating lipid globules in sporangiophores of Phycomyces

Euglena, a Gravitactic Flagellate of Multiple Usages

Human exploration of space and other celestial bodies bears a multitude of challenges. The Earth-bound supply of material and food is restricted, and in situ resource utilisation (ISRU) is a prerequisite. Excellent candidates for delivering several services are unicellular algae, such as the space-approved flagellate Euglena gracilis. This review summarizes the main characteristics of this unicellular organism. Euglena has been exposed on various platforms that alter the impact of gravity to analyse its corresponding gravity-dependent physiological and molecular genetic responses. The sensory transduction chain of gravitaxis in E. gracilis has been identified. The molecular gravi-(mechano-)receptors are mechanosensory calcium channels (TRP channels). The inward gated calcium binds specifically to one of several calmodulins (CaM.2), which, in turn, activates an adenylyl cyclase. This enzyme uses ATP to produce cAMP, which induces protein kinase A, followed by the phosphorylation of a motor protein in the flagellum, initiating a course correction, and, finally, resulting in gravitaxis. During long space missions, a considerable amount of food, oxygen, and water has to be carried, and the exhaled carbon dioxide has to be removed. In this context, E. gracilis is an excellent candidate for biological life support systems, since it produces oxygen by photosynthesis, takes up carbon dioxide, and is even edible. Various species and mutants of Euglena are utilized as a producer of commercial food items, as well as a source of medicines, as it produces a number of vitamins, contains numerous trace elements, and synthesizes dietary proteins, lipids, and the reserve molecule paramylon. Euglena has anti-inflammatory, -oxidant, and -obesity properties.

Actin cytoskeleton and organelle movement in the sporangiophore of the zygomycete Phycomyces blakesleeanus

Growth, photo- and gravitropism of sporangiophores of the zygomycete Phycomyces blakesleeanus occur within the apical growing zone, a cylindrical structure (diameter about 100 μm) that reaches about 1.5-2.5 mm below the tip and has growth rates up to 50 μm·min(-1) . To better understand morphogenesis and growth of the giant aerial hypha, we investigated with confocal microscopy and inhibitors the actin cytoskeleton and by in-vivo particle tracking the associated organelle movement. We found stage-1 sporangiophores (without sporangium) possess an actin cytoskeleton with polar zonation. (i) In the apex, abundant microfilaments without preferential orientation entangled numerous nuclei as well as a conspicious complex of some 200 lipid globules. Microfilament patches (≈ 1.6-μm diameter) are clustered in the tip and were found in the apical cortex, whereas short, curved microfilament bundles (≈ 2.3-μm long) prevailed in the subapex. (ii) In a transition zone downwards to the shaft, the microfilaments rearranged into a dense mat of longitudinal microfilaments that was parallel close to the periphery but more random towards the cell centre. Numerous microfilament patches were found near the cortex (≈ 10/100 μm(2) ); their number decreased rapidly in the subcortex. In contrast, the short, curved microfilament bundles were found only in the subcortex. (iii) The basal shaft segment of the sporangiophore (with central vacuole) exhibited bidirectional particle movement over long distances (velocity ≈ 2 μm·s(-1) ) along massive longitudinal, subcortical microfilament cables. The zonation of the cytoskeleton density correlated well with the local growth rates at the tip of the sporangiophore, and appears thus as a structural prerequisite for growth and bending.

The sporangiophore of Phycomyces blakesleeanus: a tool to investigate fungal gravireception and graviresponses

The giant sporangiophore of the single-celled fungus, Phycomyces blakesleeanus, utilises light, gravity and gases (water and ethylene) as environmental cues for spatial orientation. Even though gravitropism is ubiquitous in fungi (Naturwissenschaftliche Rundschau, 1996, 49, 174), the underlying mechanisms of gravireception are far less understood than those operating in plants. The amenability of Phycomyces to classical genetics and the availability of its genome sequence makes it essential to fill this knowledge gap and serve as a paradigm for fungal gravireception. The physiological phenomena describing the gravitropism of plants, foremost adherence to the so-called sine law, hold even for Phycomyces. Additional phenomena pertaining to gravireception, specifically adherence to the novel exponential law and non-adherence to the classical resultant law of gravitropism, were for the first time investigated for Phycomyces. Sporangiophores possess a novel type of gravisusceptor, i.e. lipid globules that act by buoyancy rather than sedimentation and that are associated with a network of actin cables (Plant Biology, 2013). Gravitropic bending is associated with ion currents generated by directed Ca(2+) and H(+) transport in the growing zone (Annals of the New York Academy of Sciences, 2005, 1048, 487; Planta, 2012, 236, 1817). A set of behavioural mutants with specific defects in gravi- and/or photoreception allowed dissection of the respective transduction chains. The complex phenotypes of these mutants led to abandoning the concept of simple linear transduction chains in favour of interacting networks with molecular modules of physically interacting proteins.

Gravitropism in Phycomyces: violation of the so-called resultant law - evidence for two response components

We investigated gravitropic bending of sporangiophores of the zygomycete fungus Phycomyces blakesleeanus in response to centrifugal accelerations to test the so-called resultant law of gravitropism ('Resultantengesetz'; Jahrbuch der wissenschaftlichen Botanik, 71, 325, 1929; Der Geotropismus der Pflanzen, Gustav Fischer, Jena, Germany, 1932), which predicts that gravitropic organs orient in a centrifuge rotor parallel to the stimulus vector resulting from the centrifugal acceleration and gravity. Sporangiophores of wild-type strain C171 carAcarR and of several gravitropism mutants were subjected for 7 h to centrifugal accelerations in a dynamic range between 0.01 and 3 × g. The stimulus-response curves that were obtained for C171 carA carR, C2 carA geo and C148 carA geo madC were complex and displayed two response components: a low-acceleration component between 0.01 and 0.5 × g and a high-acceleration component above 0.5 × g. The low acceleration component is characterised by bending angles exceeding those predicted by the resultant law and kinetics faster than that of the second component; in contrast, the high-acceleration component is characterised by bending slightly below the predicted level and kinetics slower than that of the first component. Sporangiophores of the wild-type C171 centrifuged horizontally displayed the opposite behaviour, i.e. low accelerations diminished and high accelerations slightly enhanced bending. Further proof for the existence of the two response components was provided by the phenotype of gravitropism mutants that either lacked the first response component or which caused its overexpression. The tropism mutant C148 with defective madC gene, which codes for a RasGap protein (Fungal Genetics Reports, 60 (Suppl.), Abstract # 211, 2013), displayed hypergravitropism and concomitant deviations from the resultant law that were twice as high as in the wild-type C171. Gravitropism mutants with defects in the genes madF, madG and madJ lacked the low-response component below 0.5 × g. Our data are at variance with the so-called resultant law and imply that gravitropic orientation cannot depend exclusively on the classical sine stimulus (i.e. acting perpendicularly on the side walls); it rather must also be controlled by the cosine stimulus acting parallel to the longitudinal axis of the gravisensing organ. Our studies indicate that the threshold for the cosine response is the same as that of the sine response, and thus close to 0.01 × g.

Surface tip-to-base Ca2+ and H+ ionic fluxes are involved in apical growth and graviperception of the Phycomyces stage I sporangiophore

Net fluxes of Ca(2+) and H(+) ions were measured non-invasively close to the surface of Phycomyces blakesleeanus sporangiophores stage I using ion-selective vibrating microelectrodes. The measurements were performed on a wild type (Wt) and a gravitropic mutant A909 kept in either vertical or tilted orientation. Microelectrodes were positioned 4 μm from the surface of sporangiophore, and ion fluxes were recorded from the apical (0-20 μm) and subapical (50-100 μm) regions. The magnitude and direction of ionic fluxes measured were dependent on the distance from the tip along the growing zone of sporangiophore. Vertically oriented sporangiophores displayed characteristic tip-to-base ion fluxes patterns. Ca(2+) and H(+) fluxes recorded from apical region of Wt sporangiophores were inward-directed, while ion fluxes from subapical locations occurred in both directions. In contrast to Wt, mutant A909 showed opposite (outward) direction of Ca(2+) fluxes and reduced H(+) influxes in the apical region. Following gravistimulation, the magnitude and direction of ionic fluxes were altered. Wt sporangiophore exhibited oppositely directed fluxes on the lower (influx) and the upper (efflux) sides of the cell, while mutant A909 did not show such patterns. A variable elongation growth in vertical position and reduced growth rate upon gravistimulation were observed in both strains. The data show that tip-growing sporangiophores exhibit a tip-to-base ion flux pattern which changes characteristically upon gravistimulation in Wt in contrast to the mutant A909 with a strongly reduced gravitropic response.

Gravisusception by buoyancy: a mechanism ubiquitous among fungi?

Gravitropism is ubiquitous among the fungal taxa; however, the mechanism(s) of gravisusception have overall remained obscure so far. In the vegetative sporangiophore of the zygomycete Phycomyces blakesleeanus some 200 large lipid globules form a conspicuous spherical complex which is positioned in a dense mesh of filamentous actin about 100 microm below the growing tip of the apex. Experimental suppression of that complex by transient growth at low temperature greatly diminishes the gravitropic response of the sporangiophore. With respect to size and abundance of the globules, the complex of lipid globules meets basic physical criteria for a possible function of gravisusception. Accumulations of similar lipid globules of critical size are documented in the apex of gravitropically growing hyphae of the endomycorrhizal fungus Gigaspora margarita (Glomeromycota) and have been described in the hyphal apices of members of various fungal phyla. We suppose that--in contrast to plants which use starch as a carbon storage and amyloplasts as statoliths--the fungi utilise the buoyancy of carbon-storing oil droplets for gravisusception.

Halotolerance is enhanced in carrot callus by sensing hypergravity: influence of calcium modulators and cytochalasin D

Carrot callus was centrifuged at 10 g and compared to callus growing at 1 g on agar in the presence of increasing sodium chloride concentrations. Growth after 14 days was enhanced in the centrifuged samples versus samples kept at 1 g. This effect was not found when the samples were grown on potassium chloride. At 50 mM NaCl, the calcium ionophore ionomycin was applied to centrifuged and noncentrifuged callus samples. In both experiments, the growth of callus increased with increasing ionomycin concentrations but under 10 g this increase was more enhanced. As inhibitors of calcium influx, lanthanum and gadolinium chloride were chosen in the presence of 50 mM NaCl. Both inhibitors inhibited growth at 1 g at low concentrations of around 2 microM, whereas the centrifuged samples were not or much less so inhibited. We tested an involvement of actin by application of cytochalasin D to callus grown in the presence of 50 mM NaCl. In both types of samples, growth at 1 g and growth at 10 g, cytochalasin D enhanced growth but the effect was clearly stronger at 10 g than at 1 g. As increased halotolerance was only observed in the presence of increased sodium ions, not potassium ions, and as halotolerance is known to be induced by an influx of calcium, the data suggest that a calcium influx induced by hypergravity and possibly modulated by actin caused the observed increase in halotolerance at 10 g.

Ca2+and H+Ion Fluxes near the Surface of Gravitropically StimulatedPhycomycesSporangiophore

The single-celled fungus Phycomyces blakesleeanus forms vertically oriented sporangiophores from hyphae, which display a negative gravitropic response. Longitudinal growth and gravitropic bending of these sporangiophores were measured with noninvasive H(+) and Ca(2+) ion-selective microelectrode measurements. The directions of H(+) and Ca(2+) fluxes, recorded at different locations of sporangiophores, were opposite when the sporangiophores were kept in vertical position. Ca(2+) fluxes were in most experiments positive (efflux), while H(+) fluxes were negative (influx). The direction of ion fluxes depended on developmental stage of sporangiophores and changed with gravistimulation.

Gravitropism in Phycomyces: threshold determination on a clinostat centrifuge

The absolute sensitivity of sporangiophores of Phycomyces blakesleeanus to centrifugal acceleration was determined on a clinostat centrifuge. The centrifuge provides centrifugal accelerations ranging from 10(-4) to 6 x g. The rotor of the centrifuge, which accommodates 96 culture vials with single sporangiophores, is clinostatted, that is, turning "head over", at slow speed (1 rev min(-1)) while it is running. The negative gravitropism of sporangiophores is characterized by two components: a polar angle, which is measured in the plane of bending, and an aiming-error angle, which indicates the deviation of the plane of bending from the vector of the centrifugal acceleration. Dose-response curves were generated for both angles with centrifugations lasting 3, 5, and 8 h. The threshold for the polar angle depends on the presence of statoliths, so-called octahedral protein crystals in the vacuoles. The albino strain C171 carAcarR (with crystals) has a threshold near 10(-2) x g while the albino strain C2 carAgeo-3 (without crystals) has a threshold of about 2 x 10(-1) x g. The threshold for the aiming error angle is ill defined and is between 10(-2) and 10(-1) x g. The threshold for the polar angle of the wild type NRRL 1555 (with crystals) is near 8 x 10(-2) x g.

Optospectroscopic detection of primary reactions associated with the graviperception of Phycomyces. Effects of micro- and hypergravity

The graviperception of sporangiophores of the fungus Phycomyces blakesleeanus involves gravity-induced absorbance changes (GIACs) that represent primary responses of gravitropism (Schmidt and Galland, 2000). GIACs (DeltaA(460-665)) of sporangiophores were measured in vivo with a micro-dual wavelength spectrometer at 460 and 665 nm. Sporangiophores that were placed horizontally displayed an instant increase of the GIACs while the return to the vertical position elicited an instant decrease. The GIACs are specific for graviperception, because they were absent in a gravitropism mutant with a defective madJ gene. During parabola flights hypergravity (1.8 g) elicited a decrease of the GIACs, while microgravity (0 +/- 3 x 10 (-2) g) elicited an instant increase. Hypergravity that was generated in a centrifuge (1.5-6.5 g) elicited also a decrease of the GIACs that saturated at about 5 g. The GIACs have a latency of about 20 ms or shorter and are thus the fastest graviresponses ever measured for fungi, protists, and plants. The threshold for eliciting the GIACs is near 3 x 10 (-2) g, which coincides numerically with the threshold for gravitropic bending. In contrast to gravitropic bending, which requires long-term stimulation, GIACs can be elicited by stimuli as short as 20 to 100 ms, leading to an extremely low threshold dose (acceleration x time) of about 3 x 10 (-3) g s, a value, which is four orders of magnitude below the ones described for other organisms and which makes the GIACs of Phycomyces blakesleeanus the most sensitive gravi-response in literature.