Effects of weak static magnetic fields on the development of seedlings of Arabidopsis thaliana

To study magnetoreception of Arabidopsis thaliana, we analysed several developmental responses including cryptochrome-independent seed germination and the phytochrome- and cryptochrome-dependent hypocotyl elongation and photo-accumulation of anthocyanins and chlorophylls in weak static magnetic fields ranging from near null to 122 μT. A field of 50 μT accelerated seed germination by about 20 h relative to samples maintained in a near-null field. The double mutant, cry1cry2, lacking cryptochromes 1 and 2 displayed the same magnetic field-induced germination acceleration under blue light as the wild-type strain. Magnetic field-induced germination acceleration was masked in the presence of exogenous sucrose. Stimulus-response curves for hypocotyl elongation in a range between near-null to 122 μT indicated maxima near 9 and 60 μT for the wild-type strain as well as mutant cry1cry2. The photo-accumulation of anthocyanins and chlorophylls could be effectively modulated by magnetic fields in the presence of low-irradiance red and blue light, respectively. The findings indicate that Arabidopsis thaliana possesses light-independent mechanisms of magnetic field reception, which remain presently unidentified. Our results are in better agreement with predictions of the level crossing mechanism (LCM) of magnetoreception rather than those of the cryptochrome-associated radical-pair mechanism (RPM).

coleoptile cells; a laboratory study

BACKGROUND

In this study, we investigated the effect of an electric field, with an intensity similar to that of the Earth's field, on plant cells growth. The molecular mechanism underlying this effect remains unclear.

RESULTS

It was found that the electric field, depending on the applied voltage, its duration and the polarization of the maize seedlings, stimulated or inhibited the growth of the seedling organs (root, mesocotyl and coleoptile). Moreover, it was also noticed that the gravitropic response of maize seedlings was inhibited at all voltages studied. Simultaneous measurements of growth and external medium pH show that auxin(IAA, indole-3-acetic acid)- and fusicoccin(FC)-induced elongation growth and proton extrusion of maize coleoptile segments were significantly inhibited at higher voltages. The ionic current flowing through the single coleoptile segment during voltage application was 1.7-fold lower in segments treated with cation channel blocker tetraethylammonium chloride (TEA-Cl) and 1.4-fold higher with IAA compared to the control. The electrophysiological experiments show that the electric field caused the depolarization of the membrane potential of parenchymal coleoptile cells, which was not reversible over 120 min.

CONCLUSION

It is suggested that a DC electric field inhibits the plasma membrane H⁺ pump activity and K⁺ uptake through voltage-dependent, inwardly rectifying ZMK1 channels (Zea mays K⁺ channel 1). The data presented here are discussed, taking into account the "acid growth hypothesis" of the auxin action and the mechanism of gravitropic response induction.

Single and combined effects of Cd and Pb on the growth, medium pH, membrane potential and metal contents in maize (Zea mays L.) coleoptile segments

The mechanisms of the toxic effects of Cd and Pb on plant cell growth are still poorly understood. In particular, little is known about their interactive effects, which usually occur in the environment. Moreover, the data that do exist in the literature are controversial. This study describes experiments that were performed with maize (Zea mays) coleoptile segments, which is a classical model system for studies of plant cell elongation growth. Cadmium and lead, which were added at 0.1 mM, reduced the endogenous and IAA-induced elongation growth of maize coleoptile cells. When both metals were added together or in sequence, their effect on IAA-induced growth was more toxic. The medium pH changes, which were measured simultaneously with growth, indicated that while Pb stopped IAA-induced proton extrusion, Cd only partially diminished it. Although Cd was generally more accumulated than Pb in the maize coleoptile segments, when IAA was added together with Pb, it significantly increased the accumulation of the metal. The short-term electrophysiological experiments showed that the addition of Cd caused the depolarisation of the membrane potential (E_m), whereas Pb caused membrane hyperpolarisation. In the long-term electrophysiological experiments, it was found that the Cd-induced E_m changes are complex. In conclusion, these results suggest that the effects of Cd and Pb as well as their combination on the elongation growth of maize coleoptile cells and the accumulation of the metals result, among others, from different ionic mechanisms by which each metal change the membrane potential of the cells.

A quantitative report on the impact of chloride on the kinetic coefficients of auxin-induced growth: a numerical contribution to the "acid growth hypothesis"

This work presents the application of several our own novel methods of analysing the kinetics of plant growth, which create, among others, a common platform for the comparison of experimental results. A relatively simple formula is used to parameterize the wide range of data that has been obtained for Zea mays L. in the literature, though it can also be used for different species. A biophysical/biochemical interpretation of the parameters was obtained from a theoretical model that is based on a modified Lockhart equation. The derived formula, which was extended for practical use in Zajdel et al. (Acta Physiol Plant 38:5, 2016), and which was implemented in the attached computer program (ibid.), allowed the data that was obtained from the growth-related problems to be parameterized in a simple way. As a working example that shows the robustness of our approach, we comment in detail on the qualitative assessments of the impact of chloride ions on auxin-induced growth. We note that calculated continuous curves (fits), which are rooted in the growth functional that was introduced by Pietruszka (J Theor Biol 315:119-127, 2012), were in a perfect agreement (R2 ~ 0.99998) with the raw experimental data that was published recently by Burdach et al. (Ann Bot 114:1023-1034, 2014). This fact justified the use of this strict technique, which allows for the determination of kinetic coefficients, to critically evaluate the results and suppositions (claims) therein. Moreover, we calculated the time-delay derivative of elongation growth-pH cross-correlations, and validated the "acid growth hypothesis" in figures by considering, amongst others, the magnitude of the H+-activity of elongation growth (per μm). An empirical constant (field strength), EH+ = Em/(log10 1/aH+ ∙ μm) = 0.157 ± 0.009 [V/mm] was obtained, where Em [mV] is the membrane potential in the perenchymal coleoptile cells of Zea mays L. When this relation is known, the membrane potential can not only be determined for intact growth, but also for different intervening substances exclusively from growth (or growth rate) and pH measurements, i.e. without performing electrophysiological measurements. However, the question of whether this constant is universal remains open.

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.