The testa of higher plant seeds protects the embryo against adverse environmental conditions. Its role is assumed mainly by controlling germination through dormancy imposition and by limiting the detrimental activity of physical and biological agents during seed storage. To analyze the function of the testa in the model plant Arabidopsis, we compared mutants affected in testa pigmentation and/or structure for dormancy, germination, and storability. The seeds of most mutants exhibited reduced dormancy. Moreover, unlike wild-type testas, mutant testas were permeable to tetrazolium salts. These altered dormancy and tetrazolium uptake properties were related to defects in the pigmentation of the endothelium and its neighboring crushed parenchymatic layers, as determined by vanillin staining and microscopic observations. Structural aberrations such as missing layers or a modified epidermal layer in specific mutants also affected dormancy levels and permeability to tetrazolium. Both structural and pigmentation mutants deteriorated faster than the wild types during natural aging at room temperature, with structural mutants being the most strongly affected.

Influence of the Testa on Seed Dormancy, Germination, and Longevity in Arabidopsis

Plant protoplasts show physiological perceptions and responses to hormones, metabolites, environmental cues, and pathogen-derived elicitors, similar to cell-autonomous responses in intact tissues and plants. The development of defined protoplast transient expression systems for high-throughput screening and systematic characterization of gene functions has greatly contributed to elucidating plant signal transduction pathways, in combination with genetic, genomic, and transgenic approaches.

/pdf/signal-transduction-in-maize-and-arabidopsis-mesophyll-4b65wac81t.pdf

Signal Transduction in Maize and Arabidopsis Mesophyll Protoplasts

THE molecular mechanisms by which Ca2+ controls a variety of ion transport-associated cellular functions in higher plant cells1, including movements of stomatal pores, remain unknown. Stomatal pores regulate the gas exchange in leaves. Openings of stomatal pores are mediated by an increase in the intracellular potassium and anion content of guard cells2,3. Voltage-dependent K+ channels4–7 and hyperpolarizing H+ pumps8,9 have been identified as mechanisms controlling stomatal movements. But depolarizing mechanisms required for K+ efflux during stomatal closing remain unknown. Indirect evidence suggests that Ca2+ triggers stomatal closing and inhibits stomatal opening10–12. Using patch-clamp techniques13, we show here that elevated (micromolar) concentrations of cytosolic Ca2+ in guard cells block inward rectifying K+ channels. Furthermore, elevation of cytosolic Ca2+ leads to the activation of a voltage-dependent depolarizing conductance with a permeability to anions. The Ca2+-induced modulation of ion channels reported here could provide a molecular basis for Ca2+-dependent regulation of stomatal movements in leaves.

Cytosolic calcium regulates ion channels in the plasma membrane of Vicia faba guard cells

Article de synthese sur les mecanismes moleculaires du processus de transport des ions dans les cellules des plantes superieures

The Physiology of ION Channels and Electrogenic Pumps in Higher Plants

▪ Abstract Nonselective cation channels are a diverse group of ion channels characterized by their low discrimination between many essential and toxic cations. They are ubiquitous in plant tissues and are active in the plasma membrane, tonoplast, and other endomembranes. Members of this group are likely to function in low-affinity nutrient uptake, in distribution of cations within and between cells, and as plant Ca2+ channels. They are gated by diverse mechanisms, which can include voltage, cyclic nucleotides, glutamate, reactive oxygen species, and stretch. These channels dominate tonoplast cation transport, and the selectivity and gating mechanisms of tonoplast nonselective cation channels are comprehensively reviewed here. This review presents the first classification of plant nonselective cation channels and the first full description of nonselective cation channel candidate sequences in the Arabidopsis genome.

Nonselective cation channels in plants.

Leaflet movements in Samanea saman are driven by the shrinking and swelling of cells in opposing (extensor and flexor) regions of the motor organ (pulvinus). Changes in cell volume, in turn, depend upon large changes in motor cell content of K+, Cl− and other ions. We performed patch-clamp experiments on extensor and flexor protoplasts, to determine whether their plasma membranes contain channels capable of carrying the large K+ currents that flow during leaflet movement. Recordings in the “whole-cell” mode reveal depolarization-activated K+ currents in extensor and flexor cells that increase slowly (t½ = ca. 2 seconds) and remain active for minutes. Recordings from excised patches reveal a single channel conductance of ca. 20 picosiemens in both cell types. The magnitude of the K+ currents is adequate to account quantitatively for K+ loss, previously measured in vivo during cell shrinkage. The K+ channel blockers tetraethylammonium (5 millimolar) or quinine (1 millimolar) blocked channel opening and decreased light- and dark-promoted movements of excised leaflets. These results provide evidence for the role of potassium channels in leaflet movement.

Potassium channels in motor cells of Samanea saman: a patch-clamp study

The patch-clamp technique was used to study passive movements of ions through the plasmalemma of wheat leaf protoplasts. This method overcomes the problems inherent in conventional electrophysiological study of plant cells. Changes in conductance were recorded in patches excised from the plasmalemma. Two types of patches were observed: (i) regions of low channel density, where discrete single-channel currents could be resolved and conductance ranged from 10 to 200 picosiemens and (ii) regions of high channel density, where single-channel currents could not be resolved and conductance was on the order of a few nanosiemens. The results indicate a striking similarity between animal and plant cell membranes in the basic phenomena of transport. Moreover, the approach used constitutes a new degree of refinement in the study of processes of regulation, pathology, and toxicity in plants.

http://science.sciencemag.org/content/sci/226/4676/835.full.pdf

Ion channels in plasmalemma of wheat protoplasts.

Membrane transport in animal cells still keeps many a researcher intrigued; transport in plant cells, by comparison, is virtually terra incognita in spite of the increasingly intensifying research during the last few decades (Clarkson, 1984).

Charles Bare

Papers

Potassium channels in motor cells of Samanea saman: a patch-clamp study

Ion channels in plasmalemma of wheat protoplasts.

Ionic Channels in Plant Protoplasts