Cell-type-specific calcium responses to drought, salt and cold in the Arabidopsis root

Little is known about the signalling processes involved in the response of roots to abiotic stresses. The Arabidopsis root is a model system of root anatomy with a simple architecture and is amenable to genetic manipulation. Although it is known that the root responds to cold, drought and salt stress with increases in cytoplasmic free calcium, there is currently no information about the role(s) of the functionally diverse cell types that comprise the root. Transgenic Arabidopsis with enhancer-trapped GAL4 expression in specific cell types was used to target the calcium reporting protein, aequorin, fused to a modified yellow fluorescent protein (YFP). The luminescence output of targeted aequorin enabled in vivo measurement of changes in cytosolic free calcium concentrations ([Ca2+]cyt) in specific cell types during acute cold, osmotic and salt stresses. In response to an acute cold stress, all cell types tested as well as plants constitutively expressing aequorin displayed rapid [Ca2+]cyt peaks. However, there were significant quantitative differences between different cell types in terms of their response to cold stress, osmotic stress (440 mM mannitol) and salt stress (220 mM NaCl), implying specific roles for certain cell types in the detection and/or response to these stimuli. In response to osmotic and salt stress, the endodermis and pericycle displayed prolonged oscillations in cytosolic calcium that were distinct from the responses of the other cell types tested. Targeted expression of aequorin circumvented the technical difficulties involved in fluorescent dye injection as well as the lack of cell specificity of constitutively expressed aequorin, and revealed a new level of complexity in root calcium signalling.

Role of biosurfactant and ion channel-forming activities of syringomycin in transmembrane ion flux: a model for the mechanism of action in the plant-pathogen interaction

Syringomycin is a necrosis-inducing lipopeptide toxin synthesized and secreted by the phytopathogen, Pseudomonas syringae pv. syringae. Although small quantities of syringomycin are known to activate a cascade of physiological events in plasma membranes, the mechanism of action of the phytotoxin has never been fully characterized. The objective of this study was to test the hypothesis that the primary mode of action of syringomycin is to form transmembrane pores that are permeable to cations. Accordingly, direct measurement of ion fluxes were performed using artificial bilayers. The hemolytic properties and surface activity of HPLC-purified syringomycin were quantified by use of an erythrocyte lysis assay and by the drop weight method. Assays were performed using syringomycin form SRE alone or a mixture containing all forms of the phytotoxin. At a threshold concentration of 500 ng/ml, syringomycin induced hemolysis by forming ion channels in membranes. Osmotic protection studies indicated a channel radius of between 0.6 and 1 nm. The ion channel-forming activity was insensitive and permeable to both monovalent and divalent cations, suggesting that syringomycin causes lysis of erythrocytes by colloid osmotic lysis. In addition, syringomycin, like other lipopeptide antibiotics, is a potent biosurfactant capable of lowering the interfacial tension of water to 31 mN/m. The critical micellar concentration of syringomycin was calculated to be 1.25 mg/ml and the gamma CMC was 33 mN/m. A model is presented depicting the mechanism of action of syringomycin in the plant-pathogen interaction. The model integrates known effects of the toxin on ion flux in plasma membranes with formation of ion channels and the consequential cascade of effects associated with cellular signalling.

Potassium channels from the plasma membrane of rye roots characterized following incorporation into planar lipid bilayers

Plasma membrane was purified from roots of rye (Secale cereale L. cv. Rheidol) by aqueous-polymer two-phase partitioning and incorporated into planar bilayers of 1-palmitoyl-2-oleoyl phosphatidylethanolamine by stirring with an osmotic gradient. Since plasmamembrane vesicles were predominantly oriented with their cytoplasmic face internal, when fused to the bilayer the cytoplasmic side of channels faced the trans chamber. In asymmetrical (cis:trans) 280∶100 mM KCl, five distinct K(+)-selective channels were detected with mean chord-conductances (between +30 and -30 mV; volyages cis with respect to trans) of 500 pS, 194 pS, 49 pS, 21 pS and 10 pS. The frequencies of incorporation of these K(+) channels into the bilayer were 48, 21, 50, 10 and 9%, in the order given (data from 159 bilayers). Only the 49 pS channel was characterized further in this paper, but the remarkable diversity of K(+) channels found in this preparation is noteworthy and is the subject of further study. In symmetrical KCl solutions, the 49 pS channel exhibited non-ohmic unitary-current/voltage relationships. The chord-conductance (between +30 and-30 mV) of the channel in symmetrical 100 mM KCl was 39 pS. The unitary current was greater at positive voltages than at corresponding negative voltages and showed considerable rectification with increasing positive and negative voltages. This would represent 'inward rectification' in vivo. Gating of the channel was not voltage-dependent and the channel was open for approx. 80% of the time. Presumably this is not the case in vivo, but we are at present uncertain of the in vivo controls of channel gating. The distribution of channel-open times could be approximated by the sum of two negative exponential functions, yielding two open-state time constants (τo, the apparent mean lifetime of the channel-open state) of 1.0 ms and 5.7 s. The distribution of channel-closed times was best approximated by the sum of three negative exponential functions, yielding time constants (τc, the apparent mean lifetime of the channel-closed state) of 1.1 ms, 51 ms and 11 s. This indicates at least a five-state kinetic model for the activity of the channel. The selectivity of the 49 pS channel, determined from both reversal potentials under biionic conditions (100 mM KCl∶100 mM cation chloride) and from conductance measurements in symmetrical 100 mM cation chloride, was Rb(+)≥ K(+) > Cs(+) > Na(+) > Li(+) > tetraethylammonium (TEA(+)). The 49 pS channel was reversibly inhibited by quinine (1 mM) but TEA(+) (10 mM), Ba(2+) (3 mM), Ca(2+) (1 mM), 4-aminopyridine (1 mM) and charybdotoxin (3 μM) were without effect when applied to the extracellular (cis) surface.