Effects of low-chloride solutions on action potentials of sheep cadiac purkinje fibers

The rapid repolarization during phase 1 of the action potential of sheep cardiac purkinje fibers has been attributed to a time- and voltage-dependent chloride current. In part, this conclusion was based on experiments that showed a substantial slowing of phase 1 when larger, presumably impermeant, anions were substituted for chloride in tyrode’s solution. We have re- examined the electrical effects of low-chloride solutions. We recorded action potentials of sheep cardiac purkinje fibers in normal tyrode’s solution and in low-chloride solutions made by substituting sodium propionate, acetylglycinate, methylsulfate, or methanesulfonate for the NaCl of Tyrode’s solution. Total calcium was adjusted to keep calcium ion activity of test solutions equal to that of control solutions. Propionate gave qualitatively variable results in preliminary experiments; it was not tested further. Low-chloride solutions made with the other anions gave much more consistent results: phase 1 and the notch that often occurs between phases 1 and 2 were usually unaffected, and the action potential duration usually increased. The only apparent change in the resting potential was a transient 3-6 mV depolarization when low-chloride solution was first admitted to the chamber, and a symmetrical transient hyperpolarization when chloride was returned to normal. If a time- and voltage-dependent chloride current exists in sheep cardiac purkinje fibers, our results suggest that it plays little role in generating phase 1 of the action potential.     

Determination of 1-aminocycopropane-1-carboxylic acid (ACC) to assess the effects of ACC deaminase-containing bacteria on roots of canola seedlings

Previously, it was proposed that plant growth-promoting bacteria that possess the enzyme, 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase, can reduce the amount of ethylene produced by a plant and thereby promote root elongation. To test this model, canola seeds were imbibed in the presence of the chemical ethylene inhibitor, 2-aminoethoxyvinyl glycine (AVG), various strains of plant growth-promoting bacteria, and a psychrophilic bacterium containing an ACC deaminase gene on a broad host range plasmid. The extent of root elongation and levels of ACC, the immediate precursor of ethylene, were measured in the canola seedling roots. A modification of the Waters AccQ.Tag Amino Acid Analysis Method was used to quantify ACC in the root extracts. It was found that, in the presence of the ethylene inhibitor, AVG, or any one of several ACC deaminase-containing strains of bacteria, the growth of canola seedling roots was enhanced and the ACC levels in these roots were lowered.     

Levels of ACC and related compounds in exudate and extracts of canola seeds treated with ACC deaminase-containing plant growth-promoting bacteria

It was previously proposed that plant growth-promoting bacteria that possess 1-aminocyclopropane-1-carboxylic acid (ACC) deaminase could utilize ACC that is present in the exudate of germinating canola seeds. The uptake and cleavage of ACC by these bacteria would lower the level of ACC, and thus ethylene within the plant, and reduce the extent of its inhibition on root elongation. To test part of the above mentioned model, ACC levels were monitored in canola seed tissues and exudate during germination. Lower amounts of ACC were present in the exudate and tissues of seeds treated with the plant growth-promoting bacterium Enterobacter cloacae CAL3, than in control seeds treated with MgSO4. The ACC-related compounds, alpha- and gamma-aminobutyric acids, both known to stimulate ethylene production, were also measured in the canola seed exudate and tissues. Approximately the same levels of alpha-aminobutyric acid were present in the exudates of the bacterium-treated seeds and the control seeds, but the amount of alpha-aminobutyric acid was lower in the tissues of the bacterium-treated seeds than in the control seeds. Smaller quantities of gamma-aminobutyric acid were seen in both the exudate and tissues of the E. cloacae CAL3-treated seeds than in the control seeds.     

Strategies used by rhizobia to lower plant ethylene levels and increase nodulation

Agriculture depends heavily on biologically fixed nitrogen from the symbiotic association between rhizobia and plants. Molecular nitrogen is fixed by differentiated forms of rhizobia in nodules located on plant roots. The phytohormone, ethylene, acts as a negative factor in the nodulation process. Recent discoveries suggest several strategies used by rhizobia to reduce the amount of ethylene synthesized by their legume symbionts, decreasing the negative effect of ethylene on nodulation. At least one strain of rhizobia produces rhizobitoxine, an inhibitor of ethylene synthesis. Active 1-aminocyclopropane-1-carboxylate (ACC) deaminase has been detected in a number of other rhizobial strains. This enzyme catalyzes the cleavage of ACC to alpha-ketobutyrate and ammonia. It has been shown that the inhibitory effect of ethylene on plant root elongation can be reduced by the activity of ACC deaminase.