Ca2+‐ and Mg2+‐dependent ATPase activity (EC 3.6.1.3) in a plasma membrane‐enriched fraction increased rapidly after in vivo application of physiologically active concentrations of triacontanol (TRIA) to the roots of barley (Hordeum vulgare L. cv. Conquest) seedlings. Ca2+‐ and Mg2+‐dependent ATPase activity was 64 and 85% higher, respectively, in the roots of seedlings germinated in the presence of growth‐promoting concentrations of TRIA compared to controls. The increase in vivo was concentration dependent, with the greatest increase obtained at 2.3 nM TRIA. Maximal stimulation of ATPase activity of excised tissue treated with TRIA coincided with the temperature at which the barley was grown. At this temperature the plasma membrane is primarily in a mixed gel/liquid crystalline state. Pretreatment of barley roots with cyclohexamide did not alter ATPase stimulation by TRIA. Two to three times more [14C]‐TRIA (mg membrane protein)−1 was found associated with plasma membrane‐enriched vesicles treated with TRIA than with vesicles enriched for mitochondrial membranes or for vesicles enriched for tonoplast, Golgi and rough endoplasmic reticulum. Both Ca2+‐ and Mg2+‐dependent ATPase activity increased by 40–60% within 30 min of the addition of 2.3 nM TRIA to cell‐free extracts of barley roots. The addition of octacosanol, the C28 analogue of TRIA, to cell‐free extracts did not affect metal‐dependent ATPase activity. Consistent with many studies in the green‐house, simultaneous additions of equimolar amounts of TRIA and octacosanol to cell‐free extracts resulted in inhibition of ATPase stimulation by TRIA. TRIA may directly affect plasma membrane function in barley roots.

When serial dilutions of a suspension of Bradyrhizobium japonicum strain 138 were inoculated onto both soybean and cowpea roots, the formation of nodules in the initially susceptible region of the roots of both hosts was found to be linearly dependent on the log of the inoculum dosage until an optimum dosage was reached. Approximately 30-to 100-fold higher dosages were required to elicit half-maximal nodulation on cowpea than on soybean in the initially susceptible zone of the root. However, at optimal dosages, about six times as many nodules formed in this region on cowpea roots than on soybean roots. There was no appreciable difference in the apparent rate of nodule initiation on these two hosts nor in the number of inoculum bacteria in contact with the root. These results are consistent with the possibility that cowpea roots have a substantially higher threshold of response to symbiotic signals from the bacteria than do soybean roots. Storage ofB.japonicum cells in distilled water for several weeks did not affect their viability or efficiency of nodule initiation on soybean. However, the nodulation efficiency of these same cells on cowpea diminished markedly over a 2 week period. These differential effects of water storage indicate that at least some aspects of signal production by the bacteria during nodule initiation are different on the two hosts. Mutants of B. japonicum 138 defective in synthesis of soybean lectin binding polysaccharide were defective in their efficiency of nodule initiation on soybean but not on cowpea. These results also suggest that B. japonicum may produce different substances to initiate nodules on these two hosts.Rhizobium and Bradyrhizobium are soil bacteria able to symbiotically infect and nodulate a wide diversity of legumes, reducing atmospheric nitrogen in these nodules to ammonia for the plant in exchange for fixed carbon (1,12

1989. Stimulation of ATPase activity in barley {Hordeum vulgare) root plasma membranes after treatment with triacontanol and calmodulin. -Physiol. Plant. 75: 75-80.Triacontanol (TRIA) treatment of plasma membrane-enriched vesicles from barley {Hordeum vulgare L. cv. Conquest) roots resulted in stimulation of membraneassociated, divalent cation-dependent ATPase activity (EC 3,6.1.3). The stimulation at physiologically active concentrations of TRIA (10-"-l(r'' M) occurred only when the vesicles were treated with TRIA in the presence of calmodulin. Octacosanol, the C2x-analogue of TRIA, had no effect on divalent cation-dependent ATPase activity. Consistent with in vivo studies, simultaneous treatment of vesicles with weight equivalents of TRIA and octacosanol reduced the stimulation of ATPase activity. The effect of calmodulin on the stimulation of ATPase activity was diminished by calmidazolium, a specific inhibitor of calmodulin. Circular dichroism studies did not show a change in the a-helix content of calmoduhn in the presence of TRIA. TRIA also had no apparent effect on soluble calcium-calmodulin 3',5'-cyclic nucleotide phosphodiesterase activity. Removal of excess TRIA from the medium after treatment still resulted in stimulation of divalent cation-dependent ATPase activity in the presence of calmoduiin was comparable to treated vesicles from which excess TRIA had not been removed. These data further support the contention that TRIA affects membrane structure and function.

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