Cortical Cell Flux Research Articles

Mechanical Stimulus-Sensitive Mutation, rrl3Affects the Cell Production Process in the Root Meristematic Zone in Rice

Genetic studies on the response of plant root to environmental stimuli are important for elucidating the mechanism of the stress tolerance of plants. We isolated and characterized a recessive rice mutant, rrl3,which was highly sensitive to mechanical stimulus and has short roots. No significant difference was observed between the seminal roots of rrl3mutant and wild type in the mean axial and radial length of mature cortical cells. On the other hand, meristematic zone of the root was smaller and the cortical cell flux in the growing zone of the root was significantly lower in the mutant than in the wild type. In addition, the rrl3mutant and the wild type did not differ in sensitivity to ethylene, IAA or ABA. These results suggest that the RRL3gene specifically regulates the cell production process in the root meristematic zone under a mechanically impeded condition and does not regulate the sensitivities to ethylene, IAA and ABA.

Cortical cell fluxes of cobalt in roots and transport to the shoots of ryegrass seedlings

Cortical cell fluxes of cobalt in roots and transport to the shoots of ryegrass seedlings

Cortical cell fluxes of cobalt in roots and transport to the shoots of ryegrass seedlings

Total uptake and transport of 58Co as a function of time were measured in seedlings of Lolium perenne L. cv. Premo, using nutrient solutions containing either 0.1 or 1.0 μM Co2+. After an initial shoulder, uptake was linear and about 15% of the Co absorbed was transported to the shoot after 72 h. Log total uptake and transport as a function of log Co concentration (0.01 to 1.0 μM) were also linear. Co uptake and transport markedly increased with increasing pH but were unaffected by water flux. Compartmental analysis of 58Co efflux data was used to estimate unidirectional fluxes and compartment al concentrations of Co in root cortex, cells. At both levels of external Co, influx to the cytoplasm was passive and cytoplasmic concentrations were comparable. In the 0.1 μM treatment, cytoplasm concentration was controlled by an efflux pump; fluxes across the tonoplast were passive and concentration in the vacuole was small. In the 0.1 μM treatment, the concentration of Co in the cytoplasm was regulated by both an efflux pump at the plasmalemma and an influx pump at the tonoplast. Stored Co in the vacuole was largely unavailable for transport. Factors limiting transport, and the significance of Co depletion in nutrient solutions due to uptake, were discussed. We also established that 0.1 μM Co was sufficient to provide adequate levels of ryegrass shoot Co for ruminant diets.

Cortical Cell Fluxes of Phosphate in Excised Root Segments ofAllium cepaL.

Ron, M. M., Macklon, A. E. S. and Sim, A. 1988. Cortical cell fluxes of phosphate in excised root segments of Allium cepa L.—J. exp. Bot. 39: 1699-1707. Compartmental analysis was used to study content and transmembrane fluxes of phosphate in excised root segments in a nutrient solution containing 10 mol phosphate m 3, at pH 5-6. The resulting radioactivity content versus time curve deviated from the criteria determining a relationship conforming to first order kinetics, since the final rate constant was an order of magnitude lower than that exhibited by the curve for efflux versus time. The data were manipulated so as to bring about conformation to the appropriate criteria, revealing a large, slowly exchanging pool that is considered to represent assimilated phosphate or stored polyphosphate. Subsequent calculations, using the modified data, suggested that HjPO^ was absorbed by a phosphate pump at the plasmalemma, but was actively extruded from the vacuole after passive entry. The consequences of pH effects in the ceil wall, cytoplasm and vacuole in determining the concentration of H2PC>4 in each phase are discussed.

Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L.

From compartmental analysis of radioisotope elutin measurements, fluxes of Ca(2+) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to complete nutrient solutions containing a range of calcium concentrations ([Ca0]) from 2 μeq l(-1) to 20 meq l(-1), increasing in 10-fold steps for Ca(2+). Except for the calcium counter-ion (usually NO 3 (-) , sometimes Cl(-) at the highest [Ca0]), the composition of the nutrient solution was other-wise the same at all calcium concentrations. Compartmental analysis indicated that the cytoplasm had a high content of exchangeable Ca(2+) but, in the light of evidence from animal studies, ionic activity of calcium in the cytoplasm was assumed to be no greater than 0.002 μeq ml(-1). With the Ussing-Teorell flux equation as the criterion, it was concluded that at all values of [Ca0] tested, Ca(2+) entered the cytoplasm passively and was actively pumped back into the external solution. Entry of calcium to the vacuole from the cytoplasm was active in all cases. The conclusions regarding the character of ion transport across the plasmalemma were the same as when the whole calcium content of the cytoplasm was taken to contribute to the ionic activity. However, the electrochemical activity gradient was very much steeper than formerly estimated. Calcium was transported to the stele in proportion to the calcium content of the cytoplasm and moved in the xylem almost exclusively in the basipetal direction.

Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L.

From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of Mg(2+) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to a complete nutrient solution containing 0.25 mM Mg(2+). Five compartments for Mg(2+) in the cortex were found and, in order of increasing rates of exchange, identified with the vacuoles and the cytoplasm of the cortical parenchyma, the Donnan free space, the water free space, and the superficial film of solution on the segments. With the Ussing-Teorell flux ratio equation as the criterion, it was concluded that Mg(2+) entered the cytoplasm passively and was actively pumped back across the plasmalemma. Mg(2+) concentration in the vacuole could be estimated only as lying between wide limits (1.3 to 14.3 μeq ml(-1)), but whatever the concentration within this range, it was concluded that Mg(2+) was passively distributed across the tonoplast. Net flux was zero and the vacuolar concentration commensurate with this was found to be 6.6 μeq ml(-1). The transported fraction of total efflux, appearing at the segment cut ends, was estimated separately. Magnesium was found to be transported almost exclusively in the basipetal direction.

Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L.

From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of Ca(2+) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to a complete nutrient solution containing 1 mM Ca(2+). Five compartments for Ca(2+) in the cortex were revealed. These were identified, in order of increasing rates of exchange, with the vacuole and cytoplasm of the cortical parenchyma, the Donnan free space in the cell walls, the water free space in the tissue and the superficial film of solution on the segments. With the Ussing-Teorell flux ratio equation as the criterion, it was concluded that Ca(2+) entered the cytoplasm passively and was actively pumped back to the external solution. Ca(2+) concentration in the vacuole could only be estimated as lying between wide limits (1.0 to 7.5 μeq. ml(-1)), but even at the maximum concentration, it was concluded that entry was passive and content limited by an efflux pump across the tonoplast. Net flux was zero and the vacuolar concentration of Ca(2+) compatible with this was found to be 2.6 μeq. ml(-1). The transported fraction of the total efflux, appearing at the segment cut ends, was estimated separately. Calcium was found to be transported almost exclusively in the basipetal direction.

Cortical cell fluxes and transport to the stele in excised root segments of Allium cepa L.

From compartmental analysis of radioisotope elution measurements, concentrations and fluxes of K(+), Na(+) and Cl(-) were estimated for cortical cells in root segments of onion, Allium cepa L., relative to a complete nutrient solution. The transported fraction of the total efflux was estimated separately. With the Ussing-Teorell flux ratio equation as the criterion, it was concluded that all three ions were actively accumulated from the outside medium into the cytoplasm and that only Na(+) was actively accumulated into the vacuole. K(+) and Cl(-) moved passively, in both directions across the tonoplast. Failure to account for leakage from the stele via the segment cut ends resulted in an over-estimate of exchange across the tonoplast but did not alter the conclusions qualitatively. The consequences of changing the assumed value of the tonoplast electrical potential (from 0 to+10- mV), and the effects of different experimental procedures, were also assessed, and found not to affect the main conclusions significantly. Separate measurement of ions leaking from the segment ends revealed that Na(+) was transported almost exclusively in an acropetal direction in the stele. Cl(-) appeared at both ends of the segments in similar amounts and K(+) was transported mainly in the basipetal direction. The implications of these findings for the mechanism and site of ion selectivity are discussed.