Suitability of Using Rubidium-86 as a Tracer for Potassium in Studying Potassium Uptake by Barley Plants

Mycorrhiza-mediated potassium transport in Medicago truncatula can be evaluated by using rubidium as a proxy

Arbuscular mycorrhizal (AM) fungi considerably improve plant nutrient acquisition, particularly phosphorus and nitrogen. Despite the physiological importance of potassium (K⁺) in plants, there is increasing interest in the mycorrhizal contribution to plant K⁺ nutrition. Yet, methods to track K⁺ transport are often costly and limiting evaluation opportunities. Rubidium (Rb⁺) is known to be transported through same pathways as K⁺. As such our research efforts attempt to evaluate if Rb⁺ could serve as a viable proxy for evaluating K⁺ transport in AM symbiosis. Therefore, we examined the transport of K⁺ in Medicago truncatula colonized by the AM fungus Rhizophagus irregularis isolate 09 having access to various concentrations of Rb⁺ in custom-made two-compartment systems. Plant biomass, fungal root colonization, and shoot nutrient concentrations were recorded under sufficient and limited K⁺ regimes. We report that AM plants displayed higher shoot Rb⁺ and K⁺ concentrations and a greater K⁺:Na⁺ ratio relative to non-colonized plants in both sufficient and limited K⁺ conditions. Consequently, our results indicate that Rb⁺ can be used as a proxy to assess the movement of K⁺ in AM symbiosis, and suggest the existence of a mycorrhizal uptake pathway for K⁺ nutrition in M. truncatula.

Benefits provided by four ectomycorrhizal fungi to Pinus taeda under different external potassium availabilities

Ectomycorrhizal fungi contribute to the nutrition of many woody plants, including those in the Pinaceae family. Loblolly pine (Pinus taeda L.), a native species of the Southeastern USA, can be colonized by multiple species of ectomycorrhizal fungi. The role of these symbionts in P. taeda potassium (K⁺) nutrition has not been previously investigated. Here, we assessed the contribution of four ectomycorrhizal fungi, Hebeloma cylindrosporum, Paxillus ammoniavirescens, Laccaria bicolor, and Suillus cothurnatus, in P. taeda K⁺ acquisition under different external K⁺ availabilities. Using a custom-made two-compartment system, P. taeda seedlings were inoculated with one of the four fungi, or kept non-colonized, and grown under K⁺-limited or -sufficient conditions for 8 weeks. Only the fungi had access to separate compartments in which rubidium, an analog tracer for K⁺, was supplied before harvest. Resulting effects of the fungi were recorded, including root colonization, biomass, and nutrient concentrations. We also analyzed the fungal performance in axenic conditions under varying supply of K⁺ and sodium. Our study revealed that these four ectomycorrhizal fungi are differentially affected by external K⁺ and sodium variations, that they are not able to provide similar benefits to the host P. taeda in our growing conditions, and that rubidium may be used with some limitations to estimate K⁺ transport from ectomycorrhizal fungi to colonized plants.

Potassium transport in Chlamydomonas reinhardtii: isolation and characterization of transport-deficient mutant strains

Putative potassium-transport-deficient mutant strains of Chlamydomonas reinhardtii Dang. were induced by ultra-violet mutagenesis and were identified by their dependence on abnormally high concentrations of potassium for growth. Potassium transport studies employing (86)Rb as a tracer were carried out with wild-type cells and with three independently isolated KDP (potassium-dependent phenotype) clones. Wildtype cells exhibit two transport activities. Transport activity A was expressed when cells were grown in medium supplemented with 10 mM KCl. The transporter with type-A activity does not discriminate between either Rb(+) or K(+) as a substrate and has a Km for Rb(+) equal to 1 mM and a Vmax equal to 31 nmol Rb(+) h(-1) 10(-6) cells. Transport activity B was expressed when cells were starved of potassium for 24 h. The transporter with type-B activity prefers K(+) to Rb(+) as a substrate; it has a Km for Rb(+) equal to 2.5 mM and a Vmax equal to 210 nmol Rb(+) h(-1) 10(-6) cells. All three mutant clones exhibit transport activity comparable to type-A when grown in 10 mM KCl. When starved of potassium for 24 h, two KDP clones demonstrate no transport activity and the third clone continues to exhibit only type-A activity.

Inhibition of ion uptake to barley roots by cycloheximide

Cycloheximide was shown to inhibit the transport of ions into the xylem of "salt-saturated" barley roots (Hordeum vulgare cv cape), and incorporation of L-[1-(14)C] leucine into protein within 40-60 min. Water flow across the roots of whole seedlings was not altered for at least 180 min. Uptake of ions into the cells of "salt-saturated" roots was not affected for 120 min, but was inhibited when treated with cycloheximide for more than 120 min. H(+) efflux from "lowsalt" roots was inhibited by cycloheximide at about the same time as ion uptake. Measurement of ATP levels and O2uptake indicated that cycloheximide was not acting as an uncoupler of oxidative phosphorylation. The present data are considered to support the view that secretion of ions into the xylem vessels involves a specific protein with a short effective half-life, and is a separate process from active uptake into the cortical cells.

Relation between permeability to potassium and sodium ions and fusicoccin-stimulated hydrogen-ion efflux in barley roots

Measurements are described of fusicoccin (FC)-stimulated H(+) efflux in barley (Hordeum vulgare L.) roots when K(+) and Na(+) concentrations were varied. In low-salt roots H(+) efflux was stimulated in both 5 mM KCl and NaCl. In salt-saturated roots H(+) efflux was stimulated more effectively in KCl than in NaCl solution. The stimulation of H(+) efflux thus is parallel with the selectivity of these different root preparations for K(+) and Na(+) and with estimates of permeability ratios (P Na/P K) determined from electrical measurements. It is suggested that the results support electrogenic coupling between FC-stimulated H(+) efflux and cation uptake.

Ultrastructure of xylem parenchyma cells of barley roots in relation to ion transport to the xylem

The structure of xylem parenchyma cells is examined in relation to transport of ions through the root. Measurement of uptake of (86)Rb(+) and its transport through the root at different distances from the apex show that this is a general activity along the length of the root and not confined to a limited region. Thus transport through the root is not stopped by removal of that part of the root tip containing metaxylem vessels with living contents. The structure of xylem parenchyma appears to be suitable for involvement in ion transport from the stele to the xylem. At 1 cm behind the tip, where metaxylem vessels have no living contents but ion uptake and transport are going on at high rates, xylem parenchyma cells are rich in cytoplasm with extensive rough endoplasmic reticulum and well-developed mitochondria. Their cell walls contain numerous plasmodesmata, establishing the possibility of a symplastic pathway across the stele up to the vessels. The results are discussed in relation to regulation of ion transport to the xylem vessels in roots.