Potassium transport in fungi and plants.

Publisher Summary This chapter presents the phosphorus uptake, storage, and utilization by fungi. Fungi are a diverse group of lower plants united by their heterotrophic mode of nutrition. Studies of fungal nutrition have, thus, not unnaturally been dominated by considerations of carbon metabolism. The chapter draws together some of the scattered data available on fungal Physiology and uptake. It includes the various yeasts that are sometimes segregated from the filamentous fungi. The approach is basically biochemical and orientated towards laboratory studies. Many of the studies discussed in the chapter have attempted to define the fundamental cellular processes of phosphorus (P) metabolism and have concentrated on only a few fungal species; notably on a handful of fast-growing, easily cultured yeasts (species of Saccharomyces and Candida), and the filamentous species— Aspergillus niger and Neurospora crassa . The development of X-ray microanalysis techniques for use with the electron microscope has provided a powerful tool for investigating ion distribution at the cellular level. The P content of the cell wall is highly variable, depending on the species, the structure that the wall came from, and on the growth conditions. It is striking that the cell walls of the zygomycetes, Cunninghamella and Muco r have very high P levels in comparison with those of other fungi. Extensive depletion of most P compounds of the cell is incompatible with continued growth. However, under conditions of restricted P supply, the effects have been found is a reduction in the total P content of sulfur (S). An understanding of the role of polyp in the P economy of the cell requires both the knowledge of the enzymes involved in its metabolism and its rates of turnover in the cell. The enzymes of polyp metabolism are discussed in the chapter.

Phosphorus uptake, storage and utilization by fungi.

Effects of inhibitors on the plasma membrane and mitochondrial adenosine triphosphatases of Neurospora crassa.

The effects of vanadate on the plasma membrane ATPase of Neurospora crassa.

Combined ion flux and electrophysiological measurements have been used to characterized active transport of potassium by cells of Neurospora crassa that have been moderately starved of K+ and then maintained in the presence of millimolar free calcium ions. These conditions elicit a high-affinity (K1/2 = 1-10 microM) potassium uptake system that is strongly depolarizing. Current-voltage measurements have demonstrated a K+-associated inward current exceeding (at saturation) half the total current normally driven outward through the plasma membrane proton pump. Potassium activity ratios and fluxes have been compared quantitatively with electrophysiological parameters, by using small (approximately 15 micron diam) spherical cells of Neurospora grown in ethylene glycol. All data are consistent with a transport mechanism that carries K ions inward by cotransport with H ions, which move down the electrochemical gradient created by the primary proton pump. The stoichiometry of entry is 1 K ion with 1 H ion; overall charge balance is maintained by pumped extrusion of two protons, to yield a net flux stoichiometry of 1 K+ exchanging for 1 H+. The mechanism is competent to sustain the largest stable K+ gradients that have been measured in Neurospora, with no direct contribution from phosphate hydrolysis or redox processes. Such a potassium-proton symport mechanism could account for many observations reported on K+ movement in other fungi, in algae, and in higher plants.

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A potassium-proton symport in Neurospora crassa.

Characterization of plasma membrane adenosine triphosphatase of Neurospora crassa.

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Characterization of plasma membrane adenosine triphosphatase of Neurospora crassa.