Surface Electrical Potentials of Root Cell Plasma Membranes: Implications for Ion Interactions, Rhizotoxicity, and Uptake.

Many crop plants are exposed to heavy metals and other metals that may intoxicate the crop plants themselves or consumers of the plants. The rhizotoxicity of heavy metals is influenced strongly by the root cell plasma membrane (PM) surface's electrical potential (ψ). The usually negative ψ is created by negatively charged constituents of the PM.

Delineating ion-ion interactions by electrostatic modeling for predicting rhizotoxicity of metal mixtures to lettuce Lactuca sativa.

Effects of ion-ion interactions on metal toxicity to lettuce Lactuca sativa were studied based on the electrical potential at the plasma membrane surface (ψ0 ). Surface interactions at the proximate outside of the membrane influenced ion activities at the plasma membrane surface ({M(n+)}0). At a given free Cu(2+) activity in the bulk medium ({Cu(2+)}b), additions of Na(+), K(+), Ca(2+), and Mg(2+) resulted in substantial decreases in {Cu(2+)}0.

The rhizotoxicity of metal cations is related to their strength of binding to hard ligands.

Mechanisms whereby metal cations are toxic to plant roots remain largely unknown. Aluminum, for example, has been recognized as rhizotoxic for approximately 100 yr, but there is no consensus on its mode of action. The authors contend that the primary mechanism of rhizotoxicity of many metal cations is nonspecific and that the magnitude of toxic effects is positively related to the strength with which they bind to hard ligands, especially carboxylate ligands of the cell-wall pectic matrix.

Molecular and physiological analysis of Al³⁺ and H⁺ rhizotoxicities at moderately acidic conditions.

Al³⁺ and H⁺ toxicities predicted to occur at moderately acidic conditions (pH [water] = 5-5.5) in low-Ca soils were characterized by the combined approaches of computational modeling of electrostatic interactions of ions at the root plasma membrane (PM) surface and molecular/physiological analyses in Arabidopsis (Arabidopsis thaliana). Root growth inhibition in known hypersensitive mutants was correlated with computed {Al³⁺} at the PM surface ({Al³⁺}(PM)); inhibition was alleviated by increased Ca, which also reduced {Al³⁺}(PM) and correlated with cellular Al responses based on expression analysis of genes that are markers for Al stress.

Modelling metal-metal interactions and metal toxicity to lettuce Lactuca sativa following mixture exposure (Cu²⁺-Zn²⁺ and Cu²⁺-Ag⁺).

Metal toxicity to lettuce Lactuca sativa was determined following mixture exposure based on the concepts of concentration addition (CA) and response addition (RA). On the basis of conventional models assuming no interaction between mixture components, Ag(+) was the most toxic, followed by Cu(2+) and Zn(2+). Furthermore, ion-ion interactions were included in quantitatively estimating toxicity of interactive mixtures of Cu(2+)-Zn(2+) and Cu(2+)-Ag(+) by linearly expanding the CA and RA models.