Plants and sodium ions: keeping company with the enemy

Role of ENA ATPase in Na(+) efflux at high pH in bryophytes

Potassium or Na(+) efflux ATPases, ENA ATPases, are present in all fungi and play a central role in Na(+) efflux and Na(+) tolerance. Flowering plants lack ENA ATPases but two ENA ATPases have been identified in the moss Physcomitrella patens, PpENA1 and PpENA2. PpENA1 mediates Na(+) efflux in Saccharomyces cerevisiae. To propose a general function of ENA ATPases in bryophytes it was necessary to demonstrate that these ATPases mediate Na(+) efflux in planta and that they exist in more bryophytes than P. patens. For these demonstrations (1) we cloned a third ATPase from P. patens, PpENA3, and studied the expression pattern of the three PpENA genes; (2) we constructed and studied the single and double Deltappena1 and Deltappena2 mutants; and (3) we cloned two ENA ATPases from the liverwort Marchantia polymorpha, MpENA1 and MpENA2, and expressed them in S. cerevisiae. The results from the first two approaches revealed that the expression of ENA ATPases was greatly enhanced at high pH and that Na(+) efflux at high pH depended on PpENA1. The ENA1 ATPase of M. polymorpha suppressed the defective growth of a S. cerevisiae mutant at high K(+) or Na(+) concentrations, especially at high K(+).

Cellular mechanisms of potassium transport in plants

Potassium (K(+)) is the most abundant ion in the plant cell and is required for a wide array of functions, ranging from the maintenance of electrical potential gradients across cell membranes, to the generation of turgor, to the activation of numerous enzymes. The majority of these functions depend more or less directly upon the activities and regulation of membrane-bound K(+) transport proteins, operating over a wide range of K(+) concentrations. Here, we review the physiological aspects of potassium transport systems in the plasma membrane, re-examining fundamental problems in the field such as the distinctions between high- and low-affinity transport systems, the interactions between K(+) and other ions such as NH(4)(+) and Na(+), the regulation of cellular K(+) pools, the generation of electrical potentials and the problems involved in measurement of unidirectional K(+) fluxes. We place these discussions in the context of recent discoveries in the molecular biology of K(+) acquisition and produce an overview of gene families encoding K(+) transporters.

Salt-induced respiration in Bruguiera cylindrica - role in salt transport and protection against oxidative damage

Leaves of Bruguiera cylindrica plants grown in the greenhouse and irrigated with fresh water (FW plants) and those from salt-adapted plants from the seacoast (SW plants) showed about 5-fold and 3-fold increase in respiration, respectively, when immersed in 4M NaCl solution. The increase in respiration was not due to dehydration effect of high salt concentration, since PEG-imposed dehydration stress to the leaves led to an inhibition of respiration rates in both FW and SW plants. The salt-induced increase in respiration rate was specific to monovalent cations, especially Na(+) and K(+), but not divalent or trivalent cations, and to Cl(-), but not other anions. Pretreatment of leaves of FW plants with 1mM amiloride, an inhibitor of the Na(+) / H(+) antiporter, reduced the NaCl-induced respiration surge. At least some part of the observed respiratory increase could therefore be for providing energy for ion transport, since the Na(+) / H(+) antiport activity is driven by activities of the tonoplast and plasma membrane H(+)-ATPases and H(+)-PPases. Respiration of the leaves from both FW and SW plants was accounted for by the COX pathway and was inhibited by KCN. But 4M NaCl-induced increase in FW, but not SW plants, was inhibited by the AOX inhibitor, SHAM. Also, generation of ROS was reduced by treatment with KCN, but increased with SHAM. This pointed to a protective role of AOX in reducing ROS generation during salt-induced respiration. Our results indicated that NaCl-induced increase in leaf respiration of B. cylindrica plants irrigated with fresh water was required for (a) salt transport and (b) reducing the harmful effects of ROS that are known to accompany increased respiratory activity.

The Arabidopsis kinase-associated protein phosphatase regulates adaptation to Na+ stress

The kinase-associated protein phosphatase (KAPP) is a regulator of the receptor-like kinase (RLK) signaling pathway. Loss-of-function mutations rag1-1 (root attenuated growth1-1) and rag1-2, in the locus encoding KAPP, cause NaCl hypersensitivity in Arabidopsis thaliana. The NaCl hypersensitive phenotype exhibited by rag1 seedlings includes reduced shoot and primary root growth, root tip swelling, and increased lateral root formation. The phenotype exhibited by rag1-1 seedlings is associated with a specific response to Na(+) toxicity. The sensitivity to Na(+) is Ca(2+) independent and is not due to altered intracellular K(+)/Na(+). Analysis of the genetic interaction between rag1-1 and salt overly sensitive1 (sos1-14) revealed that KAPP is not a component of the SOS signal transduction pathway, the only Na(+) homeostasis signaling pathway identified so far in plants. All together, these results implicate KAPP as a functional component of the RLK signaling pathway, which also mediates adaptation to Na(+) stress. RLK pathway components, known to be modulated by NaCl at the messenger RNA level, are constitutively down-regulated in rag1-1 mutant plants. The effect of NaCl on their expression is not altered by the rag1-1 mutation.