Potassium homeostasis in salinized plant tissues

Potassium is an essential cation, comprising ∼6% of a plants dry weight and is involved in numerous functions such as osmo- and turgor regulation, charge balance, and control of stomata and organ movement. K+?activates over 50 enzymes critical for numerous metabolic processes, including photosynthesis, oxidative metabolism and protein synthesis (Marschner 1995). Within the cytosol, K⁺ neutralizes the soluble and insoluble macromolecular anions and stabilizes the pH at the level optimal for most enzymatic reactions (pH ∼7.2). Thus, cytosolic K⁺homeostasis is crucial to optimal cell metabolism.

In contrast to K⁺, Na⁺ is not essential for plants (Marschner 1995). For the majority of crop species, Na⁺ is toxic at mM concentrations in the cytosol. With cytosolic K⁺ concentrations being around 150 mM (Leigh and Wyn Jones 1984; Leigh 2001) and cytosolic Na⁺ in a lower mM range (Carden et al. 2003), the cytosolic K⁺/Na⁺ ratio is high, enabling many K⁺-dependent metabolic processes to proceed (Rubio et al. 1995; Maathuis and Amtmann 1999). Under saline conditions, cytosolic Na⁺ levels increase dramatically, estimates varying from 10 to 30 mM, up to 200 mM (Koyro and Stelzer 1988; Flowers and Hajibagheri 2001; Carden et al. 2003). At the same time, cytosolic K⁺ content decreases dramatically. An almost 2-fold decrease in cytosolic K⁺ activity was measured in salinized roots of barley (Carden et al. 2003), and cytosolic K⁺ activity as low as 15 mM in epidermal leaf cells has been reported (Cuin et al. 2003). Thus the cytosolic K⁺/Na⁺ ratio falls dramatically under saline conditions, severely impairing cell metabolism (Maathuis and Amtmann 1999; Flowers and Hajibagheri 2001; Munns 2002). Not surprising, the ability to maintain a high cytosolic K⁺/Na⁺ ratio has often been cited as a key feature in plant salt tolerance (Gorham et al. 1990; Maathuis and Amtmann 1999; Tester and Davenport 2003; Chen et al. 2005).

Within the vacuole, K⁺ mediates osmoregulation, and within specialized cells, stomatal movements and tropisms. Here the K⁺ concentration is much more flexible and can be more readily replaced by other cations, including Na⁺ (Leigh et al. 1986). However, the vacuolar PP-ase is critically dependent on K⁺ for both hydrolytic activity and H⁺ pumping (White et al. 1990). Thus, even in this organelle, maintenance of a minimal level of K⁺ is vitally important for optimal plant performance. How is this achieved?

Molecular and ionic mechanisms of K⁺ transport have been the subject of a large number of comprehensive reviews in recent years (Maathuis and Amtmann 1999; Maathuis and Sanders 1999; Tyerman and Skerrett 1999; Schachtman 2000; Mser et al. 2001; Vry and Sentenac 2002, 2003; Shabala 2003) so are only briefly revised in our review. Many important questions, however, remain to be answered. It is not clear how the levels and ratios of K⁺ to Na⁺ are maintained within the plant, and why these ratios are different in cells within various plant tissues. It is also remains to be answered how plants distinguish between K⁺ and Na⁺, both at the root and cellular levels. This latter problem is not trivial, due to the similarity in ionic radius and ion hydration energies for K⁺ and Na⁺ (Hille 1992), factors which determine both the ion transport mode and the competition for enzyme binding sites within the cytosol. Despite a recent plethora of research (Apse et al. 1999, 2003; Hasegawa et al. 2000; Zhu 2000, 2003; Zhang and Blumwald 2001), we are still lacking full knowledge of the signal-transduction pathways involved in K⁺ homeostasis and maintenance of the critical K⁺/Na⁺ ratios under salt stressed conditions.

This review addresses some of the above issues and summarizes molecular and electrophysiological evidence regarding mechanisms regulating K⁺ homeostasis in salinized plant tissues. The main emphasis is made on the integration of K⁺ transport mechanisms at various levels of plant structural organization.