Potassium homeostasis in salinized plant tissues

Tracey A. Cuin, Sergey Shabala

Research output: Chapter in Book/Conference paperChapterpeer-review

11 Citations (Scopus)


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.

Original languageEnglish
Title of host publicationPlant Electrophysiology
Subtitle of host publicationTheory and Methods
Number of pages31
ISBN (Electronic)9783540378433
ISBN (Print)3540327177, 9783540327172
Publication statusPublished - 2006
Externally publishedYes


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