|عنوان فارسی مقاله:||انتقال +k در گیاهان: فیزیولوژی و بیولوژی مولکولی|
|عنوان انگلیسی مقاله:||K+ transport in plants: Physiology and molecular biology|
|رشته های مرتبط:||زیست شناسی، علوم سلولی و مولکولی، علوم گیاهی و میکروبیولوژی|
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|نشریه||الزویر – Elsevier|
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Potassium (K+ ) is an essential nutrient and the most abundant cation in plant cells. Plants have a wide variety of transport systems for K+ acquisition, catalyzing K+ uptake across a wide spectrum of external concentrations, and mediating K+ movement within the plant as well as its efflux into the environment. K+ transport responds to variations in external K+ supply, to the presence of other ions in the root environment, and to a range of plant stresses, via Ca2+ signaling cascades and regulatory proteins. This review will summarize the molecular identities of known K+ transporters, and examine how this information supports physiological investigations of K+ transport and studies of plant stress responses in a changing environment. & 2008 Elsevier GmbH. All rights reserved.
Potassium (K+ ) is an essential nutrient for plant growth and development. It is the most abundant cation in plant cells and can comprise as much as 10% of plant dry weight (Leigh and Wyn Jones, 1984; Ve´ry and Sentenac, 2003). Plant roots take up K+ from a wide range of external concentrations ([K+ ]ext), which typically vary from 0.1 to 10 mM (Reisenauer, 1966; Hawkesford and Miller, 2004). Occasionally, much higher [K+ ] are observed (Ramadan, 1998), while in some intensively cultivated areas such as rice fields of Southeast Asia, the depletion of soil K+ threatens to reduce crop yields (Dobermann and Cassman, 2002; Yang et al., 2004). Other environmental stresses, such as metal toxicity, salinity, and drought, are known to adversely affect K+ uptake and transport by plants (Schroeder et al., 1994; Amtmann et al., 2006; Shabala and Cuin, 2008), and such stresses can often be ameliorated by increased K+ supply (Cakmak, 2005). The link between K+ and crop production has been highlighted in two recent reviews: one on the role of K+ in reducing the effects of pests and disease on plants (Amtmann et al., 2008) and the other on the importance of K+ in the onset of sodium (Na+ ) toxicity (Shabala and Cuin, 2008). The extraction of K+ from soil and its distribution within the plant require the presence of membrane-bound transport proteins. A large number of such transporters have now been identified at the molecular level, demonstrating the complex nature of K+ transport. The physiological roles of these proteins in primary K+ influx, efflux, compartmentation, and transport within the plant have been partially characterized (Gierth and Ma¨ser, 2007; Lebaudy et al., 2007), while many putative K+ transporters and transport regulators are currently under investigation. The present review will begin with a synopsis of the functions of K+ , then discuss the known classes of K+ transporters and their regulation, with attention to special topics such as K+ -use efficiency and root zonation. Throughout, we shall assess some of the latest investigations into K+ transport at cellular and whole-plant levels. It is our hope to generate new discussion for K+ transport research by bringing together important advances in plant molecular biology and physiology.
Functions of K+
Potassium plays major biochemical and biophysical roles in plants. General maintenance of the photosynthetic apparatus demands K+ , and K+ deficiency reduces photosynthetic activity, chlorophyll content, and translocation of fixed carbon (Hartt, 1969; Pier and Berkowitz, 1987; Zhao et al., 2001). Plant movements such as closing and opening of stomata, leaf movements, and other plant tropisms are driven by K+ -generated turgor pressure (Maathuis and Sanders, 1996a; Philippar et al., 1999). The osmotic pressure brought about by K+ accumulation within cells is also used to drive cellular and leaf expansion (Maathuis and Sanders, 1996a; Elumalai et al., 2002). K+ is highly mobile within plants, exhibiting long-distance cycling between roots and shoots in the xylem and phloem. This is most evident in the cotransport of K+ with nitrate (NO3 ) to shoots and its subsequent retranslocation to roots with malate when plants are supplied with NO3 , and is also seen in the cotransport of K+ with amino acids in the xylem (Ben Zioni et al., 1971; Jeschke et al., 1985). Recirculated K+ can be an important source of K+ in roots, particularly with NO3 -grown plants, and phloemdelivered K+ from shoots may be a signal that modulates K+ influx into the root (Peuke and Jeschke, 1993; White, 1997). The relatively high permeability of plant cells to K+ confers on the ion the ability to impose short- and long-term modifications upon the electrical potential difference across the plasma membrane (DCPM) that is primarily established and maintained by the H+ – ATPase. This can be readily seen when changes in the [K+ ]ext result in permanent hyperpolarization (upon reduction of K+ ) or depolarization (upon increase in K+ ) of DCPM (Pitman et al., 1970; Cheeseman and Hanson, 1979; Kochian et al., 1989; Maathuis and Sanders, 1996a; Rodrı´guez-Navarro, 2000). Notably, in some species such as rice (Oryza sativa), or the halophyte Triglochin maritima, ammonium (NH4 + ), and sodium (respectively) can also adjust DCPM (Jefferies, 1973; Wang et al., 1994). Nevertheless, in most plants DCPM is only transiently modified by either ion (Higinbotham et al., 1964; L’Roy and Hendrix, 1980; Cheeseman, 1982; Cheeseman et al., 1985). K+ accumulates to considerable concentrations in cytosolic and vacuolar compartments. The cytosolic K+ pool appears to be relatively stable, although estimates of cytosolic K+ concentration ([K+ ]cyt) can range widely, between 30 and 320 mM, tending to a set point of around 100 mM (Walker et al., 1996). This range, and the exact stringency of [K+ ]cyt homeostasis, reflects, in part, some disagreement arising from the use of different methods (see Britto and Kronzucker, 2008). By contrast with the cytosolic pool, the concentration of the vacuolar K+ pool has been found to vary greatly, between 10 A and 500 mM, depending on the plant examined and the K+ growth condition (Leigh and Wyn Jones, 1984; Marschner, 1995). A stable [K+ ]cyt is considered necessary for optimal enzyme activity, and its disruption may underlie ion toxicities such as brought about by high sodium or ammonium provision (Mills et al., 1985; Hajibagheri et al., 1987, 1988; Speer and Kaiser, 1991; Walker et al., 1996; Flowers and Hajibagheri, 2001; Carden et al., 2003; Halperin and Lynch, 2003; Kronzucker et al., 2003, 2006; Szczerba et al., 2006, 2008a).