Mechanisms of salt tolerance in plants

The increasing prevalence of soil salinity is one of the most significant obstacles to improving crop productivity. Recent advances in genetic and molecular analysis of Arabidopsis thaliana mutants, ion transporters and stress signaling proteins have improved our understanding of the mechanisms of cellular ion homeostasis and its regulation in plants. Since Na toxicity is the principal stress component in saline soils, much research has focused on the identification of ion transporters and regulatory mechanisms that mediate Na+ homeostasis and maintenance of a high cytoplasmic K/Na ratio. The SALT OVERLY SENSITIVE (SOS) signaling pathway, composed of the SOS1, 2 and 3 proteins, has emerged as a key factor in the detection of and tolerance to salt stress. Recent evidence suggests that the SOS pathway may regulate several ion transport mechanisms critical for salt tolerance. Salt stress is a serious environmental factor limiting crop productivity and affects about 20% of irrigated agricultural land (Flowers and Yeo, 1995). Progress in breeding for salt-tolerant crops has been limited because of inadequate understanding of the molecular basis of salt tolerance mechanisms in plants. The adverse effects of saline soil include 1 nutrient constraints caused by reduced uptake of potassium, phosphorous, nitrate and calcium; 2 ion cytotoxicity due to accumulation of sodium, chloride and sulfate; 3 osmotic stress that results from relatively high solute concentrations in the soil; and 4 oxidative stress caused by the accumulation of reactive oxygen species that damage membrane lipids, proteins and nucleic acids. Ion homeostasis is a fundamental cellular mechanism by which all living cells maintain an intracellular ionic composition. One of the most detrimental effects of salt stress is to disrupt the ion homeostasis mechanisms of the plant. In particular, the similar radii of Na and K make it difficult for transport proteins to distinguish between these two ions. Thus, under conditions of high external Na, there is substantial uptake of Na through K transporters or channels (Blumwald et al. 2000). This Na entry disrupts the normally high cytosolic K/Na ratio and causes numerous detrimental effects. Therefore, maintenance of a high cytosolic K/Na concentration is a key requirement for plant salt tolerance (Glenn et al. 1999). There are several possible strategies that plants could employ to avoid a damaging decrease in the K/Na ratio: reduce entry of Na into the cell, remove Na from the cell, or compartmentalize Na into the vacuole where it cannot disrupt cellular function. In this article, we discuss recent advances in understanding the molecular mechanisms of ion homeostasis in plants and, in particular, the role of Arabidopsis thaliana SALT OVERLY SENSITIVE (SOS) pathway in the maintenance of ion homeostasis under saline conditions. Sodium uptake Both low affinity and high affinity transport systems are involved in determining the cellular K/Na ratio during salt stress. Both AKT1, an inward-rectifying K channel of the Shaker type family, as well as LCT1, a low-affinity cation transport channel in wheat, have been shown to mediate low affinity Na influx during salt stress (Schachtman, 2000). In contrast, the high-affinity carrier HKT1, a Na-K symporter, is required for continued K uptake during salt stress (Uozumi et al. 2000). High external Na concentrations inhibited HKT1-mediated K influx (Rubio et al. 1995) and transgenic wheat with reduced HKT1 expression exhibited significantly less Na uptake and enhanced tolerance to salinity (Laurie et al. 2002). This suggests that HKT1 is an important determinant of salt tolerance. Sodium extrusion and the SOS pathway The Arabidopsis sos1, 2 and 3 mutants, were isolated in a genetic screen for plants hypersensitive to NaCl. Subsequent work has shown that these three SOS proteins are components of a stress-signaling pathway controlling ion homeostasis (Zhu, 2003). SOS1 was the first protein to be identified as a plasma membrane Na/H antiporter involved in Na efflux (Shi et al. 2000, Qiu et al. 2002). SOS1 plays a crucial role in sodium efflux from root cells and long-distance Na transport from roots to shoots (Shi et al. 2002). Transgenic plants overexpressing SOS1 exhibited enhanced tolerance to NaCl because they accumulated less Na in the transpirational stream and in shoots as compared with wild-type plants (Shi et al. 2002). An additional role for SOS1 as a Na sensor, possibly involving the cytoplasmic, hydrophilic C-terminal domain of the protein, has been suggested (Zhu, 2003). SOS2 is a ser/thr protein kinase capable of autophosphorylation. It has an N-terminal catalytic domain similar to that of yeast sucrose nonfermenting 1 (SNF1) and mammalian AMP-activated protein kinase (AMPK) and a C-terminal regulatory domain (Liu et al. 2000). SOS3 is a myristoylated calcium-binding protein that is thought to respond to salt-induced Ca transients in the cytosol (Liu and Zhu, 1998). SOS3 shares significant sequence similarity with yeast calcineurin B subunit and animal neuronal calcium sensors (Liu and Zhu, 1998). SOS3 physically interacts with the regulatory region of SOS2, activates its protein kinase activity in a Ca-dependent manner and recruits it to the plasma membrane (Halfter et al. 2000). The SOS3/SOS2 complex then phosphorylates and activates SOS1, which results in the efflux of Na ions, thereby restoring cellular ionic balance (Quintero et al. 2002). Co-expression of SOS1, 2 and 3 in a salt-sensitive yeast mutant led to enhanced tolerance to Na (Qiu et al. 2002), further demonstrating the importance of the SOS signaling pathway in salt tolerance. In addition to activating Na efflux by regulating the activity of the SOS1 antiporter, the SOS pathway may also turn off Na+ influx. The salt-sensitive phenotype of sos3 was suppressed in an athkt1 mutant (Rus et al. 2001). This suggests that the SOS3/SOS2 complex may also negatively regulate HKT1 function. It is possible that additional salt tolerance factors regulated by the SOS signaling pathway remain to be identified. Vacuolar compartmentalization Vacuolar compartmentalization of Na ions is an efficient mechanism to maintain a lower concentration of Na in the cytosol. Transport of Na into the vacuole is mediated by a Na/H antiporter. The proton-motive force for this transport is provided by both a proton pumping ATPase and a H-pyrophosphatase. Overexpression of a vacuolar H-pyrophosphatase (AVP1) in Arabidopsis enhanced sequestration of Na into vacuoles and led to increased salt tolerance (Gaxiola et al. 2001). Likewise, transgenic plants overexpression the tonoplast Na/H antiporter, AtNHX1 also exhibited increased salt tolerance (Apse et al. 1999). Here again, the SOS regulatory pathway may have a role. A study by Qiu et al. (2003) showed that the addition of a constitutively active SOS2 allele into a sos2 mutant background increased tonoplast Na/H antiporter activity. However, the antiporter activity was not affected by the sos3 mutant. Regulation of tonoplast Na transport may instead occur through SOS3-like calcium binding proteins (SCaBPs that have been shown to interact with SOS2).