Understanding and Improving Salt Tolerance in Plants

even when ECe is 3.0 dS m 1 (Table 1), which in terms of osmotic potential is less than –0.117 MPa (osmotic One-fifth of irrigated agriculture is adversely affected by soil salinpotential 0.39 ECe). At these salinity levels, the ity. Hence, developing salt-tolerant crops is essential for sustaining predominant cause of crop susceptibility appears to be food production. Progress in breeding for salt-tolerant crops has been hampered by the lack of understanding of the molecular basis of salt ion toxicity rather than osmotic stress. Ion cytotoxicity tolerance and lack of availability of genes that confer salt tolerance. is caused by replacement of K by Na in biochemical Genetic evidence suggests that perception of salt stress leads to a reactions and conformational changes and loss of funccytosolic calcium-signal that activates the calcium sensor protein tion of proteins as Na and Cl ions penetrate the hydraSOS3. SOS3 binds to and activates a ser/thr protein kinase SOS2. tion shells and interfere with noncovalent interactions The activated SOS2 kinase regulates activities of SOS1, a plasma between their amino acids. Metabolic imbalances caused membrane Na /H antiporter, and NHX1, a tonoplast Na /H antiby ionic toxicity, osmotic stress, and nutritional defiporter. This results in Na efflux and vacuolar compartmentation. ciency under salinity may also lead to oxidative stress A putative osmosensory histidine kinase (AtHK1)-MAPK cascade (Zhu, 2002). Hence, engineering crops that are resistant probably regulates osmotic homeostasis and ROS scavenging. Osto salinity stress is critical for sustaining food production motic stress and ABA (abscisic acid)-mediated regulation of LEA (late-embryogenesis-abundant)-type proteins also play important and achieving future food security. Understanding the roles in plant salt tolerance. Genetic engineering of ion transporters molecular basis of salt-stress signaling and tolerance and their regulators, and of the CBF (C-repeat-binding factor) regumechanisms is essential for breeding and genetic engilons, holds promise for future development of salt-tolerant crops. neering of salt tolerance in crop plants. Here, we discuss the molecular basis of cellular ion homeostasis, osmotic homeostasis, stress damage control and repair under S is one of the major abiotic stresses that adsalt stress, and their exploitation for genetic engineering versely affect crop productivity and quality. About of salt-tolerant crop plants. 20% of irrigated agricultural land is adversely affected by salinity (Flowers and Yeo, 1995). The problem of soil Sensors of Salt Stress salinity is further increasing because of the use of poor Plants sense salt stress through both ionic (Na ) and quality water for irrigation and poor drainage. In clay osmotic stress signals. Excess Na can be sensed either soils, improper management of salinity may lead to soil on the surface of the plasma membrane by a transmemsodicity whereby sodium binds to negatively charged clay brane protein or within the cell by membrane proteins causing clay swelling and dispersal that makes the soil or Na sensitive enzymes (Zhu, 2003). In addition to less fit for crop growth. According to the USDA salinity its role as an antiporter, the plasma membrane Na /H laboratory, saline soil can be defined as soil having an antiporter SOS1 (Salt Overly Sensitive 1), having 10 to electrical conductivity of the saturated paste extract (ECe) 12 transmembrane domains and a long cytoplasmic tail, of 4 dS m 1 (4 dS m 1 ≈ 40 mM NaCl) or more. Most may act as a Na sensor (Zhu, 2003). This dual role would grain crops and vegetables are glycophytes and are highly be analogous to the sugar permease BglF in Escherichia susceptible to soil salinity even when the soil ECe is 4 coli and the yeast ammonium transporter Mep2p. When dS m 1. Different threshold tolerance ECe and different expressed in Xenopus laevis oocytes Na –K cotransrate of reduction in yield beyond threshold tolerance porters from Eucalyptus camaldulensis Dehnh. show inECe indicate variation in mechanisms of salt tolerance creased ion uptake under hypoosmotic conditions while, among crop species (Table 1). their Arabidopsis homolog do not show this osmosenSoil type and environmental factors, such as vapor sing capacity (Liu et al., 2001). Entry of Na through pressure deficit, radiation, and temperature may further nonspecific ion channels under salinity may cause memalter salt tolerance. Adverse effects of salinity on plant brane depolarization that activates Ca2 channels (Sandgrowth may be due to ion cytotoxicity (mainly due to ers et al., 1999), and thus generates Ca2 oscillations, Na , Cl , and SO 4 ), and osmotic stress (reviewed by and signals salt stress. Cell volume decreases because Zhu, 2002). Most crop plants are susceptible to salinity of turgor loss under salinity-induced hyperosmotic stress may lead to retraction of the plasma membrane from Viswanathan Chinnusamy, Water Technology Centre, Indian Agriculthe cell wall, which is probably sensed by both stretchtural Research Institute, New Delhi, India; André Jagendorf, Departactivated channels and transmembrane protein kinases, ment of Plant Biology, Cornell University, Ithaca, NY14853; Jiansuch as two component histidine kinases and wall-assoKang Zhu, Institute for Integrative Genome Biology and Department ciated kinases (Urao et al., 1999; Kreps et al., 2002; Seki of Botany and Plant Sciences, University of California, Riverside, California 92521. Received 3 Dec. 2003. Symposia. *Corresponding author et al., 2002). Salinity up-regulates the biosynthesis of ([email protected]). the plant stress hormone ABA (Jia et al., 2002; Xiong and Zhu, 2003), and causes accumulation of reactive Published in Crop Sci. 45:437–448 (2005). oxygen species (ROS) (Smirnoff, 1993; Hernandez et al., © Crop Science Society of America 677 S. Segoe Rd., Madison, WI 53711 USA 2001). ABA and ROS also regulate ionic and osmotic 437 Published online January 31, 2005