Salt Stress Tolerance of Plants
Shuji Yokoi, Ray A. Bressan and Paul Mike Hasegawa
Center for Environmental Stress Physiology, Purdue University 1165 Horticulture Building, Purdue University, West Lafayette, IN 47907-1165 USA
Salinity stress negatively impacts agricultural yield throughout the world affecting production whether it is forsubsistence or economic gain. The plant response to salinity consists of numerous processes that must function in coordination to alleviate both cellular hyperosmolarity and ion disequilibrium. In addition, crop plants must be capable of satisfactory biomass production in a saline environment (yield stability). Tolerance and yield stability are complex genetic traits that are difficult to establish incrops since salt stress may occur as a catastrophic episode, be imposed continuously or intermittently, or become gradually more severe, and at any stage during development. However, cell biology and molecular genetics research is providing new insight into the plant response to salinity and is identifying genetic determinants that effect salt tolerance. Recent confirmation that many salt tolerancedeterminants are ubiquitous in plants has led to the use of genetic models, like Arabidopsis thaliana, to further dissect the plant salt stress response. Since many of the most fundamental salt tolerance determinants are those that mediate cellular ion homeostasis, this review will focus primarily on the functional essentiality of ion homeostasis mechanisms in plant salt tolerance. The transportsystems that facilitate cellular capacity to utilize Na+ for osmotic adjustment and growth and the role of the Salt-Overly-Sensitive (SOS) signal transduction pathway in the regulation of ion homeostasis and salt tolerance will be particularly emphasized. A perspective will be presented that integrates cellular based stress signaling and ion homeostasis mechanisms into a functional paradigm forwhole plants and defines biotechnology strategies for enhancing salt tolerance of crops. Keywords: Salt adaptation, ion homeostasis, transport determinants, stress singnaling
Introduction Soil salinity is a major constraint to food production because it limits crop yield and restricts use of land previously uncultivated. The United Nations Environment Program estimates that approximately 20% ofagricultural land and 50% of cropland in the world is salt-stressed (Flowers and Yeo, 1995). Natural boundaries imposed by soil salinity also limit the caloric and the nutritional potential of agricultural production. These constraints are most acute in areas of the world where food distribution is problematic because of insufficient infrastructure or political instability. Water and soilmanagement practices have facilitated agricultural production on soils marginalized by salinity but additional gain by these approaches seems problematic. On the horizon are crop improvement strategies that are based on the use of molecular marker techniques and biotechnology, and can be used in
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conjunction with traditionalbreeding efforts (Ribaut and Hoisington, 1998). DNA markers should enhance the recovery rate of the isogenic recurrent genome after hybridisation and facilitate the introgression of quantitative trait loci necessary to increase stress tolerance. Molecular marker techniques were used successfully to transfer alleles of interest from wild relatives into commercial cultivars (Tanksley and McCouch,1997). The basic resources for biotechnology are genetic determinants of salt tolerance and yield stability. Implementation of biotechnology strategies to achieve this goal requires that substantial research effort be focused to on identify salt tolerance effectors and the regulatory components that control these during the stress episode (Hasegawa et al., 2000b). Further knowledge obtained about...