Breeding for Abiotic Stress Resistance: Challenges and Opportunities

Recent experimentation with transgenic plants has led to increased salinity tolerance, with emphasis on the areas of ion homeostasis, osmotic regulation and antioxidant protection. A case study of the major challenges and opportunities to improve stress tolerance in plants using salinity is presented. As different abiotic stresses are inter-related (e.g. salinity and osmotic stress), our ability to improve crop performance may well be determined by combining different, apparently unrelated approaches for introducing several stress tolerance mechanisms into specific crop plants. Introduction Soil salinity is one of the major abiotic stresses reducing agricultural productivity (Boyer, 1982). The levels of salt inimical to plant growth affect large terrestrial areas of the world. It is estimated that more than a third of all of the irrigated land in the world is presently affected by salinity. This is exclusive of the regions classified as arid and desert lands (which comprise 25% of the total land of our planet). The loss of farmable land due to salinization is directly in conflict with the needs of the world population which is projected to increase by 1.5 billion in the next 20 years. The damaging effects of salt accumulation in agricultural soils have influenced ancient and modern civilizations. Although famine in the world nowadays is a complex problem and often not the direct result of an insufficient production of food, there is no doubt that the gains in food production provided by the “Green Revolution” have reached the ceiling. Therefore, increasing the yield of crop plants in optimal soils and in less productive lands, including salinized lands, is essential for feeding the world. The need to produce stress tolerant crops was evident even in ancient times (Jacobsen and Adams, 1958). However, efforts to improve crop performance under environmental stresses have not been much fruitful because the fundamental mechanisms of stress tolerance in plants remain to be completely understood. Epstein et al. (1980) described technical and biological constraints to the problem of salinity. While there appears more success with the technical solutions to the problem, the biological solutions have been more difficult to develop. For the biological approach in raising salt tolerance to work, identification of the genetic basis of stress tolerance and using the requisite salt stress tolerance related genes or QTL (Quantitative Trail Loci) to develop varieties with enhanced salinity tolerance are a pre-requisite. The existence of salt-tolerant halophytes and differences in salt tolerance between genotypes within saltsensitive glycophytes species clearly indicates that there is a genetic basis to salt response. While varietal differences in salt tolerance have been known since the 1930s (Epstein, 1977, 1983) and intra-specific selection for salt tolerance reported in rice (Akbar and Yabuno, 1977) and barley (Epstein et al., 1980), there exists still a large gap in our understanding. Flowers and Yeo (1995) reviewed the evidence for the paucity of salt-tolerant cultivars and concluded that the number was likely to be fewer than 30. Since 1993, there have been just three registrations of salt-resistant cultivars in Crop Science (Owen et al., 1994; Al-Doss and Smith, 1998; Dierig et al., 2001). Two basic genetical approaches currently being utilized to improve stress tolerance include: (1) exploitation of natural genetic variations, either through direct selection in stressful environments or through the mapping of QTLs and subsequent marker-assisted selection and (2) generation of transgenic plants to introduce novel genes or alter expression levels of the existing genes to affect the degree of salt stress tolerance. We discuss these approaches in somewhat detail, focusing on the recent experimentation