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Water shortage and salinity are the most important factors limiting rice production worldwide. No drought tolerant (DT) or salt tolerance (ST) rice varieties have been commercially released in the past, due largely to the lack of breeding efforts and partially to the complexity of genetics and physiology underlying DT/ST in rice. The real challenge facing plant breeders is how to efficiently develop high yield and DT or ST cultivars for varied stress scenarios of different rice ecosystems. Progress has been recently made in developing DT/ST rice cultivars using the conventional breeding approach at IRRI and hybrid rice cultivars tend to show high yield potential and good levels of water use efficiency or DT. Tremendous QTL mapping efforts in the past decade have identified numerous QTLs affecting DT/ST in rice, but the results have not let to any successful MAS. A new and promising strategy combining BC breeding with designed QTL pyramiding have been practiced at IRRI and in China, in which exploiting useful genetic diversity for DT/ST from the primary gene pool of rice by BC breeding and developing DT/ST introgression lines in elite genetic backgrounds, discovery, allelic mining and characterization of QTL networks for DT/ST, and directed trait improvement by designed QTL pyramiding are well designed and integrated. Many promising DT and ST rice lines have been developed using this strategy, even though the theoretical aspects underlying this strategy remain to be fully established

Substantial advances have been made in breeding wheat for dry environments that will also improve performance in saline environments. These genetic gains have been made by conventional breeding. Further gains in productivity will come from the addition of traits that increase the efficiency of water use in dry soils, and control the uptake of salts from saline soils. Conventional breeding methods will continue to be important to provide farmers with higher yielding varieties in dry or saline soils which are resistant to current diseases and which have the grain quality demanded by competitive markets. Trait-based breeding approaches, which often utilize molecular markers to improve selection efficiency, are starting to deliver new and significant gains. To target the most important traits, it is important to know how they will influence yield. Is it through more water use, more efficient use of water or a higher harvest index? For example, the trait of ‘early vigour’ may be an advantage in some years but in others may lead to the exhaustion of soil water and a low yield. The challenge for breeders will be to efficiently integrate trait-based and molecular methods to increase yield in dry and saline environments

Maize production losses due to drought and salinity prominently affect economies and the livelihoods of millions of people, given the global and regional importance of maize and its pronounced susceptibility to these stress factors. Climate change and accelerating competition for irrigation water are expected to further increase the need for adaptive strategies. There is vast evidence for genetic approaches being able to significantly improve the drought and salinity tolerance of maize. Field-based breeding approaches have resulted in average breeding gains of around 100 kg ha yr under drought conditions, and there are first reports on transgenic drought and salinity tolerance mechanisms increasing maize grain yields under laboratory and field conditions. Drought and salinity tolerance are based on complex genetic systems and successful genetic enhancement programs need to consider gene-by-gene, gene-by environment and gene-by-developmental stage interactions. In the case of drought, field-based and transgenic approaches have resulted in the improvement of diverse and potentially additive tolerance mechanisms. Increasing yields and yield stability of maize in the face of climate change and scarcity of irrigation water will therefore likely be the most successful if complementary investments in field-based and transgenic breeding approaches are being made

Barley is the most tolerance cereal crop for drought and salinity and is an ideal model crop for genetic study of drought and salinity tolerance because of its early maturity, diploid and self-pollination. Selection for drought tolerance in convention breeding programs has achieved significant progress to improve yield and yield stability under drought through direct selection or indirect selection for early vigour, coleoptile length or “stay green”. A large number of Quantitative Trait Loci (QTL) were mapped for drought and salinity tolerance related traits, including physiological biochemical traits such as osmotic adjustment capacity, proline content, stomatal conductance, water-soluble carbohydrates, relative water content, leaf turgor, ABA content, transpiration efficiency, water use efficiency and carbon isotope discrimination; and developmental/ morphological traits such as height, leaf emergence, leaf area index, tiller development, flowering time, maturity rate and root characteristics. QTLs for yield and yield components were also identified under drought. Extensive research has been devoted to the characterization of genes induced or up-regulated by drought or salinity. Numerous candidate genes were identified to associate with tolerance to drought or salinity and some of the candidate genes co-located with the QTLs for drought tolerance. Wild barley was demonstrated as a key genetic resource for drought and salinity tolerance. QTLs from the wild barley increased yield by 12–22% under drought. New germplasm and molecular tools make it possible to develop better barley variety faster for drought or salinity tolerance, but challenges still remain due to complexity of drought and salinity tolerance.

Barley is the fourth largest cereal crop in the world with annual production over 140 million tonnes. It has been used as a staple food for humans, feed for animals, and a key ingredient in beer and whiskey production. Barley has a wider ecological range than any other cereals and is widespread in temperate, subtropical and artic areas, from sea level to heights of more than 4,500 m in the Andes and Himalayas (Bothmer et al., 1995). Barley can be grown on soils unsuitable for wheat, and at altitudes unsuitable for wheat or oats. Because of its salt and drought tolerance, barley thrives in nearly every corner of the earth including extremely dry areas near deserts. Barley is a short-season, early maturing, diploid and self-pollinating crop, thus it is also an ideal model plant for genetic study of drought and salinity tolerance. Several papers have summarized research on barley abiotic stress tolerance including drought and salinity tolerance (Cattivelli et al., 2002; Stanca et al., 2003). In this chapter, we will review recently progress on molecular breeding for saline and drought tolerance in barley

Citrus is a major world horticultural commodity, and most of its world-wide production depends on irrigation, which is inevitably associated with the deterioration of water quality from run-off or ground water. Citrus, like most fruit trees, is relatively salt sensitive. The deleterious effects of salt stress lead to reduction in fruit yield and quality. In recent years, only a few relatively salt-tolerant rootstocks have been obtained through selection and conventional breeding, due to a rather limited existing genetic pool and the long period of time required for experiments. Attempts to regenerate salt-tolerant citrus plants via in vitro production of salt-tolerant callus or mutagenesis have been rather limited and as of yet not in use. Therefore, efforts should be invested to identify traits/genes that have a key role in tolerance to salt in order to speed up the process and to enlarge these genetic resources

QTL analyses revealed that response to salt in citrus is a multigenic trait, as has been shown in other species, but some genes probably exist that have a major impact on salt tolerance and (or) mineral accumulation. Several robust EST databases now exist and are growing, the first microarray chips have been manufactured, and an initial genome sequencing effort is underway. These tools should allow citrus physiologists, biochemists, and geneticists to make much more rapid progress in understanding salt and water stress in the future and to design strategies to ameliorate their effects

Wine grape () is the world’s most important fruit crop both in terms of crop production and economic value. For most crops, water deficit stress has negative implications for production and quality. For wine grape, however, vegetative growth is more sensitive to water-deficit stress than fruit growth. Thus, moderate water-deficit can positively influence the quality of wine produced from grapes harvested from vines grown under regulated (water) deficit irrigation (RDI) and partial root zone drying (PRD) conditions. However, the interaction between water deficit stress and berry composition is complicated by the ability to accurately measure water potential under field conditions and can be influenced by many parameters including timing of stress application within a season and across several seasons, grapevine variety and canopy, leaf to fruit ratio, and soil structure. The mechanistic basis for observed quality differences are poorly understood. However, recent studies using integrated transcriptome and metabolome data sets have revealed potential underlying changes in gene expression and determinants of fruit characteristics that explain the major effects that water deficit treatment can be expected to have on wine quality. Major responses include gene expression changes resulting in alterations in sugar content, anthocyanin accumulation, and decreased organic acid accumulation

Salinity and drought are among the most challenging environmental constraints to crop productivity worldwide. The cultivated tomato, Mill., is moderately sensitive to both of these stresses throughout its ontogeny, including during seed germination, seedling emergence, vegetative growth and reproduction. Limited variation exists within the cultivated tomato for abiotic stress tolerance, however, the related wild species of tomato is a rich source of genetic variation which can be used for crop improvement. During the past several decades this variation has been utilized for characterization of physiological and genetic bases of tolerance to different abiotic stresses, including salinity and drought. Abiotic stress tolerance is a complex phenomenon, controlled by more than one gene and influenced by uncontrollable environmental factors. Furthermore, tomato stress tolerance is a developmentally-regulated state-specific phenomenon, such that tolerance at one stage of plant development is independent of tolerance at other stages. This has been demonstrated by analysis of response and correlated response to selection as well as identification of quantitative trait loci (QTLs) conferring tolerance at different stages. Transgenic approaches also have been employed to gain a better understanding of the genetic and physiological bases of salt and, to a lesser degree, drought tolerance in tomato, and to develop transgenic plants with improved stress tolerance. However, despite considerable traditional genetics and physiological research as well as contemporary molecular marker and transgenic studies in tomato, there is yet no report of any commercial cultivar of tomato with salt or drought tolerance. To achieve this goal, cooperation among plant geneticists, physiologists, molecular biologists and breeders engaged in tomato stress tolerance is imperative. In this chapter, I review the recent progresses in genetics and breeding of salt and drought tolerance in tomato and discuss the prospects for developing commercial cultivars with stress tolerance

Cassava is an important tropical starchy root crop that is used extensively in drought prone tropical regions. It responds to water deficit with a dehydration avoidance and growth arrest syndrome. Carbohydrate is supplied from stems via remobilization. It is very limited in its use of osmotic adjustment, compatible solute synthesis, dehydrin accumulation and other tolerance mechanisms for low water potential. Given the difficulties of conventional breeding of cassava due to its long breeding cycle, heterozygousity, and difficulties in producing seed, an important recent development is the use of molecular markers and marker assisted selection (MAS). MAS is also contributing to the introgression of traits from wild relatives

Defense systems are triggered when plants encounter environmental stresses such as high salinity or drought. Many studies have shown that these defense systems depend on protective mechanisms created by altering the expression levels of stress genes. The agricultural species is an autotetraploid with a highly complicated, quantitative inheritance pattern. Thus, breeding new potato cultivars that are tolerant of saline and drought stress by conventional methods is tedious, difficult and time-consuming, and generally requires between 10 and 15 years. Genetic engineering techniques represent a faster and more reliable way to improve potato cultivars. As a first step towards developing drought- and saline-tolerant potato plants by molecular breeding methods, numerous potato stress genes, including those that code for functional and regulatory proteins, have been isolated and characterized by homologue gene screening, differential screening, microarray analysis and proteome analysis. There have been many attempts around the world to create drought- and saline-tolerant potato plants by introducing abiotic stress genes for functional proteins, such as proline synthesis protein, osmotin-like protein, GPD, trehalose synthesis protein, and regulatory proteins such as StEREBP, CBF and StRD22

Drought and salinity are two important abiotic factors limiting soybean production worldwide and drought alone accounts for about 40% crop loss. Irrigation and soil reclamation are not economically viable options for soybean production under drought and salinity. Hence, genetic improvement for drought and salt tolerance are cost effective. Conventional breeding has made a significant contribution to soybean improvement in the last 50 years. Through conventional breeding, it is easy to manipulate simply inherited qualitative traits which are less sensitive to environmental variation, but quantitative traits like yield or tolerance to abiotic stress are significantly influenced by environment. Most agronomically important traits are quantitatively inherited and are difficult to improve through conventional breeding. Molecular marker technologies can dissect quantitative traits into individual components, known as quantitative trait loci enabling marker assisted selection of desired traits in much shorter time avoiding labor intensive, conventional, phenotypic selection. A molecular breeding approach can supplement the conventional breeding system. Well developed molecular genetic maps, functional genomic resources, and other molecular tools are available for soybean. Effective use of these resources will allow a greater understanding of basic mechanisms of tolerance to abiotic stress. Integration of these genomic tools coupled with well-designed breeding strategies will help to develop soybean varieties with higher tolerance to drought and salt