Abstract
Drought stress is a major constraint to the production and yield stability of soybean [Glycine max (L.) Merr.]. For developing high yielding varieties under drought conditions, the most widely employed criterion has traditionally been direct selection for yield stability over multiple locations. However, this approach is time consuming and labor intensive, because yield is a highly quantitative trait with low heritability, and influenced by differences arising from soil heterogeneity and environmental factors. The alternative strategy of indirect selection using secondary traits has succeeded only in a few crops, due to problems with repeatability and lack of phenotyping strategies, especially for root-related traits. Considerable efforts have been directed towards identifying traits associated with drought resistance in soybean. With the availability of the whole genome sequence, physical maps, genetics and functional genomics tools, integrated approaches using molecular breeding and genetic engineering offer new opportunities for improving drought resistance in soybean. Genetic engineering for drought resistance with candidate genes has been reported in the major food crops, and efforts for developing drought-resistant soybean lines are in progress. The objective of this review is to consolidate the current knowledge of physiology, molecular breeding and functional genomics which may be influential in integrating breeding and genetic engineering approaches for drought resistance in soybean.
Highlights
Soybean is the world’s leading economic oilseed crop
During our survey of the literature, we found that there have been only limited efforts to study soybean genotypic variation for important physiological traits related to drought tolerance, such as cell membrane thermostability, osmotic adjustment, canopy temperature depression and metabolic traits such as antioxidants and ABA
More concentrated efforts are needed to screen for resistant germplasm, discover new candidate genes and combine these genes for higher levels of drought tolerance in soybean
Summary
Soybean is the world’s leading economic oilseed crop. Processed soybeans are the largest source of vegetable oil and protein feed. In addition to being a source of macronutrients and minerals, soybeans contain secondary metabolites such as isoflavones (Sakai and Kogiso 2008), saponins, phytic acid, oligosaccharides, goitrogens (Liener 1994) and phytoestrogens (Ososki and Kennelly 2003). Global production of soybean in 2007 was around 219.8 million metric tonnes (mmt). While soybean has long been important in Japan for the production of traditional foods such as tofu, miso, shoyu and vegetable oil, the consumption of soybean-based products is increasing worldwide because of the reported beneficial effects including lowering of cholesterol, prevention of cancer, diabetes and obesity, and protection against bowel and kidney diseases (Friedman and Brandon 2001). Soybean is viewed as an attractive crop for the production of biodiesel (Pimentel and Patzek 2008). It has the ability to fix atmospheric nitrogen (Burris and Roberts 1993) and requires minimal input of nitrogen fertilizer which
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