Drought and soil salinity and acidity are among the most important threats to agricultural production, causing severe yield losses of all major food crops worldwide. In China, the north-west region is prone to drought, so water supply is a major limitation for crop production; in south and central China, soil acidity is a major limiting factor reducing crop yield; salinity occurs in large areas in the east coastal region.
Drought resistance has been the subject of many studies in several major food crops, including rice, maize and sorghum (Nguyen et al., 1998). Although many quantitative trait loci (QTLs), which explain certain genetic variations in drought tolerance in experimental populations, have been identified by molecular marker mapping, they are unlikely to have a major role to play in improving the drought tolerance of crops.
There have also been QTL studies on the tolerance of rice to acidic soil conditions, especially with respect to aluminium and ferrous iron toxicity (Wu et al., 1999), showing that major gene loci may be involved in increasing the tolerance of rice plants. This may present an opportunity for using genes from rice itself to improve the tolerance of rice varieties to acidic soils.
A more promising line of research is the use of gene coding for citrate synthase, the enzyme for biosynthesis of citric acid (de la Fuente et al., 1997). Transgenic sugar-beet plants with elevated expression of this gene show an enhanced tolerance to aluminium and also increased uptake of phosphate in the acidic soil as a result of excretion of citrate. This indicates that genetic engineering may be able to produce plants that can grow better in acidic soil even with reduced application of phosphate fertilizers. This work may have tremendous implications in crop improvement, especially for crops grown in tropical and subtropical regions.
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