Engineering salt tolerance in crop plants

Abstract

Environmental stress caused by salinity is a serious factor limiting the productivity of agricultural crops, which are predominantly sensitive to the presence of high concentrations of salts in the soil. Although drainage and the supply of high quality water can solve the problem, these measures are extremely costly and not applicable in extensive agriculture. Salt has a detrimental effect on plants because it causes a water deficit, which results in osmotic stress, and because of the effects of excess sodium ions on key biochemical processes. To tolerate high levels of salts, plants should be able to use ions for osmotic adjustment and to accumulate these ions in the vacuole to keep sodium away from the cytosol. It has been shown previously that overexpression of AtNHX1 (a vacuolar Na+–H+ antiport) in Arabidopsis plants, allowed the transgenic plants to grow in 200 mm NaCl, suggesting that crop plants with improved salt tolerance could be engineered.Hong-Xia Zhang and Eduardo Blumwald 1xTransgenic salt tolerant tomato plants accumulate salt in the foliage but not in the fruits. Zhang, H-X. and Blumwald, E. Nat. Biotechnol. 2001; 19: 765–768Crossref | PubMed | Scopus (627)See all References1 now report the generation of salt-tolerant transgenic tomato plants. Whereas wild-type plant growth was severely inhibited by salt, the transgenic plants overexpressing an Arabidopsis vacuolar Na+–H+ antiport were able to grow and produce fruit in the presence of 200 mm NaCl (Fig. 1Fig. 1). The sodium and chloride content of the leaves of the transgenic plants was 20-fold higher that in untransformed plants. This increase in sodium content in the transgenic plants correlated with an increase in the amount of antiport protein and an increase in Na+–H+ antiport activity in the tonoplast vesicles of transgenic plants compared with the untransformed plants. More significantly, whereas the leaves of the transgenic plants accumulated high sodium and chloride concentrations, the tomato fruits displayed low sodium and chloride content.Fig. 1(a) Control and (b) transgenic tomato plants growing in the presence of 200 mm NaCl. (Photographs courtesy of Eduardo Blumwald.)View Large Image | Download PowerPoint SlideIn summary, these results show that the accumulation of sodium into the vacuoles, mediated by the vacuolar Na+–H+ antiport, allows crop plants to minimize the toxic effects of salinity. In spite of many years of conventional breeding for salt tolerance, only a few varieties, with only limited salt tolerance, have been released. Salt tolerance is a complex trait, and a long list of salt-responsive genes has been reported. One school of thought has concluded that salt tolerance will be achieved only after pyramiding several characteristics into a single genotype, where each one alone could not confer a significant increase in salt tolerance. However, Zhang and Blumwald's work shows that it is possible to generate crop plants that can tolerate high salinity levels with the modification of a single gene. This result suggests that with the incorporation of transgenic modification, breeding of salt-tolerant crops will be possible with far fewer target traits than has been anticipated. A substantial area of irrigated land has been damaged by the accumulation of salt, and the process of soil salinization is a permanent threat. The generation of salt-tolerant transgenic plants is an essential step toward solving the problem of feeding the world's rapidly growing population.