27. Improving the salt tolerance of proline-accumulated rice by suppressing Na+ absorption

1) Hitachi, Ltd., Life Science Group, Kawagoe, 350-1165 Japan
2) Hitachi, Ltd., Central Research Laboratory, Hatoyama, Saitama, 350-0395 Japan

Salt stress is one of the most serious factors limiting the productivity of rice, as experiencing a water deficit. This limitation occurs as a result of osmotic stress and biochemical perturbations caused by the influx of sodium ions (Na+). The cell cytoplasm of rice accumulates substantial amounts of proline (Pro), an osmotically active compound, to protect against salt stress.

We previously reported a positive correlation between Pro accumulation and salt tolerance in rice (Igarashi et al. 1997). Moreover, studies on transgenic tobacco enhanced

Pro synthesis and transgenic Arabidopsis suppressed Pro degradation clearly demonstrated that Pro accumulation was effective in adapting to salt stress (Kavi Kishor et al. 1995; Nanjo et al. 1999).

To investigate the role of Pro accumulation on salt tolerance in rice, immediately prior to NaCl treatment, we pretreated IR28, widely used as a stress-sensitive breeding line (Akbar 1985), with Pro solution at various concentrations for five hours. Afterwards, we studied the effect on salt tolerance levels (Fig. 1(a)). In previous salt tolerance tests, IR28 did not survive a 72-hour treatment of 250 mM NaCl. By contrast, Dee-gee-woogen (DGWG), reported to be a salt-tolerant cultivar during germination (Tobita et al. 1994), survived at a rate of 90% under the same conditions (Igarashi et al. 1997). As shown in Fig. 1(b), Pro pretreatment was very effective in improving salt tolerance in rice. This was especially true for IR28 with 10 and 100 mM Pro pretreatment, in which high salt tolerance similar to DGWG was observed. In these cases, the survival ratio after 72-hour NaCl treatment increased dramatically from 0 to 90%. By contrast, concentrations of 500 mM decreased the survival ratio.

Salt stress induces biochemical perturbations as a result of the influx of Na+ and the accumulation of compatible solutes. To investigate the correlation between Pro accumulation and Na+ absorption, we compared the contents of Pro and Na+ in IR28, and DGWG and IR28 with a 5-hour, 100 mM Pro pretreatment under salt-stress conditions (Fig. 1(c)). Salt-sensitive IR28 accumulated less Pro, but absorbed more Na+ than DGWG. IR28 with Pro pretreatment, by contrast, rapidly absorbed large amounts of Pro, but Na+ absorption was suppressed to two-thirds that of the salt-sensitive IR28. The Na+ absorption was suppressed only in the leaf blades, not in the leaf sheaths, stems or roots (data not shown). These results suggest that Pro pretreatment increases salt tolerance levels and suppresses Na+ absorption in IR28, and that rapid Pro accumulation effectively improves salt tolerance in rice.


We thank Dr. Yukika Sanada and Dr. Keishiro Wada (Kanazawa Univ., Japan) for measuring Pro content, and Dr. Kazuko Yamaguchi-Shinozaki (Japan International Research Center for Agricultural Sciences, Japan) and Dr. Kazuo Shinozaki (Institute of Physical and Chemical Research, Japan) for their helpful advice.


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