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.
Acknowledgement
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.
References
Akbar, A., 1985. Breeding for salinity tolerance in rice. US-Pakistan
Biosaline Research Workshop (suppl): 1-25.
Igarashi, Y., Y. Yoshiba, Y. Sanada, K. Wada, K. Yamaguchi-Shinozaki and
K. Shinozaki, 1997. Characterization of the gene for delta1-pyrroline-5-carboxylate
synthetase and correlation between the expression of the gene and salt
tolerance in Oryza sativa L. Plant Mol. Biol. 33: 857-865.
Kavi Kishor, P.B., Z. Hong, G.-H. Miao, C.-A.A. Hu and D.P.S. Verma, 1995.
Overexpression of delta1-pyrroline-5-carboxylate synthetase
increases proline production and confers osmotolerance in transgenic plants.
Plant Physiol. 108: 1387-1394.
Nanjo, T., M. Kobayashi, Y. Yoshiba, Y. Kakubari, K. Yamaguchi-Shinozaki
and K. Shinozaki, 1999. Antisense suppression of proline degradation improves
tolerance to freezing and salinity in Arabidopsis thaliana. FEBS
Lett. 461: 205-210.
Tobita, S., I.H. Somantri, Y.-Z. Zhang, T. Nagamine and T. Senboku, 1994.
Geographical distribution of salt tolerance in rice as screened with germination
rate. Breed. Sci. 85 (suppl): 158.