Characterization of Na⁺ exclusion mechanism in rice under saline-alkaline stress conditions

This study was designed to elucidate the physiological responses of two rice genotypes to different pH levels under high saline stress. A salt-tolerant cultivar, FL478, and a salt-sensitive cultivar, IR29, were exposed to saline-alkaline solutions supplemented with 50 mM Na at pH 9 (severe), pH 8 (moderate), and pH 7 (mild) for three weeks. The results indicated that FL478 is relatively saline-alkaline tolerant compared to IR29, and this was evident from its higher dry mass production, lower Na⁺ concentration in the leaf blades, and maintenance of water balance under both mild and moderate saline-alkaline stress conditions. In both cultivars, Na⁺ concentrations in the leaf blades were considerably higher at pH 8 than at pH 7, indicating that high alkaline stress promoted Na⁺ accumulation under highly saline conditions. FL478 plants had lower Na⁺/K⁺ ratios in leaf blades and leaf sheaths than IR29 plants under saline-alkaline stress at both pH 7 and pH 8. To understand the mechanisms behind the difference in saline-alkaline tolerance between the two rice genotypes, transcript levels of the genes encoding Na⁺ transport proteins were analyzed. In response to mild and moderate saline-alkaline stress conditions, salt-tolerant FL478 had highly induced expression of the OsHKT1;5 gene in the roots, corresponding to lower Na⁺ accumulation in the leaf blades. Induction of high expression of the OsSOS1 gene in the roots of FL478 implied that Na may be effectively exported from cytosols to apoplasts in the roots resulting in sequestration of Na⁺ to outside of the roots and loading Na⁺ in xylem transpiration stream. On the other hand, the salt-sensitive IR29 had lower expression of the genes related to Na⁺ transporters, such as the OsHKT1;5 gene and the OsSOS1 gene, in the roots, leading to higher Na⁺ accumulation in the shoots. Expression of the determinant genes for alkaline tolerance, such as K⁺ and Fe acquisition and acidification of the rhizosphere was highly induced in FL478, but not in IR29. Thus, molecular analysis suggested that genes encoding Na⁺ transport proteins are involved in regulating Na⁺ transport under saline-alkaline stress in both salt-tolerant and salt-sensitive rice cultivars, and this is useful information for improving saline-alkaline tolerance traits of rice in the future.