Comparative Physiological and Transcriptomic Profiling Reveals the Characteristics of Tissue Tolerance Mechanisms in the japonica Rice Landrace Under Salt Stress

The aim of this study was to characterize the tissue tolerance mechanisms of rice under salt stress. Our preliminary experiment identified a japonica rice landrace Shuzenji-kokumai (SZK), which is considered to be tissue-tolerant because it can maintain better growth than salt-sensitive rice varieties while having a high-Na + concentration in the shoots under salt stress. These mechanisms differ from those of most salt-tolerant rice varieties, which have low Na + concentrations in the shoots. We compared the physiological and molecular characteristics of SZK with those of FL478, a salt-tolerant variety, and Kunishi, a salt-sensitive variety. Under salt stress conditions, SZK accumulated high levels of Na + in the roots, leaf sheaths, and leaf blades, which were almost as high as those in the salt-sensitive Kunishi. Simultaneously, SZK maintained better growth and physiological status, as determined by its higher dry weight, lower electrolyte leakage ratio, and lower malondialdehyde concentration. Expressions of OsNHX1 and OsNHX2 were upregulated in the leaf sheaths of SZK, suggesting that Na + might be compartmentalized in the vacuoles to avoid cytosolic Na + toxicity. In contrast, FL478 showed upregulation of OsHKT1;5 and OsSOS1 in the roots, which may exclude Na + from the shoots. RNA-seq analysis showed that 4623 and 1998 differentially expressed genes were detected in the leaf sheaths and leaf blades of SZK, respectively. Among them, the HSP (heat shock protein) gene expression was highly up-regulated only in SZK, indicating that SZK protects against the protein damage caused by Na + toxicity. Our findings suggest that SZK has atypical survival mechanisms under salt stress conditions. These mechanisms offer potential traits for improving salt tolerance in rice in terms of tissue tolerance.