Gene expression profiling of antioxidant defense enzymes and Na+ and K+ transporters
To better understand the salinity insensitive mechanism in thesitl1 , the transcription abundance of genes encoding antioxidant enzymes were analyzed using quantitative qRT-PCR (Figure S6 and Figure 6). The expression levels of antioxidant enzyme genes were not changed significantly between the sitl1 and WT under normal condition. However, the transcript abundance of the OsCAT1 showed 0.14- and 0.4-fold decrease in the root and leaf tissues of sitl1 (Figure 6a), respectively, compared to WT plants. Under salinity stress conditions, most antioxidant enzyme genes were strongly upregulated in both root and leaf tissues of the WT plants, whereas in thesitl1 , the genes did not show the upregulated expression levels, especially in leaf tissues. For examples, gene expression levels ofOsCAT2 , OsAPX1 , OsAPX2, OsPOD , OsGR1 ,OsGR2 , OsDHAR1 , and OsMDHAR2 were highly increased in leaf tissues of WT upon salinity stress. However, the expression levels of these genes in sitl1 showed only 1.2-, 1.2-, 1.2-, 1.3-, 1.7-, 2.3-, 1.6-, or 3.0- fold inductions, respectively (Figure 6a). Furthermore, we confirmed that 8 out of 15 antioxidant enzyme genes showed significant decreases in transcript abundance in both root and leaf tissues of the sitl1 compared to WT.
Alterations in Na+ and K+transporter functions were explored by comparing their transcript abundance in the sitl1 and WT. OsLti6a and OsLti6bgenes encode low-molecular weight membrane proteins required for the prevention of excess Na+ entry in the cell (Krishnamurthy, Ranathunge, Nayak, Schreiber, & Mathew, 2011). Transcript levels of OsLti6a and OsLti6b did not show any difference between the sitl1 and WT (Figure 6b). The expression of OsHKT1;5 was higher (2-fold) in the root tissues of thesitl1 compared to WT under normal condition (Figure S6c and Figure 6b). This increased transcript levels of OsHKT1;5 in thesitl1 were maintained when plants were subjected to salinity stress. In contrast, the expression of OsHKT2;1 did not change in both plants under normal condition. Under salinity stress condition, theOsHKT2;1 showed opposite expression patterns in both tissues. TheOsHKT2;1 in sitl1 showed decreased (0.5-fold) transcript accumulation in roots and increased (3.5-fold) transcription accumulation in leaves. A vacuolar Na+/H+ antiporter, OsNHX1 , showed decreased transcript abundance in both roots and leaves in thesitl1 . The transcript abundance of OsSOS1 increased in the roots of sitl1 under normal conditions but decreased transcription levels were observed in both tissues under salinity stress condition. Another known Na+ influx channel ofOsCNGC1 also showed significantly reduced transcript abundance in both tissues of the sitl1 under salinity stress condition. Lastly, OsAKT1 and OsHAK7 genes showed decreased transcript abundance in both tissues of the sitl1 under salinity stress condition.