SOS1 expression in Salicornia

The plasma membrane Na + /H + -antiporter SOS1 is essential for salt tolerance in plants. In Chapter 4, we observed a ten-fold higher constitutive expression of SOS1 in the highly salt tolerant salt-accumulating halophyte Salicornia dolichostachya in comparison with a lower inducible SOS1 expression in its glycophytic family member Spinacia oleracea. The objective of this study was to investigate the origin of these differences in SOS1 expression. Copy number variation did not contribute to the differences in expression, since S. oleracea and S. dolichostachya had both two copies of SOS1. Genome walking generated 596bp of the S. dolichostachya SOS1 promoter, and a de novo draft genome assembly of the diploid Salicornia brachystachya generated 2372bp of the SbSOS1 promoter. Analysis of the cisregulatory DNA elements in these promoters in comparison with the Arabidopsis thaliana SOS1 promoter showed that the Salicornia SOS1 promoters contained fewer cis-regulatory elements with an ABRE-variant than the AtSOS1 promoter. Moreover, the DRE/CRT element was uniquely present in the AtSOS1 promoter. These elements are the binding sites for stressresponsive transcription factors, and might be responsible for the induction of AtSOS1 in response to salt stress. Absence of these elements in the SbSOS1 promoter might explain the lack of SOS1 inducibility in Salicornia in response to salinity, but not the high-level expression of SdSOS1.