Salinity‐induced Changes in Gene Expression in the Streptophyte Alga Chara: The Critical Role of a Rare Na+‐ATPase

The primarily freshwater genus Chara is comprised of many species that exhibit a wide range of salinity tolerance. The range of salt tolerance provides a good platform for investigating the role of transport mechanisms in response to salt stress, and the close evolutionary relationship between Charophytes and land plants can provide broader insights. We investigated the response to salt stress of previously identified transport mechanisms in two species of Chara, Chara longifolia (salt‐tolerant), and Chara australis (salt‐sensitive): a cation transporter (HKT), a Na+/H+ antiport (NHX), H+‐ATPase (AHA), and a Na+‐ATPase (ENA). The presence of these candidate genes has been confirmed in both species of Chara, with the exception of the Na+‐ATPase, which is present only in salt‐tolerant Chara longifolia. Time‐course Illumina transcriptomes were created using RNA from multiple time points (0, 6, 12, 24 and 48 h) after freshwater cultures for each species were exposed to salt stress. These transcriptomes verified our hypotheses of these mechanisms conferring salt tolerance in the two species examined and also aided in identification of specific transcripts representing our genes of interest in both species. The expression of these transcripts was validated through use of qPCR, in a similar experimental set‐up used for the RNAseq data described above. The RNAseq and qPCR data showed significant changes of expression mechanisms in C. longifolia (respectively), a down‐regulation of HKT and a substantial up‐regulation of ENA. Significant responses to salt stress in salt‐sensitive C. australis show up‐regulation of NHX and AHA.