The role of ion‐transporting proteins in the evolution of salt tolerance in charophyte algae

Abstract

Species within the genus Chara have variable salinity tolerance. Their close evolutionary relationship with embryophytes makes their study crucial to understanding the evolution of salt tolerance and key evolutionary processes shared among the phyla. We examined salt‐tolerant Chara longifolia and salt‐sensitive Chara australis for mechanisms of salt tolerance and their potential role in adaptation to salt. We hypothesize that there are shared mechanisms similar to those in embryophytes, which assist in conferring salt tolerance in Chara, including a cation transporter (HKT), a Na+/H+ antiport (NHX), a H+‐ATPase (AHA), and a Na+‐ATPase (ENA). Illumina transcriptomes were created using cultures grown in freshwater and exposed to salt stress. The presence of these candidate genes, identified by comparing with genes known from embryophytes, has been confirmed in both species of Chara, with the exception of ENA, present only in salt‐tolerant C. longifolia. These transcriptomes provide evidence for the contribution of these mechanisms to differences in salt tolerance in the two species and for the independent evolution of the Na+‐ATPase. We also examined genes that may have played a role in important evolutionary processes, suggested by previous work on the Chara braunii genome. Among the genes examined, cellulose synthase protein (GT43) and response regulator (RRB) were confirmed in both species. Genes absent from all three Chara species were members of the GRAS family, microtubule‐binding protein (TANGLED1), and auxin synthesizers (YUCCA, TAA). Results from this study shed light on the evolutionary relationship between Chara and embryophytes through confirmation of shared salt tolerance mechanisms, as well as unique mechanisms that do not occur in angiosperms.

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

“…Phips et al. (2021) make the argument for the former, but further investigation into the presence of ENA in Charophytes could shed light on the important question of the evolution of salt tolerance.…”

“…This study shows no evidence for a strong up-regulation of a novel antiport, and instead, we now hypothesize that a Na + ATPase is responsible for the bulk of the enhanced Na + export under salt culture. That ENA is indeed a Na + ATPase seems reasonable, based on sequence similarities to documented PpENA (Phipps et al 2021) and on its greatly enhanced expression in response to salt stress, when we know there is large increase in Na + export (Kiegle and Bisson 1996). However, proper assignation of this function, particularly because it is rare in plants, can only be determined by appropriate physiological tests.…”

“…Transcriptome assembly. Trimming, quality control, de novo assembly, annotation, and initial quantification were performed as described in (Phipps et al (2021).…”

“…We have previously discovered genes in C. australis and C. longifolia that are homologues to genes in embryophytes that are responsible for these functions, although ENA has been identified only in C. longifolia (Phipps et al. 2021). We explore here the roles of these genes by examining their expression in response to salt stress.…”

“…The fourth mechanism, H + ATPase (AHA), does not directly associate with a sodium ion, but provides the energy source for the activity of the Na + /H + antiport (Gaxiola et al 2007). We have previously discovered genes in C. australis and C. longifolia that are homologues to genes in embryophytes that are responsible for these functions, although ENA has been identified only in C. longifolia (Phipps et al 2021). We explore here the roles of these genes by examining their expression in response to salt stress.…”