Expression of a Heat Shock Protein 70 from the Brown Alga Ectocarpus sp. Imparts Salinity Stress Tolerance in Arabidopsis thaliana

The brown alga Ectocarpus sp. belongs to Phaeophyceae, a class of macroalgae in the phylum Heterokonts, which is also known as the Stramenopiles lineage. Ectocarpus is a dominant seaweed in temperate regions around the globe, abundant mostly in the intertidal zone, a habitat with harsh environmental conditions resulting from tidal cycles. Analysis of previously generated transcriptomic data of brown alga showed the up-regulation of the Esi0379_0027 gene, encoding a HSP70 protein, in response to various abiotic stresses. Bioinformatics study demonstrated that the HSP70 protein is soluble, monomeric and well conserved compared to other organisms. Subcellular localization of EsHSP70 revealed that the protein is localized in the nucleus. To explore the role of HSP70 in stress tolerance, it was expressed in Arabidopsis under constitutive and stress inducible promoters. Transgenic Arabidopsis plants generated using both promoters exhibited higher tolerance to salinity stress compared to wild type plant. Transcript analysis of various abiotic stress-responsive genes showed that genes participating in Na⁺ efflux and sequestration, and in ABA-mediated stress tolerance were remarkably upregulated in the EsHSP70-expressing transgenic Arabidopsis. Overall, our results showed that the expression of the EsHSP70 promotes salt stress tolerance by upregulating stress responsive genes in Arabidopsis.