Genome-wide analysis of <i>HSP70</i> gene superfamily in kelp ( <i>Saccharina japonica</i> ): identification, characterization, and heat stress-responsive expression profiles

With the ongoing global climate change, high temperatures severely compromise the cultivation of large seaweeds. Saccharina japonica , one of the most economically important alga species, is a cold-temperate seaweed particularly sensitive to heat stress. However, the regulatory mechanisms underlying the response of S. japonica to heat stress remain unclear. Heat shock proteins (HSPs) play a crucial role in protecting plants against various abiotic stresses. Although individual members of the HSP70 family have been analyzed in previous studies, a comprehensive analysis of the HSP70 gene family in S. japonica has not yet been conducted. In this study, we identified fourteen putative HSP70 genes in S. japonica , of which four belong to the HSP110 subfamily and ten to the DnaK subfamily. Furthermore, we characterized the expression patterns of the SjHSP70 genes. Among them, ten HSP70s were up-regulated under heat stress. Subcellular localization analysis further revealed that SjHSP70-03, SjHSP70-06, and SjHSP70-13 were mainly localized in the nucleus/cytoplasm, whereas SjHSP70-11 was also detected in the nucleus and cytoplasm but predominantly in an undetermined compartment. These findings provide valuable insights that will facilitate future studies on the function of HSP70s in S. japonica.