Identification and characterization of glutathione S-transferase genes in Sargassum thunbergii and their response to cadmium stress

Abstract Glutathione S-transferases (GSTs) play critical roles in cellular detoxification and antioxidant defense, yet their molecular characteristics remain poorly understood in brown macroalgae. In this study, three GST genes (designated as StGST1 , StGST2 , and StGST3 ) were identified from Sargassum thunbergii based on transcriptomic data and were systematically characterized at the molecular level. Conserved domain analysis revealed that StGST1 and StGST3 share structural features typical of Sigma-type GSTs, with N-terminal thioredoxin-like domains and C-terminal GST_C domains, whereas StGST2 belongs to the dehydroascorbate reductase (DHAR)-type family containing a GstA superfamily domain. Phylogenetic analysis demonstrated that StGST1 and StGST3 clustered with Sigma-class GSTs from Ectocarpus siliculosus and Laminaria digitata , while StGST2 grouped with DHAR-type GSTs from higher plants. To examine cadmium responsiveness, a time-matched transcriptomic comparison was performed between cadmium-exposed (5 mg l −1 ) and non-exposed control thalli cultured in parallel for three days. Among the three genes, StGST1 was significantly upregulated under cadmium exposure, identifying it as the principal cadmium-responsive GST isoform in S. thunbergii . These results provide the first molecular characterization of GST genes in S. thunbergii and identify StGST1 as a cadmium-responsive GST, contributing to our understanding of heavy-metal detoxification mechanisms in brown algae.