Genome-wide identification and light-quality-dependent characterization of Glutathione S-transferase genes involved in anthocyanin accumulation in Rhododendron

BACKGROUND: Glutathione S-transferases (GSTs) mediate vacuolar sequestration of anthocyanins and are key determinants of flower color. Several Phi (GSTF) and Tau (GSTU) members have been linked to anthocyanin transport, but how GST repertoires in woody ornamentals respond to light quality is largely unknown in Rhododendron. RESULTS: We identified 87 GST genes in the Rhododendron simsii genome and classified them into seven subfamilies, with Tau and Phi predominating. Segmental duplication was the main driver of family expansion, and most promoters carried multiple light-responsive cis-elements. Twelve representative RsGST homologues were mapped to Rhododendron hybridum and showed distinct tissue- and stage-specific expression, including a group (RsGSTF2, RsGSTL7, RsGSTU1) preferentially expressed in floral organs. Under defined light-quality treatments, GST transcripts in buds were rapidly induced by UV-A/blue light, whereas red/far-red light elicited stronger and more sustained induction in fully open flowers. Total petal anthocyanin content increased markedly under red and far-red light, broadly paralleling the expression of RsGSTF2, RsGSTL7 and RsGSTU1. CONCLUSIONS: Our integrative genomic and expression analyzes indicate that the GST family in Rhododendron is tightly connected to light signaling and floral development, and highlight flower-preferential, light-quality-responsive Tau and Phi members as candidate vacuolar anthocyanin transporters for light-dependent flower color formation.