A zeaxanthin epoxidase homolog from the intertidal red alga <i>Pyropia haitanensis</i>: evolutionary divergence and stress-responsive expression

<p style="line-height: 1.6;"> Zeaxanthin epoxidase (ZEP) plays a central role in carotenoid metabolism, but its evolution and function in red algae remain unclear. Here, we comprehensively identified ZEP homologs across red algal lineages through phylogenetic reconstruction, conserved motif analysis, structural comparison, and branch‑specific selection pressure analyses to uncover adaptive evolutionary patterns. The ZEP homolog from<i> Pyropia haitanensis</i> (PhZEP) was selected for functional characterization, including promoter cis‑element analysis, heterologous expression, in vitro enzymatic assays, and expression profiling under high light and desiccation–rehydration stress. Thirteen ZEP homologs were identified and clustered into three clades, indicating early divergence and a cyanobacterial origin. Lineage‑specific structural divergence and purifying selection with relaxed constraints in Bangiophyceae were observed. Recombinant PhZEP specifically converted zeaxanthin to antheraxanthin, but not further to violaxanthin. PhZEP expression was moderately induced by high light and strongly upregulated during desiccation, peaking at 9.52‑fold at 4.8% relative water content. These results demonstrate that PhZEP exhibits specific zeaxanthin epoxidase activity and may be associated with abiotic stress responses, providing insights into carotenoid pathway evolution and photoprotection strategies in red algae.</p>