Distinct Transcriptional and Enzymatic Responses of Superoxide Dismutases to Heat Stress in Onion (Allium cepa L.)

High temperature disrupts cellular redox homeostasis by enhancing reactive oxygen species production, requiring coordinated antioxidant regulation. Superoxide dismutases (SODs) are central to this response, but their molecular organization and heat-responsive regulation remain poorly understood in onion (Allium cepa L.). Here, we combined physiological, biochemical, transcriptional, and functional analyses to characterize the onion SOD family. Following exposure to 35 °C, H2O2 accumulation and proline content increased relative to baseline, whereas total SOD activity was significantly higher after 1 d but declined to near-baseline levels after 2 d. Native PAGE showed class-specific responses, with FeSOD and CZSOD activities increasing under heat stress, whereas MnSOD activity remained largely unchanged. Genome-wide analysis identified 12 AcSOD genes comprising nine CZSODs genes, one FeSOD gene, and two MnSODs genes. Most AcSOD genes were induced after 1 d of heat exposure, with AcCZSOD2 showing the strongest transcriptional response. Importantly, transcriptional changes were not always paralleled by isozyme activity, indicating distinct but complementary regulation at the transcript and enzyme levels. Transient expression of AcFeSOD, AcMnSOD1, AcCZSOD2, and AcCZSOD7 in Nicotiana benthamiana significantly increased total SOD activity by approximately 14–22% relative to the control, supporting their contribution to SOD activity in a heterologous plant system. These findings provide an integrated view of SOD regulation during heat exposure in onion and establish a basis for future studies of isoform-specific antioxidant functions.