Foliar application of Karrikin<sub>1 </sub>enhanced heat tolerance of creeping bentgrass by activating antioxidant defense, signaling transduction, and stress responsive pathways

Karrikin (KAR), a recently identified plant growth regulator, functions in seed germination, seedling development, and abiotic stress adaptation. Despite its recognized biological functions, the regulatory role and underlying mechanisms of KAR in mediating heat tolerance of turfgrass remain largely unknown. In the present study, creeping bentgrass plants were treated with or without KAR₁ (100 nM) and subjected to two temperature treatments (25/20 and 35/30 °C, day/night) for 20 d. Physiological parameters, antioxidant enzyme activities, as well as expression levels of genes involved in stress signaling and heat stress responses were analyzed. The results showed that heat stress caused negative effects on creeping bentgrass, including leaf water loss and membrane lipid peroxidation. Exogenous application of KAR₁ alleviated heat-induced damage on creeping bentgrass, resulting in improved leaf relative water content, decreased leaf electrolyte leakage, and increased antioxidant enzyme activities (including ascorbate peroxidase, catalase, and superoxide dismutase). In addition, KAR₁ treatment up-regulated the expression of KAR/KAI2-Ligand signal transduction genes (<italic>AsKAI2</italic>, <italic>AsDLK2</italic>), phytohormone biosynthesis and signaling genes (<italic>AsSnRK2.6</italic>, <italic>AsICS1</italic>), transcription factors (<italic>AsNAC053</italic>, <italic>AsbZIP58</italic>, <italic>AsHSFB-2b</italic>, <italic>AsHSFB-4d</italic>), antioxidant defense genes (<italic>AsAPX1</italic>, <italic>AsAPX2</italic>, <italic>AsCATA</italic>, <italic>AsCATC</italic>, <italic>AsFe-SOD</italic>, <italic>AsCuZn-SOD</italic>, <italic>AsMn-SOD</italic>), and heat shock proteins (<italic>AsHSP12</italic>, <italic>AsHSP16</italic>, <italic>AsHSP70</italic>, <italic>AsHSP90.5</italic>) under heat stress. These results suggested that KAR₁ might act as an effective agent to enhance the heat tolerance of creeping bentgrass, which provides a new perspective on cool-season lawn maintenance.