Karrikin1 Enhances Drought Tolerance in Creeping Bentgrass in Association with Antioxidative Protection and Regulation of Stress-Responsive Gene Expression

Karrikins are active components of smoke that can promote seed germination and regulate seedling morphogenesis. However, the role of karrikins as alleviators of abiotic stress remains largely elusive. In this study, we examined whether exogenous application of karrikin(1) (KAR(1)) might improve drought tolerance in creeping bentgrass (Agrostis stolonifera cv. PennA4), and investigated the underlying mechanism. We found that exogenous application of 100 nM KAR(1) enhanced drought tolerance in creeping bentgrass, as manifested by significant increases in leaf relative water content, efficiency of photosystem II, leaf chlorophyll content, proline content, and membrane stability, as well as significantly enhanced activities of antioxidant enzymes. RT-PCR analysis indicated that improved drought stress tolerance by application of KAR(1) might be related to upregulation expression of karrikin-responsive genes (KAI2, MAX2 and AFL1), transcription factors (ABF3, bHLH148, MYB13 and DREB2A), antioxidant defense genes (Cu/Zn-SOD, APX2, CAT1, and POD2), and downregulation expression of chlorophyll-degradation genes (PPH and Chl-PRX). These findings suggest that KAR(1) may promote the drought tolerance of creeping bentgrass by activating karrikin-responsive genes and transcription factors, enhancing proline accumulation and antioxidant capacity, and suppressing leaf senescence under prolonged drought stress.