Strigolactone enhances aphid resistance by increasing antioxidant enzyme activity in cucumber

Abstract

Aphids are common agricultural pests that often cause significant yield losses in crops. Strigolactones, as plant hormones, play multiple roles in plant growth and development, but their involvement in biotic stress responses remains poorly understood. This study investigates the role of strigolactones in cucumber resistance against aphids. We found that application of the strigolactone analog rac-GR24 significantly enhanced cucumber resistance to aphids, whereas treatment with the strigolactone biosynthesis inhibitor TIS108 reduced this resistance. To further elucidate the mechanism underlying strigolactone-mediated aphid resistance, we examined the biological functions of two key strigolactone biosynthesis genes—carotenoid cleavage dioxygenase 7 (CsCCD7) and carotenoid cleavage dioxygenase 8 (CsCCD8). We successfully generated knockout transgenic lines for both CsCCD7 and CsCCD8. Aphid feeding bioassays revealed that knockout of either gene significantly reduced plant resistance to aphids compared with the control group. Furthermore, CsMYC2–1 and CsMYC2–2 were found to promote the expression of CsCCD7 by binding to its promoter, potentially regulating strigolactone homeostasis in cucumber through a positive feedback loop. Additional experiments showed that application of the strigolactone analog rac-GR24 following aphid infestation reduced reactive oxygen species (ROS) levels in cucumber. Measurement of antioxidant enzyme activities related to ROS scavenging revealed that, compared with the solvent control group, the strigolactone-treated and aphid-infested group exhibited significantly higher antioxidant enzyme activities in leaves. In summary, our findings indicated that CsMYC2–1 and CsMYC2–2 promoted strigolactone biosynthesis by positively regulating the expression of CsCCD7. Strigolactones then enhanced aphid resistance in cucumber by activating antioxidant enzyme activities to scavenge excessive ROS.