Thymol Stimulates Lateral Root Formation via Regulating Endogenous Reactive Oxygen Species

The application of an environmentally friendly plant growth regulator to regulate plant growth and development represents a promising strategy for sustainable agriculture. Thymol is a kind of plant-derived natural compound. We have found that thymol is a potential biostimulant with the capability to trigger plant defense against abiotic stresses. Little is known about whether and how thymol modulates plant root system architecture. In this study, physiological, histochemical, and molecular approaches were applied to identify the role of thymol in promoting lateral root development in watermelon seedlings. Thymol significantly promoted LRP (lateral root primordia) initiation and lateral root formation. Rboh (respiratory burst oxidase homolog)-dependent reactive oxygen species (ROS) generation was involved in thymol-promoted lateral root development from LRP. Then, the Rboh gene family with nine members (ClRboh1–ClRboh9) was identified from watermelon genome. Thymol significantly induced the expression of a set of ClRbohs in roots. These results suggested that thymol was able to stimulate lateral root formation by triggering Rboh-dependent ROS production. These findings may help understand the biological function of thymol as an elicitor of lateral root in both applied and fundamental study.