A Novel Plant Immune Inducer Esculetin Activates Systemic Acquired Resistance against Ralstonia solanacearum via the SA Signaling Pathway

Plant systemic resistance induced by natural compounds is a promising alternative method of disease management. Esculetin is a plant-derived coumarin with a variety of biological activities including antibacterial, anti-inflammatory, antitumor and so on. Previous studies have found that esculetin can effectively inhibit the growth of Ralstonia Solanacearum and can induce resistance to R. Solanacearum in tobacco at a lower concentration. To investigate the induced resistance imparted by esculetin, the effect of esculetin against tobacco bacterial wilt and the mechanism governing this effect were determined. The results showed that esculetin could promote reactive oxygen species (ROS) burst, activate plant defense enzymes activities and upregulate salicylic acid (SA) synthesis as well as the expression of SA response-related genes. Interestingly, the pathways involved in phenylpropanoid and flavonoid biosynthesis, along with the MAPK signaling pathway, were also activated. Thus, esculetin may induce resistance in tobacco plants against R. Solanacearum through the SA pathway. Notably, foliar spraying of esculetin was more effective than Benzothiadiazole (BTH) in controlling tobacco bacterial wilt at 9 dpi, 11 dpi and 13 dpi (66.99%, 44.73% and 39.42%, respectively) than BTH (55.74%, 32.52% and 29.86%, respectively). Moreover, esculetin had no significant effect on the growth of tobacco plants and low toxicity to non-target organisms such as silkworms and earthworms. This study indicates that esculetin has the potential to serve as an environmental-friendly plant immune activator, offering a sustainable approach for the environmentally safe management of bacterial wilt.