Dissecting the Role of Serotonin in Drought Responses of Rocket Plants: From Serotonin Accumulation to Tryptophan-ABA Associations.

In recent years, serotonin has been proposed as a signaling molecule in several plant physiological processes, although evidence supporting this function remains limited to date. In particular, its involvement under water deficit conditions is poorly characterised, especially regarding its mechanism of action. This study aimed to determine the implication of serotonin in the response to water stress in rocket plants through three experimental approaches: (1) analysis of endogenous concentrations of indoleamines, tryptophan, serotonin, and melatonin during the acclimation response to stress, (2) evaluation of the dose-dependent effect following its application in seedlings, and (3) analysis of the impact of serotonin application under field conditions. The results showed an increase in serotonin content under water deficit, which was negatively correlated with plant biomass and stomatal conductance, and positively correlated with abscisic acid (ABA) levels. Serotonin application in seedlings led to an improvement in plant water status in a dose-dependent manner but did not alter ABA content. By contrast, this positive effect was not observed under crop conditions. Moreover, when all datasets were integrated, tryptophan, the precursor of serotonin, exhibited a stronger correlation with ABA than serotonin. Taken together, these findings indicate that serotonin does not appear to play a central role in regulating ABA-mediated drought responses. Instead, the stronger association of tryptophan with ABA, together with its role as a precursor of indolamines and auxin, supports a more prominent role for tryptophan metabolism in the drought response than for serotonin itself.