Melatonin alleviates aluminum-induced root growth inhibition by interfering with nitric oxide production in Arabidopsis

Root growth inhibition is the primary symptom of aluminum (Al) toxicity. In this study, the roles of nitric oxide (NO) and melatonin, two ubiquitous signal molecules involved in diverse plant physiological processes and stress responses, were investigated in Arabidopsis responses to Al toxicity. The results showed that nitric reductase- and nitric oxide synthase-dependent NO production contributed to Al-induced root growth inhibition through interfering with the root cell cycle progression and the quiescent center cell homeostasis. The Al-induced NO production was decreased by exogenous melatonin, which was associated with abolishment of Al-induced root elongation and cell cycle arresting, and with the quiescent center activity returning to the Al-free control levels. The expression of AtSNAT (Arabidopsis thaliana serotonin N-acetyltransferase) was down-regulated by Al, which coincided with decreased melatonin accumulation in Arabidopsis. Pharmacological and genetic analyses further confirmed that a decrease in melatonin synthesis enhanced NO production and Al sensitivity. Taken together, the results presented here suggest that melatonin alleviates Al-induced root growth inhibition by interfering with NO-mediated reduction of cell division cycle progression and the quiescent center cellular activity in Arabidopsis roots.