Melatonin Antagonizes Cytokinin Responses to Stimulate Root Growth in Arabidopsis

Melatonin functions as the key growth regulator in various plant species. The mechanisms of the interactions between melatonin and cytokinins remain largely unknown. In this study, the kinetic effects of melatonin over a range of concentrations were investigated. The results showed that melatonin functioned as a positive regulator of root growth ranged from 0.1 to 100 nM. In contrast, exogenous cytokinin at 0.5–1 nM and overexpression of cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE 8 (IPT8) inhibited primary root growth. Combined treatments with melatonin and cytokinin indicated that melatonin antagonized the inhibitory effect of cytokinin 6-benzylaminopurine (6-BA) on primary root elongation. Further analysis revealed that melatonin promotes primary root growth by modulating expression and distribution of auxin efflux transporters PIN2/3 and influx transporter AUX1. Moreover, the cytokinin signaling components AHK4, AHP2/3/5, and type-A ARR15 were down-regulated after melatonin treatment. The polar auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) impaired the promotive effect of melatonin on primary root growth, indicating that auxin transport is essential in melatonin-mediated root growth. Taken together, our data provided evidence to show that melatonin regulates primary root growth in coordination with cytokinin partially through auxin-dependent pathway.