Cytokinin Modulates Responses to Phytomelatonin in Arabidopsis thaliana under High Light Stress

Fine-tuned interactions between melatonin (MT) and hormones affected by environmental inputs are crucial for plant growth. Under high light (HL) conditions, melatonin reduced photodamage in and contributed to the restoration of the expression of the cytokinin (CK) synthesis genes , and and genes for CK signal transduction , and , , and which were downregulated by stress. However, CK signaling mutants displayed no significant changes in the expression of CK genes following HL + MT treatment, implying that a fully functional cytokinin signaling pathway is a prerequisite for MT-CK interactions. In turn, cytokinin treatment increased the expression of the key melatonin synthesis gene under both moderate and HL in wild-type plants. This upregulation was further accentuated in the , , mutant which is highly sensitive to CK. In this mutant, in addition to , the melatonin synthesis genes and , as well as the putative signaling genes and , displayed elevated transcript levels. The results of the study suggest that melatonin acts synergistically with CK to cope with HL stress through melatonin-associated activation or repression of the respective hormonal genes.