Mechanism Of Interactions Between Melatonin And Cytokinin In Regulating Arabidopsis Primary Root Growth

The interactions among various phytohormones are extensive and complex.Cytokinin and auxin regulate the growth of Arabidopsis primary root by controlling the root meristem division and differentiation.Melatonin has been widely studied as antioxidants and growth regulator in plants.Related researches showed that melatonin regulated plant roots in a way similar to IAA,while,some results revealed that functions of melatonin in roots were not related with auxin.The recent research indicated that exogenous melatonin at high concentrations limited root meristem size by repressing auxin synthesis and polar transport;however,little information is available related to the role of lower levels of melatonin on these processes.In this study,we used Arabidopsis thaliana wild type Col-0 and an inducible transgenic plant of cytokinin-biosynthesis gene(AtIPTS)IPT8-OX plants as research materials,to analyze the primary root phenotypes,the expression levels of related genes,and quantification of proteins with or without treatment of melatonin,cytokinins,estradiol and polar auxin transport inhibitor(TIBA),respectively or synergistically,and expected to uncover the mechanism of melatonin and cytokinins in regulating the growth of primary root growth in Arabidopsis.The main results were as follows:(1)Melatonin at low levels(<100 nM),functioned as a positive regulator of root growth.Melatonin promoted the primary root growth of Arabidopsis thaliana by increasing the number of cells in root tip meristematic zones and antagonized the inhibitory effect of exogenous cytokinin or induced endogenous cytokinin over-production.(2)Melatonin antagonized the inhibitory effect of cytokinins by down-regulating the expression of cytokinin signaling components(AHK4,AHP2,AHP3,AHP5,ARR15).(3)Melatonin modulates auxin transport by regulating the expressions of auxin polar transport carriers(PIN1,PIN2,PIN3,AUX1 and AXR3),as well as the protein accumulations of PIN2 and AUX1 in root tips.(4)Exogenous treatment with auxin polar transport inhibitor(TIBA)showed that the regulation of melatonin and cytoknin to Arabidopsis primary root depended on auxin transport.Taken together,our data provided evidences that melatonin with low concentrations promoted primary root growth,and revealed the mechanism of melatonin in coordination with other hormones that influence plant growth and development in basal tissues and cells.