Growth activity, rooting capacity, and tropism: three auxinic precepts fulfilled by melatonin

Plant melatonin appears to be a multiregulatory molecule with multiple functions similar to those observed in animals. It induces growth in stems and stimulates root generation. It is also able to delay senescence by protecting photosynthetic systems and related processes. One of the most studied actions of melatonin is its effect on biotic and abiotic stresses in the plant, such as that produced by drought, extreme temperatures, chemical pollution, UV radiation, etc. Recent data have demonstrated its role as a modulator of gene expression in plants. In this review, we compare studies which show that melatonin behaves in a similar way to auxin, and present data that relate the physiological responses produced by melatonin with the action of auxin, such as promoting/inhibiting growth activity and rooting capacity. In addition, for the first time, the data presented demonstrate the possible involvement of melatonin in the tropic response of roots. The possible role of melatonin as a plant regulator and its relationship with auxin action and the signaling molecule nitric oxide is presented and discussed in a hypothetical model.