Melatonin and its relationship to plant hormones

Abstract

Background: Plant melatonin appears to be a multi-regulatory molecule, similar to those observed in animals, with many specific functions in plant physiology. In recent years, the number of studies on melatonin in plants has increased significantly. One of the most studied actions of melatonin in plants is its effect on biotic and abiotic stress, such as that produced by drought, extreme temperatures, salinity, chemical pollution and UV radiation, among others.

Scope: This review looks at studies in which some aspects of the relationship between melatonin and the plant hormones auxin, cytokinin, gibberellins, abscisic acid, ethylene, jasmonic acid and salicylic acid are presented. The effects that some melatonin treatments have on endogenous plant hormone levels, their related genes (biosynthesis, catabolism, receptors and transcription factors) and the physiological actions induced by melatonin, mainly in stress conditions, are discussed.

Conclusions: Melatonin is an important modulator of gene expression related to plant hormones, e.g. in auxin carrier proteins, as well as in metabolism of indole-3-acetic acid (IAA), gibberellins, cytokinins, abscisic acid and ethylene. Most of the studies performed have dealt with the auxin-like activity of melatonin which, in a similar way to IAA, is able to induce growth in shoots and roots and stimulate root generation, giving rise to new lateral and adventitious roots. Melatonin is also able to delay senescence, protecting photosynthetic systems and related sub-cellular structures and processes. Also, its role in fruit ripening and post-harvest processes as a gene regulator of ethylene-related factors is relevant. Another decisive aspect is its role in the pathogen-plant interaction. Melatonin appears to act as a key molecule in the plant immune response, together with other well-known molecules such as nitric oxide and hormones, such as jasmonic acid and salicylic acid. In this sense, the discovery of elevated levels of melatonin in endophytic organisms associated with plants has thrown light on a possible novel form of communication between beneficial endophytes and host plants via melatonin.

Keywords: ABA; JA; SA; auxin; cytokinin; ethylene; gibberellin; melatonin; phytomelatonin; plant hormone; plant pathogen; plant stress; post-harvest; rhizogenesis; senescence; tropism.

Fig. 1.

Biosynthetic pathway of melatonin in plants. The enzymes of the respective steps are: T5H, tryptophan 5-hydroxylase; TDC, tryptophan decarboxylase; SNAT, serotonin N-acetyltransferase; ASMT, acetylserotonin methyltransferase; and COMT, caffeic acid O-methyltransferase.

Fig. 2.

Model of melatonin/NO/ROS action on biotic stress responses (pathogen resistance). Pathogen attack increases NO and melatonin levels through ROS. In the model of arabidopsis/Pseudomonas syringae DC3000 (avrRpt2), the plant–pathogen interaction revealed that the MAPKKK3 and OXI1 (oxidative signal-inducible1) kinases are responsible for triggering melatonin-induced defence signalling pathways. Isochorismate synthase-1, a key enzyme in salicylic acid synthesis, was upregulated, increasing SA levels and triggering a pathogen-induced response. Also, melatonin and NO were able to induce jasmonic acid (JA) biosynthesis and increase fructose, glucose, sucrose and glycerol levels, all of which activate pathogenesis-related gene expression. Also the melatonin induction of ethylene biosynthesis, through ACC synthase (ACS6) collaborates in the induction of pathogen resistance (PR) proteins such as NPR1, plant defensin (PDF), ethylene insensitive (EIN), enhanced disease susceptibility 1 (EDS1) and phytoalexin deficient 4 (PAD4) factors, all key signalling components in the plant SA- and ethylene-mediated defence responses.

Fig. 3.

Model of melatonin/NO/ROS action on plant hormone-mediated abiotic stress responses. Abiotic stressors induce an increase in endogenous melatonin through the upregulation of melatonin biosynthetic genes. Melatonin increases the nitric oxide (NO) level through (at least) the upregulation of nitrate reductase (which usually reduces nitrate to nitrite but can also reduce nitrite to NO using NADPH as a cofactor). NO induces melatonin biosynthesis, while melatonin can also act as an NO scavenger. Melatonin and NO induce changes in hormone levels through the up-/downregulation of hormone biosynthesis/catabolism enzymes. Also, melatonin and NO change the expression of several transcription factors and hormone signalling elements, which determines the global antistress response. Some plant hormones such as IAA, CKs and ABA can stimulate NO production. As a particular case, melatonin and NO can generate changes in the auxin carriers of cells (PINs 1/3/7), and in auxin signalling transduction genes (IAA19 and IAA24, in the case of melatonin), determining auxin-like responses such as growth, rooting and tropism (localized gradients).