Cadmium Disrupts Subcellular Organelles, Including Chloroplasts, Resulting in Melatonin Induction in Plants

Abstract

Cadmium is a well-known elicitor of melatonin synthesis in plants, including rice. However, the mechanisms by which cadmium induces melatonin induction remain elusive. To investigate whether cadmium influences physical integrities in subcellular organelles, we treated tobacco leaves with either CdCl2 or AlCl3 and monitored the structures of subcellular organelles—such as chloroplasts, mitochondria, and the endoplasmic reticulum (ER)—using confocal microscopic analysis. Unlike AlCl3 treatment, CdCl2 (0.5 mM) treatment significantly disrupted chloroplasts, mitochondria, and ER. In theory, the disruption of chloroplasts enabled chloroplast-expressed serotonin N-acetyltransferase (SNAT) to encounter serotonin in the cytoplasm, leading to the synthesis of N-acetylserotonin followed by melatonin synthesis. In fact, the disruption of chloroplasts by cadmium, not by aluminum, gave rise to a huge induction of melatonin in rice leaves, which suggests that cadmium-treated chloroplast disruption plays an important role in inducing melatonin in plants by removing physical barriers, such as chloroplast double membranes, allowing SNAT to gain access to the serotonin substrate enriched in the cytoplasm.

Mitochondria: Central Organelles for Melatonin′s Antioxidant and Anti-Aging Actions

“…This would be a very fortuitous arrangement given the fact that, as already mentioned herein, mitochondria have a propensity to produce an abundance of free radicals as a byproduct of oxidative phosphorylation [ 57 , 71 , 142 , 157 ]. Recently published data supports the high likelihood that both mitochondria [ 71 , 100 , 101 , 102 , 160 ] and chloroplasts [ 161 , 162 ] are in fact sites of melatonin synthesis.…”

Melatonin ameliorates aluminum toxicity through enhancing aluminum exclusion and reestablishing redox homeostasis in roots of wheat

“…It was found that Al significantly increased melatonin content in soybean . In other plants, however, it has been reported that melatonin levels decreased, for example, in the roots of Arabidopsis after Al treatment, but not changed in rice seedlings . Most of studies have only employed a single genotype with undefined resistance to Al stress.…”

Exogenous Melatonin Confers Cadmium Tolerance by Counterbalancing the Hydrogen Peroxide Homeostasis in Wheat Seedlings

“…Subsequent studies revealed the important roles of melatonin in regulating plant growth, development, and responses to the various environmental stresses [ 14 , 15 ]. Melatonin acts as bio-stimulator in plants by enhancing the tolerance against abiotic stresses [ 16 , 17 ], including high temperature [ 18 , 19 , 20 ], cold and chilling [ 21 , 22 , 23 , 24 , 25 ], drought [ 26 ], salt [ 27 , 28 , 29 ], heavy metal [ 30 , 31 , 32 , 33 , 34 ], acid rain [ 35 ], and biotic stresses such as pathogen attacks [ 12 ]. Similar to the other abiotic stresses, cadmium pollution was a major environmental concern for its toxic actions to plants and animals [ 36 ].…”