Abstract
Although melatonin (N-acetyl-5-methoxytryptamine) could alleviate salinity stress in plants, the downstream signaling pathway is still not fully characterized. Here, we report that endogenous melatonin and thereafter nitric oxide (NO) accumulation was successively increased in NaCl-stressed rapeseed (Brassica napus L.) seedling roots. Application of melatonin and NO-releasing compound not only counteracted NaCl-induced seedling growth inhibition, but also reestablished redox and ion homeostasis, the latter of which are confirmed by the alleviation of reactive oxygen species overproduction, the decreases in thiobarbituric acid reactive substances production, and Na+/K+ ratio. Consistently, the related antioxidant defense genes, sodium hydrogen exchanger (NHX1), and salt overly sensitive 2 (SOS2) transcripts are modulated. The involvement S-nitrosylation, a redox-based posttranslational modification triggered by NO, is suggested. Further results show that in response to NaCl stress, the increased NO levels are strengthened by the addition of melatonin in seedling roots. Above responses are abolished by the removal of NO by NO scavenger. We further discover that the removal of NO does not alter endogenous melatonin content in roots supplemented with NaCl alone or together with melatonin, thus excluding the possibility of NO-triggered melatonin production. Genetic evidence reveals that, compared with wild-type Arabidopsis, the hypersensitivity to NaCl in nia1/2 and noa1 mutants (exhibiting null nitrate reductase activity and indirectly reduced endogenous NO level, respectively) cannot be rescued by melatonin supplementation. The reestablishment of redox homeostasis and induction of SOS signaling are not observed. In summary, above pharmacological, molecular, and genetic data conclude that NO operates downstream of melatonin promoting salinity tolerance.
Highlights
Soil salinity is a major factor that significantly influences global agricultural production [1]
The above effect was not found in seedlings supplemented with old so5doiuf 2m2 nitroprusside (SNP), a negative
The above effect was not found in seedlings supplemented with old SNP, a negative control of SNP, which contains no nitric oxide (NO), but nitrate, nitrite, and ferrocyanide [51,52]
Summary
Soil salinity is a major factor that significantly influences global agricultural production [1]. High salinity (mainly NaCl) provokes two primary effects on plants, including ionic and oxidative effects [1,2,3,4]. SOS signaling is a well-known pathway responsible for initiating transport of Na+ out of the cells, or activating an unknown transporter, leading to the sequestration of Na+ in the vacuole [8,9,10]. Another type of Na+/H+ antiporter belongs to the Na+/H+ exchanger (NHX) family, and constitutive overexpression of NHX can increase Na accumulation in vacuoles, and enhance salt tolerance [10]. Superoxide dismutase (SOD), catalase (CAT), and guaiacol peroxidase (POD), are very important parts of this enzymatic system, and normally, plants decrease ROS by upregulating activities of these enzymes [11,12,13]
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