Applications of nitric oxide and melatonin in improving postharvest fruit quality and the separate and crosstalk biochemical mechanisms

Traditionally, reactive oxygen intermediates (ROIs) were considered to be toxic by-products of aerobic metabolism, which were disposed of using antioxidants. However, in recent years, it has become apparent that plants actively produce ROIs as signaling molecules to control processes such as programmed cell death, abiotic stress responses, pathogen defense and systemic signaling. Recent advances including microarray studies and the development of mutants with altered ROI-scavenging mechanisms provide new insights into how the steady-state level of ROIs are controlled in cells. In addition, key steps of the signal transduction pathway that senses ROIs in plants have been identified. These raise several intriguing questions about the relationships between ROI signaling, ROI stress and the production and scavenging of ROIs in the different cellular compartments.

Melatonin is a pleiotropic molecule with many diverse actions in plants. It is considered primarily an antioxidant with important actions in the control of reactive oxygen and nitrogen species (ROS and RNS), among other free radicals, and harmful oxidative molecules present in plant cells. In addition, plant melatonin is involved in multiple physiological actions, such as growth, rooting, seed germination, photosynthesis, and protection against abiotic and/or biotic stressors. The recent identification of the first plant melatonin receptor opened the door to this regulatory molecule being considered a new plant hormone. However, due to the diversity of its actions, melatonin has also been proposed as a plant master regulator. Here, we discuss the most recent data in respect to both perspectives.

Nitric oxide (NO) serves as a signal in plants. An Arabidopsis mutant (Atnos1) was identified that had impaired NO production, organ growth, and abscisic acid-induced stomatal movements. Expression of AtNOS1 with a viral promoter in Atnos1 mutant plants resulted in overproduction of NO. Purified AtNOS1 protein used the substrates arginine and nicotinamide adenine dinucleotide phosphate and was activated by Ca2+ and calmodulin-like mammalian endothelial nitric oxide synthase and neuronal nitric oxide synthase, yet it is a distinct enzyme with no sequence similarities to any mammalian isoform. Thus, AtNOS1 encodes a distinct nitric oxide synthase that regulates growth and hormonal signaling in plants.