NO-mediated protein tyrosine nitration crosstalks with melatonin homeostasis and HO-1 activity in mitigating salt stress in sunflower seedlings

Nitric oxide- mediated tyrosine nitration of proteins influences cellular responses to internal and external stimuli. Identification of melatonin receptor in plants strongly suggests its significant role as a plant hormone. In animals, melatonin primarily functions through receptor-mediated pathways, whereas in plants, it involves a complex interaction with other plant hormones and signaling molecules. Present work aims to examine the effects of NaCl stress and heme oxygenase-1 (HO-1) activity inhibition using its potent inhibitor, zinc protoporphyrin IX (ZnPPIX) on melatonin biosynthesis, its spatial distribution, and tyrosine nitration of proteins in the cotyledons of etiolated sunflower seedlings. The findings highlight possible role/s of tyrosine nitration of cytosolic proteins and its correlation with HO-1 action and melatonin accumulation. Salt stress upregulated the activities of HO-1 and the key regulatory enzyme involved in melatonin biosynthesis [N-acetyl serotonin methyl transferase (ASMT)], leading to enhanced melatonin accumulation and its spatial distribution. This suggests a possible crosstalk between melatonin and HO-1 in mitigating salt stress in sunflower seedlings. These observations further highlight the role of melatonin as a long-distance salt-sensing molecule that regulates HO-1 function in salt stress mitigation. Accumulation of melatonin inside the nuclei of cotyledon cells further suggests the presence of probable melatonin receptors on the nuclear membrane of plant cells. These findings indicate a complex mechanism involved in salt stress tolerance in plants.