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Rice, a fundamental dietary staple food for over half the global population, particularly in developing nations. It has developed intricate and sophisticated mechanisms to confront environmental stresses such as drought, heat and salinity. In response to stress conditions, rice plants showcase diverse adaptive strategies. Melatonin and dopamine serve as pivotal signaling molecules, contributing significantly to diverse physiological processes in plants, alongside stress ameliorative potentials. The present study was aimed at deciphering the mechanistic roles of these plant neurotransmitter-like ameliorants in rice.
The first experiment was conducted at the Department of Plant Physiology, College of Agriculture, Vellanikkara laid out in a Completely Randomized Design (CRD) with nine treatments, with three replications. Rice seedlings were exposed to three stressors in petriplates namely Methyl Viologen (MV; 2.5 μM and 5 μM for oxidative stress), Dithiothreitol (DTT; 2.5 mM and 5 mM for endoplasmic reticular stress) and Antimycin A (ANT A; 10 µM and 20 µM for chloroplast and mitochondrial stress). Melatonin and dopamine, at concentrations of 100 µM and 0.4 µg/mL, respectively, were used to investigate their ameliorative potentials against the three stressors. The study examined various traits such as growth patterns, biochemical changes and stress tolerance mechanisms to elucidate the potential protective or ameliorative effects of melatonin and dopamine under the given stress conditions.
The seedling assays indicated that the growth of non-stressed ameliorant-treated seedlings exceeded that of stressed ones. Dopamine was effective in mitigating damage caused by oxidative and ER stress, while melatonin demonstrated superior performance in alleviating chloroplast and mitochondrial stress. Malondialdehyde (MDA) content increased significantly under oxidative stress induced by 5.0 μM MV treatment and decreased in the control. Under ER stress, MDA was higher in DTT treated seedlings @ 2.5 mM with (84.13%) and without (94.83%) dopamine. In chloroplast and mitochondrial stress, MDA levels were 25.6% and 82.5% lower in dopamine-treated seedlings stressed with ANT A @ 10 and 20 μM, respectively. Histochemical observations revealed that, methyl viologen induced damage elevated ROS production, as indicated by prominent staining in stressed seedlings, with an ameliorant specific scavenging activity in shoots (melatonin) and roots (dopamine). Noticeably, dopamine outperformed melatonin at higher ER stress levels, in maintaining shoot and root growth, whereas t in chloroplast and mitochondrial stress, dopamine and melatonin had distinct roles in shoot and root growth maintenance, respectively.
The second experiment was carried out in pots at the nethouse facility, Department of Plant Physiology, College of Agriculture, Vellanikkara, in Completely Randomized Design (CRD)

with three replications. One month old rice plants were subjected to four treatments namely a mock/absolute control, Tween-20 (0.05%), Melatonin (100 μM) and Dopamine (0.4 μg/mL). Both dopamine and melatonin treatments constitute Tween-20 as a surfactant. Results indicated that the ameliorants have a distinct physiological role in stress alleviation over control. Plant height variations indicated a more significant increase with Tween-20, followed by melatonin and dopamine, compared to the unsprayed control. Ameliorant treated rice showed enhanced chlorophyll levels, with the highest content observed in Tween-20-treated plants, followed by melatonin and dopamine treatments. Likewise, dopamine treatment resulted in a higher net photosynthetic rate than melatonin, while Tween-20-treated plants displayed the highest net photosynthetic rate and stomatal conductance among all treatments. Soluble sugar and protein contents were on par across the four treatments.
The research also focused on exploring the response of noble prize-winning mechanosensitive ion channel receptors such as TRPV1 and PIEZO mediating human sensory perception of temperature, touch, and cold, to exogenous application of melatonin and dopamine. Acknowledging the potential involvement of plant neurotransmitter-like molecules like melatonin and dopamine in orchestrating sensory responses, the study delved into the plant homologs of these receptors. Phylogenetic analysis of TRPV1 with diverse species indicated a 33% homology between Oryza sativa and Homo sapiens. Further, in silico expression analysis of TRPV1 under diverse stress conditions using the ePlant Stress Expression Browser was studied and interpreted. Mechanosensory receptor expression signatures (OsPIEZO, OsTRPV1, OsPMTR1) and basal transcriptional regulator responsiveness (OsTAF2, OsTAF4, OsTAF6, OsTAF9A, OsTAF9B, OsBTF3), were examined in melatonin and dopamine treated rice leaf tissues. OsTAF4, OsTAF6, OsTAF9A, OsTAF9B, OsBTF3, OsPIEZO, OsTRPV1 and OsPMTR1
were upregulated in dopamine-treated plants (0.4 µg/mL) compared to control, whereas OsTAF4 and OsTAF6 showed similar expression patterns under both dopamine and melatonin administration. OsTAF2 exhibited higher expression in Tween-20 treated leaf tissues. With dopamine treatment, OsTAF9B, OsPIEZO and OsPMTR1 displayed nearly three-fold change in transcript levels over control. OsTRPV1 exhibited one- and two-fold changes under melatonin and dopamine treatments, respectively, marking the first report of mechanosensory receptor responsiveness to plant neurotransmitter-like molecules in rice.