Abstract
Melatonin plays key roles in development and confers stress tolerance to plants. Serotonin N-acetyltransferase (SNAT) is either the enzyme involved in the last step or the penultimate enzyme of phytomelatonin biosynthesis. To date, SNAT genes have not been characterized in tobacco (Nicotiana tabacum), an economically important plant species. The sequence of the Acetyltransf_7 conserved domain was used as a query sequence, and 12 NtSNAT candidate genes were in turn identified in the genome of tobacco. These NtSNATs could be divided into two groups based on the phylogenetic tree. NtSNAT1 and NtSNAT2 clustered together with the other typical SNATs, but the other 10 NtSNATs separately clustered outside of the typical SNATs. These 10 NtSNATs have only motif 1, whereas representative SNATs, such as NtSNAT1 and NtSNAT2 or a SNAT from cyanobacteria, have five motifs. In addition, NtSNAT1 and NtSNAT2 are highly homologous to the characterized OsSNAT1, 62.95 and 71.36%, respectively; however, the homology between the other 10 NtSNAT genes and OsSNAT1 is low. Concomitantly, it is hypothesized that NtSNAT1 and NtSNAT2 are the homolog of SNATs, whereas the other 10 candidates could be considered NtSNAT-like genes. Furthermore, both Nicotiana tomentosiformis and Nicotiana sylvestris, two diploid ancestor species of N. tabacum, have two SNAT candidates; therefore, it is speculated that gene rearrangement or deletion during the process of genomic stabilization after whole-genome duplication or polyploidization led to the preservation of NtSNAT1 and NtSNAT2 during the evolution of tobacco from the ancestral diploid to the allotetraploid. NtSNAT and NtSNAT-like genes were differentially expressed in all organs under different stress conditions, indicating that these genes potentially associated with plant growth and development and stress resistance. Under different stress conditions, the expression of NtSNAT1 was significantly upregulated upon high-temperature and cadmium stresses, while the expression of NtSNAT2 did not significantly increase under any of the tested stress treatments. These results provide valuable information for elucidating the evolutionary relationship of SNAT genes in tobacco and genetic resources for improving tobacco production in the future.
Highlights
Melatonin (N-acetyl-5-methoxytryptamine) is an evolutionarily conserved pleiotropic molecule that exists ubiquitously in living organisms (Tan et al, 2011; Lee et al, 2014a; Kanwar et al, 2018)
A total of six enzymes, tryptophan decarboxylase (TDC), TPH, tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), acetylserotonin-Omethyltransferase (ASMT), and caffeic acid O-methyltransferase (COMT), participate in the synthesis of melatonin in plants, among which SNAT is either the penultimate enzyme or the last step enzyme involved in melatonin biosynthesis (Back et al, 2016; Hardeland, 2016; Wang et al, 2017; Yu et al, 2018)
Sequence analysis revealed that introns were present in the coding DNA sequences (CDSs) of these genes, except in NtSNAT7 and NtSNAT12, and the number of introns varied from 2 to 8 (Figure 1)
Summary
Melatonin (N-acetyl-5-methoxytryptamine) is an evolutionarily conserved pleiotropic molecule that exists ubiquitously in living organisms (Tan et al, 2011; Lee et al, 2014a; Kanwar et al, 2018). In addition to its roles in plant development, melatonin is involved in the tolerance of plants to a range of biotic and abiotic stresses, such as pathogen infection, drought, high temperature, cold, and salinity (Tal et al, 2011; Byeon et al, 2014; Lee et al, 2014a; Shi et al, 2015a; Zhang et al, 2015; Arnao and Hernández-Ruiz, 2017; Zhao D. et al, 2019). Melatonin is synthesized from tryptophan through four distinct reaction steps. Since the first plant SNAT gene was identified and cloned in rice, homologous SNAT genes in other species, such as cyanobacteria, alga laver, Arabidopsis thaliana, grapevine and apple, have been identified, found to be present at even higher frequencies than the frequency in rice, and enzymatically characterized (Byeon et al, 2013, 2015, 2016b; Wang et al, 2017; Yu et al, 2019)
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