Deciphering melatonin biosynthesis pathway in Chenopodium quinoa: genome-wide analysis and expression levels of the genes under salt and drought.

Main conclusion: In this study, we identified a total of ten melatonin biosynthesis genes (3 TDCs, 2 TSHs, 3 SNATs, and 2 ASMTs) in Chenopodium quinoa through bioinformatics methods, and analyzed physiological traits and gene expression levels in drought- and salt-treated plants with or without melatonin. Gene expression levels showed variations depending on tissues, genotypes, and abiotic stress. Melatonin is involved in distinct biological processes, such as growth, development, and stress response in plants. The tryptophan decarboxylase (TDC), tryptamine 5-hydroxylase (T5H), serotonin N-acetyltransferase (SNAT), and N-acetylserotonin O-methyltransferase (ASMT) enzymes are involved in melatonin biosynthesis. Exogenous melatonin reduces the adverse effects of salt stress in different plants, but the roles of melatonin biosynthesis pathway in quinoa (Chenopodium quinoa) remain elusive. This study aims to identify and characterize the melatonin biosynthetic genes encoding TDCs, T5Hs, SNATs, and ASMTs in C. quinoa genome through bioinformatics methods and determine their transcript abundances under salt and drought stress. In total, ten genes were identified in C. quinoa genome, including 3 TDCs, 2 TSHs, 3 SNATs, and 2 ASMTs. TDCs have a pyridoxal-dependent decarboxylase domain, T5Hs possess a cytochrome P450, SNAT proteins contain the Acetyltransf_1 domain, and ASMTs include the O-methyltransferase domain. We also examined some physiological characteristics such as growth and water relations along with electrolyte leakage. For that purpose, two quinoa genotypes (Salcedo and Ames 1377) were subjected to salt and drought stress, with or without melatonin. Exogenous melatonin remarkably reduced the negative effects of salt and drought on shoot length, RWC, and electrolyte leakage in the sensitive Salcedo genotype. However, it showed limited impact on the stress-tolerant Ames 1377 genotype. Expression patterns showed variations depending on tissues, genotypes, and the type of abiotic stress. Promoter analysis indicated that the cis-elements in TDC, T5H, and SNAT promoters were mostly associated with stress-response, while those in ASMT promoters were related to light response.