Abstract
Melatonin (MEL) can act as a plant growth regulator and biostimulator in stressful situations. Using MEL in seed pretreatment also affects the future growth of plants. Therefore, this research investigated the effects of MEL on seed germination and seedling growth under NaCl in in vitro conditions. The additional effects of MEL on the accumulation of steviol glycosides (SGs) and on the expression of appropriate genes were also studied. Five μM of MEL was the best concentration for seed germination, while 20 μM exerted a positive impact on the biomass of stevia plantlets. NaCl significantly decreased seed germination, but MEL alleviated this effect when seeds were germinated in 50 mM of NaCl. Under salinity, the values of almost all morphological traits decreased as MEL concentration increased. The highest amounts of stevioside and rebaudioside A (Reb A) were observed as a result of treating seeds with 5 and 20 μM of MEL, respectively. When adding NaCl, positive impacts of MEL on the accumulation of both SGs were also observed. Expression analyses of the genes involved in SGs biosynthesis was explored in seeds and leaves, and the transcripts of key enzymes occurred in both the tissues. However, quantitative polymerase chain reaction (qPCR) analysis showed that all tested genes were upregulated in younger leaves, contrary to older ones. Also in younger, rather than older, leaves SG gene expression varied according to MEL concentration. This study, therefore, presents the promising potential of MEL for improving stevia seed germination under salinity conditions and for enhancing the production of SGs in stevia plants.
Highlights
Melatonin (MEL) (N-acetyl-5-methoxytryptamine) has been detected in many plant species, including both monocotyledonous and dicotyledonous families [1,2,3,4]
This study demonstrates the positive effect soaking seeds in 5 μM and 20 μM of MEL had on stevia germination in normal conditions and under low salinity stress
Lower MEL concentrations promoted the biosynthesis of stevioside and rebaudioside A (Reb A), alleviating salt stress
Summary
Melatonin (MEL) (N-acetyl-5-methoxytryptamine) has been detected in many plant species, including both monocotyledonous and dicotyledonous families [1,2,3,4]. Melatonin enhances germination and stevia plant quality. Exogenous MEL, used for seed germination, affects the future growth of plants. The effect of MEL on leaf and root development, as well as on plant reproductivity, has been described in previous research [8, 10,11,12,13], and the positive impact of MEL on germination and growth has been especially reported under stress conditions [14, 15]. Because of the accumulation of free radicals in plants, can negatively affect seed germination, resulting in reduced plant growth [16]. Various approaches, including applying MEL, have been tested to improve plant response to salt stress [17,18,19]. Published studies have shown that MEL affects the levels of valuable secondary metabolites in in vitro cultures of Rosmarinus officinalis [20] and Leucojum aestivum [21]
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