Melatonin confers fenugreek tolerance to salinity stress by stimulating the biosynthesis processes of enzymatic, non-enzymatic antioxidants, and diosgenin content

Abstract

Salinity-induced stress is widely considered a main plant-growth-limiting factor. The positive effects of melatonin in modulating abiotic stresses have led this hormone to be referred to as a growth regulator in plants. This study aims to show how melatonin protects fenugreek against the negative effects of salt stress. Different amounts of melatonin (30, 60, and 90 ppm), salinity stress (150 mM and 300 mM), and the use of both salinity and melatonin were used as treatments. The results showed that applying different melatonin levels to salinity-treated fenugreek plants effectively prevented the degradation of chlorophyll a, chlorophyll b, total chlorophyll, and carotenoid contents compared with salinity treatment without melatonin application. Besides, melatonin increases the biosynthesis of enzymatic and non-enzymatic antioxidants, thereby adjusting the content of reactive oxygen species, free radicals, electrolyte leakage, and malondialdehyde content. It was observed that applying melatonin increased the activity of potassium-carrying channels leading to the maintenance of ionic homeostasis and increased intracellular water content under salinity stress. The results revealed that melatonin activates the defense signaling pathways in fenugreek through the nitric oxide, auxin, and abscisic acid-dependent pathways. Melatonin, in a similar vein, increased the expression of genes involved in the biosynthesis pathway of diosgenin, a highly important steroidal sapogenin in medical and food industries, and hence the diosgenin content. When 150 mM salinity stress and 60 ppm melatonin were coupled, the diosgenin concentration rose by more than 5.5 times compared to the control condition. In conclusion, our findings demonstrate the potential of melatonin to enhance the plant tolerance to salinity stress by stimulating biochemical and physiological changes.

Keywords: fenugreek and diosgenin; melatonin; physiological changes; salinity stress.

Figure 1

Biosynthesis pathway of diosgenin from farnesyl diphosphate in fenugreek. Acetyl-coenzyme A (acetyl-CoA) converts to cycloartenol via a series of multiple reactions. Cycloartenol converts to either cycloartenol, leading to cholesterol formation, or 24 methylene cycoartanol, leading to stigmasterol. This branch-point has a great influence on disogenin biosynthesis. Cholesterol then converts to 3ß -cholesta- 5-en- 3, 22-diol, which ultimately results in diosgenin production (Mohammadi et al., 2020).

Figure 2

The mean comparison effects of melatonin concentrations and different levels of NaCl on (A) Total flavonoid, (B) Total phenol, (C) Sugar content. Duncan method at 1% probability level was applied to compare mean values. The columns having similar letters had no significant difference.

Figure 3

The mean comparison effects of melatonin concentrations and different levels of NaCl on (A) Endogenous melatonin content, (B) Abscisic acid content, (C) Auxin content. Duncan method at 1% probability level was applied to compare mean values. The columns having similar letters had no significant difference.

Figure 4

The mean comparison effects of melatonin concentrations and different levels of NaCl on (A) Potassium content, (B) Chlorine content, (C) Sodium content. Duncan method at 1% probability level was applied to compare mean values. The columns having similar letters had no significant difference.

Figure 5

The mean comparison effects of melatonin concentrations and different levels of NaCl on the expression of (A) CAS, (B) SQS, (C) SEP, (D) SSR. Duncan method at 1% probability level was applied to compare mean values. The columns having similar letters had no significant difference.

Figure 6

The mean comparison effects of melatonin concentrations and different levels of NaCl on the expression of (A) SMT, (B) BGL, and (C) Diosgenin content. Duncan method at 1% probability level was applied to compare mean values. The columns having similar letters had no significant difference.