Spermidine (Spd) as a modulator of osmotic, redox and ion homeostasis in common bean seedlings under salinity stress: Physiological, biochemical and molecular aspects

Abstract

Spermidine (Spd), a naturally occurring molecule in plants, is being investigated for its potential to improve the bean’s tolerance to salinity stress. This study explores how common beans (Phaseolus vulgaris L.) can be helped to thrive in salty environments. This research involved a group receiving regular watering with half strength Hoagland’s solution (HS), and other group watered with a modified form of the same solution containing 75 mM NaCl to stimulate salt stress. Seedlings were treated with foliar applications of Spd (0, 0.5, 1 mM) at five intervals: 15, 20, 25, 30, and 35 d after sowing. Applying Spd significantly reduced damage caused by salt and protected essential pigments for photosynthesis (total chlorophyll by 33% and carotenoids by 57% over than the Spd-untreated plants under saline conditions). Additionally, 1 mM Spd enhanced the antioxidant enzyme activities, i.e., superoxide dismutase, catalase, and ascorbate peroxidase by 15%, 55% and 4.6% and promoted the buildup of beneficial compounds like proline, free amino acids and sugars. Remarkably, exogenous Spd application reduced Na⁺ accumulation while improving K⁺ content in common beans under salinity stress. Additionally, it enhanced the cell membrane stability index (CMSI) during stressful situations, the maximum CMSI (94.31%) was recorded with 1 mM Spd under non-stress conditions, whereas the minimum CMSI (72.97%) was registered under salinity without Spd. Under salinity condition, the real-time quantitative PCR (qRT-PCR) results demonstrated a significant increase in the expression of the vacuolar-localized Na⁺/H⁺ antiporter protein (NHX1), and the multifunctional osmotic protection protein (Osmotin) in comparison to the control. Furthermore, common bean treated with Spd showed significantly higher expression levels of SOS1, NHX1, and Osmotin, under normal and salinity conditions, with the highest gene expression observed under 1 mM spermidine treatment under saline conditions. Spermidine at 1 mM notably decreased malondialdehyde levels compared to 0.5 mM (10.68 vs. 12.11 nmol g^-¹ FW). Overall, the findings propose that Spd may be an effective tool for cultivating common beans in saline areas, potentially paving the way for increased food production in challenging environments.