GhSPDS11 up-regulated spermidine synthase responding to alkaline tolerance in cotton

• Spd is involved alkali tolerance in cotton. • Na 2 CO 3 stress induced the expression of GhSPDS . • The expression of GhSPDS11 regulates stomatal movement. Alkaline stress causes significant adverse effects that slows down the growth of plants and lowers the yield of crops; hence, it is a major challenge in cotton farming. Spermidine (Spd), a vital polyamine, plays a significant role in enhancing plant resistance to stress caused by various abiotic factors. The molecular mechanism of Spd biosynthesis and especially the role of spermidine synthase (SPDS) in tolerance of alkaline stress in cotton is, however, little known. In this study, a systematic comparative analysis of SPDS-associated genes was performed across four representative cotton cultivars ( Gossypium spp.), followed by preliminary functional characterization through promoter cis -acting element profiling. Virus-induced gene silencing (VIGS) was utilized to disrupt GhSPDS11 -mediated Spd biosynthesis. Under alkaline stress, GhSPDS11 -silenced seedlings exhibited 29.14% and 11.12% reductions in superoxide dismutase (SOD) and catalase (CAT) activities, 31.57% and 15.16% decreases in soluble sugar and proline (Pro) content, along with 42.38% and 38.66% increases in malondialdehyde (MDA) and hydrogen peroxide (H₂O₂) compared to controls. Concurrently, silenced plants showed 44.87% fewer open stomata and significant declines in Spd content, relative water content, and biomass. These results indicate the key importance of Spd, which is composed of GhSPDS11 , in improving alkali tolerance in cotton. This research gives good information regarding the molecular processes that take part in the tolerance of cotton to the saline-alkaline soils, and that GhSPDS11 could be a good genetic target in cotton enhancement in this tough agro-climatic condition.