Silicon upregulates NHX1 expression to enhance Na⁺ partitioning into vacuoles in leaf mesophyll cells of cucumber under salt stress

Objectives  The mechanisms for Si-mediated salt tolerance vary in different plants. The effects of Si on oxidative damage, Na⁺ accumulation and partitioning, as well as hormone levels were explored in cucumber seedlings under salt stress in this paper.

Methods  A hydroponic culture was conducted using cucumber cultivar in genotype of Mch-4 as experimental material. In the nutrient solution, the NaCl stress level was 65 mmol/L and Na₂SiO₃·9H₂O level was 0.3 mmol/L. After the seedlings were treated for 10 days under stress, the biomass, Na⁺ content and partitioning, the expression of Na⁺ transport related genes and hormone levels were investigated.

Results  Addition of Si could improve the growth of cucumber seedlings and alleviate plant oxidative damage under NaCl stress. Si addition had no significant effects on Na⁺ contents in roots and leaves, significantly reduced the expressions of plasma membrane Na⁺/H⁺ antiporter gene SOS1, had no significant effects on high-affinity K⁺ channel gene HKT1 under salt stress, while promoted the expression of vacuolar Na⁺/H⁺ exchanger gene NHX1. According to the subcellular localization of Na⁺ in leaf mesophyll cells of cucumber, silicon decreased the Na⁺ content in chloroplasts while increased that in vacuoles under salt stress. Added silicon increased the levels of gibberellin, auxin and cytokinin in roots and leaves under salt stress.

Conclusions  Si addition could upregulate the expression of vacuolar Na⁺/H⁺ exchanger gene NHX1, enhance Na⁺ partitioning into vacuoles while reducing Na⁺ content in chloroplasts, so alleviate oxidative damage of cucumber seedlings under salt stress. Si-induced increases in levels of gibberellin, auxin and cytokinin may be involved in regulating salt tolerance of cucumber, the mechanism needs further study.