Exogenous silicon enhances cadmium-stressed cucumber seedling establishment by modulating antioxidant and hormonal pathways.

It has been demonstrated that silicon (Si) has the capacity to enhance the tolerance of plants to abiotic stress; however, there is a paucity of research regarding its function in alleviating heavy metal toxicity during the processes of seed germination and early seedling growth. In the present study, we firstly examined the effects of Si on the germination and early growth of cucumber (Cucumis sativus L.) seeds under Cd stress, and further investigated the biochemical mechanisms. The results demonstrated that 2 mM Si significantly alleviated Cd-induced inhibition of radicle elongation and growth. Adding Si resulted in a reduction in H2O2 accumulation, an increase in the soluble sugar content, and elevated antioxidant enzyme (SOD, POD, CAT) activities and their related genes (CsSOD(Cu-Zn) , CsPOD-2-4 and CsCAT) expression levels. Application of Si also led to a decrease in ABA concentration and an increase in total GA content. The analysis revealed that the genes involved in ABA biosynthesis (CsNCED1, CsNCED2) were downregulated concurrently with the upregulation of a gene involved in ABA catabolism (CsCYP707A1). Concurrently, the expression of GA content was enhanced, concomitant with the upregulation of CsGA20OX, CsGA3OX2 and CsGA3OX3 . The alleviation mechanism involved in mediating redox and hormonal status was tested by adding ABA, GA biosynthesis and H2O2 scavenging inhibitors. The findings of this study highlight the potential of eco-friendly Si in alleviating Cd-induced inhibition of post-germinative growth and seedling establishment against heavy metal contamination, and the pivotal role of redox and hormonal status involved in seed germination.