Silicon reduces cadmium accumulation and toxicity by regulating transcriptional and physiological pathways, and promotes the early growth of tomato seedlings

Cadmium (Cd) pollution is a serious threat to plant growth and human health. Ample evidence suggests that silicon (Si) enhances plant Cd resistance; however, the precise underlying mechanism remains unclear. In this study, tomato (Solanum lycopersicum) was used as the test material for exploring the alleviating effect of 1 mM Na2SiO3 on Cd stress induced by 10 mu M CdCl2. The transcriptional and physiological experiments revealed that (I) Si downregulated the iron-regulated transporter (SlIRT1), zinc and iron regulated transporter-like proteins (SlZIP4), and reduced Cd absorption by tomato seedlings. (II) Si upregulated the genes related to plant Cd resistance (SlPCR), pleiotropic drug resistance (SlPDR), and ATP-binding cassette G transporters (SlABCG), which promoted Cd efflux and suppressed Cd accumulation in tomato seedlings. (III) Si increased the activities of antioxidant enzymes and contents of non-enzymatic antioxidants, reduced the contents of reactive oxygen species and malondialdehyde, and suppressed Cd-induced oxidative damages. (IV) Si inhibited Cd-induced chlorophyll degradation, alleviated damages to chloroplast ultrastructure. (V) Si enhanced the activity of enzymes related to carbon-nitrogen metabolism, and promoted the growth of tomato seedlings. (VI) Si increased the contents of auxin and gibberellin, reduced the salicylic acid content, and maintained growth and resistance. (VII) Si regulated the expression of WRKY, NAC, ERF, MYB, HSF, and bHLH transcription factors, reduced Cd accumulation and improved Cd tolerance in tomato seedlings. These results indicated that Si effectively reduced Cd accumulation in tomato seedlings and improved Cd tolerance, and may therefore be applied for alleviating the inhibitory effect of Cd on crop growth and ensuring food security.