Silicon mediates sodium transport and partitioning in maize under moderate salt stress

Silicon (Si) is known to alleviate salt stress in various crops; however, the influence of Si on sodium (Na) transport and partitioning at the tissue, cell and organelle levels is poorly understood. Maize (Zea mays L.) hybrid sensitive to salt stress was exposed to moderate salt stress (40 mM NaCl; simulating conditions in salinized agricultural soils) without or with supply of 1.5 mM Si(OH)4. We investigated the expression of SOS genes encoding Na+ efflux transporter in various root tissues of maize, paralleled by measurements of tissue Na concentration. In addition, subcellular localization of Na (using Na fluorescent dye) within the leaf mesophyll cells was also performed. Silicon supplied plants accumulate less Na in both root apex and cortex, but allocate more Na+ to the leaves via the xylem. This was accompanied by increased expression of ZmSOS1 and ZmSOS2 in the root apex and cortex facilitating Na+ exclusion, and in the root stele for enhanced Na+ loading into the xylem. Also, Si down-regulated the expression of ZmHKT1 in the root stele, which further decreased Na+ unloading from the xylem. Consequently, Si increased accumulation of Na in leaves, but also enhances sequestration of Na+ into the vacuoles thereby decreasing Na+ accumulation in the chloroplasts. In response to moderate salt stress in maize, Si shifts the typical glycophyte behavior of this species towards that of halophytes.