Low nitrate treatment promotes iron transportation from root to shoot
Under Fe adequate conditions, we found that there was no significant difference in total Fe content in new leaves and roots under either HN or LN treatment. By contrast, under Fe deficient conditions, LN treatment remarkably increased total Fe content both in newly growing leaves and roots (Fig. 3a, b). In addition, the Fe content was higher in stems of plants under LN-Fe treatment than that under HN-Fe treatment (Fig. 3c). Nitrogen content analysis suggested that seedlings under HN-Fe treatment had a higher nitrate and total nitrogen content than that of LN-Fe treatment (Fig. S3a, b, c). Furthermore, gene expression analysis found that the expression of MdFRD3 and MdMATE43 , which are associated with vascular Fe loading and unloading, were dramatically increased by LN treatment (Fig. 3d, e). Early on in Fe deficiency (1 day), LN treatment promoted the expression ofMdNAS1 (Fig. 3f), suggesting that LN-promoted iron transportation might result from LN-induced upregulation of genes related with Fe-chelator formation and transportation.