Differential regulation of cadmium-inducible expression of iron-deficiency-responsive genes in tobacco and barley

The expression of four representative iron-deficiency-responsive genes from tobacco (NtIRT1 and NtYSL1) and barley (HODS2 and HvYS1) plants were evaluated in each host plant in response to iron deficiency (Delta Fe), cadmium exposure (+Cd) or both (Delta Fe + Cd). These conditions significantly enhanced NtIRT1 and HvIDS2 expression in roots, whereas NtYSL1 and HvYS1 expression was similar in shoots and roots. NtIRT1 expression under +Cd and Delta Fe + Cd was lower than that under Delta Fe, whereas the expression of NtYSL1, HvIDS2 and HvYS1 in roots under +Cd and Delta Fe + Cd was similar or higher than that under Delta Fe. A time-course experiment showed that NtIRT1 expression under +Cd and Delta Fe was regulated similarly throughout the experiment [expressed between 3 and 21 days after treatment (DAT)]. NtYSL1 expression under +Cd and Delta Fe began at 1 DAT, expression soon disappeared under Delta Fe, whereas it continued to 21 DAT under +Cd. The timing of HvIDS2 and HvYS1 expression under +Cd (between 1 and 5 DAT) was earlier than that under Delta Fe (between 5 and 21 DAT). Notably, no Fe deficit occurred in any parts of these plants when grown under +Cd, except for tobacco shoots, even when the genes were highly expressed. Thus, some expression under +Cd differed from that under Delta Fe. It is possible that both the genuine Fe-deficiency responsive mechanism and an unidentified mechanism, which can be directly regulated by Cd, contribute to gene expression to maintain metal homeostasis within the plant.