Different pathways for exogenous ABA-mediated down-regulation of cadmium accumulation in plants under different iron supplies.

Exogenous abscisic acid (ABA) could inhibit cadmium (Cd) accumulation in plants; however, its performance in an uneven iron (Fe) background remains unknown. Here, we found that the inhibitory effects of ABA on Cd accumulation in plants were optimal under nonlimiting Fe availability (25 and 50 µM), causing a reduction of 25-50 %, whereas only a 0-29 % decrease was observed in a Fe-free or -deficient (5 µM) medium. Although ABA significantly inhibited the expression of IRT1 under different Fe supplies, the inhibitory effects of ABA on Cd accumulation were lower (or absent) in irt1-mutants than in wild-type plants growing under nonlimiting Fe availability, whereas no significant difference was found under Fe deficiency. The mechanisms by which ABA reduces Cd accumulation under different Fe environments may differ. Furthermore, under Fe sufficiency, ABA increased Fe levels of root apoplasts by 91 % without changing the activity level of root ferric reductase (FCR). In contrast, ABA resulted in a 17 % decrease in Fe concentration in apoplasts and a 37 % decrease in FCR activity under Fe-deficient conditions. Thus, under Fe sufficiency, plants may show a reduced accumulation of Cd by accumulating more Fe in the apoplasts, which in turn inhibits the expression of IRT1. However, plants are more prone to redirect apoplastic Fe to prevent Cd accumulation under Fe deficiency. The different mechanisms of inhibition of Cd accumulation by ABA under different Fe supplies revealed in this study may provide guidelines for the precise regulation of Cd accumulation in crops via ABA-based strategies.