IDI7, a new iron-regulated ABC transporter from barley roots, localizes to the tonoplast.

A new Fe-deficiency-induced cDNA, IDI7, was isolated from the roots of Fe-deficient barley (Hordeum vulgare L. cv. Ehimehadaka no. 1). The transcript levels of IDI7 in roots strongly correlated with iron nutritional status, and induction by Fe-deficiency was restricted to roots. Excess treatment with heavy metal ions, such as copper, manganese, and zinc, did not cause obvious IDI7 induction in either leaves or roots. IDI7 encodes a 644 amino acid protein, and has features typical of ATP-binding cassette (ABC) transporters. Phylogenetic analysis revealed that IDI7 is closely related to the half-type ABC protein subfamily, which includes mammalian transporters associated with antigen processing (TAPs). A transiently expressed fusion protein of IDI7 to green fluorescent protein (GFP) was localized to tonoplasts in suspension-cultured tobacco (Nicotiana tabacum L.) cells. IDI7 and its orthologues are thought to comprise a new class of ABC transporters, located in the tonoplasts of higher plants. A possible Fe-deficiency adaptation role for IDI7 in barley root cells, involving transport across the tonoplast, is proposed.