OsYSL2 is a rice metal-nicotianamine transporter that is regulated by iron and expressed in the phloem

We identified 18 putative yellow stripe 1 (YS1)-like genes (OsYSLs) in the rice genome that exhibited 36-76% sequence similarity to maize iron(III)-phytosiderophore transporter YS1. Of particular interest was OsYSL2, the transcripts of which were not detected in the roots of either iron-sufficient or iron-deficient plants, but dramatic expression was induced in the leaves by iron deficiency. Based on the nucleotide sequence, OsYSL2 was predicted to encode a polypeptide of 674 amino acids containing 14 putative transmembrane domains. OsYSL2:green fluorescent protein (GFP) was localized in the plasma membrane of onion epidermal cells. Promoter:beta-glucuronidase (GUS) analysis revealed that OsYSL2 was expressed in companion cells in iron-sufficient roots. GUS activity was increased in companion cells, but no GUS staining was observed in epidermal or cortex cells, even in iron-deficient roots. In the leaves and leaf sheaths of iron-sufficient rice, GUS staining was observed in phloem cells of the vascular bundles. In iron-deficient leaves, the OsYSL2 promoter was active in all tissues with particularly strong GUS activity evident in companion cells. The phloem-specific expression of the OsYSL2 promoter suggests that OsYSL2 is involved in the phloem transport of iron. Strong OsYSL2 promoter activity was also detected in developing seeds. Electrophysiological measurements using Xenopus laevis oocytes showed that OsYSL2 transported iron(II)-nicotianamine (NA) and manganese(II)-NA, but did not transport iron(III)-phyosiderophore. These results suggest that OsYSL2 is a rice metal-NA transporter that is responsible for the phloem transport of iron and manganese, including the translocation of iron and manganese into the grain.