Rice plants take up iron as an Fe3+‐phytosiderophore and as Fe2+

Abstract

Only graminaceous monocots possess the Strategy II iron (Fe)-uptake system in which Fe is absorbed by roots as an Fe³⁺-phytosiderophore. In spite of being a Strategy II plant, however, rice (Oryza sativa) contains the previously identified Fe²⁺ transporter OsIRT1. In this study, we isolated the OsIRT2 gene from rice, which is highly homologous to OsIRT1. Real-time PCR analysis revealed that OsIRT1 and OsIRT2 are expressed predominantly in roots, and these transporters are induced by low-Fe conditions. When expressed in yeast (Saccharomyces cerevisiae) cells, OsIRT2 cDNA reversed the growth defects of a yeast Fe-uptake mutant. This was similar to the effect of OsIRT1 cDNA. OsIRT1– and OsIRT2–green fluorescent protein fusion proteins localized to the plasma membrane when transiently expressed in onion (Allium cepa L.) epidermal cells. OsIRT1 promoter–GUS analysis revealed that OsIRT1 is expressed in the epidermis and exodermis of the elongating zone and in the inner layer of the cortex of the mature zone of Fe-deficient roots. OsIRT1 expression was also detected in the ccompanion cells. Analysis using the positron-emitting tracer imaging system showed that rice plants are able to take up both an Fe³⁺-phytosiderophore and Fe²⁺. This result indicates that, in addition to absorbing an Fe³⁺-phytosiderophore, rice possesses a novel Fe-uptake system that directly absorbs the Fe²⁺, a strategy that is advantageous for growth in submerged conditions.