OsFRDL1 Is a Citrate Transporter Required for Efficient Translocation of Iron in Rice  

Abstract

Multidrug and toxic compound extrusion (MATE) transporters represent a large family in plants, but their functions are poorly understood. Here, we report the function of a rice (Oryza sativa) MATE gene (Os03g0216700, OsFRDL1), the closest homolog of barley (Hordeum vulgare) HvAACT1 (aluminum [Al]-activated citrate transporter 1), in terms of metal stress (iron [Fe] deficiency and Al toxicity). This gene was mainly expressed in the roots and the expression level was not affected by either Fe deficiency or Al toxicity. Knockout of this gene resulted in leaf chlorosis, lower leaf Fe concentration, higher accumulation of zinc and manganese concentration in the leaves, and precipitation of Fe in the root's stele. The concentration of citrate and ferric iron in the xylem sap was lower in the knockout line compared to the wild-type rice. Heterologous expression of OsFRDL1 in Xenopus oocytes showed transport activity for citrate. Immunostaining showed that OsFRDL1 was localized at the pericycle cells of the roots. On the other hand, there was no difference in the Al-induced secretion of citrate from the roots between the knockout line and the wild-type rice. Taken together, our results indicate that OsFRDL1 is a citrate transporter localized at the pericycle cells, which is necessary for efficient translocation of Fe to the shoot as a Fe-citrate complex.

Isolation and characterisation of two MATE genes in rye

“…By contrast, other members including AtFRD3, OsFRDL1 and GmMATE are involved in the translocation of Fe from the roots to the shoots in Arabidopsis, rice and soybean (Glycine max L.), respectively (Durrett et al 2007;Rogers et al 2009;Yokosho et al 2009). These proteins are localised at the pericycle cells and mediate the release of citrate to the xylem (Durrett et al 2007;Yokosho et al 2009). Citrate is required to translocate iron from the roots to the shoots by forming a complex with iron.…”

“…HvAACT1 is localised at the epidermal cells of the root tip region to facilitate the secretion of citrate for detoxifying Al in the rhizosphere (Furukawa et al 2007). By contrast, other members including AtFRD3, OsFRDL1 and GmMATE are involved in the translocation of Fe from the roots to the shoots in Arabidopsis, rice and soybean (Glycine max L.), respectively (Durrett et al 2007;Rogers et al 2009;Yokosho et al 2009). These proteins are localised at the pericycle cells and mediate the release of citrate to the xylem (Durrett et al 2007;Yokosho et al 2009).…”

“…Recently, a group of MATE protein has been reported to transport citrate (Durrett et al 2007;Furukawa et al 2007;Magalhaes et al 2007;Yokosho et al 2009). Interestingly, they play two different physiological roles in plant.…”

“…Citrate is required to translocate iron from the roots to the shoots by forming a complex with iron. Therefore, knockout of these genes result in Fe precipitation in the root stele (Green and Rogers 2004;Durrett et al 2007;Yokosho et al 2009). …”

“…For example, in Arabidopsis thaliana (L.), there are 58 MATE orthologues (Yazaki 2005). In the rice (Oryza sativa L.) genome, there are at least 40 MATE genes (Yokosho et al 2009). However, only few members of the MATE family have been functionally characterised.…”