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

Abstract

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:b-glucuronidase (GUS) analysis revealed that OsYSL2 was expressed in companion cells in ironsufficient 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.

Phloem-Specific Methionine Recycling Fuels Polyamine Biosynthesis in a Sulfur-Dependent Manner and Promotes Flower and Seed Development

“…Regarding the latter, NA is most important for the phloem mobility of Fe, Zn, and Cu (Klatte et al, 2009;Deinlein et al, 2012;Haydon et al, 2012;Schuler et al, 2012;Zheng et al, 2012). Furthermore, several phloem localized NA transporters have been identified (Koike et al, 2004;Chu et al, 2010;Zheng et al, 2012), some of which appeared to mediate source-to-sink translocation of Fe and other metals during plant senescence (DiDonato et al, 2004). To our knowledge, cell type-specific expression of NA synthases has not been shown directly in Arabidopsis.…”

Identification and expression analysis of multiple FRO gene copies in Medicago truncatula

“…It was very interesting to note the strong expression level of the MtFRO3 and MtFRO6 in the shoots, since experiments suggest that the ferric-chelate reduction is performed in aerial tissues. The role of the chelate Fe 3+ -nicotianamine on the plant Fe transport via the phloem has been demonstrated (Takahashi et al, 2003), and it has been suggested that Fe could be internalized in the cells via the Fe 3+ -nicotianamine transporter in both strategy I and strategy II plants (Koike et al, 2004). Our results provided molecular evidence that the FRO is strongly expressed in aerial tissues and indicates that Fe is reduced before it can be internalized in the plant cells as the results of Wu et al (2005) and Mukherjee et al (2006) suggest.…”

Iron Uptake and Loading into Rice Grains

“…The protein encoded by OsYSL15 transports Fe(III)-DMA from the rhizosphere to roots (Inoue et al 2009), whereas OsYSL18 encodes a functional Fe(III)-DMA transporter involved in DMA-mediated Fe distribution in reproductive organs (Aoyama et al 2009). In comparison, the translation product of OsYSL2 has been identified as an Fe-NA or Mn-NA transporter (Koike et al 2004).…”

“…The rice genome contains 18 putative YSL family genes (Koike et al 2004), among which OsYSL2, OsYSL15, and OsYSL18 have been characterized in detail (Aoyama et al 2009;Inoue et al 2009;Ishimaru et al 2010;Koike et al 2004). The protein encoded by OsYSL15 transports Fe(III)-DMA from the rhizosphere to roots (Inoue et al 2009), whereas OsYSL18 encodes a functional Fe(III)-DMA transporter involved in DMA-mediated Fe distribution in reproductive organs (Aoyama et al 2009).…”