Arabidopsis NPCC6/NaKR1 Is a Phloem Mobile Metal Binding Protein Necessary for Phloem Function and Root Meristem Maintenance  

Abstract

SODIUM POTASSIUM ROOT DEFECTIVE1 (NaKR1; previously called NPCC6) encodes a soluble metal binding protein that is specifically expressed in companion cells of the phloem. The nakr1-1 mutant phenotype includes high Na + , K + , Rb + , and starch accumulation in leaves, short roots, late flowering, and decreased long-distance transport of sucrose. Using traditional and DNA microarray-based deletion mapping, a 7-bp deletion was found in an exon of NaKR1 that introduced a premature stop codon. The mutant phenotypes were complemented by transformation with the native gene or NaKR1-GFP (green fluorescent protein) and NaKR1-b-glucuronidase fusions driven by the native promoter. NAKR1-GFP was mobile in the phloem; it moved from companion cells into sieve elements and into a previously undiscovered symplasmic domain in the root meristem. Grafting experiments revealed that the high Na + accumulation was due mainly to loss of NaKR1 function in the leaves. This supports a role for the phloem in recirculating Na + to the roots to limit Na + accumulation in leaves. The onset of root phenotypes coincided with NaKR1 expression after germination. The nakr1-1 short root phenotype was due primarily to a decreased cell division rate in the root meristem, indicating a role in root meristem maintenance for NaKR1 expression in the phloem.

The Rice High-Affinity Potassium Transporter1;1 Is Involved in Salt Tolerance and Regulated by an MYB-Type Transcription Factor

“…Some mechanisms of controlling Na + accumulation in leaves have been identified and/or proposed. They include the control of Na + uptake from soils, the reduction of the delivery of Na + to the xylem, the storage of Na + in the lower parts of the leaf (such as the sheath), and the recirculation of Na + from shoots to roots (Davenport et al, 2005;Munns and Tester, 2008;Tian et al, 2010). In this article, we show the genetic evidence that OsHKT1;1, which is mainly in the phloem of leaf blades, is involved in limiting Na + accumulation in leaves and salt tolerance in rice.…”

Sphingolipids in the Root Play an Important Role in Regulating the Leaf Ionome inArabidopsis thaliana

“…The application of high-throughput elemental profiling of mutagenized populations of Arabidopsis thaliana, coupled to linkage mapping, for the identification of genes involved in regulating plant mineral ion homoeostasis has been termed ionomics . Such an approach has been used to successfully clone genes involved in regulating the ionome (Nublat et al, 2001;Baxter et al, 2009;Tian et al, 2010), and with the development of new genomic-based tools for mapping (for example Hodges et al, 2007;Schneeberger et al, 2009), this forward genetic approach is likely to be used increasingly in the future.…”

Overexpression of NaKR3 enhances salt tolerance in Arabidopsis

“…The HMA domain exists in proteins that function in the transfer and/or binding of heavy metals; these proteins include heavy metal transporters, metallochaperones, and enzymes that use heavy metals as cofactors (Dykema et al, 1999;Tian et al, 2010). Tian et al (2010) reported that Zn, Cu, Fe, Ni, and Co were associated with the NaKR1 protein.…”

“…The HMA domain exists in proteins that function in the transfer and/or binding of heavy metals; these proteins include heavy metal transporters, metallochaperones, and enzymes that use heavy metals as cofactors (Dykema et al, 1999;Tian et al, 2010). Tian et al (2010) reported that Zn, Cu, Fe, Ni, and Co were associated with the NaKR1 protein. On the other hand, the HMA domain of NaKR3 shares low sequence similarity with the Arabidopsis HMA domain protein CCH; this protein is found in the phloem and is thought to transport Cu out of senescing tissues (Mira et al, 2001).…”

“…Tian et al (2010) reported that loss of NaKR1 expression led to over accumulation of Na + and K + in the leaves. This finding implied that NaKR1 functions in the recirculation of Na + to the roots via the phloem, thereby limiting Na + accumulation in the leaves.…”