title: 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-β-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.
authors.affiliation: Department of Plant Biology, University of Minesota, St. Paul, Minesota 55108, USA.
authors.forename: Hui
authors.initials: H
authors.lastname: Tian
authors.forename: Ivan R
authors.initials: IR
authors.lastname: Baxter
authors.forename: Brett
authors.initials: B
authors.lastname: Lahner
authors.forename: Anke
authors.initials: A
authors.lastname: Reinders
authors.forename: David E
authors.initials: DE
authors.lastname: Salt
authors.forename: John M
authors.initials: JM
authors.lastname: Ward
mesh_terms: D017360:Arabidopsis / Q000502:physiology*; D029681:Arabidopsis Proteins / Q000235:genetics / Q000378:metabolism*; D005091:Exons; D018519:Meristem / Q000502:physiology*; D019216:Metals, Heavy / Q000378:metabolism*; D020411:Oligonucleotide Array Sequence Analysis; D052585:Phloem / Q000502:physiology*; D018515:Plant Leaves / Q000378:metabolism
chemical_list: D029681:Arabidopsis Proteins; D019216:Metals, Heavy