NAKR1 deletion analysis

ABSTRACT:The SODIUM POTASSIUM ROOT DEFECTIVE 1 (NaKR1) 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 mapping, a 7 bp deletion was found in an exon of NaKR1 that caused a premature stop. The mutant phenotypes were complemented by the native gene and by GFP and GUS fusions driven by the native promoter. NAKR1-GFP was mobile in the phloem, it moved into sieve elements and into a novel symplasmic domain of the root meristem. Grafting experiments revealed that the high Na+ accumulation was due primarily 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. Short root length was primarily due to a decrease in cell division rate in the root meristem indicating a role for NaKR1 expression in the phloem in root meristem maintenance. 3 hybridizations each of mutant NAKR1 and wildtype for deletion identification.