Our paper "Arabidopsis NPCC6/NaKR1 Is a Phloem Mobile Metal Binding Protein Necessary for Phloem Function and Root Meristem Maintenance" was just published in Plant Cell. This paper was mainly the result of the hard work of newly minted Ph.D Hui Tian from John Ward's Lab at UMN. She worked with me to clone the gene, finally finding it when the causal deletion of only 7 bp disrupted a single oligo on the Arabidopsis tiling array. It's a fascinating gene, encoding a protein that moves through the phloem, the part of the plants vasculature responsible for moving solutes away from leaves.
Here is the abstract:
SODIUM POTASSIUM ROOT DEFECTIVE1 (NaKR1; previously called NPCC6)encodes a soluble metal binding protein that is specificallyexpressed in companion cells of the phloem. The nakr1-1 mutantphenotype includes high Na+, K+, Rb+, and starch accumulationin leaves, short roots, late flowering, and decreased long-distancetransport of sucrose. Using traditional and DNA microarray-baseddeletion mapping, a 7-bp deletion was found in an exon of NaKR1that introduced a premature stop codon. The mutant phenotypeswere complemented by transformation with the native gene orNaKR1-GFP (green fluorescent protein) and NaKR1-β-glucuronidasefusions driven by the native promoter. NAKR1-GFP was mobilein the phloem; it moved from companion cells into sieve elementsand into a previously undiscovered symplasmic domain in theroot meristem. Grafting experiments revealed that the high Na+accumulation was due mainly to loss of NaKR1 function in theleaves. This supports a role for the phloem in recirculatingNa+ to the roots to limit Na+ accumulation in leaves. The onsetof root phenotypes coincided with NaKR1 expression after germination.The nakr1-1 short root phenotype was due primarily to a decreasedcell division rate in the root meristem, indicating a role inroot meristem maintenance for NaKR1 expression in the phloem.
And here is a non-technical summary:
A major problem for world agriculture is the growing decrease in avaialable arable land. More and more we are working in solils that impart a stress on the plants that make up the crops we depend on. In order for plants to survive without being able to move out of unfavorable soil environments, they adjust the biochemical composition of their tissues through a wide variety of mechanisms. One of these mechanisms is to move elements such as sodium (Na) and potassium (K) from tissue to tissue, including from the root to the shoot and back again. Understanding the molecular basis of these mechanisms will enable the production of crops that are better able to respond to the changing environment and increase yields with fewer inputs. In this study, we identified and characterized a gene which is important for loading Na into the phloem, the 'veins' of the plant responsible for moving molecules out of the leaves to the seeds and roots. The protein also moves into the phloem. Plants without a functional form of this gene, called NAKR1, have altered levels of Na, K and starch in the leaves, have shorter roots and flower later than plants with a functional copy of NAKR1. These results will lead to a better understanding of how plants distribute elements between tissues and ultimately will allow for crop improvement strategies that deal with poor soil quality.