Phloem functions revealed by the nakr1-1 mutant.

Abstract

Na+ is a non- essential element for plant growth. Na+ accumulation within plants, especially the shoot tissue causes osmotic stress and Na+-specific toxicity that threatens plant survival and reduces crop yield. The control of Na+ accumulation in the shoot is mainly at the root level, by regulating net Na+ uptake into roots and Na+ transport from root to the shoot. My thesis work is mainly on the characterization of a fast neutron mutagenized Arabidopsis mutant, nakr1-1, that accumulates Na+ and K+ in the shoot tissue and has pleiotrophic developmental phenotypes (including short roots, late flowering and loss of apical dominance). Using traditional mapping together with DNA- chip based mapping, a 7-bp deletion was identified that caused loss- of- function mutation of a gene encoding a putative heavy-metal-binding protein. The metalbinding feature of the protein was confirmed by elemental analysis of maltose binding protein (MBP)- tagged NaKR1 expressed and purified from Escherichia coli. AtNaKR1 was specifically expressed in the phloem companion cells. NaKR1 protein was phloem mobile and unloaded at the phloem terminal into the proximal root meristem region. nakr1-1 mutation caused severe phloem function defects as demonstrated by less efficient 14C- sucrose loading and starch accumulation in rosette leaves. Phloem function defects were also responsible for the Na+/K+ accumulation in the shoot tissue based on the reciprocal grafting results together with ICP- MS analyses. Moreover elemental analysis of xylem sap indicated Na+ and K+ accumulation phenotypes were not caused by increased root- to- shoot transport of Na+ and K+. nakr1-1 mutation affects root meristem maintenance after germination as revealed by study of root meristem size, cell pattern and starch accumulation in root columella cells and quiescent center activity. My work provided evidence that phloem recirculation plays more important roles than had been suggested by previous literature in controlling shoot Na+ accumulation. Understanding how Na+ and K+ redistribution is regulated might have potential application in improving salinity tolerance of crop plants and the improvement of seed quality.