Abstract
HKT1;5 loci/alleles are important determinants of crop salinity tolerance. HKT1;5s encode plasmalemma-localized Na+-transporters, retrieving xylem Na+ into xylem parenchyma cells, reducing shoot Na+ accumulation. Allelic variation in rice OsHKT1;5 sequence in specific landraces (Nona Bokra OsHKT1;5-NB/Nipponbare OsHKT1;5-Ni) correlates with variation in salt tolerance. Oryza coarctata, a halophytic wild rice, grows in fluctuating salinity at the seawater-estuarine interface in Indian and Bangladeshi coastal regions. The distinct transport characteristics of the shoots and roots expressing O. coarctata OcHKT1;5 transporter, are reported vis-à-vis OsHKT1;5-Ni. Yeast sodium extrusion-deficient cells expressing OcHKT1;5 are sensitive to increasing Na+ (10-100 mM). Electrophysiological measurements in Xenopus oocytes expressing O. coarctata or rice HKT1;5 transporters indicates that OcHKT1;5, like OsHKT1;5-Ni is a Na+-selective transporter, but displays 16-fold lower affinity for Na+ and 3.5-fold higher maximal conductance than OsHKT1;5-Ni. For Na+ concentrations greater than 10 mM, OcHKT1;5 conductance is higher than that of OsHKT1;5-Ni, indicating potential of OcHKT1;5 for increasing domesticated rice salt tolerance. Homology modeling/ simulation suggests that four key amino acid changes in OcHKT1;5 (in loops on the extracellular side; E239K, G207R, G214R, L363V), account for its lower affinity and higher Na+ conductance vis-à-vis OsHKT1;5-Ni. Of these, E239K in OcHKT1;5, confers lower affinity for Na+ transport, as evidenced by Na+ transport assays of reciprocal site directed mutants for both transporters (OcHKT1;5- K239E, OsHKT1;5-Ni- E270K) in Xenopus oocytes. Both transporters have analogous roles in xylem sap desalinization and differences in Na+ transport affinity/ conductance between the transporters is attributable to differences in xylem sap Na+ concentrations in both species.