Homology Modeling Identifies Crucial Amino-Acid Residues That Confer Higher Na+ Transport Capacity of OcHKT1;5 from Oryza coarctata Roxb.

HKT1;5 loci/alleles are important determinants of crop salinity tolerance. HKT1;5s encode plasmalemma-localized Na+ transporters, which move 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 the 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 indicate 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 >10 mM, OcHKT1;5 conductance is higher than that of OsHKT1;5-Ni, indicating the 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 likely analogous roles in xylem sap desalinization, and differences in xylem sap Na+ concentrations in both species are attributed to differences in Na+ transport affinity/conductance between the transporters.

Somasundaram S ，Véry AA ，Vinekar RS ，Ishikawa T ，Kumari K ，Pulipati S ，Kumaresan K ，Corratgé-Faillie C ，Sowdhamini R ，Parida A ，Shabala L ，Shabala S ，Venkataraman G ... -  《-》

Two types of HKT transporters with different properties of Na+ and K+ transport in Oryza sativa.

Horie T ，Yoshida K ，Nakayama H ，Yamada K ，Oiki S ，Shinmyo A ... -  《PLANT JOURNAL》

OsHKT1;4-mediated Na(+) transport in stems contributes to Na(+) exclusion from leaf blades of rice at the reproductive growth stage upon salt stress.

Suzuki K ，Yamaji N ，Costa A ，Okuma E ，Kobayashi NI ，Kashiwagi T ，Katsuhara M ，Wang C ，Tanoi K ，Murata Y ，Schroeder JI ，Ma JF ，Horie T ... -  《BMC PLANT BIOLOGY》

Changes in Expression Level of OsHKT1;5 Alters Activity of Membrane Transporters Involved in K(+) and Ca(2+) Acquisition and Homeostasis in Salinized Rice Roots.

Al Nayef M ，Solis C ，Shabala L ，Ogura T ，Chen Z ，Bose J ，Maathuis FJM ，Venkataraman G ，Tanoi K ，Yu M ，Zhou M ，Horie T ，Shabala S ... -  《INTERNATIONAL JOURNAL OF MOLECULAR SCIENCES》

A two-staged model of Na+ exclusion in rice explained by 3D modeling of HKT transporters and alternative splicing.

Cotsaftis O ，Plett D ，Shirley N ，Tester M ，Hrmova M ... -  《PLoS One》