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

Abstract

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.

Constitutive Contribution by the Rice OsHKT1;4 Na+ Transporter to Xylem Sap Desalinization and Low Na+ Accumulation in Young Leaves Under Low as High External Na+ Conditions

“…We determined that the affinity for Na + of OsHKT1;4 is still 2.5folds higher than that of OsHKT1;3 ( Figure 4C; Jabnoune et al, 2009). OsHKT1;4 affinity for Na + is also slightly higher (by 1.5 folds) than that of OsHKT1;5 (Somasundaram et al, 2020). Thus, within the rice HKT subfamily 1, OsHKT1;4 displays the highest affinity for Na + .…”

Transcriptome-Wide Analysis Revealed the Potential of the High-Affinity Potassium Transporter (HKT) Gene Family in Rice Salinity Tolerance via Ion Homeostasis

“…A 1.5 Kb genomic DNA sequence upstream of each identified OsHKT gene’s start codon was obtained from the Ensemble Plants database ( ) (accessed on 11 March 2022) [ 31 ]. The online Plant CARE ( ) (accessed on 11 March 2022) database was used to identify cis -regulatory elements for all the OsHKT genes.…”

Comparative Analysis of Root Na+ Relation under Salinity between Oryza sativa and Oryza coarctata

“…O. coarctata has long been known to maintain low leaf Na + /K + ratio [ 37 ], showing greater Na + accumulation in the root rather than the shoot under salinity [ 38 ]. Secretion of Na + via external microhairs [ 39 ], efficient performance of NHX [ 40 ], and a high transport capacity of HKT1;5 [ 41 ] are considered to contribute to superior ionic homeostasis under salinity in this species. Due to high salinity tolerance within the genus of Oryza , O. coarctata has been considered as an important resource of gene pools to improve salinity tolerance in rice cultivars [ 35 , 41 ].…”

“…Secretion of Na + via external microhairs [ 39 ], efficient performance of NHX [ 40 ], and a high transport capacity of HKT1;5 [ 41 ] are considered to contribute to superior ionic homeostasis under salinity in this species. Due to high salinity tolerance within the genus of Oryza , O. coarctata has been considered as an important resource of gene pools to improve salinity tolerance in rice cultivars [ 35 , 41 ]. However, detailed mechanisms of maintaining Na + homeostasis in this species have been less understood due to the limited number of studies at the cellular level.…”