Plant Molecular Biology Laboratory, M. S. Swaminathan Research Foundation, III Cross Street, Taramani Institutional Area, Chennai 600113, India.
Biochimie & Physiologie Moléculaire des Plantes, UMR Univ. Montpellier, CNRS, INRAE, SupAgro, 34060 Montpellier Cedex 2, France.
Plant Cell Physiol. 2020 Jul 1;61(7):1321-1334. doi: 10.1093/pcp/pcaa061.
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.
HKT1;5 基因座/等位基因是作物耐盐性的重要决定因素。HKT1;5 编码定位于质膜的 Na+转运体，将木质部 Na+转运到木质部薄壁细胞，减少地上部 Na+积累。水稻 OsHKT1;5 序列在特定地方品种（Nona Bokra OsHKT1;5-NB/Nipponbare OsHKT1;5-Ni）中的等位基因变异与耐盐性变异相关。沿海地区印度和孟加拉国的海水-河口界面处生长着盐生野生稻 Oryza coarctata。与 OsHKT1;5-Ni 相比，表达 O. coarctata OcHKT1;5 转运体的地上部和根部具有独特的转运特性。表达 OcHKT1;5 的酵母钠外排缺陷细胞对 Na+（10-100 mM）敏感。在表达 O. coarctata 或水稻 HKT1;5 转运体的非洲爪蟾卵母细胞中的电生理学测量表明，OcHKT1;5 与 OsHKT1;5-Ni 一样，是一种 Na+-选择性转运体，但对 Na+的亲和力低 16 倍，最大电导高 3.5 倍。对于 >10 mM 的 Na+浓度，OcHKT1;5 电导高于 OsHKT1;5-Ni，表明 OcHKT1;5 提高了驯化水稻的耐盐性。同源建模/模拟表明，OcHKT1;5 中的四个关键氨基酸变化（在细胞外环上的环；E239K、G207R、G214R、L363V）导致其对 Na+的亲和力降低和电导增加，与 OsHKT1;5-Ni 相比。其中，OcHKT1;5 中的 E239K 导致对 Na+转运的亲和力降低，这一点通过对两种转运体（OcHKT1;5-K239E、OsHKT1;5-Ni-E270K）的互易定点突变体在非洲爪蟾卵母细胞中的 Na+转运实验得到证实。两种转运体在木质部汁液脱盐中可能具有类似的作用，并且两种物种木质部汁液中 Na+浓度的差异归因于转运体之间 Na+转运亲和力/电导的差异。
