T-DNA Tagging-Based Gain-of-Function of OsHKT1;4 Reinforces Na Exclusion from Leaves and Stems but Triggers Na Toxicity in Roots of Rice Under Salt Stress.

Yuuka Oda,Yukiko Itou,Takashi Itou,Natsuko Kobayashi,Tomoaki Horie,Maki Katsuhara,Jian Ma,Keitaro Tanoi

doi:10.3390/ijms19010235

Abstract

The high affinity K+ transporter 1;4 (HKT1;4) in rice (Oryza sativa), which shows Na+ selective transport with little K+ transport activity, has been suggested to be involved in reducing Na in leaves and stems under salt stress. However, detailed physiological roles of OsHKT1;4 remain unknown. Here, we have characterized a transfer DNA (T-DNA) insertion mutant line of rice, which overexpresses OsHKT1;4, owing to enhancer elements in the T-DNA, to gain an insight into the impact of OsHKT1;4 on salt tolerance of rice. The homozygous mutant (the O/E line) accumulated significantly lower concentrations of Na in young leaves, stems, and seeds than the sibling WT line under salt stress. Interestingly, however, the mutation rendered the O/E plants more salt sensitive than WT plants. Together with the evaluation of biomass of rice lines, rhizosphere acidification assays using a pH indicator bromocresol purple and 22NaCl tracer experiments have led to an assumption that roots of O/E plants suffered heavier damages from Na which excessively accumulated in the root due to increased activity of Na+ uptake and Na+ exclusion in the vasculature. Implications toward the application of the HKT1-mediated Na+ exclusion system to the breeding of salt tolerant crop cultivars will be discussed.

Highlights

Soil salinization represents the excessive accumulation of water-soluble salts in the rhizosphere, and when the electrical conductivity of its saturation extract (ECe) is more than 4 dS m−1, the soil is considered as saline that can cause salt stress to glycophytic plants [1]
Salt tolerance quantitative trait locus (QTL) analyses using wheat cultivars have led to the findings that superior xylem Na+ unloading systems controlled by Nax1 and Nax2 QTL can prevent from over-accumulating Na in leaf blades of near isogenic lines of wheat cultivars suffering salt stress [15]
We were interested in a line designated PFG_3A-05753.L as the transfer DNA (T-DNA) inserted upstream of the first ATG codon of OsHKT1;4 harbors enhancer elements composed of tandem repeats of the 35S promoter core sequence [24] (Figure 1a)

Summary

Introduction

Soil salinization represents the excessive accumulation of water-soluble salts in the rhizosphere, and when the electrical conductivity of its saturation extract (ECe) is more than 4 dS m−1, the soil is considered as saline that can cause salt stress to glycophytic plants [1]. Recent reports with genetic and bioinformatic approaches further revealed that OsHKT1;1, a class I Na+ selective transporter in rice [18], contributes to Na+ exclusion from leaves of salt-stressed rice plants [19,20,21], for which detailed mechanisms are yet to be elucidated. These past findings suggest that HKT1-mediated Na exclusion from leaves appears to be mediated by multiple HKT1 transporters in at least monocot plants

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Results

Conclusion