The sodium transporter encoded by the HKT1;2 gene modulates sodium/potassium homeostasis in tomato shoots under salinity.

Excessive soil salinity diminishes crop yield and quality. In a previous study in tomato, we identified two closely linked genes encoding HKT1-like transporters, HKT1;1 and HKT1;2, as candidate genes for a major quantitative trait locus (kc7.1) related to shoot Na+ /K+ homeostasis - a major salt tolerance trait - using two populations of recombinant inbred lines (RILs). Here, we determine the effectiveness of these genes in conferring improved salt tolerance by using two near-isogenic lines (NILs) that were homozygous for either the Solanum lycopersicum allele (NIL17) or for the Solanum cheesmaniae allele (NIL14) at both HKT1 loci; transgenic lines derived from these NILs in which each HKT1;1 and HKT1;2 had been silenced by stable transformation were also used. Silencing of ScHKT1;2 and SlHKT1;2 altered the leaf Na+ /K+ ratio and caused hypersensitivity to salinity in plants cultivated under transpiring conditions, whereas silencing SlHKT1;1/ScHKT1;1 had a lesser effect. These results indicate that HKT1;2 has the more significant role in Na+ homeostasis and salinity tolerance in tomato.

Jaime-Pérez N ，Pineda B ，García-Sogo B ，Atares A ，Athman A ，Byrt CS ，Olías R ，Asins MJ ，Gilliham M ，Moreno V ，Belver A ... -  《-》

Differences in shoot Na+ accumulation between two tomato species are due to differences in ion affinity of HKT1;2.

HKT1 has been shown to be essential in Na(+) homeostasis in plants. In this paper, we report the analysis of Na(+) accumulation in different plant organs of two tomato species with contrasting salt tolerances: Solanum lycopersicum and Solanum pennellii. Furthermore, we relate these differences in Na(+) accumulation between the two species to the differences in HKT1;2 transport kinetics and HKT1;2 expression. S. lycopersicum showed "Na(+) excluder" behaviour, whereas S. pennellii showed "Na(+) includer" behaviour. SlHKT1;2 expression, in contrast to SpHKT1;2 expression showed a significant effect of NaCl treatment, especially stems had a high increase in SlHKT1;2 expression. SlHKT1;2 promoter-GUS reporter gene analysis showed that SlHKT1;2 is expressed in the vasculature surrounding the roots and shoots of transformed Arabidopsis plants. In this paper, we present HKT1;2 protein sequences of both tomato species and provide evidence that both SlHKT1;2 and SpHKT1;2 are Na(+) transporters. Our kinetic studies showed that SpHKT1;2, in comparison with SlHKT1;2, had a lower affinity for Na(+). This low affinity of SpHKT1;2 correlated with higher xylem Na(+) and higher accumulation of Na(+) in stems and leaves of S. pennellii. Our findings demonstrate the importance of the understanding of transport characteristics of HKT1;2 transporters to improve the understanding of Na(+) homeostasis in plants.

Almeida P ，de Boer GJ ，de Boer AH 《-》

Na(+) transporter HKT1;2 reduces flower Na(+) content and considerably mitigates the decline in tomato fruit yields under saline conditions.

Genes encoding HKT1-like Na+ transporters play a key role in the salinity tolerance mechanism in Arabidopsis and other plant species by retrieving Na+ from the xylem of different organs and tissues. In this study, we investigated the role of two HKT1;2 allelic variants in tomato salt tolerance in relation to vegetative growth and fruit yield in plants subjected to salt treatment in a commercial greenhouse under real production conditions. We used two near-isogenic lines (NILs), homozygous for either the Solanum lycopersicum (NIL17) or S. cheesmaniae (NIL14) allele, at HKT1;2 loci and their respective RNAi-Sl/ScHKT1;2 lines. The results obtained show that both ScHKT1;2- and SlHKT1;2-silenced lines display hypersensitivity to salinity associated with an altered leaf Na+/K+ ratio, thus confirming that HKT1;2 plays an important role in Na+ homeostasis and salinity tolerance in tomato. Both silenced lines also showed Na+ over-accumulation and a slight, but significant, reduction in K+ content in the flower tissues of salt-treated plants and consequently a higher Na+/K+ ratio as compared to the respective unsilenced lines. This altered Na+/K+ ratio in flower tissues is associated with a sharp reduction in fruit yield, measured as total fresh weight and number of fruits, in both silenced lines under salinity conditions. Our findings demonstrate that Na+ transporter HKT1;2 protects the flower against Na+ toxicity and mitigates the reduction in tomato fruit yield under salinity conditions.

Romero-Aranda MR ，González-Fernández P ，Pérez-Tienda JR ，López-Diaz MR ，Espinosa J ，Granum E ，Traverso JÁ ，Pineda B ，Garcia-Sogo B ，Moreno V ，Asins MJ ，Belver A ... -  《-》

Two closely linked tomato HKT coding genes are positional candidates for the major tomato QTL involved in Na+ /K+ homeostasis.

The location of major quantitative trait loci (QTL) contributing to stem and leaf [Na(+) ] and [K(+) ] was previously reported in chromosome 7 using two connected populations of recombinant inbred lines (RILs) of tomato. HKT1;1 and HKT1;2, two tomato Na(+) -selective class I-HKT transporters, were found to be closely linked, where the maximum logarithm of odds (LOD) score for these QTLs located. When a chromosome 7 linkage map based on 278 single-nucleotide polymorphisms (SNPs) was used, the maximum LOD score position was only 35 kb from HKT1;1 and HKT1;2. Their expression patterns and phenotypic effects were further investigated in two near-isogenic lines (NILs): 157-14 (double homozygote for the cheesmaniae alleles) and 157-17 (double homozygote for the lycopersicum alleles). The expression pattern for the HKT1;1 and HKT1;2 alleles was complex, possibly because of differences in their promoter sequences. High salinity had very little effect on root dry and fresh weight and consequently on the plant dry weight of NIL 157-14 in comparison with 157-17. A significant difference between NILs was also found for [K(+) ] and the [Na(+) ]/[K(+) ] ratio in leaf and stem but not for [Na(+) ] arising a disagreement with the corresponding RIL population. Their association with leaf [Na(+) ] and salt tolerance in tomato is also discussed.

Asins MJ ，Villalta I ，Aly MM ，Olías R ，Alvarez DE Morales P ，Huertas R ，Li J ，Jaime-Pérez N ，Haro R ，Raga V ，Carbonell EA ，Belver A ... -  《-》

HKT1;1 and HKT1;2 Na(+) Transporters from Solanum galapagense Play Different Roles in the Plant Na(+) Distribution under Salinity.

Asins MJ ，Romero-Aranda MR ，Espinosa J ，González-Fernández P ，Jaime-Fernández E ，Traverso JA ，Carbonell EA ，Belver A ... -  《-》