A role for HKT1 in sodium uptake by wheat roots

Abstract

SummaryThe high affinity potassium transporter, HKT1 from wheat was introduced into Florida wheat in sense and antisense orientation under control of a ubiquitin promoter. Ten transgenic lines expressing the transgene were identified and two of these showed strong down-regulation of the native HKT1 transcript. One line (271) was expressing the antisense construct and the other (223) was expressing a truncated sense construct. The two lines were examined further for phenotype relating to cation transport. Membrane depolarisations were measured in low (0.1 mM) K þ and high (100 mM) NaCl. Under these conditions there was no difference between line 271 and the control at low K þ , but at high Na þ there was a rapid depolarisation that was significantly larger in control plants. 22 Na uptake was measured in this line and there was a significant decrease in uptake at 100 mM NaCl in the transgenic line when compared with the control. The two transgenic lines were grown at high NaCl (200 mM) and analysed for growth and root sodium content. Lines 271 and 223 showed enhanced growth under salinity when compared with the control and had lower sodium in the root. Secondary ion mass spectrometry (SIMS) analysis of transverse sections of the root showed that Na þ and K þ were strongly localised to stelar regions when compared with other ions, and that the Na þ : K þ ratios were reduced in salt-stressed transgenic tissue when compared with the control.

Sodium as nutrient and toxicant

“…Interestingly, despite considerable effort, entry paths for Na + into roots have not as yet been successfully identified at the molecular level across taxonomic groups (Munns and Tester 2008;Zhang et al 2010;Kronzucker and Britto 2011;Cheeseman 2013), while a strong body of evidence has shown, at least in grasses, that one family of genes, HKT2 (formerly referred to as HKT1, but the latter designation is now reserved for a group of Na + transporters believed to be predominantly involved in intra-plant Na + transfer from root to shoot; Sunarpi et al 2005;Møller et al 2009), encodes transporters that can transport Na + at substantial rates across root plasma membranes, especially when K + is limiting (Horie et al 2001(Horie et al , 2011Laurie et al 2002;Munns and Tester 2008;Hauser and Horie 2010). This is instructive, given that Na + benefits tend to be at their most pronounced when K + is in short supply, and, indeed, Na + can assume some of the functions of K + .…”

“…Those from the HKT1 subfamily are believed to operate mostly in regulating root-to-shoot Na + translocation (Sunarpi et al 2005;Møller et al 2009), while those from the HKT2 subfamily have been implicated in primary Na + influx at least at lower Na + concentrations and in grasses (Horie et al 2001;Laurie et al 2002;Munns and Tester 2008;Hauser and Horie 2010;Horie et al 2011; see also Schulze et al 2012). In addition, two Na + /H + antiport systems have been identified, one of which, SOS1, is believed to be responsible predominantly for Na + efflux at the plasma membrane (Shi et al 2000), the other, NHX1, for Na + sequestration into the vacuole (Apse et al 1999).…”

The Rice Monovalent Cation Transporter OsHKT2;4: Revisited Ionic Selectivity

“…Furthermore, few indications have been obtained that HKT2;1 subgroup transporters, shown to be able to work as Na + -K + symport when heterologously expressed in yeast and Xenopus oocytes, can transport K + in planta. Indeed, varying the transcript levels of TaHKT2;1 and OsHKT2;1 genes in planta through genetic engineering did not allow to identify a role in root K + uptake for these transporters, which are highly expressed in root cortex (Laurie et al, 2002;Horie et al, 2007). These results have led to hypothesize that HKT2;1 subgroup transporters only work as Na + uniporters in planta (Haro et al, 2005;Horie et al, 2007), and that the Na + -K + symport mode evidenced in yeast or Xenopus oocytes is artifactual, resulting from the heterologous context, or anecdotal in planta.…”

AtCCX1 transports Na+ and K+ in Pitch pastoris

“…Various reports have indicated that increasing cytosolic K + levels relative to Na + , thus, increasing the K + /Na + ratio, is crucial for Na + tolerance in plants and maintaining high K + /Na + ratio in shoots is highly correlated with salinity tolerance in glycophytes (Dubcovsky et al, 1996;Ren et al, 2005;Sunarpi et al, 2005;Mason et al, 2010). The Arabidopsis AtHKT1 protein, a Na + -K + cotransporter, mediates Na + influx when expressed in heterologous systems such as Xenopus oocytes, yeast and wheat (Mason et al, 2010;Laurie et al, 2002;Baek et al, 2011;Plett et al, 2010). The novel CCXs family is also possibly correlated with maintenance of Na + /K + ratio in plant.…”