Targeted Overexpression of a Sodium Transporter in the Root Stele Increases Salinity Tolerance

Soil salinity poses a major threat to global agricultural productivity. An elevated concentration of shoot Na⁺ causes stress in many crops growing in saline soils (reviewed in Tester and Davenport, 2003). Arabidopsis HKT1;1 (for high affinity potassium transporter1;1), a plasma membrane protein expressed in the root stele, mediates salinity tolerance by retrieving Na⁺ from the transpiration stream and thereby reducing Na⁺ levels in the shoot (Mäser et al., 2002; Sunarpi et al., 2005; Davenport et al., 2007).

In an effort to boost salinity tolerance by minimizing Na⁺ accumulation in Arabidopsis shoots, Møller et al. (pages 2163–2178) produced plants that overexpress HKT1;1 either constitutively or in a cell-type–specific manner. They used an enhancer trap system to generate plants that overexpress HKT1;1 specifically in (1) the pericycle and (2) the vascular bundle of the stele of mature roots. Cell-type specificity of the transgene was confirmed by RT-PCR analysis of laser-dissected stelar root cells and by mRNA in situ hybridization. Furthermore, patch clamp analysis showed that protoplasts derived from pericycle cells overexpressing HKT1;1 had an increased electrogenic transport capacity for Na⁺, which is consistent with increased amounts of active HKT1;1 protein being present in the plasma membrane.

The researchers monitored the effect of overexpression of HKT1;1 on Na⁺ transport using radiolabeled sodium (²²Na⁺). Whereas targeted overexpression of HKT1;1 did not affect the unidirectional influx of ²²Na⁺ into the roots, stelar-specific overexpression of HKT1;1 decreased ²²Na⁺ flux from the roots to the shoot by 28 to 59%. Inductively coupled plasma atomic emission spectroscopy analysis confirmed that overexpression of HKT1;1 specifically in the stele reduced the concentration of Na⁺ in the shoot by 37 to 64% and increased retention in the root by up to twofold. X-ray microanalysis of cryogenically frozen roots demonstrated that this was due to retention of Na⁺ within xylem parenchyma and cortical cells. By contrast, constitutive overexpression of HKT1;1 increased both the unidirectional influx of ²²Na⁺ into the root and the flux of ²²Na⁺ to the shoot.

Finally, the authors found that targeted overexpression of HKT1;1 improved a plant's ability to withstand salt stress (see figure). The total dry mass of plants overexpressing HKT1;1 specifically in stelar root cells was not significantly affected by a 5-d salt treatment compared with control plants (although the stature of one of the lines was reduced by HKT1;1 overexpression itself). However, salt treatment resulted in a significant reduction in the total dry biomass of untransformed parental lines as well as transgenic plants constitutively overexpressing HKT1;1.

Figure 1.

Hydroponically grown plants overexpressing HKT1 specifically in the stele continued to thrive in the presence of 100 mM NaCl (A), whereas control plants (B) as well as plants constitutively overexpressing HKT1 (data not shown) exhibited signs of salt stress.

This study demonstrates that manipulating transport processes in specific plant cells may be more effective in modifying the accumulation of solutes in the plant than manipulating these processes indiscriminately. The authors are currently using this strategy to engineer major food crops to have increased resistance to salt stress.