Co-expression of vacuolar Na(+)/H(+) antiporter and H(+)-pyrophosphatase with an IRES-mediated dicistronic vector improves salinity tolerance and enhances potassium biofortification of tomato.

Potassium (K) deficiency is a worldwide problem. Thus, the K biofortification of crops is needed to enhance human nutrition. Tomato represents an ideal candidate for such biofortification programs thanks to its widespread distribution and its easy growth on a commercial scale. However, although tomato is moderately tolerant to abiotic stresses, the crop losses due to salinity can be severe. In this study, we generated transgenic tomato plants over-expressing a Na(+)-K(+)/H(+) exchanger gene (TNHXS1), singly or with H(+)-pyrophosphatase (H(+)-PPiase) gene using a bicistronic construct. Transgenic tomato lines co-expressing both genes (LNV) significantly showed higher salinity tolerance than the wild-type (WT) plans or those expressing the TNHXS1 gene alone (LN). Indeed, under salt stress conditions, double transgenic plants produced higher biomass and retained more chlorophyll and catalase (CAT) activity. In addition, they showed earlier flowering and produced more fruits. To address K deficiencies in humans, an increase of 50% in K content of vegetable products was proposed. In this study, ion content analysis revealed that, under salt stress, fruits from double transgenic plants accumulated 5 times more potassium and 9 times less sodium than WT counterparts. Interestingly, the ionomic analysis of tomato fruits also revealed that LNV had a distinct profile compared to WT and to LN plants. Indeed, LNV fruits accumulated less Fe(2+), Ca(2+), Mg(2+) and Zn(2+), but more Mn(2+). This study demonstrates the effectiveness of bicistronic constructs as an important tool for the enhancement of biofortification and salt stress tolerance in crops.