Expressing AtNHX1 in barley (Hordium vulgare L.) does not improve plant performance under saline conditions

Salinity is one of the main causes of yield loss in agricultural crop production worldwide. Numerous attempts have been made to produce salinity tolerant crops by manipulating the expression of genes involved in ion transport; yet, the reported results are often rather controversial. In this work, Arabidopsis AtNHX1 gene encoding vacuolar Na⁺/H⁺ exchanger was introduced into barley (Horduem vulgare L. cv. Golden Promise) plants grown under saline conditions. Transgenic plants were compared to null segregants for biomass, water content, gas exchange, and Na⁺ and K⁺ content of the leaf. It was shown that most of the lines expressing AtNHX1 have no significant difference to null segregants, suggesting the lack of beneficial effect of expressing the AtNHX1 gene in barley, at least under our experimental conditions. The lack of phenotype might be explained by: (1) low level of activity of vacuolar H⁺-inorganic pyrophosphatase and vacuolar H⁺-ATPase resulting in an insufficient proton gradient required for the vacuolar Na⁺/H⁺ exchanger; (2) the inability of transgenic plants to prevent a passive leak of sodium via Na⁺ permeable slow activating and fast activating channels; (3) insufficient ATP pool to support H⁺ pumping activity; and (4) the fact that the AtNHX1 protein might not been properly folded, inactive or incorrectly targeted. Overall, this study suggests that, while targeting vacuolar Na⁺ sequestration, at least several genes need to be pyramided to ensure the efficiency of the above process.