Major genes for Na+ exclusion, Nax1 and Nax2 (wheat HKT1;4 and HKT1;5), decrease Na+ accumulation in bread wheat leaves under saline and waterlogged conditions

Two major genes for Na+ exclusion in durum wheat, Nax1 and Nax2, that were previously identified as the Na+ transporters TmHKT1;4-A2 and TmHKT1;5-A, were transferred into bread wheat in order to increase its capacity to restrict the accumulation of Na+ in leaves. The genes were crossed from tetraploid durum wheat (Triticum turgidum ssp. durum) into hexaploid bread wheat (Triticum aestivum) by interspecific crossing and marker-assisted selection for hexaploid plants containing one or both genes. Nax1 decreased the leaf blade Na+ concentration by 50%, Nax2 decreased it by 30%, and both genes together decreased it by 60%. The signature phenotype of Nax1, the retention of Na+ in leaf sheaths resulting in a high Na+ sheath:blade ratio, was found in the Nax1 lines. This conferred an extra advantage under a combination of waterlogged and saline conditions. The effect of Nax2 on lowering the Na+ concentration in bread wheat was surprising as this gene is very similar to the TaHKT1;5-D Na+ transporter already present in bread wheat, putatively at the Kna1 locus. The results indicate that both Nax genes have the potential to improve the salt tolerance of bread wheat.