Expression levels and promoter activities of candidate salt tolerance genes in halophytic and glycophytic Brassicaceae

We compared six Brassicaceae glycophytes and halophytes for salt tolerance and the expression levels in roots and shoots of the candidate salt tolerance genes, NHX1, SOS1, and VATD, encoding the tonoplast Na+/H+ antiporter, the plasma-membrane-located Na+/H+ antiporter, and subunitD of the tonoplast W-ATPase, respectively. Salt tolerance decreased in the order of Cochlearia x hollandica >> Cochlearia danica/Thellungiella botschantzevii > Brassica oleracea> Thlaspi arvense > Arabidopsis thaliana. The highest expression levels of NHX1, SOS1, and VATD were consistently found in C. x hollandica, both in shoots and roots, and both in control plants and salt-treated ones. Salt-imposed induction of NHX1 was observed in C. danica (shoot and root) and B. oleracea (shoot). SOS1 was up-regulated by salt treatment in the shoots of C. x hollandica and C. danica, and VATD in the shoot of T. arvense. Expression of NHX1 genomic DNA under the C x hollandica NHX1 promoter in the A.t.nhx1 mutant background yielded, irrespective of the genomic DNA source, 20-fold and 2-fold enhanced expression levels, in comparison with those in wild-type A. thaliana and C x hollandica, respectively. This suggests that the high expression level in C. x hollandica is completely explained by altered cis-regulation of this gene. Promoter swap experiments showed that the C x hollandica SOS1 and VATD promoters were fivefold and two-fold more active than the corresponding A. thaliana promoters, respectively. However, particularly in the case of VATD, this is not sufficient to explain the difference in the wild-type expression levels between C. x hollandica and A. thaliana. (C) 2013 Elsevier B.V. All rights reserved.