Abstract

The cDNA and genomic DNA clones for the salt stress and abscisic acid (ABA) inducible gene Esi47 were isolated from the salt-tolerant tall wheatgrass species Lophopyrum elongatum . The Esi47 gene encodes a protein kinase with highest amino acid sequence similarity to the NAK group of protein serine/threonine kinases. Esi47 and the NAK group protein kinases have a central catalytic domain flanked by short amino and carboxyl terminal non-catalytic domains. The NAK group was named after the first gene isolated in this group, the Arabidopsis thaliana NAK gene (Hardie, 1999; Moran and Walker, 1993). However, none of the genes in this group has been implicated to be involved in plant signaling pathways except for the Arabidopsis gene ARSK1 , which has only been shown to be inducible by salt stress and ABA in plant roots (Hwang and Goodman, 1995). The protein phosphorylation activity of the Esi47 gene product was demonstrated in Escherichia coli . Arabidopsis plants transformed with Esi47 did not show any enhanced salt stress tolerance. Approximately 60 Arabidopsis genes previously shown to be regulated by salt stress or ABA were evaluated for altered expression in the transgenic plants by northern analysis, but none were found to be affected by Esi47 overexpression. However, transient expression of Esi47 in barley aleurone suppressed the induction of the barley gene for Ì-amylase by the plant hormone gibberellin, which is an ABA antagonist. This indicates that the Esi47 gene is involved in plant hormone signaling. Analysis of the primary structure of the Esi47 gene revealed that it contains in the 5 ' -untranslated region of its mRNA a small upstream open reading frame that has been demonstrated to mediate the ABA regulation of Esi47 expression in an independent study. Three Arabidopsis homologs of Esi47 were identified and different members of this clade of genes showed differential patterns of regulation by salt stress and ABA in Arabidopsis roots and leaves. At least one of the Arabidopsis homologs, F8A24.12 , contains a small open reading frame in its 5 ' -untranslated region, indicating that the unusual regulatory mechanism identified in Esi47 may be widely conserved. The identification of the Arabidopsis genes homologous to Esi47 would facilitate further functional characterization of this type of gene in plant stress and hormone signaling pathways in a model plant species.