The salt stress-inducible protein kinase gene, Esi47, from the salt-tolerant wheatgrass Lophopyrum elongatum is involved in plant hormone signaling

Protein kinases play a central role in signal transduction in all organisms and to study signal transduction in response to salt stress we have identi fied and characterized a gene encoding a protein kinase that is induced by salt stress and abscisic acid (ABA) in the salt-tolerant wild wheatgrass Lo phapyrum elongatum (Host) A. Love. The product of the early salt stress-ind uced gene, Esi47, was found to belong to the "novel Arabidopsis protein kin ase" group of plant serine/threonine protein kinases. Transient gene expres sion assays in barley aleurone tissue showed Esi47 to suppress the gibberel lin induction of the barley low-pI alpha -amylase gene promoter, thus provi ding evidence for the role of this protein kinase gene in plant hormone sig naling. Esi47 contains a small upstream open reading frame in the 5 ' -untr anslated region of its transcript that is implicated in mediating the repre ssion of the basal level of the gene expression and in regulating the ABA i nducibility of the gene, as shown in the transient gene expression assay in maize callus. Three Arabidopsis homologs of Esi47 were identified, and dif ferent members of this clade of genes showed differential patterns of regul ation by salt stress and ABA in Arabidopsis roots and leaves. At least one of the Arabidopsis homologs contains a small open reading frame in its 5 ' -untranslated region, indicating that the unusual regulatory mechanism iden tified in Esi47 may be widely conserved.