Abstract
WPK4, a gene encoding a putative protein kinase, was initially identified in wheat (Triticum aestivum) and shown to be up-regulated by light, nutrient deprivation, and cytokinins. To confirm that WPK4 has protein kinase activity, the protein was produced in Escherichia coli as a fusion protein with glutathione S-transferase. The purified protein exhibited autophosphorylation activity and phosphorylated both myelin basic protein and a peptide fragment of rice 3-hydroxy-3-methylglutaryl-coenzyme A reductase. Levels of WPK4 transcripts in wheat seedlings were increased and decreased by the removal and addition of sucrose (Suc), respectively, to the culture medium. The introduction of the N-terminal kinase region of WPK4 into the yeast snf1 mutant cells, which cannot utilize Suc as a carbon source, rescued growth in Suc-containing medium. Cytokinins up-regulated the accumulation of WPK4 transcripts, but their effects were cancelled by the addition of Suc. Our results suggest that Suc negatively regulates the signaling pathway in which transcriptional activation of WPK4 is mediated by cytokinins.