A mutant ankyrin protein kinase from Medicago sativa affects Arabidopsis adventitious roots

A family of plant kinases containing ankyrin-repeats, the Ankyrin-Protein Kinases (APKs), shows structural resemblance to mammalian Integrin-Linked Kinases (ILKs), key regulators of mammalian cell adhesion. MsAPK1 expression is induced by osmotic stress in roots of Medicago sativa (L.) plants. The Escherichia coli-purified MsAPK1 could only phosphorylate tubulin among a variety of substrates and the enzymatic activity was strictly dependent on Mn²⁺. MsAPK1 is highly related to two APK genes in Arabidopsis thaliana (L.), AtAPK1 and AtAPK2. Promoter-GUS fusions assays revealed that the Arabidopsis APK genes show distinct expression patterns in roots and hypocotyls. Although Medicago truncatula (L.) plants affected in MsAPK1 expression could not be obtained using in vitro regeneration, A. thaliana plants expressing MsAPK1 or a mutant MsAPK1 protein, in which the conserved aspartate 315 of the kinase catalytic domain was replaced by asparagines (DN-lines), developed normally. The DN mutant lines showed increased capacity to develop adventitious roots when compared with control or MsAPK1-expressing plants. APK-mediated signalling may therefore link perception of external abiotic signals and the microtubule cytoskeleton, and influence adventitious root development.