The alfalfa (Medicago sativa) TDY1 gene encodes a mitogen-activated protein kinase homolog

Development of root nodules, specifically induction of cortical cell divisi on for nodule initiation, requires expression of specific genes in the host and microsymbiont. A full-length cDNA clone and the corresponding genomic clone encoding a MAP (mitogen-activated protein) kinase homolog were isolat ed from alfalfa (Medicago sativa), The genomic clone, TDY1, encodes a 68.9- kDa protein with 47.7% identity to MMK4, a previously characterized MAP kin ase homolog from alfalfa, TDY1 is unique among the known plant MAP kinases, primarily due to a 230 amino acid C-terminal domain. The putative activati on motif, Thr-Asp-Tyr (TDY), also differs from the previously reported Thr- Glu-Tyr (TEY) motif in plant MAP kinases. TDY1 messages were found predomin antly in root nodules, roots, and root tips. Transgenic alfalfa and Medicag o truncatula containing a chimeric gene consisting of 1.8 kbp of 5' flankin g sequence of the TDY1 gene fused to the beta-glucuronidase (GUS) coding se quence exhibited GUS expression primarily in the nodule parenchyma, meriste m, and vascular bundles, root tips, and root vascular bundles. Stem interno des stained intensely in cortical parenchyma, cambial cells, and primary xy lem. GUS activity was observed in leaf mesophyll surrounding areas of mecha nical wounding and pathogen invasion. The promoter was also active in root tips and apical meristems of transgenic tobacco. Expression patterns sugges t a possible role for TDY1 in initiation and development of nodules and roo ts, and in localized responses to wounding.