A novel function of TDIF-related peptides: promotion of axillary bud formation

Small peptides derived from the CLAVATA3/EMBRYO SURROUNDING REGION-related (CLE) gene family play a key role in various cell-cell communications in land plants. Among them, tracheary element differentiation inhibition factor (TDIF; CLE41/CLE44 peptide) and CLE42 peptide of Arabidopsis have almost identical amino acid sequences and act as inhibitors of tracheary element differentiation. In this study, we report a novel function of TDIF and CLE42. We found by the GUS (β-glucuronidase) reporter gene assay that while CLE41 and CLE44 are expressed preferentially in vascular bundles, CLE42 is expressed strongly in the shoot apical meristem (SAM) and axillary meristems. Overexpression of CLE42 and CLE41 enhanced axillary bud formation in the leaf and cotyledon axils. Before floral transition, the emergence of axillary buds in these plants occurred in an acropetal order. Exogenous supply of either TDIF or CLE42 peptide to the wild type induced similar excess bud emergence. In vascular bundles, the TDIF RECEPTOR (TDR) acts as the main receptor for TDIF. The axillary bud emergence of tdr mutants was little affected by either of the peptides. It was confirmed by scanning electron microscopy that peptide-treated wild-type plants form an axillary meristem-like structure earlier than non-treated plants. SHOOT MERISTEMLESS (STM), a marker gene for meristems, was up-regulated in peptide-treated plants before the axillary meristem becomes morphologically distinguishable. These results indicate that CLE42 peptide and TDIF have an activity to enhance axillary bud formation via the TDR. Judging from its expression pattern, CLE42 may play an important role in the regulation of secondary shoot development.