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Background Plants have the lifelong ability to generate new organs due to the persistent functioning of stem cells. In seed plants, groups of stem cells are housed in the shoot apical meristem (SAM), root apical meristem (RAM), and vascular cambium (VC). In ferns, a single shoot stem cell, the apical cell, is located in the SAM, whereas each root initiates from a single shoot-derived root initial.  WUSCHEL-RELATED HOMEOBOX  ( WOX ) family transcription factors play important roles to maintain stem-cell identity.  WOX  genes are grouped phylogenetically into three clades. The T3WOX/modern clade has expanded greatly in angiosperms, with members functioning in multiple meristems and complex developmental programs. The model fern  Ceratopteris richardii  has only one well-supported  T3WOX /modern  WOX  gene,  CrWUL . Its orthologs in  Arabidopsis ,  AtWUS ,  AtWOX5 , and  AtWOX4 , function in the SAM, RAM, and VC, respectively. Identifying the function of  CrWUL  will provide insights on the progenitor function and the diversification of the modern  WOX  genes in seed plants. Results To investigate the role of CrWUL in the fern, we examined the expression and function of  CrWUL  and found it expresses during early root development and in vasculature but not in the SAM. Knockdown of CrWUL by RNAi produced plants with fewer roots and fewer phloem cells. When expressed in  Arabidopsis  cambium,  CrWUL  was able to complement  AtWOX4  function in an  atwox4  mutant, suggesting that the WOX function in VC is conserved between ferns and angiosperms. Additionally, the proposed progenitor of  T3WOX  genes from  Selaginella kraussiana  is expressed in the vasculature but not in the shoot apical meristem. In contrast to the sporophyte, the expression of  CrWUL  in the gametophyte exhibits a more general expression pattern and when knocked down, offered little discernable phenotypes. Conclusions The results presented here support the occurrence of co-option of the  T3WOX/ modern clade gene from the gametophyte to function in vasculature and root development in the sporophyte. The function in vasculature is likely to have existed in the progenitor of lycophyte  T3WOX /modern clade genes and this function predates its SAM function found in many seed plants.

Vascular function of the T3/modern clade WUSCHEL-Related HOMEOBOX transcription factor genes predate apical meristem-maintenance function