Distinct transgenic effects of poplar TDIF genes on vascular development in Arabidopsis

Poplar CLE genes encoding TDIF motifs differentially regulate vascular cambial cell division and woody tissue organization in transgenic Arabidopsis. In Arabidopsis, CLE41 and CLE44 genes encode the tracheary element differentiation inhibitory factor (TDIF) peptide, which functions as a non-cell autonomous signal to regulate vascular development, and overexpression of AtCLE41/CLE44 generate similar phenotypic defects. In poplar, there are six CLE genes (PtTDIF1-4 and PtTDIF-like1-2) encoding two TDIF peptides (TDIF and TDIF-like peptide), which exhibit nearly same activities when exogenously applied to Arabidopsis seedlings. In this work, for each TDIF peptide, we chose two poplar CLE genes (PtTDIF2 and 3 for TDIF, and PtTDIF-like1-2 for TDIF-like peptide) to compare their in vivo effects in transgenic Arabidopsis. Our results showed that transgenic Arabidopsis lines overexpressing each individual PtTDIF gene exhibited dramatically distinct phenotypes associated with vascular development, demonstrating that TDIF motif is not the only functional determinant after genetic transformation. Moreover, we revealed that overexpressed poplar TDIFs enhanced the proliferation of (pro)cambial cells only in hypocotyls, but not in inflorescence stems by differentially regulating the transcriptional levels of WOX4 and WOX14 in these two tissues.