LL3, a homolog of LONESOME HIGHWAY, regulates vascular cell proliferation in the root apical meristem

Vascular bundles, composed of various cell types, are essential for the transport of water and various molecules throughout the plant body. Transcriptional complexes consisting of LONESOME HIGHWAY (LHW) and TARGET OF MONOPTEROS5 regulate vascular development, particularly in two aspects: vascular cell proliferation, which increases the number of vascular cell files, and xylem differentiation in the Arabidopsis root. LHW has three homologs: LHW-LIKE 1 (LL1), LHW-LIKE 2 (LL2), and LHW-LIKE 3 (LL3). In our previous study, we demonstrated that LL1 predominantly contributes to xylem differentiation together with LHW, while its involvement in vascular cell proliferation appears to be limited. The involvement of homologs other than LHW in vascular cell proliferation remains unknown, despite the critical importance of vascular cell proliferation in the initial process of vascular development. Therefore, we investigated the roles of LL2 and LL3 in vascular cell proliferation in this study. Although single loss-of-function mutants of ll2 and ll3 did not exhibit obvious phenotypes, the lhw ll3 double mutant displayed severe defects in root vascular development. In lhw ll3 roots, only one or a few vascular cells were formed, where phloem differentiation was observed but xylem differentiation was absent. In addition, introducing LL3 into lhw could rescue the lhw phenotype. These results suggest that LL3 has a redundant role with LHW in root vascular cell proliferation, and that both LHW and LL3 are essential regulators for this process. Thus, our work indicates that different LHW homologs contribute to distinct functions of LHW in root vascular development.