LONESOME HIGHWAY‐TARGET OF MONOPTEROS5 transcription factor complex promotes a predifferentiation state for xylem vessel differentiation in the root apical meristem by inducing the expression of VASCULAR‐RELATED NAC‐DOMAIN genes

Summary In Arabidopsis thaliana, heterodimers comprising two bHLH family proteins, LONESOME HIGHWAY (LHW) and TARGET OF MONOPTEROS5 (TMO5) or its homolog TMO5‐LIKE 1 (T5L1) control vascular development in the root apical meristem (RAM). The LHW‐TMO5/T5L1 complex regulates vascular cell proliferation, vascular pattern organization, and xylem vessel differentiation; however, the mechanism of preparation for xylem vessel differentiation in the RAM remains elusive. We examined the relationship between LHW‐T5L1 and VASCULAR‐RELATED NAC‐DOMAIN (VND) genes, which are key regulators of vessel differentiation, using reverse genetics approaches. LHW‐T5L1 upregulated the expression of VND1, VND2, VND3, VND6, and VND7 but not that of other VNDs. The expression of VND1–VND3 in the RAM was decreased in lhw. In vnd1 vnd2 vnd3 triple loss‐of‐function mutant roots, metaxylem differentiation was delayed, and VND6 and VND7 expression was reduced. Furthermore, transcriptome analysis of VND1‐overexpressing cells revealed that VND1 upregulates genes involved in the synthesis of secondary cell wall components. These results suggest that LHW‐T5L1 upregulates VND1–VND3 at the early stages of vascular development in the RAM, and VNDs promote a predifferentiation state for xylem vessels by triggering low levels of VND6 and VND7 as well as genes for the synthesis of secondary cell wall materials.