LBD37, LBD38, and LBD39 are redundantly involved in regulating lateral root development and elongation in Arabidopsis thaliana.

The LATERAL ORGAN BOUNDARIES DOMAIN (LBD) genes encode a family of transcription factors harboring a conserved LOB domain that regulate diverse aspects of plant growth and development. Arabidopsis LBD proteins are grouped into two classes according to their domain structures. Class I proteins, which function as typical DNA-binding transcription factors, are relatively well characterized, whereas the biological functions of class II proteins remain largely unknown. Here, we show that LBD37, LBD38, and LBD39-three class II LBD genes-redundantly regulate lateral root (LR) formation and elongation. Although single and double lbd mutants exhibited LR densities comparable to wild type, the lbd37,38,39 triple mutant displayed significantly reduced LR density with no change in primary root length. Additionally, the triple mutant showed shorter LR length and reduced LR primordium density at stage I relative to wild type. Furthermore, the triple mutant exhibited a reduced meristem size and fewer cells in the LR-with no alteration in cell size-indicating that LBD37/38/39 modulate LR elongation by promoting cell proliferation, rather than cell expansion, within the LR meristem. Taken together, these results demonstrate that LBD37/38/39 redundantly regulate an early stage of LR development and LR elongation.