The establishment of asymmetry in Arabidopsis lateral root founder cells is regulated by LBD16/ASL18 and related LBD/ASL proteins

In most dicot plants, lateral root (LR) formation, which is important for the construction of the plant root system, is initiated from coordinated asymmetric cell divisions (ACD) of the primed LR founder cells in the xylem pole pericycle (XPP) of the existing roots. In Arabidopsis thaliana, two AUXIN RESPONSE FACTORs (ARFs), ARF7 and ARF19, positively regulate LR formation through activation of the plant-specific transcriptional regulators LATERAL ORGAN BOUNDARIES-DOMAIN 16/ASYMMETRIC LEAVES2-LIKE 18 (LBD16/ASL18) and the other related LBD/ASL genes. The exact biological role of these LBD/ASLs in LR formation is still unknown. Here, we demonstrate that LBD16/ASL18 is specifically expressed in the LR founder cells adjacent to the XPP before the first ACD and that it functions redundantly with the other auxin-inducible LBD/ASLs in LR initiation. The spatiotemporal expression of LBD16/ASL18 during LR initiation is dependent on the SOLITARY-ROOT (SLR)/IAA14-ARF7-ARF19 auxin signaling module. In addition, XPP-specific expression of LBD16/ASL18 in arf7 arf19 induced cell divisions at XPP, thereby restoring the LR phenotype. We also demonstrate that expression of LBD16-SRDX, a dominant repressor of LBD16/ASL18 and its related LBD/ASLs, does not interfere in the specification of LR founder cells with local activation of the auxin response, but it blocks the polar nuclear migration in LR founder cells before ACD, thereby blocking the subsequent LR initiation. Taken together, these results indicate that the localized activity of LBD16/ASL18 and its related LBD/ASLs is involved in the symmetry breaking of LR founder cells for LR initiation, a key step for constructing the plant root system.