Involved in the positive regulation of tracheary element (TE) differentiation. Involved in a positive feedback loop that maintains or promotes NAC030/VND7 expression that regulates TE differentiation-related genes (PubMed:19088331). Functions in the initiation and emergence of lateral roots, in conjunction with LBD16, downstream of ARF7 and ARF19 (PubMed:19717544, PubMed:23749813). Transcriptional activator that directly regulates EXPA14, a gene encoding a cell wall-loosening factor that promotes lateral root emergence. Activates EXPA14 by directly binding to a specific region of its promoter (PubMed:22974309). Transcriptional activator that directly regulates EXPA17, a gene encoding a cell wall-loosening factor that promotes lateral root emergence (PubMed:23872272). Acts downstream of the auxin influx carriers AUX1 and LAX1 in the regulation of lateral root initiation and development (PubMed:26059335). Homodimer and heterodimer with LBD16 (PubMed:23430048). Interacts with GIP1 (PubMed:24484953). Expressed in roots, stems, leaves and flowers (PubMed:12068116, PubMed:24484953). Expressed in vascular tissues of hypocotyls, leaves, roots, developing floral organs and siliques (PubMed:19088331). During lateral root formation, expressed in the lateral root primordia, and the developing, emerged, and mature lateral roots. By auxin. Reduced number of lateral roots. Plants over-expressing LBD18 have a dwarf and bushy phenotype, with short petioles, curled downward leaves, ectopic shoots from the adaxial side of cotyledons, shrunken root tips and disorganized columella cells. Belongs to the LOB domain-containing protein family.