Auxins mainly promote:
Auxins represent principal hormone class promoting root initiation and development, encompassing natural indole-3-acetic acid IAA synthesized via Trp dependent TAA1 YUCCA flavin monooxygenase pathway and synthetic analogues NAA, IBA, 2,4-D with higher stability and resistance to enzymatic oxidation by IAA oxidases. For rhizogenesis auxin maximum generated at basal end through PIN efflux carrier mediated polar transport triggers pericycle or phloem parenchyma cells to become root founder cells. Signaling involves TIR1/AFB auxin receptors part of SCF ubiquitin ligase complex that in presence of auxin ubiquitinates Aux/IAA repressors for degradation freeing Auxin Response Factors ARF7 ARF19 to induce downstream genes LBD16 LBD29 WOX5 defining quiescent center and columella. Cellular effect includes activation of H+ ATPase acidifying apoplast activating expansins wall loosening enabling primordia emergence. In micropropagation rooting phase exogenous IBA 0.5-2 mg per L induces adventitious roots from shoot bases within 7-14 days. High auxin represses cytokinin responses ensuring root program dominates over shoot proliferation, critical for acclimatization and field establishment.
Ref: Woodward & Bartel Ann Bot 2005 Auxin biology; Lavy & Estelle Development auxin signaling.