High auxin and low cytokinin ratio promotes:
High auxin coupled with low cytokinin specifies root regeneration from callus or shoots, principle underlying rooting stage of micropropagation and natural adventitious rooting of cuttings. Elevated auxin synthesized via TAA1/YUCCA and transported acropetally creates maximum at basal region where founder cells are specified. Auxin binds nuclear receptors TIR1/AFB forming SCF complex that ubiquitinates Aux/IAA repressors for 26S proteasome degradation, freeing ARF7 and ARF19 to transcribe LBD16, LBD18, LBD29 and PLETHORA family genes converting pericycle-like parenchyma into root primordia. Auxin also enhances H+ ATPase activity acidifying wall and induction of expansins facilitating emergence. Low cytokinin is critical because cytokinin signaling via AHK-ARR pathway induces SHY2/IAA3 which suppresses PIN expression reducing auxin accumulation and inhibits root meristem activity. In vitro typical rooting medium contains IBA or NAA 1-3 mg/L with negligible or zero BAP, yielding functional roots with quiescent center expressing WOX5 and SCROW, essential for absorptive function and successful acclimatization after transfer from heterotrophic sterile environment to soil.
Ref: Perianez-Rodriguez et al., Front Plant Sci 2014 auxin root specification; Taiz & Zeiger Chap. 19.