Practice question

Rhizogenesis denotes formation of roots from callus, shoots or explants, derived from rhiza root, complementing caulogenesis in organogenesis. It is induced predominantly by elevated auxin levels and low cytokinin, mirroring natural lateral root formation. Molecular basis involves creation of localized auxin maxima through biosynthesis via YUCCA pathway and polar transport by PIN1, PIN2, AUX1 carriers concentrating auxin in founder cells. Perception via TIR1/AFB F-box receptors triggers degradation of Aux/IAA repressors liberating ARF7 and ARF19 transcription factors that induce LBD16, LBD29 and PLETHORA genes specifying pericycle-like cells to organize quiescent center expressing WOX5. Auxin also activates plasma membrane H+ ATPase causing apoplast acidification, expansin mediated wall loosening and initiation of primordia. Cytokinin antagonism is relieved when cytokinin level drops, allowing downregulation of SHY2 inhibitor and restoration of PIN expression. In tissue culture, IBA or NAA at 0.5-3 mg per L triggers adventitious root emergence within 10 days from shoot bases, essential for final rooting stage of micropropagated plantlets before hardening and field transfer ensuring autotrophic survival.