Bud regeneration growth and proliferation of aspen (Populus tremula) roots in liquid cultures

Aspen root explants, isolated from in vitro shoots, were cultured in liquid medium on a gyratory shaker or in a bubble column bioreactor. The roots regenerated as many as seven adventitious buds per 10-15 mm long root explant. All the adventitious buds developed in close proximity to lateral roots, which originated from the pericycle of the main root. The bud meristems divided to form a multilayered tissue and were organized at the periphery of the pericycle of the lateral roots. Bud differentiation and their further growth caused the disruption and disorganization of the cortical and epidermal tissue of the roots, exposing the developing buds to the medium. Polarity along the root was a major factor in controlling bud differentiation, their number increasing basipetally. Root explants, 15-20 mm long including the tip, inoculated to 1.21 bioreactor cultures in 1/2 MS minerals with full organic medium, regenerated several buds. Paclobutrazol (PAC), a gibberellin biosynthesis inhibitor, reduced shoot elongation of the differentiating buds and promoted shoot cluster formation. A higher number of the root explants regenerated buds, although the number of buds per explant was similar to the control. Cluster formation was observed also in liquid cultured axillary buds or adventitious buds on liquid cultured roots, in the presence of PAC. The results of the present work indicating the potential of roots as a regenerative system in bioreactor cultures will be discussed.