In vitro propagation of Holostemma annulare (Roxb.) K. Schum., a rare medicinal plant

A rapid micropropagation system was established for Holostemma annulare (Roxb.) K. Schum., (H. ada-kodien R. Br. ex Schult; Asclepiadaceae), a rare medicinal plant. Shoot tips (0.5-0.8 cm) and terminal and basal nodes (1.0-1.5 cm) harvested from actively growing shoots of conventionally raised plants were cultured on Murashige and Skoog (MS) medium supplemented with various concentrations of 6-benzyladenine (BA) and α-naphthaleneacetic acid (NAA). Multiple shoot formation (3.8) was observed in 68% of basal nodes cultured on medium with optimum concentration of 4.43 μM BA and 0.54 μM NAA after 8 wk. Terminal nodes were not suitable for inducing multiple shoots. Irrespective of the orientation (vertical/horizontal), all shoot tip explants responded with a single shoot in all the combinations of plant growth regulators tried. Effects of other cytokinins (kinetin and 2-isopentenyladenine) and auxins [indole-3-acetic acid and indole-3-butyric acid (IBA)] to enhance the regeneration potential of basal nodes were analyzed. Shoots were multiplied by subculture of basal nodes and stumps (the original explant tissue free of shoots, but with remnant axillary meristem and two or three protruding buds) in a reduced concentration of BA (2.21 μM and NAA (0.27 μM). Liquid medium for multiplication was found to be ineffective due to a high degree of hyperhydricity. To make the multiplication process cost effective, culture bottles with polypropylene caps were used for multiplication. The best root induction (75%) and survival (80%) was achieved on 0.5 strength MS medium supplemented with 1.48 μM IBA. Field-established plants had uniform growth habit traits in terms of height of plants and number, length, and weight of the tuberous roots.