Cytokinin-facilitated plant regeneration of three Brachystelma species with different conservation status

Abstract

In Africa and Asia, members of the genus Brachystelma are well-known for their diverse uses, especially their medicinal and nutritional values. However, the use of many Brachystelma species as a valuable resource is generally accompanied by the concern of over-exploitation attributed to their slow growth and general small size. The aim of the current study was to establish efficient micropropagation protocols for three Brachystelma species, namely Brachystelma ngomense (endangered), Brachystelma pulchellum (vulnerable) and Brachystelma pygmaeum (least concern), as a means of ensuring their conservation and survival. This was achieved using nodal segments (~10 mm in length) as the source of explants in the presence of different concentrations of three cytokinins (CK) namely N6-benzyladenine (BA), isopentenyladenine (iP) and meta-topolin riboside (mTR), over a period of 6 weeks. The highest (25 µM) concentration of cytokinin treatments typically resulted in significantly higher shoot proliferation. However, each species differed in its response to specific CK: the optimal concentrations were 25 µM mTR, 25 µM iP and 25 µM BA for Brachystelma ngomense, Brachystelma pulchellum and Brachystelma pygmaeum, respectively. During the in vitro propagation, both Brachystelma ngomense and Brachystelma pygmaeum rooted poorly while regenerated Brachystelma pulchellum generally lacked roots regardless of the CK treatments. Following pulsing (dipping) treatment of in vitro-regenerated shoots with indole-3-butyric acid (IBA), acclimatization of all three Brachystelma species remained extremely limited due to poor rooting ex vitro. To the best of our knowledge, the current protocols provide the first successful report for these Brachystelma species. However, further research remains essential to enhance the efficiency of the devised protocol.