Morpho- and histodifferentiation of shoot regeneration of Billbergia zebrina (Helbert) Lindley nodular cultures

Regeneration of nodular cultures involves the production of new individuals from organogenic nodules. Despite the existence of well-established protocols for bromeliads species, many gaps in understanding the mechanisms of this route still remain, particularly the regeneration of new shoots, a stage little explored. Therefore, this work aimed to monitor the morpho and histodifferentiation processes of shoot regeneration from nodular cultures of Billbergia zebrina . To accomplish this, nodular cultures previously obtained from nodal segments of B. zebrina were transferred to regeneration induction medium consisting of semi-solid MS medium supplemented with 1.0 µM of naphthalene acetic acid and 2.0 µM of benzylaminopurine, respectively. Cultures were maintained in a growth chamber over 8 weeks, and samples were collected weekly for analysis under light, transmission electron, and scanning electron microscopy. Cells in the shoot formation region presented characteristics of mitotically active cells, such as the presence of numerous plasmodesmata and conspicuous microtubules. New shoots maintained vascular connection to the node. Cells binding the nodule vascular bundle toward the shoot showed degeneration of organelles, which, in turn, originated vascular neoelements. The dome presented a depressed or prominent surface, depending on shoot development. Rounded stomata were conspicuous in the leaf primordia and generally higher in number as compared to the epidermal surface. This study elucidated the nodular culture in vitro system in aspects associated to the regeneration of shoots and it is anticipated that such data will lead to the optimization of present nodular cultures based protocols with the aim of conserving endangered bromeliad species.