The Transition between Shoot Regeneration Competence and Callus Determination in Internodal Stem Explants of Populus deltoides
Experiments were conducted to monitor the competence status of internodal stem expiants of 15 Populus deltoides genotypes in in vitro culture. The focus of this study was to investigate the transition from shoot regeneration competent to callus determined growth when in the presence of the inducer zeatin. Shoot regeneration competence and callus determination were measured by transferring expiant tissue from callus inducing medium (CIM: WNA medium supplemented with 0.5 mgl-1 2,4-D) to shoot inducing medium (SIM: WNA medium supplemented with 0.5 mgl-1 zeatin). Transfers from CIM to SIM were made at 1, 2, 4, 6, 8, and 10 day intervals. The number of regenerated shoots per expiant and the percent of expiants regenerating at least one shoot were determined after 60 days.
Three general expiant competence responses were observed among the 15 Populus deltoides genotypes: (1) two genotypes were initially competent, with little increase in shoot regeneration by culture on CIM before transfer to SIM; (2) seven genotypes were not initially competent for shoot regeneration, but competence was acquired by initially culturing the explants on CIM before transfer to SIM, and this resulted in marked increases in shoot regeneration; and (3) six genotypes were not initially competent, and showed only slight competence enhancement after initial culture on CIM and produced relatively few adventitious shoots. The competence state transition from high levels of shoot regeneration to callus determination was very marked for the initially competent and competence acquired genotypes. Expiants cultured on CIM for 6 days before transfer to SIM produced a relatively large number of shoots. However, expiants subjected to CIM for 8 days before transfer to SIM produced few if any shoots, and the expiants became determined for callus growth regardless of how long they remained on SIM. Genotypic responses to the different treatments will be discussed, in addition to preliminary results of analysis of protein differences associated with the competence state changes.
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