Competence, determination, and meristemoid plasticity in tobacco organogenesis in vitro

Tobacco leaf explants can produce both shoots and roots depending on the phytohormones in the medium. These arise directly via meristemoids (meristematic centers), which form distinct primordia and then organs. In this study it was found that shoot primordia arose from the palisade mesophyll cells at the adaxial surface, while root primordia arose from the rib parenchyma cells, near the existing vascular bundles. In studies on competency and determination, it was found that the tobacco leaf explants required 4-6 days in culture on a shoot-inducing medium (SIM) to become determined for shoot formation, while the explants were competent for rooting at excision and needed only 1 day on the root-inducing medium (RIM) to become determined for root formation. Transfer of explants from SIM or RIM to basal medium (BM without phytohormones) and vice versa supported the above findings. Transfer of explants from SIM to RIM and vice versa, delayed the timing of root and shoot formation, but not the position in the explant from which the organs arose. On transfer from SIM to RIM or vice versa, meristemoids that were already determined for shoot or root formation continued to develop, while those not yet determined were inhibited and (or) reverted to parenchymatous tissue. Thus under our culture conditions meristemoids in tobacco leaf explants are not plastic.