A comparative structural analysis of direct and indirect shoot regeneration of Papaver somniferum L. in vitro

Cellular origin of shoot buds, cell morphogenesis and differentiation were studied during direct and indirect shoot regeneration of Papaver somniferum L. in vitro. Direct shoot organogenesis was induced in immature somatic embryos, where cell division and protomeristem formation started in sub-epidermal and epidermal cell layers of hypocotyl. Indirect shoot regeneration was initiated from callus culture using auxins and cytokinins, where compact globular meristemoids were produced. The common morphogenetic event of direct and indirect shoot regeneration was an establishment of non-random cell division and restricted cell expansion within the group of competent cells during protomeristem formation. However, in contrast to direct regeneration, where all activated cells became competent, in indirect regeneration, only peripheral cells of meristemoids acquired morphogenetic competence. The second difference occurred in shoot tunica formation: original hypocotyl epidermal cells participated in tunica formation during direct organogenesis, while this layer regenerated de novo in meristemoids. These results indicate that cell morphogenesis during shoot regeneration is independent of the developmental history of the competent cells.