Auxin and root initiation in somatic embryos of Arabidopsis.

Somatic embryos of Arabidopsis thaliana can be produced from explants of developing zygotic embryos. Cultivation of explants on maturation medium results in development of three main classes of regenerative structures: adventitious shoots, fused shoots, and complete somatic embryos. The ontogeny and anatomy of these structures was examined using serial plastic sections. Furthermore, two molecular markers were assayed to monitor transcriptional auxin responses and formation of a root meristem in this process: the LENNY allele of PIN4, a transposon insertion creating a fusion to the reporter gene GUS; and DR5::GUS, a synthetic reporter of auxin-induced transcription. In zygotic embryogenesis, PIN4 expression is confined to the center of the root meristem and begins to be detectable by the globular stage of embryogenesis, while DR5::GUS expression marks an "auxin perception maximum" in the more distal regions of the root. Adventitious and fused shoots develop no anatomically recognizable root meristem and do not express either of the two markers at their basal pole. Instead, the vasculature of their axis is directly connected to the vasculature of the explant. By contrast, complete somatic embryos were only loosely attached to the explant, had an anatomically defined root meristem and showed expression of both markers at their root pole. Our results suggest that the establishment of a root meristem in somatic embryos required appropriate auxin levels during the course of their development.