Spatial expression of a sunflower SERK gene during induction of somatic embryogenesis and shoot organogenesis.
Organogenesis or somatic embryogenesis can be induced on immature zygotic embryos (IZE) of sunflower depending on the culture conditions. Both morphogenic processes originate from the same group of cells and show identical kinetics. Using real-time PCR and in situ hybridisation, we showed that somatic embryogenesis receptor-like kinase (SERK) transcripts accumulate early after the beginning of the culture in the morphogenic zone of IZE explants whatever the induction conditions used, i.e. organogenic, embryogenic or highly embryogenic conditions. Quantitative analyses failed to show any correlation between the SERK expression level during the period decisive for the orientation of the morphogenic pathway, i.e. the first 2 days of culture, and the type of morphogenesis induced. However, after 2 days of culture on the organogenic medium, the SERK gene expression level was severely down-regulated in the IZE explants. At 4 days of culture, SERK transcripts were no longer detectable by in situ hybridisation in the developing shoot structures whereas they still continued to accumulate in the embryonic structures induced on both embryogenic and highly embryogenic culture media. The significance of these expression analyses was addressed by transfer medium experiments. Results revealed that IZE cultured on the organogenic medium were able to form somatic embryos when transferred on the highly embryogenic medium as long as the SERK transcripts accumulated at a high level in their morphogenic zone, i.e. first 2 days of culture. Passt this delay, explants rapidly lost their embryogenic competence. Indeed, after 4 days of culture on the organogenic medium, IZE were definitely oriented towards shoot organogenesis. Taken together, these data suggest that reactive cells of IZE develop the competence to somatic embryogenesis during the first day of culture whatever the morphogenic induction conditions used.