Session 4: Embryogenesis
Chair: Gerd Juergens, Universitaet Tuebingen, Federal Republic of Germany
email: geju@fserv1.mpib-tuebingen.mpg.de
Various aspects of embryogenesis were addressed in the talks and posters of this session: pattern formation, morphogenesis, cell expansion, cytokinesis, promoter traps and somatic embryogenesis.K. Barton (Madison) gave a detailed description of the localised expression in the embryo of the STM gene involved in shoot meristem development (Long et al., 1996, Nature 379, 66-69). STM expression starts in the globular embryo and then the pattern evolves in a specific way. No STM expression was observed in topless embryos, while pinhead affects the later refinement, and clv1 and clv3 enlarge the STM expressing region. Conversely, UFO is not expressed in stm heart-stage embryos. These results suggest that STM plays an important early role in shoot meristem development. Related posters presented shoot meristem mutants (zll, wus, stm; T. Laux lab, Tuebingen) and root meristem mutants (hbt ; B. Scheres lab, Utrecht).
R.A. Torres Ruiz (Munich) described mutations in several genes affecting apical development of the embryo and reported on progress in cloning two of the genes, GK and PEP. The MP gene involved in basal development of the embryo was subject of a poster (T. Berleth lab, Munich). The MP gene which is also required for postembryonic development may play a role in cell "axialisation". This idea will soon be put to test as cloning of the MP gene is almost done.
Two talks addressed the problem of embryo polarity. J.Z. Zhang (C. Somerville lab, Stanford) reported on the twn2 mutation which produces twin embryos. The embryogenic apical daughter cell of the zygote does not divide but persists on top of a new embryo which is "regenerated" from the extra-embryogenic basal cell. Surprisingly, the TWN2 gene appears to encode a valyl tRNA synthese-like protein. G. Juergens (Tuebingen) in his introductory talk summarised what is known about the GNOM/EMB30 gene and its role in embryogenesis. The 163 kDa gene product shows limited sequence similarity ("sec7 domain") to the yeast secretory protein sec7p (Shevell et al., 1994, Cell 77, 1051-1062). More similarity was found to another yeast ORF which is not essential for cell viability in yeast (Busch et al., 1996, Molec. Gen. Genet. 250, 681-691). While the primary function of the GN protein is still unclear, a molecular marker for apical development is variably located in gn embryos (Vroemen et al., 1996, Plant Cell 8, 783-791).
Molecular markers would aid in analysing developmental mutants, and two posters presented studies of promoter trap lines with interesting GUS expression patterns (K. Lindsey lab, Durham, and P. Gallois lab, Perpignan). Another approach uses the visualisation of the cytoskeleton with tubulin antibodies (U. Mayer lab, Tuebingen) which may particularly be useful to characterise morphogenesis (embryo shape) mutants, such as fass, knopf, enano (R.A. Torres Ruiz lab, Munich), mickey and obelix (Beeckman et al, Gent/Munich) and cytokinesis mutants, such as keule (F. Assaad et al., Munich/Tuebingen), cyt1 (formerly: emb101; D. Meinke lab, Stillwater OK), and knolle. The KNOLLE gene which encodes a cytokinesis-specific syntaxin-related protein (Lukowitz et al., 1996, Cell 84, 61-71) was discussed in more detail by G. Juergens (Tuebingen).
In conclusion, the study of embryogenesis has become more diverse, still lacking a unifying conceptual framework. New approaches and unexpected findings will certainly continue to challenge established views. For example, somatic embryogenesis may soon become a routine procedure for testing developmental mutants as well as a source of new molecules, such as the somatic embryogenesis receptor kinase (SERK) associated with embryogenic potential (S. De Vries lab, Wageningen).