Session 9 : Vegetative Development II
Chair: B. Scheres, University of Utrecht, The Netherlands
email: b.scheres@cc.ruu.nl
The "vegetative development II" session centered on the functions of vegetative meristemsin development. The first three talks illustrated intimate links between the post-embryonic activities of vegetative meristems and their initiation during embryogenesis, a recurring theme in the meeting: genes involved in diverse aspects of meristem function appear to confer embryonic phenotypes upon mutation. Two aspects of meristem function were explicitly addressed in the session: cell specification, and cell proliferation.Ben Scheres reported on cell specification in the root meristem. Laser ablation studies revealed that cell specification, despite the virtual invariance of the root cell lineage, depends on the continuous presence of positional information. The specification of the distal-most root cap cell types, which arise from the early-embryonic hypophyseal cell, was studied genetically. Plants homozygous for mutations in the HOBBIT gene lack specified cells in the root cap region. Mutants deviate from normal development in the hypophyseal cell region at the pre-globular embryo stage. The connection between early-acting genes and the positional signals that reside in the root meristem will be an important issue in future studies.
Steve Clark addressed the control of cell proliferation in the shoot meristem by studying the CLAVATA1 gene, which appears to encode a putative transmembrane receptor-kinase containing leucine-rich repeats. Mutations in the STM (see Kathy Barton, embryogenesis) and CLV1 genes, both yielding embryonic phenotypes, are mutual suppressors. This was interpreted to signify that STM promotes the undifferentiated state of cells within the meristem, and CLV1 promotes the differentiated state from the embryonic and the post-embryonic shoot meristem. The possibility that the CLV1 protein acts as a receptor for an extracellular ligand opens up genetic (e.g. interactions with two other CLV loci and the suppressor screens that were described) and molecular means to identify signals that may be involved in creating "differentiation boundaries" within the meristem.
Thomas Laux reported on the analysis of the WUSCHEL and ZWILLE genes, which upon mutation affect the proliferation of cells within the shoot meristem. Analysis of floral and shoot meristems in wus single and double mutants suggests that the WUS gene is involved in establishing "central zone" identity both embryonically and post-embryonically. zll mutants are defective in primary shoot formation but they can make normal adventitious shoots post-embryonically. However, zll mutants post-embryonically enhance the phenotypes of other shoot meristem mutants, perhaps illustrating the complexity of interactions that yet have to be uncovered. It will be interesting to learn how WUS and ZLL fit together with STM and the CLV loci in a molecular pathway that defines zones of cellular proliferation.
The embryonic specification of primary meristems, which were the subject of the first three talks in the session, is followed by axillary meristem formation which determines to a large extent the architecture of the mature plant. Vojislava Grbic utilised the STM probe as a marker to follow the formation of axillary meristems in different genetic backgrounds, and demonstrated that it is feasible to identify axillary meristem sites prior to the occurrence of anatomical changes.
A consequence of shoot meristem activity is the formation of leaves in a process that is not yet well-defined genetically. Christoph Fabri genetically addressed a particular aspect of leaf development, which is torsional growth. Three tortifolia loci were described which display intriguing left- and right handed torsion phenotypes as single and double mutants. It was suggested that the wildtype function of these genes may be to orient the leaf. Progress in gene cloning was reported as a first step to reveal the molecular mechanisms behind torsional growth.