From the Somerville Lab, June 2004Signaling a switch between cell types How do plants make specialized cell types? How do they make sure these specialized cells are formed at the right time, in the right place and in the optimal amounts? Recent work from Carnegie researchers Dominique Bergmann, Wolfgang Lukowitz and Chris Somerville shows how a universally conserved signaling cascade is used to make important developmental decisions during the life of a plant. The epidermis of a plant serves as the interface between the plant and the world, and is comprised of cell types that reflect this role. The shoot epidermis of the model plant Arabidopsis thaliana is a simple tissue consisting of three major cell types: (1) pavement cells that interdigitate to form a strong, impermeable "skin", (2) trichomes, small hairs that serve a protective role, and (3) stomatal guard cells. Stomata are specialized structures through which carbon dioxide enters the plant and water vapor exits and are comprised of a pair of guard cells that flank a pore in the epidermis.
 Figure legend: 1(A) Confocal image of the epidermis of a 5-day old developing Arabidopsis cotyledon. Stomatal guard cells are indicated by * (B) Confocal image of equivalently staged yoda mutant epidermis with the massive over-proliferation of guard cells visible. (C) Confocal image of the gain-of-function (N-terminal deletion) YODA phenotype. All cells have been converted into pavement cells. (D) DIC image of a cleared leaf of the FAMA mutant whose transcript was identified in the YODA microarray experiment. Although pavement cells are clearly formed, the guard cells have been replaced by large caterpillar-like clusters of incompletely differentiated cells. In a recent report in Science (304:1494-1497) Bergmann et al. found a key regulator in stomatal identity. They found that mutations in a MAPKK kinase gene called YODA resulted in plants whose epidermis consisted almost entirely of stomatal guard cells. By removing a negative regulatory domain of the MAPKK kinase and reintroducing it into Arabidopsis, they showed that over-activity of the kinase resulted in plants with absolutely no stomata. This suggested that YODA acts as a switch in stomatal cell development. Too much YODA and no stomata were formed. Too little YODA activity and the epidermis over-filled with clusters of these specialized cells. This provided the researchers with a helpful tool to study stomatal development more generally. Reasoning that genes that are required to make stomata would be expressed in stomata (or the cells about to become them), Bergmann et al. used a microarray analysis to compare the expression of all the genes in the Arabidopsis genome in plants that had no stomata vs. plants with an overabundance of stomata. They found about 100 genes that were up-regulated in plants with extra stomata and down-regulated in plants without stomata. They further showed that disruptioning one of these genes resulted in plants that could not produce mature guard cells. The idea of a MAPKK kinase acting as a cell fate switch is not a new one. Homologous MAPK signaling pathways are used extensively in animal developmental decisions, and a previous report from the Somerville group (Lukowitz et al , Cell 116:109-119) describes the role of YODA in embryonic cell fate decisions. The challenge now is to understand the tissue-specific upstream factors and downstream targets that transmit the YODA MAPKKK signal in these different developmental contexts. |
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