Control of Stomatal Formation in Arabidopsis Thaliana Inflorescence Stem
Stomata are cellular structures that control water loss and gas exchange through the plant epidermis. Stomata arise from special stem cells called meristemoids through a series of programmed asymmetric divisions that are controlled by cell signaling, or via multitude of regulatory pathways and intercellular communication between epidermal cells. In Arabidopsis thaliana, stomata are spaced non-randomly in the epidermis by cell-cell signaling of the receptor-like protein TOO MANY MOUTHS (TMM) as well as other proteins. Point mutation of the TMM gene prevents the development of stomata in some tissues like inflorescence stems. Investigation of tmm mutant stems showed that self-renewing stem cell-like precursors form by dividing asymmetrically but fail to form stomata. This is further supported by molecular markers of stomatal cell fate that show stomatal precursors form but do not differentiate as stomata. Therefore, TMM signaling is likely required to control expression of genes that are essential for the formation of stomata in stems. As a second approach, gene expression profiling was used to identify candidate genes involved in stomatal biogenesis. Differentially expressed genes were categorized by gene ontology and analyzed for statistically overrepresented classes to gain insight into functional processes. Comparison of stem expression data with previously published microarray data was used to narrow the list to genes involved in stomatal patterning. Mutants in these target genes have been obtained and phenotypic analysis revealed new stomatal regulators. Comparison of epidermal cells of the stem tip region from wild-type and tmm revealed that there are significantly more meristemoids formed in tmm stems compared to wild-type stems. In addition, the orientation of meristemoids formed in wild-type stems was random with respect to stem polarity and followed a spiral pattern of asymmetric divisions similar to leaves. This showed that stomatal patterning in dicots does not follow orientation in asymmetric cell division for spacing the adjacent stomata like monocots.
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