Cellular differentiation in the shoot epidermis

Stomatal development in Arabidopsis

Stomata consist of two guard cells around a pore and act as turgor-operated valves for gas exchange. Arabidopsis stomata develop from one or more asymmetric divisions followed by the symmetric division of the guard mother cell. Stomatal number is partly a function of the availability of smaller epidermal cells that are competent to divide asymmetrically. Stomata are spaced apart from each other by at least one neighbor cell. Pattern generation may involve cell-cell signaling that transmits spatial cues used to orient specific classes of asymmetric divisions. TOO MANY MOUTHS may function in receiving or transducing these cues to orient asymmetric divisions. TMM also is a negative or positive regulator of entry into the stomatal pathway, with the direction of the response dependent on organ and location. STOMATAL DENSITY AND DISTRIBUTION1 is a negative regulator of stomatal formation throughout the shoot and encodes a processing protease that may function in intercellular communication. FOUR LIPS apparently controls the number symmetric divisions at the guard mother cell stage. In some organs, such as the hypocotyl, the placement of stomata may be coordinated with internal features and involves genes that also regulate root hair and trichome formation. Other mutations affect guard cell morphogenesis, cytokinesis, and stomatal number in response to carbon dioxide concentration. The molecular analysis of stomatal development promises advances in understanding intercellular signaling, the control of the plane and polarity of asymmetric division, the specification of cell fate, and the regulation of cell differentiation and shape.

Tissue patterning of Arabidopsis cotyledons

•  Trichome and stomatal patterning are not independent events because trichomes form before stomata. We thought trichome genes might provide spatial referents to ensure proper distribution of stomata for gas exchange, and therefore studied mutants of GL1 and TRY using stomatal pattern of the entire cotyledon surface as the indicator. •  Mature cotyledons were imaged by SEM, stomatal maps were generated, and data were spatially analysed. Expression of GL1 and TRY was determined in wild type and mutant samples by reverse transcriptase-PCR (RT-PCR) analysis. •  At the tissue level, findings showed wild type cotyledons had a random stomatal pattern, whereas gl1-1 and try240 cotyledons had ordered and clustered stomatal patterns, respectively. Regardless of overall pattern type, c. 10% of the stomatal population - those closest to one another - were always ordered, the result of genes regulating cellular differentiation. •  These results indicate epidermal cells respond to GL1 and TRY signals that affect distribution of both stomata and trichomes in postembryogenic events. The GL1 and TRY genes play dual roles in the epidermis, one role regulating epidermal tissue patterning and a second role connected with trichome development.

Transactivation of BARNASE under the AtLTP1 promoter affects the basal pole of the embryo and shoot development of the adult plant in Arabidopsis

Genetically controlled expression of a toxin provides a tool to remove a specific structure and consequently study its role during a developmental process. The availability of many tissue-specific promoters is a good argument for the development of such a strategy in plants. We have developed a conditional system for targeted toxin expression and demonstrated its use for generating embryo phenotypes that can bring valuable information about signalling during embryogenesis. The BARNASE gene was expressed in the Arabidopsis embryo under the control of two promoters, one from the cyclin AtCYCB1 gene and one from the AtLTP1 gene (Lipid Transfer Protein 1). One-hundred percent seed abortion was obtained with the cyclin promoter. Surprisingly however, the embryos displayed a range of lethal phenotypes instead of a single arrested stage as expected from this promoter. We also show that BARNASE expression under the control of the AtLTP1 promoter affects the basal pole of the globular embryo. Together with reporter expression studies, this result suggests a role of the epidermis in controlling the development of the lower tier of the embryo. This defect was not embryo-lethal and we show that the seedlings displayed a severe shoot phenotype correlated to epidermal defects. Therefore, the epidermis does not play an active role during organogenesis in seedlings but is important for the postgermination development of a viable plant.