Given that hypocotyl stomata and trichoblasts in roots have the same positional relationship with respect to underlying cells, a common mechanism exists for epidermal patterning ( Masucci et al., 1996 Berger et al., 1998 Hung et al., 1998). Stomatal development in the hypocotyl is regulated by genes active in the root epidermis ( Masucci et al., 1996 Berger et al., 1998 Hung et al., 1998). Precedent for this possibility is known from studies of roots ( Scheres, 2000), hypocotyls ( Berger et al., 1998), and the leaf epidermis ( Benfey, 1999). Here we examined that possibility using genes with known effects in the Arabidopsis trichome pathway. Since trichomes arise earlier than stomata, which need to be appropriately placed for effective gas exchange, we felt genes that influence the siting of trichomes might also influence the siting of stomata.
Misplaced fate decisions disrupt local pattern but do not reflect tissue pattern. Some defects are strictly concerned with differentiation ( Glover, 2000 Martin & Glover, 1998 Walker et al., 1999), but others, such as clustered trichomes and stomata, reveal disruptions in cell fate decisions ( Hülskamp et al., 1994 Yang & Sack, 1995 Schnittger et al., 1999 Geisler et al., 2000). This approach differs significantly from the molecular genetic studies of trichomes and stomata, which identify defects in cellular patterning ( Oppenheimer et al., 1991 Larkin et al., 1994 Larkin et al., 1996 Larkin et al., 1997 Marks, 1997 Szymanski et al., 1998a,b Perazza et al., 1999 Berger & Altmann, 2000 Szymanski et al., 2000).
Studies of tissue spatial organization require examination of the entire cellular field and of entire populations of stomata and trichomes, with each serving as an indicator of tissue spatial organization. Leaves are ideal for the study of tissue pattern because they present two-dimensional patterns, their surface is accessible, and two specialized cells – trichomes and stomata – can be used to monitor spatial organization of the epidermis. How this organization occurs and what molecular agents are involved is not known, although there are hints from published work. Within the field, cells commit to one of two differentiation pathways on discrete temporal and spatial bases. Patterning of stomata and trichomes in the epidermis are not independent events, but represent the interaction of spatial organization in a single field of cells. 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. These results indicate epidermal cells respond to GL1 and TRY signals that affect distribution of both stomata and trichomes in postembryogenic events. 10% of the stomatal population – those closest to one another – were always ordered, the result of genes regulating cellular differentiation. Expression of GL1 and TRY was determined in wild type and mutant samples by reverse transcriptase-PCR (RT-PCR) analysis.Īt 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.
Mature cotyledons were imaged by SEM, stomatal maps were generated, and data were spatially analysed. 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. Trichome and stomatal patterning are not independent events because trichomes form before stomata.
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