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During embryogenesis in higher plants, special tissues called the shoot and root meristems are formed at the upper and lower ends, respectively. After germination, the shoot meristem forms above-ground organs such as leaves, stems and floral organs, while the root meristem produces underground roots. Both genetic controls and diverse external environmental factors such as light or gravity influence the formation of the plant body. By using Arabidopsis thaliana, a model crucifer plant suitable for molecular genetic analyses, we are studying molecular mechanisms that regulate plant development.
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- Formation and function of the shoot meristem
Nearly all above-ground organs such as leaves, stems and floral organs originate from the shoot meristem formed at the upper end of the embryo. The base of leaves may form new meristems called axillary meristems, resulting in gbranchingh. We have identified (and still try to identify) many genes involved in shoot development by isolating mutants showing abnormal shoot morphology. By this approach, we found some important regulatory genes that control meristem initiation, positioning or specification of shoot organs, or specification of organ boundaries (Fig. 1).
- Molecular mechanism for gravitropism
The stem of plants grows upwards, while the roots grow downwards. We have isolated many sgr (shoot gravitropism) mutants showing abnormal gravitropism, and demonstrated that the stem perceives gravitational force at the endodermal cells and that the vacuoles and/or vesicular transport within endodermal cells are closely involved in the gravitropism of stem. At present, we are attempting to clarify the molecular mechanism for gravitropism through functional analysis of all SGR genes. Analysis of sgr mutants unveiled that vesicular transport plays an important role also in morphogenesis of plants. At present, we are studying the relationship between these higher physiological functions and vesicular transport, using methods of molecular genetics and cell biology (Fig. 2).
- Mechanism of polar auxin transport
Polar auxin transport is involved in various growth responses including axis formation during embryogenesis, organogenesis, vascular patterning, apical dominance, and tropism. In the process, auxin efflux carriers play a key role. Auxin efflux carriers are localized to the plasma membrane with polarity in one cell and arranged in a uniform orientation in organs and tissues, which allow directional auxin transport through them (Fig. 3). We have isolated and identified genes involved in localization of auxin efflux carrier with polarity by molecular genetic methods. The analyses of these genes demonstrate an important role of vesicle trafficking and signaling pathway in the establishment of polar localization of auxin efflux carrier. At present, we attempt to investigate the intercellular molecular network to establish the polarity.
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- Hashiguchi Y. et al., Plant Cell, 22, 159-172, 2010
- Niihama M. et al., Plant Cell Physiol., 50, 2057-2068, 2010
- Karim M. R. et al., Plant Cell, 21, 1360-1372, 2009
- Igari, K. et al. Plant J., 55, 14-27, 2008
- Furutani M. et al., Development, 134, 3849-3859, 2007
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Fig. 1 Wild-type Arabidopsis normally forms one axillary mersitem (AM) at each leaf axil (arrowhead) and as a result one branch elongates there (arrow). On the other hand, uni-1D mutants form multiple AMs at each axil (arrowheads), resulting in formation of many branches. Hence, the shoot apical meristem (SAM) at tips of branches of uni-1D plants displays low activity, resulting in short branches.
Fig. 2 (A) Gravitropism of the flower stem of Arabidopsis thaliana. Pictures taken every 10 minutes, beginning 30 minutes after the plant was placed horizontally are overlapped. (B) Schematic representation of the longitudinal section of the flower stem. (C) Electron micrograph of the endodermal cells which perceive gravitational force. The picture additionally includes amyloplasts which precipitate in the direction of gravitation. Amyloplasts are surrounding by the vacuolar membrane with thin cytoplasm. (D) Confocal micrograph. The dynamics of vacuolar membrane in endodermal cells can be observed while the cells remain viable, by using a protein (green) on the vacuolar membrane fused to GFP. Red indicates autofluorescence from the amyloplast.
Fig. 3 (A) Auxin efflux carriers (green) are localized in the plasma membrane with polarity and auxin is transported to a neighboring cell (orange arrow). (B) Localization of GFP-tagged auxin efflux carrier (green) and presumptive auxin flows (orange arrows) in Arabidopsisembryo.
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