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Functional characterization of tomato Sl-IAA3 and Sl-hls genes. Role ...

Functional characterization of tomato Sl-IAA3 and Sl-hls genes. Role ...

ChapitreI: Bibliographic

ChapitreI: Bibliographic review stimuli, but homeostasis is critical for both optimal development and to keep the system primed for stimulatory responses. At present, the feedback mechanisms for homeostasis are not well characterized. However, it is clear that auxin transport plays a crucial role in both establishing and perturbing homeostasis (Figure 12). VI. Role of auxin Auxin is an absolute requirement for plant cell division, as well as to many other developmental and environmental responses that are mediated through auxin movements. Of these, a good number are commercially relevant, such as branching, rooting, and fruiting, but the biggest market for synthetic auxins is as selective herbicides. Figure 12. Auxin homestasis. (Paciorek and Friml, 2006) 16

VI.1 Cell division and tissue culture ChapitreI: Bibliographic review One of the most profound actions of auxin on plants is the control of cell division. A primary event in the stimulation of cell division is the cytokinin- mediated initiation of cyclin D transcription (Cockcroft et al., 2000). The newly synthesized cyclin D associates with a cyclin-dependent kinase (CDK) to create an active complex at the G1 to S transition. The CDK-a/cyclin D complex leads to phosphorylation of the retinoblastoma tumor suppressor protein (Rb). Inactivation of Rb in late G1 provokes release of the transcription factor E2F, and genes controlled by E2F factors are therefore activated, driving cells into DNA replication and committing them to the cell cycle. Cells can be arrested either in G1 or in G2 phase after auxin deprivation (Planchais et al., 1997). Generally, higher auxin concentrations stimulate cell division, and low auxin concentrations drive cell elongation, cell enlargement, and cell differentiation (Winicur et al., 1998; Zazimalova et al., 1995). Among the most striking examples of this critical control over division is in cell and tissue culture where auxin must be included in almost all media. Furthermore, auxin and cytokinin concentrations can be manipulated to promote proliferative callus growth, regeneration of vegetative tissue, or root induction. Elevated auxin/cytokinin in ratio is favorable for rooting, whereas a reduced ratio facilitates buds regeneration. Horticultural practices, such as cutting and clonal micropropagation, are directly based on these characteristics. VI.2 Organ patterning VI.2.a Phyllotaxis Phyllotaxis is the regular arrangement of leaves on a stem. Polar auxin transport inhibitors and Arabidopsis mutants defective in polar transport like pin1 have helped us to illustrate that accumulation of IAA in cells at the side of the shoot apical meristem initiates organogenesis and determines the position of the next leaf primordium. Moreover, microapplications of transport inhibitors or IAA to 17

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