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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 (Woodward and Bartel.,2005) Figure 2. Potential pathways of IAA biosynthesis in Arabidopsis. De novo IAA biosynthetic pathways initiate from Trp. Compounds quantified in Arabidopsis are in blue, enzymes for which the Arabidopsis genes are identified are in red, and Arabidopsis mutants are in lower-case italics. Suggested conversions for which genes are not identified are indicated with question marks. II. 3 Desactivation Control of auxin action can be mediated by removing IAA as well as by synthesis. Auxins are removed by conjugation into inactive storage compounds and by oxidation. The primary oxidation auxins product of IAA in Arabidopsis and other plants is likely to be 2-oxo-indoleacetic acid (OxIAA), although quantities of oxidized conjugates, OxIAAaspartate, and O-glucoside are also found (Östin et al., 1998). Free IAA is metabolized by decarboxylation by a variety of plant peroxidases in vitro, but the quantities of such products from intact tissues are generally very low suggesting that these are less important catabolic pathways in vivo. 4

III. Auxin distribution and transport: the road network ChapitreI: Bibliographic review All plant hormones are moved around the plant in the vasculature where, once loaded, they move passively as passenger molecules. Auxin is so far unique among plant hormones in being actively moved around the plant by a series of transmembrane pumps or pump components (Blakeslee et al., 2005). The chemiosmotic hypothesis is a long-standing and widely accepted model for the basic operation of this system (Goldsmith et al., 1981). Auxin is a weak acid, and at the extra-cellular (apoplastic) pH a significant fraction is protonated and hence apolar. As such auxin can freely diffuse into the cell, where the pH is higher, resulting in ionisation. The auxin ions are then trapped in the cell and can only leave through active transport, energised by the electrochemical gradient across the plasma membrane. The auxin efflux activity can be localised to a specific part of the cell surface. Thus, in a file of cells that are all polarised in the same direction, auxin movement will be unidirectional. Polar auxin transport contributes to many of the important auxin-dependant response as cell division, cambial development, apical dominance and gravitropism but is not correct to suppose that all auxin movement is through the polar transport system. Considerable quantities of free IAA and auxin conjugates are carried in the vasculature, particularly the phloem. (Baker, 2000; Cambridge and Morris, 1996; Else et al., 2004). Many of the auxin-driven responses fail if auxin is absent or in excess, or if polar auxin transport is defective. Certainly, hormone delivery is as important as the hormone itself. As a result, auxin physiology has benefited from the use of drugs that act specifically to inhibit polar auxin transport. Of these, naphthylphthalamic acid (NPA) is the most response-specific and has been used widely. III.1 Update carriers All Candidate proteins described for both uptake and efflux have been identified from screening mutant populations and the application of molecular genetics. The influx carrier gene, known as AUX1, was isolated from seedlings 5

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