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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 and indole-3-butyric acid (IBA), but it is likely that IBA is converted to IAA by oxidation in peroxisomes (Bartel et al., 2001). II. 1 Auxin conjugates In germinating seeds, IAA is produced from the breakdown of stored forms of the hormone, conjugates of amino acids, proteins, and sugars (Figure 1). The most abundant storage products in dicotyledonous plants are amino acid conjugates. Hydrolysis of these conjugates during germination precedes or (Woodward and Bartel., 2005) Figure 1. Potential pathways of IAA metabolism. Compounds quantified in Arabidopsis are in blue, enzymes for which the arabidopsis genes are cloned are in red, and Arabidopsis mutants are in lower-case italics. A family of amidohydrolases can release IAA from IAA conjugates. ILR1 has specificity for IAA–Leu (Bartel and Fink, 1995), whereas IAR3 prefers IAA–Ala (Davies et al., 1999). Arabidopsis UGT84B1 esterify IAA to glucose (Jackson et al., 2001). IBA is likely to be converted to IAA–CoA in a peroxisomal process that parallels fatty acid b-oxidation to acetyl-CoA (Bartel et al., 2001). IAA can be inactivated by oxidation (oxIAA) or by formation of nonhydrolysable conjugates (IAA–Asp and IAA–Glu). IAA–amino acid conjugates can be formed by members of the GH3/JAR1 family (Staswick et al., 2002, 2005). OxIAA can be conjugated to hexose, and IAA–Asp can be further oxidized (Östin et al., 1998). IBA and hydrolysable IAA conjugates are presumably derived from IAA; biosynthesis of these compounds may contribute to IAA inactivation. Formation and hydrolysis of IBA conjugates may also contribute to IAA homeostasis. coincides with the start of root extension. The storage endosperm of seeds is not the only site of IAA conjugate synthesis. Conjugate synthesis is developmentally 2

ChapitreI: Bibliographic review regulated and can be switched on at any time by exogenous application of auxins. Conjugates are likely to be moved into storage compartments such as the plant vacuole and, possibly, the endoplasmic reticulum. II. 2 De novo synthesis There are parallel biosynthetic pathways referred to as the tryptophan- dependent and tryptophan-independent pathways. Several Trp-dependent pathways, which are generally named after an intermediate, have been proposed: the indole-3-pyruvic acid (IPA) pathway, the indole-3- acetamide (IAM) pathway and the tryptamine pathway (Figure2). An Arabidopsis mutant named yucca accumulates more IAA than the wild type and exhibits characteristic auxin-enriched phenotypes such as elongated hypocotyls, epinastic leaves, and increased apical dominance (Zhao et al., 2001). The YUCCA protein is a cytosolic flavin monooxygenase that has tryptamine as substrate (Figure2). There is also evidence for a pathway through indole-3- pyruvate (Bartel et al., 2001; Ljung et al., 2002). Moreover, in Arabidopsis, an interesting pathway using a set of cytochrome P450s and a C-S lyase (Mikkelsen et al., 2004) has been identified through the analysis of a set of auxin-enriched mutants such as the superroot lines (sur1 and sur2) implicated in the synthesis of glucosinolates. All parts of young, growing plants appear to participate in IAA synthesis, although this synthesis is tightly controlled in order to maintain homeostasis (Ljung et al., 2001). Very young leaves produce large amounts of IAA. These early high concentrations and high synthetic capacity help drive leaf cell division. As the leaf expands towards its final size, synthesis and concentrations fall. In fully grown Arabidopsis plants, the highest concentrations of IAA are found in expanding fruiting bodies, the siliques (Müller et al., 2002). It is likely that most of this is synthesized by seeds during embryo development. The inflorescence stalk contains more IAA than most other parts of the plant, but it is not clear if this is synthesized in situ or if it is in transit (Figure3). 3

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