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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 from auxin response mutants, all support this hypothesis. Other data, such as kinetic measurements of gravistimulated roots and experiments in which seedling shoots are incubated in excess IAA that are still found to be able to respond by differential growth (Edelmann., 2001), argue against the first hypothesis. Either way, it is clear that auxin is a requirement for differential and tropic growth, and many auxin sensitive mutants like aux1 have agravitropic root phenotypes (Bennett et al., 1996). VI.5. Adventitious rooting Addition of auxin to cut or damaged stems often induces a strong adventitious rooting response, and horticultural industries relying on clonal propagation make good use of this response for ornamentals, trees, flowers, and general garden plants. The initiation of root primordia in stem tissues requires a redifferentiation response, and, although auxin promotes this, the molecular or genetic mechanisms are unclear. There are mutants of Arabidopsis that show precocious rooting along the seedling hypocotyl, superroot (sur) 1 and 2, for example (Boerjanet al., 1995). In both, auxin levels are elevated due to accumulation of endogenous aldoxime (IAOx) that is channeled into auxin (Figure 14). (Mikkelsen et al., 2004) Figure 14. The role of SUR genes in auxin homeostasis. Hight auxin mutant are schown in red. VI.6. Fruit growth Auxin plays a vital role in all stages of reproductive growth. Some of the highest auxin concentrations have been found in developing fruit (Müller et al., 20

ChapitreI: Bibliographic review 2002). It has been postulated that auxin is first produced by elongating pollen tubes and then by the embryo and endosperm in the developing seeds. Subsequent development of the fruit appears to depend on these sources of auxin. In the early 1950s, Nitsch showed that achene (seed) removal from strawberry receptacles inhibited receptacle enlargement (Pennazio., 2002). Replacing the achene with a supply of auxin maintained fruit growth. Cessation of auxin supply also led to ripening in this nonclimacteric fruit. Supporting this hypothesis, the auxin-resistant tomato mutant, diageotropica, which encodes a cyclophilin has reduced fruit set, fruit weight and seed production (Balbi and Lomax., 2003; Oh et al., 2006) and the application of either auxin or auxin transport inhibitors that cause an increase in auxin in the ovary stimulate fruit set and the development of parthenocarpic fruit (Gustafson., 1937; Beyer and Quebedeaux., 1974). Parthenocarpy has also been induced in tomato and other fruit by ovary- targeted ectopic expression of Agrobacterium iaaM under the control of carpel- specific promoter, which confers higher auxin production and induces seed free fruit (Ficcadenti et al., 1999). Some apple and cherry crops are sprayed with auxin (mixed with other hormones) at flowering stage to induce fruit set. Treatments of orchards to delay fruit abscission is becoming common to facilitate automated harvesting, Thus maximizing the yield of ripe fruit collected in the minimum number of passes by harvesters. In 2002, Jones demonstrated for the first time that down-regulation of DR12, an auxin-response-factor homolog, in tomato, results in a pleiotropic fruit phenotype including dark green, blotchy ripening fruit and enhanced fruit firmness (Figure 15). a b I (Jones et al., 2002) Figure 15. Altered phenotypes of DR12-inhibited plants. (a) Dark-green phenotype of DR12 antisense fruit (AS) at anthesis +30 days compared to wild-type fruit (WT) at the same stage. (b) 21

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