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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 continuous hydrophobic core among ligand, substrate and receptor. The authors characterize this as a ‘molecular glue’ that effectively strengthens the binding of the substrate to the receptor (Figure 6). Two others F-box proteins had been identified earlier [now named Auxin signalling F-Box proteins (AFB) 1 and 2] but not studied in detail (Ruegger et al., 1998). One other is named as AFB3 (Dharmasiri et al., 2005). Each AFB protein has been shown to bind Aux/IAAs in an auxin-dependent manner (Dharmasiri et al., 2005). Seedlings of each single TIR1 and AFB mutant line resemble wild-type seedlings in light or dark, but some do display auxin-tolerant phenotypes such as a partial tolerance to auxin-induced inhibition of root elongation (Dharmasiri et al., 2005). Far greater additive effects are found in double, triple and quadruple mutants. Triple and quadruple mutant lines show high levels of arrest after germination, although some seedlings do develop and a range of phenotypes develops in the quadruple mutants (Figure8). Classified into three groups, the most severe group fails to develop a root and develops only a single cotyledon. The least severe group has seedlings with roots that respond poorly to gravity and lack hairs. In more mature plants, the triple and quadruple mutants develop rosettes with reduced highly curled leaves and the inflorescences are dwarfed and highly branched (Dharmasiri et al., 2005). (Badescu and Napier., 2006) Figure 8. Seedlings of the quadruple mutant tir1 afb1 afb2 afb3 show three classes of phenotype. Seven-day-old seedlings were grown on vertical plates in the light. The most extreme phenotype (right) is rootless, shows no hypocotyl extension and often has only a single cotyledon, resembling monopteros plants at the same age. The least extreme (left) shows impaired gravitropism and few root hairs. 10

ChapitreI: Bibliographic review All these observations are consistent with the assertion that the TIR1 family acts as a set of auxin receptors with overlapping redundant functionality between members. IV. 2 Signalling IV.2.a .Auxin induced transcripts Auxin rapidly and transiently induced accumulation of at least three families of transcripts: SMALL AUXIN-UP RNAs (SAURs), GRETCHENHAGEN (GH3)- related transcripts and Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) family members. SAUR transcripts accumulate rapidly after auxin exposure in soybean (Walker and Key., 1982) and many other species, including Arabidopsis (Gil et al., 1994). Maize ZmSAUR2 is a small nuclear protein that, like the encoding transcript, is rapidly degraded (Knauss et al., 2003). The short half-lives of SAUR mRNAs appear to be conferred by downstream elements (DSTs) in the 3’ untranslated region of the message (Sullivan and Green., 1996). Arabidopsis mutants that stabilize DST-containing RNAs, and thus stabilize SAUR transcripts, have no reported morphological phenotype (Johnson et al., 2000), and the function of these small RNAs remains unknown. GH3 transcript accumulation is also induced by auxin (Hagen et al., 1984). At least some IAA-induced GH3 genes encode IAA–amino acid conjugating enzymes (Staswick et al., 2005) (Figure 1), whereas several GH3-related proteins that are not auxin regulated function to adenylate or conjugate amino acids to molecules other than IAA, including jasmonic acid (Staswick et al., 2002; Staswick and Tiryaki., 2004). Thus, the auxin induction of GH3 genes likely serves to dampen the auxin signal by inactivating IAA via conjugation. Aux/IAA family includes 29 proteins in Arabidopsis. Induction of some Aux/IAA genes occurs within minutes of auxin application and does not require new protein synthesis (Abel et al., 1994; Abel and Theologis., 1996). Aux/IAA genes encode proteins that generally have nuclear localization and four conserved domains (I–IV). Domain I is a transcriptional repressor (Tiwari et al., 2004). Domain II is critical for Aux/IAA instability; several mutations in this domain 11

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