Complex regulation of the TIR1/AFB family of auxin receptors

Abstract

Auxin regulates most aspects of plant growth and development. The hormone is perceived by the TIR1/AFB family of F-box proteins acting in concert with the Aux/IAA transcriptional repressors. Arabidopsis plants that lack members of the TIR1/AFB family are auxin resistant and display a variety of growth defects. However, little is known about the functional differences between individual members of the family. Phylogenetic studies reveal that the TIR1/ AFB proteins are conserved across land plant lineages and fall into four clades. Three of these subgroups emerged before separation of angiosperms and gymnosperms whereas the last emerged before the monocot-eudicot split. This evolutionary history suggests that the members of each clade have distinct functions. To explore this possibility in Arabidopsis, we have analyzed a range of mutant genotypes, generated promoter swap transgenic lines, and performed in vitro binding assays between individual TIR1/ AFB and Aux/IAA proteins. Our results indicate that the TIR1/AFB proteins have distinct biochemical activities and that TIR1 and AFB2 are the dominant auxin receptors in the seedling root. Further, we demonstrate that TIR1, AFB2, and AFB3, but not AFB1 exhibit significant posttranscriptional regulation. The microRNA miR393 is expressed in a pattern complementary to that of the auxin receptors and appears to regulate TIR1/AFB expression. However our data suggest that this regulation is complex. Our results suggest that differences between members of the auxin receptor family may contribute to the complexity of auxin response.plant hormone ͉ plant development

COP1 mediates the coordination of root and shoot growth by light through modulation of PIN1- and PIN2-dependent auxin transport in Arabidopsis

“…S4A). In addition, the tissue-specific expression along the hypocotyl axis or in the RAMs of GUS translational fusions to the TIR1/AFB1-3 auxin co-receptors (Parry et al, 2009) was not affected by the light conditions (supplementary material Fig. S4B), indicating that the differences in DII-VENUS fluorescence are not due to altered auxin sensitivity in response to light.…”

Auxin transport and activity regulate stomatal patterning and development

“…Stomatal patterning thus probably requires PIN-mediated transport of auxin. However, stomatal phenotypic defects are also present in tir1, afb1, 2, 3 quadruple mutants 25,26 , thus reinforcing the involvement of auxin signalling in stomatal patterning ( Fig. 2c and Supplementary Fig.…”

Auxin-Binding Protein 1 is a negative regulator of the SCFTIR1/AFB pathway

“…1a). The differential effect of double mutations confirms predominance of TIR1 and AFB2 in roots 19 ; however, AFB3 is also clearly involved. Conversely, postembryonic ABP1 knockdown had no effect on the variability of phenotypes reported for the triple tir1-1afb2-1afb3-1 mutant 9 but the typical long wavy primary root phenotype resulting from group III triple tir/afb mutations was observed (Fig.…”

“…We then investigated whether ABP1 inactivation affects the stability of other wellknown UPS-dependent regulatory proteins starting with JAZ repressors of the jasmonate pathway. The COI1 gene that triggers ubiquitination of JAZ is the closest gene to TIR1 and AFBs within the LRR subfamily of F-box proteins 9,19 . We analysed the effect of ABP1 inactivation on JAZ1-GUS protein half-life 25 .…”