Department of Biology, Indiana University, Bloomington, IN 47401, USA.
Proc Natl Acad Sci U S A. 2009 Dec 29;106(52):22540-5. doi: 10.1073/pnas.0911967106. Epub 2009 Dec 16.
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.
生长素调节植物生长和发育的大多数方面。激素被 TIR1/AFB 家族的 F-box 蛋白感知，这些蛋白与 Aux/IAA 转录抑制子协同作用。缺乏 TIR1/AFB 家族成员的拟南芥植物对生长素具有抗性，并表现出多种生长缺陷。然而，关于该家族成员之间的功能差异知之甚少。系统发育研究表明，TIR1/AFB 蛋白在陆地植物谱系中是保守的，并分为四个分支。这三个亚组在被子植物和裸子植物分离之前出现，而最后一个亚组在单子叶植物和双子叶植物分裂之前出现。这种进化历史表明，每个分支的成员具有不同的功能。为了在拟南芥中探索这种可能性，我们分析了一系列突变体基因型，生成了启动子交换转基因系，并进行了单个 TIR1/AFB 和 Aux/IAA 蛋白之间的体外结合测定。我们的结果表明，TIR1/AFB 蛋白具有不同的生化活性，并且 TIR1 和 AFB2 是幼苗根中的主要生长素受体。此外，我们证明 TIR1、AFB2 和 AFB3，但不是 AFB1，表现出明显的转录后调节。miR393 微 RNA 在与生长素受体互补的模式中表达，似乎调节 TIR1/AFB 的表达。然而，我们的数据表明，这种调节很复杂。我们的结果表明，生长素受体家族成员之间的差异可能导致生长素反应的复杂性。
