Degradation of Aux/IAA proteins is essential for normal auxin signalling

The growth substance auxin mediates many cellular processes, including divi sion, elongation and differentiation. PSIAA6 is a member of the Aux/IAA fam ily of short-lived putative transcriptional regulators that share four cons erved domains and whose mRNAs are rapidly induced in the presence of auxin. Here PSIAA6 was shown to serve as a dominant transferable degradation sign al when present as a translational fusion with firefly luciferase (LUC), wi th an in vivo half-life of 13.5 min in transgenic Arabidopsis seedlings. In a transient assay system in tobacco protoplasts using steady-state differe nces as an indirect measure of protein half-life, LUC fusions with full-len gth PSIAA6 and IAA1, an Aux/IAA protein from Arabidopsis, resulted in prote in accumulations that were 3.5 and 1.0%, respectively, of that with LUC alo ne. An N-terminal region spanning conserved domain II of PSIAA6 containing amino acids 18-73 was shown to contain the necessary cis-acting element to confer low protein accumulation onto LUC, while a fusion protein with PSIAA 6 amino acids 71-179 had only a slight effect. Single amino acid substituti ons of PSIAA6 in conserved domain II, equivalent to those found in two alle les of axr3, a gene that encodes Aux/IAA protein IAA17, resulted in a great er than 50-fold increase in protein accumulation. Thus, the same mutations resulting in an altered auxin response phenotype increase Aux/IAA protein a ccumulation, providing a direct link between these two processes. In suppor t of this model, transgenic plants engineered to over-express IAA17 have an axr3-like phenotype. Together, these data suggest that rapid degradation o f Aux/IAA proteins is necessary for a normal auxin response.