TheArabidopsisAux/IAA Protein Family Has Diversified in Degradation and Auxin Responsiveness

Abstract

Rapid, auxin-responsive degradation of multiple auxin/indole-3-acetic acid (Aux/IAA) proteins is essential for plant growth and development. Domain II residues were previously shown to be required for the degradation of several Arabidopsis thaliana Aux/IAA proteins. We examined the degradation of additional full-length family members and the proteolytic importance of N-terminal residues outside domain II using luciferase (LUC) fusions. Elimination of domain I did not affect degradation. However, substituting an Arg for a conserved Lys between domains I and II specifically impaired basal degradation without compromising the auxin-mediated acceleration of degradation. IAA8, IAA9, and IAA28 contain domain II and a conserved Lys, but they were degraded more slowly than previously characterized family members when expressed as LUC fusions, suggesting that sequences outside domain II influence proteolysis. We analyzed the degradation of IAA31, with a region somewhat similar to domain II but without the conserved Lys, and of IAA20, which lacks domain II and the conserved Lys. Both IAA20:LUC and epitope-tagged IAA20 were long-lived, and their longevity was not influenced by auxin. Epitope-tagged IAA31 was long-lived, like IAA20, but by contrast, it showed accelerated degradation in response to auxin. The existence of long-lived and auxin-insensitive Aux/IAA proteins suggeststhat they may play a novel role in auxin signaling.

The Ectomycorrhizal FungusLaccaria bicolorStimulates Lateral Root Formation in Poplar and Arabidopsis through Auxin Transport and Signaling

“…During LRI, once an auxin maximum is formed in pericycle founder cells, auxin triggers its signaling cascade by binding to its receptors (Badescu and Napier, 2006). This activates auxin-dependent transcription factors (AUX/IAA and ARF) that act on cell cycle-regulating targets (Fukaki et al, 2005;Dreher et al, 2006) to elicit cell cycle reactivation. The mechanism that confers founder cell identity to pericycle cells in the basal meristem is distinct from LRI.…”

Context, Specificity, and Self-Organization in Auxin Response

“…For example, two AFB proteins, AFB4 and AFB5 can be distinguished from the rest of the AFB family both structurally by a long amino-terminal extension and genetically (in the case of AFB5) because in contrast to loss-of-function in TIR1, AFB1, AFB2, and AFB3, which all confer resistance to exogenous auxin, afb5 mutants are slightly hypersensitive to IAA (Dharmasiri et al 2005b;Walsh et al 2006). In the case of the Aux/IAAs, several members (IAAs 20,30,32,33, and 34) lack a recognizable domain II degron motif suggesting that these Aux/IAAs might be long-lived proteins, something that has been confirmed in the case of IAA20 (Dreher et al 2006). Lastly, of the 23 ARFs in Arabidopsis ARFs 3/ ETTIN, 13, 17 and 23 lack carboxy-terminal dimerization domains (CTDDs) (Guilfoyle and Hagen 2007).…”

“…This is perhaps not surprising given that among the 24 or so Aux/IAAs with recognizable domain II degron motifs there is significant variation at more than half of the degron positions (Dreher et al 2006). Thus it is highly likely that this structural variation among both receptors and Aux/IAAs will be reflected in a range of affinities for each receptor-auxinAux/IAA complex.…”

“…The principal mechanism of change here is the modulation of Aux/ IAA levels and the fact that the rate of change of concentrations of different Aux/IAAs in response to auxin varies considerably across the family (Dreher et al 2006). As well as variation in Aux/IAA half-lives, Aux/IAA genes are induced by exogenous auxin over a range of time and dose response profiles, some gene transcripts taking minutes to reach maximal levels with others taking hours (Abel et al 1995).…”

“…Thus it is highly likely that this structural variation among both receptors and Aux/IAAs will be reflected in a range of affinities for each receptor-auxinAux/IAA complex. In turn, such differences in affinity would be predicted to translate to differences in Aux/IAA stability and indeed the half-lives of Aux/IAA proteins range from extremely short at 6 min to much longer at 80 min (Dreher et al 2006). Therefore this specificity in the very first step of TIR1/AFBmediated auxin signal transduction has the potential to affect profoundly the balance of downstream interactions between Aux/IAAs and ARFs and so, how the auxin signal in that particular cell is interpreted.…”