What's the physiological role of domain II-less Aux/IAA proteins?

Arabidopsis has Aux/IAA proteins that lack domain II which is a binding site for the TIR1 auxin receptor. These proteins have been shown to be more stable than the canonical Aux/IAA proteins. We investigated the phenotypes of overexpression lines of domain II-less Aux/IAA proteins, IAA20 and IAA30, by the use of 35S promoter. The transgenic lines showed many aberrant phenotypes in auxin physiology. The most conspicuous phenotype was collapse of the root apical meristem as occurred in plt1 plt2 double mutants. Because IAA20 and IAA30 were early auxin-inducible and were expressed in the root apical meristem, they may have a physiological role in maintaining the stem cell niche of root, by keeping the activity of MP/ARF5 and NPH4/ARF7 at an acceptable level. On the other hand, domain-II less Aux/IAA proteins are not present in balck cottonwood tree or grapevine, suggesting wide diversification of Aux/IAA proteins in higher plants.