Minisymposium 19: Gene regulation

Abs # 34002: Analysis of Aux/IAA Proteins Domain I Function in Arabidopsis

Presenter:	Tiwari, Shiv , Tiwaris@missouri.edu
Authors	Tiwari, Shiv  (A)   Hagen, Gretchen  (A)   Guilfoyle, Tom  (A)
Affiliations:	(A): University Of Missouri,Columbia

Aux/IAA proteins are repressors that contain four conserved domains, I, II, II and IV. Domain II functions as an instability domain. Domains III and IV make up a dimerization domain. We have recently shown that domain I functions an active repression domain (Tiwari et. al., 2004, Plant Cell). To evaluate the in planta function of domain I, IAA7 and IAA17 genes containing mutations in domain I and/or II were overexpressed as 35S promotor constructs in Arabidopsis. Mutations in domain I compromise, but do not totally eliminate the repressive capacity of Aux/IAA proteins. Mutations in domain II result in increased stability of the Aux/IAA proteins. Wild-type and mutant Aux/IAA proteins were also tested after fusion to the VP16 activation domain. Our results show that plants overexpressing Aux/IAA proteins with mutations in domain I alone had a phenotype similar to wild-type plants, while Aux/IAA proteins with mutations in domain II had pleiotropic phenotype ranging from monopteros-like seedlings to stunted plants with defective siliques and low fertility. Fusion to VP16 had little, if any, effect on the phenotypes displayed by these mutations. Overexpression of Aux/IAA proteins with mutations in both domains I and II resulted in seedlings with long, unbranched primary roots, short hypocotyls, and small, rounded leaves. The primary roots were agravitropic. As adults, these plants were dwarfed, had a poor root system, were bushy, and had short siliques. When crossed with a DR5:GUS, seedlings had reduced levels of reporter gene expression in response to auxin. VP16 fusion to the Aux/IAA proteins containing mutations in both domains I and II generated plants with phenotypes that were opposite those of domain I and II mutant plants. These seedlings had short primary roots, long hypocotyls, and thin, elongated leaves. The roots showed a normal gravitropic response. As adults, these plants had a good root system, strong apical dominance, anthocyanin-rich floral stalks with a zigzag appearance, and reduced fertility. When crossed with DR5:GUS, seedlings had enhanced expression of the reporter gene with or without auxin treatment. Our in planta results are consistent with domain I functioning as an active repression domain and suggest that different Aux/IAA proteins will produce similar phenotypes if expressed from the same promoter, even though they may have very different phenotypes when expressed from their native promoters.