Abstract
Aux/IAA proteins are transcriptional repressors that control auxin signaling by interacting with auxin response factors (ARFs). So far all of the identified Aux/IAA mutants with auxin-related phenotypes in Arabidopsis and rice (Oryza sativa) are dominant gain-of-function mutants, with mutations in Domain II that affected stability of the corresponding Aux/IAA proteins. On the other hand, morphological changes were observed in knock-down mutants of Aux/IAA genes in tomato (Solanum lycopersicum), suggesting that functions of Aux/IAA proteins may be specific for certain plant species. We report here the characterization of PtrIAA14.1, a poplar (Populus trichocarpa) homolog of IAA7. Bioinformatics analysis showed that PtrIAA14.1 is a classic Aux/IAA protein. It contains four conserved domains with the repressor motif in Domain I, the degron in Domain II, and the conserved amino acid signatures for protein–protein interactions in Domain III and Domain IV. Protoplast transfection assays showed that PtrIAA14.1 is localized in nucleus. It is unable in the presence of auxin, and it represses auxin response reporter gene expression. Expression of wild-type PtrIAA14.1 in Arabidopsis resulted in auxin-related phenotypes including down-curling leaves, semi-draft with increased number of branches, and greatly reduced fertility, but expression of the Arabidopsis Aux/IAA genes tested remain largely unchanged in the transgenic plants. Protein–protein interaction assays in yeast and protoplasts showed that PtrIAA14.1 interacted with ARF5, but not other ARFs. Consistent with this observation, vascular patterning was altered in the transgenic plants, and the expression of AtHB8 (Arabidopsis thaliana homeobox gene 8) was reduced in transgenic plants.
Highlights
Auxin regulates most, if not all, aspects of plant growth and development, such as organ formation, lateral root initiation, stem and root elongation, vascular tissue differentiation, and apical dominance (Davies, 1995)
We found that PoptrIAA7.1 and PoptrIAA7.2 are homologs of IAA7 but they have been renamed PtrIAA14.1 (Potri.008G161200) and PtrIAA14.2 (Potri.010G078300), respectively
PtrIAA14.1 is a Transcription Repressor Because PtrIAA14.1 is highly similar to IAA7 at amino acid sequence level, and it contains all the conserved motifs and amino acid signatures presented in IAA7 (Figure 1B), we examined if PtrIAA14.1 functions as transcriptional repressor as well with protoplast transfection assays
Summary
If not all, aspects of plant growth and development, such as organ formation, lateral root initiation, stem and root elongation, vascular tissue differentiation, and apical dominance (Davies, 1995). Auxin-regulated plant growth and development is likely initiated by the rapid response of specific genes to local changes in auxin concentration. These genes are referred to as early auxin response genes. Aux/IAA proteins are transcription repressors that involve in the regulation of auxin signaling. Based on their amino acid sequence similarity, Aux/IAA proteins generally contain four conserved domains, Domain I, II, III, and IV. Domains III and IV are related to the conserved C-terminal dimerization domains of auxin response factors (ARFs), and contain conserved amino acids that are required for homo and hetero interactions among Aux/IAA proteins and ARFs (Ulmasov et al, 1997, 1999; Ramos et al, 2001; Tiwari et al, 2001, 2003, 2004; Dreher et al, 2006; Nanao et al, 2014)
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