FUNCTIONS OF IAA28 IN GROWTH AND DEVELOPMENT IN ARABIDOPSIS THALIANA
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The phyto-hormone auxin plays a vital role in regulating plant growth and development throughout the plant’s lifecycle. It implicates in most aspects of plant growth and development and influence overall size and shape of a plant. Auxin modulates gene transcription through the degradation of a group of transcriptional repressor proteins called Aux/IAAs. These repressor proteins interact with auxin co-receptors, TIR1/AFBs, through highly conserved domain II of Aux/IAAs in the presence of auxin, and targeted for ubquitin mediated degradation. Arabidopsis iaa28-2 is a gain-of-function mutation in domain II of Aux/IAA28. The domain II mutation in iaa28-2 drastically interferes with its interaction with auxin co-receptors, TIR1/AFBs in response to auxin resulting in stabilization of the mutant protein. This eventually leads to pleiotropic developmental defects such as defective lateral root development, stunted growth and reduced fertility in the mutant. iaa28-2 mutant is severely defective in lateral root development but produces lateral root primordia in response to high levels of 2,4-D or IBA. The mutant seedlings produce adventitious roots in response to high level of picloram, a synthetic auxin. Results of this study suggest that reduced auxin transport from hypocotyl to root and the degradation of mutant iaa28 protein through auxin co-receptor AFB1 in the hypocotyl in response to picloram may lead to induction of down-stream genes such as LBD16 that is required for adventitious root development. Additionally, this study shows that unlike other Aux/IAA proteins, IAA28 is localized to both the nucleus and the cytoplasm and exhibit a peculiar subcellular localization pattern along the primary root. IAA28 protein is mainly localized to the cytoplasm in the basal meristem and then gradually localizes to nucleus in cell elongation and differentiation zones. Further, the data indicate that IAA28 expression is induced by light and the IAA28 protein is modified in response to light perhaps through phytochromes. Biological significance of this localization pattern or IAA28 modification by light is currently unclear.