Abstract
The coordinated action of the auxin-sensitive Aux/IAA transcriptional repressors and ARF transcription factors produces complex gene-regulatory networks in plants. Despite their importance, our knowledge of these two protein families is largely based on analysis of stabilized forms of the Aux/IAAs, and studies of a subgroup of ARFs that function as transcriptional activators. To understand how auxin regulates gene expression we generated a Physcomitrella patens line that completely lacks Aux/IAAs. Loss of the repressors causes massive changes in transcription with misregulation of over a third of the annotated genes. Further, we find that the aux/iaa mutant is blind to auxin indicating that auxin regulation of transcription occurs exclusively through Aux/IAA function. We used the aux/iaa mutant as a simplified platform for studies of ARF function and demonstrate that repressing ARFs regulate auxin-induced genes and fine-tune their expression. Further the repressing ARFs coordinate gene induction jointly with activating ARFs and the Aux/IAAs.
Highlights
The plant hormone auxin plays a central role in plant growth and development
Very low levels of GUS staining was detected in the PIAA1B:GUS line, whereas GUS staining was high in the PIAA1B:GUS aux/iaaD line both in the absence and presence of applied auxin, indicating that this line displays a constitutive auxin response (Figure 1B)
The Auxin/ INDOLE-3-ACETIC-ACID (Aux/IAA) proteins have a central role in auxin signaling, serving as both auxin co-receptors and transcriptional repressors
Summary
The cellular response to auxin can be very different, including changes in cell division, cell expansion, and differentiation. The hormone acts by regulating the transcription of auxin responsive genes. Auxin-regulated gene sets can vary significantly between different cell types consistent with cell specific cellular responses (Bargmann et al, 2013). Repression is relieved when auxin binds to a co-receptor complex consisting of a TRANSPORT INHIBITOR RESISTANT 1/AUXIN F-BOX (TIR1/AFB) F-box protein and an Aux/IAA protein. The elucidation of the auxin co-receptor mechanism provided the molecular link between auxin perception at the cellular level and subsequent changes in gene expression. The mechanisms by which the interactions between TIR1/AFBs, ARFs and Aux/IAAs results in the induction of specific gene sets, as well as the establishment of gene regulatory networks, are not known
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