Small GTP-binding proteins of the Rac family are important signaling switches that regulate plant growth and development due to their ability to shuttle between the inactive GDP-bound and the active GTP-bound form. The ratio between the two forms is tightly regulated in the cell. The Arabidopsis genome encodes eleven Rac proteins designated Arac1-11. Based on the C-terminus, Aracs can be divided into typeI and typeII. TypeI Aracs associate with the membrane by prenylation while typeII Aracs associate with the membrane by palmitoylation. Differences in the effector-binding region and the C-terminal hypervariable region place Arac7 in a separate phylogenetic group than the other typeII Aracs. The work described in this dissertation examines the expression patterns, functions and regulation of the Arac7 protein and provides evidence that supports a distinct role for Arac7 within type II Aracs. Arac7 expression is shown to repress auxin-induced gene expression and to enhance ABA-stimulated gene expression in Arabidopsis. Plants overexpressing Arac7 are less responsive to auxin but show increased responses to ABA while the opposite is observed in plants with decreased levels of Arac7 mRNA. Arac7 expression is high in the lateral root primordia and can be traced back to the first pericicle divisions that give rise to these primordia. This, together with the increased number of lateral roots of plants overexpressing Arac7, is consistent with a role in lateral root formation for this small GTPase. Interestingly, transcription from the Arac7 promoter is stimulated by auxin but repressed by ABA. Together with the observed functions on auxin and ABA signaling, the regulated expression of Arac7 provides a feedback mechanism to ensure that cells maintain sensitivity to signals. This work also shows that Arac7 is constitutively associated with the plasma membrane where it partitions into detergent-resistant membrane domains (DRMs). Moreover, at the plasma membrane, the majority of Arac7 partitions into high molecular weight complexes. These properties contrast with those observed for the typeI Arac5 which is distributed between the cytosol and non-DRMs regions of the plasma membrane and exists predominantly in low molecular weight forms. Together, these observations suggest different functions and regulatory mechanisms for typeI and typeII Aracs.
| Identifer | oai:union.ndltd.org:UMASS/oai:scholarworks.umass.edu:dissertations-4123 |
| Date | 01 January 2005 |
| Creators | Nibau, Candida Sofia Nobre |
| Publisher | ScholarWorks@UMass Amherst |
| Source Sets | University of Massachusetts, Amherst |
| Language | English |
| Detected Language | English |
| Type | text |
| Source | Doctoral Dissertations Available from Proquest |
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