In plants, the TIR1/AFB receptor proteins regulate responses to the plant hormone auxin by stimulating the proteolytic degradation of AUX/IAA repressors of auxin-dependent gene transcription. Almost all auxin responses have been shown to involve specific pairings of TIR1/AFBs, AUX/IAAs, and auxin-responsive transcriptional activators. Experiments with the prolyl cis-trans isomerase (PPIase) inhibitor juglone suggest that a PPIase may control the rate of AUX/IAA degradation in an auxin-independent manner. Recently, the investigators have discovered that Arabidopsis APP1, which possesses juglone-sensitive X-prolyl aminopeptidase and PPIase activities, directly interacts with AUX/IAA proteins via their interaction domains. As compared to wild type plants, these app1 mutants exhibit reduced auxin sensitivity; they have reduced elongation growth, as well as sharply decreased expression of auxin responsive genes and synthetic reporters after auxin treatment. Increased APP1 activity results in increased auxin-responsive gene expression and growth similar to that seen in plants treated with low concentrations of auxin. The overall goal of the project is to elucidate the role of APP1 in TIR1/AFB-mediated auxin signaling. The specific objectives of the project are as follows. 1) The project will determine which enzymatic activity of APP1 regulates auxin responses. The investigators hypothesize that the PPIase activity mediatesTIR1-AUX/IAA interactions and that the proteolytic activity contributes to subsequent AUX/IAA processing. 2) The project will determine the function of APP1 in the auxin response pathway. The model underlying the work proposes that APP1 stabilizes the interaction domain of AUX/IAAs to promote TIR1 binding. This project is expected to characterize an unrecognized global component of plant hormone signaling. The project is transformative, as it will help determine whether the PPIase activity discovered in AtAPP1 is found in APP orthologs from other species and whether APPs may play a previously uncontemplated role in animal and fungal signaling mechanisms. The project will also involve students from a summer undergraduate research program run by the PI and Dr. Wendy Peer that recruits students into the life sciences from underserved populations.
