Fig. 1.

The Skp1-Cullin1-F-box transport inhibitor response protein 1 (SCF^TIR1) ubiquitin ligase functions as the receptor for the plant hormone auxin. A: chemical structures of the natural auxin, 2 auxin analogs [2,4-dichlorophenoxyacetic acid (2,4-D) and 1-naphthalene acetic acid (1-NAA)], and jasmonate-isoleucine (JA-Ile). B: the role of SCF^TIR1 ubiquitin ligase in the auxin-signaling pathway in plants. In the absence of auxin, auxin/indole-3-acetic acids (AUX/IAAs) repress the expression of auxin response genes (AuxRE) by binding to the transcription activator auxin response factors (ARFs) (left). In the presence of auxin, the multisubunit SCF^TIR1 ubiquitin ligase recognizes the hormone and promotes the polyubiquitination and proteasomal degradation of AUX/IAAs in a hormone-dependent manner (middle). Auxin-induced degradation of AUX/IAAs frees ARFs, which activate the transcription of auxin response genes (right). U, ubiquitin protein.