Fig. 2.
The central pathways for auxin signal transduction. Nuclear auxin signal transduction is mediated by the TRANSPORT INHIBITOR RESPONSE1/AUXIN SIGNALING F-BOX PROTEINS (TIR1/AFBs) pathway, which includes the AUXIN RESPONSE FACTOR (ARF) transcription factors and Auxin/INDOLE-3-ACETIC ACID (Aux/IAA) repressor proteins. The E3 ubiquitin ligase TIR1/AFBs are complexed with the S PHASE KINASE-ASSOCIATED PROTEIN1 (SKP1) (or ASK1 in plants), CULLIN1 (CUL1), and RING BOX1 (RBX1), forming an SCFTIR1/AFB complex. Aux/IAAs interact with ARFs to prevent auxin signaling under low levels of auxin. As the auxin levels increase, auxin induces Aux/IAA binding to the SCFTIR1/AFB complex, causing the ubiquitination and degradation of Aux/IAA proteins through the 26S proteasome. The degradation of Aux/IAA proteins relieves ARF repression to allow ARF-mediated transcription to proceed. In parallel, the E3 ubiquitin ligase AUXIN RESPONSE FACTOR F-BOX1 (AFF1) facilitates the ubiquitylation of ARF7 and ARF19 for degradation through the 26S proteasome, regulating the auxin signaling transduction. Additionally, the transmembrane kinase (TMK) proteins localized in the plasma membrane mediate cell surface auxin signal transduction by binding to the non-canonical IAA32 and IAA34. The auxin-binding protein 1 (ABP1) also medicates auxin signaling through the TMK1-based cell surface pathway.