Nucleotidyl cyclase activities of TIR1/AFB auxin receptors: new insights into the mechanism of auxin signaling

As an important plant hormone to regulate growth and development, auxin has been investigated for more than a century. It had been clearly demonstrated and well-accepted that the intracellular auxin receptors, TIR1/AFBs, are F-box proteins mediating transcriptional auxin signaling by their E3 ubiquitin ligase activity, which targets and sends for degradation the Aux/IAA transcriptional repressors. The recent discovery of adenylate cyclase (AC) and guanylate cyclase (GC) activities for TIR1/AFBs open entirely new perspectives on how auxin signaling can operate. This chapter traces back the history of how canonical transcriptional auxin signaling was established and introduces the discovery of the TIR1/AFBs-mediated nontranscriptional signaling branch. Finally, the current understanding and open questions of how TIR1/AFBs’ AC and GC activities contribute to the transcriptional and nontranscriptional auxin signaling are discussed, highlighting the possibility that cyclic adenosine monophosphate (cAMP) and cyclic guanosine monophosphate (cGMP) act as second messengers in auxin signal transduction.