Emerging roles of cAMP: a transcriptional master regulator in the canonical TIR1/AFB-mediated auxin signaling

Recent breakthroughs have revealed that the auxin receptors TIR1/AFB possess adenylate cyclase (AC) activity, producing the second messenger 3',5'-cyclic adenosine monophosphate (cAMP) to regulate transcription. This fundamentally reshapes our understanding of auxin signaling. This review synthesizes current knowledge on the dual functionality of TIR1/AFBs: mediating both the canonical SCF^TIR1/AFB-Aux/IAA-ARF cascade and auxin-induced cAMP production. Genetic and biochemical evidence demonstrates that Aux/IAA interaction enhances TIR1/AFB's AC activity, while locally generated cAMP near ARFs can activate transcription independently of Aux/IAA degradation, suggesting the presence of a parallel signaling axis. As a central hub, cAMP further integrates auxin responses with other hormonal pathways (e.g., ABA, JA, SA, and GA3) through bidirectional crosstalk. In this review, two mechanistic models are proposed for cAMP-mediated ARF activation: (1) kinase cascades via cAMP-dependent phosphorylation, and (2) chromatin remodeling through histone acetylation. Key unresolved questions include spatiotemporal control of cAMP gradients, identification of plant phosphodiesterases (PDEs), and coordination between fast and canonical auxin signaling responses. These findings establish cAMP as a pivotal regulator in auxin signaling and hormonal crosstalk, providing novel insights into plant development and stress adaptation.