Adenylate cyclase activity of TIR1/AFB auxin receptors in plants

The phytohormone auxin is the major coordinative signal in plant development(1), mediating transcriptional reprogramming by a well-established canonical signalling pathway. TRANSPORT INHIBITOR RESPONSE 1 (TIR1)/AUXIN-SIGNALING F-BOX (AFB) auxin receptors are F-box subunits of ubiquitin ligase complexes. In response to auxin, they associate with Aux/IAA transcriptional repressors and target them for degradation via ubiquitination(2,3). Here we identify adenylate cyclase (AC) activity as an additional function of TIR1/AFB receptors across land plants. Auxin, together with Aux/IAAs, stimulates cAMP production. Three separate mutations in the AC motif of the TIR1 C-terminal region, all of which abolish the AC activity, each render TIR1 ineffective in mediating gravitropism and sustained auxin-induced root growth inhibition, and also affect auxin-induced transcriptional regulation. These results highlight the importance of TIR1/AFB AC activity in canonical auxin signalling. They also identify a unique phytohormone receptor cassette combining F-box and AC motifs, and the role of cAMP as a second messenger in plants.

Automated summary

The phytohormone auxin is a crucial signal in plant development, and its transcriptional reprogramming is mediated through a canonical signaling pathway. The TIR1/AFB receptors, in addition to their known function as auxin receptors, also possess adenylate cyclase (AC) activity. Mutations in the AC motif of the TIR1 C-terminal region affect the receptor's function, leading to impaired gravitropism and auxin-induced growth inhibition in plants.