Introduction:Auxin, a key hormone regulating plant growth and development, has seen a major breakthrough in the study of its signaling mechanisms! Professor Qi Linlin from Faculty of Synthetic Biology at Shenzhen University of Advanced Technology, as co-first author, collaborated with Jiří Friml’s team from the Institute of Science and Technology Austria (ISTA) to publish a groundbreaking study in Nature.This research has revised the classical theoretical model of auxin signaling and has confirmed that cyclic adenosine monophosphate (cAMP) serves as a second messenger in plants.

Research Background:The Classical Model of Auxin Signaling

Research Background: The Classical Model of Auxin Signaling Auxin, one of the most critical plant hormones, regulates nearly all stages of plant life, from embryonic development to reproduction. Research on auxin signaling has long been at the forefront of plant science, providing insights into other signaling pathways. The classical model of auxin signaling posits that auxin promotes the interaction between the TIR1/AFB receptors and the Aux/IAA co-receptors. The E3 ubiquitin ligase activity of TIR1/AFB triggers the ubiquitination and degradation of Aux/IAA, relieving Aux/IAA’s repression of ARF transcription factors and ultimately activating downstream gene expression. This model considers the degradation of Aux/IAA proteins as both a sufficient and necessary condition for mediating downstream transcriptional responses to auxin. Although alternative mechanisms, such as the extracellular ABP1/ABLs-TMKs-mediated auxin signaling pathway and direct regulation of the ARF3 transcription factor by IAA, have been identified, the TIR1/AFB-Aux/IAA-ARF pathway is widely regarded as the core mechanism mediating most auxin effects. For nearly two decades, this classical model has remained conceptually unchanged.

In 2022, during his postdoctoral research with Jiří Friml’s team at ISTA, Dr. Qi Linlin published a first-author paper in Nature, reporting for the first time that the TIR1/AFB receptor family also possesses adenylate cyclase (AC) activity. This activity catalyzes ATP to produce cAMP, regulating root growth inhibition and gravitropism (Qi et al., Nature, 2022), opening a new direction in auxin signaling research. This earlier study highlighted the AC activity of TIR1/AFB and its importance in transcriptional regulation. However, the relationship between the AC activity and E3 ubiquitin ligase activity of TIR1/AFB remained unclear, and whether cAMP, the product of AC activity, truly functions as a second messenger in the auxin signaling pathway required further validation.

Research Breakthrough: cAMP as a Second Messenger in Auxin Signaling Pathway

Recently, Professor Qi Linlin’s team, in collaboration with Jiří Friml’s group, published a study in Nature titled TIR1-produced cAMP as a second messenger in transcriptional auxin signalling. This research revises the classical auxin signaling model, demonstrating that Aux/IAA degradation is not a sufficient and necessary condition for mediating downstream transcriptional regulation. Instead, the AC activity of TIR1 and its product, cAMP, play an indispensable role in auxin signaling. The study provides the first genetic evidence confirming cAMP as a second messenger in plant cells, resolving a long-standing historical controversy.

The research demonstrates that the loss of AC activity in the auxin receptor TIR1/AFB does not impair Aux/IAA protein degradation but disrupts auxin-induced transcriptional regulation, indicating that Aux/IAA degradation alone is insufficient to fully mediate the auxin transcriptional response. Furthermore, by inducing the expression of artificially constructed fusion proteins to produce cAMP near the Aux/IAA-ARF complex, the study shows that this approach can bypass TIR1/AFB auxin perception and Aux/IAA degradation pathways. Even in the presence of stable Aux/IAA transcriptional repressors, cAMP can still activate ARF transcription factor-mediated transcriptional reprogramming, mimicking auxin’s biological effects (see figure below). This suggests that, in this experimental system, cAMP is sufficient to initiate the auxin transcriptional response. The study not only provides genetic evidence confirming cAMP as a second messenger in plants—a topic of historical debate—but also revises the theoretical framework of the classical auxin signaling pathway. Notably, the core assertion of the classical model, that “Aux/IAA degradation is a sufficient and necessary condition for mediating auxin transcriptional responses”, shifts to a scientific question requiring re-examination, paving the way for new theoretical explorations in future research.

Jiří Friml is the corresponding author of this paper, with ISTA doctoral student Chen Huihuang, Qi Linlin (now a professor at Shenzhen University of Advanced Technology), and Dr. Zou Minxia as co-first authors. The authors thank Mark Estelle and Stefan Kepinski for their constructive comments. This project was supported by the European Research Council (ERC), the Austrian Science Fund (FWF), the National Natural Science Foundation of China (General Program), ISTA’s Interdisciplinary Project Committee (IPC), and an EMBO Postdoctoral Fellowship.