@article{19421,
  abstract     = {The phytohormone auxin (Aux) is a principal endogenous developmental signal in plants. It mediates transcriptional reprogramming by a well-established canonical signalling mechanism. TIR1/AFB auxin receptors are F-box subunits of an ubiquitin ligase complex; after auxin perception, they associate with Aux/IAA transcriptional repressors and ubiquitinate them for degradation, thus enabling the activation of auxin response factor (ARF) transcription factors1,2,3. Here we revise this paradigm by showing that without TIR1 adenylate cyclase (AC) activity4, auxin-induced degradation of Aux/IAAs is not sufficient to mediate the transcriptional auxin response. Abolishing the TIR1 AC activity does not affect auxin-induced degradation of Aux/IAAs but renders TIR1 non-functional in mediating transcriptional reprogramming and auxin-regulated development, including shoot, root, root hair growth and lateral root formation. Transgenic plants show that local cAMP production in the vicinity of the Aux/IAA–ARF complex by unrelated AC enzymes bypasses the need for auxin perception and is sufficient to induce ARF-mediated transcription. These discoveries revise the canonical model of auxin signalling and establish TIR1/AFB-produced cAMP as a second messenger essential for transcriptional reprograming.},
  author       = {Chen, Huihuang and Qi, Linlin and Zou, Minxia and Lu, Mengting and Kwiatkowski, M and Pei, Yuanrong and Jaworski, K and Friml, Jiří},
  issn         = {1476-4687},
  journal      = {Nature},
  pages        = {1011--1016},
  publisher    = {Springer Nature},
  title        = {{TIR1-produced cAMP as a second messenger in transcriptional auxin signalling}},
  doi          = {10.1038/s41586-025-08669-w},
  volume       = {640},
  year         = {2025},
}

@phdthesis{19478,
  author       = {Chen, Huihuang},
  issn         = {2663-337X},
  pages        = {118},
  publisher    = {Institute of Science and Technology Austria},
  title        = {{The cAMP second messenger in auxin signalling}},
  doi          = {10.15479/AT-ISTA-19478},
  year         = {2025},
}

@article{17141,
  abstract     = {The TIR1/AFB–Aux/IAA–ARF canonical auxin signaling pathway is widely accepted to (de)active transcriptional regulation, thus controlling auxin-associated developmental processes. However, the theme of a rapid auxin response has emerged since the 2018 Auxins and Cytokinin in Plant Development conference. To date, a few signaling components have been identified to mediate both slow and rapid auxin responses, which unveils the complexity of auxin signaling.},
  author       = {Zhang, Zilin and Chen, Huihuang and Peng, Shuaiying and Han, Huibin},
  issn         = {0022-0957},
  journal      = {Journal of Experimental Botany},
  number       = {18},
  publisher    = {Oxford University Press},
  title        = {{Slow and rapid auxin responses in Arabidopsis}},
  doi          = {10.1093/jxb/erae246},
  volume       = {75},
  year         = {2024},
}

@article{13212,
  abstract     = {Auxin is the major plant hormone regulating growth and development (Friml, 2022). Forward genetic approaches in the model plant Arabidopsis thaliana have identified major components of auxin signalling and established the canonical mechanism mediating transcriptional and thus developmental reprogramming. In this textbook view, TRANSPORT INHIBITOR RESPONSE 1 (TIR1)/AUXIN-SIGNALING F-BOX (AFBs) are auxin receptors, which act as F-box subunits determining the substrate specificity of the Skp1-Cullin1-F box protein (SCF) type E3 ubiquitin ligase complex. Auxin acts as a “molecular glue” increasing the affinity between TIR1/AFBs and the Aux/IAA repressors. Subsequently, Aux/IAAs are ubiquitinated and degraded, thus releasing auxin transcription factors from their repression making them free to mediate transcription of auxin response genes (Yu et al., 2022). Nonetheless, accumulating evidence suggests existence of rapid, non-transcriptional responses downstream of TIR1/AFBs such as auxin-induced cytosolic calcium (Ca2+) transients, plasma membrane depolarization and apoplast alkalinisation, all converging on the process of root growth inhibition and root gravitropism (Li et al., 2022). Particularly, these rapid responses are mostly contributed by predominantly cytosolic AFB1, while the long-term growth responses are mediated by mainly nuclear TIR1 and AFB2-AFB5 (Li et al., 2021; Prigge et al., 2020; Serre et al., 2021). How AFB1 conducts auxin-triggered rapid responses and how it is different from TIR1 and AFB2-AFB5 remains elusive. Here, we compare the roles of TIR1 and AFB1 in transcriptional and rapid responses by modulating their subcellular localization in Arabidopsis and by testing their ability to mediate transcriptional responses when part of the minimal auxin circuit reconstituted in yeast.},
  author       = {Chen, Huihuang and Li, Lanxin and Zou, Minxia and Qi, Linlin and Friml, Jiří},
  issn         = {1674-2052},
  journal      = {Molecular Plant},
  number       = {7},
  pages        = {1117--1119},
  publisher    = {Elsevier },
  title        = {{Distinct functions of TIR1 and AFB1 receptors in auxin signalling.}},
  doi          = {10.1016/j.molp.2023.06.007},
  volume       = {16},
  year         = {2023},
}

@article{11723,
  abstract     = {Plant cell growth responds rapidly to various stimuli, adapting architecture to environmental changes. Two major endogenous signals regulating growth are the phytohormone auxin and the secreted peptides rapid alkalinization factors (RALFs). Both trigger very rapid cellular responses and also exert long-term effects [Du et al., Annu. Rev. Plant Biol. 71, 379–402 (2020); Blackburn et al., Plant Physiol. 182, 1657–1666 (2020)]. However, the way, in which these distinct signaling pathways converge to regulate growth, remains unknown. Here, using vertical confocal microscopy combined with a microfluidic chip, we addressed the mechanism of RALF action on growth. We observed correlation between RALF1-induced rapid Arabidopsis thaliana root growth inhibition and apoplast alkalinization during the initial phase of the response, and revealed that RALF1 reversibly inhibits primary root growth through apoplast alkalinization faster than within 1 min. This rapid apoplast alkalinization was the result of RALF1-induced net H+ influx and was mediated by the receptor FERONIA (FER). Furthermore, we investigated the cross-talk between RALF1 and the auxin signaling pathways during root growth regulation. The results showed that RALF-FER signaling triggered auxin signaling with a delay of approximately 1 h by up-regulating auxin biosynthesis, thus contributing to sustained RALF1-induced growth inhibition. This biphasic RALF1 action on growth allows plants to respond rapidly to environmental stimuli and also reprogram growth and development in the long term.},
  author       = {Li, Lanxin and Chen, Huihuang and Alotaibi, Saqer S. and Pěnčík, Aleš and Adamowski, Maciek and Novák, Ondřej and Friml, Jiří},
  issn         = {1091-6490},
  journal      = {Proceedings of the National Academy of Sciences of the United States of America},
  keywords     = {Multidisciplinary},
  number       = {31},
  publisher    = {National Academy of Sciences},
  title        = {{RALF1 peptide triggers biphasic root growth inhibition upstream of auxin biosynthesis}},
  doi          = {10.1073/pnas.2121058119},
  volume       = {119},
  year         = {2022},
}

@article{12144,
  abstract     = {The phytohormone auxin is the major coordinative signal in plant development1, 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 ubiquitination2,3. 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.},
  author       = {Qi, Linlin and Kwiatkowski, Mateusz and Chen, Huihuang and Hörmayer, Lukas and Sinclair, Scott A and Zou, Minxia and del Genio, Charo I. and Kubeš, Martin F. and Napier, Richard and Jaworski, Krzysztof and Friml, Jiří},
  issn         = {1476-4687},
  journal      = {Nature},
  number       = {7934},
  pages        = {133--138},
  publisher    = {Springer Nature},
  title        = {{Adenylate cyclase activity of TIR1/AFB auxin receptors in plants}},
  doi          = {10.1038/s41586-022-05369-7},
  volume       = {611},
  year         = {2022},
}