To grow up or not: SUMOylation of IAA7 acts as a key molecular switch of auxin signaling

Auxin controls plant growth and development by determining the rate of cell division, differentiation, and elongation. Auxin regulates diverse developmental processes, and plants have developed sophisticated mechanisms to perceive and respond to dynamic levels of auxin (Yu et al. 2022). However, much less is known about the molecular mechanisms facilitating rapid switching between transcriptional repression and activation of auxin-responsive genes.

In the current model (Fig. 1), auxin/indole-3-acetic acid (Aux/IAA) family proteins act as repressors of the auxin-signaling pathway. Under low auxin conditions, Aux/IAAs directly interact with auxin response factors (ARFs) and the TOPLESS corepressor to repress the dimerization and activity of ARFs, thereby abolishing the transcription of downstream auxin-responsive genes. When auxin levels increase, Aux/IAAs interact with the auxin receptor transport inhibitor response 1 (TIR1), which also serves as a core subunit of the Skp1-cullin 1-F-box ubiquitin ligase complex. Auxin strengthens the interaction between Aux/IAA and TIR1, leading to the proteolysis of Aux/IAAs via the action of the 26S proteasome and enabling ARF-ARF dimerization/transcriptional activation. Thus, high concentrations of auxin effectively turn on the signaling pathway by inducing the degradation of Aux/IAAs and in turn activating the transcription of auxin-responsive genes.

Figure 1.

A role for MMS21-mediated SUMOylation of IAA17 in auxin signaling. Under low auxin conditions, the SUMO E3 ligase MMS21 mediates SUMOylation of Aux/IAA17 at the position of Lysine 41 (K41). This modification stabilizes IAA17 by reducing the ubiquitination of IAA17. When high levels of auxin are present, auxin dampens IAA17 SUMOylation and strengthens the interaction between IAA17 and auxin-bound TIR1, which triggers the degradation of IAA17. Meanwhile, ARF6 is released from IAA17 and forms homodimers to activate expression of auxin-responsive genes. Adapted from Zhang et al. (2023).

SUMOylation is an essential posttranslational protein modification that controls protein stability and activity by attaching small ubiquitin-like modifier (SUMO) peptides to target proteins. How SUMOylation contributes to auxin perception in plants remains unclear. The SUMO E3 ligase methyl methanesulfonate sensitivity gene 21 (MMS21) regulates auxin signaling and root development. However, the relevant molecular mechanism is still unclear (Duan et al. 2009; Huang et al. 2009; Xu et al. 2013).

In this issue of Plant Physiology, Zhang et al. (2023) address the function of MMS21-mediated SUMOylation in auxin signaling. The authors first searched for the interaction partners of MMS21 in Arabidopsis (Arabidopsis thaliana) using a large-scale yeast two-hybrid screen. They discovered that IAA17, a member of the Aux/IAA protein family (Paponov et al. 2008), physically interacts with MMS21 (Zhang et al. 2023). They further validated that MMS21 catalyzes SUMOylation of IAA17 at the position of Lys-41 both in vivo and in vitro (Zhang et al. 2023). To further verify the roles of MMS21-mediated IAA17 SUMOylation in auxin signaling and root growth, Zhang et al. conducted elegant genetic analyses by comparing root growth in mms21 mutant, IAA17 overexpression (IAA17-OE), IAA17^K41R-OE, and wild-type lines. They revealed that compromised root elongation induced by the constitutive expression of IAA17 depends on MMS12-mediated SUMOylation of IAA17 at Lys-41 (Zhang et al. 2023).

Next, the authors elaborated on how SUMOylation affects the action of IAA17 in auxin signaling. Among the four conserved domains found in the IAA proteins, domain II contains a conserved “degron” motif, which can be recognized by TIR1 to trigger the ubiquitin-based degradation of IAA17 (Calderon-Villalobos et al. 2010). Zhang et al. demonstrated that synthetic auxin, 1-naphthylacetic acid (NAA), induces degradation of IAA17, whereas the 26S proteasome inhibitor MG132 efficiently prevents the degradation, confirming that auxin triggers ubiquitin-dependent degradation of IAA17. To further examine whether MMS21-mediated SUMOylation regulates the degradation of IAA17, the authors compared the stability of IAA17 in the transgenic plants with the constitutive expression of IAA17 or IAA17^K41R in the wild type or mms21 mutant. Notably, NAA-induced IAA17 degradation was enhanced by the Lys41→Arg mutation in IAA17 or by a lack of MMS21, indicating that MM21-induced SUMOylation at K41 impedes the ubiquitin-based degradation of IAA17 (Zhang et al. 2023).

How does SUMOylation prevent IAA17 degradation? While TIR1 interacts with the highly conserved domain II of IAA17 and promotes the ubiquitination and degradation of IAA17, the SUMOylation site K41 is not localized in the domain II of IAA17. Interestingly, Zhang et al. found that MMS21-mediated SUMOylation inhibits the ubiquitination of IAA17 (Zhang et al. 2023). The SUMO chain may interfere with the interaction between IAA17 and TIR1, thus inhibiting the ubiquitination-dependent degradation of IAA17. It will be fascinating to further test this regulatory mechanism in the future. It will also be interesting to investigate whether K41 SUMOylation is conserved among IAA proteins in Arabidopsis and other organisms.

IAA17 inhibits auxin signaling by directly repressing ARFs (Salehin et al. 2015). Zhang et al. point out that this regulation substantially relies on the SUMOylation of IAA17 as both IAA17^K41R-OE in wild type and IAA17-OE in mms21 failed to suppress the expression of ARF6-targeted genes (Zhang et al. 2023). In addition, a co-immunoprecipitation assay indicated SUMOylation of IAA17 does not affect its interaction with ARF6 (Zhang et al. 2023).

Finally, the authors explored the physiological link between IAA17 SUMOylation and auxin signaling. They observed that the level of IAA17 SUMOylation was substantially reduced after NAA treatments, suggesting that plants sense high auxin levels that inhibit IAA17 SUMOylation, finally inducing IAA17 degradation and activation of ARFs (Zhang et al. 2023). This hypothesis is further supported by the reduced difference in root length between the IAA17-OE and IAA17^K41R-OE lines after NAA treatment.

In summary, this study depicts a molecular mechanism by which MMS21-mediated IAA17 SUMOylation stabilizes IAA17 by inhibiting the ubiquitination of IAA17 under low auxin concentration conditions (Fig. 1). In contrast, high levels of auxin can diminish SUMOylation of IAA17, leading to the ubiquitin-based degradation of IAA17 and thus alleviating the transcriptional repression of ARF6-targeted genes. This regulation is important for plants to establish normal root architecture. Nevertheless, auxin not only regulates plant growth and development but also plays a crucial role during abiotic stress, such as drought and high salinity, and biotic stress, including defense against various plant pathogens and insects (Yuan et al. 2017; Liu et al. 2022). The contribution of protein SUMOylation during stress response remains to be addressed in the future.

Funding

This work was supported by the Seed Funding Program for Basic Research (66620211115918), and the Hong Kong Research Grants Council Early Career Scheme (27118022). Any opinions, findings, conclusions, or recommendations expressed in this publication do not reflect the views of the Government of the Hong Kong Special Administrative Region or the Innovation and Technology Commission.