COP1 destabilizes DELLA proteins in Arabidopsis

Background

Gibberellic acid (GA) is one of the key phytohormones that contribute to plant growth. The canonical model of the GA signalling pathway in angiosperms includes the recognition of the hormone by the receptor GID1 (GIBBERELLIN INSENSITIVE1) and the further ubiquitination and degradation of the DELLA proteins by the proteasome. The DELLA proteins interact with transcription factors and work as coactivators helping to regulate downstream genes (Djakovic-Petrovic et al., 2007). Additionally, environmental cues like light and temperature affect GA metabolism and are tightly connected with the biological processes regulated by GA. However, the cross-talk between GA and environmental signalling is not fully understood.

Major findings

In this paper, Blanco-Touriñan and colleagues observed that in Arabidopsis thaliana  the abundance of the DELLA protein REPRESSOR OF ga1-3 (RGA) decreases under warm temperatures (28°C) and shade. They showed that in the presence of the GA inhibitor (PAC), the reduction in environmental conditions is still observed, indicating that this effect is independent of GA/GID1 mediated degradation. ‘They obtained similar results with an RGA version with an RGA version resistant to GA-mediated degradation that cannot interact with GID1 (Dill et al., 2001).

Interestingly, they found that this environmental degradation of RGA is mediated by the E3 ubiquitin ligase CONSTITUTIVELY PHOTOMORPHOGENIC1 (COP1), as cop1-4 mutant plants or exogenous addition of the proteasome inhibitor MG132 are unable to degrade RGA under shade and warm conditions. This degradation is rapid and precedes the reduction of GA observed in cop1-4 under warm conditions.

The authors further show that both in yeast and in planta, the DELLA proteins RGA and GAI (GIBBERELLIC ACID INSENSITIVE) can directly interact with COP1 and SPA1 (SUPRESSOR OF phyA-105 1), which forms a complex with COP1. In vitro analysis also supports that COP1 can ubiquitinate the DELLA proteins RGA and GAI.

Hypocotyl growth is one of the phenotypes that connects GA and light signalling. On one hand, exogenous GA addition increases hypocotyl growth independently of COP1 under shade and warm conditions. On the other hand, COP1’s effect on hypocotyl elongation depends on DELLA proteins. These results highlight the physiological relevance of this interaction.

What I like about this pre-print

Findings described in this paper question the canonical signalling pathways established in plants. At the same time, they propose novel interactions between well-known proteins involved in important developmental processes and environmental regulation. These interactions were probably overlooked in the past and will impact the current knowledge in the field.

Take-home messages

This work describes an alternative pathway of DELLA degradation mediated by COP1/SPA1 and independent of GA/GID1.

The alternative degradation produces a fast reduction of DELLA abundance regulated by environmental cues and independent of the canonical GA pathway.

Future directions

The GA/GID1 pathway is conserved in all vascular plants (tracheophytes), while it is not present in other plant lineages as algae and bryophytes. At the same time, DELLA and COP1 proteins are more ancestral in conserved in all land plants (Bowman et al., 2017). Their role in these lineages is still unknown, but this recently discovered interaction could give a clue of what they are doing. This alternative regulation of DELLA stability could represent an ancient mechanism and could help to understand the origin and evolution of GA signalling in land plants. Future studies on other plant species will shed light on these possible implications.

Acknowledgement

I would like to thank for answer the questions and participate in the discussion.

References

Dill, H. S. Jung, T. P. Sun (2001). The DELLA motif is essential for gibberellin-induced degradation of RGA. Proceedings of the National Academy of Sciences of the United States of America 98, 14162–14167.

Djakovic-Petrovic, M. de Wit, L. A. Voesenek, R. Pierik (2007). DELLA protein function in growth responses to canopy signals. Plant J 51, 117–126.

Bowman, J.L., Kohchi, T., Yamato, K.T., Jenkins, J., Shu, S., Ishizaki, K., Yamaoka, S., Nishihama, R., Nakamura, Y., Berger, F., et al. (2017). Insights into land plant evolution garnered from the Marchantia polymorpha genome. Cell 171, 287–304.

 

Tags: arabidopsis, gibberelins, molecular biology, plant hormones, stress

doi: https://doi.org/10.1242/prelights.18982

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