SUMO wrestles with photosynthesis in plant chloroplasts

New research from the University of Oxford, published recently in the journal eLife, sheds fresh light on plant chloroplasts, and the proteins inside them. The regulation of chloroplast proteins is important for plant development and stress acclimation, and this is increasingly significant as plants (including our staple crops) are having to respond to unanticipated environments due to climate change.

All green plants grow by converting light energy into chemical energy via a process known as photosynthesis. Photosynthesis occurs within specialised compartments of plant cells known as chloroplasts. Chloroplasts require thousands of different proteins to function, and these are imported into the chloroplast via a specialised machinery known as the TOC complex. The TOC complex is, itself, made of proteins.

Recent studies revealed that the TOC complex is rapidly destroyed when plants encounter environmental stress; this protects plant cells from damage by limiting photosynthesis, which can generate toxic by-products under adverse conditions. In this process, the TOC complex is first marked with a small protein called ubiquitin. This ‘ubiquitination’ promotes the destruction of the complex, and thus suppresses chloroplast protein import and photosynthesis.

In this study, researchers asked whether the TOC complex is SUMOylated. SUMO is another small tag that is similar to ubiquitin, and ‘SUMOylation’ can either speed-up or slow-down protein destruction, depending on the context. The study also aimed to determine whether such SUMOylation is functionally important, and whether it influences the stability of the TOC complex.

The researchers started by showing that there is a functional link between chloroplast protein import and SUMOylation, in a series of genetic experiments. Then, they went on to demonstrate that TOC proteins are SUMOylated in biochemical experiments using purified proteins. All together, these experiments demonstrated that TOC protein SUMOylation is important for plant growth and development.

The genetic interaction between chloroplast protein import and the SUMO system is intriguing, as it is highly similar to the previously-observed interaction between chloroplast protein import and ubiquitination. In the latter, an E3 ubiquitin ligase known as SP1 plays a key role, and operates within the recently-described CHLORAD pathway.

The CHLORAD pathway promotes the degradation of the TOC complex in response to developmental cues or environmental stress. The observed similarity with CHLORAD implies that SUMOylation may regulate the activity of the CHLORAD pathway. This is very interesting, as SUMOylation is induced by various forms of environmental stress and is a key driver of plant stress acclimation.

Professor Paul Jarvis, who supervised the research, said: “This study has uncovered another layer of complexity within the systems that plants use to control their chloroplasts. It was remarkable when the role for ubiquitination, and CHLORAD, was discovered a few years ago, and this new role for SUMO just adds to the intrigue. It’s becoming increasingly clear that such post-translational regulation of chloroplast proteins is vital for plant growth and productivity, and thus of course for food production”.

Dr Samuel Watson, the paper’s lead author, said: “It was very satisfying to bring this work to publication. As the planet warms, it will be increasingly urgent to understand the molecular basis of plant stress tolerance. I hope that this work has provided a small contribution to this important area of research”.

Building on these discoveries, the researchers are currently exploring how the CHLORAD pathway can be manipulated to improve crop performance. Better understanding of the regulation of chloroplast protein import and/or the CHLORAD pathway, delivered as a result of the new findings reported here, will help to guide these efforts.

To read more about this research, published in eLife, please visit