We use the model plant Arabidopsis thaliana (thale cress) to study the molecular mechanisms underlying chloroplast protein import. Our recent work demonstrated that the TOC machine is selectively targeted for a type of modification called ubiquitination, by a protein named SP1 (a RING-type ubiquitin E3 ligase) that is also located in the chloroplast outer envelope membrane. Such modification promotes the removal and breakdown of unwanted TOC components as part of a regulatory process.
To better understand the function of SP1, we compared the growth of natural, wild-type plants with that of mutant plants lacking SP1, and that of engineered plants that possesses higher levels of SP1. Under high-salinity and water-stress conditions, the mutant plants failed to develop normally, whereas the plants with higher levels of SP1 were more stress-tolerant than the wild type. By contrast, the different plants were indistinguishable when grown under non-stressful conditions. These results indicated that SP1 acts to promote resilience under stressful conditions, and we were able to show that it achieves this by limiting the accumulation of ROS.
Concerning the underlying mechanism, we observed that TOC proteins are depleted under stress conditions in an SP1-dependent manner: levels of TOC proteins declined markedly in the wild type under stress, but such a decline did not occur in mutant plants; in contrast, TOC components reached even lower levels in plants with more SP1. These results were intriguing, as they suggested a hypothesis in which SP1 acts to deplete TOC components under stress in order to restrict the import of new protein components of the photosynthetic machinery (see Figure 1). This hypothesis was confirmed by measuring the import capabilities of chloroplasts isolated from osmotically stressed wild-type plants, mutant plants, and plants with more SP1.
SP1 has considerable potential as a technology.
Our research has revealed a role for the SP1 protein in the regulation of chloroplast protein import in response to environmental stresses. Such regulation helps to finely tune the protein composition of the chloroplasts, ensuring that it is optimally matched to varying environmental circumstances. Current work in our lab aims to elucidate the molecular mechanisms of such regulation . Since all plants have the SP1 gene, we hope that by modifying SP1 expression we will be able to generate crops that can better deal with sub-optimal growing conditions to sustainably meet increasing demands for global food and bioenergy production. We are currently exploring this possibility by using a range of different crop species as models.
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- Ling, Q., Huang, W., Baldwin, A. & Jarvis, P. Chloroplast biogenesis is regulated by direct action of the ubiquitin-proteasome system. Science 338, 655-659 (2012).
- Ling, Q. & Jarvis, P. Regulation of chloroplast protein import by the ubiquitin E3 ligase SP1 is important for stress tolerance in plants. Curr. Biol. 25, 2527-2534 (2015).
- Ling, Q., Broad, W., Trösch, R., Töpel, M., Demiral Sert, T., Lymperopoulos, P., Baldwin, A., Jarvis, RP. Ubiquitin-dependent chloroplast-associated protein degradation in plants. Science. 363, eaav4467 (2019).