The human population is growing rapidly and set to reach over 9 billion by 2050, and there is ever increasing pressure on natural resources. Therefore, the drivers for increased crop yields are stronger than ever. To meet these demands it will be essential to develop improved crop varieties – plants with enhanced nutritional value or resilience to adverse environments. The chloroplast, a defining feature of the plant cell, will be an important target of our efforts to address this aim.
Chloroplasts are green plant organelles (tiny compartments within cells) which perform photosynthesis – a process that provides us with both food to eat and oxygen to breathe. This means they are vital to essentially all life on the planet. However, that is not all they do. Chloroplasts actually belong to a broader family of related, but structurally diverse, organelles called the plastids . These include, for example: etioplasts in plants grown in darkness; chromoplasts containing red, orange or yellow carotenoid pigments in flowers and fruits; and amyloplasts in starch storage organs such as roots, tubers and seeds. Remarkably, these different types of plastid can interconvert, such as during seed germination, fruit ripening, or leaf ageing. Moreover, such transitions are critically important for the yield and quality of crops, not only because photosynthesis provides the energy for plant growth, but also because substances determining flavour and nutritional properties are produced inside different plastids. In addition, plastids are at the heart of the mechanisms that plants use to respond to environmental challenges, such as abiotic stress tolerance (e.g., drought, salinity) and pathogen attack (disease). To achieve such diversity of form and function, a process called “plastid protein import” is needed.
Chloroplasts are vital to essentially all life on the planet.
The majority of chloroplast proteins are encoded by genes in the cell nucleus, and produced as precursor proteins in the cytosol (a jelly-like matrix that the cell components are suspended in) that must be imported into chloroplasts. Such import occurs through a sophisticated protein transport (or translocation) system located in a chloroplast’s two envelope membranes. One part of this import system is a multi-protein machine called the TOC complex; this machine exists at the surface of the chloroplasts, in the outer membrane, and it acts to receive and initiate the translocation of chloroplast precursor proteins .
Interestingly, the TOC complex contains a variety of different receptor components (these are the subunits responsible for recognizing client precursor proteins). This variety of receptors means that many different types of precursor protein (e.g., those with photosynthesis-related functions, and others with “house-keeping” functions) can be recognized efficiently. Our recent work has helped to clarify how the TOC machine, and its different receptor subunits, are regulated in order to optimize protein import to achieve the required plastid functions, and the dramatic plastid type interconversions (or transitions) mentioned earlier.
The different types of plastid can interconvert.
We use the model plant Arabidopsis thaliana (thale cress) to study the regulatory mechanisms governing plastid protein import. Our research has demonstrated that TOC components are selectively targeted for a type of modification called ubiquitination by a protein named SP1; this protein is a RING-type ubiquitin E3 ligase of the chloroplast outer envelope membrane . Such modification triggers the degradation (breakdown by proteolysis) of TOC components. Remarkablely, such degradation is critical for plastid transitions. For example, plants lacking the SP1 gene cannot turn green when transferred from darkness to the light, due to the failure of the plastids to transition from etioplasts to chloroplasts.
Recently, we achieved a breakthrough in our work on this “SP1 pathway”: we discovered two new components called SP2 and CDC48 . SP2 forms a channel across the chloroplast’s outer membrane, while CDC48 is a molecular chaperone (a molecular motor) that resides outside of the chloroplasts in the cytosol (jelly-like cell matrix). Both SP2 and CDC48 are involved in the “SP1 pathway” for TOC component break-down; more specifically, they function to extract TOC components from the chloroplasts once they have been ubiquitinated by SP1, in a process called “retrotranslocation”. The SP2 component provides a channel or conduit through which TOC proteins are extracted, while CDC48 acts by pulling on TOC proteins to drive their extraction to the cytosol. On arrival in the cytsol, following extraction, the TOC proteins are degraded. We named this new TOC protein degradation system CHLORAD (for “Chloroplast-Associated Protein Degradation”). We found that SP2, like SP1, is important for plastid type transitions, for example during leaf ageing: in plants lacking SP2, the leaves stay green for longer than those of normal plants, indicating delayed transition of chloroplasts to gerontoplasts.
CHLORAD stands for "Chloroplast-Associated Protein Degradation".
Our new research provides a detailed mechanistic basis (CHLORAD) for the regulation of plastid protein import, and thereby presents a means by which the protein composition (or proteome) of plastids can be adjusted. It also shows that CHLORAD is important for the orchestration of plastid type interconversions, providing a further opportunity for the improvement of agricultural and horticultural plants. Many important agronomic traits are strongly related to plastid transitions. For example, leaf ageing involves the chloroplast-to-gerontoplast transition, fruit ripening requires the chloroplast-to-chromoplast transition, and grain development involves amyloplast differentiation. Because CHLORAD is a conserved system in all higher plants, it offers broad application prospects for the generation of new crop varieties with improved yield and quality characteristics, which may be delivered through the manipulation of CHLORAD components in plants.
By manipulating the CHLORAD system, we could produce crops with increased yield and nutritional value.
 Ling, Q., Broad, W., Trösch, R., Töpel, M., Demiral Sert, T., Lymperopoulos, P., Baldwin, A. and Jarvis, R.P. (2019) Ubiquitin-dependent chloroplast-associated protein degradation in plants. Science 363: eaav4467.