A plant’s ability to turn green and start photosynthesising depends on a molecular delivery system that scientists still do not fully understand how to build. This project aims to solve that puzzle. Chloroplasts—the tiny structures inside plant cells that capture sunlight and convert it into energy—are built from thousands of different proteins. Most of these proteins are made elsewhere in the cell and must be imported through a two-part gate called TOC-TIC. Researchers know what the gate’s components are and how they are regulated, but they have almost no idea how those components assemble themselves into a working machine during the rapid greening process. Without that assembly, chloroplasts cannot form properly, and plants cannot grow. If the team succeeds in identifying the assembly factors and step-by-step events that build TOC-TIC, it will fill a fundamental gap in plant biology. This is primarily curiosity-driven fundamental science. However, a deeper understanding of chloroplast development could eventually inform efforts to breed or engineer crops with better photosynthetic performance, higher yields, or greater resilience to environmental stress—traits that matter for food security as the global population approaches 9.8 billion by 2050.
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Context Current agricultural outputs are insufficient to meet the needs of a rapidly-growing human population (set to reach 9.8 billion by 2050), especially in the context of anthropogenic climate-change, leading to increasing concerns about food-security. Thus, delivering improved agricultural yields with resilience to stress and disease, while alleviating pressure on natural resources, is a key challenge of our era. To meet this challenge, we must develop improved crop varieties by delivering and then exploiting advances in our understanding in key areas of plant biology. Photosynthesis, chloroplasts, and protein import Photosynthesis is the process whereby sunlight energy is harnessed to power cellular activities and growth. It is the energetic basis for essentially all life on Earth, including our food systems, and thus is of inestimable importance. In plants, photosynthesis occurs inside tiny, subcellular structures (“organelles”) called chloroplasts, which are rich in the green, light-absorbing pigment chlorophyll. Chloroplasts are built-up from thousands of different proteins which must be rapidly assembled during plant greening. Because most of these proteins are encoded by nuclear genes and made outside of the organelle (in the area of the cell called the cytosol), sophisticated machinery is needed to import these proteins into the organelle. This import machinery is vital for chloroplast development, photosynthetic establishment, and plant growth; and it has two parts, TOC and TIC (“translocon of the outer/inner chloroplast membrane”), each of which is a multiprotein complex comprising several different subunits. The project: aim, challenges and applications The aim of this project is to elucidate how the TOC-TIC multiprotein machines are assembled from individual subunits, to enable the massive delivery of proteins into chloroplasts during greening and photosynthetic establishment. Our research group has a long track-record of studying chloroplast protein import mechanisms1,2. For example, we discovered how the TOC is regulated by a novel mechanism called CHLORAD (“chloroplast-associated protein degradation”)3; and showed how such regulation is vital for diverse aspects of plant growth, including resilience to adverse environments4 and fruit ripening5. However, while the composition and regulation of the import machinery are well established, our understanding of the molecular assembly processes that lead to its formation is rudimentary. This project will directly address this major knowledge gap. Within multiprotein complexes like TOC and TIC, the organization of subunits is crucial for function; and this is established through a highly-controlled assembly process involving accessory proteins called “assembly-factors”. These processes are vital, because any defects in them can result in dysfunctional or unstable complexes with severe consequences. We will undercover the assembly-factors and step-wise events that deliver the TOC complex and its subsequent association with TIC. The main challenges in this area are the low abundance of the complexes and the transient nature of the relevant assembly-intermediates and interactions, making it difficult to purify them for analysis. Fortunately, we have now substantially overcome these hurdles as our preliminary results show. Thus, we are in an exciting and unprecedented position to deliver a step-change in our understanding of this vital component of plant greening and photosynthetic establishment. This new information on chloroplast development and function may offer applications in the future development of crops with improved photosynthetic performance, stress resilience, and yields. The project therefore has strong relevance to BBSRC’s long-term research and innovation priorities in the area of plant health and agri-food. [1]_Jarvis_2013_Nat.Rev.Mol.CellBiol._14:787_doi:10.1038/nrm3702. [2]_Sun_2023_Annu.Rev.PlantBiol._74:259_doi:10.1146/annurev-arplant-070122-032532. [3]_Ling_2019_Science_363:eaav4467_doi:10.1126/science.aav4467. [4]_Ling_2015_Curr.Biol._25:2527_doi:10.1016/j.cub.2015.08.015. [5]_Ling_2021_Nat.Plants_7:655_doi:10.1038/s41477-021-00916-y.
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