description
- Feeding the rapidly growing human population requires continuous increases in crop yield. An important strategy towards higher crop productivity is to improve the efficiency of photosynthesis in plants, so that plants capture CO2 more efficiently from the atmosphere and sequester it into biomass. A small number of plant species already perform a more efficient type of photosynthesis than others - called C4 photosynthesis. C4 photosynthesis evolved independently over 60 times in different plant lineages and involves modified biochemistry that occurs between two cell types in the leaf: the mesophyll and bundle sheath cells. In C4 plants, CO2 is initially fixed in mesophyll cells into a four-carbon acid, which is then transferred into bundle sheath cells. There, the acid is decarboxylated and CO2 is regenerated, thus forming a pump action that concentrates CO2 in the bundle sheath cells. The CO2 is then fixed primarily within these bundle sheath cells, where the high CO2 concentration increases photosynthetic efficiency by up to 50% in comparison to non-C4 plants. It is therefore a major goal of plant biotechnology to engineer C4 photosynthesis into non-C4 crop species (e.g: rice, wheat, potatoes) to improve their productivity and yield. The exchange of metabolites between the mesophyll and bundle sheath cells is central to the biochemistry of C4 photosynthesis. Typical C4 plants therefore have specialised leaf anatomy that maximises physical connections between these two cell types, with numerous pores named plasmodesmata that mediate metabolite exchange. Despite the critical importance of cell-to-cell connectivity in facilitating C4 metabolism, almost nothing is known about how the large numbers of plasmodesmata form between the mesophyll and bundle sheath cells. My research is focused on this process in the C4 plant Gynandropsis gynandra, where I discovered that light is the cue that triggers the rapid and numerous formation of plasmodesmata at the mesophyll-bundle sheath interface during leaf development. In my fellowship project, I will aim to understand the cell biological mechanisms underpinning plasmodesmata formation at the mesophyll-bundle sheath interface in G. gynandra and its importance to photosynthetic efficiency. Since G. gynandra is the closest known C4 relative to the non-C4 model plant Arabidopsis thaliana, I will use the latest findings on plasmodesmata formation in Arabidopsis as a starting point. Plasmodesmata are known to contain components of the cytoskeleton, as well as plasma membrane with specialised lipid composition (lipid microdomains), and previous work in Arabidopsis suggests that both cytoskeleton and lipid microdomains are involved in plasmodesmata regulation. I will further explore this in the context of the mesophyll-bundle sheath interface in G. gynandra, testing the impact of specific chemical inhibitors of the cytoskeleton and lipid microdomains on plasmodesmata formation. I will also use gene editing in G. gynandra to generate knockout mutants in three candidate genes that could link plasmodesmata formation to the cytoskeleton and lipid biosynthesis. The effect of the gene knockouts on plasmodesmata formation at the mesophyll-bundle sheath interface, and consequences on photosynthetic efficiency, will be assessed. Finally, I also aim to discover novel candidate genes in plasmodesmata formation in G. gynandra using proximity labelling to find novel proteins associated with known plasmodesmata localised proteins. The role of these proteins will be investigated in future independent work. Overall, my findings will not only create new understanding of how plasmodesmata form in C4 plants, but reveal new cell biological and biochemical mechanisms that underpin plasmodesmata formation in plants. The knowledge can be used in the global effort to engineer C4 photosynthesis in major crops to increase photosynthesis and yield.