Inhibition of Carbon Assimilation by excess Radiation: Understanding maize weak Spot (ICARUS)

Light is both a resource and a potent stress factor for plants. Photosynthesis harnesses energy from the sun to fix carbon dioxide into sugars and the protein building blocks of life, However, leaves routinely receive up to 5 times more light than can be processed via photosynthesis. To avoid damage by too much light, plants have evolved a range of ways to protect themselves. However, light levels are highly dynamic due to shading by other leaves, clouds passing over and changing direction of sunlight. As a result, plants need to be able to adjust very rapidly. Over-protection leads to a loss of photosynthesis and growth, whereas under-protection renders the leaves susceptible to getting damaged. Theoretical predictions indicated that more rapid changes in protection levels might benefit growth and productivity. Indeed, I recently demonstrated that when changes in protection levels are accelerated, this dramatically improves photosynthetic efficiency and productivity under field conditions in a model crop. Theoretical analysis suggests that maize could benefit even more from improving protection against high light and accelerate recovery rates, but we lack understanding to pin-point the specific targets that need improving.
Maize is the most dominant crop globally, with a world-wide production volume of 1.09 billion metric tons. Maize is increasingly being grown in temperate regions and in the UK is a popular crop for animal fodder and for use in anaerobic digesters to provide bioenergy, with a total production area of 195.000 ha in 2017. Intriguingly, maize seems to have a difficult time protecting itself against too much light. Light-inflicted damage strongly limits productivity of maize, especially during bright cool days in early season, when plants can often look yellow and stunted, but also under very hot or dry conditions. Improving the protection level and adjustment rate in maize is predicted to improve productivity by up to 30%, and would hold great economic value, especially for agriculture in temperate climates such as the UK.
The reasons for the sensitivity to damage by light and slow adjustment in maize are unclear. Maize photosynthesis utilizes a biochemical pump to concentrate carbon dioxide, the substrate for photosynthesis, in specialized bundle sheath cells. This so-called C4 pathway increases the efficiency of photosynthesis, but also makes photosynthetic limitations more difficult to study than in plants with conventional photosynthesis. At the photosystem level, we don't know which photosynthetic complex is most vulnerable, and at the cellular level, we don't know which of the two photosynthetic cell types is most easily inhibited. The aim of this project is therefore to identify the weaknesses in maize photosynthesis leading to its vulnerability to photoinhibition, using a novel combination of non-invasive measurement techniques.
The proposed research programme will focus on:
1) which specific photosynthetic electron transport proteins are the main bottlenecks underlying maize photoinhibition
2) which photosynthetic cell type is most impacted by photoinhibition
2) which light conditions are most likely to give rise to photoinhibition
3) whether the capacity of maize to protect against too much light is insufficient
The results will provide detailed understanding of the weak links in maize photosynthesis in response to too much light and will be synthesized into a priority list of improvements in maize photosynthesis and crop improvement.

The most dominant global crop maize is highly sensitive to photo-inhibition and recovery time is remarkably slow. Photo-inhibition drastically limits the productivity of maize, especially during bright cool days in early season, when plants seem to be distinctly under-protected. Improving the protection level and adjustment rate in maize may boost productivity by up to 30%, and would hold great economic value, especially for agriculture in temperate climates such as the UK.
The reasons for maize' high susceptibility to photo-inhibition and slow recovery are unclear and the presence of the C4 pathway makes them less tractable than in C3 species. The overall aim of this project is to identify the reasons for the vulnerability of maize photosynthesis to photo-inhibition and the subsequent slow recovery. We will use a combination of chlorophyll fluorescence and near-infrared differential absorption techniques to simultaneously assess photosystem I and II donor and acceptor side and quantum yields. We will spatially resolve these optical measurements using light sheet microscopy and a leaf radiative transfer model. The most challenging aspect of characterizing photo-inhibition in maize is the distribution of photosynthetic capacity across two contrasting photosynthetic cell types, mesophyll (M) and bundle sheath (BS). We will perform parallel measurements of stable isotope fractionation online with photosynthetic gas exchange to determine the balance between photoinhibition in M and BS cells. The combination of these measurements will provide a truly integrated view of maize photosynthetic performance in response to excessive light, which will be used to systematically evaluate the action spectrum and frequency response of maize photoinhibition, and quantify photoprotective capacity of chloroplast movements, nonphotochemical quenching and photorespiration. The results will provide detailed understanding of photo-inhibition in maize and highlight targets for improvement.

In the short to mid-term maize breeding and agricultural biotechnology companies such as KWS, Limagrain, Bayer CropScience and Syngenta may benefit from this research, as it will produce key targets for the improvement of photosynthesis in maize and potentially other C4 plants. The PI and PDRA will actively approach these companies in the early stages of the project to discuss how the results from the proposed research could be used to screen their germplasm and cross beneficial traits into new varieties. The results will also pave the way to capitalize on the truly disruptive potential of increasing maize yields via improved photosynthesis, as well as enhanced chilling tolerance and drought tolerance. The importance of maize as a global food, feed and fuel crop emphasizes the potential impact in terms of economic value.

Companies will have access to the data through publications, through attendance and presentations at national and international meetings. In addition, I am a non-funded member of the Realizing Increased Photosynthetic Efficiency consortium which is already actively engaged in photosynthesis research to improve crop yields, supported by the Bill and Melinda Gates foundation, Foundation for Food and Agricultural Research (FFAR) and UKAID and the projects results will be presented in their annual project meetings to complement their ongoing research activities. The results of the project will be discussed annually or as soon as relevant with Cambridge Enterprise to make sure intellectual property arising from the results is adequately dealt with.

The PI and PDRA will actively engage with a local network of plant scientists via the Cambridge University hub for all those involved in Plant Sciences (CambPlants), which consists of an extensive network of academics, non-governmental organizations and private industry, and the Cambridge Global Food Security research centre, which is an interdisciplinary centre bringing together scientists involved in future food security.

Members of the public will appreciate the understanding of the intricate workings of a crop species they encounter very frequently and the utility of this knowledge for improving future crop yields. They will have access to our findings through: 1) University of Cambridge outreach via the University website and press releases, 2) communication through the Departmental website and Twitter, 3) Outreach activities during the Festival of Plants and the Cambridge Science Festival.

The project will give early career scientists training opportunities and lab experience. It is expected to provide short research placements for several undergraduate and high school students and we will present and discuss the project in its broadest context at the Cambridge Food Security Forum, a student-led body aiming to promote discussion and spread knowledge about food security.