Activation of Non-Photosynthetic Leaf Cells for Improved Productivity

This proposal aims to answer a fundamental question in biology that will enable increased photosynthetic capacity to be engineered and therefore productivity to be improved in a wide range of plant species. Specifically, it is proposed to elucidate the genetic mechanisms that determine whether a cell in a leaf becomes photosynthetic or not and exploit these mechanisms to activate photosynthesis in inactive cells within the leaf. This utilization of conventionally inactive leaf cells for photosynthesis could lead to a step-change in agricultural productivity.

The form of photosynthesis used by the majority of plants is referred to as C3 photosynthesis because the initial product of CO2 fixation contains three carbons. Whilst mesophyll cells of C3 species green up in response to light, other cells in the leaf such as the bundle sheath do not. This proposal aims to elucidate the genetic basis of this distinction. To understand factors regulating differential photosynthetic competence, rice will be used as a model system. Rice is the most appropriate system to use because it has a relatively small genome that is better annotated than any other cereal genome. These features make genome-wide analysis of gene expression profiles and other computational analyses straightforward. In addition, the rice leaf has a developmental trajectory that is perfectly suited to the biological question being addressed. Specifically, photosynthesis is activated along the rice leaf, with inactive cells at the base of the leaf and fully active cells at the tip. This gradient can be used to dissect the dynamics and mechanisms by which photosynthesis is activated. Moreover as photosynthetically active and inactive cells develop side-by-side in the same gradient it is an ideal system in which to compare the development of photosynthetic versus non-photosynthetic cell-types.

The research programme will be split into six work packages (WP). WP1 will compute a Gene Regulatory Network for Photosynthesis (GRN-Ps). This will identify the likely regulatory components involved in the photosynthesis activation gradient. WP2 will use the GRN-Ps to identify novel regulators of photosynthetic development in the C3 mesophyll and elucidate the spatial and temporal interactions between these regulatory components. Functional analyses in rice will then test the extent to which these regulators can be modified and recruited to function in the bundle sheath. WP3 will test the hypothesis that photosynthesis is limited in the C3 bundle sheath because normal light-induced expression of photosynthesis genes is repressed in this cell type, and WP4 will discover the components of the repression mechanism. WP5 will then generate a toolkit of candidate cis and trans regulators of photosynthetic activation (or derepression) that will be tested in pairwise combinations in a rapid transient assay system. Finally, WP6 will build minimal synthetic circuits to activate and maintain photosynthesis in non-photosynthetic cells of rice leaves. Together the outputs of this research will provide design parameters for a synthetic approach to improving photosynthetic efficiency for the future.

This proposal aims to discover genetic mechanisms that prevent bundle sheath cells in C3 plants from becoming fully photosynthetic, and then to manipulate these mechanisms to activate photosynthesis in the C3 leaf. The rice leaf will be used because it provides a dynamic system to compare ontogeny of photosynthetic versus non-photosynthetic cells. Work-package 1 (WP1) generates a Gene Regulatory Network for Photosynthesis. Only ~4% of transcription factors in Arabidopsis have direct orthologues in rice, and so a monocot specific network is critical. WP2 tests the extent to which known photosynthesis activators and repressors can be manipulated. Rice containing transgenes will be phenotyped to quantify the impact of each manipulation on photosynthetic development. WP3 determines the impact of a JMJ14 histone demethylase that we have recently identified as repressing photosynthesis. WP3 will also develop a dynamic gene expression atlas for rice bundle sheath and mesophyll cells that encompasses the transition from a non-photosynthetic leaf primordium to a photosynthetic expanded leaf. In so doing, the stage at which photosynthetic development is repressed in the rice bundle sheath will be identified. WP4 provides global and unbiased insight into the extent to which photosynthesis genes in bundle sheath cells are associated with repressive histone marks, and identifies regions of DNA that are bound by transcription factors. Elucidation of these binding sites provides a platform to identify the transcription factors that bind to photosynthesis genes in bundle sheath cells, and also generate a library of cis-elements that can be tested for their capacity to activate or repress gene expression in the bundle sheath. WP5 uses a high throughput screen in planta to identify cognate pairs of regulatory DNA and transcription factors that interact. WP6 integrates WP1-5 to generate synthetic circuits for activation and maintenance of photosynthesis in non-photosynthetic cells.

The primary and immediate impact of this research will be enhanced knowledge and understanding of a fundamental biological process, namely how cells become photosynthetic. This knowledge will be communicated to the general public in a number of ways; for example in seminars at the Botanic Gardens of both the University of Cambridge and University of Oxford, and during University Open Days. Press releases will accompany the publication of high impact research, and the PIs will strengthen existing relationships with journalists and the BBC. Advances in understanding will also be disseminated at secondary education level by engaging in dialogue with both students and teachers. Together these knowledge exchange activities will enrich societal understanding of the scientific method and of biosciences research in particular.

More broadly, the training of highly skilled personnel and the publication of high impact science will contribute to the UK's position as a leading country for R & D, and will help sustain the 'Knowledge Based BioEconomy'. The BBSRC has noted the need for "succession planning" in vulnerable areas where few people are active, and this project falls in one of those areas. The proposed training platform will therefore provide a much-needed skills base for future research in both academia and industry.

In the long-term, the outputs of this project should have commercial applications. If mechanisms that prevent the bundle sheath cells in C3 plants from becoming photosynthetic are understood, it will be possible to engineer crops in which those mechanisms are repressed. The consequent induction of photosynthesis in the bundle sheath cells could increase photosynthetic efficiency in the canopy by 15%, and would also pave the way for a 50% increase through engineering of the C4 pathway into C3 plants. To provide an economic context, in 2012 the global rice and wheat crops were estimated by FAOStat to be worth 333 and 187 Billion USD respectively, and thus a modest 15% increase in yield could add around 78 Billion USD a year to the world economy. Such an increase would have global impact in both developed and developing economies by improving on-farm yields, income, supply chain stability, and food security. The beneficiaries would thus range from small farmers to multinational biotechnology companies, and from individual consumers to corporate food and energy suppliers. Furthermore, given that increased photosynthetic efficiency would be a truly transferable trait, equally applicable to crops from cereals to trees, the potential to enhance future sustainability is enormous.