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.