16-ERACAPS. Designing C4 breeding strategies using genetic enablers of C4 evolution

C4 photosynthesis is used by the most productive crops and native vegetation on the planet. However, the molecular mechanisms and the genetic architecture underlying this complex trait are poorly understood and this impedes efforts to introduce this desirable trait into C3 crops. Recently, we and others have made significant progress both in understanding the mechanisms that prime species to evolve C4 photosynthesis, and also the evolutionary trajectories that then lead to the full C4 trait. This proposal builds on these recent findings, but combines our expertise in C4 photosynthesis with a synthetic experimental evolution approach that aims to experimentally recapitulate the initial steps towards C4 evolution. Using this approach, we will translate the mechanistic understanding of the molecular evolution of a complex trait into an innovative breeding approach that is informed by evolutionary biology and enabled by synthetic biology and genome editing. To achieve this, we have assembled a new team made up of international authorities on: wide-crossing to breed complex traits (Stich), DNA editing technologies and synthetic biology (Voytas), and combined this with the expertise on C4 photosynthesis (Hibberd and Weber). Our specific aims are to test the hypothesis that establishment of a rudimentary photorespiratory carbon pump reduces the carbon dioxide compensation point and induces a primordial C4-like carbon cycle; to leverage the power of inter-species crosses between C3 and C3-C4 intermediate Brassicaceae species to identify key anatomical and biochemical enablers of C4 evolution by quantitative genetics; and lastly, to employ genome editing and wide intra-species crosses to introduce C4 enablers into C3 Brassicaceaen models and crops, such as oilseed rape.

C4 photosynthesis is used by the most productive crops and native vegetation on the planet. However, the molecular mechanisms and the genetic architecture underlying this complex trait are poorly understood and this impedes efforts to introduce this desirable trait into C3 crops. Recently, we and others have made significant progress both in understanding the mechanisms that prime species to evolve C4 photosynthesis, and also the evolutionary trajectories that then lead to the full C4 trait. This proposal builds on these recent findings, but combines our expertise in C4 photosynthesis with a synthetic experimental evolution approach that aims to experimentally recapitulate the initial steps towards C4 evolution. Using this approach, we will translate the mechanistic understanding of the molecular evolution of a complex trait into an innovative breeding approach that is informed by evolutionary biology and enabled by synthetic biology and genome editing. To achieve this, we have assembled a new team made up of international authorities on: wide-crossing to breed complex traits (Stich), DNA editing technologies and synthetic biology (Voytas), and combined this with the expertise on C4 photosynthesis (Hibberd and Weber). Our specific aims are to test the hypothesis that establishment of a rudimentary photorespiratory carbon pump reduces the carbon dioxide compensation point and induces a primordial C4-like carbon cycle; to leverage the power of inter-species crosses between C3 and C3-C4 intermediate Brassicaceae species to identify key anatomical and biochemical enablers of C4 evolution by quantitative genetics; and lastly, to employ genome editing and wide intra-species crosses to introduce C4 enablers into C3 Brassicaceaen models and crops, such as oilseed rape.

The economic and environmental impacts of placing characteristics of C4 photosynthesis into C3 crops are huge, given a potential increase of photosynthetic carbon conversion efficiency by approx. 30%, while requiring lower inputs, such as nitrogen fertilizer and water. To our knowledge, C4BREED is the first project aiming at developing a strategy for implementing C4-like photosynthesis in a dicotyledonous crop species, and the first taking an introgression/next-generation breeding approach, as compared to large-scale transgene insertion strategies in complementary projects.
The approach taken here is innovative and clearly distinct from other C4 engineering efforts as we aim at implementing a C4 breeding strategy that is guided by models explaining the path of C4 evolution via C3-C4 intermediate stages. That is, we do not aim to engineer a C3 species into a C4 plant but rather to implement the first requisite for further evolution of C3-C4 intermediacy. Once this has been achieved, further optimization towards C4 photosynthesis could likely be achieved by strong selection under appropriate selective conditions, as indicated by predictive models of the C4 evolutionary trajectory. Albeit we will be using genome engineering approaches and transgenic plants to test hypotheses from predictive computational models and quantitative genetics, the knowledge generated will potentially allow for advanced breeding approaches towards C4 through interspecies introgression of C4 enablers, as identified in the proposed work. Thus, the strategies for introduction of the C4-trait shadowing a naturally occurring evolutionary trajectory is more likely to be met with societal approval than conventional transgenic approaches since they follow classical breeding and domestication principles.
Given the phylogenetic proximity of oilseed rape to C3-C4 intermediate sister species in the Brassicaceae, the previously demonstrated capacity for inter-species crosses between Brassica and its C4-intermediate sister species, and the power of gene editing and gene replacement enabled through the RNA-guided Cas9-technology, it is highly likely that the knowledge generated within this consortium can be directly applied to introducing C4-enabling traits into Brassica germplasm.