With world population projected to exceed eight billion people by 2025, food security is already a pressing global problem. Part of the solution will come from efforts to increase the yields of current food crops, but the global need for food only continues to grow, and could eventually outpace farmers' ability to increase production via conventional means such as breeding, application of fertilizers and improved land management. Martin Jonikas, an assistant professor in Princeton's Department of Molecular Biology, is collaborating with Alistair McCormick from the University of Edinburgh, Luke Mackinder from the University of York and Howard Griffiths from Cambridge University on a novel approach to boost crop productivity.
The team aims to use synthetic biology to increase the efficiency of the chloroplast, the plant organelle that conducts photosynthesis. During photosynthesis, chloroplasts transmute light energy into chemical energy by converting carbon dioxide and water into sugar. The chloroplasts of all photosynthetic eukaryotes - from the single-celled green algae to the massive sycamore - use an enzyme called Rubisco to extract carbon atoms from carbon dioxide in the air and incorporate them into starches. In many crop plants, this process (known as carbon fixation) is slow and limits growth because Rubisco catalysis runs very slowly. But algae don't face this limitation because their chloroplasts contain a structure called a pyrenoid, which feeds densely packed Rubisco with concentrated CO2, making the enzyme run faster.
"The algal CO2 concentrating mechanism contributes significantly to global carbon fixation and has the potential to enhance crop productivity if successfully introduced into higher plant chloroplasts," Jonikas said. Thanks to a grant jointly awarded by the U.S. National Science Foundation and the United Kingdom's Biotechnology and Biological Sciences Research Council, Jonikas and colleagues now plan to do just that, by expressing the genes essential for a functional pyrenoid in Arabidopsis thaliana, a small flowering plant commonly used in studies of plant biology.
"We hypothesize that a minimal algal-type CO2 concentrating mechanism can be produced in a higher plant such as Arabidopsis by expressing as few as six already known algal proteins," Jonikas said.
The effort to engineer a CO2 concentrating mechanism into higher plants builds on discoveries the team has made over the past 10 years, which have transformed our understanding of pyrenoid protein composition, structure and biogenesis. The work will be guided by a computational model of pyrenoid function being developed in Jonikas' lab. The researchers plan to begin by investigating whether Rubisco clusters can be created in Arabidopsis. Next, it will be important to show that chloroplasts can be engineered to supply their new Rubisco clusters with concentrated CO2, and that this improves plant growth. Jonikas and colleagues expect that this work will set the stage for future efforts to add a pyrenoid to crop plants, and lead to new revelations about how the pyrenoid functions and how it is regulated in algae.