How plastic are the C4 subtypes? Investigating the plasticity of C4 grasses through exposure to low light and low carbon dioxide

Watson-Lazowski A, Sagun J, Koller F, Papanicolaou A and Ghannoum O

Hawkesbury Institute for the Environment, Western Sydney University, Locked Bag 1797, Penrith, NSW 2753, Australia.

C4 photosynthesis is an essential process which accounts for ~25% of total plant productivity, generated from a relatively small subset of plant species (~3%). This increased productivity is due to the evolution of a carbon concentrating mechanism between the mesophyll and bundle sheath cells of C4 plants, which allows for an increased concentration of carbon dioxide (CO2) around Rubisco. Three subtypes of C4 photosynthesis exist, each named after the main decarboxylase they utilise to release CO2: NADP-ME, NAD-ME and PEPCK. A number of grass species which utilise C4 photosynthesis are essential to maintaining increasing food and energy demands, highlighting them as an important subset of species for investigation. Firstly, the genetic make-up of the C4 cycle within each subtype requires some elucidation, especially in regards to the NAD-ME subtype. Therefore, we provide an updated model for NAD-ME photosynthesis within grasses via analysis of transcript expression and gene evolution. We can then begin to understand the plasticity of each subtype, both within the C4 cycle and on a whole plant basis. Understanding this plasticity is key to identifying both beneficial traits and targets for improvement. Utilising multiple C4 grass species, spanning two independent origins and all three subtypes, we subjected plants to ambient, low CO2 (180 ppm) and low light (200 PAR) conditions. By comparing phenotype, physiology, biochemistry and transcript expression we are able to dissect the plasticity of C4 grasses, as well as identify candidate genes which may be utilised to improve C4 crop productivity.