Tissue Succulence Engineering for CAM Biodesign

Crassulacean acid metabolism (CAM) is a photosynthetic adaptation present in 6-7% of vascular plant species that reduces water loss through stomata by shifting the diel rhythm of primary atmospheric CO₂ fixation from the day to the night, which results in improved water-use efficiency (WUE) and drought tolerance. Introducing the CAM photosynthetic machinery into C₃ plants (CAM biodesign) is expected to confer improved WUE in order to allow plants to withstand long episodes of drought or perhaps even assist in the expansion of crop production into semi-arid regions. Some degree of tissue succulence is typically correlated with the optimal performance of CAM.  Thus, for CAM biodesign, tissue succulence engineering in C₃ plants represents a key anatomical prerequisite for enhancing the efficient operation of engineered CAM in C₃ photosynthesis species. Tissue succulence engineering is expected to afford increased mesophyll cell size in order to increase malate storage capacity in the vacuoles of the larger cells and to reduce intercellular air space (IAS) to limit the diffusion of CO₂ out of the leaf upon its release during day. A method for the genetic engineering of tissue succulence was developed involving the overexpression of a modified basic helix-loop-helix (bHLHL) transcription factor from Vitis vinifera. The engineered tissue succulence in Arabidopsis results in reduced IAS and increased cell size, organ size, organ number, vegetative biomass, and seed production.