Ancient atmospheric changes could favour C3-to-CAM transition: A simulation-based evidence

Abstract

Crassulacean Acid Metabolism (CAM) plants have very low water demand and can survive in arid environment. It is hypothesized to evolve from C3 metabolism about 20 to 30 million years ago (MYA) when the Earth faced declining atmospheric CO2 concentration ([CO2]a), increasing aridity and decreasing temperature. Understanding whether and how these atmospheric changes favour C3-to-CAM transition will help in bioengineering CAM into crop plants to tackle the threat of global climate change and ensure food security. Our simulations of plant cellular metabolism under the changing atmospheric conditions of ancient times observe C3-to-CAM transition, capture possible temporal alterations of active metabolic pathways and further confirm that both the reduced [CO2]a and increased water scarcity associated with higher aridity, can act as evolutionary agents, driving the C3-to-CAM transition. Although the predicted elevated [CO2]a of future reveals a reversion towards C3-like behaviour, drought always favours CAM regardless of [CO2]a and temperature levels. Moreover, a minimum oxygen concentration is required to sustain elevated nocturnal respiration necessary for CAM, reflected by the increased activities of enzymes involved in the TCA cycle and the mitochondrial electron transport chain.