Carbon concentrating mechanisms: in rescue of Rubisco inefficiency

Agricultural yields have kept pace with the rising demands in the recent past as a result of the breeding and improved farming practices, but these practices alone will not be able to meet the demands of the future. The focus is now on the enhancement of the photosynthetic machinery. In photosynthesis, the rate limiting step is the one catalyzed by RuBisCO- Ribulose-1,5-bisphosphate carboxylase/oxygenase (4.1.1.39), which, because of its loose specificity and low turnover rate, is the primary target of most research programs directed towards improved photosynthesis. The other avenues of photosynthetic machinery that are under investigation to enhance it include—improved stomatal regulation and membrane permeability, RuBisCO with high specificity for CO₂ and higher catalytic turnover; bypass of photorespiration and introduction of carbon concentrating mechanism (CCM) into the C₃ plants. Carbon concentrating mechanisms cause accumulation of carbon dioxide in vicinity of RuBisCO producing a high CO₂/O₂ ratio and hence an environment more suitable for carboxylation reactions than oxygenation reactions. This article includes the basic details of the major naturally occurring CCMs in various photosynthetic organisms to identify the knowledge gaps in each which could help study the prospects of its possible introduction into a non-native system as C₃ plants which are devoid of any CCM.