What happens during photorespiration.
2 and CO 2 on the Rubisco enzyme of the Calvin-Benson cycle
To study the evolution of the expression of photorespiratory and C4 cycle genes during the transition from C3 to C4 photosynthesis in the genus Flaveria, nine species reflecting the evolutionary trajectory taken were selected, including two C3 (F. robusta and F. pringlei), two C4 (F. bidentis and F. trinervia), and five C3–C4 intermediate species (). According to their CO2 compensation points and the percentage of carbon initially fixed into malate and aspartate, F. chloraefolia and F. pubescens were earlier classified as type I C3–C4 intermediates. F. anomala and F. ramosissima belong to the type II C3–C4 intermediates and F. brownii is classified as a C4-like species (; ; ; ). Type I C3–C4 intermediates are defined as solely relying on the photorespiratory CO2 concentration cycle whereas a basal C4 cycle activity is present in type II C3–C4 intermediates species. C4-like species exhibit much higher C4 cycle activities but lack complete bundle sheath compartmentation of Rubisco activity ().
What is the role of rubisco in the Calvin Cycle
In hot and dry environments and under low atmospheric CO2 conditions, when the oxygenation activity of Rubisco is increased, the high rate of photorespiration becomes unfavorable for the plants (, ). C4 plants possess a mechanism that minimizes the oxygenase function of Rubisco and thereby reduces photorespiration and decreases the loss of carbon. C4 photosynthesis is based on a division of labor between two different cell types, mesophyll and bundle sheath cells, which are organized in a wreath-like structure called ‘Kranz Anatomy’ (; ). Atmospheric CO2 is initially fixed in the mesophyll by phosphoenolpyruvate carboxylase (PEPC), and the resulting four-carbon compound is transported to the bundle sheath cells and decarboxylated by NADP/NAD malic enzyme or phosphoenolpyruvate carboxykinase (). Thereby CO2 is concentrated at the site of the Rubisco in the bundle sheath cells (), outcompeting the molecular oxygen. As a consequence, photorespiration is drastically reduced as compared to C3 plants, and C4 plants are characterized by a high photosynthetic efficiency ().