The close association between mitochondria and chloroplasts also facilitates diffusion of photorespired CO 2 into the chloroplasts, and thus concentrates CO 2 surrounding Rubisco (Sage et al., 2014). Therefore, a major feature of C 2 anatomy is increased abundance of mitochondria with high GDC activity, and aggregation of mitochondria and chloroplasts in the inner side of bundle sheath cells to minimize gas exchange with mesophyll cells (Fig. Glycine generated by oxygenation reaction of Rubisco in mesophyll cells is transported to the bundle sheath cells and then converted to serine and CO 2 by the mitochondrial glycine decarboxylase complex (GDC) (Fig. (2019) compared various anatomical, ultrastructural and biochemical traits in nine species of the subtribe Neurachninae of the grass family, and identified new C 3-C 4 intermediate species.Ĭ 2 photosynthesis differs from C 4 in biochemical reactions in that C 2 plants concentrate CO 2 by limiting the photorespiratory reactions in the mitochondria of bundle sheath cells. In this issue of Plant Physiology, Khoshravesh et al. It is worth noting that although grasses and sedges dominate C 4 species, only less than 20% of known intermediate species are monocots, hindering our understanding of C 4 evolution within monocots (Sage et al., 2014). Indeed, C 2 photosynthesis that utilizes glycine (two-carbon compound, C 2) as a shuttle to concentrate CO 2 is proposed as an intermediate state during C 4 evolution. If the evolution of these traits is stepwise or progressive, we may expect to observe species with intermediate traits between C 3 and C 4 during evolutionary history. One or both types of sheath cells may be co-opted for C 4 function, although one of them is often lost during C 4 evolution (Lundgren et al., 2014).Ī few anatomical and cellular adaptations are required for efficient C 4 physiology: (a) compartmentation of CO 2 assimilation and fixation by increasing PEPC activity in the mesophyll cells and limiting Rubisco only to bundle sheath chloroplasts, (b) increased size and number of chloroplasts in the bundle sheath, (c) short distance between mesophyll and bundle sheath cells by increasing vein density and/or enlarging bundle sheath and mesophyll cell size, and (d) enrichment of plasmodesmata between mesophyll and bundle sheath cells to facilitate metabolic exchange (Fig. In grasses and sedges, there are two layers of sheath cells, an inner bundle sheath (also called mestome sheath) and an outer bundle sheath.

The mesophyll and bundle sheath are arranged in a “wreath-like” shape surrounding the vascular bundle, a structure referred to as the Kranz anatomy. Then, the C 4 compound is transported to the chloroplasts of the bundle sheath cells where it releases CO 2, resulting in increased CO 2 concentration and enhanced carboxylase activity of ribulose-1,5-bisphosphate carboxylase/oxygenase (Rubisco), the rate limiting enzyme of photosynthesis. In C 4 photosynthesis, CO 2 is initially taken up by mesophyll cells and converted into a four-carbon (C 4) compound by phosphoenolpyruvate carboxylase (PEPC), which has high affinity for CO 2. In spite of their multiple origins, C 4 plants are only present in certain angiosperm families, including about 5000 grasses, 1300 sedges, and 1800 dicot species, indicating developmental and anatomical constraints for C 4 evolution (Sage, 2016). The transition from C 3 to C 4 photosynthesis has occurred more than 62 times independently in a complex process with a combination of anatomical, biochemical and physiological changes in leaves. Although accounting for only 3% of the vascular plants on earth, C 4 plants contribute 25% of terrestrial photosynthesis (Westhoff and Gowik, 2010), thus providing a highly desirable system to increase carbon fixation efficiency and enhance crop yield. C 4 photosynthesis originated as an adaptation to low atmospheric CO 2 concentrations 30-35 million years ago (Sage, 2016).

C3 or c4 middle c