Discovery of the C4 Photosynthetic Pathway in Developing Wheat Grains

Sugarcane, maize, wheat, rice and potatoes are the five major crops on the basis of global production. The first two of these are well-known C₄ plants. Despite early reports of the presence of malic acid and C₄ enzymes in wheat grains but not in leaves; the lack of Kranz anatomy in wheat grains ruled out the possibility of C₄ photosynthesis in wheat. Much later, the occurrence of C₄ photosynthesis without Kranz anatomy has been reported in other plants. Our study explored the possibility of C₄ photosynthesis in developing wheat grains in a transcriptome based RNA-sequencing and differential gene expression study conducted across 35 hexaploid wheat genotypes between the early- and late-grain filling stages. The NGS based deep sequencing approach led us to uncover the occurrence of a C₄ photosynthetic pathway and a complete set of C₄ specific genes being differentially regulated in the pericarp between the early- and late- grain filling stages. Based upon this result and the reports of C₄ photosynthesis without Kranz anatomy; we have gathered taxonomical and evolutionary evidence to propose a novel NAD-ME type C₄ photosynthetic pathway occurring in the cross- and tube-cell layers of developing wheat grains. These cross- and tube-cell layers parallel mesophyll and bundle sheath layers to accomplish NAD-ME type C₄ photosynthesis in wheat grains. Although wheat and rice are known to be C₃ plants with much research progressing to develop a strategy to incorporate a C₄ photosynthetic pathway to enhance crop productivity; here, we report the discovery of a complete C₄ photosynthetic pathway in the developing wheat grains which is absent in leaves.