C4 leaf development and evolution

Abstract

C₄ photosynthesis is more efficient than C₃ photosynthesis for two reasons. First, C₄ plants have evolved efficient C₄ enzymes to suppress wasteful photorespiration and enhance CO₂ fixation. Second, C₄ leaves have Kranz anatomy in which the veins are surrounded by one layer of bundle sheath (BS) cells and one layer of mesophyll (M) cells. The BS and M cells are functionally well differentiated and also well coordinated for rapid assimilation of atmospheric CO₂ and transport of photo-assimilates between the two types of cells. Recent comparative transcriptomics of developing M and BS cells in young maize embryonic leaves revealed not only potential regulators of BS and M cell differentiation but also rapid early BS cell differentiation whereas slower, more prolonged M cell differentiation, contrary to the traditional view of a far simpler process of M cell development. Moreover, new upstream regulators of Kranz anatomy development have been identified and a number of gene co-expression modules for early vascular development have been inferred. Also, a candidate gene regulatory network associated with Kranz anatomy and vascular development has been constructed. Additionally, how whole genome duplication (WGD) may facilitate C₄ evolution has been studied and the reasons for why the same WGD event led to successful C₄ evolution in Gynandropsis gynandra but not in the sister species Tarenaya hassleriana have been proposed. Finally, new future research directions are suggested.

Keywords: C4 photosynthesis; Genome duplication; Kranz anatomy; Leaf development.