Significance of C4 Leaf Structure at the Tissue and Cellular Levels

Abstract

The CO2 concentrating mechanism (CCM) in C4 plants requires a complex coordination of both leaf anatomical and biochemical traits. While there are key traits common across the 60 plus C4 lineages, there is also significant structural and biochemical variation. Traditionally, C4 plants are described as one of three biochemical subtypes based on the primary enzyme used for C4 acid decarboxylation: NADP-malic enzyme (NADP-ME), NAD-malic enzyme (NAD-ME), and phosphoenolpyruvate carboxykinase (PCK). However, there may be biochemical flexibility and overlap between these subtypes. C4 plants typically rely on Kranz-type anatomy that partitions the C4 cycle into the mesophyll (M) cells and the majority of C3 cycle into the bundle-sheath (BS) cells. However, within the succulent Chenopods some NAD-ME type C4 plants use one of two single-cell arrangements to partition and compartmentalize the C4 and C3 cycles. Here we discuss key leaf anatomical traits at the tissue, cellular, and sub-cellular level that influence the efficiency and effectiveness of C4 photosynthesis. Specifically, we discuss preconditioning of leaf traits that increase the evolvability of C4 photosynthesis, the evolutionary transition of organelles from C3 to a C4 leaf, gas and metabolite movement within the leaf, the positioning and maintenance of organelles in M and BS cells, and the movement of M chloroplasts.