The Differences between NAD-ME and NADP-ME Subtypes of C₄ Photosynthesis: More than Decarboxylating Enzymes

As an adaptation to changing climatic conditions that caused high rates of photorespiration, C₄ plants have evolved to display higher photosynthetic efficiency than C₃ plants under elevated temperature, high light intensities, and drought. The C₄ plants independently evolved more than 60 times in 19 families of angiosperms to establish similar but not uniform C₄ mechanisms to concentrate CO₂ around the carboxylating enzyme Rubisco (ribulose bisphosphate carboxylase oxygenase). C₄ photosynthesis is divided into at least two basic biochemical subtypes based on the primary decarboxylating enzymes, NAD-dependent malic enzyme (NAD-ME) and NADP-dependent malic enzyme (NADP-ME). The multiple polygenetic origins of these subtypes raise questions about the association of C₄ variation between biochemical subtypes and diverse lineages. This review addresses the differences in evolutionary scenario, leaf anatomy, and especially C₄ metabolic flow, C₄ transporters, and cell-specific function deduced from recently reported cell-specific transcriptomic, proteomic, and metabolic analyses of NAD-ME and NADP-ME subtypes. Current omic analysis has revealed the extent to which component abundances differ between the two biochemical subtypes, leading to a better understanding of C₄ photosynthetic mechanisms in NAD-ME and NADP-ME subtypes.