The Differences between NAD-ME and NADP-ME Subtypes of C(4) Photosynthesis: More than Decarboxylating Enzymes

As an adaptation to changing climatic conditions that caused high rates of photorespiration, C(4) plants have evolved to display higher photosynthetic efficiency than C(3) plants under elevated temperature, high light intensities, and drought. The C(4) plants independently evolved more than 60 times in 19 families of angiosperms to establish similar but not uniform C(4) mechanisms to concentrate CO(2) around the carboxylating enzyme Rubisco (ribulose bisphosphate carboxylase oxygenase). C(4) 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 C(4) variation between biochemical subtypes and diverse lineages. This review addresses the differences in evolutionary scenario, leaf anatomy, and especially C(4) metabolic flow, C(4) 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 C(4) photosynthetic mechanisms in NAD-ME and NADP-ME subtypes.