Redundancy is sometimes seen only by the uncritical: Does Arabidopsis need six malic enzyme isoforms?

Arabidopsis thaliana contains six genes encoding active malic enzymes (AtMEs). But, are all of them just involved in l-malate degradation? The presence of the six AtMEs can be easily attributed to genetic redundancy as none of the single knock-out mutants show a visible phenotype under normal conditions. However, the AtMEs display differential patterns of expression and well-distinct biochemical properties. In this regard, four AtMEs use NADP, three of which are cytosolic and one plastidic. While one cytosolic and the plastidic isoforms are constitutively expressed, the other cytosolic isoforms are exclusively found in roots, trichomes or pollen. The other two AtMEs use NAD, are localized in the mitochondria and are active as homo- and hetero-oligomers. Although AtMEs share a high degree of identity (up to 90%) they display different kinetic properties and metabolite regulation and some of the isoforms are active in the l-malate synthesis direction. Thus, the physiological context might also be controlling the functional specificity in planta, due to differences in metabolite concentrations in the compartments in which each AtME is expressed. As a whole, the divergent properties of the AtMEs allow us to propose that each ME fulfils an exclusive metabolic function in vivo. Moreover, due to the well-distinct properties of each AtME, the co-expression of some MEs in the same cellular compartment would not imply redundancy but represents specificity of function.