Abstract

The malate dehydrogenase (MDH) family contains multiple conserved isoforms with different subcellular localisations. MDHs catalyse the reversible interconversion of oxaloacetate (OAA) to malate using NAD H as cofactor. pdNAD-MDH is one of the two isoforms located within the chloroplast. It was previously assumed to play a role in maintaining redox homeostasis, particularly in the dark and in non-green plastids, but its exact function was unknown. I will demonstrate in this talk that pdNAD-MDH plays an important role in early chloroplast development and is therefore essential for proper embryo development. The pdnad-mdh knock-out mutant of Arabidopsis is embryo-lethal – a phenotype that could not be complemented by targeting other isoforms of the NAD -MDH family into the chloroplast. This demonstrated that restoring MDH activity alone was not sufficient to support normal embryo development. However, expressing mutated versions of pdNAD-MDH with no catalytic activity complemented the embryo-lethal phenotype of pdnad-mdh, and the complemented plants grew normally. This suggests that the pdNAD-MDH protein, but not its enzymatic activity, is required for normal embryo and chloroplast development. I propose a vital moonlighting function for the pdNAD-MDH protein in stabilising a large protein complex involved in protein import at the chloroplast envelope membrane.