Genome-wide Identification, Classification, Molecular Evolution and Expression Analysis of Malate Dehydrogenases in Apple

Malate dehydrogenase plays crucial roles in energy homeostasis, plant development and cold and salt tolerance, as it mediates the reversible conversion of malate to oxaloacetate. However, the evolutionary pattern of MDH genes in apple remains elusive. In this study, a total of 20 MDH genes were identified from the Golden Delicious apple draft genome. We revealed the physiological and biochemical properties, gene structure, and conserved motifs of MdMDH genes. Chromosomal localization and Ka/Ks ratio analysis of MdMDH genes revealed different selective pressures acted on duplicated MdMDH genes. Exploration of the phylogenetic relationships revealed six clades and similar frequencies between old and recent duplications, and significant differences in the evolutionary rates of the MDH gene family were observed. One MdMDH gene, MDP0000807458, which was highly expressed during apple fruit development and flower bud differentiation, was under positive selection. Thus, we speculated that MDP0000807458 is a likely candidate gene involved in regulation of flower bud differentiation and organic acid metabolism in apple fruits. This study provides a foundation for improved understanding of the molecular evolution of MdMDH genes and further facilitates the functional analysis of MDP0000807458 to unravel its exact role in flower bud differentiation and organic acid metabolism.