Isolation and Preliminary Functional Analysis of MxCS2: a Gene Encoding a Citrate Synthase in Malus xiaojinensis

Iron (Fe) is one of the essential micronutrients required by all plants. Citric acid (CA) is considered as the chelate substance in the long-distance transport of Fe. In the present study, a gene encoding putative citrate synthase was isolated from Malus xiaojinensis and designated as MxCS2. The MxCS2 gene encodes a protein of 296 amino acid residues with a predicted molecular mass of 33.2 kDa and a theoretical isoelectric point of 7.79. Subcellular localization has revealed that MxCS2 is preferentially localized in the mitochondrion and cytoplasmic membrane. The expression of MxCS2 was enriched in leaf, root, and phloem, which was highly affected by Fe stress and indoleacetic acid treatment in M. xiaojinensis seedlings. When MxCS2 was introduced into Arabidopsis, it promoted the synthesis of citrate synthase and increased CA content. Overexpression of MxCS2 improved the tolerance to Fe stress in transgenic Arabidopsis, but also led to increased fresh weight, root length, CS activity, and contents of chlorophyll, citrate acid, and Fe, especially when dealing with Fe stress.