Function of a citrate synthase gene (MaGCS) during postharvest banana fruit ripening

• A full-length citrate synthase gene MaGCS was isolated from a banana fruit cDNA library.
• MaGCS expression was greatly induced by exogenous ethylene and suppressed by 1-MCP.
• Oxaloacetic acid (OA) stimulated the expression of MaGCS, MaACO1, MaACS1 and accelerated fruit ripening.
• Citric acid (CA) inhibited the expression of MaGCS, MaACO1, MaACS1 and delayed fruit ripening.

A full-length citrate synthase gene designated MaGCS was isolated from a banana fruit cDNA library, which contained a 1885 bp fragment carrying a 1542 bp open reading frame (ORF) that encodes a 513 amino acid protein. MaGCS includes the glyoxysomal targeting signal PST2, an oxaloacetic acid binding site and a citrate synthase active site. Southern blotting showed that the banana genome has two copies of MaGCS. MaGCS is constitutively expressed in all organs with high levels in the fruit and the lowest levels in the leaves. With natural ripening, the MaGCS expression pattern was similar to that of ethylene production and respiration rate. MaGCS expression could be induced by ethylene and inhibited by the ethylene receptor inhibitor 1-methylcyclopropene (1-MCP), which is in accordance with ripening-related ethylene synthesis and respiration rate, and demonstrates that MaGCS is associated with ethylene biosynthesis. Oxaloacetic acid (OA) accelerated fruit ripening, and in contrast, citric acid (CA) delayed it. The expression of MaGCS, the 1-aminocyclopropane-1-carboxylic acid (ACC) oxidase gene 1 (MaACO1, AJ223232) and the ACC synthase gene 1 (MaACS1, AB021906) was evaluated by real-time quantitative PCR (QPCR), which showed that the expression of MaGCS, MaACO1, and MaACS1 were greatly improved by OA and suppressed by CA. These results indicate that MaGCS plays an important role in postharvest banana fruit ripening, and that OA and CA could affect fruit ripening. This effect on climacteric ethylene and ripening progress was due to the regulation of MaGCS, MaACO1, and MaACS1 expression.