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Banana (Musa acuminata) is a common starch-conversion fruit. Starch is rapidly degraded and converted to soluble sugars during postharvest banana fruit ripening. The conversion efficiency directly influences fruit firmness, sweetness, and shelf life. Glucan, water dikinase (GWD) is the first rate-limiting enzyme of starch metabolism. Musa acuminata glucan water dikinase 1 (MaGWD1) plays an important role in banana starch metabolism and its expression is regulated by transcription factors (TFs). However, the regulation of MaGWD1 by MADS-box TFs remains unclear. In this study, the molecular mechanism underlying the cooperative regulation of MaGWD1-mediated starch degradation by MaMADS36 and MaMADS55 was investigated. The results indicated that MaMADS36 interacts with MaMADS55. Both MaMADS36 and MaMADS55 could directly bind to the same position on the CA/T(r)G box within the MaGWD1 promoter. MaMADS36 and MaMADS55 cooperatively regulate banana fruit starch degradation via transcriptional control of MaGWD1.These findings improve our understanding of how TFs regulate starch degradation; this information may be used for banana fruit quality improvement and to support the development of the banana industry.

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.

The banana (Musa acuminata L. AAA group), a typical climacteric fruit, undergoes a postharvest ripening process that plays a crucial role in commercial banana fruit quality and shelf-life. To gain a better understanding of the postharvest ripening of the banana at the molecular level, we conducted a cDNA microarray analysis on banana fruit 10 d after harvest, which was the time of ripening initiation by ethylene biosynthesis. A total of 16 cDNAs were found to be up-regulated and six down-regulated. These results were confirmed by using reverse transcriptase-polymerase chain reaction (RT-PCR). The cDNAs identified are involved in signal transduction, amino acid metabolism, lipid metabolism, proteolysis, citrate biosynthesis and metabolism, and the uptake and transport of potassium. The information generated in this study provides new clues to aid in understanding the regulation of ethylene biosynthesis initiation in postharvest banana ripening.