CmMYB44 might interact with CmAPS2-2 to regulate starch metabolism in oriental melon fruit.

Sugar content is one of the determining factors for melon fruit maturity. Studies have shown that starch gradually degrades during fruit ripening, resulting in sugar accumulation. But the specific relationship between starch metabolism and sucrose accumulation was still unknown. Here, the starch and sugar contents, the activities of key enzymes and the expression patterns of genes related to starch-sucrose metabolism were determined in the fruit of high sugar and starch variety 'HS' and low sugar and starch variety 'LW'. It was found that starch accumulated during fruit development process, and then degraded at 30 days after anthesis (DAA), which was synchronized with sucrose accumulation in 'HS' fruit, while starch and sucrose contents were always at a lower level during 'LW' fruit maturation. Furthermore, starch metabolism-related enzymes (Adenine dinucleotide phosphate -glucose pyrophosphorylase (AGPase), α-amylase (AMY), β-amylase (BMY)) and the key enzymes for sucrose accumulation (sucrose phosphate synthase (SPS) and sucrose synthase (SS)) were significantly increased at ripening stage of 'HS' fruit, and their activities were consistent with the expressions of CmAPS2-2, CmAMY2, CmBAM1, CmBAM9 and CmSPS1. However, the contents of starch and sucrose and the activities of AGPase and SPS in 'LW' fruit didn't change significantly. We discovered an R2R3-type MYB transcription factor, CmMYB44, screened from yeast one hybrid library, could directly bind to the promoter of CmAPS2-2 to inhibit its transcription. These results revealed that the targeted down-regulation of CmAPS2-2 by CmMYB44 might be involved in the starch accumulation process, which affect the flavor quality of oriental melon fruit.