MaMYB4 is involved in the accumulation of naringenin chalcone, phloretin and dihydrokaempferol in the peels of banana fruit under chilling injury

Bananas are good sources of nutrients, magnesium and vitamins, and they are also rich in many antioxidant compounds, such as flavonoids. Flavonoids are secondary metabolites with antioxidant properties that play an effective role in detoxification of reactive oxygen species (ROS). Stresses, such as low temperature stress, lead to excessive accumulation of ROS and stimulate the increase of activity of enzymes and genes related to flavonoid biosynthesis and thereby the flavonoid contents in a variety of plants. However, the mechanism underlying the transcriptional regulation of flavonoids production in banana fruit under chilling injury (CI) remains largely unknown. In this study, we found that both the contents of phloretin, naringenin chalcone and dihydrokaempferol (a member of the flavanonols) and the expression levels of chalcone and flavanonol biosynthesis genes MaCHSs and MaF3Hb significantly increased in banana peels during CI. It is vital that these genes expression levels and flavonoid contants contain remarkabe positive correlations. Additionally, a key myeloblastosis (MYB) transcriptional inhibitor MaMYB4, a homologue of AtMYB4 involved in flavonoids biosynthesis, was negatively correlated to the accumulated phloretin, naringenin chalcone, and dihydrokaempferol. MaMYB4 could bind to AC-rich elements in the promoters of MaCHSs and MaF3Hb to repress their transcriptional activities. However, the expression level of MaMYB4 is down-regulated in banana peels during CI. As a result, MaCHSs and MaF3Hb were released and activated, leading to the production of naringenin chalcone and dihydrokaempferol in banana peels. Taken together, our findings unveil an interesting transcriptional module in relation to flavonoid accumulation in peels of banana under chilling stress.