Banana MabZIP21 positively regulates MaBAM4, MaBAM7 and MaAMY3 expression to mediate starch degradation during postharvest ripening

Starch is the major constituent of green banana fruit and is a resource for banana quality as it is degraded to soluble sugars during postharvest ripening. However, the molecular basis of starch degradation in different banana cultivars during fruit ripening remains largely unclear. In this study, the starch degradation of four banana cultivars (Baxi, Dajiao, Fengjiao, Gongjiao) during ripening was investigated. The contents of total starch, amylase, amylopectin, the amylose/amylopectin ratio, and the crystallinity were all decreased in all the four banana cultivars tested. In addition, the morphological structures of starch granules changed as ripening progressed due to starch degradation. The activity and gene expression of starch hydrolytic enzymes were increased. Importantly, a transcriptional regulator MabZIP21 was identified, which functions as an activator of starch degradation by upregulating the transcription of MaBAM4, MaBAM7, and MaAMY3. In addition, transient overexpression of MabZIP21 in banana or overexpression of MabZIP21 in tomato increased starch corruption by elevating the accumulation of starch degrading genes. Through this study, we expect to show that distinct patterns of starch degradation exist in different banana cultivars, and that MabZIP21 mediates starch breakdown by transcriptional activation of MaBAM4, MaBAM7, and MaAMY3.