Up-regulated glycolysis, TCA, fermentation and energy metabolism promoted the sugar receding in ‘Shixia’ longan (Dimocarpus longan Lour.) pulp

Sugar receding, a drastic decrease of total soluble solid (TSS) in pulp, leads to decreased sweetness and deterioration of longan fruits during on-tree preservation. Although soluble acid invertases were proved as key factors regulating sugar receding, systematic analysis of the other pathways regulating this process was still lacked. Significant decreased TSS (by 40.61%) and sucrose (by 66.71%) but slight decreased fructose and glucose during 95–127 days after flowering (DAF) indicated a rapid conversion of sucrose to hexose and consumption of hexose in ‘Shixia’ longan pulp. TMT-labeled proteomic analysis of longan pulp at 95, 110 and 127 DAF identified 575 proteins differently expressed (≧1.2-fold or ≦0.833-fold) in at least two stages (BDEPs). GO annotation of the BDEPs showed that the enriched biological processes terms were: response to inorganic substance, cadmium ion, stimulus and chemical, catabolic processes of carbohydrate, glucose, monosaccharide and hexose, and monosaccharide metabolic process. Mapman visualization of the BDEPs indicated that down-regulation of the key proteins involved in starch synthesis & degradation (ie. Starch phosphorylase L & isoamylase), sucrose synthesis (ie. sucrose-phosphate synthase & sucrose-6-phosphatase) and cytoplasmic sucrose degradation (ie. neutral invertase, sucrose synthase) but up-regulation of the enzymes for vacuolar sucrose degradation (soluble acid invertase) and most of the proteins involved in the cytosolic and plastidial glycolysis (Calvin cycle), fermentation, TCA, and energy metabolism promoted the consumption of hexose. Taken together, the up-regulated glycolysis, fermentation, TCA and energy metabolism might promote sucrose decomposition and thus sugar receding of longan pulp.