SnRK1 phosphorylation of SDH positively regulates sorbitol metabolism and promotes sugar accumulation in peach fruit

Abstract

Fruit quality depends largely on the type and amount of sugar accumulated in the fruit. In peach [Prunus persica (L.) Batsch], sorbitol is the main photosynthetic product and plays a crucial role in sugar metabolism. As a conserved energy sensor, SNF1-related kinase 1 (SnRK1) is involved in the regulation of carbon metabolism. In this study, SnRK1 was able to respond to induction by treatment with exogenous trehalose and sorbitol on 'Ruipan 17' peach fruit. After treatment with 100-mM trehalose for 3 h, the SnRK1 activity decreased by 18% and the activities of sorbitol dehydrogenase (SDH) and sucrose synthase (SS) also decreased significantly, but sucrose phosphate synthase (SPS) activity increased significantly; whereas sorbitol treatment under the same conditions resulted in a 12.6% increase in SnRK1 activity and the activities of SDH and SS synthase also increased significantly, compared with the control. The contents of glucose, fructose and sucrose in peach fruit increased significantly after 3 h of sorbitol treatment. In addition, the interactions between PpSnRK1α and enzymes PpSDH and PpSPS were confirmed by yeast two-hybrid method and the phosphorylation of PpSnRK1α and PpSDH was detected in vitro. Taken together, these results suggest that SnRK1 promotes sorbitol metabolism by activating SDH and it also regulates the activities of SS and SPS that enhance sucrose accumulation in peach fruit. SnRK1 protein kinase is involved in sugar metabolism and has the potential to be used for improving fruit quality.

Keywords: Prunus persica; SDH activity; SnRK1 protein kinase; fruit quality; sorbitol; sugar metabolism.

Figure 1.

Effect of trehalose and sorbitol treatments on SnRK1 activity of peach fruit. (A) The content of trehalose-6-phosphate in peach fruit after the treatment with 100-mM trehalose. An asterisk (^*) on top of the error bar designates a significant difference compared with the control at P < 0.05. (B) SnRK1 activity under the trehalose treatment. (C) SnRK1 activity under the sorbitol treatment. (D) SnRK1 activity under the trehalose and sorbitol co-treatment. Error bars represent the SD based on three independent biological replicates. The various small letters indicate significant differences compared with the control separately at each sampling time at P < 0.05 level.

Figure 2.

Effect of trehalose and sorbitol treatments on activities of SDH, SOX, SPS and SS in peach fruit. (A) SDH activity; (B) SOX activity; (C) SPS activity; (D) SS synthase activity; (E) SS decomposition activity. Error bars represent the SD based on three independent biological replicates. An asterisk (^*) on top of the error bar designates a significant difference compared with the control separately at each sampling time at P < 0.05.

Figure 3.

Effect of trehalose and sorbitol treatments on contents of sorbitol, fructose, glucose and sucrose in peach fruit. (A) The content of sorbitol; (B) the content of fructose; (C) the content of glucose; (D) the content of sucrose. Error bars represent the SD based on three independent biological replicates. An asterisk (^*) on top of the error bar designates a significant difference compared with the control separately at each sampling time at P < 0.05.

Figure 4.

Yeast two-hybrid experiments showing the interactions between SnRK1 and the SDH, SS and SPS proteins. Results show a representative experiment out of three independent biological replicates.

Figure 5.

Phosphorylation detection of PpSnRK1α and PpSDH in vitro. (A) The purified PpSnRK1α protein. MK: molecular weight marker; PpSnRK1α: PpSnRK1α protein. (B) The purified PpSDH protein. MK: molecular weight marker; PpSDH: PpSDH protein. (C) PpSnRK1α phosphorylates PpSDH in vitro. The asterisk indicates the autophosphorylation of the purified HIS-PpSnRK1α, while the triangle indicates the phosphorylation of the purified HIS-PpSDH by HIS-PpSnRK1α in the autoradiogram.

Figure 6.

Effect of PpSnRK1α phosphorylation on PpSDH activity. Error bars represent the SD. Different small letters indicate significant differences at P < 0.05 level.