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Article|01 Dec 2018|OPEN
Increased activity of MdFRK2, a high-affinity fructokinase, leads to upregulation of sorbitol metabolism and downregulation of sucrose metabolism in apple leaves
Jingjing Yang1, Lingcheng Zhu1, Weifang Cui1, Chen Zhang1, Dongxia Li1, Baiquan Ma1, Lailiang Cheng2, Yong-Ling Ruan3, Fengwang Ma1 & Mingjun Li1,
1State Key Laboratory of Crop Stress Biology for Arid Areas/Shaanxi Key Laboratory of Apple, College of Horticulture, Northwest A&F University, Yangling, Shaanxi 712100, China
2Section of Horticulture, School of Integrative Plant Science, Cornell University, Ithaca, NY 14853, USA
3School of Environmental and Life Sciences, The University of Newcastle, Callaghan, NSW 2308, Australia

Horticulture Research 5,
Article number: 71 (2018)
doi: 10.1038/hortres.2018.71
Views: 633

Received: 06 Jun 2018
Revised: 29 Aug 2018
Accepted: 14 Sep 2018
Published online: 01 Dec 2018

Abstract

To investigate the functions of fructokinase (FRK) in apple (Malus domestica) carbohydrate metabolism, we cloned the coding sequences of MdFRK1 and MdFRK2 from the ‘Royal Gala’ apple. The results showed that MdFRK2 expression was extremely high in shoot tips and young fruit. Analyses of heterologously expressed proteins revealed that MdFRK2 had a higher affinity for fructose than did MdFRK1, with Km values of 0.1 and 0.62 mM for MdFRK2 and MdFRK1, respectively. The two proteins, however, exhibited similar Vmax values when their activities were significantly inhibited by high concentrations of fructose. MdFRK2 ectopic expression was associated with a general decrease in fructose concentration in transgenic lines. In leaves, increased FRK activity similarly resulted in reduced concentrations of glucose and sucrose but no alterations in sorbitol concentration. When compared with those in the untransformed control, genes involved in sorbitol synthesis (A6PR) and the degradation pathway (SDH1/2) were significantly upregulated in transgenic lines, whereas those involved in sucrose synthesis (SPS1) and other degradation processes (SUSY4, NINV1/2, and HxK2) were downregulated. The activity of enzymes participating in carbohydrate metabolism was proportional to the level of gene expression. However, the growth performance and photosynthetic efficiency did not differ between the transgenic and wild-type plants. These results provide new genetic evidence to support the view that FRK plays roles in regulating sugar and sorbitol metabolism in Rosaceae plants.