Analysis of the fruit quality and the metabolic characteristics of sucrose in fruit of Actinidia eriantha at ripening stage

Abstract: 【Objective】Actinidia eriantha is a unique kiwifruit species of Actinidia spp. The fruit of A. eriantha is rich in nutrients and ascorbic acid (vitamin C). However, the flavor quality of most wild A. eriantha resources is lower than that of the cultivars from A. chinensis or A. deliciosa mainly due to the low accumulations of soluble sugars. Fortunately, a high sugar type kiwifruit cultivar, A. eriantha ‘Ganlü No. 1’, has been developed by our research group recently. In order to explore the formational mechanism of the high accumulation of soluble sugars, and to identify the related key metabolic enzyme and key gene in the fruit of A. eriantha‘Ganlü No. 1’at the ripening stage, A. eriantha‘Ganlü No. 1’and another low sugar type kiwifruit cultivar, A. eriantha‘Ganlü No. 6’, were selected as the research materials in this study. These two cultivars possess similar genetic background and they both originated from the wild A. eriantha resources of Magu Mountain in Nancheng County, Jiangxi Province.【Methods】The grafted vines of A. eriantha‘Ganlü No. 1’and A. eriantha‘Ganlü No. 6’were planted in an orchard of the Fengxin County Bureau of Agriculture and Rural Affairs, which was located in Fengxin County, Jiangxi Province. In 2021, at physiological maturity stage when the soluble solids content of fruits of each cultivar reached over 7.0%, fruits without pests, diseases and mechanical damage were collected randomly from selected vines. After collection, the fruits were placed in an ice box and transported to the laboratory for further treatment. For each cultivar, three uniform vines were used as three biological replicates. Twenty fruits were collected from each vine. For fresh fruits, the fruit weight was measured using an electronic balance; the fruit longitudinal diameter and fruit trans-verse diameter were measured respectively with a vernier caliper. The fruit shape index was obtained using the ratio of the longitudinal diameter to the transverse diameter. Then those fruits were stored at room temperature until soft- ripe stage. For those fresh fruits, soluble solids content (SSC) was measured with a portable refractometer, and dry matter content (DM) was measured using fruit slices. For the remaining fruits, after removing the peel, seeds and core, the flesh was separated, sliced, frozen with liquid nitrogen, and stored in a refrigerator at -80 ℃. For the frozen flesh samples, the soluble sugar content (SS) was determined using anthrone sulfuric acid colorimetry; the titratable acidity (TA) was determined by titration with NaOH; and the ascorbic acid (AsA) content was determined by the 2, 6-dichlorphenolindophenol titration method. The sugar- acid ratio was calculated by the ratio of the SS to the TA. The glucose (Glu), fructose (Fru), and sucrose (Suc) were also extracted from the frozen samples in 80% ethanol, and their contents were determined by high-performance liquid chromatography. The total soluble sugar content (TS) was obtained by calculating the sum of Glu, Fru, and Suc. The sweetness value (SV) was calculated based on the contents of Glu, Fru, Suc, as well as their corresponding coefficients. Besides, for the frozen samples, the activity assays of the main enzymes involved in Suc metabolism, including sucrose phosphate synthase (SPS), sucrose synthase (SUSY), acid invertase (AINV), and neutral invertase (NINV), were carried out using their corresponding kits. Furthermore, the relative expression levels of the related genes encoding those enzymes, including AeSPS, AeSUSY, AeAINV, and AeNINV, were analyzed by qRT-PCR method for A. eriantha‘Ganlü No. 1’and A. eriantha‘Ganlü No. 6’. In addition, the correlation relationships among different indicators were analyzed using bivariable analysis following the Pearson method. Those indicators included SS, SSC, SV, TS, Glu, Fru, Suc, the activities of SPS, SUSY, AINV, NINV, and the relative expression levels of AeSPS, AeSUSY, AeAINV, and AeNINV.【Results】The fruit size, fruit shape index, and fruit weight were significantly higher in A. eriantha‘Ganlü No. 1’than those in A. eriantha‘Ganlü No. 6’. Most of the indicators related to internal fruit quality were also significantly higher in A. eriantha‘Ganlü No. 1’than those in A. eriantha‘Ganlü No. 6’. Those indicators included SSC, SS, sugar-acid ratio, and AsA content. Besides, higher contents of Fru, Suc, TS, and higher SV were detected in the fruits of A. eriantha ‘Ganlü No. 1’at the ripening stage. Additionally, in the fruits of Ganlü No. 1, the activities of SPS and SUSY were both significantly higher than those of Ganlü No. 6; however, the activities of AINV and NINV were significantly lower in the fruits of Ganlü No. 1. Meanwhile, the relative expression levels of the AeSPS and AeNINV were significantly higher in the fruits of Ganlü No. 1 than those of Ganlü No. 6. Moreover, the soluble sugar content was positively correlated to SSC, SV, the contents of TS, Fru, Suc, and the activities of SPS and SUSY; while it was negatively correlated to the Glu content, and the activities of AINV and NINV. Furthermore, the relative expression level of the AeSPS was found to be positively correlated to both the Suc content and SPS activity.【Conclusion】The difference in sucrose accumulation level was the dominant factor that results in the great difference in soluble sugar content between the cultivars of A. eriantha‘Ganlü No. 1’and A. eriantha‘Ganlü No. 6’. And the sucrose phosphate synthase was the key metabolic enzyme that could positively regulate sucrose accumulation in the ripe fruit of A. eriantha culitvars at the ripening stage. Besides, we speculated that the AeSPS would be the key gene which could contribute to the biosynthesis of sucrose by increasing SPS activity in the fruits of A. eriantha‘Ganlü No. 1’at the ripening stage.