title: Sucrose phloem unloading follows an apoplastic pathway with high sucrose synthase in Actinidia fruit.
abstract: Phloem unloading plays a pivotal role in photoassimilate partitioning and the accumulation of sugars in sink organs, e.g. fruit. Here, we investigated the pathway of sucrose unloading in kiwifruit (Actinidia deliciasa cv. Qinmei) using a combination of electron microscopy, transport of the phloem-mobile symplastic tracer carboxyfluorescein and enzyme activity and gene expression assays. Our structural investigation revealed that the sieve element-companion cell complex of bundles feeding the fruit flesh was symplastically isolated from its surrounding parenchyma cells throughout fruit development, whereas numerous plasmodesmata were present between the phloem parenchyma cells. Consistent with this, carboxyfluorescein unloading showed that the dye remained confined in the phloem strands during fruit development. The activities and expression of cell wall acid invertase in fruit flesh were lower than those of other enzymes that catalyze sucrose dissociation. However, sucrose synthase showed higher enzyme activities and mRNA expression in fruit flesh compared with other detected enzymes. These results imply that, in kiwifruit flesh, phloem unloading of sucrose is predominantly an apoplastic pathway during fruit development, and that sucrose synthase is a key enzyme for sucrose post-unloading pathways.
authors.affiliation: State Key Laboratory of Crop Stress Biology in Arid Areas/College of Horticulture, Northwest A&F University, Yangling 712100, Shaanxi, China.
authors.forename: Cheng
authors.initials: C
authors.lastname: Chen
authors.affiliation: State Key Laboratory of Crop Stress Biology in Arid Areas/College of Horticulture, Northwest A&F University, Yangling 712100, Shaanxi, China.
authors.forename: Yulin
authors.initials: Y
authors.lastname: Yuan
authors.affiliation: State Key Laboratory of Crop Stress Biology in Arid Areas/College of Horticulture, Northwest A&F University, Yangling 712100, Shaanxi, China.
authors.forename: Chen
authors.initials: C
authors.lastname: Zhang
authors.affiliation: State Key Laboratory of Crop Stress Biology in Arid Areas/College of Horticulture, Northwest A&F University, Yangling 712100, Shaanxi, China.
authors.forename: Huixia
authors.initials: H
authors.lastname: Li
authors.affiliation: State Key Laboratory of Crop Stress Biology in Arid Areas/College of Horticulture, Northwest A&F University, Yangling 712100, Shaanxi, China.
authors.forename: Fengwang
authors.initials: F
authors.lastname: Ma
authors.affiliation: State Key Laboratory of Crop Stress Biology in Arid Areas/College of Horticulture, Northwest A&F University, Yangling 712100, Shaanxi, China. Electronic address: limingjun@nwsuaf.edu.cn.
authors.forename: Mingjun
authors.initials: M
authors.lastname: Li
mesh_terms: D029042:Actinidia / Q000378:metabolism*; D005452:Fluoresceins / Q000378:metabolism; D005638:Fruit / Q000378:metabolism*; D005964:Glucosyltransferases / Q000235:genetics / Q000378:metabolism*; D052585:Phloem / Q000378:metabolism*; D010940:Plant Proteins / Q000378:metabolism*; D031425:Plasmodesmata; D012333:RNA, Messenger / Q000378:metabolism; D013395:Sucrose / Q000378:metabolism*; D043324:beta-Fructofuranosidase / Q000378:metabolism
chemical_list: D005452:Fluoresceins; D010940:Plant Proteins; D012333:RNA, Messenger; C024098:6-carboxyfluorescein; D013395:Sucrose; D005964:Glucosyltransferases; C022699:sucrose synthase; D043324:beta-Fructofuranosidase
keywords: Cell wall invertase; Kiwifruit; Sucrose; Sucrose synthase; Unloading