Sugar Uptake in the Aril of Litchi Fruit Depends on the Apoplasmic Post-Phloem Transport and the Activity of Proton Pumps and the Putative Transporter LcSUT4

Abstract

The post-phloem unloading pathway and the mechanism of sugar accumulation remain unclear in litchi fruit. A combination of electron microscopy, transport of phloem-mobile symplasmic tracer (carboxyfluorescein, CF) and biochemical and molecular assays was used to explore the post-phloem transport pathway and the mechanism of aril sugar accumulation in litchi. In the funicle, where the aril originates, abundant plasmodesmata were observed, and CF introduced from the peduncle diffused to the parenchyma cells. In addition, abundant starch and pentasaccharide were detected and the sugar concentration was positively correlated with activities of sucrose hydrolysis enzymes. These results clearly showed that the phloem unloading and post-phloem transport in the funicle were symplastic. On the other hand, imaging of CF showed that it remained confined to the parenchyma cells in funicle tissues connecting the aril. Infiltration of both an ATPase inhibitor [eosin B (EB)] and a sucrose transporter inhibitor [p-chloromercuribenzene sulfonate (PCMBS)] inhibited sugar accumulation in the aril. These results indicated an apoplasmic post-phloem sugar transport from the funicle to the aril. Although facilitated diffusion might help sucrose uptake from the cytosol to the vacuole in cultivars with high soluble invertase, membrane ATPases in the aril, especially tonoplast ATPase, are crucial for aril sugar accumulation. The expression of a putative aril vacuolar membrane sucrose transporter gene (LcSUT4) was highly correlated with the sugar accumulation in the aril of litchi. These data suggest that apoplasmic transport is critical for sugar accumulation in litchi aril and that LcSUT4 is involved in this step.

New Insights into Phloem Unloading and Expression of Sucrose Transporters in Vegetative Sinks of the Parasitic Plant Phelipanche ramosa L. (Pomel)

“…This was confirmed in the present study concerning the parasitic plant species P . ramosa parasitizing B. napus through the use of the symplastic tracer CF which was previously used for analysing the phloem unloading pathways in plants (Zhang et al, 2006; Bederska et al, 2012; Wang et al, 2015). CFDA experiments here provided evidence for the symplastic phloem continuity from host leaves to both tubercle and shoot of the parasitic plant, hence explaining how the latter can divert resources from the host plant, especially sucrose, to support parasite growth driven by high sugar accumulation in vegetative sinks (Aber et al, 1983; Delavault et al, 2002; Draie et al, 2011).…”

Transcriptomes of Arbuscular Mycorrhizal Fungi and Litchi Host Interaction after Tree Girdling

“…Soluble sugars were measured using HPLC analyses. Determinations were performed as described by Wang et al (2013) and Wang T.D. et al (2014) .…”

“…The precipitate was washed twice with 5 mL of water, and the water was also transferred to a 50 mL volumetric flask. The solution in the 50 mL volumetric flask was analyzed for starch content according to Wang T.D. et al (2014) .…”

A Tonoplast Sugar Transporter Underlies a Sugar Accumulation QTL in Watermelon

“…TST genes associated with vacuole transport have been characterized and cloned from Arabidopsis (Arabidopsis thaliana) (Wingenter et al, 2010;Cho et al, 2010), berry, apple, and litchi (Afoufa-Bastien et al, 2010;Li et al, 2012;Wang et al, 2015) fruit development. However, TSTs and the mechanisms regulating their expression have not been studied in detail in plant species that produce fruits consumed by humans.…”