Elucidating the sugar import pathway into developing kiwifruit berries (Actinidia deliciosa)

Abstract

The aim of this work was to investigate the roles of the apoplasmic and symplasmic pathways in the delivery of solutes to the outer pericarp of developing green-fleshed kiwifruit berries (Actinidia deliciosa [A. Chev.] C. F. Liang et A. R. Ferguson var. deliciosa 'Hayward'). Experiments with the symplasmic tracer 5(6)-carboxyfluorescein (CF) showed symplasmic movement of dye during the initial rapid growth stage of the fruit, but as fruit growth slowed and starch content increased, a reduction in symplasmic spread of CF was observed ('126 DAA [days after anthesis]). Measurements of sugar uptake rates from the apoplasm were sufficient to account for the dry matter accumulation in growing fruit at each stage of fruit growth, and could potentially operate alongside a symplasmic pathway for post-phloem unloading of sugars during the rapid growth period. Energy requirements driving uptake from the apoplasm appear to depend on the sugar concentration gradients across the cell membrane throughout fruit development. Together, these data suggest: 1) the post-phloem symplasmic pathway is important during the fruit rapid growth stage, 2) the uptake of sugars from the apoplasm has a role in the transport of carbohydrates in the fruit, especially later in fruit development when the symplasmic pathway is reduced, and 3) apoplasmic sugar has an important role in the flow of water into the apoplasm of the fruit, providing a mechanism to maintain hydrostatic pressure gradients along the phloem supplying the fruit.

Modelling grape growth in relation to whole-plant carbon and water fluxes

“…At the same time, the main water transport pathway changes from xylem to phloem (Lang and Thorpe, 1989; Greenspan et al , 1996; Keller et al , 2015), and sugar transport shifts from the symplastic to apoplastic pathway (Zhang et al , 2006). Similar changes are observed in other fleshy fruits (Morandi et al , 2010; Clearwater et al , 2012; Gould et al , 2013; Brüggenwirth et al , 2016).…”

Metabolic analysis of kiwifruit (Actinidia deliciosa) berries from extreme genotypes reveals hallmarks for fruit starch metabolism

“…2A , B , G , H ), were the dominant Suc-cleaving enzymes. The presence of symplasmic connectivity between phloem cells and vascular parenchyma cells ( Gould et al , 2013 ) and the lack of both cell wall invertase activity ( Moscatello et al , 2011 ) and the gene for a fruit-specific cell wall invertase (this study) in kiwifruit berries support this hypothesis. Tomato phloem unloading is still debated, and evidence supporting two contrasting theories has been produced using different systems.…”

Fruit Salad in the Lab: Comparing Botanical Species to Help Deciphering Fruit Primary Metabolism

“…For kiwifruit, Chen et al (2017) showed that sucrose phloem unloading occurs mainly through the apoplasmic route along fruit development (44–135 days after blooming). However, Gould et al (2013), working from 22 to 200 days after anthesis, found that phloem unloading dominantly appeared via symplasmic route in early fruit development, while an apoplasmic route becomes important during the later developmental stages. The dominant symplastic import of sugar at the initial stages of fruit development allows a high inflow of carbon input via mass flow.…”