Apoplastic sugar may be lost from grape berries and retrieved in pedicels

Abstract

In ripening grape (Vitis sp.) berries, the combination of rapid sugar import, apoplastic phloem unloading, and water discharge via the xylem creates a potential risk for apoplastic sugar to be lost from the berries. We investigated the likelihood of such sugar loss and a possible sugar retrieval mechanism in the pedicels of different Vitis genotypes. Infusion of D-glucose-1-13C or L-glucose-1-13C to the stylar end of attached berries demonstrated that both sugars can be leached from the berries, but only the non-transport sugar L-glucose moved beyond the pedicels. No 13C enrichment was found in peduncles and leaves. Genes encoding 10 sugar transporters were expressed in the pedicels throughout grape ripening. Using an immunofluorescence technique, we localized the sucrose transporter SUC27 to pedicel xylem parenchyma cells. These results indicate that pedicels possess the molecular machinery for sugar retrieval from the apoplast. Plasmodesmata were observed between vascular parenchyma cells in pedicels, and movement of the symplastically mobile dye carboxyfluorescein demonstrated that the symplastic connection is physiologically functional. Taken together, the chemical, molecular, and anatomical evidence gathered here supports the idea that some apoplastic sugar can be leached from grape berries and is effectively retrieved in a two-step process in the pedicels. First, sugar transporters may actively retrieve leached sugar from the xylem. Second, retrieved sugar may move symplastically to the pedicel parenchyma for local use or storage, or to the phloem for recycling back to the berry.

From acidity to sweetness: a comprehensive review of carbon accumulation in grape berries

“…It involves two pathways that are not mutually exclusive: symplastic and apoplastic (Ruan et al 2001 ; Viola et al 2001 ; Zhang et al 2006 ; Nie et al 2010 ; Braun et al 2014 ; Ren et al 2023 ). Symplastic unloading through the plasmodesmata predominate during the early and mid-stages of grape berry development, while apoplastic unloading through the membranes becomes prominent at véraison (Zhang et al 2022 ; Zhou et al 2023 ). The plasmodesmata play a significant role in the switch from symplastic to apoplastic pathways (Zhang et al 2006 ; Li et al 2021 ; Zhou et al 2023 ).…”