Phloem loading in peach: Symplastic or apoplastic?

Abstract

Sorbitol and sucrose are the two main soluble carbohydrates in mature peach leaves. Both are translocated in the phloem, in peach as in other rosaceous trees. The respective role of these two soluble carbohydrates in the leaf carbon budget, and their phloem loading pathway, remain poorly documented. Though many studies have been carried out on the compartmentation and export of sucrose in sucrose-transporting species, far less is known about sorbitol in species transporting both sucrose and sorbitol. Sorbitol and sucrose concentrations were measured in several tissues and in sap, in 2-month-old peach (Prunus persica L. Batsch) seedlings, i.e. leaf blade, leaf main vein, petiole, xylem sap collected using a pressure bomb, and phloem sap collected by aphid stylets. The sorbitol to sucrose molar ratio depended on the tissue or sap, the highest value (about 7) found in the leaf main vein. Sorbitol concentration in the phloem sap was about 560 mM, whereas that of sucrose was about 140 mM. The lowest sorbitol and sucrose concentrations were observed in xylem sap collected from the shoot. The volume of the leaf apoplast, estimated by infiltration with ^H-inulin, represented about 17% ofthe leaf blade water content. This volume was used to calculate a global intracellular concentration for each carbohydrate in the leaf blade. Following these simplifying assumptions, the calculated concentration gradient between the leaf's intracellular compartment and phloem sap is nil for sorbitol and could thus allow for the sympiastic loading of the phloem of this alditol. However, infiltration of ''^C-labelled source leaves with 2 mM p-chloromercuribenzenesulfonic acid (PC-MBS), a potent inhibitor of the sucrose carrier responsible for phloem loading in sucrose-transporting plants, had a significant effect on the exudation of both labelled sucrose and sorbitol from the phloem. Therefore, in peach, which is a putative sympiastic loader according to minor vein anatomy and sorbitol concentration gradients, apoplastic loading may predominate.

Changes in the Activity and Gene Expression of Sorbitol- and Sucrose-related Enzymes Associated with Development of 'La France' Pear fruit

“…However, the maximum activities of both enzymes in vitro may not necessarily reflect the activities of both enzymes in vivo since they are regulated by many factors in vivo. The translocating sugars in peach were composed of sorbitol (about 80%) and sucrose (about 20%) (Moing et al, 1997). Similarly, the phloem exudate from the peduncle of apple fruit was composed about 70% sorbitol and about 30% sucrose (Klages et al, 2001).…”

The high-quality draft genome of peach (Prunus persica) identifies unique patterns of genetic diversity, domestication and genome evolution

“…In Rosaceae, polyol biosynthesis 15 has a more prominent role than what is seen in other plant families. For example, in apple and peach, ~70% of translocated carbon is in the form of sorbitol 16,17 . Integrating the most recent Rosaceae molecular phylogeny 18 with data on sorbitol content 19 , it is evident that leaf sorbitol synthesis and accumulation are restricted to the subfamily Spiraeoideae (for example, apple, peach and cherry), whereas in the subfamilies Rosoideae and Dryoideae, this polyol is comparatively absent 20,21 .…”

Phloem Loading Strategies in Three Plant Species That Transport Sugar Alcohols

“…Although it is reasonable to assume that sugar alcohols are loaded by the same species-specific strategies as Suc, these strategies have not been well documented. In particular, there is a debate over the possibility that sugar alcohols load through the symplast (Moing et al, 1997;Nadwodnik and Lohaus, 2008).…”