Phloem loading strategies in three plant species that transport sugar alcohols

Abstract

Many plants translocate sugar alcohols in the phloem. However, the mechanism(s) of sugar alcohol loading in the minor veins of leaves are debated. We characterized the loading strategies of two species that transport sorbitol (Plantago major and apple [Malus domestica]), and one that transports mannitol (Asarina scandens). Plasmodesmata are abundant at all interfaces in the minor vein phloem of apple, and in one of two types of phloem in the minor veins of A. scandens. Few plasmodesmata are present in the minor veins of P. major. Apple differs from the other two species in that sugar alcohol and sucrose (Suc) are present in much higher concentrations in leaves. Apple leaf tissue exposed to exogenous [(14)C]sorbitol, [(14)C]Suc, or (14)CO(2) did not accumulate radiolabel in the minor veins, as determined by macroautoradiography. P. major minor veins accumulated radiolabel from [(14)C]Suc, [(14)C]sorbitol, and (14)CO(2). A. scandens minor veins accumulated (14)C from [(14)C]Suc and (14)CO(2), but not from [(14)C]mannitol. We conclude that the movement of sugar alcohol from the mesophyll into the phloem in apple and A. scandens is symplastic and passive, but in P. major it involves an apoplastic step and is energized. We also suggest that apple leaves transport sorbitol in high concentrations to avoid the feedback limitation of photosynthesis that would result from driving passive movement of solute into the phloem with high levels of Suc alone. The loading pathways and the mechanisms by which hydrostatic pressure is maintained in the minor vein phloem of these species are discussed.

Figure 1.

Electron micrographs of apple (A and B) and P. major (C–E) minor veins in transverse sections. A, SEs and CCs are surrounded by PP cells. Numerous plasmodesmata are present at BS-PP and PP-CC interfaces (arrows), providing symplastic continuity into the phloem. B, Plasmodesmata between PP cells. C, Plasmodesmata between PP cells and CCs. D, SEs are surrounded by a ring of CCs and PP cells. Few plasmodesmata are visible at any interface. E, PP cells have transfer cell wall ingrowths (arrow) at the interfaces with SEs and CCs. Scale bars: A and E = 2 μm; B and C = 0.2 μm; D = 4 μm.

Figure 2.

Autoradiographs of leaf tissue from apple, P. major, and A. scandens. Abraded leaf discs were floated on either [¹⁴C]Suc or [¹⁴C]sugar alcohol for 1 h, then washed. Apple and P. major discs were floated on [¹⁴C]sorbitol, A. scandens discs on [¹⁴C]mannitol. Whole leaves were exposed to ¹⁴CO₂ for 5 min, followed by a chase in room atmosphere for 55 min. Tissue was flash frozen, lyophilized, pressed thin, and autoradiographed. Leaf discs are 8 mm diameter. Scale bar for ¹⁴CO₂ panels = 2 mm.

Figure 3.

Transverse section of an apple leaf. Note the minor vein (between arrowheads) converging with a large vein. The large vein has BS extensions above and below that reach almost to the epidermal layers and are composed primarily of sclerenchyma cells (asterisks). Arrows indicate the phloem of the minor and large veins. Scale bar = 100 μm. [See online article for color version of this figure.]

Figure 4.

Schematic diagrams of hypothetical phloem-loading schemes. Relative concentrations of Suc and sugar alcohol are represented in mesophyll cells, CCs, and SEs. Solutes pass between cells through plasmodesmata (gaps in walls) or are transferred into the apoplast and are pumped into CCs by transporters (red and blue stars). A, Apoplastic loading of both sugar alcohol and Suc. B, Symplastic loading of both sugar alcohol and Suc, driven by their respective concentration gradients. C, Symplastic loading of sugar alcohol and Suc into intermediary cells, where Suc is converted into raffinose and stachyose. [See online article for color version of this figure.]