Phloem Loading. A Reevaluation of the Relationship between Plasmodesmatal Frequencies and Loading Strategies

Abstract

(R.T.); and Electron Microscopy Services and Consultants, Phoenix, Arizona 85022 (R.M.)The incidence of plasmodesmata in the minor vein phloem of leaves varies widely between species. On this basis, two pathways of phloem loading have been proposed: symplastic where frequencies are high, and apoplastic where they are low. However, putative symplastic-loading species fall into at least two categories. In one, the plants translocate raffinose-family oligosaccharides (RFOs). In the other, the primary sugar in the phloem sap is sucrose (Suc). While a thermodynamically feasible mechanism of symplastic loading has been postulated for species that transport RFOs, no such mechanism is known for Suc transporters. We used p-chloromercuribenzenesulfonic acid inhibition of apoplastic loading to distinguish between the two pathways in three species that have abundant minor vein plasmodesmata and are therefore putative symplastic loaders. Clethra barbinervis and Liquidambar styraciflua transport Suc, while Catalpa speciosa transports RFOs. The results indicate that, contrary to the hypothesis that all species with abundant minor vein plasmodesmata load symplastically, C. barbinervis and L. styraciflua load from the apoplast. C. speciosa, being an RFO transporter, loads from the symplast, as expected. Data from these three species, and from the literature, also indicate that plants with abundant plasmodesmata in the minor vein phloem have abundant plasmodesmata between mesophyll cells. Thus, plasmodesmatal frequencies in the minor veins may be a reflection of overall frequencies in the lamina and may have limited relevance to phloem loading. We suggest that symplastic loading is restricted to plants that translocate oligosaccharides larger than Suc, such as RFOs, and that other plants, no matter how many plasmodesmata they have in the minor vein phloem, load via the apoplast.Phloem loading is an energy-requiring process that elevates the solute content of sieve elements (SEs) and companion cells (CCs) to levels far above those of surrounding cells (Geiger et al., 1973;van Bel, 1993;Grusak et al., 1996;Turgeon, 1996Turgeon, , 2000Beebe and Russin, 1999;Hellmann et al., 2000;Komor, 2000;Lalonde et al., 2003). The pressure gradient between source and sink organs drives long-distance transport. It has been suggested that there are two loading strategies-apoplastic and symplastic. This hypothesis derived originally from the observation that there is great variation in the number of plasmodesmata in the phloem of minor veins (Turgeon et al., 1975;Gamalei, 1989Gamalei, , 1990Gamalei, , 1991Gamalei, , 2000van Bel, 1993). Gamalei has classified over 1,000 species on this basis and finds that it is a relatively consistent feature at the family level. He terms those with the highest number of minor vein plasmodesmata as type 1.The physical basis for phloem loading via the apoplast is reasonably well understood; Suc enters the cell wall space and is driven across the plasma membranes of CCs and/or SEs by cotransport with protons (Lalonde ...

A comprehensive picture of phloem loading strategies

“…Counterintuitively, the remaining 6 species are type 1 or type 1-2a, with more abundant plasmodesmata, which would seem to preclude the possibility of active phloem loading. The ability of some type 1 plants to actively load from the apoplast has been previously described (27,28). One reasonable explanation is that, in these species, the plasmodesmata linking CCs to surrounding cells in the minor veins are temporarily or permanently sealed to prevent sucrose leakage.…”

Anatomical and photosynthetic acclimation to the light environment in species with differing mechanisms of phloem loading

“…However, further work is needed to determine whether this strategy for up-regulating loading capacity may be available to other apoplastic loaders. In the few studies that have characterized vein density in sun vs. shade leaves (36)(37)(38), the mode of phloem loading was not firmly established for most of the species investigated, although those species capable of adjusting vein density appear to have frequent plasmodesmatal connections into the minor vein phloem (39)(40)(41).…”

Functional Characterization of the Arabidopsis AtSUC2 Sucrose/H+ Symporter by Tissue-Specific Complementation Reveals an Essential Role in Phloem Loading But Not in Long-Distance Transport

“…To assess the efficiency of photoassimilate transport out of the leaf, excised leaves were photosynthetically labeled with 14 CO 2 , and phloem sap was collected over 20 h by an EDTA exudation method (Turgeon and Medville, 2004). At the end of the exudation experiment, the amount of isotope exuded from the leaves was determined relative to the amount retained by the leaves in soluble and insoluble fractions (Table IV, top three entries).…”

“…The cDNA was purified with the Wizard SV gel and PCR clean-up system (Promega), digested with KpnI and SacI, and ligated into the same sites of pGemTGUTTT7CO, which is identical to pUCTGUTTT7CO but with a pGEM3zf1 backbone replacing the pUC19 backbone (Ayre and Turgeon, 2004), creating pGEMTCmGAS1pTcSUC2. Two kilobases of AtSUC2 promoter was PCR amplified using oligonucleotides AtSUC2p5 (5#-TCAATCCTG-CAGGTCGACGGT-3#; PstI/SbfI site underlined) and AtSUC2p3 (5#-TCA-ATGGTACCTTCCCGGGAT-3#; KpnI site underlined), digested with PstI and KpnI, and ligated into the same sites of pGEMTCmGAS1pTcSUC2 to create pGEMTSUC2pTcSUC2.…”