Symplastic continuity between companion cells and the translocation stream: long-distance transport is controlled by retention and retrieval mechanisms in the phloem

Abstract

Substantial symplastic continuity appears to exist between companion cells (CCs) and sieve elements of the phloem, which suggests that small solutes within the CC are subject to indiscriminate long-distance transport via the translocation stream. To test this hypothesis, the distributions of exotic and endogenous solutes synthesized in the CCs of minor veins were studied. Octopine, a charged molecule derived from arginine and pyruvate, was efficiently transported through the phloem but was also transferred in substantial amounts to the apoplast, and presumably other non-phloem compartments. The disaccharide galactinol also accumulated in non-phloem compartments, but long-distance transport was limited. Conversely, sucrose, raffinose, and especially stachyose demonstrated reduced accumulation and efficient transport out of the leaf. We conclude that small metabolites in the cytosol of CCs do enter the translocation stream indiscriminately but are also subject to distributive forces, such as nonselective and carrier-mediated membrane transport and symplastic dispersal, that may effectively clear a compound from the phloem or retain it for long-distance transport. A model is proposed in which the transport of oligosaccharides is an adaptive strategy to improve photoassimilate retention, and consequently translocation efficiency, in the phloem.

Figure 1

Quantitative bioassay for octopine, and distribution patterns throughout transgenic tobacco. A, Representative standard curve for detecting octopine in the growth medium of Agrobacterium tumefaciens strain KYC16 in the absence of wild-type plant extract (♦), with the addition of extract from 10 mg fresh weight wild-type leaf tissue (▴), and with the addition of extract from 50 mg fresh weight wild-type leaf tissue (●). B, Leaf section of NtGOG-12 tobacco plant stained with the β-glucuronidase (GUS) substrate 5-bromo-4-chloro-3-indolyl-β-d-GlcUA (X-GlcA) and cleared of chlorophyll. Staining in a minor vein is indicated with an arrow, whereas an arrowhead indicates the absence of staining in a larger vein. C, Representative distribution of octopine in grafted tobacco plants. Stock, Mature leaf tissue from the indicated transgenic tobacco plant; solid bars, NtGOG-12; and hollow bars, NtGOG-20. Apoplast, Apoplast washing fluid collected from a mature leaf sample of the indicated transgenic plant, corrected for dilution, and assuming a specific gravity of one. Scion, Apical bud tissue of a wild-type scion grafted to the indicated transgenic stock plant.

Figure 2

Schematic representation of the distribution of solutes originating in the CC of minor veins. Net diffusion of small solutes in the CCs is in the direction of the SEs and the translocation stream (dashed arrows). Solutes may also diffuse across membranes as a function of their permeability coefficients. The relative rates of transmembrane diffusion are represented by the size of the solid arrows. The relative amounts of each solute remaining in the symplast along the phloem pathway are represented by the size of the respective symbols. Octopine (squares) is ionized, and is efficiently retained in and translocated through the phloem. Gradual leakage and accumulation in the apoplast does occur. Suc (solid circles) is pumped into the SECCC with protons (+) by Suc transporters (white circles). Suc leaks from the phloem symplast at appreciable rates but is retrieved by transporters situated throughout the phloem network. Galactinol (triangles) also leaks at appreciable rates but is not retrieved, and it is eventually depleted from the translocation stream. The larger saccharide stachyose (rectangles) leaks minimally and is efficiently transported to sink tissues. Although not represented, diffusion of some solutes from the SECCC to mesophyll cells through plasmodesmata may also occur. For simplicity, CCs in the path and sink tissues are not represented.