Phloem

Phloem (from Greek phloios, meaning "bark," coined by Carl Nägeli in 1858) is the living vascular tissue in tracheophytes (vascular plants) responsible for the long-distance translocation of organic photoassimilates—predominantly non-reducing sucrose, raffinose-family oligosaccharides (RFOs), sugar alcohols (polyols such as mannitol and sorbitol), amino acids (glutamine, glutamate, asparagine, aspartate), inorganic ions (K⁺, Mg²⁺, PO₄³⁻), phytohormones, and systemic macromolecular signals (mRNAs, small interfering RNAs, and proteins such as the florigen hormone FT). Whereas xylem conducts water and mineral solutes unidirectionally upward from roots to leaves through dead, lignified tracheary conduits operating under negative hydrostatic tension driven by transpiration, phloem conducts sap bidirectionally through living, membrane-bounded conduits under positive turgor pressure from sources (net carbon exporters, such as mature photosynthesizing leaves or mobilizing storage tubers in spring) to sinks (net carbon importers, such as growing root and shoot apical meristems, developing fruits, seeds, and storage organs).

During primary growth, primary phloem differentiates from the procambium on the outer (abaxial) side of vascular bundles. In woody dicots and gymnosperms undergoing secondary thickening, the vascular cambium generates secondary phloem outward, forming the inner living bark layer—which is explains why circumferential girdling (removing a ring of bark down to the cambium, as performed by Marcello Malpighi in 1675) halts downward sugar transport and starves the root system while leaving upward xylem water flow initially intact.

Ontogeny and Selective Autolysis of Sieve Tube Elements

To serve as low-resistance microfluidic conduits while retaining a semipermeable cell membrane capable of sustaining osmotic turgor gradients, angiosperm conducting cells—termed sieve tube elements (STEs)—undergo a tightly programmed partial selective autolysis during differentiation. A mature sieve tube element retains an intact plasma membrane, a parietally anchored smooth endoplasmic reticulum (sER), plastids, and modified mitochondria, but completely degrades its nucleus, central vacuole (tonoplast), ribosomes, Golgi apparatus, and cytoskeleton. This clearing of the central lumen creates an unobstructed cytoplasmic flow path linked end-to-end with adjacent STEs via perforated end walls called sieve plates, whose pores (0.5 to 15 μm in diameter, lined by a thin cylinder of β-1,3-glucan callose) represent widened plasmodesmata.

Because an enucleate, ribosome-free sieve tube element cannot transcribe genes or synthesize proteins, its survival over months or years depends entirely on an ontogenetically sister companion cell (CC) derived from the same asymmetric division of a phloem mother cell. The sieve tube element and companion cell form an obligate functional unit—the sieve element–companion cell (SE–CC) complex—interconnected by specialized, branched pore-plasmodesma units (PPUs) that traffic ATP, chaperones, and metabolic enzymes from the metabolically hyperactive, densely cytoplasmic companion cell into the sieve tube lumen.

Evolutionary Comparison of Angiosperm vs. Gymnosperm Phloem

Surrounding the SE–CC complexes are phloem parenchyma cells (which store starch, tannins, and resins and participate in radial solute transfer) and thick-walled, lignified phloem sclerenchyma (bast fibers—the commercial source of linen flax, hemp, and jute—and sclereids) that provide tensile mechanical reinforcement.

Proposed by the German plant physiologist Ernst Münch in 1930, the pressure-flow hypothesis explains long-distance phloem transport as an osmotically generated, bulk Poiseuille mass flow of solution driven by a hydrostatic turgor pressure gradient (Δ*P*) between source and sink without requiring direct metabolic propulsion along the length of the sieve tube path:

1. Phloem Loading at the Source: In mature photosynthetic leaves (where C₃ or C₄ mesophyll cells fix CO₂ via stomata and export triose phosphates to the cytosol to synthesize sucrose), sucrose is actively concentrated into the minor-vein SE–CC complex to molarities of 0.5 to 1.2 M (15–30% w/v dry matter). This massive solute accumulation drops the sieve-tube solute potential (Ψ_s) to approximately −1.5 to −2.5 MPa.
2. Osmotic Water Influx from the Xylem: Because adjacent xylem vessels carry dilute sap at a much higher (less negative) water potential (Ψ_w ≈ −0.3 to −1.0 MPa), water flows osmotically across aquaporins from the xylem into the source sieve tubes, building a high positive hydrostatic turgor pressure (Ψ_p,source ≈ 1.0 to 2.0 MPa).
3. Bulk Mass Flow Along the Sieve Tube Conduit: Assuming laminar Poiseuille flow through a cylindrical tube of radius r, length L, and sap viscosity η (typically 1.5–2.0 mPa·s at 20% w/v sucrose), the volumetric flux J_v (or average velocity v) obeys the Hagen–Poiseuille relation:

v = (r² / 8η) · (Δ*P* / L) Driven by a source-to-sink pressure drop Δ*P* ≈ 0.5 to 1.2 MPa, the entire column of phloem sap—water, sugars, amino acids, ions, and RNAs—moves in bulk toward the sink at velocities of 0.3 to 1.5 m h⁻¹.

1. Phloem Unloading and Xylem Water Recycling at the Sink: At metabolic or storage sinks, sucrose is unloaded out of the sieve tubes and immediately metabolized via cellular respiration or polymerized into osmotically inactive starch inside amyloplasts. The removal of osmolytes raises the sink sieve-tube water potential, lowering sink turgor (Ψ_p,sink ≈ 0.2 to 0.5 MPa), and the excess water recycles back into the adjacent xylem transpiration stream.

Apoplastic Loading vs. Symplastic Polymer Trapping

Plants employ three distinct evolutionary strategies to load sucrose from mesophyll cells into minor-vein SE–CC complexes against or down a concentration gradient:

• Active Apoplastic Loading (Herbaceous Eudicots and Grasses): Mesophyll cells transport sucrose symplastically to phloem parenchyma cells, which export sucrose into the cell wall space (apoplast) via SWEET11 and SWEET12 uniporter efflux proteins. Companion cells (often modified into transfer cells with labyrinthine cell-wall ingrowths that amplify membrane surface area) then use a plasma-membrane P-type H⁺-ATPase transmembrane pump to establish a steep proton-motive force (ΔpH and membrane potential), driving secondary active SUT1 / SUC2 H⁺–sucrose symporters to concentrate sucrose into the SE–CC complex up to 10-fold higher than in the mesophyll.
• Active Symplastic Polymer-Trap Loading (Cucurbits, Lamiaceae, Onagraceae): Specialized companion cells termed intermediary cells are connected to bundle-sheath cells by abundant, narrow plasmodesmata. Sucrose diffuses down its concentration gradient into the intermediary cell, where galactinol synthase and raffinose/stachyose synthases enzymatically convert it into larger raffinose (trisaccharide) and stachyose (tetrasaccharide) molecules. Because these oligosaccharides have larger hydrodynamic radii than sucrose, they cannot diffuse backward through the narrow bundle-sheath plasmodesmata, trapping them to enter the wider sieve tube pores.
• Passive Symplastic Diffusion (Many Woody Trees): High sucrose concentrations are maintained in the mesophyll cytosol, allowing sucrose to diffuse passively through plasmodesmata all the way into the sieve tubes down a continuous concentration gradient.

P-Protein and Callose Defense Against Sap Loss and Aphids

Because sieve tubes operate under high positive pressure (1–2 MPa) and contain nutrient-rich sap, mechanical severing or stylet penetration by phloem-feeding hemipteran insects (aphids, whiteflies, psyllids) threatens catastrophic sap bleeding. Angiosperms deploy a dual-stage sealing mechanism:

1. Rapid P-Protein Plugging (<1 second): Sudden pressure drop or influx of apoplastic Ca²⁺ through mechanosensitive or glutamate-receptor-like ion channels triggers phloem proteins (P-proteins)—such as crystalline forisomes in legumes (Fabaceae), which undergo an ATP-independent, Ca²⁺-driven conformational swelling from condensed spindles into dispersed plugs—to surge with the escaping sap and clog the downstream sieve plate pores within milliseconds. Aphids counteract this defense by injecting watery saliva containing Ca²⁺-binding proteins into the punctured sieve element to chelate Ca²⁺ and prevent P-protein dispersion.
2. Enzymatic Callose Constriction (Minutes): Sustained Ca²⁺ elevation activates plasma-membrane callose synthase (β-1,3-glucan synthase), which deposits rings of callose around the sieve plate pores and PPUs until the punctured conduit is completely sealed off.

Why is sucrose (rather than glucose) the universal transport sugar in phloem sap?

Monosaccharides like glucose and fructose are reducing sugars that possess a free hemiacetal or hemiketal carbonyl group capable of reacting non-enzymatically with free amino groups on proteins via the Maillard/glycation reaction, or being prematurely phosphorylated by hexokinases along the transport path. Sucrose (α-D-glucopyranosyl-(1→2)-β-D-fructofuranoside) links the anomeric carbons of both glucose and fructose together in a glycosidic bond, rendering it a chemically inert non-reducing disaccharide (a property shared by raffinose, stachyose, and sugar alcohols) that can be transported at ~1 M concentration without damaging sieve-tube membranes.

How can Mature Sieve Tube Elements stay alive for years without a nucleus or ribosomes?

Although angiosperm sieve tube elements undergo selective autolysis of their nucleus, vacuole, and ribosomes to minimize hydraulic drag in the flow lumen, they remain enclosed by a living, semipermeable plasma membrane and are metabolically sustained by their sister companion cells. Extensive branched pore-plasmodesma units (PPUs) with an enlarged size-exclusion limit (>30–60 kDa) allow the nucleated companion cell to continuously supply the sieve tube element with ATP, antioxidant enzymes, membrane lipids, and turnover proteins.

How do researchers collect pure phloem sap without triggering P-protein wound plugging?

If a stem is cut with a razor blade, sudden turgor loss triggers P-protein and callose plugging within seconds, and broken parenchyma cells contaminate the exudate. Plant physiologists exploit aphid stylectomy: an aphid is allowed to insert its sub-micrometer chitinous stylet directly into a single living sieve tube element and inject its anti-clotting Ca²⁺-chelating saliva; a micro-laser or radiofrequency microcautery needle then severs the aphid's body from its embedded stylet, leaving the microscopic tube as a pressure-driven cannula that exudes pure microliter droplets of sieve-tube sap for hours.

1. Knoblauch, M., & Peters, W. S. (2017). "What actually is the Münch hypothesis? A short history of assimilate transport by mass flow." Journal of Integrative Plant Biology, 59(5), 292–310. https://doi.org/10.1111/jipb.12532
2. Turgeon, R., & Wolf, S. (2009). "Phloem transport: cellular pathways and molecular trafficking." Annual Review of Plant Biology, 60, 207–221. https://doi.org/10.1146/annurev.arplant.043008.092045
3. Chen, L. Q., et al. (2012). "Sucrose efflux mediated by SWEET proteins as a key step for phloem transport." Science, 335(6065), 207–211. https://doi.org/10.1126/science.1213351
4. van Bel, A. J. E. (2003). "The phloem, a miracle of ingenuity." Plant, Cell & Environment, 26(1), 125–149. https://doi.org/10.1046/j.1365-3040.2003.00963.x