Plants have two long-distance transport systems: the xylem (water and minerals from roots to shoots) and the phloem. The phloem transports sugars (photosynthates), amino acids, hormones, signaling molecules, and even RNA from where they are produced (sources) to where they are needed (sinks) [citation:1].
Key concept: Unlike xylem, which consists of dead cells, phloem cells are alive and require energy to function. This living nature is essential for the active loading and unloading of sugars that drives transport [citation:2].
Several classic experiments provided evidence that phloem is the tissue responsible for sugar transport [citation:2][citation:5]:
Phloem tissue contains several specialized cell types working together [citation:2][citation:9]:
The conducting cells. These elongated cells are joined end-to-end to form sieve tubes [citation:2]. They lose their nucleus, ribosomes, and tonoplast during maturation, but retain their plasma membrane [citation:5].
Sieve plates: The end walls are perforated with pores that allow bulk flow of sap [citation:10].
The "life support" cells. Each sieve element is connected to one or more companion cells via specialized plasmodesmata [citation:7]. Companion cells have dense cytoplasm, many mitochondria, and a nucleus—they provide metabolic support and energy for loading/unloading sugars [citation:2][citation:10].
Storage cells involved in lateral transport and exchange with surrounding tissues [citation:6].
Structural support cells (sclerenchyma) that give mechanical strength [citation:2].
Sieve elements and companion cells develop from the same mother cell and remain intimately connected through numerous plasmodesmata [citation:2][citation:5]. This SE-CC complex is the functional unit of phloem transport. The companion cell provides the sieve element with proteins, ATP, and other essential molecules that the enucleate sieve element cannot produce itself [citation:7].
Analysis of phloem exudates reveals a complex mixture [citation:4][citation:1]:
| Component | Examples | Concentration/Notes |
|---|---|---|
| Sugars | Sucrose (primary), raffinose, stachyose in some species | 10-30% (w/v) — very high concentration [citation:10] |
| Amino acids | Glutamine, glutamate, aspartate, others | For nitrogen transport [citation:4] |
| Proteins | Phloem proteins (P-proteins), chaperones, proteases | Involved in wound sealing, signaling [citation:1][citation:9] |
| Lipids | Phosphatidic acid (PA), phosphatidylcholine (PC), jasmonic acid | Signaling molecules; transported bound to lipid-binding proteins [citation:1] |
| Hormones | Auxin, cytokinins, ABA, jasmonates | Long-distance signaling [citation:4] |
| RNA | mRNA, siRNA, miRNA | Systemic gene silencing and developmental regulation [citation:1][citation:9] |
| Ions | K⁺, Mg²⁺, Cl⁻, PO₄³⁻ | Maintain osmotic balance [citation:4] |
Phloem contains many proteins with diverse functions [citation:9]:
In pumpkin and other cucurbits, two major phloem proteins—PP1 (96 kDa filamentous protein) and PP2 (48 kDa dimeric lectin)—dominate the phloem exudate. PP2 has lectin activity (binds chitin) and may play roles in defense against pathogens. These proteins can form disulfide bridges, creating large filaments that help seal damaged sieve tubes [citation:9].
| Feature | Phloem | Xylem |
|---|---|---|
| Primary function | Transport sugars, amino acids, signals | Transport water and minerals |
| Direction of flow | Source → sink (can be up or down) [citation:3] | Roots → shoots (unidirectional) [citation:3] |
| Cells alive at maturity? | Yes — sieve elements are living [citation:2] | No — vessel elements and tracheids are dead [citation:3] |
| Cell types | Sieve elements, companion cells, parenchyma, fibers [citation:2] | Vessels, tracheids, parenchyma, fibers |
| Driving force | Osmotic pressure gradient (turgor) [citation:3] | Transpiration pull (negative pressure) [citation:3] |
| Energy requirement | Active loading/unloading requires ATP [citation:3] | Passive (cohesion-tension theory) |
| Contents under pressure? | Positive pressure (turgor) [citation:3] | Negative pressure (tension) [citation:3] |
Key insight: The phloem's living nature means it can be regulated—plants can control which sinks receive sugars by adjusting loading and unloading rates. This is impossible in the passive xylem [citation:3].
Because phloem is under positive pressure (5-30 atmospheres!), damage would cause massive sap loss if not quickly sealed. Plants have evolved rapid wound responses [citation:9]:
Beyond sugar transport, the phloem carries critical signaling molecules that coordinate plant development and stress responses [citation:1][citation:8]:
Recent research highlights the phloem's critical role in determining crop yield through [citation:8]:
Understanding phloem function is therefore essential for crop improvement [citation:8].
In some fruit trees (e.g., lychee, citrus), growers practice trunk girdling—removing a ring of bark to interrupt phloem transport. This causes sugars to accumulate above the girdle, which can [citation:2]:
However, girdling must be done carefully to avoid killing the tree.
Some systemic pesticides are designed to move through the phloem, allowing them to reach all parts of the plant, including new growth and roots [citation:4].
Successful grafting requires that the phloem tissues of scion and rootstock connect properly to allow sugar transport. Incompatible grafts often fail because phloem connections don't form [citation:8].
| Cell type | Function | Key features |
|---|---|---|
| Sieve elements | Conduct sugars and signals | Living but enucleate; connected by sieve plates; form sieve tubes [citation:2] |
| Companion cells | Support sieve elements | Dense cytoplasm, many mitochondria; connected via plasmodesmata [citation:10] |
| Phloem parenchyma | Storage, lateral transport | Ordinary parenchyma cells [citation:6] |
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