BIOLOGY Volume 2 - A Guide to General Biology - 2004

13. PLANT TRANSPORT

13.8. Translocation of organic substances in the phloem

13.8.4. Mechanism of phloem translocation

Sections 13.8.1 and 13.8.3 outlined the facts that any hypothesis explaining this transport must account for; essentially, large quantities of substances move relatively rapidly through very narrow sieve tubes. Within the tubes, there are obvious obstacles—the sieve plates—as well as other structural features, such as P-protein, whose role remains unknown. When one adds to this that the entire system is quite fragile and easily damaged by any outside intervention, it becomes easy to understand the difficulties researchers have faced in unraveling The Mechanism of phloem translocation.

Many now believe that mass flow of solution takes place through the sieve tubes. Diffusion is much too slow a process to account for the observed transport rates. Below, we list the evidence supporting the mass flow hypothesis in the phloem.

1. When the phloem is cut, sap begins to exude from the cut; clearly, it moves by mass flow. This phenomenon is sometimes exploited practically to harvest sugar. For example, the sugar palm "weeps," yielding up to 10 liters of sugary sap daily.

2. The continuous exudation of sucrose solution from the aphid's stylet in the experiment described above (Section 13.8.3) indicates the presence of hydrostatic pressure within the sieve tubes.

3. Certain Viruses are transported throughout the plant via the phloem stream. Viruses are incapable of active movement and, being non-soluble, cannot diffuse. The only explanation for their movement is transport within a mass fluid flow.

Münch's mass flow and pressure-flow hypotheses

In 1930, Münch proposed a purely physical hypothesis to explain The formation of mass flow in sieve tubes. This hypothesis is illustrated by the model in Fig. 13.26.

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Fig. 13.26. Physical model illustrating the Münch mass-flow hypothesis for the translocation of solutes through the phloem. Modeled parts of a living plant: A — source of assimilates, e.g., a leaf; B — phloem; C — sink, e.g., roots, Meristems, fruits; D — xylem, apoplast, and intercellular spaces.

In this model, Water initially tends to enter vessels A and C via osmosis, but this tendency is stronger for A because the solution in A is more concentrated than in C. The influx of water into A increases the hydrostatic pressure within the closed A-B-C system, forcing water out of C. A mass flow is thus established in tube B, moving along the pressure gradient. An osmotic gradient also exists between A and C. Ultimately, as the solution in A becomes diluted and the solute is pumped into C, the system reaches equilibrium.

This model can be applied to living plants. Vessel A corresponds to the leaves. The production of sugar within them via Photosynthesis makes both the osmotic potential (ψ₀) and, consequently, The water potential more negative. Water arriving at the leaves via the xylem (D) enters the mesophyll by osmosis, increasing its hydrostatic potential (ψ_p). Concurrently, sugars are consumed at their "destination points," such as the roots (C), for various purposes including Respiration and Cellulose synthesis. This raises the osmotic potential (ψ₀) there, making it less negative and thereby inhibiting osmosis. As a result, a hydrostatic gradient is established, decreasing from the leaves to the roots—or, more generally, from the source of assimilates to the sink areas—which drives a mass flow of liquid between them. In a living plant, equilibrium is never attained because solutes are continuously consumed by sink Tissues (C) and produced by source tissues (A).

The Münch hypothesis is purely physical and fails to explain why sieve elements must remain living and metabolically active. Nor does it account for how leaf mesophyll Cells are able to load assimilates into sieve tubes against an osmotic gradient, given that the phloem's ψ₀ is known to be more negative than that of the photosynthetic tissue. In light of this, the Münch hypothesis was subsequently refined to incorporate a mechanism for the active loading of solutes into the sieve tubes. This implies that the osmotic and hydrostatic gradients originate not in the photosynthetic mesophyll, but directly within the phloem. Furthermore, phloem unloading at the sink level is also believed to be an active process. This modern version of the Münch hypothesis is referred to as the pressure-flow hypothesis.

Sieve Tube Loading

It has been demonstrated that the sucrose concentration in leaf sieve tubes typically ranges from 10 to 30%, whereas in photosynthetic cells it remains at about 0.5%. Consequently, as noted earlier, phloem loading proceeds against an osmotic gradient. This mechanism has been extensively studied in recent years. First, dissolved Organic compounds must move from the METABOLISM/14.html">Chloroplasts into the phloem; this pathway spans no more than 3 mm, with substances capable of moving via both the symplast and the apoplast. The symplastic pathway involves movement through plasmodesmata, while the apoplastic pathway occurs across Cell walls via diffusion or mass flow in the Transpiration stream.

In 1968, Gunning and his colleagues described a novel type of modified companion cells termed transfer cells. As seen in Fig. 13.27, these cells lie adjacent to the sieve tubes. Due to additional uneven Cell wall thickenings, they develop numerous internal wall ingrowths, which increase the surface area of The Plasma Membrane lining the wall nearly tenfold. This cellular modification is believed to be associated with the need to actively absorb solutes from neighboring cells. Energy for all active processes is supplied by the numerous Mitochondria within the transfer cells. Transfer cells are not found in all plants, but they are common in legumes and certain other families. Nevertheless, Active Transport is thought to occur even in their absence.

Fig. 13.27. Cytology/practical/72.html">Cross section of a minor leaf vein of common ragwort (Senecio vulgaris). In the phloem (upper half of the micrograph), six cells surrounding two sieve tubes are visible: four companion cells (CC) with dense Cytoplasm, transformed into transfer cells, and two phloem parenchyma cells (PP) with a more translucent cytoplasm and cell wall ingrowths restricted to the side facing the sieve tubes. The latter are typically connected to companion cells via plasmodesmata, whereas phloem parenchyma rarely forms symplastic connections with the conducting elements. In the lower half of the micrograph, two xylem vessels are visible in the center, flanked by two large bundle sheath cells. The arrows indicate potential pathways of solute movement into the sieve tubes, including the apoplastic transport of certain xylem sap components. Magnification ×6560.

The loading of sucrose (along with various other solutes such as Amino Acids, phosphates, potassium, and ammonium ions) into companion cells is believed to be mediated by specific carrier Proteins localized in the plasma membrane of these cells. The transport systems formed by these proteins resemble analogous systems in animal and bacterial cells, where The transport of organic molecules is coupled with the transport of protons (H⁺). Protons are pumped out of The Cell by a carrier utilizing ATP energy (Fig. 13.28). This establishes an H⁺ concentration gradient that generates potential energy. Protons rapidly diffuse back into the cell with the aid of other carrier proteins, which simultaneously co-transport sucrose molecules or other organic substances (symport mechanism). Thus, the driving force of this process is the establishment of a proton gradient across the plasma membrane, characterized by a lower pH (higher H⁺ concentration) outside the cell.

Fig. 13.28. Sucrose loading into a companion cell. H⁺ ions (protons) are pumped out of the cell by a proton pump. Located in the plasma membrane, it requires ATP energy for its operation and exhibits ATPase activity itself. Protons return to the cell via Facilitated Diffusion: a specialized carrier protein translocates them back simultaneously with sucrose molecules (symport).

As a result of the active Transport of substances into the companion cells, their osmotic potential drops significantly. This drives an influx of water via osmosis, an increase in pressure, and the movement of solutions (including the sucrose solution) via mass flow through the numerous plasmodesmata connecting the companion cells to the sieve tubes. Thus, high pressure and mass flow are generated within the sieve tubes rather than in the leaf mesophyll cells, as Münch had assumed. It is also possible that the mechanism of active transport via plasmodesmata from companion cells into sieve tubes is identical to the mechanism driving sucrose into the companion cells.

During phloem unloading, dissolved solutes likely leave the sieve tubes and enter transfer cells via plasmodesmata. As a result, the WATER POTENTIAL OF the sieve tubes increases (becomes less negative). This impedes potential osmosis from the outside and maintains the hydrostatic gradient between the phloem loading and unloading sites. It is believed that the latter process may also occur across the plasma membrane of the sieve tubes into their cell walls and subsequently via the apoplastic pathway.

Critical Evaluation of the Pressure-Flow Hypothesis

1. According to this hypothesis, a mass flow is established within the sieve tubes, which is consistent with experimental evidence (see data above).

2. The hypothesis postulates the existence of an osmotic gradient and high pressure within the phloem. This has been demonstrated for a wide range of plants.

The hydrostatic gradient required to drive solute transport through sieve tubes at the observed rates must be relatively steep. Even with all sieve pores open, theoretical calculations yield a gradient value of 13 kPa/m, which until recently was considered unlikely to be attainable. Direct measurement of phloem pressure is a formidable challenge, yet it has been successfully accomplished in recent years. Pressure levels ranging from 1000–2000 kPa with gradients up to 20 kPa/m have been recorded. Thus, the hypothesis is substantiated here as well.

3. The pressure-flow hypothesis has been criticized for failing to explain why sieve tubes remain living, unlike the dead xylem vessel elements.

However, only living cells possess a plasma membrane, which is essential to prevent sucrose leakage into the external environment. Recent studies indicate that the metabolic rate in sieve tubes is low, suggesting passive solute movement through them, which is entirely consistent with the pressure-flow hypothesis.

4. It is thought that sieve plates, despite the resistance they offer to flow, are necessary because they provide structural reinforcement to the sieve tubes, preventing them from bulging, cracking, or bursting under high internal pressure.

5. The pH of the sieve tube contents is close to 7.5–8 (mildly alkaline). This can be explained by the pumping of protons out of the tubes, as described above. The presence of a proton pump also accounts for the high ATP concentration observed in sieve tubes. The overall scheme of sucrose translocation throughout the plant is shown in Fig. 13.29.

Fig. 13.29. Translocation of soluble organic substances, such as sucrose, through the phloem of a green plant. This transport occurs in three stages: the movement of solutes from photosynthesizing cells to the sieve tubes (phloem loading), translocation through the phloem, and unloading at utilization or storage sites.