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Phloem is the vascular tissue responsible for the transport of sugars from source tissues (ex. photosynthetic leaf cells) to sink tissues (ex. non-photosynthetic root cells or developing flowers). Other molecules such as proteins and mRNAs are also transported throughout the plant via phloem.






Phloem Structure


Phloem is composed of several cell types including
sclerenchyma, parenchyma, sieve elements and companion cells. The sieve element and companion cell are found closely associated with each other in what is referred to as the sieve element/companion cell complex. One or more companion cells may be associated with a single sieve element.

The so called “sieve element” may be more specifically referred to as a sieve tube member (angiosperms) or sieve cell (gymnosperms and ferns). The sieve cells of gymnosperms lack a sieve plate and instead have sieve pores throughout the cell wall which allow flow between adjacent cells. The sieve tube members found in flowering plants are generally wider than sieve cells and have sieve plates connecting the ends of adjacent cells. These sieve plates are areas with many pores through which adjacent cells are connected by a continuous cytoplasm.


Phloem cell types

Blue: Sieve element- conducting element of the phloem

Yellow: Companion Cell- "life support" cell for the sieve element

Red: Fibers- made of sclerenchyma cells and provides structural support for the plant

Green: Parenchyma- acts as packing material between other cell types and helps transfer materials to the SE/CC complex













Sieve Element/Companion Cell Development


Not a great deal is known about the genetic mechanisms involved in the specification of phloem cells during differentiation. What is known is that ALTERED PHLOEM DEVELOPMENT (APL), a MYB-transcription factor, plays a part in inhibiting xylem cells and promoting the formation of phloem cells. Mutations in APL give rise to plants with cells showing xylem characteristics where phloem cells should be. Over-expressing APL inhibits the formation of xylem. Cytokinin signaling is also required to maintain cell identities other than xylem and is therefore important for the formation of phloem. VAHOX1, a homeobox gene from tomato, shows phloem specific expression during secondary growth and is therefore a candidate gene playing a role in phloem specification from the vascular cambium.









It is known that the sieve element (SE) and companion cell (CC) arise from an unequal division of a common “phloem mother cell.” This mother cell may be found in the
procambium in the case of primary phloem or in the vascular cambium in secondary phloem.












The SE then undergoes a “partial programmed cell death.” This highly selective degradation of cellular organalles eliminates the vacuole, cytoskeleton, ribosomes, Golgi bodies and nucleus. The endoplasmic reticulum becomes modified to form the sieve endoplasmic reticulum (SER) which lacks ribosomes. The plasma membrane survives the degradation process as does the SER, mitochondria (although they may become swollen), P-proteins, and plastids. These few remaining organelles take a parietal position along the edge of the SE. This emptying of the SE is essential to allow the unimpeded flow of water, signal proteins, mRNA, and photoassimilates which travel through the SE. During SE maturation, the cell walls connecting adjacent SEs become modified to form sieve plates. These sieve plates are modified cell walls with plasma membrane lined pores which allow the phloem stream to pass from one SE to the next. The plasmodesmata, which symplastically connect the SE to the CC, become modified to form the pore-plasmodesma (PPUs). These PPUs are branched tunnels on the CC end and converge to form a single tunnel on the SE end. The PPUs play an integral role in maintaining the SE in a partially dead state by connecting it to the CC. The CC remains in a fully intact state and plays a life support role by channeling necessary biomolecules from the fully functioning CC to the SE.

phloem







P-Proteins and Callose: The Damage Control Team

P-Proteins
The P-proteins are phloem proteins whose function is not yet completely understood. They are believed to play in “damage control” when phloem is damaged. Upon the disruption of a sieve element, the P-proteins quickly aggregate at the sieve plate to form a “clot” which prevents the leakage of phloem exudates through the wound. PP1 and PP2, phloem proteins from pumpkins, are examples of P-proteins which appear to be synthesized in CC and transported to SE. PP2 is not present in early stages of development but begins to be produced by the CC and transported to the SE later in development.

Callose
Callose, a Beta-1-3-Glucan, is a carbohydrate which is also deposited at the sieve plate in response to injury. However, there is some question as to whether the callose is deposited immediately in response to injury or if it is a secondary response to the manipulation required to view a sieve element microscopically. Accumulation of callose also occurs in old, non-functional sieve elements and likely seals the old elements closed to keep the phloem translocation stream active.








This animation depicts the response of a sieve element to mechanical damage. Damage to the sieve element causes a reduction in turgor pressure as phloem contents (which are under pressure according to the "mass flow" transport) escape from the cells. An immediate response by the sieve element is necessary to prevent the loss of phloem contents. First, the P-plastids rupture and release starch grains which accumulate on the sieve plate. Next, the P-proteins are released from their parietal position and accumulate at the sieve plate forming a "clot."











Phloem Loading/Unloading: Why the plasma membrane must remain intact.

In contrast to xylem transport, the “mass flow” mode of phloem transport requires that the plasma membrane remain intact. At the source end of the phloem (area where sugar is synthesized), sugars are moved into the phloem sieve elements. This increase in solute decreases the water potential of the cell and causes water to flow in from surrounding areas by osmosis. The increase in the volume of water in the cell causes an increase in pressure which forces the sugar/water/amino acid solution to move toward the sink tissue. At the sink tissue, the sugars are off-loaded which increases the water potential and causes water to flow out of the phloem by osmosis. The sieve elements must remain at least partially alive and keep a functioning plasma membrane in order to help control the flow of sugars into and out of the sieve element.

Symplastic or Apoplastic Loading?

Two distinct methods can be employed by plants to move sugars into the phloem. Symplastic loading involves the movement of sugars through the plasmodesmata from one cell to another. Apoplastic loading involves the movement of sugars from the apoplast (the extracellular cell wall space) across the plasma membrane and into the cell. This movement of sugar against a concentration gradient is accomplished by sugar transporters in the plasma membrane such as SUC2.

SUC2 is a phloem specific plasma membrane sucrose transporter localized to the plasma membranes of sieve elements and/or companion cells depending on species and plays a role in apoplastic sucrose loading. In Arabidopsis, this transporter is required for the completion of the plant’s normal life cycle. Loss-of-function of the SUC2 transporter results in severely impaired sugar transport.

Another sucrose transporter, SUT1, is localized to the plasma membrane of sieve elements. SUT1 is synthesized in the companion cells, but must move through the plasmodesmata to the sieve element before it can perform its function.







Phloem loading diagram depicting two pathways for sucrose to enter the sieve element. In path A, sucrose is pumped across the plasma membrane from the cell wall space by sucrose transporters (apoplastic loading). In path B, sucrose is moved into the companion cell and sieve element through plasmodesmata (symplastic loading).