Topic: phloem loading

Presented by: Nasira Bashir

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CONTENT

• Phloem• Phloem loading• Munch hypothesis• SE-CC complex• Mechanisms for phloem loading• Apoplastic loading• Symplastic loading• Passive symplastic loading• Summary/conclusion• References

Phloem

Vascular tissue

Transportation of photosynthates from source to sink

Composed of many cells

Sclerenchyma

Parenchyma

Sieve element

Companion cells

laticifers

Internal structure of phloem

The movement of the sugar at the source is referred as phloem loading

ORIt is a process whereby carbohydrates (sugars) enter the sieve

tube at source.

Munch hypothesis

• A high concentration of sugar develops in phloem cell near the source.

• Phloem loading result in lowered water potential compared with adjacent xylem cells causing water to move from xylem to phloem by osmosis

• This influx of water creates a high turgor pressure near the source and lower turgor pressure near sink

• This causing the movement of water and sugar from the source to sink.

• Removal of sucrose at sink increases water potential causing water to move out of the sieve tube at the sink.

• Solutes move to sink cell and water goes back to xylem.

• Sieve element and companion cells are considered functional units.

• Numerous plasmodesmata connect sieve element to companion cells and the two cells are referred as sieve element/companion cell complex

Sieve element/companion cell complex

Mechanisms for phloem loading

• Source cell release sugar into the apoplast, from there it is actively loaded into the SE/CC complex.

Apoplastic loading

• Sugar diffuses via plasmodesmata from source cells to the SE/CC complex.

Symplastic loading

Apoplastic vs. symplastic loading

Apoplastic loading

• Sugar moves through plasmodesmata frommesophyll cells up to companion cells where itmoves into the apoplast and is actively loadedinto ordinary companion cells.

• Ordinary companion cells with cell wallingrowths have very few plasmodesmata andload sucrose from the apoplast

• Active uptake of sucrose from the apoplast by a companion cell takes place.

• The plasma membrane H+-ATPase pumps protons out of the cell, creating a proton gradient.

• The energy of this gradient drives the uptake of sucrose via an H+/sucrose sympoter.

Apoplastic and symplastic loading

Symplastic loading

• Intermediate companion cells are characteristic of symplasmic loader

• Many plasmodesmata that connect them photosynthesizing cells.

Polymer trapping model

• This model explains symplastic loading in plants with intermediary cells

• Sucrose, synthesized in the mesophyll, diffuses from the bundle sheath cells into the intermediary cells through the abundant plasmodesmata

• In the intermediary cells, raffinose is synthesized from sucrose and galaticnol, thus maintaining the diffusion gradient for sucose.

Polymer trapping model

• Because of large size, raffinose is not able to diffuse back into the mesophyll

• As a result the concentration of transport sugar rises in the intermediary cells and sieve elements.

Polymer trapping model

Passive symplastic loading

• Tran locate sucrose and have ordinary companion cells

• Possess abundant connections b/w the sieve element-companion cell complex and surrounding cells.

• Species with passive symplastic loading are characterized by high overall sugar concentrations in the source leaves, which maintain a concentration gradient b/w the mesophyll and SE/CC complex.

• The high sugar concentration give rise to the high turgor pressures in the sieve elements of the source leaves, generating the driving force for long-distance transport.

• Example apple ( Malus domestica )

• willow( Salix babylonica)

Patterns in apoplastic, symplastic loading and passive symplastic loading

Features Apoplastic loading Symplastic polymer trapping

Passive symplastic lo

Transport sugar sucrose raffinose and sytachyose

sucrose

Characteristic companion cells

ordinary intermediary ordinary

Number and conductivity of plasmodesmata connecting the SE-CC complex to surrounding cells

low high high

Dependence on active carriers in SE-CC complex

Transporters driven Independent of transporters

Independent of transporters

Overall concentration of sugar in source leaves

low low high

Cell types in which driving force for long-distance transport is generated

SE-CC complex intermediary cells mesophyll

Growth habit mainly herbaceous Herbs and woody species

mainly trees

Conclusion

• Phloem loading is very important phenomenon for the movement of sugars at source.

• Loading of sieve tubes from the cell walls requires energy which is derived by the proton gradient.

• Different mechanisms are involved in phloem loading

References

• L. Taiz, E. Zeiger Plant Physiology, 5th Edition, SinauerAssociates, Sunderland, U.S.A

• http://biologyforums.com/gallery/33_25_07_11_12_58_57.jpeg

• http://digital.library.unt.edu/ark:/67531/metadc30441/m1/21/med_res

• http://bio1903.nicerweb.com/Locked/media/ch36/36_17SucroseLoading.jpg