10-Translocation in the Phloem

8/12/2019 10-Translocation in the Phloem

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Translocation in

the Phloem


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Land colonization Prompted greater shoot growth to reach and

compete for sunlight Prompted development of a deeper root system SEPARATES PH OTOSYNTH ESI ZI NG

REGI ONS F ROM AREAS WH ERE SUGARSARE USED

REQUI RES A DRI VI NG FORCE F OR TH I SL ONG-DI STANCE TRANSPORT


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Phloem transport

A highly specialized process for redistributing: Photosynthesis products Other organic compounds (metabolites,

hormones) some mineral nutrients

Redistr ibuted from SOURCE SINK


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Phloem transport: Sources andsinks

Source : Any exporting region that produces photosynthate

above and beyond that of its own needs Sink :

any non-photosynthetic organ or an organ thatdoes not produce enough photosynthate to meetsits own needs


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Source-sink pathways followpatterns

Although the overall pattern of transport can bestated as source to sink

Not all sources supply all sinks in a plant

Certain sources preferentially supply specific sinks

In the case of herbaceous plant, such as sugar-beet,

the following occurs:


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Proximity :

of source to sink is a significant factor. Upper nature leaves usually provide photosynthesis

products to growing shoot tip and young, immatureleaves

Lower leaves supply predominantly the root system Intermediate leaves export in both directions

Development : Importance of various sinks mayshift during plant development

Roots and shoots major sinks during vegetative growth But fruits become dominant sinks during reproductive

development


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Vascular connections :

Source leaves preferentiallysupply sinks with direct vascular connections

A given leaf is connected via vascular system to leavesabove and below it on the stem

M odif ications of translocation pathways : -Interference with a translocation pathway bymechanical wounding (or pruning)

vascular interconnections can provide alternate pathwaysfor phloem transport


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Exactly what istransported in

phloem?


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What is transported in phloem?


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Carbohydrates transported in phloem are all nonreducing sugars.

This is because they are lessreactive

Reducing sugars, such asGlucose , M annose andFructose contain an exposedaldehyde or ketone group

Too chemically reactive to betransported in the phloem

Sugars that are not generally inphloem


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ther compounds Water!!!!!!!!! Nitrogen is found in the phloem mainly in:

amino acids (Glutamic acid) Amides (Glutamine)

Proteins (see later)


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Phloem Structure

The main components of phloem are sieve elements companion cells .

Sieve elements have no nucleus and onlya sparse collection of other organelles .Companion cel l provides energy

so-named because end walls are perforated - allows cytoplasmic

connections between vertically-stackedcells .

conducts sugars and amino acids - fromthe leaves, to the rest of the plant


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Phloem transport requiresspecialized, living cells

Sieve tubes elements join toform continuous tube Pores in sieve plate between

sieve tube elements are open

channels for transport Each sieve tube element is

associated with one or morecompani on cel ls .

Many plasmodesmata penetratewalls between sieve tube elementsand companion cells

Close relationshi p, have a r eadyexchange of solutes between thetwo cells


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Phloem transport requiresspecialized, living cells

Compani on cel ls : Role in transport of

photosynthesis products from producing cells in matureleaves to sieve plates of thesmall vein of the leaf

Synthesis of the various proteins used in the phloem

Contain many, manymitochondria for cellularrespiration to provide thecellular energy required foractive transport

There ate three types Ordinary companion cells Transfer cells Intermediary cells


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Types of companion cells

Ordinary Companion cel ls : Chloroplasts with well developed thylakoids, smooth

inner cell wall, relatively few plasmodesmata. Connected only to its own sieve plate

Transfer cel ls : Well developed thylakoids Have fingerlike cell wall ingrowths increase surface

area of plasma membrane for better solute transfer. Both of these types are specialized for taking up

solutes from apoplast or cell wall space


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Types of companion cells

I ntermediary cel ls : Appear well suited for taking up solutes via cytoplasmic

connections Have many plasmodesmata connects to surrounding cells

M ost character istic f eature Contain many small vacuoles Lack starch grains in chloroplast Poorly developed thylakoids

Function in symplastic transport of sugars frommesophyll cells to sieve elements where no

apoplast pathway exists


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Types of sieve elements


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Protective mechanisms inphloem

Sieve elements are under high internal turgor pressure

When damaged the release of pressure causes the

contents of sieve elements to surge towards the damagesite Plant coul d lose too much of the hard worked for sugars if not f ixed

Damaged is caused by Insects feeding on manufactured sugars Wind damage, temperature (hot and cold) Pollution causing a change in light wavelength


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Protective mechanisms inphloem

P proteins : Occurs in many forms (tubular, fibrillar, chrystaline

depends on plant species and age of cell) Seal off damaged sieve elements by plugging up the

sieve plate pores Short term solution

Callose : Long term solution This is a b-(1,3)-glucan, made in functioning sieve

elements by their plasma membranes and seals offdamaged sieve elements


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The mechanism ofphloem transport

The Pressure-Flow Model


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The Pressure-Flow Model Translocation is thought to move at

1 meter per hour Diffusion too slow for thi s speed

The flow is driven by anosmotically generated pressuregradient between the source and the sink .

Source Sugars (red dots) is actively

loaded into the sieve element-

companion cell complex Call ed phl oem loading

Sink Sugars are unloaded

Call ed phloem unl oading


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The Pressure-Flow Model So, the translocation pathway has

cross walls Allow water to move from xylem to

phloem and back again If absent- pressur e dif ference f rom

source to sink would qui ckly

equilibrate Water is moving in the phloem

by Bulk F low No membranes are crossed from

one sieve tube to another Solutes are moving at the same rate

as the water Water movement is driven by

pressure gradient and NOT water potential gradient


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Phloem Loading:Where do the solutes come from?

Triose phosphate formed from photosynthesis during the day ismoved from chloroplast to cytosol

At night, this compound, togetherwith glucose from stored starch,is converted to sucrose

Both these steps occur in amesophyll cell

Sucrose then moves from themesophyll cell via the smallestveins in the leaf to near the sieveelements

Known as short distance pathway only moves two or three cells


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In a process called sieve elementloading , sugars are transported intothe sieve elements and companioncells

Sugars become more concentratedin sieve elements and companioncells than in mesophyll cells

Once in the sieve element/companion cell complex sugars aretransported away from the sourcetissue called export

Translocation to the sink ti ssue iscall ed long distance transport


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Movement is via either apoplast orsymplast

Via apoplastic pathway requires

Active transport against itschemical potential gradient

I nvolves a sucrose-H + sympor ter The energy dissipated by protons

moving back into the cel l is coupledto the uptake of sucrose


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Symplastic phloem loading Depends on plant species

Dependant on species that tr ansport sugars other than sucrose Requires the presence of open plasmodesmata between

different cells in the pathway Dependant on plant species with intermediary companion

cells


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Phloem unloading Three steps (1) Sieve element unloading :

Transported sugars leave the sieve elements of sink tissue (2) Short distance tr ansport :

After sieve element unloading, sugars transported to cells in the sink by meansof a short distance pathway

(3) stor age and metabol ism : Sugars are stored or metabolized in sink cells


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Phloem unloading Also can occur by symplastic or apoplatic pathways

Varies greatly from growing vegetative organs (root tips and youngleaves) to storage tissue (roots and stems) to reproductive organs Symplastic : Appears to be a completely symplastic pathway in young dicot leaves

Again, moves through open plasmodesmata


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Phloem unloading Apoplastic : three types

(1) [B] One step, transport from the sieve element-companion cell complex to successive sink cells, occurs inthe apoplast.

Once sugars are taken back into the symplast of adjoining

cells transport is symplastic


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Phloem unloading Apoplastic : three types

(2) [A] involves an apoplastic step close to the sieveelement companion cell.

(3) [B] involves an apoplastic step father from the sieveelement companion cell

Both involve movement through the plant cel l wall


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Summary

Pathway of translocation : Sugars and other organic materials are conducted

throughout the plant in the phloem by means of sieveelements

Sieve elements display a var iety of str uctural adaptati onsthat make them well sui ted for transpor t

Patterns of translocation : Materials are translocated in the phloem from sources

(usually mature leaves) to sinks (roots, immatureleaves)


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Summary

M ater ials translocated in phloem : Translocated solutes are mainly carbohydrates Sucrose is the most common translocated sugar

Phloem also contains: Amino acids, proteins, inorgani c ions, and plant hormones

Rate of translocation :

Movement in the phloem is rapid, well in excess of ratesof diffusion Average velocity is 1 meter per hour

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10-Translocation in the Phloem