Phloem Transport in Plants

Hypothesis:

The development of a highly specialised transport system was essential in order to enable plant species to develop, diversify and occupy the many different niches that they do.

In Roots….

Phloem transport requires the establishment of functional SIEVE TUBES , which must connect the SOURCE to the (local) SINK

In this onion root, the first formed PROTOPHLOEM sieve tubes mature quite close to the ROOT APEX. So, within about 1000 m m of the tip, carbon skeletons are delivered to the rapidly dividing and expanding cells within this root.

In Stems...

Development of the vascular system requires formation of VASCULAR BUNDLES. Here in

Cucurbita pepo

, you see

SIEVE TUBES

, with large

SIEVE PLATES

and underlying these, is the P Protein (phloem proteins) associated with many functional sieve tube members.

In Leaves…..

Vascular bundles develop from

PROCAMBIUM

Leaves may develop specialised photosynthetic layers such as the PALISADE, and

SPONGY

mesophyll depicted here, as well as PARAVEINAL MESOPHYLL is highly specialised in assimilation and solute retrieval

Phloem Transport Mechanisms

What do we know?

 Solutes move from source to sink  That sinks may be local or distant  That sink strength is a contributes to controlling or is perhaps,

the

controlling factor in regulation of transport capacity •That the system could be symplastic , apoplastic or mixed mode

Imperative to distinguish between

1.

Phloem loading

mechanisms 2.

Phloem transport

mechanisms 3.

Phloem unloading

mechanisms

The Loading Process:

Essentially, can follow a passive pathway .

or

could involve an active (accumulating) step .

In the first instance,

there may be no energy or thermodynamic demands

placed upon the system.

In the second instance,

ATP, NADPH +

would be needed directly to drive co-transport across membranes

Uphill….

The Transport process:

Phloem transport can be viewed as an entirely passive process , which makes no demands upon the energy cycles of the plant, other than energy required for the maintenance of plant membranes

The Transport process..

If transport is does not require energy input, then one could envisage an entirely bulk (

passive

) flow system, driven by concentration gradients established and maintained between the source and the sink Transport would thus be along, or down a

concentration gradient

An Active Transport process..

The alternative, is a mechanism of phloem transport which is

an active process This requires energy

( physiological or thermodynamic) in order to drive and maintain it. Here one would envisage ATP NADPH + or H + K + ion exchange as the driving force

NB.

Metabolic inhibitors effect upon the process

WOULD

have an

A Passive Transport process..

Metabolic inhibitors would/should not have an effect upon the process

Conundrum!

But, there can be little argument that

some energy has to be expended along the way

else a “leaky” system would develop, in which solute loss leads to << Yp , and hence, turgor-related changes

The makings of the channel:

Phloem sieve tubes:

Highly specialised Function under pressure (why?) therefore need control and regulatory mechanisms. Callose is one controlling mechanism Callose formation on sieve plates in the phloem of

Saccharum officinarum

This is fun!

Developmental Sequence

Complex interrelationship during the early stages. A MOTHER cell differentiates, to give rise to a member , and a sieve tube companion cell

Structural considerations of the mature phloem

Long files of cells are formed, joined by their cross walls. Cells designed for rapid longitudinal transport.

Structural considerations of the mature phloem

Sieve tubes are highly specialised cells essentially devoid of protoplasm at maturity (everything is parietally located) - the end walls of the cells are highly modified, and contain a number of sieve plate pores, through which substances travel from cell to cell.

Sieve tube Companion cells Phloem Functionality Sieve tubes are composed of files of

sieve tube members

, joined end to end via their cross walls These cross walls are highly specialised and form

sieve plates

, each of which contains many sieve plate pores

P ST

Vascular Tissue in Roots

E X

In the Root, sieve tubes are larger in diameter than their corresponding companion cells. This is typified, in this cross-section of a young

Rannunculus

root. This section typifies

UNLOADING PHLOEM

Sieve tube members Fibers P-Protein From: Raven, Evert and Eichhorn.

In Stems, the relationship of the sieve tube members to their companion cells is clearer. Here CC’s are narrower than their corresponding STM as in this example of

Tilia americana

Note the inclined, compound sieve plates This view, typifies

,

(stained blue) and large number of lateral sieve area pores in the sieve tube member to the right

TRANSPORT PHLOEM,

where there may be many connections between the companion cells and sieve tube members

Remember..

There is a requirement for transport between all organs within the plant. Here we see the similarity between the transport pathway in salt glands, the leaf, and the root

Phloem-related Transport

Phloem Loading…

Can follow an entirely symplastic pathway or have a specific apoplastic disjunction

Plasmodesmata in short-distance transport.

In actively-loading and unloading systems, sugars are loaded at a SOURCE, then transferred to the loading phloem, then moved into the long-distance transport phloem, and are released at metabolically active SINKS.

Local Sinks can occur along the way.

The potato tuber (

Solanum tuberosum L

.) acts as a SOURCE and a SINK, depending on requirements

Mechanisms ?

Simple or Complex?

Clearly can be placed in one of two categories, 1. Those where OSMOTIC POTENTIAL is the driving force 2. Those where ENERGY TRANSFORMATIONS are necessary Distinguish between loading, transport and unloading parenchyma and the sieve element companion cell complex?

Ultrastructural Investigations Barley is one of the most studied crop plants, world-wide. Yet, it is only recently that we have gained clear knowledge of cell structure, and the plasmodesmatal frequencies, along the loading pathway from mesophyll to sieve tube. We now recognise thick-walled (solid dots) and thin-walled (open circles) metaphloem.

Plasmodesmatal frequencies

tell us a great deal about the cell-cell pathway. Clearly, low frequencies at CC-ST interfaces, indicate that is apoplasmic.

phloem loading

Phloem transport - visualization

Phloem transport – Organization & mechanics Tying things down… From Ehlers, et al., 2000 Protoplasma 214: 80 92. Used with author’s permission

Variations in structure

young tissues show this ‘good fixation shows this not seen in any EM micrographs Poor fixation shows this

Phloem transport mechanisms

There are several, some require energy inputs, others do not.

Integrated Transport..

Xylem and phloem dependency.

Electro osmosis

Note ion gradient is necessary, else system will not function

Trancellular strands

Strands “peristaltic” squeeze substances along tubules

Facilitation of bi-directional transport

Sucrose, co-transport

So, what works?

It is simple

…

Pressure flow

, regulated by a

difference in osmotic potential

, along the transport gradient. This will work, PROVIDED accumulation does not attain equilibrium along the gradient.

Well ain’t that something!!

Finis