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.

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 of the tip, carbon skeletons are delivered to the rapidly dividing and expanding cells within this root. In Roots….

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….. PROCAMBIUM 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

What do we know?  Solutes move from source to sink sinkslocal or distant  That sinks may be local or distant sink strength  That sink strength is a contributes to controlling or is perhaps, the controlling factor in regulation of transport capacity symplastic apoplastic mixed modeThat the system could be symplastic, apoplastic or mixed mode Phloem Transport Mechanisms

Phloem loading 1. Phloem loading mechanisms Phloem transport 2. Phloem transport mechanisms Phloem unloading 3. Phloem unloading mechanisms Imperative to distinguish between

The Loading Process: Essentially, can follow a passive pathway. or 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 WOULD have an effect upon the process

A Passive Transport process.. Metabolic inhibitors would/should not have an effect upon the process

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 << , and hence, turgor-related changes Conundrum!

The makings of the channel: Callose formation on sieve plates in the phloem of Saccharum officinarum Phloem sieve tubes: Highly specialised - Function under pressure (why?) therefore need control and regulatory mechanisms. Callose is one controlling mechanism This is fun!

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

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 Companion cells Sieve tube

Vascular Tissue in Roots 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 E P X ST

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, (stained blue) and large number of lateral sieve area pores in the sieve tube member to the right This view, typifies TRANSPORT PHLOEM, where there may be many connections between the companion cells and sieve tube members Sieve tube members Fibers P-Protein From: Raven, Evert and Eichhorn.

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

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. 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. Plasmodesmata in short-distance transport.

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, OSMOTIC POTENTIAL 1. Those where OSMOTIC POTENTIAL is the driving force ENERGY TRANSFORMATIONS 2. Those where ENERGY TRANSFORMATIONS are necessary Distinguish between loading, transport and unloading parenchyma and the sieve element- companion cell complex?

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 phloem loading is apoplasmic. Ultrastructural Investigations

Phloem transport - visualization

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

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

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

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

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. So, what works?

Finis Well ain’t that something!!