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Phloem and Sugar Transport

Phloem and Sugar Transport

Like many living organisms, plants have tissues that specialize in specific plant functions. For example, shoots are well adapted to rapid growth, while roots are structured to acquire resources efficiently. However, sugar production is primarily restricted to the photosynthetic cells that reside in the leaves of angiosperm plants. Sugar and other resources are transported from photosynthetic tissues to other specialized tissues by a process called translocation.

The Apoplast and Symplast

The Apoplast and Symplast

Plant growth depends on its ability to take up water and dissolved minerals from the soil. The root system of every plant is equipped with the necessary tissues to facilitate the entry of water and solutes. The plant tissues involved in the transport of water and minerals have two major compartments - the apoplast and the symplast. The apoplast includes everything outside the plasma membrane of living cells and consists of cell walls, extracellular spaces, xylem, phloem, and tracheids. The...

Xylem and Transpiration-driven Transport of Resources

Xylem and Transpiration-driven Transport of Resources

The xylem of vascular plants distributes water and dissolved minerals that are taken up by the roots to the rest of the plant. The cells that transport xylem sap are dead upon maturity, and the movement of xylem sap is a passive process.

Short-distance Transport of Resources

Short-distance Transport of Resources

Short-distance transport refers to transport that occurs over a distance of just 2-3 cells, crossing the plasma membrane in the process. Small uncharged molecules, such as oxygen, carbon dioxide, and water, can diffuse across the plasma membrane on their own. In contrast, ions and larger molecules require the assistance of transport proteins due to their charge or size. Transport across membranes also occurs within individual cells, playing a variety of essential roles for the plant as a whole.

Water and Mineral Acquisition

Water and Mineral Acquisition

Specialized tissues in plant roots have evolved to capture water, minerals, and some ions from the soil. Roots exhibit a variety of branching patterns that facilitate this process. The outermost root cells have specialized structures called root hairs that increase the root surface, thus increasing soil contact. Water can passively cross into roots, as the concentration of water in the soil is higher than that of the root tissue. Minerals, in contrast, are actively transported into root cells.

Tonicity in Plants

Tonicity in Plants

Tonicity describes the capacity of a cell to lose or gain water. It depends on the quantity of solute that does not penetrate the membrane. Tonicity delimits the magnitude and direction of osmosis and results in three possible scenarios that alter the volume of a cell: hypertonicity, hypotonicity, and isotonicity. Due to differences in structure and physiology, tonicity of plant cells is different from that of animal cells in some scenarios.

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Symplastic phloem loading in poplar.

Cankui Zhang¹, Lu Han², Thomas L Slewinski²

¹Department of Plant Biology, Cornell University, Ithaca, New York 14853 (C.Z., T.L.S., J.S., J.Z., R.T.); andForage Improvement Division, The Noble Foundation, Ardmore, Oklahoma 73401 (L.H., Z-Y.W.) cz46@cornell.edu ert2@cornell.edu.

Plant Physiology

|July 25, 2014

Summary

This summary is machine-generated.

Gray poplar transports sucrose into phloem sieve tubes via plasmodesmata, not active loading. This passive sugar transport mechanism was confirmed using genetically modified plants, contrasting with alfalfa

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Published on: September 15, 2015

Area of Science:

Plant Biology
Plant Physiology
Molecular Biology

Background:

Phloem transport drives sap movement via osmotic pressure gradients.
Sucrose loading into phloem sieve tubes can be active (energy-dependent) or passive.
Understanding these mechanisms is crucial for plant growth and resource allocation.

Purpose of the Study:

To experimentally determine the mechanism of sucrose loading in gray poplar (Populus tremula × Populus alba).
To investigate the role of plasmodesmata in phloem transport in trees.
To compare sucrose loading pathways between poplar and alfalfa (Medicago sativa).

Main Methods:

Generation of transgenic poplar and alfalfa plants expressing yeast invertase in cell walls to block sucrose entry.
Utilizing constitutive (35S) and minor vein-specific (galactinol synthase) promoters for invertase expression.
Assessing plant growth, carbohydrate accumulation, leaf chlorosis, and photosynthetic rates in transgenic and wild-type plants.

Main Results:

Transgenic poplar plants showed no inhibition of growth or loading, indicating sucrose uptake independent of the targeted pathway.
Alfalfa plants, with fewer plasmodesmata, exhibited typical loading inhibition symptoms when transformed.
Poplar leaves displayed defense responses at high temperatures, suggesting hexose involvement in the hypersensitive response.

Conclusions:

Sucrose enters poplar phloem passively through plasmodesmata, supporting a passive loading mechanism in trees.
Alfalfa utilizes a different, likely more active, sucrose loading pathway.
Hexose sugars may play a role in plant defense signaling.