The ecophysiology of phloem loading

Key reference: Factfile # 8: The Ecophysiology of Phloem Loading

frequency

The number of plasmodesma, along or associated with a particular cell wall or cell wall interface, and expressed as plasmodesma per m cell wall interface or plasmodesma per m wall contact area.

Definition of transport zones

sieve element-companion cell complex

Frequency relative to loading type

loading strategiesDecreasing plasmodesmal frequency (A -> C) allows for increased concentration of sugars within the sieve element companion cell complex.

A

B

C

symplasmic:polymer trap model (Turgeon)

apoplasmic

symplasmic

Apoplasmic vs. symplasmic pathways

Mesophyll cell

Bundle Sheath cell

VascularParenchymacell

Sieve element/companionCell complex with pore pd.

Symplasmic disjunction. ‘uploading’ againstConcentration gradient is possible here

Raffinose converters vs sucrose loaders

In many species with high pd frequencies between the vascular parenchyma cells and the cc-se complex, evidence suggests that sucrose is converted to raffinose-family sugars. This so called ‘polymer trap’ loading method was first described by Turgeon. Conversion to a larger form of sugar means that the molecule is ‘trapped’ within the cc-se complex, effectively increasing osmotic potential and transport capacity

Definitions of phloem loading types

TYPES PLASMODESMAL ABUNDANCE

COMPANION CELL ULTRASTRUCTURE

LOADING METHOD

1 Many plasmodesma at all cell interfaces between Mesophyll and cc-se complex

Intermediary cells symplasmic

2a Few plasmodesma Companion cells may have extensive vesicular substructure

apoplasmic

2b cc-se is apparently apoplasmically isolated

Companion cells modified with wall ingrowths (transfer cells)

apoplasmic

2c High frequencies of plasmodesmata to bundle sheath, few internal of this and the cc-se may be isolated

Companion cells apoplasmic

Symplasmic vs. apoplasmic?

Loading from the mesophyll

In symplasmic loading, all sugars are assumed to travel down a concentration gradient, and is transported as sucrose.

Apoplasmic phloem loading

In apoplasmic phloem loading species, the sugars must be loaded metabolically, either at a vascular parenchyma-vascular parenchyma cell interface, or at a VP to cc-se interface.

Hordeum vulgare

Loading occurs along a gradient that exists between cells and can involve a number of individual compartments

Clearly, it is possible that DISJUNCTIONS can force particular pathways, or different loading processes. This leads to the evolution of a LOADING STRATEGY which may have ecological significance.

Stress-driven loading processes.

symplasmic

apoplasmic

apoplasmic

apoplasmic

Sieve elementCompanion cellcomplex

UNLOADING(sink)

LOADING(source)

A final word…mirror images & seasonal shift