1. The ecophysiology of phloem loading Key reference: Factfile # 8: The Ecophysiology of Phloem Loading

2. 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.

3. Definition of transport zones sieve element- companion cell complex

4. Frequency relative to loading type

5. loading strategies Decreasing 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

6. Apoplasmic vs. symplasmic pathways Mesophyll cell Bundle Sheath cell Vascular Parenchyma cell Sieve element/companion Cell complex with pore pd. Symplasmic disjunction. ‘uploading’ against Concentration gradient is possible here

7. 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

8. Definitions of phloem loading types TYPESPLASMODESMAL ABUNDANCE COMPANION CELL ULTRASTRUCTURE LOADING METHOD 1 Many plasmodesma at all cell interfaces between Mesophyll and cc-se complex Intermediary cellssymplasmic 2a Few plasmodesmaCompanion 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

9. Symplasmic vs. apoplasmic?

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

11. 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.

12. 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.

13. Stress-driven loading processes.

14. symplasmic apoplasmic

15. Sieve element Companion cell complex UNLOADING (sink) LOADING (source) A final word…mirror images & seasonal shift