Do starving plants salvage carbon from their cell-wall polysaccharides?

Plant cell-wall polysaccharides (cellulose, hemicelluloses, pectin) account for a high percentage of the plant’s total carbon [1]. They are mainly ‘structural’ end-products of metabolism, whereas starch is a readily mobilisable reserve. However, this is an over-simplification.
When carbon-starved, plants salvage carbon (as glucose) from stored starch, and use it for respiration, ensuring survival. This project explores the emerging idea that structural polysaccharides (especially hemicelluloses) are also mobilised during severe carbon starvation and the released sugars used for staying alive.
Limited hemicellulose degradation is well documented in the thin primary walls, as required for cell-wall loosening during primary growth, e.g. in auxin-treated cereal coleoptiles. Also, certain seeds have hemicellulose-rich endosperms, providing carbon stores that are drawn on after germination, enhancing seedling growth before adequate photosynthesis becomes feasible.
It is less widely recognised that hemicelluloses in mature vegetative tissues can supply extra carbon when required. Nevertheless, this concept is supported in the literature [2,3]. For example, hemicellulose concentrations diminish in older conifer stem wood and in previous years’ needles, contributing carbon needed for new shoot growth.
Hemicelluloses are major plant constituents, constituting 10–30% of the plant’s total dry weight, and being the biosphere’s second most abundant organic material (after cellulose). Therefore, even moderate hemicellulose recycling within plants would be important for a stressed plant’s carbon relations as well as for the global carbon cycle.

This project will provide solid new data on hemicellulose mobilisation (especially in mature leaves) in ‘starving’ plants, and discover when, where and to what extent this occurs. Additionally, we will investigate the less explored idea that cellulose and pectin are also salvaged during carbon starvation.
When is a plant carbon-starved? Examples that the student can explore in this project are:
• Plants in deep shade with insufficient light for photosynthesis—
o A young tree ‘trying’ to establish in an existing forest might be expected to salvage as much carbon as possible from its leaves (for storage as starch in twigs) prior to autumnal leaf-fall, thus available for next spring’s bud-break [2].
o Herbs on the floor of a deciduous forest (e.g. bluebells and wood anemones), where there is only transiently (during spring) enough light before tree leaves expand and block the light. Such herbs might be expected to salvage maximal carbon from their leaves for storage in bulbs or rhizomes prior to leaf senescence.
• A plant which has lost foliage, e.g. by herbivory, so that insufficient photosynthetic machinery is left. [The ‘herbivore’ will be a pair of scissors, wielded by the student!]
• Finally, in model experiments, plant cell-suspension cultures will be grown with and without adequate carbon (sugar). Carbon-limited cultures end up with abnormally thin cell walls, suggesting possible wall polysaccharide mobilisation. We will use radiolabelling with 14C to distinguish whether this is due to decreased polysaccharide synthesis or increased degradation.
The project will thus make an important new contribution to our understanding of plant eco-physiology. Possibly unfamiliar techniques, e.g. polysaccharide analysis and radiolabelling, will be taught by Professor Fry during the project.

For more information on the Edinburgh Cell Wall Group, please see http://fry.bio.ed.ac.uk/index.html

Potential funding sources include:
The Darwin Trust, Edinburgh.
Commonwealth Scholarships.
BBSRC (Biotechnology & Biological Sciences Research Council)
See also http://fry.bio.ed.ac.uk/links.html