Researchers prove how plants transport sugars - could lead to plants with increased photosynthetic rates, yields, CO2 intake

Using genetic engineering techniques, Cornell University researchers have finally proven a long-standing theory of how plants ship sugars from their leaves to flowers, roots, fruits and other parts of their structure. The findings could lead to the development of plants with increased photosynthetic rates, yields and carbon dioxide intake - all of which are highly relevant to the bioenergy and bioproducts sector.

The theory of transporting sugar, the 'polymer trap model', was first proposed in 1991 by Robert Turgeon, Cornell professor of plant biology. He is also the senior author of the latest research published in the Proceedings of the National Academy of Sciences. Ashlee McCaskill, Ph.D. '07, who worked in Turgeon's lab, is the paper's lead author.

Turgeon's theory suggested that as sucrose, a form of sugar, accumulates in leaves as a product of photosynthesis, it diffuses into the plant's tubelike transport tissue, called phloem, along with other nutrients to move to other areas of the plant. Once in the phloem, small molecules of sucrose polymerize, or combine, to form larger, more complex sugars, which become too large to flow back into the leaf. The polymerized sugars are then forced to move away from the leaf to parts of the plant where they may be used or stored.

To prove the theory, Turgeon and McCaskill genetically engineered a plant closely related to a member of the figwort family, purple mullein (Verbascum phoeneceum L.), so that two genes involved with polymerizing sucrose into larger molecules were silenced. When they did so, sugars backed up in the leaves.

In normal plants, when sugars (made from water and carbon dioxide during photosynthesis) accumulate in the leaves, photosynthesis slows down, and the plant does not take in as much carbon dioxide from the air. Likewise, when the sugars move out of the leaves, the rate of photosynthesis and carbon intake increases, McCaskill said:
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"If we could increase the plant's phloem-loading rate, the potential would be to increase photosynthetic rate and yield, but that is theoretical right now," said McCaskill.

A 2006 article in the journal Science, McCaskill said, showed that when atmospheric carbon dioxide increases, plants do not take in the excess due to a series of feedback loops that constrain the plant.

"Phloem loading is one of these feedbacks that have an effect on the ability of plants to intake carbon dioxide at the highest level," said McCaskill. Carbon dioxide, which is increasing in the Earth's atmosphere, is the major greenhouse gas that traps heat and warms the planet, McCaskill noted.


The new insights add to knowledge that could lead to the development of next-generation dedicated energy crops suited for both the production of fuels and energy, as well as for the production of negative emissions.

When crops are designed to take up more CO2 in their tissue, they become interesting for use in 'carbon negative' energy systems. Such systems utilize biomass to produce energy, while the carbon contained in it is captured before, during or after the combustion of the fuel, after which the greenhouse gas is sequestered geologically.

Such 'bio-energy with carbon storage' (BECS) systems go much further than wind, solar, nuclear or ordinary bioenergy - which are merely 'carbon neutral' energy sources. In fact, BECS systems take CO2 out of the atmosphere. Now the higher the CO2 uptake of energy crops used in BECS systems, the more carbon dioxide can be taken out of the atmosphere and thus the more value is created (as CO2 fetches a price on the market).


Picture: The plant on the left is the normal (wild type) Verbascum phoenecium L. The plant on the right is a transgenic Verbascum, modified so that sugars do not form into larger, more complex sugars. The yellowing is the result of the plant's inability to export sugars from the leaf. Credit: Ashlee McCaskill.

References:
Ashlee McCaskill and Robert Turgeon, "Phloem loading in Verbascum phoeniceum L. depends on the synthesis of raffinose-family oligosaccharides", PNAS, December 4, 2007, vol. 104, no. 49, pp. 19619-19624, DOI: 10.1073/pnas.0707368104

Cornell Chronicle: Research on how plants transport sugars could be of critical importance in era of global warming - December 20, 2007.