Life's a Gas - if you're a plant
How can we squeeze more from photosynthesis?
The majority of life on Earth is sustained by photosynthesis - plants’ ability to trap the energy in sunlight and use it to power the conversion of water and carbon dioxide into oxygen and sugar. It is the primary route for converting energy from the sun into chemical energy stored in molecules such as sugars, starches and oils. This stored energy can then be released, as needed, to power growth and development. Photosynthesis annually produces 100,000 million tons of plant material (much of it made only from carbon dioxide and water) and a similar quantity of oxygen. It consumes about 300 trillion k/cal per year (or 30 times the energy consumption of all the machines on the planet), but even so traps only 0.02% of the Sun’s energy that falls on the Earth.
Photosynthesis is a very efficient biological process and making it more efficient is a difficult challenge. We can effectively improve the efficiency of photosynthesis by changing the way that the products of photosynthesis (photosynthates) are used and distributed around the plant. Understanding the first steps of how sugars from photosynthesis are stored and processed is essential if we want to redirect sugar use. Although the general process has been known for some time it is only in the last few months that JIC scientists have discovered the actual pathways that plants use to channel sugars from photosynthesis into their general metabolism. We can now study how these pathways are controlled and how they each lead to particular end-products. Seeds and tubers typically contain a mixture of starches, oils and proteins, with the proportions varying among different plant species. Human intervention, through selection and breeding, has altered the proportions of these main components to suit our needs, for example increasing the seed oil content at the expense of starch. A better understanding of how these proportions are controlled will allow breeders to continue to increase the amount and quality of useful products that we can harvest from a crop, while reducing the proportion of by-products and waste.
Another route to improving the efficiency of photosynthesis is to select crop plants where more of the photosynthates are stored in parts of the plant we harvest. This approach has been very successful in breeding high-yielding wheat varieties and was the basis of the ‘Green Revolution’ of the 60s and 70s, when global cereal production doubled. By selecting for shorter (dwarf or semi-dwarf) plants breeders found they could dramatically increase grain yield because in shorter plants more energy is available for making grain, as less energy is used to produce stems and leaves.
Understanding how some plants, such as maize, use carbon dioxide and water
more efficiently than other plants (wheat and oilseed rape for example) may
be a third route to increasing the efficiency of photosynthesis in some crops.
Maize is a so-called C4 plant; C4 plants’ more efficient use of carbon
dioxide and water than C3 plants (wheat, oilseed rape) could make a C4 version
of a crop higher yielding than its C3 equivalent – especially under
drought conditions. The Moricandia family, relatives of the Brassicas,
includes some species that are C3 and others, such as Moricandia arvensis,
that are intermediate between C3 and C4 plants. We know that some of the cells
in the leaves of C3 – C4 intermediate species have a distinctive internal
structure that is not seen in C3 plants. Also a key enzyme, involved in respiration,
is active in some cells and not others, again a pattern not seen in C3 plants.
Together, these characteristics enable the C3 - C4 intermediate plants to
use a greater proportion of the carbon dioxide in the air than C3 plants can.
Under drought conditions the stomata in the leaf are closed to reduce water
loss, but this also reduces air exchange with the outside of the leaf. Because
C3 – C4 intermediate plants can use a greater proportion of the carbon
dioxide in the air they can continue to photosynthesise while saving water
– unlike C3 plants. Understanding how the C3 – C4 character is
controlled is the first step in potentially using it to develop versions of
common crop plants that may be higher yielding and require less water.