Ancient bacteria improve photosynthesis

Photosynthesis is the process by which plants and some bacteria use water, carbon dioxide and the energy in sunlight to produce sugars to feed themselves, giving off oxygen as a by-product. In plants this process takes place in structures in the leaves called chloroplasts, which contain the green-coloured substance chlorophyll. But the chlorophyll plants make can capture only a certain part of the light spectrum, meaning some light is wasted. Certain ancient microbes though - called cyanobacteria - have a different form of chlorophyll that can collect this light and putting it into modern plants might make them grow much more efficiently. Connie Orbach spoke to Penn State University's Donald Bryant to hear how...

Donald - Cyanobacteria are a group of photosynthetic bacteria that make oxygen from water like higher plants. They are the organisms which gave rise to chloroplasts in higher plants and they continue today to be the most successful of all photosynthetic bacteria. Some cyanobacteria have the capacity to grow in far red light. Those are longer wavelengths, lower energy than we can see with our eyes, and use that light energy also to drive water oxidation and oxygen evolution. So, they're capable of doing something that plants can't do presently and do it in such a way that it's beneficial to them.

Connie - So, plants only use the visible spectrum. Is that right?

Donald - Yes, that's generally correct. They use blue light to red light, the visible spectrum, and presently, plants very inefficiently use any wavelengths longer or shorter than those. So, one of the goals of plant molecular biology is to increase light utilisation. One way to accomplish that would be to introduce the capacity to use far red light into plants.

Connie - So, how do the cyanobacteria use this different part of the light? Well, they use a special type of chlorophyll, the molecule that traps and absorbs sunlight. This is known as chlorophyll F and it can absorb far red light. But what Donald and his team have now discovered and what takes us one step further in actually using this amazing property is the enzyme required to make it, chlorophyll F synthase.

Donald - We identified candidate genes by making mutations in the cyanobacteria that can perform this far red light photosynthesis with the expectation that if we found the correct gene, they would not synthesis chlorophyll F and they would be unable to grow in far red light. We identified in two separate organisms genes that had those properties, taking that gene then and expressing it in a cyanobacterium which normally cannot grow in far red light, allowed that organism to synthesise chlorophyll F, confirming unequivocally that the gene that we had identified as responsible for making chlorophyll F synthase.

Connie - So, something just as simple as this one gene will confer into another plant the ability to create chlorophyll F.

Donald - It is a simple single gene product and that's one of the beauties of this is that it should be relatively simple to produce chlorophyll F in plants. Whether or not those plants will be able to productively use the chlorophyll F that is made is something that will have to be studied and perhaps adjusted. But nevertheless, making the chlorophyll should not be all that difficult.

Connie - And if you could get this gene working in higher plants to a point that they could use it and they could then use the far red light, what sorts of difference is this going to make in terms of us growing plants globally? How much difference can it really make?

Donald - That's an excellent question. The amount of light that's available between 700 and 800 nanometres, the region that is covered by the absorption of chlorophyll F, accounts for about 25 per cent of the visible light. So in principle, it would add about 25 per cent more light available to the plant for growth. So, I would say that potential is substantial.