A plant in which all naturally occurring pigments have been replaced by a non-natural pigment found in fish can still convert light into energy through the process of photosynthesis, according to a recent study published in eLife.
The result overturns the accepted theory that organisms cannot photosynthesise without the pigment beta-carotene and raises the possibility of engineering photosynthesis to maximise the productivity of crops.
Carotenoids, the pigments responsible for the orange, yellow and red colour in fruit and vegetables, are essential components of our diets. They are precursors to vitamin A, which supports healthy vision, immunity and reproduction. These pigments are also essential for photosynthesis – the process that sustains virtually all life on Earth by transforming solar energy into food and fuel and providing the oxygen we need to breathe.
Two types of carotenoids are present in cells’ photosynthetic structures – carotenes and xanthophylls. Xanthophylls are associated with the light-harvesting part of the photosynthetic machinery, while carotenes are linked to the components that convert the light into energy. Although organisms with photosynthetic systems lacking xanthophylls have been reported, the same cannot be said for carotenes.
“There are around 1,000 different carotenoid species in nature, but one of them – beta-carotene – is present in all organisms that photosynthesise, leading to the belief that this pigment is essential for photosynthesis,” explains lead author Pengqi Xu, who was a PhD student at Vrije Universiteit Amsterdam, The Netherlands, at the time the study was carried out. “However, this assumption has never been verified, because until now we have not had mutant organisms available to study that completely lack all carotenoids – both xanthophylls and carotenes.”
For this study, the team engineered a tobacco plant called Asta to produce only a xanthophyll called astaxanthin (the pigment that gives salmon and shrimps their typical pink colour) and no carotenes at all.
They found that all the main photosynthesis proteins were present in the engineered plant, but in a different ratio as compared to the normal tobacco plants. Further analysis of the individual proteins in these systems showed that in the absence of beta-carotene, astaxanthin could substitute by binding to the proteins in some but not all of the places where beta-carotene would normally be. This means that photosynthesis can still work when some of the carotenoid binding sites remain ‘empty’.
When they compared the performance of photosynthesis in the engineered Asta tobacco plants with that of the naturally occurring, carotene-containing tobacco plants, they found that, at all light intensities and in both wild and mutant plants, the balance between the two photosystems was maintained.
As well as converting light, carotenes play an important role in protecting the sensitive photosynthesis machinery from light damage, so the team looked at whether the lack of all naturally occurring carotenoids affected the protective non-photochemical quenching (NPQ) mechanisms. They found that the onset and recovery of NPQ were identical in both engineered and natural plants, which they noted as being a surprising discovery because the Asta lacks both carotenoids thought to be essential for this process.
“Our results show that the photosynthetic system is exceptionally flexible and is able to respond to changes in the functionality of some of its components in a very efficient way,” concludes senior author Roberta Croce, Head of the Biophysics of Photosynthesis and Energy Group, at Vrije Universiteit Amsterdam. “This discovery is particularly important considering that the redesign of photosynthesis is one of the most promising avenues for improving the productivity of our crops.”
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