Multi-tasking pigments

Carotenoids are organic pigments that play contrasting roles during photosynthesis in absorbing light energy and protecting plants from excess of light. Roberto Bassi’s research group now reveals that carotenoids have yet a new trick up their sleeve.

Plants and other photosynthetic organisms live in a catch-22 situation. “Plants produce oxygen but are also poisoned by oxygen,” says Roberto Bassi, an Italian plant physiologist who has been passionate about photosynthesis since his graduate degree at the Padua University Botanical Garden. Bassi’s research group at Verona University played a pivotal role in understanding the dual function of carotenoid pigments in absorbing light energy and protecting the photosynthetic machinery against light-induced damage by oxygen. Now his team has identified a new unexpected function for carotenoids in controlling the production of photosynthetic proteins. 

Carotenoids are organic pigments made by plants, algae, fungi and cyanobacteria that are found in all organisms (animals obtain them from food). Besides their fundamental roles in photosynthesis, carotenoids can act as plant hormones, vitamins, odours, colours (in fruits, flowers and bird feathers, for instance) and, in the eye retina, photo-protection. There are two types of carotenoids: carotenes, which give carrots their orange colour, and their oxygenated offshoots, the yellow xanthophylls. In plant leaves, carotenoids are normally masked by chlorophyll, but they put on a show in autumn as chlorophyll gets degraded and their striking orange and yellow colours are revealed.

In autumn, chlorophyll gets degraded and the yellow and orange colours of carotenoids are exposed.
(Credit: wikipediacommons)

Poisoned by light

Over the past 20 years, studies of plants and algae lacking each of the four main xanthophylls showed they have specific functions in photo-protection. Photosynthesis generates a waste product without which we couldn’t live: oxygen (O₂). But in the presence of excessive light, toxic forms of oxygen, called reactive oxygen species (ROS), are produced. Photosynthetic organisms have come up with clever ways of protecting themselves against photo-damage induced by ROS. Some plants can simply turn their leaves to minimise light absorption, but most invest in dissipating excess photons and getting rid of ROS. 

Xanthophylls protect the photosynthetic apparatus from photo-damage by scavenging ROS, by preventing its formation or by converting the excess light energy into heat. They are mostly found in the so-called ‘antenna’ protein-pigment structures, which absorb and transfer light energy to the photosynthetic reaction centres. And this is where the magic happens: light energy is converted into chemical energy, used to make organic matter from carbon dioxide removed from the atmosphere.

Without each of the xanthophylls, plants become photosensitive but are still able to grow. However, a new study led by Bassi and his colleague Luca Dall'Osto shows that simultaneously removing all four xanthophylls, but not carotenes, has dramatic effects for plants. “If you avoid the synthesis of xanthophylls the plant is not viable anymore,” Bassi says “It’s something really new and was completely unexpected.” The researchers blocked the synthesis of all xanthophylls in Arabidopsis thaliana, a weed widely used in research, by using a combination of genetic mutations they called nox (for ‘no xanthophylls’). They found that, in addition to their role in light absorption and photo-protection, xanthophylls are essential for plant development- the nox plants simply couldn’t grow. But this didn’t make sense. Previous studies showed that removing each xanthophyll at the time, or removing the antenna proteins to which they bind to, doesn’t affect plant growth. So why do these nox plants have such severe developmental defects?

Plants without xanthophylls (right panel) can be 'forced' to grow on a suger-rich medium, but they have severe growth defects and white leaves because they lack photosynthetic complexes. Left: normal plants. (Credit: Luca Dall'Osto)

A function at the core

To try and understand this, the researchers rolled up their sleeves and plunged into some serious biochemistry. When they analysed the proteins in the two photosynthetic reaction centres - photosystems I (PSI) and II (PSII) - they faced yet another surprise: the PSI core proteins had nearly completely vanished in the nox plants. This was strange because xanthophylls bind mainly PSII antenna proteins, which were mostly unaffected, but not PSI. “It was so surprising that we spent two years repeating the experiments to be sure we didn’t make a mistake,” Bassi recalls.

Chloroplasts, the home of photosynthesis, contain about 120 genes encoding for proteins of the photosynthetic machinery. Bassi and colleagues found that xanthophylls are important for the translation of some of this genetic information into PSI core proteins, such as the PsaA/PsaB unit, within chloroplasts. It was known that without the PsaA/PsaB central unit the PSI super-structure can’t assemble properly and eventually gets degraded, and this is exactly what the researchers see in the nox plants.

The absence of a functional PSI reaction centre in plants lacking xanthophylls explains their developmental defects, but it remains unclear why this pigment acts specifically on the synthesis of certain PSI core proteins. It is also difficult to imagine how xanthophylls, which are embedded in the chloroplast membranes, can act on the cell protein factories, the ribosomes, dispersed inside the chloroplast. Bassi believes that xanthophylls are chopped into smaller molecules that leave the membrane and interact with ribosomes to control the production of PSI core proteins, and his team is currently searching for these molecules. “There is some evidence that products from cleavage of other carotenoids are involved in development, now we know that products of carotenoids are needed for translation of genes,” he explains.

Roberto Bassi's research team. (Credit: Luca Dall'Osto)

An enlightened career

While part of Bassi’s team continues investigating carotenoids in Arabidopsis, the rest studies photosynthesis using his favourite model system: Chlamydomonas reinhardtii. This single-cell green alga is used in photosynthesis research because it grows faster than plants but their photosynthetic machineries are nearly identical. After a first postdoc in Copenhagen isolating antenna proteins from barley, Bassi reckoned that biochemistry would only take him so far in his research, so he spent the next years learning biophysics with Pierre Joliot in Paris and then molecular biology with Jean David Rochaix in Geneva, both experts and pioneers in Chlamydomonas photosynthesis research. Bassi then returned to Italy to set up his own lab to study photosynthesis in algae at Padua University, but finding little support there for his research he decided to leave and finally settled at Verona University. The idea of working with algae was not appreciated by the Italian research funding agencies, however, so eventually he was forced to switch model system from algae to plants.

Science funding in Italy: a political tragedy

“The funding situation for science in Italy is extremely bad and we live with money from Europe,” Bassi says. His team of 15 people currently runs on three European grants and some funding from the private sector. For over a decade, under the government of former Prime Minister Silvio Berlusconi, public research funding and university recruitments in Italy lacked transparency and a clear strategy. With the election of a new government in 2011, Italian scientists were hopeful the situation would improve, but because of the global economical crisis and the country’s huge public debt, Italy’s modest science budget suffered further cuts as part of the government’s austerity plan. As a result, despite recent reforms in the public science funding system, Italian research continues to rely considerably on European grants. Increasingly aware of this grim situation, thousands of Italian researchers leave the country every year with little expectations of going back. Did Bassi ever feel tempted to move abroad? In 2002 he took the plunge and moved to France to set up a lab in Marseille, but after three years he had to return to Verona for personal reasons.

Algae photo-bioreactor
(Credit: kaibara87/everystockphoto)

From algae to biofuels

In recent years the Italian government began to show some interest in the production of biofuels from algae, so after a hiatus of many years Bassi could work with Chlamydomonas again. “I like the field of bioenergy because the applied research and the basic research are very much the same thing,” he says. His team studies photosynthesis while trying to improve the efficiency of biofuel production in algae. Biofuels made from plants, such as corn or soybean, while initially regarded as a good alternative to fossil fuels like petrol, in reality cause great damage to the economy. The problem is that these crops take up a lot of land that would otherwise be used for agriculture, leading to food price inflation in the long run. Bassi’s team is part of the Sunbiopath international research consortium funded by the European Union, which aims at optimising biofuel production by photosynthetic algae for commercialisation. His team has already succeeded in genetically engineering algae strains that use light energy more efficiently, making them grow faster and thus generate more biofuel. The researchers are now trying to expand these results to the industrial scale. “I think this is a promising accomplishment and I’m very positive we could produce biofuels from algae.”

Reference:

Dall'Osto L., Piques M., Ronzani M., Molesini B., Alboresi A., Cazzaniga S. & Bassi R. (2013). The Arabidopsis nox Mutant Lacking Carotene Hydroxylase Activity Reveals a Critical Role for Xanthophylls in Photosystem I Biogenesis, The Plant Cell, 25 (2) 591-608. DOI: 10.1105/tpc.112.108621 

This article was published in Lab Times on 3-05-13. You can read it here.