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Plant Physiology

A novel shuttle for fatty acids

München, 02/04/2015

LMU biologists have discovered a previously unknown transport system for fatty acids in plant cells. The work may open new routes to improve the production of biofuels.

Unfertilized flower of an Arabidopsis plant, which forms defective pollen grains when FAX1 proteins are absent (Picture: Nannan Li/Katrin Philippar)

Fats and fat-like compounds – generically referred to as lipids – are essential for the growth and development of all organisms. They are involved in a wide range of important biological functions – as basic components of membrane systems, as signal molecules and energy stores and they form the protective wax layers found on plant leaves. Fatty acids are the major constituents of lipids. In plants, fatty acids are synthesized exclusively in the chloroplasts, and must be exported into the cell cytoplasm – the aqueous interior of the cell in which the chloroplasts are suspended – before they can be incorporated into lipid molecules. “However, the mechanism how fatty acids are transported across the envelope membranes of the chloroplast was so far unclear,” says LMU biologist PD Dr. Katrin Philippar. She and members of her research group have now shown that a member of a previously uncharacterized protein family plays a central role in this process.

The researchers identified an integral membrane protein in the inner chloroplast envelope membrane of the model plant Arabidopsis thaliana. They subsequently named the protein FAX1 (for ‘fatty acid export 1’) on the basis of studies on Arabidopsis strains that had either lost the capacity to produce FAX1 or synthesized FAX1 proteins in excess amounts. These investigations revealed that FAX1 is essential for the synthesis of the fatty acid- and lipid-rich layers that coat pollen grains and form the general waxy layer on the plant surface. In the absence of FAX1, the outer cell wall of the pollen grain is defective, with the result that male fertility is significantly reduced. In addition, in plants that are unable to express FAX1, the lipid content outside chloroplasts is reduced. In lines that overproduced FAX1, on the other hand, the researchers observed the opposite effect. In particular, levels of so-called triacylglycerol lipids (TAGs) were found to be significantly higher in leaves and flowers of these strains than in wild-type plants.

FAX1 is a chloroplast fatty-acid exporter
“Furthermore, we were able to show that, when introduced into yeast cells, FAX1 can transport fatty acids. On the basis of all these findings, we conclude that FAX1 in Arabidopsis mediates export of fatty acids across the inner envelope membrane of the chloroplast. Thus, our work elucidates a novel and previously entirely unknown mechanism of fatty acid transport,” Philippar explains. “FAX1 also has an influence – probably indirect – on carbohydrate metabolism in the cell, since lipids and carbohydrates serve as the primary sources of metabolic energy in plants,” she adds.

Philippar and her team now plan to characterize the rest of the FAX protein family in plants, with particular reference to their role in the metabolism of fatty acids, lipids and carbohydrate energy sources. Interestingly, the so-called TMEM14 proteins found in vertebrate mitochondria, whose biological role has remained unknown, are related to the FAX family. Further study of the FAX proteins therefore promises to throw new light on the function of these enigmatic gene products as well.

The significance of FAX1 function for the synthesis of TAGs is of special interest, because TAG-rich plant oils provide the basis for the production of biofuels. “Our experiments indicate that overexpression of FAX1 increases the overall level of TAGs in Arabidopsis. So further investigation of the members of this protein family may lead to new strategies for the manufacture of biofuels,” Philippar concludes.

(PLOS Biology 2015)                     göd