New bioenergetic organelle found in plants

A chromoplast is a plant organelle characterised by accumulating carotenoids, the pigments that confer yellow, orange and red colours to many flowers, fruits and roots. Besides their role in carotenoid synthesis, the chromoplast is a very active organelle involved in different biosynthetic processes during fruit ripening. According to the new study, chromoplasts are also able to produce chemical energy -- in other words, to synthetize molecules of adenosine triphosphate (ATP) -- by means of a respiratory process named chromorespiration.

Chemical energy: mitochondrion, chloroplast and chromoplast

To date, it was thought that the energy needed by the chromoplast came from the mitochondrion. However, the new study -- emphasized on the section On the Inside of the journal Plant Physiology -- reveals that the chromoplast is the third component that behaves as bioenergetic organelle in plant cells."

Professor Azcón-Bieto explains that "chromorespiration or respiratory activity in chromoplasts is a process of chemiosmotic ATP synthesis. In other words, it is a membrane process based on a respiratory chain that creates a proton electrochemical potential gradient and joins ATP synthesis like in mitochondria and chloroplasts."

Traditionally, chromoplasts were considered senescent cell components, but they are not. In previous studies, the UB research team described how chromoplasts are able to synthetize lipids without ATP. "The chromoplast -- affirms Professor Albert Boronat -- is an organelle that derives from chloroplasts; it is photosynthetically inactive but metabolically very active, even if its functions are not well known." The present study first links the mechanism of ATP synthesis to a complete respiratory process in the inner part of the organelle.

Chromoplasts: maximal activity during fruit ripening

Chromoplasts are abundant in fruits like tomato, pepper and citrus. In the study, UB experts studied tomato (Solanum lycopersicum) fruit ripening, a process in which chromoplasts' metabolic activity shoots up. During tomato ripening, energetic production (ATP) in chromoplasts is used to produce molecules (carotenoids such as lycopene and beta-carotene, scents, etc.) in order to improve fruit's properties. Signs of chromoplasts' energetic production have been also found in red bell pepper.

Although it shares some characteristics with mitochondria and chloroplasts, chromoplasts' ATP synthesis process has some particular traits. According to UB researcher Marta Renato, first author of the paper and CRAG member, "in the chromoplast, there is a change in a regulating subunit of ATP synthase, which is the enzyme that catalyses ATP synthesis. In the case of chloroplast, which has photosynthetic ability, this enzyme is always active, and it seems that the change in the sub-unit is what makes it possible."

An evolutive view to cyanobacterium

In chromoplasts, researchers have also identified the presence of cytochrome c₆, a characteristic element of cyanobacterium. Marta Renato points out that "the discovery leads to an interesting evolutive view because cyanobacterium are the origin of plastids (chloroplasts, chromoplasts, etc.), and they have photosynthetic and respiratory abilities. "In chromoplasts' respiratory chain, we observed -- highlights the researcher -- that at least one element which is part of the respiratory machinery of cyanobacterium has been recovered."