Researchers from the Institute of Molecular and Cellular Biology of Plants (IBMCP, CSIC-UPV) participate in a study that reveals that a system involved in the ageing of the leaves of the plant also regulates the ripening of the fruit.
An international research group involving researchers from the Institute of Molecular and Cellular Biology of Plants (IBMCP), a joint centre of the Spanish National Research Council (CSIC) and the Universitat Politècnica de València (UPV), has discovered that a genetic mechanism involved in the ageing of plant leaves, known as CHLORAD, plays a decisive role in the tomato ripening process. Thus, tomatoes with an activated CHLORAD system turn red faster and accumulate more lycopene, a health-promoting compound. The results are published in the latest issue of the journal Nature Plants and will lead to better quality tomatoes.
The ripening of most fleshy fruits gives them attractive colours and smells, a ‘trick’ of the plant to distribute its seeds and colonise new territories. In tomatoes, ripening changes their colour from green to orange and red. The green is due to the presence of chlorophylls (the photosynthesis pigment) in the chloroplasts of the immature fruits. When they ripen, the chloroplasts (the organs in charge of photosynthesis) lose chlorophylls and produce large quantities of other pigments called carotenoids.
Tomato carotenoids are orange (due to beta-carotene) and red (due to lycopene), which causes the fruit to change colour when ripe. In addition, these carotenoids form aromas that contribute to the characteristic smell of ripe tomatoes. For all this to happen, the chloroplasts need to be transformed into a new type of carotenoid storage compartment called a chromoplast.
Until recently, it was not known how the tomato plant controls the transformation of chloroplasts into chromoplasts. Now, a research group from the University of Oxford (UK), in collaboration with the Institute of Molecular and Cell Biology of Plants (IBMCP) in Valencia, has unravelled part of this mystery in an article published in the journal Nature Plants.
The key to this work comes from Arabidopsis, a plant used as a study model that does not naturally develop chromoplasts, but which does transform its chloroplasts during the process known as ‘leaf senescence’, in which the leaves age, lose chlorophyll and stop photosynthesising. During this process, a molecular mechanism called CHLORAD removes complexes in the outer layer of chloroplasts that import proteins needed for photosynthesis.
Faster red tomatoes
Researchers have found that the CHLORAD system also works during tomato ripening. When activated, it prevents the import of photosynthetic proteins but promotes the incorporation of other proteins necessary for the production and storage of carotenoids during the transformation of chloroplasts into chromoplasts. Thus, fruits with an activated CHLORAD system turn red faster and accumulate more of the health-promoting carotenoid lycopene, while fruits with a deficient CHLORAD system take longer to ripen.
“In addition to better understanding how chloroplasts are transformed into chromoplasts, we now know that this process not only regulates fruit pigmentation but also affects many other aspects linked to ripening that affect the firmness or aroma of tomatoes,” says Manuel Rodríguez Concepción, a CSIC researcher at the IBMCP who is participating in the study. The challenge now is to understand the connections between these mechanisms in order to produce tomatoes of higher commercial and nutritional quality without sacrificing their characteristic colour, aroma and flavour.