The ripening hormone ethylene helps green tomatoes become red after harvesting. In contrast, regular and Spanish peppers are not influenced by this plant hormone. This is even more surprising when one considers that they are closely related to each other. Researchers from the Max Planck institute for Molecular Plant Physiology in Potsdam researched this phenomenon and compared the gene expression levels and the metabolism methods of the plants.
The metabolism was tested at different moments: before and after the 'breaker point', the day where the fruit has a visible color change during the ripening process. Tomatoes release a large quantity of ethylene specifically on this day. The gas form photo-hormone ethylene activates its own synthesis as soon as the plant comes in contact with ethylene externally. Two enzymes play a central role in the synthesis of ethylene: ACC-synthase and ACC-oxidase. Fully ripened tomato fruits produce much more of this enzyme during the ripening process which leads to increasingly larger ethylene levels. The ethylene brings about a cascade of signals to the tomato which causes the fruit to ripen. Green chloroplasts become coloured chromoplasts, the hard cell walls break apart, sugars are made and the nutritional level changes.
It is a bit different with regular and Spanish peppers. "It appears that ethylene doesn't have any influence on the gene expression or the metabolism of the Spanish and regular peppers,"says Dr. Alisdair Fernie, group leader of the research team. Funny enough, the genes a bit further down in the ethylene-signal chain were pretty active. "De genes causing the break down of the plant cell wall or the carotenoid bio-synthesis were, for the most part, formed in both tomatoes as well as peppers during the normal ripening process on the plant" states Fernie. The researchers are still searching for the molecule that works against the ripening process in peppers and other non-ripened fruits.