Scientists find out how tomatoes turn to red when seeds are ripe

CSIC scientists at the Center for Research in Agricultural Genomics (CRAG) unveil that tomatoes have ‘recycled’ a light-dependent mechanism to modulate fruit ripening.

Carotenoids are a group of pigments which are essential for plants as they protect them from the excess of light (photo protectors) and are precursors of hormone synthesis. They also have a role on fruit ripening: when they are ripe, fruits turn from green to red due to the accumulation of carotenoids such as beta carotene, a precursor of vitamin A, or lycopene, a powerful anticarcinogenic compound.

Previous studies in Arabidopsis plant had shown that carotenoid synthesis in leaves was regulated by phytochromes, light receptors that allow plants to sense the quality of light they are receiving and thus to get environmental information.

What plants can “see”

As phytochromes can detect the wavelength of the light received, they can differentiate between red light -which means the plant is on the sunlight- and far red light –which means the plant is shaded, surrounded by other plants whose chlorophyll is absorbing the radiation of red light. Depending on these signals, the plant can ‘see’ the environment and adapt its growth to ‘escape’ from the shade.

Until now, it was unclear if the same mechanisms were present in fruits. Now, a group of scientists led by Manuel Rodríguez-Concepción, CSIC researcher at the Center for Research in Agricultural Genomics (CRAG), with the contribution of the Institut Català de Ciències Fotòniques (ICFO), unveils some answers in an article published in The Plant Journal.

A repressor protein

The scientists hypothesized that, in the case of fruits, carotenoid biosynthesis also had some relation with phytochromes. But they have found something else: the same mechanism occurs in fruits not to get environmental information but to get information about what is happening inside the fruit itself.

In this case, phytochromes detect changes in the composition of light that passes through the fruit flesh. When the fruit is green, due to the accumulation of chlorophyll, the latter absorbs and retains the red light radiation. But when the fruit and the seeds are fully developed, the chlorophyll starts to degrade, which in turn increases the amount of red light that reaches phytochromes. As a result, phytochromes are activated and degrade the transcription factor PIF1 a. Transcription factors are proteins that ‘switch on’ or ‘swith off’ genes. PIF1a switches-off (represses) carotenoid production.Therefore when PIF1a breaks down, the accumulation of carotenoids increases, which makes the fruit to acquire its characteristic red colour.

"It is", says Manuel Rodriguez-Concepción, "a completely new function. The fruits have recycled a mechanism, which was “developed” by plants to get information on the environment, and have adapted it to ‘see’ inside them and adjust their colour to the ripening state”.

Scientists explain that colour in fruits has a communication role in nature. Fruits get ripe with a colour change, which help animals to better identify the fruits that are ready to eat. This colour change happens when the seeds are ready for germination, not before. Image: Briardo Llorente i Manuel Rodríguez-Concepción /CRAG.

A sign when seeds are ready

The scientists believe this mechanism is essential in an ecological context of plant-animal interaction. Briardo Llorente, first and co-corresponding author of the work, explains: “We think that colour in fruits has a communication role in nature. Fruits get ripe with a colour change, which help animals to better identify the fruits that are ready to eat. This colour change happens when the seeds are ready for germination, not before.” When animals eat the fruits, they also disperse the seeds. So it is, scientists explain, a sign in the mutualistic relationship between animals and plants, where both of them have benefits.

The study also shows how to modify the discovered mechanism in order to obtain carotenoid-enriched fruits, wich are healthier and commercially more eye-catching.

Briardo Llorente, Lucio D'Andrea, M. Aguila Ruiz-Sola, Esther Botterweg, Pablo Pulido, Jordi Andilla, Pablo Loza-Alvarez, Manuel Rodriguez-Concepcion. Tomato fruit carotenoid biosynthesis is adjusted to actual ripening progression by a light-dependent mechanism Plant J 85(1): 107-119.