How plants detect the direction of light

How plants detect the direction of light

The focus on the direction of lighting becomes particularly clear with young plants. © /iStock

Always optimally facing the energy source. Researchers have now further elucidated how plants detect the direction of light. They have discovered an optical element of the light-sensitive plant tissue that is used to detect the radiation gradient: By specifically influencing the light, tiny air channels apparently increase the ability of the plant photoreceptors to detect the light gradient.

Where does the light come from? From microbes to plants and animals, many living beings have different systems to determine the direction of origin of radiation. This information can be important for various reasons. But it has a special meaning for plants because light is their source of energy. They use their ability to recognize the direction of light to align themselves as best as possible. This allows them to optimally illuminate their photosynthesis system to build chemical energy sources. This light-dependent movement system in plants is called phototropism.

On the trail of directed light sensors

In principle, it is already known that this system is based on the function of so-called photoreceptors, which are located in light-sensitive plant tissues. Depending on the irradiation intensity, these sensors emit physiological signals that cause changes in the plant tissue in certain areas, which lead to bending movements of the stem, leaf, etc. In this system, when irradiated from the side, the photoreceptors facing the light are activated more strongly than those facing away, which results in phototropism. Until now, however, it was not known to what extent optical features of the plant tissue that precede detection by the photoreceptors also play a role.

As the research team led by senior author Christian Fankhauser from Universalität Lausanne reports, the study began with a questioning look at a special mutant of the model plant Arabidopsis thaliana: This line does not show the normal alignment behavior towards light sources. Another striking feature is that the stems of the seedlings appear transparent, whereas in “normal” specimens they appear more milky-matt. So the team decided to investigate whether this striking feature had something to do with the mutant’s impaired perception of light.

First, microscopic studies revealed: “The natural milky appearance of the stems of normal plants is due to the presence of air in intercellular canals. In the mutant specimens, however, this air is replaced by a watery liquid, which gives them the strikingly translucent appearance,” reports Fankhauser. In normal plants, the tiny air spaces have a significant influence on the light – but less so in the stems of the mutant. To examine more closely the extent to which the intercellular air spaces are necessary for phototropism, the researchers flooded them in normal seedlings using vacuum infiltration. The subsequent investigations then confirmed that these plants were no longer able to align themselves with a light source due to their transparent stems.

The light gradient becomes more clearly visible

But what effect do the air-filled channels convey? As the researchers explain, these elements influence light in a way that leads to an increase in the radiation gradient in the tissue. This effect can then apparently be used particularly well by the plant to determine the direction of light. “Air and water have different refractive indices. This leads to a special influence on the radiation when it passes through the plant tissue,” explains co-author Martina Legris from Universalität Lausanne. The light gradient enhanced by the air channels is apparently so important that it is a prerequisite for the efficient phototropic reaction, say the scientists. At least in the case of Arabidopsis and brassicas, they have now been able to demonstrate this importance.

The team was also able to show which peculiarities in the plant mutant lead to the lack of air channels. They therefore do not form certain structural elements, which leads to fluid retention. This, in turn, is based on certain genetic variations, according to their research. These results could now be used for further research into the importance of aerial structural elements in plants. This could reveal other cases of functional influence on light and more. Air channels probably also play an important role in gas exchange and in plant tolerance to a lack of oxygen during flooding.

Source: University of Lausanne, specialist article: Science, doi: 10.1126/science.adh9384

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