Recycling, a way for plants to survive in case of shortage

In order to secure a place in the sun and to guarantee their growth, plants have developed different strategies. But what happens when plant density is so high that resources, especially light, run out? Plants go into survival mode and activate a recycling mechanism: autophagy. This process is at the heart of a study published on October 10, 2022 in "Nature Communications" by the team of Prof. Christian Fankhauser at the Integrative Genomics Center of UNIL.

Plants compete fiercely to optimize their access to light. When plant density is high, several adaptations are put in place. Reallocation of resources, which boosts stem growth and thus brings the leaves closer to the sun’s energy, is one of them. The acceleration of reproduction is another, with an advanced flowering.

The group led by Christian Fankhauser, full professor at the Centre for Integrative Genomics (CIG) of the Faculty of Biology and Medicine of the University of Lausanne (UNIL), compared the reaction of plants to two more or less stressful situations: in the first case, a threat of light deprivation and, in the second, an actual decrease in light. To do this, the biologists used the plant models Arabidopsis thaliana, the lady’s slipper, and Brassica rapa, the field cabbage, a cousin of rapeseed.

In both scenarios, the Lausanne researchers observed, not surprisingly, an elongation of the seedling. However, the most vivid response of crucifers under the worst conditions (heavily shaded environment) turned out to be unexpected: autophagy, a form of molecular recycling. The results of five years of research are published in the October 10, 2022 issue of the journal Nature Communications.

Anticipating or dealing with danger

Prof. Christian Fankhauser, director of the study, analyses the two contexts in a very different way. In the first case, the plant is not yet in the shade. It still has access to the sun’s rays and thus to an abundant source of energy, even if it perceives the threat of its competing neighbors. It will then lengthen its stem in anticipation of the coming danger.

In the second experiment, the stakes are different. In the shadow of its competitors, the plant sees its energy source dry up. Despite this, it increases the growth of its stem, but how does it do this? This is precisely where autophagy comes in," explains the professor. This molecular recycling, which can be compared to a kind of self-digestion, ensures the growth of the seedling even in a hostile environment. It is as if the plant destroyed some of its components in simple units, a sort of LEGO, from which different elements can be reconstructed to allow the elongation of the stem.’

Autophagy is induced by a danger signal emitted by the shading, thus making available the necessary compounds to get out of this difficult situation. However, a surprising element remains: crucifers use molecular recycling to promote their growth, while in general other organisms (animals, fungi) that use it are in a very economical mode and reduce their growth. This is probably due to the essential nature of sufficient access to sunlight for plants," suggests Christian Fankhauser.

A genetic and cellular signature

The researchers looked more closely at what was happening from a genetic and cellular perspective. By analyzing the expression levels of the plant’s genes, they were able to observe a difference between the scenario ’I am threatened by shade - how to avoid it’ and the scenario ’I am in the shade - how to get out of it’. In the second case, the plant modifies its expression program and several catabolic phenomena (molecular degradation reactions) take place. A cellular analysis of the plant, through biomarkers, has confirmed this molecular strategy. We were thus able to highlight not only a genetic but also a cellular signature of the autophagy process", summarizes Christian Fankhauser.

Identify the switch

The next step for the IGC team will be to understand how a change in light can trigger molecular recycling. In other words, to find the switch!