Plants grow mostly in the afternoon

It has long been known that plants are able to perceive photoperiod, that is, the length of the day. For some of them, flowering begins when the photoperiod becomes long (eg more than sixteen hours). These so-called “long day” plants do not flower when the nights are as long as in winter, however, even a short flash of weak light emitted at the right time can be enough to make them flower. This should arrive at a time in the biological cycle when the plant is particularly sensitive, such as late afternoon. The genetic mechanisms involved are well known. However, there are other processes besides flowering that are controlled by photoperiod and whose mechanisms are not well understood. This is the case for growth control. Qingqing Wang, from Yale University in the US, and her colleagues have demonstrated an unexpected mechanism of photoperiod perception that controls precisely this process.

The researchers started from observing women's guidance (Arabidopsis thaliana), a favorite model plant of plant biology researchers, grows fastest when days are long. Unlike flowering, it requires fairly intense light to have an effect, because the photosynthesis process must be able to work efficiently. Therefore this strong light should not arrive at any time. If it is present in the morning, growth is not stimulated. If it arrives in the late afternoon such as on long summer days, the plant grows faster. As with the beginning of flowering, at the end of the afternoon the plant is particularly sensitive to the light signal.

The researchers then studied several genes involved in growth, whose expression changes as the days get longer. Focus on MIPS1 (myositol phosphate synthase 1) Which produces sugar. They generated mutant plants by modifying their DNA sequences to affect expression MIPS1 And I noticed the consequences. “This approach is classic,” notes François Barsi, a geneticist at the Laboratory of Cellular and Plant Physiology in Grenoble. But the result is surprising! » When the gene MIPS1 It mutates, no matter how much light comes, the plant grows little, as if the days are short. Therefore, it turns out that this gene is essential for understanding the photoperiod that directs plant growth, and in some way the efficiency of photosynthesis.

This finding is surprising, because this gene is unrelated to those involved in flowering, it acts in parallel and independently: the mutants do not have flowering defects (and conversely, flowering mutants do not have a growth deficiency). “This article will likely serve as a reference for later work,” posits François Barsi. Because the entire regulatory mechanism has not yet been discovered. The researchers also speculate that plants may have many other mechanisms for measuring different aspects of day length.

“By understanding these mechanisms better, we will be able to select plants to enhance their flowering or growth depending on whether they are grown to produce seeds, fruits or biomass,” says François Barsi. Assuming this is practiced virtuously, without increasing input. » Researchers are already proposing the idea of ​​developing plants whose growth is insensitive to photoperiod to promote rapid biomass production in developing countries where the growing season is short. A perspective aligned with a new crop transformation pathway that emerged a few years ago: chronoculture, which aims to exploit the genetic basis of plants' circadian clocks to improve crop productivity and sustainability.