Researchers at the Center for Research in Agricultural Genomics (CRAG) discover how plants know when they have to grow

Anyone who has a plant at home knows that light is key for plant growth. The amount of light gives information about the season and the time of the day, and plants adapt their responses, such as flowering or stem growth, to it. Therefore, to develop and grow normally plants must coordinate the perception of light with their endogenous circadian cycle. A new study, led by the CSIC researcher at CRAG, Elena Monte, describes for the first time the molecular mechanism that plants use to coordinate these activities to time growth.

Light sensors

Plants have several photoreceptor systems, i.e. proteins that change their conformation in response to light. Among these, the phytochromes, which detect red and far-red light, are important for seed germination and seedling growth. Although phytochromes were discovered in the fifties, it was not until 1998 that the proteins that transmit the light signal from the phytochrome to the cell nucleus (allowing the gene expression to change in response to light), were discovered. These proteins, which are called Phytochrome-Interacting Factors (PIF), were discovered at the University of California in Berkeley, by professor Peter Quail’s group, who has also collaborated in this study led by Elena Monte.

PIF proteins are key to regulate the expression of genes required for plant development. It is known that when phytochromes are activated by light, they attach to PIF proteins promoting their degradation. Thus, during daylight hours, the amount of PIF proteins in the cell remains low. During the night, and especially on long winter nights, PIF proteins are synthesized and just before dawn, when its accumulation in the cell is at maximum level, they give the instructions for plant growth to the cell nucleus. This molecular mechanism controls the elongation of the of the young plant stem, allowing its growth in the pre-dawn hours in the long winter nights. But how does the plant know that this is the right time to grow?

The gate

The study now published in the American journal PNAS answers this question. CRAG and UC Berkeley researchers have discovered that PIF proteins are also able to interact with a transcriptional repressor TOC1, a protein that at the same time is involved in circadian clock control. Unlike PIF, TOC1 protein is more abundant in the plant when the night is just beginning and its levels decrease during the hours of darkness.

Using the model plant Arabidopsis thaliana, the researchers observed that in TOC1-mutant plants, stem lengthened earlier and faster at night than in wild-type plants; whereas it did not occur in plants having two mutated genes (TOC1 and PIF3). The study now published in PNAS describes for the first time that TOC1 acts as a gate for the stem growth, repressing PIF action during the night and preventing the stem elongation. When the nights are long, TOC1 levels decrease just before the dawn opening the way to PIF activity and stem growth just in this particular period. Thus, during the long nights, the plant limits its growth period just before dawn, when the environmental conditions are optimal for elongation.

"We finally understand the mechanism that explains why plants such as Arabidopsis grow during the winter, when they do not produce flowers, while in summer the growth stops to give way to flowering, which is activated when the nights are short”, explains the researcher Elena Monte.

"Understanding this mechanism may be useful in the future to optimize crops to face climate change," added Judit Soy and Pablo Leivar, pre-doctoral and post-doctoral CRAG students, respectively, who also authored this work.

Reference article:

Judit Soy, Pablo Leivar, Nahuel González-Schain, Guiomar Martín, Céline Diaz, Maria Sentandreu, Bassem Al-Sady, Peter H. Quail, Elena Monte “Molecular convergence of clock and photosensory pathways through PIF3-TOC1 interaction and co-occupancy of target promoters” Proc. Natl. Acad. Sci. U.S.A. April, 2016