TotM: Arabidopsis thaliana CRY2 and how flowers know about spring

prague_clock.JPGWe live in a world where complex and mundane devices in our pockets or wrists are able to track the passage of time with an error of one second in centuries. Time is an absolute framework for human activities, our minds chained forever to the concept of “when”. While these modern wonders are a recent invention, time itself is deeply ingrained within our bodies. Night and day, the movements of the moon and the seasons are the main sources of time information we can easily find in nature, all of them easily recognized just by looking through a window. Seasons in particular are key to the survival of our species, since the growth and reproduction of our food sources depends quite often of these cycles. Human civilization began when they discovered this and started applying this knowledge to obtain food in a controlled environment, a process we call agriculture. Plants are usually dormant in winter, they produce flowers in spring that culminate with fruits in summer, only to slowly return to the dormant state all over the course of autumn. However, this raises an important question: How do plants perceive time?



Seasons are produced by the inclination in the rotation axis of our planet, which tilts cyclically along a period of a year respect to the plane the planet produces traveling around the sun. This, in turn, produces sunlight to hit the surface of our planet with a varying angle. The angle of incidence affects the intensity, which in turn affects the transference of heat through radiation. Also, the angle determines the number of sunlight hours. And finally, and more important for our topic, the angle determines the color of the sunlight. The atmosphere of our planet is not perfectly transparent and it scatters light with certain wavelengths preferentially. With lower angles, light must traverse a longer distance through the atmosphere, which leaves us light that is enriched in red colours, less affected by this rite of passage.


By using the relation between blue and red environmental light, as well as measuring the day length and other parameters, plants are able to plan ahead of the seasons. This information allows them to produce flowers and fruits, a process that requires huge amounts of time and resources, in predicted optimal conditions of temperature and water availability. And while plants lack proper eyes, they are no strangers to light sensitivity. They require light to live, after all. By using cryptochromes, pigments that absorb blue light, plants are able to predict and adapt to seasonal cycles. The cryptochrome is associated to a pigment and changes its conformation when excited. After this change, the protein forms a homodimer and binds to DNA, recruiting a many other transcriptional regulators.



Arabidopsis_thaliana.jpgIn the case of Arabidopsis thaliana, there are three cryptochromes. CRY1 and CRY2, are associated to the nucleus of the cells, and control many aspects of the plant development. CRY2 in particular is highly involved in regulating the timing of flower production. The third one is apparently associated to the organelles. CRY1 and CRY2 are paralogous proteins arising through a duplication in the base of angiosperms, as we can observe in our tree of the month, from the Arabidopsis thaliana phylome. The cryptochrome family is actually highly conserved across eukaryotes and can be found virtually in all main lineages.






Astronomical clock of Prague, Czech Republic. Public domain.
Schematic representation of the cryptochrome signalling pathway. Extracted from Liu et al (2011).

Photography of a flower of Arabidopsis thaliana. Pulic domain.