For all but a handful of plants, sunlight is vital to their existence. It provides the energy needed to break molecules of CO2 and water in order to synthesize carbohydrates. It is no wonder then that plants are incredibly attuned to their light environment. They grow towards it, they compete for it, they simply can't live without it. Exactly how plants (as well as many bacteria and even some fungi) perceive light is quite fascinating. It involves a small family of proteins called "phytochromes" whose chemical properties function like an on and off switch. Today I would like to briefly introduce you to this system.
The activity of phytochrome proteins can be quite complex. In fact, many aspects of this system are still awaiting discovery. Still, we know pretty well how the phytochrome system functions with light and it all comes down to the color red. Pure sunlight is white light. It contains all of the wavelengths in the visible spectrum and then some. Phytochrome responds to two areas of this spectrum: red wavelengths of around 667 nm and far-red wavelengths around 730 nm.
This range of the spectrum is quite useful when it comes to assessing whether or not there is enough light for photosynthesis. Unfiltered sunlight contains the most red light. As sunlight passes through the leaves of the canopy or as the sun sets, the ratio of far-red light increases. Far-red light is not conducive to photosynthesis. As such, the long-term survival of photosynthetic organisms is tied to figuring out the relative abundance of red and far-red light.
This is where the phytochrome system comes in. It comes in two forms - an active form and an inactive form. When the inactive form absorbs red wavelengths, it is converted to its active form. This is the form that signals to the plant that there is enough light for physiological activity. When the ratio of wavelengths hitting the active form becomes dominated by far-red wavelengths (as it does when a plant is shaded or when the sun sets), the phytochrome is converted back into its inactive form. This in turn signals the plant to shut down many of the physiological activities within.
The structure of phytochrome in its inactive form (left) and active form (right).
This on and off switch is how plants regulate everything from growth to flowering. The ratio of active to inactive forms can tell some plants what time of year it is. If there is more inactive form within its tissues, the plant "knows" that the days are growing shorter. Phytochrome is also involved in the number and the size of leaves that a plant will produce. Similarly, it is how plants know when they are being shaded out by their neighbors. The more neighboring plants there are, the more filtered the sunlight becomes and the ratio of far-red light increases. It is even involved in the process of seed germination. Small seeds that don't have enough food reserves (think lettuce seeds) will only germinate once their phytochrome is converted to its active form. In doing so, they ensure that they aren't germinating in an environment with too much shade or deep under the soil.
Scientists are still working out exactly how the phytochrome system is able to regulate so many functions in plants. In some cases it can directly interact with molecules in the cytoplasm of plant cells. In other cases, it is transported into the nucleus where it can activate or deactivate particular genes. What we do know is that the phytochrome system is vitally important not just for the organisms that produce it, but for life as we know it. Without plants there could be no life on this planet.