Plants are sessile organisms, thus, they have developed considerable plasticity to respond to changes in the natural environment. They use light, a highly variable environmental factor, not only as the main energy source for photosynthesis, but also as an environmental cue. Light is arguably the most important environmental signal for plants and regulates a wide range of physiological and developmental processes, from seed germination to flowering. For light perception, plants have evolved a large set of specialized photoreceptors that enable them to sense the quality, quantity, direction, and duration of light. In the model plant Arabidopsis thaliana, three classes of photoreceptors can be distinguished by the wavelength of light they perceive: phytochromes, which absorb in the red/far-red (~620–750 nm); cryptochromes and phototropins, which absorb in the blue/UV-A (320–500 nm); and the UV-B sensing receptor(s), which have not been identified.
On the Earth's surface, most environmental factors that affect the activity of living organisms, such as temperature, the availability of food, and so forth, depend on sunlight and therefore exert their effects in a periodic, predictable fashion. Most organisms—including higher plants—have an internal time-measuring system that allows them to anticipate these regular changes in the environment and to entrain their biological processes to a part of the cycle that benefits from external light or warmth or to the absence of inhibitory internal processes. This time-measuring system is called the circadian clock.
The core of the clock is the central oscillator. The core oscillation is relayed to diverse clock-controlled processes via an output pathway. Under constant conditions, the clock runs free with a period close to, but never exactly, 24 hours. In the natural environment, the circadian clock has no survival value unless the endogenous biological time is adjusted to local time. To provide precise temporal information, the biological clock is entrained to environmental cycles via an input pathway. This resetting occurs daily in response to periodic environmental cues. Circadian rhythms in nature are always entrained, and the clock is reset by changing the phase of the rhythm every day. In the environment, the factor representing the progression of day and night most reliably is light, because the amount of light and its duration, spectral composition, and direction change in a systematic way over a day. When reaching the oscillator, the input signal causes an acute change in the level/activity of certain clock components, resulting in a phase shift of the oscillator and, consequently, of the overt rhythms.
Ultraviolet light regulates plant gene expression, development, and growth
Substantial knowledge has accumulated on the perception of visible light. In contrast, our comprehension of the sensory mechanisms activated in response to UV-B irradiation is far morelimited, regardless of the fact that the UV radiation is an important and increasing part of the sunlight reaching the Earth's surface. This region of the light spectrum is divided into three classes: UV-A (320–400 nm), UV-B (280–320 nm), and UV-C (<280 nm). The stratospheric ozone layer absorbs UV-C, but UV-A and part of UV-B reach and affect most organisms living on Earth. A number of factors determine surface UV-B levels, making it a highly dynamic environmental component. Although biologically active UV-B radiation constitutes less than 0.5 percent of solar energy, it has a serious impact on various biological processes. The UV-B portion of solar radiation can act both as an environmental stress factor and as an informational signal in plants, and the resulting effects can be broadly divided into two classes: damage (stress) and non-damage (regulatory or morphogenic) responses. UV-B as a stress factor has been the subject of numerous investigations; however, research on the characterization of UV-B action on plant growth and development started only in the past few years.
Our laboratory showed that exposing plants to non-damaging UV-B radiation leads to transcriptional activation and repression of numerous genes. In microarray experiments monitoring genomewide expression changes due to low-level UV-B irradiation, we identified several genes that are immediately up- or downregulated by a brief pulse of UV-B light. Such UV-responsive genes are grouped into different functional classes, and a substantial number of the early-induced and repressed genes encode different classes of transcription factors. The latter group contains the well-known basic domain/leucine zipper transcription factor Elongated Hypocotyl5 (HY5), one of the key regulators of photomorphogenesismediating a number of physiological responses that are controlled by red- and blue-light photoreceptors.
Analysis of double and triple photoreceptor mutants for inducibility of selected UV-regulated genes revealed that the UV-B–mediated transcriptional regulation of these genes is independent of the main photoreceptors acting in the visible range of the light spectrum. It was demonstrated, however, that HY5, which acts as a positive regulator of red- and blue-light–induced photomorphogenesis, is also required for an appropriate UV-B response. In the dark, the HY5 protein is turned over in the nucleus by the E3 ubiquitin ligase Constitutively Photomorphogenic1 (COP1), which is a crucial repressor of light signaling. In light, activation of photoreceptors leads to the inactivation and nuclear exclusion of COP1, allowing HY5 stabilization and activation of certain light-responsive genes. We have demonstrated that COP1 acts as a positive regulator of the morphogenic, low-level UV-B–mediated response in Arabidopsis, in contrast with its repressor function in visible light–induced photomorphogenesis.
To summarize our results on UV-B light signaling, we have ascertained that the UV-B– and visible light–induced signal transduction pathways share some common elements (for example, COP1 and HY5). However, these proteins display quite different action mechanisms—even opposite regulatory effects—in the two distinct signaling pathways; they are thus endowed with specialized functions for UV-B–induced signaling.
Light-regulated entrainment of the plant circadian clock
Circadian clocks provide the endogenous timetable for development, behavior, physiology, and biochemistry as well as photoperiodic events in most organisms. Circadian clocks thus lend great plasticity and significant adaptive potential to living organisms. Given that light is the most prominent environmental signal involved in clock resetting, our laboratory is interested in identifying those signal transduction pathways by which different photoreceptors, especially the red/far-red light-absorbing phytochromes that reset or entrain the circadian clock. We have shown that light-induced translocation of phytochromes is an essential and rate-limiting step in this process. Phytochromes of nuclear localization associate with large protein complexes, termed nuclear bodies. We recently began biochemical characterization of these photoreceptor-containing nuclear complexes and isolation of specific mutants displaying perturbed nuclear body formation to reveal their function in light-induced signaling toward the central circadian clock. The UV-B content of sunlight changes very rapidly in the morning and in the evening; therefore, it is possible that circadian clocks are able to use UV-B to adjust their internal rhythms to the environmental cycles, suggesting that UV-B, like red and blue light, can act as a natural signal to the clock. We are testing this hypothesis by using transgenic and mutant Arabidopsis plants expressing the firefly luciferase enzyme as a circadian reporter.
Last updated September 2009