UV-B radiation is an integral component of sunlight that has wide-ranging effects on organisms. Most of the UV-B that reaches the earth is absorbed by the stratospheric ozone layer and therefore UV-B wavelengths are only a small fraction of sunlight at the earth's surface. Nevertheless, since UV-B is the most energetic part of the daylight spectrum it has the potential to damage macromolecules such as DNA and proteins, generate reactive oxygen species (ROS) and impair cellular processes.
However, UV-B is not solely an agent of damage and has an important role as a regulatory signal. In particular, the perception of low levels of UV-B by plants actively promotes survival because it stimulates responses that help to protect against and repair UV-damage. Plants are unavoidably exposed to UV-B because they need to capture sunlight for photosynthesis. The fact that plants rarely display signs of UV-damage in the natural environment demonstrates that they have evolved very effective mechanisms for UV-protection and repair. The protective mechanisms include the deposition of UV-absorbing phenolic compounds in the outer epidermal tissues and the production of anti-oxidant systems. Repair of UV-damage involves enzymes such as DNA photolyases. Furthermore, responses to UV-B modify the biochemical composition of plants, influence plant morphology and help to deter pests and pathogens. It is well established that many plant responses to UV-B involve the regulation of gene expression. UV-B exposure stimulates the expression of hundreds of genes, including those involved in UV-protection and repair.
It is important to understand how plants respond to UV-B and to determine the contribution of UV-B responses to normal plant growth and development. In fact, it will not be possible to obtain a complete understanding of the role of light in controlling plant development without knowledge of the regulatory effects of UV-B. Much remains to be learnt about the cellular and molecular mechanisms of UV-B perception and signal transduction leading to the control of gene expression.
