UV-B radiation, its effects and defense mechanisms in terrestrial plants

Abstract

The UV-B is an important component of solar radiation to which all terrestrial and aquatic plants were exposed during the early evolutionary phase of the Earth. Hence the plants, principally terrestrial, have evolved different mechanisms to avoid and repair the UV-B damage; therefore, it is not surprising that photomorphogenic responses to the solar UV-B are erroneously assumed to be adaptations to the harmful UV radiation. The responses to UV-B enhancement include changes in the leaf area, leaf thickness, stomatal density, wax deposition, stem elongation, and branching pattern, as well as in the synthesis of secondary metabolites, alterations in plant–pathogen and plant–predator interactions, and in gene expression. However, under field conditions the ambient solar UV-B provides an important signal for the normal plant development and may be perceived by the plants through nondestructive processes involving both UV-B specific and UV-B nonspecific signaling pathways. The specific signaling pathways include the components UVR8 and COP1 which regulate the expression of a set of genes that are essential for the plants' protection. The nonspecific signaling pathways involve DNA damage, reactive oxygen species (ROS), hormones, and wound/defense signaling molecules. Indeed under the field conditions, the ambient UV-B might more properly be viewed as a photomorphogenic signal than as a stressor. Therefore, it might not be appropriate to evaluate the adaptive roles of plant responses to UV-B cues upon stress tolerance by the simultaneous application of both solar radiation and supplemental UV-B. In this chapter, we analyzed the information regarding physiological and morphogenic responses of the terrestrial plants to the UV-B radiation, as well as the events related to UV-B perception, signal transduction, gene expression, and ROS formation from different studies carried out in greenhouses, growth chambers, and field conditions.