Molecular responses of plants to solar UV-B and UV-A radiation

Abstract: Plant responses to solar ultraviolet radiation (UV, 280-400 nm) were assessed at different molecular levels using Betula pendula Roth (silver birch) and Arabidopsis thaliana (Arabidopsis) as model species in outdoor experiments to assess the possibly interacting roles of the UV-B and UV-A wavebands in acclimation to sunlight. Solar UV-B (280-315 nm) and UV-A (315-400 nm) irradiance was attenuated with plastic films. Both solar UV-B and UV-A promoted the acclimation of silver birch and Arabidopsis to UV in sunlight by regulating the expression of genes with functions in UV protection and also by inducing the accumulation of phenolic compounds in the leaves. Solar UV also regulated transcript accumulation of genes involved in the signaling and biosynthesis of auxin, brassinosteroids and jasmonic acid (JA) in Arabidopsis. A new role of Arabidopsis UV-B photoreceptor UV RESISTANCE LOCUS8 (UVR8) in the regulation of some responses to solar UV-A radiation was observed in addition to its previously described role in UV-B perception. High UV-A irradiance as present in sunlight, had a large effect on plant metabolism and modulated some of the previously characterized UV-B responses most probably through interaction between UVR8 and CRY pathways. In contrast to UVR8, under UV-B irradiation conditions not inducing stress, RADICAL-INDUCED CELL DEATH1 (RCD1) played no active role in UV signaling and acclimation, but rather modulated UV responses under sunlight. We demonstrated that solar UV-A makes an important contribution to acclimation of plants to sunlight, independently and interacting with UV-B.