Arabidopsis thaliana plants acclimated to low dose rates of ultraviolet B radiation show specific changes in morphology and gene expression in the absence of stress symptoms

• Ultraviolet B (UV-B) acclimation comprises complex and poorly understood changes in plant metabolism. The effects of chronic and ecologically relevant UV-B dose rates on Arabidopsis thaliana were determined. • The UV-B acclimation process was studied by measuring radiation effects on morphology, physiology, biochemistry and gene expression. • Chronic UV-B radiation did not affect photosynthesis or the expression of stress responsive genes, which indicated that the UV-acclimated plants were not stressed. UV-induced morphological changes in acclimated plants included decreased rosette diameter, decreased inflorescence height and increased numbers of flowering stems, indicating that chronic UV-B treatment caused a redistribution rather than a cessation of growth. Gene expression profiling indicated that UV-induced morphogenesis was associated with subtle changes in phytohormone (auxins, brassinosteroids and gibberellins) homeostasis and the cell wall. • Based on the comparison of gene expression profiles, it is concluded that acclimation to low, chronic dose rates of UV-B is distinct from that to acute, stress-inducing UV-B dose rates. Hence, UV-B-induced morphogenesis is functionally uncoupled from stress responses.