Re‐interpreting plant morphological responses to UV‐B radiation

There is a need to reappraise the effects of UV‐B radiation on plant morphology in light of improved mechanistic understanding of UV‐B effects, particularly elucidation of the UV RESISTANCE LOCUS 8 (UVR8) photoreceptor. We review responses at cell and organismal levels, and explore their underlying regulatory mechanisms, function in UV protection and consequences for plant fitness. UV‐induced morphological changes include thicker leaves, shorter petioles, shorter stems, increased axillary branching and altered root:shoot ratios. At the cellular level, UV‐B morphogenesis comprises changes in cell division, elongation and/or differentiation. However, notwithstanding substantial new knowledge of molecular, cellular and organismal UV‐B responses, there remains a clear gap in our understanding of the interactions between these organizational levels, and how they control plant architecture. Furthermore, despite a broad consensus that UV‐B induces relatively compact architecture, we note substantial diversity in reported phenotypes. This may relate to UV‐induced morphological changes being underpinned by different mechanisms at high and low UV‐B doses. It remains unproven whether UV‐induced morphological changes have a protective function involving shading and decreased leaf penetration of UV‐B, counterbalancing trade‐offs such as decreased photosynthetic light capture and plant‐competitive abilities. Future research will need to disentangle seemingly contradictory interactions occurring at the threshold UV dose where regulation and stress‐induced morphogenesis overlap.