In the course of evolution, plants have developed mechanisms that orient their organs toward the incoming light. At the seedling stage, positive phototropism is mainly regulated by phototropin photoreceptors in blue and UV wavelengths. Contrasting with this, we report that UV RESISTANCE LOCUS8 (UVR8) serves as the predominant photoreceptor of UV-B–induced phototropic responses in Arabidopsis (Arabidopsis thaliana) inflorescence stems. We examined the molecular mechanisms underlying this response and our findings support the Blaauw theory (Blaauw, 1919), suggesting rapid differential growth through unilateral photomorphogenic growth inhibition. UVR8-dependent UV-B light perception occurs mainly in the epidermis and cortex, but deeper tissues such as endodermis can also contribute. Within stems, a spatial difference of UVR8 signal causes a transcript and protein increase of transcription factors ELONGATED HYPOCOTYL5 (HY5) and its homolog HY5 HOMOLOG at the UV-B–exposed side. The irradiated side shows (1) strong activation of flavonoid synthesis genes and flavonoid accumulation; (2) increased gibberellin (GA)2-oxidase expression, diminished GA1 levels, and accumulation of the DELLA protein REPRESSOR OF GA1; and (3) increased expression of the auxin transport regulator PINOID, contributing to diminished auxin signaling. Together, the data suggest a mechanism of phototropin-independent inflorescence phototropism through multiple, locally UVR8-regulated hormone pathways.

The UV Resistance Locus 8 (UVR8) photoreceptor controls UV-B mediated photomorphogenesis in Arabidopsis. The aim of this work is to collect and characterize different molecular reporters of photomorphogenic UV-B responses. Browsing available transcriptome databases, we identified sets of genes responding specifically to this radiation and are controlled by pathways initiated from the UVR8 photoreceptor. We tested the transcriptional changes of several reporters and found that they are regulated differently in different parts of the plant. Our experimental system led us to conclude that the examined genes are not controlled by light piping of UV-B from the shoot to the root or signalling molecules which may travel between different parts of the plant body but by local UVR8 signalling. The initiation of these universal signalling steps can be the induction of Elongated Hypocotyl 5 (HY5) and its homologue, HYH transcription factors. We found that their transcript and protein accumulation strictly depends on UVR8 and happens in a tissue autonomous manner. Whereas HY5 accumulation correlates well with the UVR8 signal across cell layers, the induction of flavonoids depends on both UVR8 signal and a yet to be identified tissue-dependent or developmental determinant.

Tomato (Solanum lycopersicum L.) is globally recognised as a high-value crop both for commercial profit and nutritional benefits. In contrast to the extensive data regarding the changes in the metabolism of tomato fruit exposed to UV radiation, less is known about the foliar and root metabolome. Using an untargeted metabolomic approach through UHPLC-ESI-QTOF-MS analysis, we detected thousands of metabolites in the leaves (3000) and roots (2800) of Micro-Tom tomato plants exposed to 11 days of short daily UV radiation, applied only on the aboveground organs. Multivariate statistical analysis, such as OPLS-DA and volcano, were performed to allow a better understanding of the modifications caused by the treatment. Based on the unexpected modulation to the secondary metabolism, especially the phenylpropanoid pathway, of which compounds were down and up accumulated respectively in leaves and roots of treated plants, a phenolic profiling was carried out for both organs. The phenolic profile was associated with a gene expression analysis to check the transcription trend of genes involved in the UVR8 signalling pathway and the early steps of the phenolic biosynthesis. The retention of the modifications at metabolic and phenolic levels was also investigated 3 days after the UV treatment, showing a prolonged effect on the modulation once the UV treatment had ceased.