Phototropin mediated ultraviolet-B phototropism in etiolated seedlings

Low doses of ultraviolet B (UV-B) light have significant effects on plant morphology [1]. In Arabidopsis, many UV-B induced morphological modifications have been ascribed to the UV-B specific receptor UV resistance locus 8 (UVR8). Recent findings in etiolated Arabidopsis seedlings indicate that UVR8 regulated signaling can induce phototropin independent directional bending towards UV-B light [2]. Here, we study the relative contribution of each of these pathways in UV-B regulated phototropism through kinetic analysis of seedlings. The role of phototropins is favored under reduced light conditions and the higher UVR8 response in the UV-B hypersensitive rup1rup2 mutants is interfering with the fast phototropin-regulated phototropic response. Our data suggest that phototropins are the primary receptors for UV-B induced phototropism in etiolated Arabidopsis seedlings, and the RUP-mediated negative feedback pathway prevents UVR8-mediated signaling to affect the phototropin-dependent response. In conclusion, phototropins are the most important receptors for UV-B induced phototropism in etiolated seedlings, and a RUP-mediated negative feedback pathway prevents UVR8 signaling to interfere with the phototropin dependent response

Ultraviolet radiation from a plant perspective : the plant-microorganism context

Ultraviolet (UV) radiation directly affects plants and microorganisms, but also alters the species-specific interactions between them. The distinct bands of UV radiation, UV-A, UV-B, and UV-C have different effects on plants and their associated microorganisms. While UV-A and UV-B mainly affect morphogenesis and phototropism, UV-B and UV-C strongly trigger secondary metabolite production. Short wave (<350 nm) UV radiation negatively affects plant pathogens in direct and indirect ways. Direct effects can be ascribed to DNA damage, protein polymerization, enzyme inactivation and increased cell membrane permeability. UV-C is the most energetic radiation and is thus more effective at lower doses to kill microorganisms, but by consequence also often causes plant damage. Indirect effects can be ascribed to UV-B specific pathways such as the UVR8-dependent upregulated defense responses in plants, UV-B and UV-C upregulated ROS accumulation, and secondary metabolite production such as phenolic compounds. In this review, we summarize the physiological and molecular effects of UV radiation on plants, microorganisms and their interactions. Considerations for the use of UV radiation to control microorganisms, pathogenic as well as non-pathogenic, are listed. Effects can be indirect by increasing specialized metabolites with plant pre-treatment, or by directly affecting microorganisms

Ultraviolet-B induced phototropism in Arabidopsis seedlings and inflorescence stems

Ultraviolet-B radiation (UV-B - 280-315nm) was only recently described as a phototropism-inducing type of radiation. We have identified that phototropic growth towards UV-B of Arabidopsis hypocotyls and inflorescence stems are both regulated by the same photoreceptor pathways but display a shift in dominance during plant development. The phototropin pathway is dominant in etiolated seedlings (Vanhaelewyn et al., 2016a), while the UVR8 pathway is predominant in inflorescence stems. The role of key-players in the UVR8 pathway, such as HY5 and HYH has been evaluated for their importance in this phototropic response. Unilaterally UV-B irradiated inflorescence stem tissue demonstrates a lateral UVR8-mediated signal gradient. In addition, the function of UVR8 in different cell types was validated by use of cell type specific complementation lines of UVR8 in a uvr8-6 mutant background. This reveals that UVR8 signaling is important in various cell types within the inflorescence stem. As UV-B is known to affect plant hormones (Vanhaelewyn et al., 2016b), we investigated their involvement in this phototropic response. By means of reporter line analysis, mutant analysis, gene expression assays and pharmacological assays, we designate a role for both gibberellins and auxins to UV-B induced phototropism of Arabidopsis inflorescence stems. These combined data provide a mechanistic framework for UV-B induced phototropism

An ultraviolet B condition that affects growth and defense in Arabidopsis

Ultraviolet B light (UV-B, 280-315 nm) is the shortest wavelength of the solar spectrum reaching the surface of the Earth. It has profound effects on plants, ranging from growth regulation to severe metabolic changes. Low level UV-B mainly causes photomorphogenic effects while higher levels can induce stress, yet these effects tend to overlap. Here we identified a condition that allows growth reduction without obvious detrimental stress in wild type Arabidopsis rosette plants. This condition was used to study the effects of a daily UV-B dose on plant characteristics of UV-B adapted plants in detail. Exploration of the transcriptome of developing leaves indicated downregulation of genes involved in stomata formation by UV-B, while at the same time genes involved in photoprotective pigment biosynthesis were upregulated. These findings correspond with a decreased stomatal density and increased UV-B absorbing pigments. Gene ontology analysis revealed upregulation of defense related genes and meta-analysis showed substantial overlap of the UV-B regulated transcriptome with transcriptomes of salicylate and jasmonate treated as well as herbivore exposed plants. Feeding experiments showed that caterpillars of Spodoptera littoralis are directly affected by UV-B, while performance of the aphid Myzus persicae is diminished by a plant mediated process

Cryptochromes are the key photoreceptors mediating Arabidopsis inflorescence stem movements under natural sunlight

Inflorescence stem movements in response to natural sunlight are widely spread across angiosperm species and are suggested to increase reproductive success. However, the underlying mechanisms that mediate this phototropic response to natural irradiation are unclear. We studied phototropic responses of Arabidopsis inflorescences in both laboratory and field conditions and report an action spectrum at wavelengths below 500 nm, mediated by several photoreceptor families. In controlled conditions, UVR8 is the key photoreceptor for narrowband UV-B radiation while phototropins and cryptochromes are crucial for phototropic responses to narrowband blue light. At low blue irradiances, phototropins are dominant while cryptochromes are essential during high blue irradiances but subjected to the negative control of phototropins. Importantly, cryptochromes are the principal photoreceptors mediating inflorescence stem bending under full sunlight

Progressive alterations in ultraviolet-B induced phototropism during Arabidopsis development

Low fluence rate ultraviolet-B radiation (280-315 nm) substantially affects plant morphology. Numerous UV-B induced morphological adaptations in Arabidopsis are ascribed to the UV-B specific photoreceptor UV RESISTANCE LOCUS 8 (UVR8). Well documented examples are shorter petioles and shorter stems. Alterations are also observed at the cellular level such as changes in cell elongation, division and differentiation. Notwithstanding this extensive knowledge of UV-B responses, the mechanisms by which UV-B radiation controls plant architecture are poorly understood. Our recent research in Arabidopsis revealed that unilateral narrow-band UV-B radiation can induce reorientation of etiolated hypocotyls through UVR8 mediated signaling. This response is triggered by unilateral radiation of wavelengths shorter than 340 nm and is temporally distinct from phototropin-mediated phototropic bending. Analysis of the kinetics of plant reorientation allowed us to quantify the relative contribution of UVR8 and phototropins in steering this UV-B induced phototropic movement of etiolated hypocotyls. These data indicate that in etiolated seedlings, phototropins are more sensitive to UV-B for regulating phototropism than UVR8 and therefore mask the effect of UVR8. Phototropin signaling under UV-B is mechanistically similar to that in blue light, involving phototropin autophosphorylation and NPH3 dephosphorylation. Furthermore, the negative feedback controlled by REPRESSOR OF UV-B PHOTOMORPHOGENESIS prevents UVR8-mediated fast phototropin-dependent bending. The UVR8-phototropin relationship described for etiolated seedlings is not universally applicable. We found that the main photoreceptor for UV-B-induced phototropism in inflorescence stems is UVR8, with a less significant role for phototropins. The contribution of UVR8 expressed in different cell layers to this response is currently being examined. Based on pharmacological assays, mutant analysis and reporter lines, this shifting role of UVR8 and phototropins during plant development will be presented and discussed

UVR8-dependent reporters reveal spatial characteristics of signal spreading in plant tissues

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

Differential UVR8 Signal across the Stem Controls UV-B-Induced Inflorescence Phototropism

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 UVB–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.Fil: Vanhaelewyn, Lucas. University of Ghent; BélgicaFil: Viczián, András. Institute of Plant Biology, Biological Research Centre; HungríaFil: Prinsen, Els. Universiteit Antwerp; BélgicaFil: Bernula, Péter. Institute of Plant Biology, Biological Research Centre; Hungría. University of Szeged; HungríaFil: Serrano, Alejandro Miguel. Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Provincia de Mendoza. Instituto Argentino de Investigaciones de las Zonas Áridas. Universidad Nacional de Cuyo. Instituto Argentino de Investigaciones de las Zonas Áridas; ArgentinaFil: Arana, Maria Veronica. Instituto Nacional de Tecnología Agropecuaria. Centro Regional Patagonia Norte. Estación Experimental Agropecuaria San Carlos de Bariloche. Instituto de Investigaciones Forestales y Agropecuarias Bariloche. - Consejo Nacional de Investigaciones Científicas y Técnicas. Centro Científico Tecnológico Conicet - Patagonia Norte. Instituto de Investigaciones Forestales y Agropecuarias Bariloche; ArgentinaFil: Ballare, Carlos Luis. Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura. Universidad de Buenos Aires. Facultad de Agronomía. Instituto de Investigaciones Fisiológicas y Ecológicas Vinculadas a la Agricultura; Argentina. Universidad Nacional de San Martín. Instituto de Investigaciones Biotecnológicas. - Consejo Nacional de Investigaciones Científicas y Técnicas. Oficina de Coordinación Administrativa Parque Centenario. Instituto de Investigaciones Biotecnológicas; ArgentinaFil: Nagy, Ferenc. Institute of Plant Biology, Biological Research Centre; HungríaFil: Van Der Straeten, Dominique. University of Ghent; BélgicaFil: Vandenbussche, Filip. University of Ghent; Bélgic

UVR8 mediated spatial differences as a prerequisite for UV-B induced inflorescence phototropism

In Arabidopsis hypocotyls, phototropins are the dominant photoreceptors for the positive phototropism response towards unilateral ultraviolet-B (UV-B) radiation. We report a stark contrast of response mechanism with inflorescence stems with a central role for UV RESISTANCE LOCUS 8 (UVR8). The perception of UV-B occurs mainly in the epidermis and cortex with a lesser contribution of the endodermis. Unilateral UV-B exposure does not lead to a spatial difference in UVR8 protein levels but does cause differential UVR8 signal throughout the stem with at the irradiated side 1) increase of the transcription factor ELONGATED HYPOCOTYL 5 (HY5), 2) an associated strong activation of flavonoid biosynthesis genes and flavonoid accumulation, 3) increased GA2oxidase expression, diminished gibberellin1 levels and accumulation of DELLA protein REPRESSOR OF GA1 (RGA) and, 4) increased expression of the auxin transport regulator, PINOID, contributing to local diminished auxin signalling. Our molecular findings are in support of the Blaauw theory (1919), suggesting that differential growth occurs trough unilateral photomorphogenic growth inhibition. Together the data indicate phototropin independent inflorescence phototropism through multiple locally UVR8-regulated hormone pathways

REPRESSOR OF ULTRAVIOLET-B PHOTOMORPHOGENESIS function allows efficient phototropin mediated ultraviolet-B phototropism in etiolated seedlings.

Ultraviolet B (UV-B) light is a part of the solar radiation which has significant effects on plant morphology, even at low doses. In Arabidopsis, many of these morphological changes have been attributed to a specific UV-B receptor, UV resistance locus 8 (UVR8). Recent findings showed that next to phototropin regulated phototropism, UVR8 mediated signaling is able of inducing directional bending towards UV-B light in etiolated seedlings of Arabidopsis, in a phototropin independent manner. In this study, kinetic analysis of phototropic bending was used to evaluate the relative contribution of each of these pathways in UV-B mediated phototropism. Diminishing UV-B light intensity favors the importance of phototropins. Molecular and genetic analyses suggest that UV-B is capable of inducing phototropin signaling relying on phototropin kinase activity and regulation of NPH3. Moreover, enhanced UVR8 responses in the UV-B hypersensitive rup1rup2 mutants interferes with the fast phototropin mediated phototropism. Together the data suggest that phototropins are the most important receptors for UV-B induced phototropism in etiolated seedlings, and a RUP mediated negative feedback pathway prevents UVR8 signaling to interfere with the phototropin dependent response