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

Uncovering a novel function of the CCR4-NOT complex in phytochrome A-mediated light signalling in plants

Phytochromes are photoreceptors regulating growth and development in plants. Using the model plant Arabidopsis, we identified a novel signalling pathway downstream of the farred light-sensing phytochrome, phyA, that depends on the highly conserved CCR4-NOT complex. CCR4-NOT is integral to RNA metabolism in yeast and animals, but its function in plants is largely unknown. NOT9B, an Arabidopsis homologue of human CNOT9, is a component of the CCR4-NOT complex, and acts as negative regulator of phyA-specific light signalling when bound to NOT1, the scaffold protein of the complex. Light-activated phyA interacts with and displaces NOT9B from NOT1, suggesting a potential mechanism for light signalling through CCR4-NOT. ARGONAUTE 1 and proteins involved in splicing associate with NOT9B and we show that NOT9B is required for specific phyA-dependent alternative splicing events. Furthermore, association with nuclear localised ARGONAUTE 1 raises the possibility that NOT9B and CCR4-NOT are involved in phyA-modulated gene expression

Arabidopsis thaliana Circadian clock is regulated by the small GTPase LIP1

Background: At the core of the eukaryotic circadian network, clock genes/proteins form multiple transcriptional/translational negative-feed back loops and generate a basic -24 hr oscillation, which provides daily regulation for a wide range of processes. This temporal organization enhances the fitness of the organism only if it corresponds to the natural day/night cycles. Light is the most effective signal in synchronizing the oscillator to environmental cycles. Results: The lip1-1 (light insensitive period 1) mutant isolated from the model plant Arabidopsis thaliana displays novel circadian phenotypes arising from specific defects in the light input pathway to the oscillator. In wild-type plants, period length shortens with increasing light fluence rates and the phase of rhythms can be shifted by light pulses administered to dark-adapted plants. In contrast, in lip1-1, period length is nearly insensitive to light intensity and significantly larger phase shifts (delays) can be induced during the subjective night. The mutant also displays elevatedphotomorphogenic responses to red and blue light, which cannot be explained by the circadian defect, suggesting distinct functions for LIP1 in the circadian light input and photomorphogenesis. The LIP1 gene encodes a functional, plant-specific atypical small GTPase, and therefore we postulate that it acts similarly to ZEITLUPE at postranscriptional level. Conclusions: LIP1 represents the first small GTPase implicated in the circadian system of plants. LIP1 plays a unique negative role in controlling circadian light input and is required for precise entrainment of the plant clock

Forward genetic analysis of the circadian clock separates the multiple functions of ZEITLUPE

The circadian system of Arabidopsis ( Arabidopsis thaliana) includes feedback loops of gene regulation that generate 24-h oscillations. Components of these loops remain to be identified; none of the known components is completely understood, including ZEITLUPE (ZTL), a gene implicated in regulated protein degradation. ztl mutations affect both circadian and developmental responses to red light, possibly through ZTL interaction with PHYTOCHROME B (PHYB). We conducted a large-scale genetic screen that identified additional clock-affecting loci. Other mutants recovered include 11 new ztl alleles encompassing mutations in each of the ZTL protein domains. Each mutation lengthened the circadian period, even in darkgrown seedlings entrained to temperature cycles. A mutation of the LIGHT, OXYGEN, VOLTAGE (LOV)/Period-ARNT-Sim ( PAS) domain was unique in retaining wild-type responses to red light both for the circadian period and for control of hypocotyl elongation. This uncoupling of ztl phenotypes indicates that interactions of ZTL protein with multiple factors must be disrupted to generate the full ztl mutant phenotype. Protein interaction assays showed that the ztl mutant phenotypes were not fully explained by impaired interactions with previously described partner proteins Arabidopsis S-phase kinase-related protein 1, TIMING OF CAB EXPRESSION 1, and PHYB. Interaction with PHYB was unaffected by mutation of any ZTL domain. Mutation of the kelch repeat domain affected protein binding at both the LOV/PAS and the F-box domains, indicating that interaction among ZTL domains leads to the strong phenotypes of kelch mutations. Forward genetics continues to provide insight regarding both known and newly discovered components of the circadian system, although current approaches have saturated mutations at some loci