Emerging Hubs in Plant Light and Temperature Signaling

Due to their nature as sessile organisms, plants must accurately sense their surroundings and then translate this information into efficient acclimation responses to maximize development. Light and temperature are two major stimuli that provide immediate cues regarding energy availability, daylength, proximity of other species and seasonal changes. Both cues are sensed by complex systems and the integration of these signals is of very high value to properly respond to environmental changes without being disguised by random changes. For instance a cold day has a different significance if it occurs during the illuminated phase of the day or during the night, or when days are shortening during the fall instead of a long‐day in spring. Here, we summarize recent advances in the nature of signaling components that operate as connectors of light and temperature signaling, with emphasis on the emerging hubs. Despite the nature of the thermosensors is still in its infancy compared to an important body of knowledge about plant sensory photoreceptors, the interaction of both types of signaling will not only bring clues of how plants integrate environmental information, but also will help in leading research in the nature of the thermosensors themselves.     

Controlling the Origins of Inflammation with a Photoactive Lipopeptide Immunopotentiator

Inflammatory immune responses are mediated by signaling molecules that are both produced by and recognized across highly heterogeneous cell populations. As such, the study of inflammation using traditional immunostimulants is complicated by paracrine and autocrine signaling, which obscures the origin of a propagating response. To address this challenge, we developed a small‐molecule probe that can photosensitize immune cells, thus allowing light‐mediated inflammation. This probe was used to control the origin of inflammation using light. Following this motif, inflammation was initiated from fibroblasts or dendritic cells. The contributions of fibroblasts and dendritic cells in initiating inflammation in heterogeneous co‐culture are reported, thus providing insights into the future development of vaccines and treatment of inflammation.     

LATERAL INTERACTIONS BETWEEN VERTEBRATE PHOTORECEPTORS

Vertebrate photoreceptors generate electrical signals across their cell membrane when they absorb light. Recent studies show, moreover, that the membrane potential of an individual photoreceptor may be also modified by illumination of its neighbors. Two types of lateral interactions have been described: a direct interaction mediated by electrical synapses between adjacent photoreceptors, and a recurrent interaction mediated by a feedback circuit involving horizontal cells. In the first mechanism, photoreceptors summate their responses over short retinal distances. In the feedback circuit photoreceptors may develop light responses of opposite polarity to those induced by direct illumination. In general, these 'antagonistic' responses are best elicited by large area retinal illumination. As a consequence of these photoreceptor interactions rather complex processing of visual information, in both spatial and chromatic domains, occurs at the first stage of the retinal network.     

More Than Just Light: Clinical Relevance of Light Perception in the Nosocomial Pathogen Acinetobacter baumannii and Other Members of the Genus Acinetobacter

A summary of the major findings concerning light modulation in Acinetobacter baumannii, which governs aspects related to the success of this microorganism as a nosocomial pathogen, is presented. Particularly, the evidence shows that light modulates the ability of the bacteria to persist in the environment, its virulence against eukaryotic hosts and even susceptibility to certain antibiotics. The light signal is sensed through different mechanisms, in some cases involving specialized photoreceptors of the BLUF‐type, whereas in others, directly by a photosensitizer molecule. We also provide new data concerning the genomic context of BLUF‐domain containing proteins within the genus Acinetobacter, as well as further insights into the mechanism of light‐mediated reduction in susceptibility to antibiotics. The overall information points toward light being a crucial stimulus in the lifestyle of members of the genus Acinetobacter as well as in other clinically relevant species, such as members of the ESKAPE group, playing therefore an important role in the clinical settings.     

Light‐Induced Conformational Changes in the Plant Cryptochrome Photolyase Homology Region Resolved by Selective Isotope Labeling and Infrared Spectroscopy

Plant cryptochromes are photoreceptors that regulate flowering, circadian rhythm and photomorphogenesis in response to blue and UV‐A light. It has been demonstrated that the oxidized flavin cofactor is photoreduced to the neutral radical state via separate electron and proton transfer. Conformational changes have been found in the C‐terminal extension, but few studies have addressed the changes in secondary structure in the sensory photolyase homology region (PHR). Here, we investigated the PHR of the plant cryptochrome from the green alga Chlamydomonas reinhardtii by light‐induced infrared difference spectroscopy in combination with global ¹³C and ¹⁵N isotope labeling. Assignment of the signals is achieved by establishing a labeling strategy for cryptochromes that preserves the flavin at natural abundance. We demonstrate by UV/vis spectroscopy that the integrity of the sample is maintained and by mass spectrometry that the global labeling was highly efficient. As a result, difference bands are resolved at full intensity that at natural abundance are compensated by the overlap of flavin and protein signals. These bands are assigned to prominent conformational changes in the PHR by blue light illumination. We postulate that not only the partial unfolding of the C‐terminal extension but also changes in the PHR may mediate signaling events.     

Emetine and the Alternative Splicing of Bcl-X: Where to next?

Interference with the alternative splicing of apoptotic factors offers an innovative and specific mechanism to target malignant cells. In this issue of Chemistry & Biology, Zhou et al. report on the regulation of the alternative splicing of Bcl-x pre-mRNA in response to emetine, a potent protein synthesis inhibitor, as well as define a major player in the signaling mechanism.     

Green Light to Plant Responses to Pathogens: The Role of Chloroplast Light‐Dependent Signaling in Biotic Stress

Light has a key impact on the outcome of biotic stress responses in plants by providing most of the energy and many signals for the deployment of defensive barriers. Within this context, chloroplasts are not only the major source of energy in the light; they also host biosynthetic pathways for the production of stress hormones and secondary metabolites, as well as reactive oxygen species and other signals which modulate nuclear gene expression and plant resistance to pathogens. Environmental, and in particular, light‐dependent regulation of immune responses may allow plants to anticipate and react more effectively to pathogen threats. As more information is gathered, increasingly complex models are developed to explain how light and reactive oxygen species signaling could interact with endogenous defense pathways to elicit efficient protective responses against invading microorganisms. The emerging picture places chloroplasts in a key position of an intricate regulatory network which involves several other cellular compartments. This article reviews current knowledge on the extent and the main features of chloroplast contribution to plant defensive strategies against biotic stress.     

Transformation of Receptor Tyrosine Kinases into Glutamate Receptors and Photoreceptors

Receptor tyrosine kinases (RTKs) are key regulators of cellular functions in metazoans. In vertebrates, RTKs are mostly activated by polypeptides but are not naturally sensitive to amino acids or light. Taking inspiration from Venus kinase receptors (VKRs), an atypical family of RTKs found in nature, we have transformed the human insulin (hIR) and hepatocyte growth factor receptor (hMET) into glutamate receptors by replacing their extracellular binding domains with the ligand‐binding domain of metabotropic glutamate receptor type 2 (mGluR2). We then imparted light sensitivity through covalent attachment of a synthetic glutamate‐based photoswitch via a self‐labelling SNAP tag. By employing a Xenopus laevis oocyte kinase activity assay, we demonstrate how these chimeric RTKs, termed light‐controlled human insulin receptor (LihIR) and light‐controlled human MET receptor (LihMET), can be used to exert optical control over the insulin or MET signaling pathways. Our results outline a potentially general strategy to convert RTKs into photoreceptors.     

PHOTOMORPHOGENIC PROCESSES IN THE AGRICULTURAL ENVIRONMENT

We review recent advances in our understanding of how photomorphogenic mechanisms influence processes of significance for agriculture, including weed seed germination, acclimation to solar UV-B radiation, and plant interactions in canopies. It has been demonstrated recently that seeds of some weed species acquire the ability to germinate in response to very low photon fluences during burial. Short sunlight pulses perceived by seeds during tillage appear to be important signals of soil disturbance. Regarding UV-B acclimation there is increasing evidence for the involvement of a specific UV-B photoreceptor in pigmentation and morphological responses to UV-B. The protective role played by UV-B-induced flavonoids has been experimentally demonstrated. Concerning plant interactions in canopies it has been shown that, by perceiving light signals through phytochrome and blue-UV-A photoreceptors, plants obtain information about the characteristics of the surrounding vegetation well before their resources are reduced as a consequence of neighbors' activities. There appears to be a large potential for improving cropping systems by manipulating the light environment or by changing the photomorphogenic behavior of crop plants. However, relatively little is known about how the expression and agronomic significance of photomorphogenic responses are influenced by other factors of the plant's environment.     

Identification of a Conformational Equilibrium That Determines the Efficacy and Functional Selectivity of the μ‐Opioid Receptor

G‐protein‐coupled receptor (GPCR) ligands impart differing degrees of signaling in the G‐protein and arrestin pathways, in phenomena called “biased signaling”. However, the mechanism underlying the biased signaling of GPCRs is still unclear, although crystal structures of GPCRs bound to the G protein or arrestin are available. In this study, we observed the NMR signals from methionine residues of the μ‐opioid receptor (μOR) in the balanced‐ and biased‐ligand‐bound states. We found that the intracellular cavity of μOR exists in an equilibrium between closed and multiple open conformations with coupled conformational changes on the transmembrane helices 3, 5, 6, and 7, and that the population of each open conformation determines the G‐protein‐ and arrestin‐mediated signaling levels in each ligand‐bound state. These findings provide insight into the biased signaling of GPCRs and will be helpful for development of analgesics that stimulate μOR with reduced tolerance and dependence.     

The Involvement of Splicing Factor hnRNP A1 in UVB‐induced Alternative Splicing of hdm2

Human homolog double minute 2 (hdm2), an oncoprotein, which binds to tumor suppressor p53 to facilitate its degradation, has been known to contribute to tumorigenesis. Its splicing variants are reported to be highly expressed in many cancers and can be induced by ultraviolet B light (UVB). However, the mechanisms of how UVB radiation induces hdm2 alternative splicing still remain unclear. In this study, we investigated the roles of two common splicing factors, heterogeneous nuclear ribonucleoproteins (hnRNP) A1 and serine/arginine‐rich splicing factor 1 (SRSF1), in regulating UVB‐induced hdm2 splicing. Our study indicated that while the expression of both hnRNP A1 and SRSF1 are induced, only hnRNP A1 is involved in hdm2 alternative splicing upon UVB irradiation. Overexpression of hnRNP A1 resulted in decrease of full‐length hdm2 (hdm2‐FL) and increase of hdm2B, one of hdm2 alternate‐splicing forms; while down‐regulated hnRNP A1 expression led to the decrease of the hdm2‐FL and hdm2B in HaCaT cells. Protein‐mRNA binding assay confirmed that UVB irradiation could increase the binding of hnRNP A1 to hdm2 pre‐mRNA. In conclusion, we elucidated that UVB induces alternative splicing of hdm2 by increasing the expression and the binding of hnRNP A1 to hdm2 full‐length mRNA.