Cellular Functions In Plants Research Articles

Arabidopsis thaliana ubiquitin-associated protein 2 (AtUAP2) functions as an E4 ubiquitin factor and negatively modulates dehydration stress response.

E4, a ubiquitin (Ub) chain assembly factor and post-translational modification protein, plays a key role in the regulation of multiple cellular functions in plants during biotic or abiotic stress. We have more recently reported that E4 factor AtUAP1 is a negative regulator of the osmotic stress response and enhances the multi-Ub chain assembly of E3 ligase Arabidopsis thaliana RING Zinc Finger 1 (AtRZF1). To further investigate the function of other E4 Ub factors in osmotic stress, we isolated AtUAP2, an AtUAP1 homolog, which interacted with AtRZF1, using pull-down assay and bimolecular fluorescence complementation analysis. AtUAP2, a Ub-associated motif-containing protein, interacts with oligo-Ub5, -Ub6, and -Ub7 chains. The yeast functional complementation experiment revealed that AtUAP2 functions as an E4 Ub factor. In addition, AtUAP2 is localized in the cytoplasm, different from AtUAP1. The activity of AtUAP2 was relatively strongly induced in the leaf tissue of AtUAP2 promoter-β-glucuronidase transgenic plants by abscisic acid, dehydration, and oxidative stress. atuap2 RNAi lines were more insensitive to osmotic stress condition than wild-type during the early growth of seedlings, whereas the AtUAP2-overexpressing line exhibited relatively more sensitive responses. Analyses of molecular and physiological experiments showed that AtUAP2 could negatively mediate the osmotic stress-induced signaling. Genetic studies showed that AtRZF1 mutation could suppress the dehydration-induced sensitive phenotype of the AtUAP2-overexpressing line, suggesting that AtRZF1 acts genetically downstream of AtUAP2 during osmotic stress. Taken together, our findings show that the AtRZF1-AtUAP2 complex may play important roles in the ubiquitination pathway, which controls the osmotic stress response in Arabidopsis.

Microtubule-associated phase separation of MIDD1 tunes cell wall spacing in xylem vessels in Arabidopsis thaliana.

Properly patterned cell walls specify cellular functions in plants. Differentiating protoxylem and metaxylem vessel cells exhibit thick secondary cell walls in striped and pitted patterns, respectively. Cortical microtubules are arranged in distinct patterns to direct cell wall deposition. The scaffold protein MIDD1 promotes microtubule depletion by interacting with ROP GTPases and KINESIN-13A in metaxylem vessels. Here we show that the phase separation of MIDD1 fine-tunes cell wall spacing in protoxylem vessels in Arabidopsis thaliana. Compared with wild-type, midd1 mutants exhibited narrower gaps and smaller pits in the secondary cell walls of protoxylem and metaxylem vessel cells, respectively. Live imaging of ectopically induced protoxylem vessels revealed that MIDD1 forms condensations along the depolymerizing microtubules, which in turn caused massive catastrophe of microtubules. The MIDD1 condensates exhibited rapid turnover and were susceptible to 1,6-hexanediol. Loss of ROP abolished the condensation of MIDD1 and resulted in narrow cell wall gaps in protoxylem vessels. These results suggest that the microtubule-associated phase separation of MIDD1 facilitates microtubule arrangement to regulate the size of gaps in secondary cell walls. This study reveals a new biological role of phase separation in the fine-tuning of cell wall patterning.

Plants' Response Mechanisms to Salinity Stress.

Soil salinization is a severe abiotic stress that negatively affects plant growth and development, leading to physiological abnormalities and ultimately threatening global food security. The condition arises from excessive salt accumulation in the soil, primarily due to anthropogenic activities such as irrigation, improper land uses, and overfertilization. The presence of Na⁺, Cl-, and other related ions in the soil above normal levels can disrupt plant cellular functions and lead to alterations in essential metabolic processes such as seed germination and photosynthesis, causing severe damage to plant tissues and even plant death in the worst circumstances. To counteract the effects of salt stress, plants have developed various mechanisms, including modulating ion homeostasis, ion compartmentalization and export, and the biosynthesis of osmoprotectants. Recent advances in genomic and proteomic technologies have enabled the identification of genes and proteins involved in plant salt-tolerance mechanisms. This review provides a short overview of the impact of salinity stress on plants and the underlying mechanisms of salt-stress tolerance, particularly the functions of salt-stress-responsive genes associated with these mechanisms. This review aims at summarizing recent advances in our understanding of salt-stress tolerance mechanisms, providing the key background knowledge for improving crops' salt tolerance, which could contribute to the yield and quality enhancement in major crops grown under saline conditions or in arid and semiarid regions of the world.

A bacterial type III effector targets plant vesicle-associated membrane proteins.

The soilborne bacterial pathogen Ralstonia solanacearum is one of the most destructive plant pathogens worldwide, and its infection process involves the manipulation of numerous plant cellular functions. In this work, we found that the R. solanacearum effector protein RipD partially suppressed different levels of plant immunity triggered by R. solanacearum elicitors, including specific responses triggered by pathogen-associated molecular patterns and secreted effectors. RipD localized in different subcellular compartments in plant cells, including vesicles, and its vesicular localization was enriched in cells undergoing R. solanacearum infection, suggesting that this specific localization may be particularly relevant during infection. Among RipD-interacting proteins, we identified plant vesicle-associated membrane proteins (VAMPs). We also found that overexpression of Arabidopsis thaliana VAMP721 and VAMP722 in Nicotiana benthamiana leaves promoted resistance to R. solanacearum, and this was abolished by the simultaneous expression of RipD, suggesting that RipD targets VAMPs to contribute to R. solanacearum virulence. Among proteins secreted in VAMP721/722-containing vesicles, CCOAOMT1 is an enzyme required for lignin biosynthesis, and mutation of CCOAOMT1 enhanced plant susceptibility to R. solanacearum. Altogether our results reveal the contribution of VAMPs to plant resistance against R. solanacearum and their targeting by a bacterial effector as a pathogen virulence strategy.

Molecular Mechanisms of Regulation of Root Development by Plant Peptides

Peptides perform many functions, participating in the regulation of cell differentiation, regulating plant growth and development, and also involved in the response to stress factors and in antimicrobial defense. Peptides are an important class biomolecules for intercellular communication and in the transmission of various signals. The intercellular communication system based on the ligand-receptor bond is one of the most important molecular bases for creating complex multicellular organisms. Peptide-mediated intercellular communication plays a critical role in the coordination and determination of cellular functions in plants. The intercellular communication system based on the receptor-ligand is one of the most important molecular foundations for creating complex multicellular organisms. Peptide-mediated intercellular communication plays a critical role in the coordination and determination of cellular functions in plants. The identification of peptide hormones, their interaction with receptors, and the molecular mechanisms of peptide functioning are important for understanding the mechanisms of both intercellular communications and for regulating plant development. In this review, we drew attention to some peptides involved in the regulation of root development, which implement this regulation by the mechanism of a negative feedback loop.

Chemical control of receptor kinase signaling by rapamycin-induced dimerization

Membrane-localized leucine-rich repeat receptor kinases (LRR-RKs) sense diverse extracellular signals, and coordinate and specify cellular functions in plants. However, functional understanding and identification of the cellular signaling of most LRR-RKs remain a major challenge owing to their genetic redundancy, the lack of ligand information, and subtle phenotypes of LRR-RK overexpression. Here, we report an engineered rapamycin-inducible dimerization (RiD) receptor system that triggers a receptor-specific LRR-RK signaling independent of their cognate ligands or endogenous receptors. Using the RiD-receptors, we demonstrated that the rapamycin-mediated association of chimeric cytosolic kinase domains from the BRI1/BAK1 receptor/co-receptor, but not the BRI1/BRI1 or BAK1/BAK1 homodimer, is sufficient to activate downstream brassinosteroid signaling and physiological responses. Furthermore, we showed that the engineered RiD-FLS2/BAK1 could activate flagellin-22-mediated immune signaling and responses. Using the RiD system, we also identified the potential function of an unknown orphan receptor in immune signaling and revealed the differential activities of SERK co-receptors of LRR-RKs. Our results indicate that the RiD method can serve as a synthetic biology tool for precise temporal manipulation of LRR-RK signaling and for understanding LRR-RK biology.

Plant Aquaporins

Abstract This mini-review briefly presents the main types of plant aquaporins, highlighting their importance for different plant species and for plant cellular functions. Aquaporins (AQPs), families of water channel proteins (WCPs) are transmembrane proteins that are present in prokaryotes, animals, plants, and humans. The plant aquaporins are part of the Major Intrinsic Proteins (MIPs) family which resides in the following plant organs: roots, stems, leaves, flowers, fruits, and seeds. According to the sub-cellular localization, to their sequence homologies and to their phylogenetic distribution, plant aquaporins have been divided in five subgroups: (a) plasma membrane intrinsic proteins (PIPs); (b) tonoplast intrinsic proteins (TIPs); (c) Nodulin26-like intrinsic membrane proteins (NIPs); (d) small basic intrinsic proteins (SIPs) and (e) uncharacterized intrinsic proteins (XIPs). Different subclasses of the plant aquaporins allow several types of transport using: water, glycerol, urea, hydrogen peroxide, organic acids, ethanol, methanol, arsenite, lactic acid, and gaseous compounds. Plant aquaporins have a significant role in cell response to cold stress, photosynthesis, plant growth, cell elongation, reproduction, and seed germination.

An outlook on lysine methylation of non-histone proteins in plants.

Protein methylation is a very diverse, widespread, and important post-translational modification affecting all aspects of cellular biology in eukaryotes. Methylation on the side-chain of lysine residues in histones has received considerable attention due to its major role in determining chromatin structure and the epigenetic regulation of gene expression. Over the last 20 years, lysine methylation of non-histone proteins has been recognized as a very common modification that contributes to the fine-tuned regulation of protein function. In plants, our knowledge in this field is much more fragmentary than in yeast and animal cells. In this review, we describe the plant enzymes involved in the methylation of non-histone substrates, and we consider historical and recent advances in the identification of non-histone lysine-methylated proteins in photosynthetic organisms. Finally, we discuss our current knowledge about the role of protein lysine methylation in regulating molecular and cellular functions in plants, and consider challenges for future research.

Kinesins and Myosins: Molecular Motors that Coordinate Cellular Functions in Plants.

Kinesins and myosins are motor proteins that can move actively along microtubules and actin filaments, respectively. Plants have evolved a unique set of motors that function as regulators and organizers of the cytoskeleton and as drivers of long-distance transport of various cellular components. Recent progress has established the full complement of motors encoded in plant genomes and has revealed valuable insights into the cellular functions of many kinesin and myosin isoforms. Interestingly, several of the motors were found to functionally connect the two cytoskeletal systems and thereby to coordinate their activities. In this review, we discuss the available genetic, cell biological, and biochemical data for each of the plant kinesin and myosin families from the context of their subcellular mechanism of action as well as their physiological function in the whole plant. We particularly emphasize work that illustrates mechanisms by which kinesins and myosins coordinate the activities of the cytoskeletal system.

Polyamines: Bio-Molecules with Diverse Functions in Plant and Human Health and Disease.

Biogenic amines—polyamines (PAs), particularly putrescine, spermidine and spermine are ubiquitous in all living cells. Their indispensable roles in many biochemical and physiological processes are becoming commonly known, including promoters of plant life and differential roles in human health and disease. PAs positively impact cellular functions in plants—exemplified by increasing longevity, reviving physiological memory, enhancing carbon and nitrogen resource allocation/signaling, as well as in plant development and responses to extreme environments. Thus, one or more PAs are commonly found in genomic and metabolomics studies using plants, particulary during different abiotic stresses. In humans, a general decline in PA levels with aging occurs parallel with some human health disorders. Also, high PA dose is detrimental to patients suffering from cancer, aging, innate immunity and cognitive impairment during Alzheimer and Parkinson diseases. A dichotomy exists in that while PAs may increase longevity and reduce some age-associated cardiovascular diseases, in disease conditions involving higher cellular proliferation, their intake has negative consequences. Thus, it is essential that PA levels be rigorously quantified in edible plant sources as well as in dietary meats. Such a database can be a guide for medical experts in order to recommend which foods/meats a patient may consume and which ones to avoid. Accordingly, designing both high and low polyamine diets for human consumption are in vogue, particularly in medical conditions where PA intake may be detrimental, for instance, cancer patients. In this review, literature data has been collated for the levels of the three main PAs, putrescine, spermidine and spermine, in different edible sources—vegetables, fruits, cereals, nuts, meat, sea food, cheese, milk, and eggs. Based on our analysis of vast literature, the effects of PAs in human/animal health fall into two broad, Yang and Yin, categories: beneficial for the physiological processes in healthy cells and detrimental under pathological conditions.

Biomass production and salinity response in plants: role of MicroRNAs

Small non-coding RNAs are one of the major contributors for diverse cellular functions in plants. MicroRNAs (miRNAs) are one such class of small non-coding RNAs playing crucial role in normal plant growth and development as well as under environmental stresses. miRNA modulates the transcripts level by directly binding to the transcript generated from the target gene and mediate either the cleavage of the target mRNA transcript or the inhibitition of translation of the target transcript or inhibition of target gene expression through epigenetic modification. In the current scenario, understanding the link between the diverse miRNAs and their orchestrated functioning for regulation of stress signaling as well as biomass production is a major challenge. In the present review, we have explored the current knowledge about the plant miRNA families and their functional aspects, particularly in the context of salinity stress tolerance and higher biomass production. We conclude that miRNA families such as miR156, 159, 164, 166, 319, 393, 396 and 414 are possibly mediating the cross talk between biomass production and salinity stress response. The present review may improve the current knowledge about the plant miRNAs and thus may help in generating crop plants for marginal lands.

The Ralstonia solanacearum Type III Effector RipAY Is Phosphorylated in Plant Cells to Modulate Its Enzymatic Activity.

Most bacterial pathogens subvert plant cellular functions using effector proteins delivered inside plant cells. In the plant pathogen Ralstonia solanacearum, several of these effectors contain domains with predicted enzymatic activities, including acetyltransferases, phosphatases, and proteases, among others. How these enzymatic activities get activated inside plant cells, but not in the bacterial cell, remains unknown in most cases. In this work, we found that the R. solanacearum effector RipAY is phosphorylated in plant cells. One phosphorylated serine residue, S131, is required for the reported gamma-glutamyl cyclotransferase activity of RipAY, responsible for the degradation of gamma-glutamyl compounds (such as glutathione) inside host cells. Accordingly, non-phosphorylable mutants in S131 abolish RipAY-mediated degradation of glutathione in plant cells and the subsequent suppression of plant immune responses. In this article, we examine our results in relation to the recent reports on the biochemical activities of RipAY, and discuss the potential implications of phosphorylation in plant cells as a mechanism to modulate the enzymatic activity of RipAY.

Post-translational modifications of Arabidopsis E3 SUMO ligase AtSIZ1 are controlled by environmental conditions.

Sumoylation regulates numerous cellular functions in plants as well as in other eukaryotic systems. However, the regulatory mechanisms controlling E3 small ubiquitin‐related modifier (SUMO) ligase are not well understood. Here, post‐translational modification of the Arabidopsis E3 SUMO ligase AtSIZ1 was shown to be specifically controlled by abiotic stresses. AtSIZ1 ubiquitination was induced by exposure to heat stress in transgenic plants overexpressing the E3 ubiquitin ligase COP1. In addition, AtSIZ1 ubiquitination was strongly enhanced in transgenic plants overexpressing SUMO isopeptidase ESD4 under heat stress. By contrast, drought stress induced sumoylation rather than ubiquitination of AtSIZ1 and sumoylated forms of AtSIZ1 accumulated in esd4 and cop1–4 mutants. Moreover, siz1 mutants were found to be tolerant to heat and drought stresses. Taken together, these results indicate that ubiquitination and sumoylation of AtSIZ1 in response to abiotic stresses depend on the activities of COP1 and ESD4 and that the activity and stability of AtSIZ1 can be specifically controlled by different abiotic stresses.

The Ralstonia solanacearum type III effector RipAY targets plant redox regulators to suppress immune responses.

The subversion of plant cellular functions is essential for bacterial pathogens to proliferate in host plants and cause disease. Most bacterial plant pathogens employ a type III secretion system to inject type III effector (T3E) proteins inside plant cells, where they contribute to the pathogen-induced alteration of plant physiology. In this work, we found that the Ralstonia solanacearum T3E RipAY suppresses plant immune responses triggered by bacterial elicitors and by the phytohormone salicylic acid. Further biochemical analysis indicated that RipAY associates inplanta with thioredoxins from Nicotiana benthamiana and Arabidopsis. Interestingly, RipAY displays γ-glutamyl cyclotransferase (GGCT) activity to degrade glutathione in plant cells, which is required for the reported suppression of immune responses. Given the importance of thioredoxins and glutathione as major redox regulators in eukaryotic cells, RipAY activity may constitute a novel and powerful virulence strategy employed by R.solanacearum to suppress immune responses and potentially alter general redox signalling in host cells.

Signature motif-guided identification of receptors for peptide hormones essential for root meristem growth.

Peptide-mediated cell-to-cell signaling has crucial roles in coordination and definition of cellular functions in plants. Peptide-receptor matching is important for understanding the mechanisms underlying peptide-mediated signaling. Here we report the structure-guided identification of root meristem growth factor (RGF) receptors important for plant development. An assay based on a signature ligand recognition motif (Arg-x-Arg) conserved in a subfamily of leucine-rich repeat receptor kinases (LRR-RKs) identified the functionally uncharacterized LRR-RK At4g26540 as a receptor of RGF1 (RGFR1). We further solved the crystal structure of RGF1 in complex with the LRR domain of RGFR1 at a resolution of 2.6 Å, which reveals that the Arg-x-Gly-Gly (RxGG) motif is responsible for specific recognition of the sulfate group of RGF1 by RGFR1. Based on the RxGG motif, we identified additional four RGFRs. Participation of the five RGFRs in RGF-induced signaling is supported by biochemical and genetic data. We also offer evidence showing that SERKs function as co-receptors for RGFs. Taken together, our study identifies RGF receptors and co-receptors that can link RGF signals with their downstream components and provides a proof of principle for structure-based matching of LRR-RKs with their peptide ligands.

Feedback Regulation of DYT1 by Interactions with Downstream bHLH Factors Promotes DYT1 Nuclear Localization and Anther Development.

Transcriptional regulation is one of the most important mechanisms controlling development and cellular functions in plants and animals. The Arabidopsis thaliana bHLH transcription factor (TF) DYSFUNCTIONL TAPETUM1 (DYT1) is required for normal male fertility and anther development and activates the expression of the bHLH010/bHLH089/bHLH091 genes. Here, we showed that DYT1 is localized to both the cytoplasm and nucleus at anther stage 5 but specifically to the nucleus at anther stage 6 and onward. The bHLH010/bHLH089/bHLH091 proteins have strong nuclear localization signals, interact with DYT1, and facilitate the nuclear localization of DYT1. We further found that the conserved C-terminal BIF domain of DYT1 is required for its dimerization, nuclear localization, transcriptional activation activity, and function in anther development. Interestingly, when the BIF domain of DYT1 was replaced with that of bHLH010, the DYT1(N)-bHLH010(BIF) chimeric protein shows nuclear-preferential localization at anther stage 5 but could not fully rescue the dyt1-3 phenotype, suggesting that the normal spatio-temporal subcellular localization of DYT1 is important for DYT1 function and/or that the BIF domains from different bHLH members might be functionally distinct. Our results support an important positive feedback regulatory mechanism whereby downstream TFs increase the function of an upstream TF by enhancing its nucleus localization through the BIF domain.

Nitric Oxide Level Is Self-Regulating and Also Regulates Its ROS Partners.

Nitric oxide (NO) is a free radical recognized as an omnipresent inter- and intra-cellular signaling molecule involved in the regulation of an extraordinary range of diverse cellular functions in plants (Besson-Bard et al., 2008). All NO derivatives are called reactive nitrogen species (RNS) which include more favorable structures such as nitrosonium cation (NO+) and nitroxyl radical (NO−) as a result of NO gaining or losing electrons, and the products of the reaction between NO and its close partners, reactive oxygen species (ROS), such as peroxynitrite (ONOO−), and the NOx compounds (NO2, N2O3, and N2O4; Bellin et al., 2013). Although it is known that NO may regulate gene transcription and activate secondary messengers, the ways in which NO functions are still mostly unidentified (Besson-Bard et al., 2008; Palmieri et al., 2008; Gaupels et al., 2011). In the last decade, however, it has been shown that NO is also able to control different biological processes in plants by directly modifying proteins through covalent post-translational modifications (PTMs) giving rise to nitration, nitrosylation or S-nitrosylation (Romero-Puertas et al., 2013). S-nitrosylation is the covalent binding of a NO group to a cysteine residue and probably the better known NO-dependent PTM as more than 1000 proteins have been shown as targets of S-nitrosylation (Kovacs and Lindermayr, 2013), although the functional effect of this modification has only been analyzed in around 2% of these proteins (Astier et al., 2012; Kovacs and Lindermayr, 2013; Romero-Puertas et al., 2013). Nitration, in which 3-nitrotyrosine is produced after a nitrite group is added to the ortho-position of Tyr residues, is another NO-dependent PTM, although so far analyzed to a lesser degree than S-nitrosylation (N-Tyr; Vandelle and Delledonne, 2011). These PTMs are able to modify the activity, location, aggregation, or even stability of proteins (de Pinto et al., 2013; Gibbs et al., 2014; Albertos et al., 2015). We should bear in mind that NO function depends on the rate and location of its production and that NO level will determine a cytotoxic or stimulating effect (Beligni and Lamattina, 2001; Serrano et al., 2012). Thus, a precise control of NO level by switching the NO signaling off or not seems to be a crucial event for plant survival, and it appears that plants have developed many strategies to achieve it. Reactive oxygen species (ROS) comprises oxygen derivatives species produced by the reduction of oxygen, such as superoxide radicals (O2.-), hydroxyl radicals (·OH), peroxyl radicals (ROO·), and alkoxyl radicals (RO·) and also some non-radical compounds such as hydrogen peroxide (H2O2), the singlet oxygen (1O2), ozone (O3), and hypochlorous acid (HOCl−) (Halliwell and Gutteridge, 2007). Although research on ROS, which have strong oxidizing potential, initially focused on cytotoxicity, in recent years, it has become clear that they can also function as signaling molecules in most cellular processes (Baxter et al., 2014). Thus, plants have developed a means of utilizing lower concentrations of ROS as signaling molecules under certain physiological and stress conditions (Petrov and Van Breusegem, 2012). Genetic and pharmacological techniques have demonstrated that different ROS species can affect nuclear gene expression by responding to a variety of environmental stimuli (Sandalio and Romero-Puertas, 2015). However, a finely tuned balance between ROS scavenging and ROS production is necessary to determine their level and impact as damaging and signaling molecules (de Pinto et al., 2012; Baxter et al., 2014).

Subversion of plant cellular functions by bacterial type-III effectors: beyond suppression of immunity.

Most bacterial plant pathogens employ a type-III secretion system to inject type-III effector (T3E) proteins directly inside plant cells. These T3Es manipulate host cellular processes in order to create a permissive niche for bacterial proliferation, allowing development of the disease. An important role of T3Es in plant pathogenic bacteria is the suppression of plant immune responses. However, in recent years, research has uncovered T3E functions different from direct immune suppression, including the modulation of plant hormone signaling, metabolism or organelle function. This insight article discusses T3E functions other than suppression of immunity, which may contribute to the modulation of plant cells in order to promote bacterial survival, nutrient release, and bacterial replication and dissemination.

Analysis of putative apoplastic effectors from the nematode, Globodera rostochiensis, and identification of an expansin-like protein that can induce and suppress host defenses.

The potato cyst nematode, Globodera rostochiensis, is an important pest of potato. Like other pathogens, plant parasitic nematodes are presumed to employ effector proteins, secreted into the apoplast as well as the host cytoplasm, to alter plant cellular functions and successfully infect their hosts. We have generated a library of ORFs encoding putative G. rostochiensis putative apoplastic effectors in vectors for expression in planta. These clones were assessed for morphological and developmental effects on plants as well as their ability to induce or suppress plant defenses. Several CLAVATA3/ESR-like proteins induced developmental phenotypes, whereas predicted cell wall-modifying proteins induced necrosis and chlorosis, consistent with roles in cell fate alteration and tissue invasion, respectively. When directed to the apoplast with a signal peptide, two effectors, an ubiquitin extension protein (GrUBCEP12) and an expansin-like protein (GrEXPB2), suppressed defense responses including NB-LRR signaling induced in the cytoplasm. GrEXPB2 also elicited defense response in species- and sequence-specific manner. Our results are consistent with the scenario whereby potato cyst nematodes secrete effectors that modulate host cell fate and metabolism as well as modifying host cell walls. Furthermore, we show a novel role for an apoplastic expansin-like protein in suppressing intra-cellular defense responses.

Signal amplification and transduction in phytochrome photosensors

Sensory proteins must relay structural signals from the sensory site over large distances to regulatory output domains. Phytochromes are a major family of red-light sensing kinases that control diverse cellular functions in plants, bacteria, and fungi.1-9 Bacterial phytochromes consist of a photosensory core and a C-terminal regulatory domain.10,11 Structures of photosensory cores are reported in the resting state12-18 and conformational responses to light activation have been proposed in the vicinity of the chromophore.19-23 However, the structure of the signalling state and the mechanism of downstream signal relay through the photosensory core remain elusive. Here, we report crystal and solution structures of the resting and active states of the photosensory core of the bacteriophytochrome from Deinococcus radiodurans. The structures reveal an open and closed form of the dimeric protein for the signalling and resting state, respectively. This nanometre scale rearrangement is controlled by refolding of an evolutionarily conserved “tongue”, which is in contact with the chromophore. The findings reveal an unusual mechanism where atomic scale conformational changes around the chromophore are first amplified into an Ångström scale distance change in the tongue, and further grow into a nanometre scale conformational signal. The structural mechanism is a blueprint for understanding how the sensor proteins connect to the cellular signalling network.