Molecular cloning and characterization of a novel adenylyl cyclase gene, HpAC1, involved in stress signaling in Hippeastrum x hybridum

Cyclic nucleotides - the rise of a family

Cyclic nucleotides 3',5'-cAMP and 3',5'-cGMP are now established signaling components of the plant cell while their 2',3' positional isomers are increasingly recognized as such. 3',5'-cAMP/cGMP is generated by adenylate cyclases (ACs) or guanylate cyclases (GCs) from ATP or GTP, respectively, whereas 2',3'-cAMP/cGMP is produced through the hydrolysis of double-stranded DNA or RNA by synthetases. Recent evidence suggests that the cyclic nucleotide generating and inactivating enzymes moonlight in proteins with diverse domain architecture operating as molecular tuners to enable dynamic and compartmentalized regulation of cellular signals. Further characterization of such moonlighting enzymes and extending the studies to noncanonical cyclic nucleotides promises new insights into the complex regulatory networks that underlie plant development and responses, thus offering exciting opportunities for crop improvement.

Haplotypes at the sorghum ARG4 and ARG5 NLR loci confer resistance to anthracnose

Sorghum anthracnose caused by the fungus Colletotrichum sublineola (Cs) is a damaging disease of the crop. Here, we describe the identification of ANTHRACNOSE RESISTANCE GENES (ARG4 and ARG5) encoding canonical nucleotide-binding leucine-rich repeat (NLR) receptors. ARG4 and ARG5 are dominant resistance genes identified in the sorghum lines SAP135 and P9830, respectively, that show broad-spectrum resistance to Cs. Independent genetic studies using populations generated by crossing SAP135 and P9830 with TAM428, fine mapping using molecular markers, comparative genomics and gene expression studies determined that ARG4 and ARG5 are resistance genes against Cs strains. Interestingly, ARG4 and ARG5 are both located within clusters of duplicate NLR genes at linked loci separated by ~1 Mb genomic region. SAP135 and P9830 each carry only one of the ARG genes while having the recessive allele at the second locus. Only two copies of the ARG5 candidate genes were present in the resistant P9830 line while five non-functional copies were identified in the susceptible line. The resistant parents and their recombinant inbred lines carrying either ARG4 or ARG5 are resistant to strains Csgl1 and Csgrg suggesting that these genes have overlapping specificities. The role of ARG4 and ARG5 in resistance was validated through sorghum lines carrying independent recessive alleles that show increased susceptibility. ARG4 and ARG5 are located within complex loci displaying interesting haplotype structures and copy number variation that may have resulted from duplication. Overall, the identification of anthracnose resistance genes with unique haplotype stucture provides a foundation for genetic studies and resistance breeding.

Plant adenylate cyclases have come full circle

In bacteria, fungi and animals, 3'-5'-cyclic adenosine monophosphate (cAMP) and adenylate cyclases (ACs), enzymes that catalyse the formation of 3',5'-cAMP from ATP, are recognized as key signalling components. In contrast, the presence of cAMP and its biological roles in higher plants have long been a matter of controversy due to the generally lower amounts in plant tissues compared with that in animal and bacterial cells, and a lack of clarity on the molecular nature of the generating and degrading enzymes, as well as downstream effectors. While treatment with 3',5'-cAMP elicited many plant responses, ACs were, however, somewhat elusive. This changed when systematic searches with amino acid motifs deduced from the conserved catalytic centres of annotated ACs from animals and bacteria identified candidate proteins in higher plants that were subsequently shown to have AC activities in vitro and in vivo. The identification of active ACs moonlighting within complex multifunctional proteins is consistent with their roles as molecular tuners and regulators of cellular and physiological functions. Furthermore, the increasing number of ACs identified as part of proteins with different domain architectures suggests that there are many more hidden ACs in plant proteomes and they may affect a multitude of mechanisms and processes at the molecular and systems levels.

Establishment and characterization of a new class of adenylate cyclases (class VII ACs) in plants

Adenylate cyclase is the key enzyme in the synthesis of cAMP. Now, more and more plant genes which possessing AC function are being identified, but the classification of plant ACs has not yet been systematically studied and the relationship of plant ACs with other existing six classes ACs in animals and microorganisms is still unclear. In this study, we found that 7 of the 15 reported plant ACs with conserved CYTH-like_AC_Ⅳ-like domain were clustered into a group with high confidence (Group Ⅳ), while the other plant ACs were clustered into other three groups with no common domain. In addition, we also found that the Group Ⅳ plant ACs were grouped into an independent and specific class (Class VII), separated from the existing six classes of ACs. The Group Ⅳ plant ACs, compared to the existing six classes of ACs, own unique CYTH-like_AC_Ⅳ-like conserved domain and EXEXK signature motif, characteristic protein tertiary structures, specific subcellular localization and catalytic conditions. In view of the above, we regarded the Group Ⅳ plant ACs as the seventh class of AC (VII AC). This study does the systematic classification of plant ACs which could lay a foundation for further identification and study of the biological functions of the plant-specific VII ACs.

A triphosphate tunnel metalloenzyme from pear (PbrTTM1) moonlights as an adenylate cyclase

Adenylyl cyclase (AC) is the vital enzyme for generating 3',5'-cyclic adenosine monophosphate, an important signaling molecule with profound nutritional and medicinal values. However, merely, a dozen of AC proteins have been reported in plants so far. Here, a protein annotated as triphosphate tunnel metalloenzyme (PbrTTM1) in pear, the important worldwide fruit plant, was firstly identified to possess AC activity with both in vivo and in vitro methods. It exhibited a relatively low AC activity but was capable of complementing AC functional deficiencies in the E. coli SP850 strain. Its protein conformation and potential catalytic mechanism were analyzed by means of biocomputing. The active site of PbrTTM1 is a closed tunnel constructed by nine antiparallel β-folds surrounded with seven helices. Inside the tunnel, the charged residues were possibly involved in the catalytic process by coordinating with divalent cation and ligand. The hydrolysis activity of PbrTTM1 was tested as well. Compared to the much higher capacity of hydrolyzing, the AC activity of PbrTTM1 tends to be a moonlight function. Through a comparison of protein structures in various plant TTMs, it is reasonable to speculate that many plant TTMs might possess AC activity as a form of moonlighting enzyme function.

Three Novel Adenylate Cyclase Genes Show Significant Biological Functions in Plant

Adenylate cyclase is the key enzyme solely synthesizing cAMP which participates in cell metabolism regulations and functions as an intracellular second messenger. However, the biological functions of plant ACs have not been elucidated clearly for their poor conservative sequences and low detectable cAMP. We performed a systematic study of plant ACs by using Chinese jujube, whose fruit exhibits the highest cAMP content among plants. Three novel ACs were identified from Chinese jujube, and two types of methods including in vitro and in vivo were used to certificate ZjAC1-3 which can catalyze the conversion of ATP into cAMP. The biological functions of significant accelerations of seed germination, root growth, and flowering were found via overexpression of these AC genes in Arabidopsis, and these functions of ACs were further demonstrated by treating the AC-overexpressing transgenic lines and wild type Arabidopsis with bithionol and dibutyryl-cAMP. At last, transcriptome data revealed that the underlying mechanism of the biological functions of ACs might be regulation of the key genes involved in the circadian rhythm pathway and the hormone signal transduction pathway. This research established a foundation for further investigating plant AC genes and provided strong evidence for cAMP serving as a signaling molecule in plants.

Adenylate cyclase activity of TIR1/AFB auxin receptors in plants

The phytohormone auxin is the major coordinative signal in plant development1, mediating transcriptional reprogramming by a well-established canonical signalling pathway. TRANSPORT INHIBITOR RESPONSE 1 (TIR1)/AUXIN-SIGNALING F-BOX (AFB) auxin receptors are F-box subunits of ubiquitin ligase complexes. In response to auxin, they associate with Aux/IAA transcriptional repressors and target them for degradation via ubiquitination2,3. Here we identify adenylate cyclase (AC) activity as an additional function of TIR1/AFB receptors across land plants. Auxin, together with Aux/IAAs, stimulates cAMP production. Three separate mutations in the AC motif of the TIR1 C-terminal region, all of which abolish the AC activity, each render TIR1 ineffective in mediating gravitropism and sustained auxin-induced root growth inhibition, and also affect auxin-induced transcriptional regulation. These results highlight the importance of TIR1/AFB AC activity in canonical auxin signalling. They also identify a unique phytohormone receptor cassette combining F-box and AC motifs, and the role of cAMP as a second messenger in plants.

Two triphosphate tunnel metalloenzymes from apple exhibit adenylyl cyclase activity

Adenylyl cyclase (AC) is the key catalytic enzyme for the synthesis of 3',5'-cyclic adenosine monophosphate. Various ACs have been identified in microorganisms and mammals, but studies on plant ACs are still limited. No AC in woody plants has been reported until now. Based on the information on HpAC1, three enzymes were screened out from the woody fruit tree apple, and two of them (MdTTM1 and MdTTM2) were verified and confirmed to display AC activity. Interestingly, in the apple genome, these two genes were annotated as triphosphate tunnel metalloenzymes (TTMs) which were widely found in three superkingdoms of life with multiple substrate specificities and enzymatic activities, especially triphosphate hydrolase. In addition, the predicted structures of these two proteins were parallel, especially of the catalytic tunnel, including conserved domains, motifs, and folded structures. Their tertiary structures exhibited classic TTM properties, like the characteristic EXEXK motif and β-stranded anti-parallel tunnel capable of coordinating divalent cations. Moreover, MdTTM2 and HpAC1 displayed powerful hydrolase activity to triphosphate and restricted AC activity. All of these findings showed that MdTTMs had hydrolysis and AC activity, which could provide new solid evidence for AC distribution in woody plants as well as insights into the relationship between ACs and TTMs.

cAMP Is a Promising Regulatory Molecule for Plant Adaptation to Heat Stress

With gradual warming or increased frequency and magnitude of high temperature, heat stress adversely affects plant growth and eventually reduces plant productivity and quality. Plants have evolved complex mechanisms to sense and respond to heat stress which are crucial to avoiding cell damage and maintaining cellular homeostasis. Recently, 33″,55″-cyclic adenosine monophosphate (cAMP) has been proved to be an important signaling molecule participating in plant adaptation to heat stress by affecting multi-level regulatory networks. Significant progress has been made on many fronts of cAMP research, particularly in understanding the downstream signaling events that culminate in the activation of stress-responsive genes, mRNA translation initiation, vesicle trafficking, the ubiquitin-proteasome system, autophagy, HSPs-assisted protein processing, and cellular ion homeostasis to prevent heat-related damage and to preserve cellular and metabolic functions. In this present review, we summarize recent works on the genetic and molecular mechanisms of cAMP in plant response to heat stress which could be useful in finding thermotolerant key genes to develop heat stress-resistant varieties and that have the potential for utilizing cAMP as a chemical regulator to improve plant thermotolerance. New directions for future studies on cAMP are discussed.

The archaeal triphosphate tunnel metalloenzyme SaTTM defines structural determinants for the diverse activities in the CYTH protein family

CYTH proteins make up a large superfamily that is conserved in all three domains of life. These enzymes have a triphosphate tunnel metalloenzyme (TTM) fold, which typically results in phosphatase functions, e.g., RNA triphosphatase, inorganic polyphosphatase, or thiamine triphosphatase. Some CYTH orthologs cyclize nucleotide triphosphates to 3′,5′-cyclic nucleotides. So far, archaeal CYTH proteins have been annotated as adenylyl cyclases, although experimental evidence to support these annotations is lacking. To address this gap, we characterized a CYTH ortholog, SaTTM, from the crenarchaeote Sulfolobus acidocaldarius. Our in silico studies derived ten major subclasses within the CYTH family implying a close relationship between these archaeal CYTH enzymes and class IV adenylyl cyclases. However, initial biochemical characterization reveals inability of SaTTM to produce any cyclic nucleotides. Instead, our structural and functional analyses show a classical TTM behavior, i.e., triphosphatase activity, where pyrophosphate causes product inhibition. The Ca²⁺-inhibited Michaelis complex indicates a two-metal-ion reaction mechanism analogous to other TTMs. Cocrystal structures of SaTTM further reveal conformational dynamics in SaTTM that suggest feedback inhibition in TTMs due to tunnel closure in the product state. These structural insights combined with further sequence similarity network–based in silico analyses provide a firm molecular basis for distinguishing CYTH orthologs with phosphatase activities from class IV adenylyl cyclases.

Molecular Targets and Biological Functions of cAMP Signaling in Arabidopsis

Cyclic AMP (cAMP) is a pivotal signaling molecule existing in almost all living organisms. However, the mechanism of cAMP signaling in plants remains very poorly understood. Here, we employ the engineered activity of soluble adenylate cyclase to induce cellular cAMP elevation in Arabidopsis thaliana plants and identify 427 cAMP-responsive genes (CRGs) through RNA-seq analysis. Induction of cellular cAMP elevation inhibits seed germination, disturbs phytohormone contents, promotes leaf senescence, impairs ethylene response, and compromises salt stress tolerance and pathogen resistance. A set of 62 transcription factors are among the CRGs, supporting a prominent role of cAMP in transcriptional regulation. The CRGs are significantly overrepresented in the pathways of plant hormone signal transduction, MAPK signaling, and diterpenoid biosynthesis, but they are also implicated in lipid, sugar, K⁺, nitrate signaling, and beyond. Our results provide a basic framework of cAMP signaling for the community to explore. The regulatory roles of cAMP signaling in plant plasticity are discussed.

A new adenylyl cyclase, putative disease-resistance RPP13-like protein 3, participates in abscisic acid-mediated resistance to heat stress in maize

In plants, 3´,5´-cyclic adenosine monophosphate (cAMP) is an important second messenger with varied functions; however, only a few adenylyl cyclases (ACs) that synthesize cAMP have been identified. Moreover, the biological roles of ACs/cAMP in response to stress remain largely unclear. In this study, we used quantitative proteomics techniques to identify a maize heat-induced putative disease-resistance RPP13-like protein 3 (ZmRPP13-LK3), which has three conserved catalytic AC centres. The AC activity of ZmRPP13-LK3 was confirmed by in vitro enzyme activity analysis, in vivo RNAi experiments, and functional complementation in the E. coli cyaA mutant. ZmRPP13-LK3 is located in the mitochondria. The results of in vitro and in vivo experiments indicated that ZmRPP13-LK3 interacts with ZmABC2, a possible cAMP exporter. Under heat stress, the concentrations of ZmRPP13-LK3 and cAMP in the ABA-deficient mutant vp5 were significantly less than those in the wild-type, and treatment with ABA and an ABA inhibitor affected ZmRPP13-LK3 expression in the wild-type. Application of 8-Br-cAMP, a cAMP analogue, increased heat-induced expression of heat-shock proteins in wild-type plants and alleviated heat-activated oxidative stress. Taken together, our results indicate that ZmRPP13-LK3, a new AC, can catalyse ATP for the production of cAMP and may be involved in ABA-regulated heat resistance.

Cyclic AMP mediates heat stress response by the control of redox homeostasis and ubiquitin-proteasome system

Heat stress (HS), causing impairment in several physiological processes, is one of the most damaging environmental cues for plants. To counteract the harmful effects of high temperatures, plants activate complex signalling networks, indicated as HS response (HSR). Expression of heat shock proteins (HSPs) and adjustment of redox homeostasis are crucial events of HSR, required for thermotolerance. By pharmacological approaches, the involvement of cAMP in triggering plant HSR has been recently proposed. In this study, to investigate the role of cAMP in HSR signalling, tobacco BY-2 cells overexpressing the 'cAMP-sponge', a genetic tool that reduces intracellular cAMP levels, have been used. in vivo cAMP dampening increased HS susceptibility in a HSPs-independent way. The failure in cAMP elevation during HS caused a high accumulation of reactive oxygen species, due to increased levels of respiratory burst oxidase homolog D, decreased activities of catalase and ascorbate peroxidase, as well as down-accumulation of proteins involved in the control of redox homeostasis. In addition, cAMP deficiency impaired proteasome activity and prevented the accumulation of many proteins of ubiquitin-proteasome system (UPS). By a large-scale proteomic approach together with in silico analyses, these UPS proteins were identified in a specific cAMP-dependent network of HSR.

Cyclic AMP: A Polyhedral Signalling Molecule in Plants

The cyclic nucleotide cAMP (3',5'-cyclic adenosine monophosphate) is nowadays recognised as an important signalling molecule in plants, involved in many molecular processes, including sensing and response to biotic and abiotic environmental stresses. The validation of a functional cAMP-dependent signalling system in higher plants has spurred a great scientific interest on the polyhedral role of cAMP, as it actively participates in plant adaptation to external stimuli, in addition to the regulation of physiological processes. The complex architecture of cAMP-dependent pathways is far from being fully understood, because the actors of these pathways and their downstream target proteins remain largely unidentified. Recently, a genetic strategy was effectively used to lower cAMP cytosolic levels and hence shed light on the consequences of cAMP deficiency in plant cells. This review aims to provide an integrated overview of the current state of knowledge on cAMP's role in plant growth and response to environmental stress. Current knowledge of the molecular components and the mechanisms of cAMP signalling events is summarised.

An Arabidopsis thaliana leucine-rich repeat protein harbors an adenylyl cyclase catalytic center and affects responses to pathogens

Adenylyl cyclases (ACs) catalyze the formation of the second messenger cAMP from ATP. Here we report the characterization of an Arabidopsis thaliana leucine-rich repeat (LRR) protein (At3g14460; AtLRRAC1) as an adenylyl cyclase. Using an AC-specific search motif supported by computational assessments of protein models we identify an AC catalytic center within the N-terminus and demonstrate that AtLRRAC1 can generate cAMP in vitro. Knock-out mutants of AtLRRAC1 have compromised immune responses to the biotrophic fungus Golovinomyces orontii and the hemibiotrophic bacteria Pseudomonas syringae, but not against the necrotrophic fungus Botrytis cinerea. These findings are consistent with a role of cAMP-dependent pathways in the defense against biotrophic and hemibiotrophic plant pathogens.

The Arabidopsis thaliana K+-Uptake Permease 5 (AtKUP5) Contains a Functional Cytosolic Adenylate Cyclase Essential for K+ Transport

Potassium (K⁺) is the most abundant cation in plants, and its uptake and transport are key to growth, development and responses to the environment. Here, we report that Arabidopsis thaliana K⁺ uptake permease 5 (AtKUP5) contains an adenylate cyclase (AC) catalytic center embedded in its N-terminal cytosolic domain. The purified recombinant AC domain generates cAMP in vitro; and when expressed in Escherichia coli, increases cAMP levels in vivo. Both the AC domain and full length AtKUP5 rescue an AC-deficient E. coli mutant, cyaA, and together these data provide evidence that AtKUP5 functions as an AC. Furthermore, full length AtKUP5 complements the Saccharomyces cerevisiae K⁺ transport impaired mutant, trk1 trk2, demonstrating its function as a K⁺ transporter. Surprisingly, a point mutation in the AC center that impairs AC activity, also abolishes complementation of trk1 trk2, suggesting that a functional catalytic AC domain is essential for K⁺ uptake. AtKUP5-mediated K⁺ uptake is not affected by cAMP, the catalytic product of the AC, but, interestingly, causes cytosolic cAMP accumulation. These findings are consistent with a role for AtKUP5 as K⁺ flux sensor, where the flux-dependent cAMP increases modulate downstream components essential for K⁺ homeostasis, such as cyclic nucleotide gated channels.

An Arabidopsis Clathrin Assembly Protein with a Predicted Role in Plant Defense Can Function as an Adenylate Cyclase

Adenylate cyclases (ACs), much like guanylate cyclases (GCs), are increasingly recognized as essential parts of many plant processes including biotic and abiotic stress responses. In order to identify novel ACs, we have applied a search motif derived from experimentally tested GCs and identified a number of Arabidopsis thaliana candidates including a clathrin assembly protein (AT1G68110; AtClAP). AtClAP contains a catalytic centre that can complement the AC-deficient mutant cyaA in E. coli, and a recombinant AtClAP fragment (AtClAP^261–379) can produce cyclic adenosine 3′,5′ monophosphate (cAMP) from adenosine triphosphate (ATP) in vitro. Furthermore, an integrated analysis of gene expression and expression correlation implicate cAMP in pathogen defense and in actin cytoskeletal remodeling during endocytic internalization.

Downstream Targets of Cyclic Nucleotides in Plants

Efficient integration of various external and internal signals is required to maintain adaptive cellular function. Numerous distinct signal transduction systems have evolved to allow cells to receive these inputs, to translate their codes and, subsequently, to expand and integrate their meanings. Two of these, cyclic AMP and cyclic GMP, together referred to as the cyclic nucleotide signaling system, are between them. The cyclic nucleotides regulate a vast number of processes in almost all living organisms. Once synthesized by adenylyl or guanylyl cyclases, cyclic nucleotides transduce signals by acting through a number of cellular effectors. Because the activities of several of these effectors are altered simultaneously in response to temporal changes in cyclic nucleotide levels, agents that increase cAMP/cGMP levels can trigger multiple signaling events that markedly affect numerous cellular functions. In this mini review, we summarize recent evidence supporting the existence of cNMP effectors in plant cells. Specifically, we highlight cAMP-dependent protein kinase A (PKA), cGMP-dependent kinase G (PKG), and cyclic nucleotide phosphodiesterases (PDEs). Essentially this manuscript documents the progress that has been achieved in recent decades in improving our understanding of the regulation and function of cNMPs in plants and emphasizes the current gaps and unanswered questions in this field of plant signaling research.

An adenylyl cyclase with a phosphodiesterase domain in basal plants with a motile sperm system

Adenylyl cyclase (AC), which produces the signalling molecule cAMP, has numerous important cellular functions in diverse organisms from prokaryotes to eukaryotes. Here we report the identification and characterization of an AC gene from the liverwort Marchantia polymorpha. The encoded protein has both a C-terminal AC catalytic domain similar to those of class III ACs and an N-terminal cyclic nucleotide phosphodiesterase (PDE) domain that degrades cyclic nucleotides, thus we designated the gene MpCAPE (COMBINED AC with PDE). Biochemical analyses of recombinant proteins showed that MpCAPE has both AC and PDE activities. In MpCAPE-promoter-GUS lines, GUS activity was specifically detected in the male sexual organ, the antheridium, suggesting MpCAPE and thus cAMP signalling may be involved in the male reproductive process. CAPE orthologues are distributed only in basal land plants and charophytes that use motile sperm as the male gamete. CAPE is a subclass of class III AC and may be important in male organ and cell development in basal plants.

Identification of a Hippeastrum hybridum guanylyl cyclase responsive to wounding and pathogen infection

Guanosine 3',5'-cyclic monophosphate (cGMP) is a critical component of many (patho)physiological processes in plants whilst guanylyl cyclases (GCs) which catalyse the formation of cGMP from GTP have remained somewhat elusive. Consequently, the two major aims are the discovery of novel guanylyl cyclases and the identification of GC/cGMP mediated processes. To identify a novel GC from Hippeastrum hybridum plant and facilitate the preparation of guanylyl cyclase in an amount sufficient for further crystallographic studies, we have constructed an overproduction system for this enzyme. This gene encodes a protein of 256 amino acids, with a calculated molecular mass of 28kD. The predicted amino acid sequence contains all the typical features and shows a high identity to other plant GCs. The GST-HpGC1 was catalytically active in Escherichia coli cells and the purified, recombinant HpGC1 was able to convert GTP to cGMP in the presence of divalent cations. The used overexpression system yields a guanylyl cyclase as 6% of the bacterial cytosolic protein. Besides the identification of HpGC1 as a guanylyl cyclase, the study has shown that the level of HpCG1 mRNA changed during stress conditions. Both mechanical damage and a Peyronellaea curtisii (=Phoma narcissi) fungi infection led to an initial decrease in the HpGC1 transcript level, followed by a substantial increase during the remainder of the 48-h test cycle. Moreover, significant changes in cyclic GMP level were observed, taking the form of oscillations. In conclusion, our data unequivocally identified the product of the HpGC1 gene as a guanylyl cyclase and demonstrates that such an overproduction system can be successfully used in enzyme synthesis. Furthermore, they indicate a link between the causing stimulus (wounding, infection) and guanylyl cyclase expression and the increase in cGMP amplitude. Therefore, it is concluded that appearance of cyclic GMP as a mediator in defense and wound-healing mechanisms provides a clue to the regulation of these processes.

Exogenous adenosine 5'-phosphoramidate behaves as a signal molecule in plants; it augments metabolism of phenylpropanoids and salicylic acid in Arabidopsis thaliana seedlings

Cells contain various congeners of the canonical nucleotides. Some of these accumulate in cells under stress and may function as signal molecules. Their cellular levels are enzymatically controlled. Previously, we demonstrated a signaling function for diadenosine polyphosphates and cyclic nucleotides in Arabidopsis thaliana and grape, Vitis vinifera. These compounds increased the expression of genes for and the specific activity of enzymes of phenylpropanoid pathways resulting in the accumulation of certain products of these pathways. Here, we show that adenosine 5'-phosphoramidate, whose level can be controlled by HIT-family proteins, induced similar effects. This natural nucleotide, when added to A. thaliana seedlings, activated the genes for phenylalanine:ammonia lyase, 4-coumarate:coenzyme A ligase, cinnamate-4-hydroxylase, chalcone synthase, cinnamoyl-coenzyme A:NADP oxidoreductase and isochorismate synthase, which encode proteins catalyzing key reactions of phenylpropanoid pathways, and caused accumulation of lignins, anthocyanins and salicylic acid. Adenosine 5'-phosphofluoridate, a synthetic congener of adenosine 5'-phosphoramidate, behaved similarly. The results allow us to postulate that adenosine 5'-phosphoramidate should be considered as a novel signaling molecule.