Plant hormone perception and action: a role for G-protein signal transduction?

Plants perceive and respond to a profusion of environmental and endogenous signals that influence their growth and development. The G-protein signalling pathway is a mechanism for transducing extracellular signals that is highly conserved in a range of eukaryotes and prokaryotes. Evidence for the existence of G-protein signalling pathways in higher plants is reviewed, and their potential involvement in plant hormone signal transduction evaluated. A range of biochemical and molecular studies have identified potential components of G-protein signalling in plants, most notably a homologue of the G-protein coupled receptor superfamily (GCR1) and the G alpha and G beta subunits of heterotrimeric G-proteins. G-protein agonists and antagonists are known to influence a variety of signalling events in plants and have been used to implicate heterotrimeric G-proteins in gibberellin and possibly auxin signalling. Antisense suppression of GCR1 in Arabidopsis leads to a phenotype which supports a role for this receptor in cytokinin signalling. These observations suggest that higher plants have at least some of the components of G-protein signalling pathways and that these might be involved in the action of certain plant hormones.     

Evidence for the localization of the Arabidopsis cytokinin receptors AHK3 and AHK4 in the endoplasmic reticulum

Cytokinins are hormones that are involved in various processes of plant growth and development. The model of cytokinin signalling starts with hormone perception through membrane-localized histidine kinase receptors. Although the biochemical properties and functions of these receptors have been extensively studied, there is no solid proof of their subcellular localization. Here, cell biological and biochemical evidence for the localization of functional fluorophor-tagged fusions of Arabidopsis histidine kinase 3 (AHK3) and 4 (AHK4), members of the cytokinin receptor family, in the endoplasmic reticulum (ER) is provided. Furthermore, membrane-bound AHK3 interacts with AHK4 in vivo. The ER localization and putative function of cytokinin receptors from the ER have major impacts on the concept of cytokinin perception and signalling, and hormonal cross-talk in plants.     

Hormonal input in plant meristems: A balancing act

Plant hormones are a group of chemically diverse molecules that control virtually all aspects of plant development. Classical plant hormones were identified many decades ago in physiology studies that addressed plant growth regulation. In recent years, biochemical and genetic approaches led to the identification of many molecular components that mediate hormone activity, such as hormone receptors and hormone-regulated genes. This has greatly contributed to the understanding of the mechanisms underlying hormone activity and highlighted the intricate crosstalk and integration of hormone signalling and developmental pathways. Here we review and discuss recent findings on how hormones regulate the activity of shoot and root apical meristems.     

Protein complexes mediate signalling in plant responses to hormones,light, sucrose and pathogens

Living organisms use complex pathways of signal perception and transduction to respond to stimuli in their environments. In plants, putative signal transduction components have been identified through mutant screens and comparative analysis of genome sequences of model eukaryotes. Several pieces in a large series of puzzles have now been identified and a current challenge is to determine how these pieces interconnect. Functional analysis of the encoded proteins has necessitated a change from genetic to biochemical approaches. In recent years, the application of techniques such as two-hybrid screening and epitope tagging has facilitated the study of protein-protein interactions and has increased our understanding of cellular signalling mechanisms. One focus of present research is the ubiquitin/proteasome-mediated degradation of proteins. Increasing evidence suggests this is a control common to many plant signalling pathways including development and responsiveness to hormones, light and sucrose. A central challenge in the study of plant disease resistance has been to identify protein complexes that contain host defence proteins and pathogenicity factors. In this review we summarize the latest developments in these areas where the existence of protein complexes has been demonstrated to be of fundamental importance in plant signalling.     

Brassinosteroids and plant function: some clues, more puzzles

The role of brassinosteroids (BRs) in plant function has been intensively studied in the last few years. Mutant analysis has demonstrated that the ability to synthesize, perceive and respond to BRs is essential to normal plant growth and development. Several key elements of BR response have been identified using both genetic and biochemical approaches, and molecular models that parallel Wingless (Wnt), transforming growth factor beta (TGF beta) and receptor tyrosine kinase (RTK) signalling in animals have been proposed. Many studies have demonstrated the role of BRs, alone and in interaction with other plant hormones, in processes such as cell elongation and seed germination. In contrast, little is known about how the sensing of BRs is connected to specific physiological responses such as stress resistance. There remain many open questions about how these connections are made.     

A role for G proteins in plant hormone signalling?

The G protein signalling pathway is one of the most highly conserved mechanisms that enables cells to sense and respond to changes in their environment. Essential components of this are cell surface G protein-coupled receptors (GPCRs) that perceive extracellular ligands, and heterotrimeric G proteins (G proteins) that transduce information from activated GPCRs to down-stream effectors such as enzymes or ion channels. It is now clear from a range of biochemical and molecular studies that some potential G protein signalling components exist in plants. The best examples of these are the seven transmembrane receptor homologue GCR1 and the G_α (GPA1) and G_β (Gβ1) subunit homologues of heterotrimeric G proteins. G protein agonists and antagonists are known to influence a variety of signalling events in plants and have been used to implicate G proteins in a range of signalling pathways that include the plant hormones gibberellin and auxin. Furthermore, antisense suppression of GCR1 expression in Arabidopsis leads to a phenotype that supports a role for this receptor in cytokinin signalling. This review considers the current evidence for and against functional G protein signalling pathways in higher plants and questions whether or not these might be involved in the action of certain plant hormones.     

Genetic approaches to understanding sugar-response pathways

Plants as photoautotrophic organisms are able to produce the carbohydrates they require and have developed mechanisms to co-ordinate carbohydrate production and its metabolism. Carbohydrate-derived signals regulate the expression of genes involved in both photosynthesis and metabolism, and control carbohydrate partitioning. A number of genetic approaches have been initiated to understand sugar-response pathways in plants and identify the components involved. Screening strategies to date have been based on the effects of high sugar media on early seedling development or on changes in the enzyme activity or expression of sugar-responsive genes. These screens have established roles for plant hormones in sugar-response pathways, in particular for abscisic acid. The present emphasis on the role of plant hormones in sugar responses is due to the fact that mutants could be readily identified as belonging to these established pathways, but also results from the nature of the mutant screens in use. Progress is being made on the identification of mutants and genes that may be specific to sugar-signalling pathways. It is also expected that the modification of existing screens may target sugar-signalling pathways more directly. Genetic approaches may be especially useful in identifying components of novel signalling pathways unique to plants, and their combination with genomic and molecular approaches will guide future research.     

植物一氧化氮(NO)研究进展

一氧化氮(NO)是植物的重要生物活性分子,它参与植物生长发育的许多过程,如种子萌发、下胚轴伸长、叶扩展、根生长、侧根形成、细胞凋亡以及植物抗逆反应等。大量的证据表明,植物可以通过与动物NO合酶类似的酶产生NO。此外,植物还可通过硝酸还原酶产生NO。NO在植物中的信号传递途径仍不十分清楚,植物有可能采用与动物相类似的机制。由于植物的大多数生长发育现象都受到植物激素的调节和控制,NO与植物激素之间的关系也受到越来越多的关注。通过激素起作用可能是植物内源NO作用的机理之一。     

植物一氧化氮(NO)研究进展

一氧化氮（NO）是植物的重要生物活性分子,它参与植物生长发育的许多过程,如种子萌发、下胚轴伸长、叶扩展、根生长、侧根形成、细胞凋亡以及植物抗逆反应等。大量的证据表明,植物可以通过与动物NO合酶类似的酶产生NO。此外,植物还可通过硝酸还原酶产生NO。NO在植物中的信号传递途径仍不十分清楚,植物有可能采用与动物相类似的机制。由于植物的大多数生长发育现象都受到植物激素的调节和控制,NO与植物激素之间的关系也受到越来越多的关注。通过激素起作用可能是植物内源NO作用的机理之一。     

Ligand–receptor interactions in plant hormone signaling

Small-molecule plant hormones principally control plant growth, development, differentiation, and environmental responses. Nine types of plant hormones are ubiquitous in angiosperms, and the molecular mechanisms of their hormone actions have been elucidated during the last two decades by genomic decoding of model plants with genetic mutants. In particular, the discovery of hormone receptors has greatly contributed to the understanding of signal transduction systems. The three-dimensional structure of the ligand–receptor complex has been determined for eight of the nine hormones by X-ray crystal structure analysis, and ligand perception mechanisms have been revealed at the atomic level. Collective research has revealed the molecular function of plant hormones that act as either molecular glue or an allosteric regulator for activation of receptors. In this review, we present an overview of the respective hormone signal transduction and describe the structural bases of ligand–receptor interactions.     

Pathological hormone imbalances

Plant hormones play important roles in regulating developmental processes and signalling networks involved in plant responses to a wide range of biotic and abiotic stresses. Salicylic acid (SA), jasmonates (JA) and ethylene (ET) are well known to play crucial roles in plant disease and pest resistance. However, the roles of other hormones such as abscisic acid (ABA), auxin, gibberellin (GA), cytokinin (CK) and brassinosteroid (BL) in plant defence are less well known. Much progress has been made in understanding plant hormone signalling and plant disease resistance. However, these studies have mostly proceeded independently of each other, and there is limited knowledge regarding interactions between plant hormone-mediated signalling and responses to various pathogens. Here, we review the roles of hormones other than SA, JA and ET in plant defence and the interactions between hormone-mediated signalling, plant defence and pathogen virulence. We propose that these hormones may influence disease outcomes through their effect on SA or JA signalling.     

Molecular characterisation of a serum-responsive, DAF-12-like nuclear hormone receptor of the fox-tapeworm Echinococcus multilocularis

As the primary mediators of lipophilic and steroid hormone signalling, the family of nuclear receptors (NRs) plays a central role in the regulation of metazoan development. Lipophilic hormones are also thought to be important players in the molecular interaction between larval cestodes and their hosts but no member of the NR family has yet been characterised in this group of parasites. In this work, we provide for the first time evidence for the presence of NRs in cestodes of the genus Echinococcus. By bioinformatic analyses, we identified a set of 17 NRs in the genomes of E. multilocularis and E. granulosus which broadly overlapped with the set of NRs that is expressed by schistosomes, but also contained several members that are unique to cestodes. One of these receptors, EmNHR1, displayed structural homologies to the DAF-12/HR-96 subfamily of NRs that regulates cholesterol homeostasis and longevity in metazoans. By RT-PCR analyses, we demonstrate that the EmNHR1 encoding gene is expressed in all Echinococcus larval stages that are involved in the infection of the intermediate host. By yeast two-hybrid analyses, we further demonstrate cross-communication between EmNHR1 and TGF-β signalling pathways in Echinococcus and that mammalian serum contains a ligand that induces homodimerisation of the EmNHR1 ligand-binding domain. EmNHR1 could thus play an important role in hormonal host-parasite cross-communication mechanisms during an infection. On the basis of our results, further investigations into the role of NR signalling in cestode development and host-parasite interaction will be greatly facilitated.     

The complex role of NOTCH receptors and their ligands in the development of hepatoblastoma,cholangiocarcinoma and hepatocellular carcinoma

The NOTCH signalling pathway is one of the key molecular pathways of embryonic development and adult tissues homeostasis in mammals. Mammals have four NOTCH receptors and various ligands that modulate their activity. Many cell disorders, whose genesis involves the NOTCH signalling pathway, have been discovered, including cancer. The mechanisms by which these receptors and their ligands affect liver cell transformation are not yet well understood, and they seem to behave as both oncogenes and tumour‐suppressor proteins. In this review, we discuss the published data regarding the role of these proteins in the development of hepatoblastoma, cholangiocarcinoma and hepatocellular carcinoma malignancies. The alteration of the NOTCH signalling pathway may be one of the main drivers of hepatic neoplastic growth. However, this signalling pathway might also modulate the development of specific liver tumour features. The complexity of the function of NOTCH receptors and their ligands may be due to their interactions with many other cell signalling pathways. Furthermore, the different levels of expression and activation of these receptors could be a reason for their distinct and sometimes contradictory effects.     

Cytokinins: Biosynthesis metabolism and perception

Summary Cytokinins are essential hormones for plant growth and development. They are also of vital importance for in vitro manipulations of plant cells and tissues. The biological activities and chemistry of cytokinins are well defined but very little is known about their mode of action and it is only recently that cytokinin genes have been identified in plants. This review summarizes the current status of knowledge on cytokinin biosynthesis, metabolism and signal transduction, with an emphasis on genes encoding metabolic enzymes and putative receptors, and genes rapidly induced by cytokinins.     

Quo vadis plant hormone analysis?

Plant hormones act as chemical messengers in the regulation of myriads of physiological processes that occur in plants. To date, nine groups of plant hormones have been identified and more will probably be discovered. Furthermore, members of each group may participate in the regulation of physiological responses in planta both alone and in concert with members of either the same group or other groups. The ideal way to study biochemical processes involving these signalling molecules is ‘hormone profiling’, i.e. quantification of not only the hormones themselves, but also their biosynthetic precursors and metabolites in plant tissues. However, this is highly challenging since trace amounts of all of these substances are present in highly complex plant matrices. Here, we review advances, current trends and future perspectives in the analysis of all currently known plant hormones and the associated problems of extracting them from plant tissues and separating them from the numerous potentially interfering compounds.     

Rhizobacterial mediation of plant hormone status

Plant growth-promoting rhizobacteria are commonly found in the rhizosphere (adjacent to the root surface) and may promote plant growth via several diverse mechanisms, including the production or degradation of the major groups of plant hormones that regulate plant growth and development. Although rhizobacterial production of plant hormones seems relatively widespread (as judged from physico-chemical measurements of hormones in bacterial culture media), evidence continues to accumulate, particularly from seedlings grown under gnotobiotic conditions, that rhizobacteria can modify plant hormone status. Since many rhizobacteria can impact on more than one hormone group, bacterial mutants in hormone production/degradation and plant mutants in hormone sensitivity have been useful to establish the importance of particular signalling pathways. Although plant roots exude many potential substrates for rhizobacterial growth, including plant hormones or their precursors, limited progress has been made in determining whether root hormone efflux can select for particular rhizobacterial traits. Rhizobacterial mediation of plant hormone status not only has local effects on root elongation and architecture, thus mediating water and nutrient capture, but can also affect plant root-to-shoot hormonal signalling that regulates leaf growth and gas exchange. Renewed emphasis on providing sufficient food for a growing world population, while minimising environmental impacts of agriculture because of overuse of fertilisers and irrigation water, will stimulate the commercialisation of rhizobacterial inoculants (including those that alter plant hormone status) to sustain crop growth and yield. Combining rhizobacterial traits (or species) that impact on plant hormone status thereby modifying root architecture (to capture existing soil resources) with traits that make additional resources available (e.g. nitrogen fixation, phosphate solubilisation) may enhance the sustainability of agriculture.     

Signalling pathways and the host-parasite relationship: putative targets for control interventions against schistosomiasis: signalling pathways and future anti-schistosome therapies

A better understanding of how schistosomes exploit host nutrients, neuro-endocrine hormones and signalling pathways for growth, development and maturation may provide new insights for improved interventions in the control of schistosomiasis. This paper describes recent advances in the identification and characterisation of schistosome tyrosine kinase and signalling pathways. It discusses the potential intervention value of insulin signalling, which may play an important role in glucose uptake and carbohydrate metabolism in schistosomes, providing the nutrients essential for parasite growth, development and, notably, female fecundity. Significant progress has also been made in the characterisation of other schistosome growth factor receptors, such as transforming growth factor beta receptor and epidermal growth factor receptor, and in our understanding of their roles in the host-parasite molecular dialogue and parasite development. The use of parasite signal transduction components as novel vaccine or drug targets may prove invaluable in prevention, treatment and control strategies to combat schistosomiasis.