Current understanding of the interplays between host hormones and plant viral infections

Phytohormones mediate plant development and responses to stresses caused by biotic agents or abiotic factors. The functions of phytohormones in responses to viral infection have been intensively studied, and the emerging picture of complex mechanisms provides insights into the roles that phytohormones play in defense regulation as a whole. These hormone signaling pathways are not simple linear or isolated cascades, but exhibit crosstalk with each other. Here, we summarized the current understanding of recent advances for the classical defense hormones salicylic acid (SA), jasmonic acid (JA), and ethylene (ET) and also the roles of abscisic acid (ABA), auxin, gibberellic acid (GA), cytokinins (CKs), and brassinosteroids (BRs) in modulating plant–virus interactions.     

Signs of change: hormone receptors that regulate plant development

Hormonal signalling plays a pivotal role in almost every aspect of plant development, and of high priority has been to identify the receptors that perceive these hormones. In the past seven months, the receptors for the plant hormones auxin, gibberellins and abscisic acid have been identified. These join the receptors that have previously been identified for ethylene, brassinosteroids and cytokinins. This review therefore comes at an exciting time for plant developmental biology, as the new findings shed light on our current understanding of the structure and function of the various hormone receptors, their related signalling pathways and their role in regulating plant development.     

Auxin: a master regulator in plant root development

The demand for increased crop productivity and the predicted challenges related to plant survival under adverse environmental conditions have renewed the interest in research in root biology. Various physiological and genetic studies have provided ample evidence in support of the role of plant growth regulators in root development. The biosynthesis and transport of auxin and its signaling play a crucial role in controlling root growth and development. The univocal role of auxin in root development has established it as a master regulator. Other plant hormones, such as cytokinins, brassinosteroids, ethylene, abscisic acid, gibberellins, jasmonic acid, polyamines and strigolactones interact either synergistically or antagonistically with auxin to trigger cascades of events leading to root morphogenesis and development. In recent years, the availability of biological resources, development of modern tools and experimental approaches have led to the advancement of knowledge in root development. Research in the areas of hormone signal perception, understanding network of events involved in hormone action and the transport of plant hormones has added a new dimension to root biology. The present review highlights some of the important conceptual developments in the interplay of auxin and other plant hormones and associated downstream events affecting root development.     

Histones and Plant Hormones: New Evidence for an Interesting Interplay

As any living organism, plants have to respond to a wide variety of biotic and abiotic signals. One of the most important challenges for the plant cell is the response to plant hormones, compounds that regulate almost all aspects of plant development. This process involves the appropriate alteration of chromatin structure and function which can be facilitated by a number of different mechanisms, including histone covalent modifications, incorporation of distinct histone variants, chromatin remodeling and epigenetic regulation of gene expression. On the other hand, distinct chromatin states may in turn influence plant hormone biosynthesis and signaling. This article aims to review the evidence accumulated the last years concerning the effect of the major plant hormones - auxin, gibberellins, cytokinins, ethylene, abscisic acid, brassinosteroids and jasmonic acid - signaling on histone variants and modification pattern and to discuss the interplay between histones and hormones at plant chromatin.     

Role of plant hormones in plant defence responses

Plant hormones play important roles in regulating developmental processes and signaling networks involved in plant responses to a wide range of biotic and abiotic stresses. Significant progress has been made in identifying the key components and understanding the role of salicylic acid (SA), jasmonates (JA) and ethylene (ET) in plant responses to biotic stresses. Recent studies indicate that other hormones such as abscisic acid (ABA), auxin, gibberellic acid (GA), cytokinin (CK), brassinosteroids (BR) and peptide hormones are also implicated in plant defence signaling pathways but their role in plant defence is less well studied. Here, we review recent advances made in understanding the role of these hormones in modulating plant defence responses against various diseases and pests.     

Plant ubiquitylation and proteolytic systems, the key elements in hormonal signaling pathways

The general function of the ubiquitylation systems is to conjugate ubiquitin to lysine residues within substrate proteins, thus targeting them for degradation by the proteasome. In Arabidopsis thaliana more than 1300 genes (approximately 5% of the proteome) encode components of the ubiquitin/26S proteasome pathway. Approximately 90% of these genes encode subunits of the E3 ubiquitin ligases, which confer substrate specificity to the ubiquitin/26S proteasome pathway. The plant E3 ubiquitin ligases comprise a large and diverse family of proteins or protein complexes containing either a HECT domain, a RING-finger or U-box domain. The SCF class of E3 ligases is the most thoroughly studied in plants because some of them participate in regulation of hormone signaling pathways. The role of the SCF is to ubiquitylate repressors of hormone response (auxin, gibberellins), whereas in response to ethylene, abscisic acid and brassinosteroids the SCF participate in degradation of positive regulators in the absence of the hormone.     

IBR5, a dual-specificity phosphatase-like protein modulating auxin and abscisic acid responsiveness in Arabidopsis

Auxin is an important plant hormone that plays significant roles in plant growth and development. Although numerous auxin-response mutants have been identified, auxin signal transduction pathways remain to be fully elucidated. We isolated ibr5 as an Arabidopsis indole-3-butyric acid-response mutant, but it also is less responsive to indole-3-acetic acid, synthetic auxins, auxin transport inhibitors, and the phytohormone abscisic acid. Like certain other auxin-response mutants, ibr5 has a long root and a short hypocotyl when grown in the light. In addition, ibr5 displays aberrant vascular patterning, increased leaf serration, and reduced accumulation of an auxin-inducible reporter. We used positional information to determine that the gene defective in ibr5 encodes an apparent dual-specificity phosphatase. Using immunoblot and promoter-reporter gene analyses, we found that IBR5 is expressed throughout the plant. The identification of IBR5 relatives in other flowering plants suggests that IBR5 function is conserved throughout angiosperms. Our results suggest that IBR5 is a phosphatase that modulates phytohormone signal transduction and support a link between auxin and abscisic acid signaling pathways.     

Developing a model of plant hormone interactions

Plant growth and development is influenced by mutual interactions among plant hormones. The five classical plant hormones are auxins, cytokinins, gibberellins, abscisic acid and ethylene. They are small diffusible molecules that easily penetrate between cells. In addition, newer classes of plant hormones have been identified such as brassinosteroids, jasmonic acid, salicylic acid and various small proteins or peptides. These hormones also play important roles in the regulation of plant growth and development. This review begins with a brief summary of the current findings on plant hormones. Based on this knowledge, a conceptual model about interactions among plant hormones is built so as to link and develop an understanding of the diverse functions of different plant hormones as a whole in plants.Key words: abscisic acid, auxin, brassinosteroids, cytokinins, ethylene, gibberellins, jasmonic acid, salicylic acid, plant peptide hormones     

Hormone interactions during lateral root formation

Lateral root (LR) formation, the production of new roots from parent roots, is a hormone- and environmentally-regulated developmental process in higher plants. Physiological and genetic studies using Arabidopsis thaliana and other plant species have revealed the roles of several plant hormones in LR formation, particularly the role of auxin in LR initiation and primordium development, resulting in much progress toward understanding the mechanisms of auxin-mediated LR formation. However, hormone interactions during LR formation have been relatively underexamined. Recent studies have shown that the plant hormones, cytokinin and abscisic acid negatively regulate LR formation whereas brassinosteroids positively regulate LR formation. On the other hand, ethylene has positive and negative roles during LR formation. This review summarizes recent findings on hormone-regulated LR formation in higher plants, focusing on auxin as a trigger and on the other hormones in LR formation, and discusses the possible interactions among plant hormones in this developmental process.     

Temperature-regulation of plant architecture

As sessile organisms, plants have evolved great plasticity to adapt to their surrounding environment. Temperature signals regulate the timing of multiple developmental processes and have dramatic effects on plant architecture and biomass. The modulation of plant architecture by temperature is of increasing relevance with regard to crop productivity and global climate change. Unlike many other organisms, the mechanisms through which plants sense changes in ambient temperature remain elusive. Multiple studies have identified crosstalk between ambient temperature sensing, light signaling, cold acclimation and pathogen response pathways. The regulation of plant architecture by temperature appears to involve the complex integration of multiple hormone signaling networks. Gibberellin (GA), Salicylic Acid (SA) and cytokinin have been implicated in the regulation of plant growth during chilling, whilst a predominant role for auxin is observed at high temperatures. This mini-review summarizes current knowledge of plant growth regulation by temperature and crosstalk with other abiotic and biotic stress signaling pathways.Key words: temperature, architecture, elongation, growth, hormone, auxin, gibberellin, salicylic acid, biomass     

Comparison of phytohormone signaling mechanisms

Plant hormones are crucial signaling molecules that coordinate all aspects of plant growth, development and defense. A great deal of attention has been attracted from biologists to study the molecular mechanisms for perception and signal transduction of plant hormones during the last two decades. Tremendous progress has been made in identifying receptors and key signaling components of plant hormones. The holistic picture of hormone signaling pathways is extremely complicated, this review will give a general overview of perception and signal transduction mechanisms of auxin, gibberellin, cytokinin, abscisic acid, ethylene, brassinosteroid, and jasmonate.     

Hormone modulation of legume‐rhizobial symbiosis

Leguminous plants can establish symbiotic associations with diazotropic rhizobia to form nitrogenfixating nodules, which are classified as determinate or indeterminate based on the persistence of nodule meristem. The formation of nitrogen-fixing nodules requires coordinating rhizobial infection and root nodule organogenesis. The formation of an infection thread and the extent of nodule formation are largely under plant control, but vary with environmental conditions and the physiological state of the host plants. Many achievements in these two areas have been made in recent decades.Phytohormone signaling pathways have gradually emerged as important regulators of root nodule symbiosis. Cytokinin, strigolactones(SLs) and local accumulation of auxin can promote nodule development. Ethylene,jasmonic acid(JA), abscisic acid(ABA) and gibberellic acid(GA) all negatively regulate infection thread formation and nodule development. However, salicylic acid(SA) and brassinosteroids(BRs) have different effects on the formation of these two nodule types. Some peptide hormones are also involved in nodulation. This review summarizes recent findings on the roles of these plant hormones in legume-rhizobial symbiosis, and we propose that DELLA proteins may function as a node to integrate plant hormones to regulate nodulation.     

A comparative genomic analysis of plant hormone related genes in different species

Plant hormones are small molecules that play important roles throughout the life span of a plant,known as auxin,gibberellin,cyto-kinin,abscisic acid,ethylene,jasmonic acid,salicylic acid,and brassinosteroid.Genetic and molecular studies in the model organism Arabidopsis thaliana have revealed the individual pathways of various plant hormone responses.In this study,we selected 479 genes that were convincingly associated with various hormone actions based on genetic evidence.By using these 479 genes as querie...     

小分子RNA在植物激素信号通路中的调控功能

植物激素是调控植物生长发育的信号分子。近年来的研究发现,小分子RNA作为基因表达调控网络的组分,参与植物激素信号途径,在植物生长发育和胁迫反应方面发挥重要作用。本文综述了miRNA和次级siRNA(Short interfering RNAs)介导的基因调控与植物激素信号通路相互作用的研究进展,主要包括生长素、赤霉素、油菜素内酯和脱落酸途径涉及的miRNA及其功能,并对不同发育过程中miRNA参与的不同激素信号通路的交叉和互作进行了讨论。     

Bacteria-responsive microRNAs regulate plant innate immunity by modulating plant hormone networks

MicroRNAs (miRNAs) are key regulators of gene expression in development and stress responses in most eukaryotes. We globally profiled plant miRNAs in response to infection of bacterial pathogen Pseudomonas syringae pv. tomato (Pst). We sequenced 13 small-RNA libraries constructed from Arabidopsis at 6 and 14 h post infection of non-pathogenic, virulent and avirulent strains of Pst. We identified 15, 27 and 20 miRNA families being differentially expressed upon Pst DC3000 hrcC, Pst DC3000 EV and Pst DC3000 avrRpt2 infections, respectively. In particular, a group of bacteria-regulated miRNAs targets protein-coding genes that are involved in plant hormone biosynthesis and signaling pathways, including those in auxin, abscisic acid, and jasmonic acid pathways. Our results suggest important roles of miRNAs in plant defense signaling by regulating and fine-tuning multiple plant hormone pathways. In addition, we compared the results from sequencing-based profiling of a small set of miRNAs with the results from small RNA Northern blot and that from miRNA quantitative RT-PCR. Our results showed that although the deep-sequencing profiling results are highly reproducible across technical and biological replicates, the results from deep sequencing may not always be consistent with the results from Northern blot or miRNA quantitative RT-PCR. We discussed the procedural differences between these techniques that may cause the inconsistency.     

Function of Heterotrimeric G Protein in Gibberellin Signaling

The importance of plant heterotrimeric G protein functions has recently been recognized. Rice and Arabidopsis mutants of genes coding the subunits of the G proteins have been isolated and physiological studies on these mutants have suggested that plant heterotrimeric G proteins are involved in several intra-signaling pathways driven by external signals, such as gibberellin, auxin, abscisic acid, brassinolide, ethylene, light, and elicitor. The possible functions of rice heterotrimeric G proteins in gibberellin signaling are discussed here.     

Novel signals for plant development

Classic signal molecules such as auxin, cytokinin, gibberellins, abscisic acid and more recently brassinosteroids have been extensively studied in the context of their role in morphogenetic processes in plants. In the past five years, it has become apparent that there are novel signaling molecules, such as N-acylethanolamides, alkamides, glutamate and nitric oxide, that might play important roles in the regulation of morphogenetic and adaptive processes. There is information pointing out that these molecules might be involved in diverse processes, including seed germination, pathogenesis, modulation of plant architecture and response to abiotic factors. In animals, alkamides and N-acylethanolamides act as endogenous signaling molecules that activate cannabinoid receptors, which are coupled to signal transduction cascades involving glutamate and nitric oxide. Hence, there is a possibility that cannabinoid signaling represents an evolutionary conserved pathway that modulates cellular and physiological processes in eukaryotes.     

Focus on Ethylene: Ethylene and Hormonal Cross Talk in Vegetative Growth and Development

Ethylene is a gaseous plant hormone that most likely became a functional hormone during the evolution of charophyte green algae, prior to land colonization. From this ancient origin, ethylene evolved into an important growth regulator that is essential for myriad plant developmental processes. In vegetative growth, ethylene appears to have a dual role, stimulating and inhibiting growth, depending on the species, tissue, and cell type, developmental stage, hormonal status, and environmental conditions. Moreover, ethylene signaling and response are part of an intricate network in cross talk with internal and external cues. Besides being a crucial factor in the growth control of roots and shoots, ethylene can promote flowering, fruit ripening and abscission, as well as leaf and petal senescence and abscission and, hence, plays a role in virtually every phase of plant life. Last but not least, together with jasmonates, salicylate, and abscisic acid, ethylene is important in steering stress responses.This Update provides recent insights into the role of ethylene on vegetative growth, both at the cellular and the whole-plant levels, with special attention to hormonal cross talk. Due to space restrictions, this Update is mainly focused on Arabidopsis (Arabidopsis thaliana).