Class Of Plant Hormones Research Articles

Progress in the analysis of brassinosteroids

Brassinosteroids have been considered as a class of plant hormones with high activity. However, the complex matrix of the plant samples and the ultra-trace level of naturally occurring brassinosteroids make their separation and determination very difficult. This review summarizes the progress in the development of sample pretreatment and determination of brassinosteroids.

Signal integration in the control of shoot branching

Shoot branching is a highly plastic developmental process in which axillary buds are formed in the axil of each leaf and may subsequently be activated to give branches. Three classes of plant hormones, auxins, cytokinins and strigolactones (or strigolactone derivatives) are central to the control of bud activation. These hormones move throughout the plant forming a network of systemic signals. The past decade brought great progress in understanding the mechanisms of shoot branching control. Biological and computational studies have led to the proposal of two models, the auxin transport canalization-based model and the second messenger model, which provide mechanistic explanations for apical dominance.

Strigolactones regulate protonema branching and act as a quorum sensing-like signal in the moss Physcomitrella patens

Strigolactones are a novel class of plant hormones controlling shoot branching in seed plants. They also signal host root proximity during symbiotic and parasitic interactions. To gain a better understanding of the origin of strigolactone functions, we characterised a moss mutant strongly affected in strigolactone biosynthesis following deletion of the CAROTENOID CLEAVAGE DIOXYGENASE 8 (CCD8) gene. Here, we show that wild-type Physcomitrella patens produces and releases strigolactones into the medium where they control branching of protonemal filaments and colony extension. We further show that Ppccd8 mutant colonies fail to sense the proximity of neighbouring colonies, which in wild-type plants causes the arrest of colony extension. The mutant phenotype is rescued when grown in the proximity of wild-type colonies, by exogenous supply of synthetic strigolactones or by ectopic expression of seed plant CCD8. Thus, our data demonstrate for the first time that Bryophytes (P. patens) produce strigolactones that act as signalling factors controlling developmental and potentially ecophysiological processes. We propose that in P. patens, strigolactones are reminiscent of quorum-sensing molecules used by bacteria to communicate with one another.

Strigolactone, a key regulator of nutrient allocation in plants

Strigolactones (SLs) are a group of terpenoid lactones that are derived from carotenoids. SLs have been found in a number of plant species and appear to serve several diverse physiological functions. SLs were first identified by their ability to stimulate seed germination of root-parasitic plants. Later, SLs were isolated as hyphal-branching inducers of arbuscular mycorrhizal fungi, which facilitate the uptake of soil nutrients by plants. Most recently, SLs (or their derivatives) were found to be a new class of plant hormones that inhibit shoot branching. Considering these three roles of SLs, it was unclear at first why communication signals in the rhizosphere would regulate shoot branching in the host plant. Recent reports, however, suggest that plants produce SLs in response to nitrogen and phosphorus deficiency, stimulating changes in plant shoot and root architecture that enable them to adapt to environmental conditions. Excess SLs produced in roots are released into the soil, where they stimulate the growth of arbuscular mycorrhizal fungi. These symbiotic fungi supply inorganic nutrients that can be used by the plant. This review paper focuses on the physiological roles of SLs as a key regulator of nutrient allocation in plants.

Synthetic studies on the solanacol ABC ring system by cation-initiated cascade cyclization: implications for strigolactone biosynthesis

We report a new method for constructing the ABC ring system of strigolactones, in a single step from a simple linear precursor by acid-catalyzed double cyclization. The reaction proceeds with a high degree of stereochemical control, which can be qualitatively rationalized using DFT calculations. Our concise synthetic approach offers a new model for thinking about the (as yet) unknown chemistry that is employed in the biosynthetic pathways leading to this class of plant hormones.

Strigolactones Are Transported through the Xylem and Play a Key Role in Shoot Architectural Response to Phosphate Deficiency in Nonarbuscular Mycorrhizal Host Arabidopsis

The biosynthesis of the recently identified novel class of plant hormones, strigolactones, is up-regulated upon phosphate deficiency in many plant species. It is generally accepted that the evolutionary origin of strigolactone up-regulation is their function as a rhizosphere signal that stimulates hyphal branching of arbuscular mycorrhizal fungi. In this work, we demonstrate that this induction is conserved in Arabidopsis (Arabidopsis thaliana), although Arabidopsis is not a host for arbuscular mycorrhizal fungi. We demonstrate that the increase in strigolactone production contributes to the changes in shoot architecture observed in response to phosphate deficiency. Using high-performance liquid chromatography, column chromatography, and multiple reaction monitoring-liquid chromatography-tandem mass spectrometry analysis, we identified two strigolactones (orobanchol and orobanchyl acetate) in Arabidopsis and have evidence of the presence of a third (5-deoxystrigol). We show that at least one of them (orobanchol) is strongly reduced in the putative strigolactone biosynthetic mutants more axillary growth1 (max1) and max4 but not in the signal transduction mutant max2. Orobanchol was also detected in xylem sap and up-regulated under phosphate deficiency, which is consistent with the idea that root-derived strigolactones are transported to the shoot, where they regulate branching. Moreover, two additional putative strigolactone-like compounds were detected in xylem sap, one of which was not detected in root exudates. Together, these results show that xylem-transported strigolactones contribute to the regulation of shoot architectural response to phosphate-limiting conditions.

Arbuscular mycorrhizal symbiosis decreases strigolactone production in tomato

Strigolactones are a new class of plant hormones emerging as important signals in the control of plant architecture. In addition, they are key elements in plant communication with several rhizosphere organisms. Strigolactones are exuded into the soil, where they act as host detection signals for arbuscular mycorrhizal (AM) fungi, but also as germination stimulants for root parasitic plant seeds. Under phosphate limiting conditions, plants up-regulate the secretion of strigolactones into the rhizosphere to promote the formation of AM symbiosis. Using tomato as a model plant, we have recently shown that AM symbiosis induces changes in transcriptional and hormonal profiles. Using the same model system, here we analytically demonstrate, using liquid chromatography–tandem mass spectrometry, that strigolactone production is also significantly reduced upon AM symbiosis. Considering the dual role of the strigolactones in the rhizosphere as signals for AM fungi and parasitic plants, we discuss the potential implications of these changes in the plant interaction with both organisms.

The Strigolactone Story

Strigolactones (SLs) were originally isolated from plant root exudates as germination stimulants for root parasitic plants of the family Orobanchaceae, including witchweeds (Striga spp.), broomrapes (Orobanche and Phelipanche spp.), and Alectra spp., and so were regarded as detrimental to the producing plants. Their role as indispensable chemical signals for root colonization by symbiotic arbuscular mycorrhizal fungi was subsequently unveiled, and SLs then became recognized as beneficial plant metabolites. In addition to these functions in the rhizosphere, it has been recently shown that SLs or their metabolites are a novel class of plant hormones that inhibit shoot branching. Furthermore, SLs are suggested to have other biological functions in rhizosphere communications and in plant growth and development.

The Physiological, Biochemical and Molecular Roles of Brassinosteroids and Salicylic Acid in Plant Processes and Salt Tolerance

Plant hormones regulate plant growth and development by affecting an array of cellular, physiological, and developmental processes, including, but not limited to, cell division and elongation, stomatal regulation, photosynthesis, transpiration, ion uptake and transport, initiation of leaf, flower and fruit development, and senescence. Environmental factors such as salinity, drought, and extreme temperatures may cause a reduction in plant growth and productivity by altering the endogenous levels of plant hormones, sensitivity to plant hormones, and/or signaling pathways. Molecular and physiological studies have determined that plant hormones and abiotic stresses have interactive effects on a number of basic biochemical and physiological processes, leading to reduced plant growth and development. Various strategies have been considered or employed to maximize plant growth and productivity under environmental stresses such as salt-stress. A fundamental approach is to develop salt-tolerant plants through genetic means. Breeding for salt tolerance, however, is a long-term endeavor with its own complexities and inherent difficulties. The success of this approach depends, among others, on the availability of genetic sources of tolerance and reliable screening techniques, identification and successful transfer of genetic components of tolerance to desired genetic backgrounds, and development of elite breeding lines and cultivars with salt tolerance and other desirable agricultural characteristics. Such extensive processes have delayed development of successful salt-tolerant cultivars in most crop species. An alternative and technically simpler approach is to induce salt tolerance through exogenous application of certain plant growth–regulating compounds. This approach has gained significant interest during the past decade, when a wealth of new knowledge has become available on the beneficial roles of the six classes of plant hormones (auxins, gibberellins, cytokinins, abscisic acid, ethylene, and brassinosteroids) as well as several other plant growth–regulating substances (jasmonates, salicylates, polyamines, triacontanol, ascorbic acid, and tocopherols) on plant stress tolerance. Among these, brassinosteroids (BRs) and salicylic acid (SA) have been studied most extensively. Both BRs and SA are ubiquitous in the plant kingdom, affecting plant growth and development in many different ways, and are known to improve plant stress tolerance. In this article, we review and discuss the current knowledge and possible applications of BRs and SA that could be used to mitigate the harmful effects of salt-stress in plants. We also discuss the roles of exogenous applications of BRs and SA in the regulation of various biochemical and physiological processes leading to improved salt tolerance in plants.

Does abscisic acid affect strigolactone biosynthesis?

*Strigolactones are considered a novel class of plant hormones that, in addition to their endogenous signalling function, are exuded into the rhizosphere acting as a signal to stimulate hyphal branching of arbuscular mycorrhizal (AM) fungi and germination of root parasitic plant seeds. Considering the importance of the strigolactones and their biosynthetic origin (from carotenoids), we investigated the relationship with the plant hormone abscisic acid (ABA). *Strigolactone production and ABA content in the presence of specific inhibitors of oxidative carotenoid cleavage enzymes and in several tomato ABA-deficient mutants were analysed by LC-MS/MS. In addition, the expression of two genes involved in strigolactone biosynthesis was studied. *The carotenoid cleavage dioxygenase (CCD) inhibitor D2 reduced strigolactone but not ABA content of roots. However, in abamineSG-treated plants, an inhibitor of 9-cis-epoxycarotenoid dioxygenase (NCED), and the ABA mutants notabilis, sitiens and flacca, ABA and strigolactones were greatly reduced. The reduction in strigolactone production correlated with the downregulation of LeCCD7 and LeCCD8 genes in all three mutants. *The results show a correlation between ABA levels and strigolactone production, and suggest a role for ABA in the regulation of strigolactone biosynthesis.

Strigolactones as germination stimulants for root parasitic plants.

Witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.) are the two most devastating root parasitic plants belonging to the family Orobanchaceae and are causing enormous crop losses throughout the world. Seeds of these root parasites will not germinate unless they are exposed to chemical stimuli, ‘germination stimulants’ produced by and released from plant roots. Most of the germination stimulants identified so far are strigolactones (SLs), which also function as host recognition signals for arbuscular mycorrhizal fungi and a novel class of plant hormones inhibiting shoot branching. In this review, we focus on SLs as germination stimulants for root parasitic plants. In addition, we discuss how quantitative and qualitative differences in SL exudation among sorghum cultivars influence their susceptibility to Striga.

Proteomics Study of COI1‐regulated Proteins in Arabidopsis Flower

Jasmonates (JAs) are a new class of plant hormone that regulate expression of diverse genes to mediate various plant responses. The Arabidopsis F-box protein COI1 is required for plant defense and male fertility in JA signal pathway. To further investigate the regulatory role of COI1 in male fertility, we compared the proteomics profiles of Arabidopsis wild type (WT) flowers with coi1-1 mutant male-sterile flowers using two-dimensional difference gel electrophoresis coupled with matrix-assisted laser desoption/ionization-time-of-flight mass spectrometry. Sixteen proteins with potential function in specific biological processes such as metabolism processes and defense/stress responses were differentially expressed in WT and coi1-1 mutant flowers. Verification on a phi class glutathione transferase AtGSTF9, one out of these 16 identified proteins, revealed that the expression of AtGSTF9 was severely downregulated in flowers of coi1-1 mutant compared with that of WT. Further function analyses of these genes would provide new insights into the molecular basis of COI1-regulated male fertility.

Strigolactones, a novel class of plant hormone controlling shoot branching

For several decades, auxin and cytokinin were the only two hormones known to be involved in the control of shoot branching through apical dominance, a process where the shoot apex producing auxin inhibits the outgrowth of axillary buds located below. Grafting studies with high branching mutants and cloning of the mutated genes demonstrated the existence of a novel long distance carotenoid derived signal which acted as a branching inhibitor. Recently, this branching inhibitor has been shown to belong to the strigolactones, a group of small molecules already known to be produced by roots, exuded in the rhizosphere and as having a role in both parasitic and symbiotic interactions.

Exploring the impact of wounding and jasmonates on ascorbate metabolism

Vitamin C (ascorbate, AsA) is the most abundant water-soluble antioxidant in plants. Ascorbate provides the first line of defense against damaging reactive oxygen species (ROS), and helps protect plant cells from many factors that induce oxidative stress, including wounding, ozone, high salinity, and pathogen attack. Plant defenses against these stresses are also dependent upon jasmonates (JAs), a class of plant hormones that promote ROS accumulation. Here, we review evidence showing that wounding and JAs influence AsA accumulation in various plant species, and we report new data from Arabidopsis and tomato testing the influence of JAs on AsA levels in wounded and unwounded plants. In both species, certain mutations that impair JA metabolism and signaling influence foliar AsA levels, suggesting that endogenous JAs may regulate steady-state AsA. However, the impact of wounding on AsA accumulation was similar in JA mutants and wild type controls, indicating that this wound response does not require JAs. Our findings also indicate that the effects of wounding and JAs on AsA accumulation differ between species; these factors both enhanced AsA accumulation in Arabidopsis, but depressed AsA levels in tomato. These results underscore the importance of obtaining data from more than one model species, and demonstrate the complexity of AsA regulation.

Effect of Methyl Jasmonate on antioxidative enzyme activities and on the contents of ROS and H2O2 in Ricinus communis leaves

Jasmonates are a class of plant hormones that mediate various aspects in gene and metabolic regulation, defense, stress responses, reproduction and, possibly, communication. Oxidative stress stimulates synthesis of antioxidant metabolites and enhances antioxidant enzyme activities that could protect plant tissues. The aim of this study was to verify the effects of methyl jasmonate (JAME) treatment on the reactive oxygen species (ROS) and on the activities of H2O2 scavenging enzymes, such as superoxide dismutase (SOD; EC 1.15.1.1), catalase (CAT; EC 1.11.1.6), ascorbate peroxidase (APX EC; 1.11.1.1), and guaiacol peroxidase (GPX; EC 1.11.1.7) in Ricinus communis leaves. The activity of CAT and GPX was transient while SOD activity decreased and APX increased after treatment with JAME. In addition, JAME exposure induced ROS accumulation.

Strigolactones, a novel class of plant hormones controlling branching

Plant architecture is a major trait for plant survival and plant fitness and has a huge influence on the agronomical value for most crops. The classical theory of apical dominance based on decapitation experiments suggested that two major plant hormones, auxin and cytokinins, were acting antagonistically on bud outgrowth to promote or repress branching. However this theory was challenged in the late 1930's by Snow who suggested the existence of a second messenger to auxin, as auxin was not acting directly to repress branching. The use of branching mutants in pea, Arabidopsis and rice led to the discovery of a new carotenoid-derived signal repressing branching. Genes involved in synthesis (RMS1, RMS5) as well as in response (RMS4) to this new signal have been identified and have given rise to a new model of the branching control. Two independent group have recently shown, one on pea, the other on rice, that strigolactones correspond to this novel signal which represses branching and to the secondary messenger in the theory of apical dominance. Strigolactones have been first identified for their role in germination of parasitic plants like Striga or Orobanche. They also play a critical role in the widespread association between 80% of plants and fungi, the arbuscular mycorrhizal symbiosis, as they are necessary for interaction between certain plants and fungi in the rhizosphere.

Studies on the Host Recognition Mechanism of Root Parasitic Plants

Among parasitic angiosperms, witchweeds (Striga spp.) and broomrapes (Orobanche and Phelipanche spp.) are the two most devastating root parasitic plants, causing enormous crop losses around the world. These root parasites have evolved special strategies to ensure their survival; the seeds germinate only when they are within the host rhizosphere so that after germination the seedlings can reach and parasitize the host roots. To locate host roots, root parasites exploit strigolactones (SLs), host-derived chemical signals that are released by host plants for symbiotic arbuscular mycorrhizal (AM) fungi. In addition to these functions as rhizosphere signaling molecules, SLs act as a class of plant hormones regulating shoot branching. Using high performance liquid chromatography-tandem mass spectrometry (LC-MS/MS), SLs in root exudates from various plants have been characterized. During studies, in addition to the known SLs, more than 10 novel SLs have been isolated and their structures determined. Plants produce and exude mixtures of SLs, and nutrient availability affects SL production and exudation. These results suggest that quantitative and qualitative differences in SL production and/or exudation are important in host recognition of root parasitic plants.

Strigolactones are a new-defined class of plant hormones which inhibit shoot branching and mediate the interaction of plant-AM fungi and plant-parasitic weeds

Because plants are sessile organisms, the ability to adapt to a wide range of environmental conditions is critical for their survival. As a consequence, plants use hormones to regulate growth, mitigate biotic and abiotic stresses, and to communicate with other organisms. Many plant hormones function pleiotropically in vivo, and often work in tandem with other hormones that are chemically distinct. A newly-defined class of plant hormones, the strigolactones, cooperate with auxins and cytokinins to control shoot branching and the outgrowth of lateral buds. Strigolactones were originally identified as compounds that stimulated the germination of parasitic plant seeds, and were also demonstrated to induce hyphal branching in arbuscular mycorrhizal (AM) fungi. AM fungi form symbioses with higher plant roots and mainly facilitate the absorption of phosphate from the soil. Conforming to the classical definition of a plant hormone, strigolactones are produced in the roots and translocated to the shoots where they inhibit shoot outgrowth and branching. The biosynthesis of this class of compounds is regulated by soil nutrient availability, i.e. the plant will increase its production of strigolactones when the soil phosphate concentration is limited, and decrease production when phosphates are in ample supply. Strigolactones that affect plant shoot branching, AM fungal hyphal branching, and seed germination in parasitic plants facilitate chemical synthesis of similar compounds to control these and other biological processes by exogenous application.

Development of a method for comprehensive and quantitative analysis of plant hormones by highly sensitive nanoflow liquid chromatography–electrospray ionization-ion trap mass spectrometry

In recent plant hormone research, there is an increased demand for a highly sensitive and comprehensive analytical approach to elucidate the hormonal signaling networks, functions, and dynamics. We have demonstrated the high sensitivity of a comprehensive and quantitative analytical method developed with nanoflow liquid chromatography–electrospray ionization-ion trap mass spectrometry (LC–ESI-IT-MS/MS) under multiple-reaction monitoring (MRM) in plant hormone profiling. Unlabeled and deuterium-labeled isotopomers of four classes of plant hormones and their derivatives, auxins, cytokinins (CK), abscisic acid (ABA), and gibberellins (GA), were analyzed by this method. The optimized nanoflow-LC–ESI-IT-MS/MS method showed ca. 5–10-fold greater sensitivity than capillary-LC–ESI-IT-MS/MS, and the detection limits (S/N = 3) of several plant hormones were in the sub-fmol range. The results showed excellent linearity ( R 2 values of 0.9937–1.0000) and reproducibility of elution times (relative standard deviations, RSDs, <1.1%) and peak areas (RSDs, <10.7%) for all target compounds. Further, sample purification using Oasis HLB and Oasis MCX cartridges significantly decreased the ion-suppressing effects of biological matrix as compared to the purification using only Oasis HLB cartridge. The optimized nanoflow-LC–ESI-IT-MS/MS method was successfully used to analyze endogenous plant hormones in Arabidopsis and tobacco samples. The samples used in this analysis were extracted from only 17 tobacco dry seeds (1 mg DW), indicating that the efficiency of analysis of endogenous plant hormones strongly depends on the detection sensitivity of the method. Our analytical approach will be useful for in-depth studies on complex plant hormonal metabolism.

Chasing the structures of small molecules in arbuscular mycorrhizal signaling

The arbuscular mycorrhiza (AM) is a symbiosis between most terrestrial plants and fungi of the ancient phylum Glomeromycota. AM improves the uptake of water and mineral nutrients, such as phosphorus (P) and nitrogen (N), of the host plant in exchange for photosynthetically fixed carbon. Successful colonization and a functional interaction between host plant and mycobiont are based upon exchange of signaling molecules at different stages of symbiosis development. Strigolactones, a novel class of plant hormones, are secreted by plant roots stimulating presymbiotic growth of AM fungi. Fungi release soluble signaling molecules, the enigmatic 'Myc factors', that activate early symbiotic root responses. Lysophosphatidylcholine is a lipophilic intraradical mycorrhizal signal triggering plant phosphate transporter gene expression late in AM development through a P-controlled transcriptional mechanism. This enables uptake of orthophosphate released from the AM fungus.