Phytohormone Conjugates: Nature and Function

Reversible hormone conjugations in plants may represent physiologically and biochemically essential pathways in the regulation of endogenous levels of biologically active pools of phytohormones. Conjugates of auxins, gibberellins, and cytokinins are now widely recognized as serving a storage function for rapid (im)mobilization of these phytohormones, depending on a variety of environmental, developmental, and physiological factors. The significance of conjugates of other phytohormones (abscisic acid, ethylene, jasmonic acid, and salicylic acid) is less well understood. Recent developments in studies on phytohormone conjugation, involving both biochemical and molecular biology approaches, are presented here. The nature and possible functions of the conjugates are discussed. Conjugates of other compounds (e.g., anthranilate-glucosides) are also known (for review, see Hösel, 1981). However, it is not known whether these compounds have a signaling function.     

Conjugates of auxin and cytokinin

Plant growth and developmental processes as well as environmental responses require the action and cross talk of phytohormones including auxins and cytokinins. Active phytohormones are changed into multiple forms by acylation, esterification or glycosylation, for example. It seems that conjugated compounds could serve as pool of inactive phytohormones that can be converted to active forms by de-conjugation reactions. The concept of reversible conjugation of auxins and cytokinins suggests that under changeable environmental, developmental or physiological conditions these compounds can be a source of free hormones. Phytohormones metabolism may result in a loss of activity and decrease the size of the bioactive pool. All metabolic steps are in principle irreversible, except for some processes such as the formation of ester, glucoside and amide conjugates, where the free compound can be liberated by enzymatic hydrolysis. The role, chemistry, synthesis and hydrolysis of conjugated forms of two classes of plant hormones are discussed.     

Biosynthesis and beneficial effects of microbial gibberellins on crops for sustainable agriculture

Soil microbes promote plant growth through several mechanisms such as secretion of chemical compounds including plant growth hormones. Among the phytohormones, auxins, ethylene, cytokinins, abscisic acid and gibberellins are the best understood compounds. Gibberellins were first isolated in 1935 from the fungus Gibberella fujikuroi and are synthesized by several soil microbes. The effect of gibberellins on plant growth and development has been studied, as has the biosynthesis pathways, enzymes, genes and their regulation. This review revisits the history of gibberellin research highlighting microbial gibberellins and their effects on plant health with an emphasis on the early discoveries and current advances that can find vital applications in agricultural practices.     

Cytokinin and auxin inhibit abscisic acid-induced stomatal closure by enhancing ethylene production in Arabidopsis

Cytokinins and auxins are major phytohormones involved in various aspects of plant growth and development. These phytohormones are also known to antagonize the effects of abscisic acid (ABA) on stomatal movement, and to affect ethylene biosynthesis. As ethylene has an antagonistic effect on ABA-induced stomatal closure, the possibility that the antagonistic effects of these phytohormones on ABA were mediated through ethylene biosynthesis was investigated. Both the cytokinin, 6-benzyladenine (BA), and the auxin, 1-naphthaleneacetic acid (NAA), antagonized ABA-induced stomatal closure in a manner similar to that following application of the ethylene precursor, 1-aminocyclopropane-1-carboxylic acid (ACC). However, these effects were negated when ethylene signalling, perception, or biosynthesis were blocked. As stomatal aperture is regulated by changes in guard cell volume, ABA application was found to reduce the volume of the guard cell protoplasts (GCP). It was found that BA, NAA, or ACC application compensated perfectly for the reduction in GCP volume by ABA application in WT plants. The above observations suggest that cytokinins and auxins inhibit ABA-induced stomatal closure through the modulation of ethylene biosynthesis, and that ethylene inhibits the ABA-induced reduction of osmotic pressure in the guard cells.     

Development of Agrobacterium tumefaciens C58-induced plant tumors and impact on host shoots are controlled by a cascade of jasmonic acid,auxin, cytokinin,ethylene and abscisic acid

The development of Agrobacterium tumefaciens-induced plant tumors primarily depends on the excessive production of auxin and cytokinin by enzymes encoded on T-DNA genes integrated into the plant genome. The aim of the present study was to investigate the involvement of additional phytohormone signals in the vascularization required for rapid tumor proliferation. In stem tumors of Ricinus communis L., free auxin and zeatin riboside concentrations increased within 2 weeks to 15-fold the concentrations in control stem tissue. Auxin and cytokinin immunolocalization revealed the highest concentrations within and around tumor vascular bundles with concentration gradients. The time-course of changes in free auxin concentration in roots was inversely correlated with that in the tumors. The high ethylene emission induced by increased auxin- and cytokinin correlated with a 36-fold accumulation of abscisic acid in tumors. Ethylene emitted from tumors and exogenously applied ethylene caused an increase in abscisic acid concentrations also in the host leaves, with a diminution in leaf water vapor conductance. Jasmonic acid concentration reached a maximum already within the first week of bacterial infection. A wound effect could be excluded. The results demonstrate the concerted interaction of a cascade of transiently induced, non-T-DNA-encoded phytohormones jasmonic acid, ethylene and abscisic acid with T-DNA-encoded auxin and zeatin riboside plus trans-zeatin, all of which are required for successful plant tumor vascularization and development together with inhibition of host plant growth.     

Plant Peptide Hormones

Russian Journal of Plant Physiology - In addition to classic phytohormones, such as auxin, cytokinin, ethylene, gibberellin, and abscisic acid, plant peptide hormones are also involved in various...     

Sugar and phytohormone response pathways: navigating a signalling network

Many plant developmental, physiological and metabolic processes are regulated, at least in part, by nutrient availability. In particular, alterations in the availability of soluble sugars, such as glucose and sucrose, help regulate a diverse array of processes. Multiple lines of evidence indicate that many of these processes are also regulated in response to other signalling molecules, such as phytohormones. This review draws examples from a variety of plant systems, including bean, Arabidopsis, potato, and cereals. Five of the most interesting and best developed examples of processes regulated via 'interactions' or 'crosstalk' between sugars and phytohormones are described, including embryogenesis, seed germination, early seedling development, tuberization, and the regulation of alpha-amylase activity. The types of mechanisms by which different response pathways are known or postulated to interact are also described. These mechanisms include regulation of the metabolism and/or transport of a signalling molecule by a different response pathway. For example, sugars have been postulated to help regulate the synthesis, conjugation and/or transport of phytohormones, such as gibberellins and abscisic acid. Conversely, phytohormones, such as abscisic acid, gibberellins and cytokinins have been shown to help regulate sugar metabolism and/or transport. Similarly, sugars have been shown to regulate the expression of components of phytohormone-response pathways and phytohormones regulate the expression of some genes encoding possible components of sugar-response pathways. Examples of proteins and second messengers that appear to act in multiple response pathways are also described.     

What is known about phytohormones in halophytes? A review

Phytohormones participate in many aspects of the plant life cycle, including responses to biotic and abiotic stresses. They play a key role in plant responses to the environment with direct bearing on a plant’s fitness for adaptation and reproduction. In recent years, there have been major advances in our understanding of the role of phytohormones in halophytic plants. The variability in maximal salinity level that halophytes can tolerate makes it difficult to characterize the specific traits responsible for salt tolerance. However, the most evident effect of salinity is growth disturbance, and growth is directly governed by phytohormones. Phytohormones such as abscisic acid, salicylic acid ethylene and jasmonates are traditionally related to stress responses, while the involvement of cytokinins, gibberellins and auxins has started to be analyzed. Polyamines, although they can’t be considered phytohormones because of the high concentrations required for cell responses, have been proposed as a new category of plant growth regulators involved in several plant processes and stress responses. This review integrates the advances in the knowledge about phytohormones in halophytes and their participation in salt tolerance.     

Hormones and hormone-like substances of microorganisms: A review

Data from the literature on the ability of microorganisms to form plant hormones have been reviewed. The substances covered include abscisic acid, ethylene and other compounds with phytohormone-like properties (brassinosteroids, oligosaccharines) and analogues of animal neurotransmitters (biogenic amines). Pathways whereby the substances are metabolized and their effects on the development and activity (physiological and biochemical) of the microorganisms are considered. The role of phytohormones and hormone-like substances in the formation of association (microorganism-host) interactions are analyzed. The potential utilities of microorganisms producing hormones and hormone-like substances are discussed.     

Hormones and hormone-like substances of microorganisms: a review

Data from the literature on the ability of microorganisms to form plant hormones have been reviewed. The substances covered include abscisic acid, ethylene and other compounds with phytohormone-like properties (brassinosteroids, oligosaccharines) and analogues of animal neurotransmitters (biogenic amines). Pathways whereby the substances are metabolized and their effects on the development and activity (physiological and biochemical) of the microorganisms are considered. The role of phytohormones and hormone-like substances in the formation of association (microorganism-host) interactions are analyzed. The potential utilities of microorganisms producing hormones and hormone-like substances are discussed.     

Simultaneous quantification of phytohormones in fermentation extracts of Botryodiplodia theobromae by liquid chromatography–electrospray tandem mass spectrometry

Fermentation broth and biomass from three strains of Botryodiplodia theobromae were characterized by high performance liquid chromatography–electrospray tandem mass spectrometry (HPLC–ESI–MS/MS) method, in order to quantify different phytohormones and to identify amino acid conjugates of jasmonic acid (JA) present in fermentation broths. A liquid–liquid extraction with ethyl acetate was used as sample preparation. The separation was carried out on a C18 reversed-phase HPLC column followed by analysis via ESI–MS/MS. The multiple reaction monitoring mode was used for quantitative measurement. For the first time, indole-3-acetic acid, indole-3-propionic acid, indole-3-butyric acid and JA were identified and quantified in the ethyl acetate extracts from the biomass, after the separation of mycelium from supernatant. The fermentation broths showed significantly higher levels of JA in relation to the other phytohormones. This is the first report of the presence of gibberellic acid, abscisic acid, salicylic acid and the cytokinins zeatin, and zeatin riboside in fermentation broths of Botryodiplodia sp. The presence of JA-serine and JA-threonine conjugates in fermentation broth was confirmed using HPLC-ESI tandem mass spectrometry in negative ionization mode, while the occurrence of JA-glycine and JA-isoleucine conjugates was evidenced with the same technique but with positive ionization. The results demonstrated that the used HPLC–ESI–MS/MS method was effective for analysing phytohormones in fermentation samples.     

Structure-Activity Correlations with Compounds Related to Abscisic Acid

Inhibition of cell expansion of excised embryonic axes of Phaseolus vulgaris was used to evaluate the growth-inhibiting activity of abscisic acid and related compounds. None of the 13 compounds tested was as active as abscisic acid. 4-Hydroxyisophorone, a substance representative of the abscisic acid ring system was essentially inactive; cis, trans-3-methylsorbic acid, a compound resembling the side chain of abscisic acid, had low activity; and cis, trans-beta-ionylideneacetic acid was one-sixth as active. Loss of the ring double bond results in a drastic decrease in biological activity. Comparison of our results with those reported previously leads to the suggestion that the double bond of the cyclohexyl moiety may have an important function in determining the degree of activity of cis, trans-ionylideneacetic acids. Two modes of action are discussed. It seems possible that the ring double bond is involved in covalent bonding in binding of the abscisic acid analogue to macromolecules. This may require formation of an intermediate epoxide. It can also be argued that stereochemical differences between cyclohexane derivatives are important factors in determining the degree of biological activity.     

Vitamin E and defense-related phytohormones are reliable markers of embryo growth in macaw palm fruits exposed to various storage conditions

The fruit of the macaw palm (Acrocomia aculeata) may be used for biofuel production, but its exploitation as a crop is currently limited by its low germinability. Therefore, obtaining plantlets in vitro is an excellent way to solve this problem. Here we aimed to identify the optimal conditions for storing the fruit before obtaining plantlets and testing to what extent vitamin E and defense-related phytohormones are good indicators of embryo growth in vitro. We tested the effects of four storage conditions (nursery, laboratory, cold chamber and freezer) on seed germinability and embryo growth, and evaluated endogenous levels of vitamin E and defense-related phytohormones (abscisic acid, salicylic acid and jasmonic acid) in the endosperm and embryos. Low temperatures [both cold chamber (5 °C) and freezer (?18 °C) methods] killed the embryos, while storing the fruit in the laboratory was the most efficient method of obtaining plantlets, even after a year. Vitamin E and abscisic acid turned out to be good indicators of embryo growth. Enhanced vitamin E and abscisic acid levels had a strong positive correlation with successful embryo growth, thus indicating that these compounds are needed to protect the embryo during fruit storage. Furthermore, abscisic acid levels had a negative correlation with the percentage of contaminated embryos, thus suggesting that the endogenous physiological stage of the seeds affects subsequent contamination in in vitro cultures. We concluded that (1) storing fruit under laboratory conditions is the most efficient means of obtaining plantlets successfully, and (2) vitamin E and abscisic acid can be used as reliable indicators of embryo growth during in vitro culture.     

Hormone interactions in stomatal function

Research in recent years on the biology of guard cells has shown that these specialized cells integrate both extra- and intra-cellular signals in the control of stomatal apertures. Among the phytohormones, abscisic acid (ABA) is one of the key players regulating stomatal function. In addition, auxin, cytokinin, ethylene, brassinosteroids, jasmonates, and salicylic acid also contribute to stomatal aperture regulation. The interaction of multiple hormones can serve to determine the size of stomatal apertures in a condition-specific manner. Here, we discuss the roles of different phytohormones and the effects of their interactions on guard cell physiology and function.     

Role of Phytohormones in Regulating Heat Stress Acclimation in Agricultural Crops

Heat stress (HS) seriously affects crop growth, causing significant crop yield losses worldwide. The regulatory mechanisms controlling HS tolerance in plants are not well understood. Phytohormones are important molecules for coordinating myriad of phenomena related to plant growth and development. They are also essential endogenous signaling molecules that actively mediate numerous physiological responses under abiotic stress by triggering stress-responsive regulatory genes involved in plant growth. This review updates the central role of various phytohormones—indole acetic acid, gibberellic acid, abscisic acid, cytokinins, ethylene, salicylic acid, brassinosteroids, strigolactone, and jasmonic acid—in regulating the HS response so that plants can adapt to increasing temperature stress. We also reveal how these stress-responsive phytohormones switch on various regulatory gene(s) and genes encoding antioxidants and heat shock proteins (HSPs) to combat HS in various plant species.

     

Indole-3-acetic acid, ethylene, and abscisic acid metabolism in developing muskmelon (Cucumis melo L.) fruit

Hormonal metabolism associated with fruit development in muskmelon was investigated by measuring IAA, ABA, and ACC levels in several tissues at various stages of development. In addition, levels of conjugated IAA and ABA were determined in the same tissues. Ethylene production, which is believed to signal the ripening and senescence of mature fruit, was also measured. Ethylene production was highest in the outer tissue near the rind and gradually declined during maturation, except for a dramatic increase in all fruit tissues at the climacteric. In contrast to ethylene production, ACC levels increased during maturation and remained equal throughout the fruit until the climacteric, when levels in the outer tissues increased nearly 5-fold over levels in the inner tissues. The consistent presence of ACC indicates that ACC oxidase rather than the availability of ACC regulates ethylene production in developing fruits. ABA and ABA esters generally declined during maturation, however an increase in ABA esters associated with the outer mesocarp tissue was observed in fully mature, climacteric fruit. IAA and IAA conjugates were only found in the outer tissue near the rind, and their levels remained low until the fruit was fully mature and entering the climacteric. At that time, increased levels of conjugates were detected. The late burst of hormonal metabolism in the outer mesocarp tissue appeared to signal its degeneration and the deterioration that typically occurs in ripening fruit. The tissue-specific conjugation of IAA and ABA, in addition to the production of climacteric ethylene, may represent part of the signaling mechanism initiating ripening and eventual deterioration of tissues in muskmelon fruits.Abbreviations ABA abscisic acid - ACC 1-aminocylopropane-1-carboxylic acid - DAP days after pollination - IAA indole-3-acetic acid     

Effects of phytohormones on the cytoskeleton of the plant cell

This review highlights the effects of ??classic?? phytohormones (auxins, cytokinins, gibberellins, abscisic acid, ethylene, and brassinosteroids) and also of important signaling molecules, such as jasmonic acid, strigolactones, and nitric oxide, on the main components of the plant cytoskeleton, microtubules and microfilaments. The effects of these growth regulators on orientation and organization of microtubules and actin filaments, realization of cytoskeleton-dependent processes, expression of tubulin and actin genes, and interaction of various phytohormones in their influence on the cytoskeleton are discussed.     

Hyaluronic acid-paclitaxel conjugate micelles: synthesis, characterization, and antitumor activity

Chemical conjugates of paclitaxel and hyaluronic acid (HA) were synthesized by utilizing a novel HA solubilization method in a single organic phase. Hydrophilic HA was completely dissolved in anhydrous DMSO with addition of poly(ethylene glycol) (PEG) by forming nanocomplexes. Paclitaxel was then chemically conjugated to HA in the DMSO phase via an ester linkage without modifying extremely hydrophilic HA. A series of HA-paclitaxel conjugates with different conjugation percentages were synthesized and characterized. HA-paclitaxel conjugates self-assembled in aqueous solution to form nanosized micellar aggregates, as characterized by dynamic light scattering (DLS), atomic force microscopy (AFM), and transmission electron microscopy (TEM). An intact form of paclitaxel was regenerated from HA-paclitaxel conjugate micelles at acidic pH conditions. HA-paclitaxel conjugate micelles exhibited more pronounced cytotoxic effect for HA receptor overexpressing cancer cells than for HA receptor deficient cells, suggesting that they can be potentially utilized as tumor-specific nanoparticulate therapeutic agents.