Melatonin Antagonizes Cytokinin Responses to Stimulate Root Growth in Arabidopsis

Melatonin functions as the key growth regulator in various plant species. The mechanisms of the interactions between melatonin and cytokinins remain largely unknown. In this study, the kinetic effects of melatonin over a range of concentrations were investigated. The results showed that melatonin functioned as a positive regulator of root growth ranged from 0.1 to 100 nM. In contrast, exogenous cytokinin at 0.5–1 nM and overexpression of cytokinin biosynthesis gene ISOPENTENYLTRANSFERASE 8 (IPT8) inhibited primary root growth. Combined treatments with melatonin and cytokinin indicated that melatonin antagonized the inhibitory effect of cytokinin 6-benzylaminopurine (6-BA) on primary root elongation. Further analysis revealed that melatonin promotes primary root growth by modulating expression and distribution of auxin efflux transporters PIN2/3 and influx transporter AUX1. Moreover, the cytokinin signaling components AHK4, AHP2/3/5, and type-A ARR15 were down-regulated after melatonin treatment. The polar auxin transport inhibitor 2,3,5-triiodobenzoic acid (TIBA) impaired the promotive effect of melatonin on primary root growth, indicating that auxin transport is essential in melatonin-mediated root growth. Taken together, our data provided evidence to show that melatonin regulates primary root growth in coordination with cytokinin partially through auxin-dependent pathway.

     

植物细胞分裂素的糖苷物生物活性分析

用附加细胞分裂素及其糖苷物的MS培养基,垂直培养拟南芥并测定根的伸长,以此分析细胞分裂素及其糖苷物的生物学活性的结果表明,细胞分裂素的N-糖苷物几乎完全失去细胞分裂素活性,而D-糖苷仍具有细胞分裂素活性。推测两类糖苷物对植物体内细胞分裂素活性可能有不同的调控作用。     

Trichoderma Synthesizes Cytokinins and Alters Cytokinin Dynamics of Inoculated Arabidopsis Seedlings

Journal of Plant Growth Regulation - Trichoderma is an important genus of symbiotic fungi, commonly used around the world as biocontrol agents and as biofertilizer. Although their beneficial...     

The Yin-Yang of Hormones: Cytokinin and Auxin Interactions in Plant Development

The phytohormones auxin and cytokinin interact to regulate many plant growth and developmental processes. Elements involved in the biosynthesis, inactivation, transport, perception, and signaling of these hormones have been elucidated, revealing the variety of mechanisms by which signal output from these pathways can be regulated. Recent studies shed light on how these hormones interact with each other to promote and maintain plant growth and development. In this review, we focus on the interaction of auxin and cytokinin in several developmental contexts, including its role in regulating apical meristems, the patterning of the root, the development of the gynoecium and female gametophyte, and organogenesis and phyllotaxy in the shoot.     

Methyl Jasmonate and Cytokinin Mitigate the Salinity-Induced Oxidative Injury in Wheat Seedlings

Journal of Plant Growth Regulation - Application of methyl jasmonate (MeJA) has been shown to be effective in improving plant tolerance to various environmental stresses. Present research was...     

Functional Roles of Three Cutin Biosynthetic Acyltransferases in Cytokinin Responses and Skotomorphogenesis

Cytokinins (CKs) regulate plant development and growth via a two-component signaling pathway. By forward genetic screening, we isolated an Arabidopsis mutant named grow fast on cytokinins 1 (gfc1), whose seedlings grew larger aerial parts on MS medium with CK. gfc1 is allelic to a previously reported cutin mutant defective in cuticular ridges (dcr). GFC1/DCR encodes a soluble BAHD acyltransferase (a name based on the first four enzymes characterized in this family: Benzylalcohol O-acetyltransferase, Anthocyanin O-hydroxycinnamoyltransferase, anthranilate N-hydroxycinnamoyl/benzoyltransferase and Deacetylvindoline 4-O-acetyltransferase) with diacylglycerol acyltransferase (DGAT) activity in vitro and is necessary for normal cuticle formation on epidermis in vivo. Here we show that gfc1 was a CK-insensitive mutant, as revealed by its low regeneration frequency in vitro and resistance to CK in adventitious root formation and dark-grown hypocotyl inhibition assays. In addition, gfc1 had de-etiolated phenotypes in darkness and was therefore defective in skotomorphogenesis. The background expression levels of most type-A Arabidopsis Response Regulator (ARR) genes were higher in the gfc1 mutant. The gfc1-associated phenotypes were also observed in the cutin-deficient glycerol-3-phosphate acyltransferase 4/8 (gpat4/8) double mutant [defective in glycerol-3-phosphate (G3P) acyltransferase enzymes GPAT4 and GPAT8, which redundantly catalyze the acylation of G3P by hydroxyl fatty acid (OH-FA)], but not in the cutin-deficient mutant cytochrome p450, family 86, subfamily A, polypeptide 2/aberrant induction of type three 1 (cyp86A2/att1), which affects the biosynthesis of some OH-FAs. Our results indicate that some acyltransferases associated with cutin formation are involved in CK responses and skotomorphogenesis in Arabidopsis.     

Characteristics of Root-to-Shoot Transport of Cytokinin 6-Benzylaminopurine in Intact Seedlings of Citrus aurantium

Transport of the cytokinin 6-benzylaminopurine-8-¹⁴C in the root and shoot of intact Citrus aurantium L. seedlings was studied by “replacing” the 0.5 cm root tip with the uptake solution. The cytokinin was transported basipetally in the root and was distributed in an acropetal direction in the stem and into the leaves. Kinetic analysis of the transport for periods of up to 96 h revealed a characteristic advancing front of the label along the axis of the seedling. The estimated velocity of transport of 6-benzylamino-purine-8-¹⁴C in various regions of the intact root was 2.6 to 5.1 mm/h. The transport of 6-benzylaminopurine was predominantly in the transpiration stream, in stelar tissues of the root. Conditions of high transpiration favored enhanced transport to the shoot and an overall greater accumulation of the label. The total accumulation of 6-benzylaminopurine in roots of intact seedlings after 48 h of transport was 354% of that in roots of shoot-less seedlings. Root girdling and treatment of the root with KCN did not reduce the basipetal transport of the label in the root and into the shoot. Radiochromalogram scanning of root extracts and analysis of the ethanol insoluble-NaOH soluble fraction revealed considerable metabolic changes in the translocated cytokinin. Only 51% of the radioactivity remained in the original 6-benzylaminopurine peak after 24 h of incubation. Two other, unidentified, metabolites were detected. It is suggested that all the factors that affect the ascent of sap are involved in the long-distance transport of cytokinins, and that the rate and mode of transport of cytokinins from the root system to the shoot may be a major factor in the expression of their physiological activity.     

Biochemical Plant Responses to Ozone : II. Induction of Stilbene Biosynthesis in Scots Pine (Pinus sylvestris L.) Seedlings

Formation of the stilbenes pinosylvin and pinosylvin 3-methyl ether, as well as the activity of the biosynthetic enzyme stilbene synthase (pinosylvin-forming), were induced several hundred- to thousandfold in primary needles of 6-week-old pine (Pinus sylvestris L.) seedlings upon exposure to a single pulse of ozone of at least 0.15 microliters per liter. The seedlings required 4 hours of exposure as a minimum for the induction of stilbene biosynthesis when exposed to 0.2 microliters per liter ozone. Both stilbene synthase activity and stilbene accumulation increased with the duration of ozone treatment. The activity of phenylalanine ammonia-lyase and the activity of chalcone synthase, a key enzyme of the flavonoid pathway that uses the same substrates as stilbene synthase, were also stimulated about twofold by ozone. Stilbene biosynthesis appears to represent the first example of a dose-dependent biochemical response to ozone in a conifer species and may serve as a useful biomarker to study stress impacts on pine trees.     

RIN3 Is a Negative Regulator of Mast Cell Responses to SCF

Stimulation of the receptor tyrosine kinase KIT by Stem Cell Factor (SCF) triggers activation of RAS and its downstream effectors. Proper KIT activation is essential for the maturation, survival and proliferation of mast cells. In addition, SCF activation of KIT is critical for recruiting mast cells to sites of infection or injury, where they release a mix of pro-inflammatory substances. RIN3, a RAS effector and RAB5-directed guanine nucleotide exchange factor (GEF), is highly expressed and enriched in human mast cells. SCF treatment of mast cells increased the amount of GTP-bound RAB5, and the degree of RAB5 activation correlated with the expression level of RIN3. At the same time, SCF caused the dissociation of a pre-formed complex of RIN3 with BIN2, a membrane bending protein implicated in endocytosis. Silencing of RIN3 increased the rate of SCF-induced KIT internalization, while persistent RIN3 over-expression led to KIT down regulation. These observations strongly support a role for RIN3 in coordinating the early steps of KIT endocytosis. Importantly, RIN3 also functioned as an inhibitor of mast cell migration toward SCF. Finally, we demonstrate that elevated RIN3 levels sensitize mastocytosis cells to treatment with a KIT tyrosine kinase inhibitor, suggesting the value of a two-pronged inhibitor approach for this difficult to treat malignancy. These findings directly connect KIT activation with a mast cell-specific RAS effector that regulates the cellular response to SCF and provide new insight for the development of more effective mastocytosis treatments.     

A Plant Growth-Promoting Bacterium That Decreases Nickel Toxicity in Seedlings

A plant growth-promoting bacterium, Kluyvera ascorbata SUD165, that contained high levels of heavy metals was isolated from soil collected near Sudbury, Ontario, Canada. The bacterium was resistant to the toxic effects of Ni²⁺, Pb²⁺, Zn²⁺, and CrO₄⁻, produced a siderophore(s), and displayed 1-aminocyclopropane-1-carboxylic acid deaminase activity. Canola seeds inoculated with this bacterium and then grown under gnotobiotic conditions in the presence of high concentrations of nickel chloride were partially protected against nickel toxicity. In addition, protection by the bacterium against nickel toxicity was evident in pot experiments with canola and tomato seeds. The presence of K. ascorbata SUD165 had no measurable influence on the amount of nickel accumulated per milligram (dry weight) of either roots or shoots of canola plants. Therefore, the bacterial plant growth-promoting effect in the presence of nickel was probably not attributable to the reduction of nickel uptake by seedlings. Rather, it may reflect the ability of the bacterium to lower the level of stress ethylene induced by the nickel.     

Role of Indole-3-acetic Acid in Modification of Geotropic Responses in Clinostat Rotated Avena Seedlings

Oat seedlings were grown in a sand medium on clinostats with horizontal axes of rotation to nullify the directional component of the gravity-force vector. Coleoptile segments from such seedlings showed an enhanced absorption of apically applied exogenous auxin (indole-3-acetic acid), compared to segments from vertically rotated or stationary controls. Absorption of basally applied auxin and auxin transport were unaffected by the gravity treatments. Horizontal rotation did not materially change the amount of auxin produced and transported from excised coleoptile tips: however, plants so rotated showed an enhanced curvature response to unilaterally applied auxin.

Collectively, these experiments indicate that enhanced plant responses to horizontal clinostat rotation, where rates of rotation are sufficient to nullify the directional component of the gravity-force vector, are caused primarily by increases in metabolism and not by a modification of auxin availability. These data do not support recently advanced hypotheses that the polarity of auxin transport is based on gravitational sedimentation of cell inclusions.

     

Positive Allosteric Modulator of GABA Lowers BOLD Responses in the Cingulate Cortex

Knowledge about the neural underpinnings of the negative blood oxygen level dependent (BOLD) responses in functional magnetic resonance imaging (fMRI) is still limited. We hypothesized that pharmacological GABAergic modulation attenuates BOLD responses, and that blood concentrations of a positive allosteric modulator of GABA correlate inversely with BOLD responses in the cingulate cortex. We investigated whether or not pure task-related negative BOLD responses were co-localized with pharmacologically modulated BOLD responses. Twenty healthy adults received either 5 mg diazepam or placebo in a double blind, randomized design. During fMRI the subjects performed a working memory task. Results showed that BOLD responses in the cingulate cortex were inversely correlated with diazepam blood concentrations; that is, the higher the blood diazepam concentration, the lower the BOLD response. This inverse correlation was most pronounced in the pregenual anterior cingulate cortex and the anterior mid-cingulate cortex. For subjects with diazepam plasma concentration > 0.1 mg/L we observed negative BOLD responses with respect to fixation baseline. There was minor overlap between cingulate regions with task-related negative BOLD responses and regions where the BOLD responses were inversely correlated with diazepam concentration. We interpret that the inverse correlation between the BOLD response and diazepam was caused by GABA-related neural inhibition. Thus, this study supports the hypothesis that GABA attenuates BOLD responses in fMRI. The minimal overlap between task-related negative BOLD responses and responses attenuated by diazepam suggests that these responses might be caused by different mechanisms.     

Regulators of Cell Division in Plant Tissues XII. A Cytokinin Bioassay Using Excised Radish Cotyledons

A rapid bioassay for cytokinins is described. This is based on the ability of these compounds to promote markedly the expansion of radish cotyledons excised soon after seed germination. The response, which is due largely to a promotion of cell enlargement, is inhibited by abscisic acid. Gibberellins, certain amino acids and calcium salts, naturally occurring substances without cytokinin activity, induce small growth increments in the assay. Cytokinin structure-activity relationships observed for the assay are very similar to those which apply in the tobacco-pith tissue culture assay.     

Effect of Temperature on Phytochrome-mediated Responses in Seedlings of Mustard

Anthocyanin synthesis, hair formation, and the synthesis of ascorbic acid oxidase are all phytochrome-mediated reactions occurring in the hypocotyl of mustard (Sinapis alba L.). An investigation was conducted into the effects of temperature on the light promotion of these three photoresponses. When given prior to a light exposure there was either no difference between a 20 and 30 C temperature treatment, or the 30 C treatment resulted in a greater photoresponse. When given subsequent to a light exposure 20 C was far more effective in promoting the responses than was a 30 C treatment. These results are in accordance with the known effects of low temperature on delaying phytochrome-far red absorbing form (Pfr) destruction, thus resulting in more Pfr being available to promote the responses. The difference between temperature treatments was greatest following a short light exposure, and much smaller following a long light exposure. When light was given as intermittent illumination the difference between subsequent temperature treatments was much less than when the same amount of light was given as continuous illumination.