Growth,graviresponsiveness and abscisic-acid content of Zea mays seedlings treated with Fluridone

Ten-d-old seedlings of Zea mays L. cv. Tx 5855 treated with 1-methyl-3-phenyl-5-(3-[trifluoromethyl]phenyl)-4-(1H)-pyridinone (Fluridone) were analyzed for abscisic acid (ABA) content using high-performance liquid chromatography with an analysis sensitivity of 2.5 ng ABA g^-1 fresh weight (FW). Seedlings were divided into three portions: leaves, detipped roots, and root tips (terminal 1.5 mm). Control plants (water treatment only; no Fluridone) were characterized by the following amounts of ABA: leaves, 0.114±0.024 (standard deviation) g ABA g^-1 FW; detipped roots, 0.260±0.039±g ABA g^-1 FW; root tips, no ABA detected. We did not detect any ABA in tissues of Fluridone-treated plants. Primary roots of treated and untreated seedlings were strongly graviresponsive, with no significant differences between the curvatures or the growth rates of primary roots of Fluridone-treated and control seedlings. These results indicate that 1) Fluridone completely inhibits ABA synthesis, and 2) ABA is not necessary for positive gravitropism by primary roots of Zea mays.Abbreviations ABA abscisic acid - Fluridone 1-methyl-3-phenyl-5-(3-[trifluoromethyl]phenyl)-4-(1H)-pyridinone - FW fresh weight - SD standard deviation     

Growth inhibitors in roots of light- and dark-grown seedlings of Zea mays

Abscisic acid and 6-methoxy-2-benzoxazolinone have been identified as growth-inhibitory compounds in the primary roots of Zea mays.     

Immunocytochemical localization of indole-3-acetic acid in primary roots of Zea mays

Colloidal gold-labelled antibody was used to localize indole-3-acetic acid in caps of primary roots of Z. mays cv. Kys. Gold particles accumulated on the nucleus, vacuoles, mitochondria, and some dictyosomes and dictyosome-derived vesicles. This is the first localization of indole-3-acetic acid in dictyosomes and dictyosome-derived vesicles and indicates that dictyosomes and vesicles constitute a pathway for indole-3-acetic acid movement in and secretion from root cap cells. Our findings provide cytochemical evidence to support the hypothesis that indole-3-acetic acid plays an important role in root gravitropism.     

Graviresponsiveness of surgically altered primary roots of Zea mays

We examined the gravitropic responses of surgically altered primary roots of Zea mays to determine the route by which gravitropic inhibitors move from the root tip to the elongating zone. Horizontally oriented roots, from which a 1-mm-wide girdle of epidermis plus 2-10 layers of cortex were removed from the apex of the elongating zone, curve downward. However, curvature occurred only apical to the girdle. Filling the girdle with mucilage-like material transmits curvature beyond the girdle. Vertically oriented roots with a half-girdle' (i.e. the epidermis and 2-10 layers of the cortex removed from half of the circumference of the apex of the elongating zone) curve away from the girdle. Inserting the half-girdle at the base of the elongating zone induces curvature towards the girdle. Filling the half-circumference girdles with mucilage-like material reduced curvature significantly. Stripping the epidermis and outer 2-5 layers of cortex from the terminal 1.5 cm of one side of a primary root induces curvature towards the cut, irrespective of the root's orientation to gravity. This effect is not due to desiccation since treated roots submerged in water also curved towards their cut surface. Coating a root's cut surface with a mucilage-like substance minimizes curvature. These results suggest that the outer cell-layers of the root, especially the epidermis, play an important role in root gravicurvature, and the gravitropic signals emanating from the root tip can move apoplastically through mucilage.     

Exogenous application of abscisic acid or triadimefon affects the recovery of Zea mays seedlings from heat shock

Maize seedlings ( Zea mays L. cv. DK 246) grown for 1–4 days in the presence of abscisic acid (ABA) or triadimefon (a fungicide) demonstrated an enhanced ability to withstand the effects of a 3-h sub-lethal (40°C) or lethal (45°C) heat shock. Both the ABA and triadimefon treatments were applied solely to the roots of seedlings; however, the ability to withstand a heat shock was induced in both the root and the shoot. The level of protection provided by these agents was dependent upon the time that plants were exposed to them; prolonged exposure reduced tolerance to subsequent stress.     

Inhibition of gravitropism in primary roots of Zea mays by chloramphenicol

Primary roots of Zea mays seedlings germinated and grown in 0.1 mM chloramphenicol (CMP) were significantly less graviresponsive than primary roots of seedlings germinated and grown in distilled water. Elongation rates of roots treated with CMP were significantly greater than those grown in distilled water. Caps of control and CMP-treated roots possessed extensive columella tissues comprised of cells containing numerous sedimented amyloplasts. These results indicate that the reduced graviresponsiveness of CMP-treated roots is not due to reduced rates of elongation, the absence of the presumed gravireceptors (i.e., amyloplasts in columella cells), or reduced amounts of columella tissue. These results are consistent with CMP altering the production and/or transport of effectors that mediate gravitropism.     

Radial transport of water and abscisic acid (ABA) in roots of Zea mays under conditions of nutrient deficiency

Radial water (J(V)) and abscisic acid (ABA) flows (J(ABA)) through maize root seedlings have been investigated under different conditions of nutrient deficiency. Whereas J(V) was reduced under nitrogen deficiency, potassium deficiency stimulated J(V). A substantial increase of J(ABA) was observed in roots kept under potassium deficiency. The observed changes of J(V) might have resulted from changed barrier properties of the endodermis. Nitrogen and potassium deficiency also caused an accumulation of endogenous ABA in root tissues. Under all conditions studied, except under K(+)-deficiency, external ABA (100 nM) caused an increase of J(V). The data of this study were used to analyse the relations between internal and endogenous root ABA, J(V), and J(ABA). The internal ABA of root tissues was positively correlated with J(V) and was highly significant (P <0.001 for internal and P=0.03 for endogenous root ABA) within the range 2-300 pmol g(-1) FW. It was also highly positively correlated to the radial ABA flows. There was also a highly positive correlation between J(V) and J(ABA). The data of this study indicate, for the first time, the relations between internal ABA, water, and ABA flows. Independent of treatment with external ABA, an ABA transport by solvent drag across the endodermis is confirmed.     

Signal transmission during gravitropic curvature of primary roots of Zea mays

Primary roots of Zea mays cv. Ageotropic are nonresponsive to gravity and elongate approximately 0.80 mm h^?1. Applying mucilage-like material (K-Y Jelly) to the terminal 1.5 cm of these roots induces graviresponsiveness and slow elongation 28% (i.e. from 0.80 to 0.58mm h^?1). Applying mucilage-like material to one side of the terminal 1.5 cm of the root induces curvature toward the mucilage, irrespective of the root's orientation to gravity. Applying a 2-mm-wideband of mucilage-like material to a root's circumference 8 to 10 mm behind the root cap neither induces gravicurvature nor affects elongation significantly. Similarly, applying mucilage-like material to only the root cap does not significantly affect elongation or graviresponsiveness. Gravicurvature of mutant roots occurs only when mucilage-like material is applied to the root/root-cap junction. Reversing the caps of wild-type and mutant roots produces gravitropic responses characteristic of the root cap rather than the host root. These results are consistent with the suggestion that gravitropic effectors are growth inhibitors that move apoplastically through mucilage between the root cap and root.     

Interaction of coumarin, gibberellic acid and abscisic acid in the translocation of auxin in bean seedlings

The effects of coumarin on the translocation of 2,4,5-trichlorophenoxyaceticacid (2,4,5-T) in bean (Phaseolus vulgaris L. cv. StringlessGreenpod) seedlings was determined. ¹⁴C-labeled 2,4,5-T wasinjected in the stem tissue at the cotyledonary node along withthe coumarin or the coumarin was added to the nutrient solutionprior to, or at the time of, 2,4,5-T treatment. The amount oftranslocation of radioactive 2,4,5-T to plant parts was thendetermined at various times after treatment. An immediate effectof coumarin was to enhance acropetal 2,4,5-T translocation tothe young shoots. This effect occurred at low 2,4,5-T treatmentlevels and appeared to be specific for 2,4,5-T since sucrosetranslocation was not affected. Prolonged treatment with coumarininhibited acropetal and basipetal 2,4,5-T translocation in amanner similar to prolonged treatment with abscisic acid (ABA).Gibberellin A₃ (GA) reversed the inhibitory effects of coumarinand ABA on 2,4,5-T acropetal translocation. ¹ Journal article 3244 of the Agricultural Experiment Station,Oklahoma State University. (Received December 6, 1976; )     

Effect of abscisic acid on exudation from deficient and aged sunflower roots

The effect of abscisic acid (ABA) was observed on exudation from roots of sunflower ( Helianthus annuus L. cv. Habad) plants whose mineral nutrition was cut off or which were deprived of K⁺ or NO⁻₃ for 90 h prior to excision. In spite of a marked decrease in exudation rate, the magnitude of the promotive effect of ABA on both volume flow and release of ions to the xylem was similar to that obtained in roots of plants grown in full nutrient solution. Application of ABA to the medium at different times after excision increased the promotive effect of ABA as the time from excision increased. The magnitude of the ratio ABA-treated/control in roots which were treated 74 h after excision was twice that in freshly-excised roots. The effect of ABA lasted up to 50 h and during that period it followed the endogenous rhythm in exudation from the control roots. It is concluded that since a steady promotive effect of ABA persists under a variety of experimental conditions, this may be considered a genera! phenomenon in sunflower roots.     

Identification and localisation of auxin in primary roots of Zea mays by mass spectrometry

Summary Roots of 3.5-day-old seedlings of Zea mays L. var. Giant White Horsetooth contain an extractable auxin which has been identified unequivocally as IAA. A mass spectrometric technique has been used to determine quantitatively the levels of IAA in the cortical, stelar and apical regions of the roots. The IAA is predominantly located in the steles.     

The influence of calcium and pH on growth in primary roots of Zea mays

Hasenstein, K. H. and Evans, M. L. 1988. The influence of calcium and pH on growth in primary roots of Zea mays. - Physiol. Plant. 72: 466–470.
We investigated the interaction of Ca²⁺ and pH on root elongation in Zea mays L. cv. B73 × Missouri 17 and cv. Merit. Seedlings were raised to contain high levels of Ca²⁺ (HC, imbibed and raised in 10 m M CaCl₂) or low levels of Ca²⁺ (LC, imbibed and raised in distilled water). In HC roots, lowering the pH (5 m M MES/Tris) from 6.5 to 4.5 resulted in strong, long-lasting growth promotion. Surprisingly, increasing the pH from 6.5 to 8.5 also resulted in strong growth promotion. In LC roots acidification of the medium (pH 6.5 to 4.5) resulted in transient growth stimulation followed by a gradual decline in the growth rate toward zero. Exposure of LC roots to high pH (pH shift from 6.5 to 8.5) also promoted growth. Addition of EGTA resulted in strong growth promotion in both LC and HC roots. The ability of EGTA to stimulate growth appeared not to be related to H⁺ release from EGTA upon Ca²⁺ chelation since, 1) LC roots showed a strong and prolonged response to EGTA, but only a transient response to acid pH, and 2) promotion of growth by EGTA was observed in strongly buffered solutions. We also examined the pH dependence of the release of ⁴⁵Ca²⁺ from roots of 3-day-old seedlings grown from grains imbibed in ⁴⁵Ca²⁺. Release of ⁴⁵Ca²⁺ from the root into agar blocks placed on the root surface was greater the more acidic the pH of the blocks. The results indicate that Ca²⁺ may be necessary for the acid growth response in roots.     

Oxygen incorporation in the biosynthesis of p-coumaric acid in seedlings of Zea mays

The results of experiments in which seedlings of Zea mays were grown in the light in an atmosphere enriched with oxygen-18 indicated that the hydroxyl oxygen atom in p-coumaric acid is derived from molecular oxygen.     

Jasmonic acid and abscisic acid in shoots,coleoptiles, and roots of wheat seedlings

The endogenous levels of abscisic acid (ABA) and jasmonic acid (JA) were analyzed in wheat seedlings grown in water, a system which in the past has been used to test the effects of these plant growth inhibitors. The levels in different plant parts and in the medium were measured by gas chromatography-mass spectrometry-selected ion monitoring, using [²H₃]ABA and [²H₆]JA as internal standards. In every plant part, JA levels were about 2 orders of magnitude greater than those of ABA. The exudation of JA from roots per seedling was about 14,000-fold greater than that of ABA, although the roots contained only about 170 times more JA than ABA. It is suggested that JA is a possible allelopathic compound.     

Gravity perception in decapped roots of Zea mays

Time-lapse photography and light microscopy were used to determine whether or not sedimentation of the newly developed amyloplasts in the apex of Zea mays L. roots occurred at the time when geotropic responsiveness reappears following removal of the cap. All decapped roots exhibiting a geotropic response had some amyloplast sedimentation in the apical cortical cells. Exposing decapped roots to a centrifugal acceleration of 25 g for 4 h showed that amyloplasts of a similar size and development were not displaced within the cytoplasm when this treatment began 12 h after decapping, whereas displacement did occur when the treatment began 24 h after decapping. This finding indicates the occurrence of a change in the physical characteristics of the cytoplasm between 12 h and 24 h after removing of the cap, which allows amyloplast movement and thus restores gravity perception.