Oat Roots Research Articles

Rhizobium Attachment and Curling in Asparagus, Rice and Oat Plants

The attachment of rhizobia to non-host monocotyledonous plants was examined by electron microscopy using a variety of rhizobial strains and plant materials. The effect of hair curling in oat roots caused by non-manipulated rhizobial strains, on oat root hair are also reported

Characterization of a Polyphosphoinositide Phospholipase C from the Plasma Membrane of Avena sativa

A phosphoinositide-specific phospholipase C activity was identified in oat root (Avena sativa, cv Victory) plasma membranes purified by separation in an aqueous two-phase polymer system. The enzyme is highly active toward inositol phospholipids but only minimally active toward phosphatidylethanolamine and phosphatidylcholine. Activity approaches maximal levels at 200 micromolar phosphatidylinositol 4-phosphate (PIP) and is highly dependent on calcium; it is inhibited by 1 millimolar EGTA and is activated by calcium with an apparent activation constant of 2 micromolar. At 10 micromolar calcium and 200 micromolar inositol phospholipid, the enzyme is specific for phosphatidylinositol 4,5-bisphosphate (PIP(2)) and PIP, which are hydrolyzed at 10 and 4 times, respectively, the rate of phosphatidylinositol (PI) hydrolysis. The principle water soluble products of hydrolysis, as determined by high performance liquid chromatography, are inositol 1,4,5-trisphosphate from PIP(2), inositol 1,4-bisphosphate from PIP, and inositol phosphate from PI.

Diclofop-Methyl Increases the Proton Permeability of Isolated Oat-Root Tonoplast

Diclofop-methyl (methyl ester of 2-[4-(2',4'-dichlorophenoxy)phenoxy]propionate; 100 micromolar) and diclofop (100 micromolar) inhibited both ATP- and PPi-dependent formation of H(+) gradients by tonoplast vesicles isolated from oat (Avena sativa L., cv Dal) roots. Diclofop-methyl (1 micromolar) significantly reduced the steady-state H(+) gradient generated in the presence of ATP. The ester (diclofop-methyl) was more inhibitory than the free acid (diclofop) at pH 7.4, but this relative activity was reversed at pH 5.7. Neither compound affected the rate of ATP or PPi hydrolysis by the proton-pumping enzymes. Diclofop-methyl (50, 100 micromolar), but not diclofop (100 micromolar), accelerated the decay of nonmetabolic H(+) gradients established across vesicle membranes. Diclofop-methyl (100 micromolar) did not collapse K(+) gradients across vesicle membranes. Both the (+)- and (-)-enantiomers of diclofop-methyl dissipated nonmetabolic H(+) gradients established across vesicle membranes. Diclofop-methyl, but not diclofop (each 100 micromolar), accelerated the decay of H(+) gradients imposed across liposomal membranes. These results show that diclofop-methyl causes a specific increase in the H(+) permeability of tonoplast.

Growth promotion of rotation crop species by a sterile fungus from wheat and effect of soil temperature and water potential on its suppression of take-all

Two temperatures (15 and 20 °C) and two levels of soil water potential (−0·0015 MPa and −0·001 MPa) were used to test the ability of a sterile red fungus (SRF) to protect wheat ( Triticium aestivum cv. Gamenya) and rye-grass ( Lolium rigidum cv. Wimmera) from infection by the take-all fungus ( Gaeumannomyces graminis var. tritici (Ggt)). The SRF protected wheat and rye-grass plants from infection by Ggt at both levels of soil moisture and temperature. The SRF promoted the growth of wheat and rye-grass in both the SRF alone and SRF + Ggt treatments at −0·0015 MPa and −0·001 MPa, and 15 and 20°. Shoot and root weights were greatest at −0·001 MPa and 15° for wheat, and −0·001 MPa and 20° for rye-grass, in all treatments. At both temperatures the percentage of dead plants of wheat or rye-grass was greater at −0·0015 MPa than at −0·001 MPa in the Ggt-alone treatment. The reduction of shoot and root weights of wheat and rye-grass by the take-all fungus was most severe at −0·0015 MPa and 20°. The SRF promoted the growth of the barley ( Hordeum vulgare cv. Stirling), great brome ( Bromus diandrus ), chick pea ( Cicer arientinum cv. Opal), lupins ( Lupinus angustifolius cv. Yandee), medic ( Medicago polymorpha L. cv. Santiago), oats ( Avena saliva cv. Swan), peas ( Pisum sativum cv. Dundale) rape ( Brassica napus cv. Wesbrook), rye-grass ( Lolium rigidum cv. Wimmera) and subterranean clover ( Trifolium subterraneum cv. Nungarin) which are used as rotation crops with the wheat. Although the roots of all these plants were found to be infected and covered by the SRF, the recovery of the SRF was more frequent from the roots of wheat, rye-grass, oats, barley and great brome than peas, lupins, medic subterranean clover and chick pea.

Selective Delipidation of the Plasma Membrane by Surfactants

The influence of plasma membrane lipid components on the activity of the H(+)-ATPase has been studied by determining the effect of surfactants on membrane lipids and ATPase activity of oat (Avena sativa L.) root plasma membrane vesicles purified by a two-phase partitioning procedure. Triton X-100, at 25 to 1 (weight/weight) Triton to plasma membrane protein, an amount that causes maximal activation of the ATPase in the ATPase assay, extracted 59% of the membrane protein but did not solubilize the bulk of the ATPase. The Triton-insoluble proteins had associated with them, on a micromole per milligram protein basis, only 14% as much phospholipid, but 38% of the glycolipids and sterols, as compared with the native membranes. The Triton insoluble ATPase could still be activated by Triton X-100. When solubilized by lysolecithin, there were still sterols associated with the ATPase fraction. Free sterols were found associated with the ATPase in the same relative proportions, whether treated with surfactants or not. We suggest that surfactants activate the ATPase by altering the hydrophobic environment around the enzyme. We propose that sterols, through their interaction with the ATPase, may be essential for ATPase activity.

Lysophosphatidylcholine Stimulates ATP Dependent Proton Accumulation in Isolated Oat Root Plasma Membrane Vesicles

Lysophosphatidylcholine at concentrations of 30 micromolar stimulated the rate of MgATP-dependent H(+)-accumulation in oat (Avena sativa L. cv Rhiannon) root plasma membrane vesicles about 85% while the passive permeability of H(+) was unchanged. Activation was dependent on chain length, degree of saturation, and head group of the lysophospholipid. A H(+)-ATPase assay was developed that allowed the simultaneous measurement of proton pumping and ATPase activity in the same sample. ATP hydrolysis was also stimulated by lysophospholipids and showed the same lipid specificity, but stimulation was only about 25% at 30 micromolar. At higher concentrations of lysophosphatidylcholine the ATPase activity in a latency-free system could be stimulated about 150%. The enzymic properties of proton pumping and ATP hydrolysis were otherwise identical with respect to vanadate sensitivity, K(m) for ATP and pH optimum. The stimulatory effect of lysophospholipids suggests that these compounds could be part of the regulatory system for plant plasma membrane H(+)-ATPase activity in vivo.

Purification and identification of the fusicoccin binding protein from oat root plasma membrane.

Fusicoccin (FC), a fungal phytotoxin, stimulates the H(+) -ATPase located in the plasma membrane (PM) of higher plants. The first event in the reaction chain leading to enhanced H(+) -efflux seems to be the binding of FC to a FC-binding protein (FCBP) in the PM. We solubilized 90% of the FCBP from oat (Avena sativa L. cv Victory) root PM in an active form with 1% octyl-glucoside. The FCBP was stabilized by the presence of protease inhibitors. The FCBP was purified by affinity chromatography using FC-linked adipic acid dihydrazide agarose (FC-AADA). Upon elution with 8 molar urea, two major protein bands on sodium dodecyl sulfate-polyaerylamide gel electrophoresis with molecular weights of 29,700 and 31,000 were obtained. Successive chromatography on BBAB Bio-Gel A, hexyl agarose, and FC-AADA resulted in the same two bands when the FC-AADA was eluted with sodium dodecyl sulfate. A direct correlation was made between 3H-FC-binding activity and the presence of the two protein bands. The stoichiometry of the 29,700 and 31,000 molecular weight bands was 1:2. This suggests that the FCBP occurs in the native form as a heterotrimer with an apparent molecular weight of approximately 92,000.

Peripheral and integral subunits of the tonoplast H+-ATPase from oat roots.

The subunit organization of the tonoplast H+-pumping ATPase from oat roots (Avena sativa L. var. Lang) was investigated. Tonoplast vesicles were treated with low ionic strength solutions (0.1 mM 4-(2-hydroxyethyl)-1-piperazineethanesulfonic acid buffer or 0.1 mM Na EDTA), carbonate, or a chaotropic reagent (KI), and then centrifuged to give a soluble fraction and a pellet. Treatments with low ionic strength solutions or KI resulted in 70-80% reduction in the membrane-associated ATPase activity, but did not affect the K+-stimulated pyrophosphatase activity. Polypeptides of 72, 60, and 41 kDa were solubilized from tonoplast vesicles by these wash treatments. These polypeptides reacted with polyclonal antibodies against the holoenzyme of tonoplast ATPase (anti-ATPase) and copurified with the tonoplast ATPase activity during gel filtration chromatography (Sepharose CL-6B). Mono-specific antibody against the 72- or 60-kDa polypeptide reacted with the solubilized 72- or 60-kDa polypeptide, respectively. However, the N,N-[14C]dicyclohexylcarbodiimide-binding 16-kDa polypeptide and a 13-kDa polypeptide that also reacted with anti-ATPase and copurified with the tonoplast ATPase activity during gel filtration remained in the pellets after the wash treatments. We conclude that the 72- and 60-kDa polypeptides appear to be peripheral subunits of the tonoplast ATPase and that the 16-kDa polypeptide is probably embedded in the membrane bilayer. Additional subunits of the ATPase complex may include a 41-kDa (peripheral) and a 13-kDa (integral) polypeptide. Based on these results, a working model of the tonoplast ATPase analogous to the F1F0-ATPase is proposed.

Factors Affecting Preemergence Bioactivity of Diclofop: Rainfall, Straw Retention, and Plant Growth Stage

AbstractDiclofop {(±)‐2‐[4‐(2,4‐dichlorophenoxy)phenoxy]propanoic acid} controls downy brome (Bromus tectorum L.) when mechanically incorporated after application, but preemergence surface applications result in erratic control of downy brome. To determine the physical factors that enhance diclofop bioactivity, laboratory and greenhouse studies were conducted with two soils: sand (Ustic Torripsamment) and sandy clay loam (Aridic Paleustoll). An oat (Avena sativa) root bioassay was used to measure bioactivity of diclofop. Movement of diclofop downward by simulated rainfall was affected by soil type and time of rainfall event. Oat root growth of seed planted 25 mm deep was reduced by >80% in the sand when 25 mm of rain was applied, but less than 30% in the sandy clay loam. Diclofop movement was reduced if the rainfall event was delayed by 4 d after application to the sand soil, indicating the need for precipitation within a few days after diclofop application. Surface straw interception and retention reduced diclofop activity by 20 to 30%, but simulated rainfall reduced this retention. Diclofop bioactivity was greatest when applied before oat seed germination. Diclofop applied to the soil surface after oat seedlings had emerged reduced plant growth 20 to 30%, while diclofop applied before oat germination reduced plant growth 75 to 85%. Preemergence surface applications of diclofop to the sand soil would be suitable for irrigated wheat (Triticum aestivum L.) production on the sand, provided that diclofop was applied before weed germination and an irrigation of 25 mm occurred within 2 to 4 d after application.

Modulation of plasma membrane H+‐ATPase from oat roots by lysophosphatidylcholine, free fatty acids and phospholipase A2

Plasma membrane vesicles were purified from 8‐day‐old oat (Avena sativa L. cv. Brighton) roots in an aqueous polymer two‐phase system. The plasma membranes possessed high specific ATPase activity [ca 4 μmol P1 (mg protein)−1 min−1 at 37°C]. Addition of lysophosphatidylcholine (lyso‐PC) produced a 2–3 fold activation of the plasma membrane ATPase, an effect due both to exposure of latent ATP binding sites and to a true activation of the enzyme. Lipid activation increased the affinity for ATP and caused a shift of the pH optimum of the H+ ‐ATPase activity to 6.75 as compared to pH 6.45 for the negative H+‐ATPase. Activation was dependent on the chain length of the acyl group of the lyso‐PC, with maximal activition obtained by palmitoyl lyso‐PC. Free fatty acids also activated the membrane‐bound H+‐ATPase. This activation was also dependent on chain length and to the degree of unsaturation, with linolenic and arachidonic acid as the most efficient fatty acids. Exogenously added PC was hydrolyzed to lyso‐PC and free fatty acids by an enzyme in the plasma membrane preparation, presumably of the phospholipase A type. Both lyso‐PC and free fatty acids are products of phospholipase A2 (EC 3.1.1.4) action, and addition of phospholipase A2 from animal sources increased the H+‐ATPase activity within seconds. Interaction with lipids and fatty acids could thus be part of the regulatory system for H+‐ATPase activity in vivo, and the endogenous phospholipase may be involved in the regulation of the H+‐ATPase activity in the plasma membranne.

Herbicidal disruption of proton gradient development and maintenance by plasmalemma and tonoplast vesicles from oat root

Quinacrine fluorescence quenching was used to evaluate the effects of 23 herbicides (100 μ M) on ATP-dependent proton gradient development and maintenance by low-density (tonoplast-enriched) and high-density (plasmalemma-enriched) vesicles from oat ( Avena sativa L.) roots. Naptalam stimulated the rate of proton gradient formation but did not stimulate total ATPase activity of high-density vesicles. Naptalam action was specific for this vesicle type. (2-Methyl-4-chlorophenoxy)acetic acid, dinitramine, chlorpropham, and S-ethyl dipropylthiocarbamate inhibited the initial rate of proton gradient formation and decreased the magnitude of the equilibrium proton gradient formed by low-density vesicles. Dinitramine and chlorpropham were also effective disruptors of proton gradient formation by high-density vesicles. Diclofop-methyl, fluazifop-butyl, and flamprop-isopropyl altered proton gradient generation and/or maintenance across both vesicle types. The free acid forms of these compounds were less effective disruptors than the ester forms. ATPase activity of both high- and low-density vesicles was either unaffected or slightly stimulated by these compounds. Both acifluorfen and alachlor accelerated the rate of decay of proton gradients formed across low-density but not high-density vesicles and stimulated the KCl, Mg 2+-ATPase activity of low-density but not high-density vesicles. Atrazine, metribuzin, dalapon, glyphosate, amitrole, pyrazon, picloram, and bentazon did not affect proton-gradient formation or maintenance by either vesicle type. These results support the hypothesis that some herbicides can have rapid, direct effects on proton gradient maintenance by the plasmalemma and the tonoplast in plant cells.

Dissipation of pH Gradients in Tonoplast Vesicles and Liposomes by Mixtures of Acridine Orange and Anions

Acridine orange altered the response to anions of both ATP and in-organic pyrophosphate-dependent pH gradient formation in tonoplast vesicles isolated from oat (Avena sativa L.) roots and red beet (Beta vulgaris L.) storage tissue. When used as a fluorescent pH probe in the presence of I(-), ClO(3) (-), NO(3) (-), Br(-), or SCN(-), acridine orange reported lower pH gradients than either quinacrine or [(14)C]methylamine. Acridine orange, but not quinacrine, reduced [(14)C]methylamine accumulation when NO(3) (-) was present indicating that the effect was due to a real decrease in the size of the pH gradient, not a misreporting of the gradient by acridine orange. Other experiments indicated that acridine orange and NO(3) (-) increased the rate of pH gradient collapse both in tonoplast vesicles and in liposomes of phosphatidylcholine and that the effect in tonoplast vesicles was greater at 24 degrees C than at 12 degrees C. It is suggested that acridine orange and certain anions increase the permeability of membranes to H(+), possibly because protonated acridine orange and the anions form a lipophilic ion pair within the vesicle which diffuses across the membrane thus discharging the pH gradient. The results are discussed in relation to the use of acridine orange as a pH probe. It is concluded that the recently published evidence for a NO(3) (-)/H(+) symport involved in the export of NO(3) (-) from the vacuole is probably an artefact caused by acridine orange.

Cytochemical Localization of ATPase Activity in Oat Roots Localizes a Plasma Membrane-Associated Soluble Phosphatase, Not the Proton Pump

Cytochemical techniques employing lead-precipitation of enzymically released inorganic phosphate have been widely used in attempts to localize the plasma membrane proton pump (H(+)-ATPase) in electron micrographs. Using Avena sativa root tissue we have performed a side-by-side comparison of ATPase activity observed in electron micrographs with that observed in in vitro assays using ATPases found in the soluble and plasma membrane fractions of homogenates. Cytochemical analysis of oat roots, which had been fixed in glutaraldehyde in order to preserve subcellular structures, identifies an ATPase located at or near the plasma membrane. However, the substrate specificity and inhibitor sensitivity of the in situ localized ATPase appear identical to those of an in vitro ATPase activity found in the soluble fraction, and are completely unlike those of the plasma membrane proton pump. Further studies demonstrated that the plasma membrane H(+)-ATPase is particularly sensitive to inactivation by the fixatives glutaraldehyde and formaldehyde and by lead. In contrast, the predominant soluble ATPase activity in oat root homogenates is less sensitive to fixation and is completely insensitive to lead. Based on these results, we propose a set of criteria for evaluating whether a cytochemically localized ATPase activity is, in fact, due to the plasma membrane proton pump.

Relationship of lipophilicity to influx and efflux of triazine herbicides in oat roots

Influx and efflux of ametryn, atrazine, and atratone occurred rapidly in excised oat root segments. By 7 min of influx, the concentration of each analog had come to equilibrium in the tissue. By 15 min of efflux, over 90% of each absorbed analog had diffused out of the tissue. In contrast, hydroxyatrazine moved into and out of the tissue more slowly. The rate of hydroxyatrazine efflux was intermediate between the other triazine analogs and K +( 86Rb +), which diffused out of the tissue very slowly. Efflux of K +( 86Rb +) was resolved into four first-order kinetic processes representing efflux from the individual pools of K +( 86Rb +) in surface film, free space, cytoplasm, and vacuole of the tissue. Efflux of ametryn, atrazine, and atratone was resolved into three first-order processes, representing one tightly bound and two freely diffusible pools of these analogs in the tissue. However, the sizes of the three pools were inconsistent with their being surface film, free space, cytoplasm, or vacuole. Similar to K +, hydroxyatrazine efflux was resolved into four pools, but the pool sizes were quite different than for K +( 86Rb +). The octanol/water partition coefficient and the microsomal membrane/water distribution coefficient for the triazine analogs and K +( 86Rb +) provided the same ranking of high to low relative lipophilicity for the solutes, ametryn > atrazine > atratone > hydroxyatrazine > K +( 86Rb +). The results indicate that both influx and efflux of these triazine analogs are related positively to their relative solubilities in membrane lipids. However, for those analogs with sufficiently high lipophilicity, diffusion across membranes is so rapid that the rates of influx and efflux are indistinguishable among the analogs.

Isolation and Sequence of Tryptic Peptides from the Proton-Pumping ATPase of the Oat Plasma Membrane

In crude extracts of plant tissue, the M(r) = 100,000 proton-pumping ATPase constitutes less than 0.01% of the total cell protein. A large-scale purification procedure is described that has been used to obtain extensive protein sequence information from this enzyme. Plasma membrane vesicles enriched in ATPase activity were obtained from extracts of oat roots by routine differential and density gradient centrifugation. Following a detergent wash, the ATPase was resolved from other integral membrane proteins by size fractionation at 4 degrees C in the presence of lithium dodecyl sulfate. After carboxymethylation of cysteine residues and removal of detergent, the ATPase was digested with trypsin and resultant peptide fragments separated by reverse phase high performance liquid chromatography. Peptides were recovered with high yield and were readily sequenced by automated Edman degradation on a gas-phase sequencer. Of the eight peptides sequenced, six showed strong homology with known amino acid sequences of the fungal proton-pumping and other cation-transporting ATPases.

Phosphorylation of the plasma-membrane H+-ATPase of oat roots by a calcium-stimulated protein kinase

When plasma-membrane vesicles isolated from oat (Avena sativa L.) root cells were incubated with [γ-(32)P]ATP, the H(+)-ATPase was found to be phosphorylated at serine and threonine residues. Phosphotyrosine was not detected. Endogenous ATPase kinase activity was also observed in plasma-membrane vesicles isolated from potato (Solanum tuberosum L.) root cells as well as from yeast (Saccharomyces cerevisiae). Identity of the phosphorylated oat root Mr=100 000 polypeptide as the ATPase was confirmed using conventional glycerol density-gradient centrifugation to purify the native enzyme and by a new procedure for purifying the denatured polypeptide using reversephase high-performance liquid chromatography. Kinase-mediated phosphorylation of the oat root plasma-membrane H(+)-ATPase was stimulated by the addition of low concentrations of Ca(2+) and by a decrease in pH, from 7.2 to 6.2. These results demonstrate that kinase-mediated phosphorylation of the H(+)-ATPase is a plausible mechanism for regulating activity. They further indicate that changes in the cytoplasmic [Ca(2+)] and pH are potentially important elements in modulating the kinase-mediated phosphorylation.

Lipid requirements of the plasma membrane ATPases from oat roots and yeast

The lipid specificity of the plasma membrane ATPases from oat roots and yeast has been investigated by reconstituting delipidated enzyme with phospholipid vesicles and with micelles of lysophospholipids and other detergents. The plant ATPase is activated by Triton X-100 and by all phospholipid and lysophospholipid species, exhibiting only a slight preference for zwitterionic polar heads (phosphorylcholine and phosphorylethanolamine). No unsaturation is required on the hydrophobic chain. On the other hand, the yeast ATPase requires a negatively charged polar head (with preference for phosphorylglycerol and phosphorylinositol) and an unsaturated hydrophobic chain.

N,N'-dicyclohexylcarbodiimide-binding proteolipid of the vacuolar H+-ATPase from oat roots.

The inhibitor N,N'-dicyclohexylcarbodiimide (DCCD) was used to probe the structure and function of the vacuolar H+-translocating ATPase from oat roots (Avena sativa var. Lang). The second-order rate constant for DCCD inhibition was inversely related to the concentration of membrane, indicating that DCCD reached the inhibitory site by concentrating in the hydrophobic environment. [14C]DCCD preferentially labeled a 16-kDa polypeptide of tonoplast vesicles, and the amount of [14C]DCCD bound to the 16-kDa peptide was directly proportional to inhibition of ATPase activity. A 16-kDa polypeptide had previously been shown to be part of the purified tonoplast ATPase. As predicted from the observed noncooperative inhibition, binding studies showed that 1 mol of DCCD was bound per mol of ATPase when the enzyme was completely inactivated. The DCCD-binding 16-kDa polypeptide was purified 12-fold by chloroform/methanol extraction. This protein was thus classified as a proteolipid, and its identity as part of the ATPase was confirmed by positive reaction with the antibody to the purified ATPase on immunoblots. From the purification studies, we estimated that the 16-kDa subunit was present in multiple (4-8) copies/holoenzyme. The purification of the proteolipid is a first step towards testing its proposed role in H+ translocation.

Latency of Plasma Membrane H+-ATPase in Vesicles Isolated by Aqueous Phase Partitioning

The properties of the plasma membrane H(+)-ATPase and the cause of its latency have been studied using a highly purified plasma membrane fraction from oat (Avena sativa L., cv Victory) roots, prepared by aqueous two-phase partitioning. The ATPase has a maximum specific activity (at 37 degrees C) in excess of 4 micromoles inorganic phosphate per milligram protein per minute in the presence of nondenaturing surfactants. It is inhibited by more than 90% by vanadate, is specific for ATP, has a pH optimum of 6.5, and is stimulated more than 4-fold by 50 millimolar K(+) in the presence of low levels of the nondenaturing surfactants Triton X-100 and lysolecithin. This ;latent' activity is usually explained as being a result of the inability of ATP to reach the ATPase in right-side out, sealed vesicles, until they are disrupted by surfactants. Consistent with this idea, trypsin digestion significantly inhibited the ATPase only in the presence of the surfactants. Electron spin resonance spectroscopy volume measurements confirmed that surfactant-free vesicles were mostly sealed to molecules similar to ATP. However, the Triton to protein ratio required to disrupt vesicle integrity completely is 10-fold less than that needed to promote maximum ATPase activity. We propose that plasma membrane ATPase activation is due not solely to vesicle disruption and accessibility of ATP to the ATPase but to the surfactants activating the ATPase by altering the lipid environment in its vicinity or by removing an inhibitory subunit.

Effects of Haloxyfop and CGA-82725 on Cell Cycle and Cell Division of Oat (Avena sativa) Root Tips

The mode of action of two experimental pyridinyloxyphenoxy propionate herbicides, haloxyfop {2-[4-[[3-chloro-5-(trifluoromethyl)-2-pyridinyl]oxy] phenoxy] propanoic acid} and CGA-82725 {2-propynyl-2-[4-[(3,5-dichloro-2-pyridinyl)oxy] phenoxy] propanoic acid} was studied. Oat (Avena sativaL.) roots showed a rate-dependent sensitivity to both herbicides. Consequently, a series of assays was performed to study cell division, cell cycle dynamics, and nucleic acid and protein synthesis in oat root tips in order to characterize growth inhibition. Both herbicides inhibited cell division, apparently by inhibiting protein synthesis in the G2 stage of interphase. Supporting evidence for this assumption comes from the following observations: a) There was almost complete absence of leucine incorporation within 1 h of exposure to 2.7 × 10−4M of either herbicide; b) the number of3H-labeled dividing cells was reduced to about 20% of the control within 4 h, and essentially to zero within 8 h, after exposure to 2.7 × 10−4M of herbicide; c) the number of unlabeled cells was reduced to less than 10% of the control within 8 h of exposure to 2.7 × 10−4M of herbicide; d) during this 8-h period, the number of3H-labeled cells in interphase which had been exposed to herbicide greatly exceeded the control; e) thymidine incorporation into DNA was inhibited but was not proportional to reduction in cell division; f) uridine incorporation was enhanced during the first 8 h of exposure before a reduction was evident at 12 h; and g) an oat coleoptile elongation bioassay showed that cell enlargement was not inhibited at herbicide concentrations less than 2.7 × 10−4M.