Growth and development of embryo parts during the germination of caryopses of the wild oat (Avena fatua L.)

Wild oat (Avena fatua L.) caryopses were germinated on moist filter paper and under water in the presence and absence of hydrogen peroxide (H₂O₂). The sequential growth and development of embryo parts were studied. Germination, as indicated by radicle emergence, was least and slowest in caryopses submerged in deoxygenated water. The coleorhiza in such caryopses elongated much earlier than the root, in contrast to the other treatments where the coleorhiza and the root emerged at about the same time. In caryopses incubated on moist filter paper all embryo parts showed considerable growth. In H₂O₂ treated caryopses only the epicotyl showed substantial growth over the experimental period. In all treatments the first mitotic peaks were noticed at the same period. The occurrence of these early nuclear divisions may be due to release of 4 C nuclei from inhibition by the uptake of water during caryopsis imbibition. The mitosis continued in the radicle of the embryo in those caryopses germinating on moist filter paper, indicating occurrence of DNA synthesis. In the other two treatments, however, few divisions were detected. Here the early growth of the root, causing caryopsis germination, was due to cell elongation, especially in the proximal part of the root.     

Gibberellin-photoaffinity labeling of wild oat (Avena fatua L.) aleurone protoplasts

Aleurone protoplasts of wild oat (Avena fatua L.), and subcellular fractions isolated from them, were photoaffinity labeled using the synthetic gibberellin (GA) derivative GA₄-17-yl-1-(1-thia)propan-3-ol-4-azido-5-[¹²⁵I]iodosalicylate. Labeled polypeptides were identified by electrophoresis under denaturing conditions followed by autoradiography. GA-photoaffinity labeling of both intact protoplasts and isolated subcellular fractions led to the covalent attachment of the reagent to many polypeptides. A 50 kD polypeptide in the soluble fraction of homogenates of aleurone protoplasts GA-photoaffinity labeled in vivo showed specific binding. The biologically active GA₁, GA₄ and GA₄-17-yl-1(1-thia)propan-3-ol-4-azidosalicylate completed for binding whereas the biologically inactive GA₈ and GA₃₄ did not. The GA-photoaffinity labeling characteristics of this polypeptide suggested that it might interact specifically with biologically active GAs in vivo. Attempts to detect specific GA-binding in in vitro GA-photoaffinity labeling experiments met with only limited success perhaps indicating the labile nature of specific binding observed in vivo. The potential of GA-photoaffinity labeling for identifying GA-binding proteins in aleurone and other GA-responsive tissues is discussed.Abbreviations azido IAA = 5-azido-7-[³H]indole-3-acetic acid - azido NPA = 5-azido-[3,6-³H]1-N-napthylpthalamic acid - BTP = 1,3-bis(Tris(hydroxymethyl)methylamino)-propane - GA₄-O-ASA = GA₄-17-yl-1-(1-thia)propane-3-ol-4-azidosalicylate - [¹²⁵I]GA₄-O-ASA = GA₄-17-yl-1-(1-thia)propan-3-ol-4-azido-5-[¹²⁵I]iodosalicylate - NPA = 1-Naphthylphthalmic acid - PAGE = Polyacrylamide gel electrophoresis - PMSF = phenylmethylsulfonyl fluoride - SDS = Sodium dodecyl sulphate - TLCK = L-1-Chloro-3-(4-tosylamido)-7-amino-2-heptanone-HCl     

Membrane turnover in imbibed dormant embryos of the wild oat (Avena fatua L.)

Germinating non-dormant (ND) embryos of wild oat incorporate [³H]glycerol into phospholipid, and a 250% increase in total extractable phospholipid occurs within 72 h. During germination, leveles of phosphatidyl inositol showed the greatest change, increasing approximately 5-fold.Imbibed dormant (D) embryos of the wild oat also incorporate [³H]gycerol into phospholipids, but there is no net synthesis. A continuous turnover of membrane phospholipids could be demonstrated in pulse chase experiments, and although the proportions of most phospholipids does not change, there was a decrease of 50% in phosphatidyl serine.The half-life of [³H]glycerol in the extracted phospholipids of D and ND embryos varies between 35 and 57 h, and in membrane fractions separated on sucrose density gradients the half-lives vary between 26 and 56 h.D embryos induced to germinate with GA and ND embryos in which germination is repressed by ABA show similar phospholipid changes to ND and D embryos respectively, with the exception that the proportion of phosphatidyl serine remained unchanged in the ND-ABA embryos.It is concluded that the continual turnover of membranes of imbibed dormant embryos is consistent with the maintenance of cellular integrity determining the longevity of the seed under natural conditions.Abbreviations D dormant - ND nondormant - ABA abscisic acid - GA gibberellic acid (GA₃)     

Membrene turnover in imbibed and dormant embryos of the wild oat (Avena fatua L.)

A comparative study of protein synthesis has been carried out with embryos excised from dormant (D) and non-dormant (ND) caryopses of the wild oat. Although D embryos imbibed in water or ND embryos imbibed in abscisic acid do not germinate, they incorporate [¹⁴C]leucine into TCA-insoluble material for the first 48 h as readily as embryos that do germinate (ND embryos imbibed in water, or D embryos imbibed in gibberellic acid). Pulsechase experiments with [¹⁴]leucine show that in both D and ND embryos the proteins associated with the membranes undergo turnover. The rates of decay of incorporated radioactivity are similar in both dormant and germinating embryos up to 98 h following embryo excision. Fractionation of the membrane proteins in SDS-polyacrylamide gels indicates that the different polypeptides have different rates of turnover. It is concluded that membrane proteins in imbibed D embryos are in a state of constant turnover, and that this is a part of the replacement processes necessary to maintain the integrity of hydrated cells. The continuation of such synthetic events could account for long term survival of dormant Avena fatua in the imbibed state.Abbreviations CCRSE cytochrome relative stain equivalents - D dormant - ND nondormant - ABA abscisic acid - GA gibberellic acid GA₃     

A genetic model and molecular markers for wild oat (Avena fatua L.) seed dormancy

Seed dormancy allows weed seeds to persist in agricultural soils. Wild oat (Avena fatua L.) is a major weed of cereal grains and expresses a range of seed dormancy phenotypes. Genetic analysis of wild oat dormancy has been complicated by the difficulty of phenotypic classification in segregating populations. Therefore, little is known about the nature of the genes that regulate dormancy in wild oat. The objectives of our studies were to develop methods to classify the germination responses of segregating wild oat populations and to find molecular markers linked to quantitative trait loci (QTL) that regulate seed dormancy in wild oat. RAPD markers OPX-06 and OPT-04 explained 12.6% and 6.8% respectively, of the F₂ phenotypic variance. OPF-17 was not significant in a simple regression model, but it was linked in repulsion to OPT-04. A three-locus model of seed dormancy in wild oat is presented based on the 41-day germination profiles of F₁, F₂, F₃, BC₁P₁F₁, BC₁P₁F₂, and BC₁P₂F₁ generations, and the 113 day germination profile of 126 F₇ recombinant inbred lines. Loci G ₁ and G ₂ promote early germination, and the D locus promotes late germination. If at least one copy of the dominant G ₁ or G ₂ alleles are present regardless of the genotype at D locus, then the individual will be nondormant. If the genotype is g ₁ g ₁ g ₂ g ₂ D_, then the phenotype will be dormant. Received: 1 December 1998 / Accepted: 1 February 1999     

Protoplasts isolated from aleurone layers of wild oat (Avena fatua L.) exhibit the classic response to gibberellic acid

Viable, long-lived, gibberellic acid (GA₃)-responsive protoplasts have, for the first time, been isolated from aleurone layers of mature wild oat (Avena fatua L.) grain. More than 90% of the cells of aleurone layers are recovered as protoplasts, and these respond to treatment with GA₃ in essentially the same manner as the tissue from which they were derived. Protoplasts become vacuolate during incubation in vitro and, although not dependent upon GA₃, vacuolation is markedly stimulated by the hormone. Amylase and ribonuclease (RNase) are produced and secreted only in the presence of GA₃ and only after lag periods of 3 d and 4 d respectively. The amounts of amylase produced and secreted are proportional to GA₃ concentrations as low as 1.61·10^-13 M. With increasing concentrations of mannitol in the culture medium both vacuolation and the GA₃-induced production and secretion of enzymes are inhibited progressively, the latter being precluded by 0.6 M to 0.7 M mannitol.Abbreviations GA₃ gibberellic acid₃ - RNase ribonuclease     

Water uptake and splitting of wild oat caryopsis

Imbibition and germination experiments were conducted on the caryopses of wild oats (Avena fatua L.). The embryo envelopes, pericarp and aleurone layer, which completely cover the embryo-endosperm, do not form barriers against water uptake. The initial uptake of water is passive and the water moves across the pericarp with ease as it contains cracks; it is, however, transported across the aleurone layer through its cell walls into the endosperm and embryo of the caryopsis. The starchy endosperm enlarges due to water uptake causing the pericarp to rupture, thus exposing the aleuronelayer-covered seed. The aleurone layer is structurally heterogenous consistings of radially compressed irregular cells and cuboidal or radiallys tretched cells; the latter contains thicker walls. The former type is present along the abaxial side of the embryo and in the crease on the adaxial side of the caryopsis; the latter type covers the endosperm. The physical distention of the endosperm due to water uptake causes the rupture of the pericarp and the aleurone layer, and facilitates the emergence of the radicle and coleorhiza of the embryo during caryopsis germination.     

Effects of imazamethabenz on the main shoot growth and tillering of wild oat (Avena fatua L.)

Foliar application of imazamethabenz at sublethal doses of 100 and 200 g a.i./ha to wild oat plants at the two-leaf stage without tillers greatly inhibited the growth of the main shoot but increased tillering. The near cessation of sheath and the main stem elongation indicated that the major effect of imazamethabenz on the main shoot was inhibition of intercalary growth. Low doses of imazameth-abenz treatment resulted in more leaves (including leaf primordia) in the main stem but did not affect mature first and second leaves. Sublethal doses of imazamethabenz only briefly inhibited tiller growth. A later increase in tillering in treated plants resulted from the stimulated resumed growth of tillers and the increased initiation of tiller buds. Such enhanced tillering mainly resulted from the release of apical dominance due to the inhibition or cessation of the main stem growth with imazamethabenz treatment. Both doses of imazamethabenz (100 and 200 g a.i./ha) significantly reduced the biomass of shoots and roots, but increased the ratio of roots/ shoots dry weight.     

Effects of salicylhydroxamate on respiration, seed germination and seedling growth in Avena fatua

The effects of inhibitors of alternative respiration [salicylhydroxamate (SHAM) and propyl gallate (PG)] on germination, seedling growth and O₂ uptake in Avena fatua L. (wild oats) were studied. SHAM did not inhibit germination or O₂ uptake prior to germination. SHAM-sensitive (alternative) respiration, therefore, cannot be a pre-requisite for germination. Following germination, both chemicals inhibited seedling growth with the root being more susceptible than the shoot. SHAM concentrations that inhibited root growth by 90 to 95%, inhibited O₂ uptake of 1 cm root apices by less than 15%. While sodium azide (a cytochrome-oxidase inhibitor; 1 m M ) alone inhibited O₂ uptake by only 40 to 50%, in the simultaneous presence of SHAM (or PG), O₂ uptake was inhibited by 90 to 99%. Thus: 1) respiration of wild oat seedling root apices is predominantly cytochrome-mediated and incomplete inhibition of O₂ uptake in the presence of azide alone is due to diversion of electrons to the alternative pathway and 2) even though these roots have little alternative respiration, they maintain the capacity to support a much greater flux of electrons via this path way. SHAM and PG at concentrations (0.05 to 0.4 m M ) which inhibited O₂ uptake significantly in the presence (but not in the absence) of azide had little effect on root growth suggesting that an effect(s) other than that on respiration is involved in the inhibition of root growth at higher concentrations. The effect of SHAM on wild oat root growth is not selective as it also inhibits growth of a number of crop species.     

Associations between isozyme phenotypes and environment in the slender wild oat (Avena barbata) in Israel

Summary Collections from 31 populations of A. barbata from diverse habitats in Israel were assayed electrophoretically for seven enzyme systems. Phenotype frequencies were scored in nine enzyme zones, probably representing 27 loci, to determine isozyme variability within and among populations. Many different isozyme phenotypes were found in all of the populations; also the array of isozyme phenotypes found in each population differed distinctly from that found in each other population. Overlays of phenotypic frequencies on map locations showed that isozyme variability is distributed in mosaic patterns not related to geographical distance. Principal-component and multiple-regression analyses revealed that temperature and moisture-related variables are significantly correlated with particular isozyme phenotypes. Further, the mosaic patterns of isozyme variation were found to correspond closely to mosaic patterns of the habitat. This structuring of the genetic variability into multilocus combinations was attributed to the combined effects of directional and diversifying selection. Comparisons of patterns and extent of genetic variation in Israel and California led to the conclusion that the evolution of ecotypes, each adapted to a specific habitat and marked by a particular set of enzyme alleles, has proceeded further in Israel, where A. barbata is endemic, than in California, where it is a recent introduction.This study was supported in part by NSF Grant BMS-01113-A01. Seed collections were supported by a United States-Israel Binational Science Foundation Grant     

Isolation and identification of a light-induced growth inhibitor in diffusates from blue light-illuminated oat (Avena sativa L.) coleoptile tips

The amounts of two growth inhibitors in diffusates from illuminatedhalves of phototropically stimulated oat (Avena sativa L.)coleoptile tips were larger than those from shaded halves. The less polarinhibitor was isolated from diffusates from oat coleoptile tips illuminatedwithblue light, and identified as uridine from ¹H NMR spectrum. Thedistribution of endogenous uridine in diffusates from the illuminated andshadedsides of coleoptile tips unilaterally exposed to blue light for 3, causing a first positive phototropic curvature, and fromdark-control tips, was determined using a physicochemical assay. The uridineconcentration was significantly higher in the diffusates from the illuminatedside than in those from the shaded side and the dark-control. Uridine inhibitedthe growth of etiolated oat coleoptile tips at concentrations of 30 and above. These results suggest that uridine plays a role inthe phototropism of oat coleoptiles.     

After-ripening and phytochrome action in seeds of dormant lines of wild oat (Avena fatua)

The effects of a short exposure to red, far-red or alternate red/far-red light on the germination of seeds after-ripened for different periods of time were studied in dormant lines of wild oat ( Avena fatua L.). Three stages were distinguishable in the after-ripening period in the response of germination to light. Seeds stayed dormant and showed no response to light during stage I. Phytochrome-mediated germination was observed in seeds during stage II. The phytochrome action disappeared during stage III, i.e. seeds fully germinated following treatments of all light qualities. When the seeds were imbibed in polyethylene glycol solutions, dark germination was reduced and phytochrome again had an effect, which suggested the involvement of phytochrome in water uptake of the seed.     

Water uptake and distribution in non-dormant and dormant wild oat (Avena fatua L.) caryopses

The influence of seed coat modification and light quality onwater uptake and distribution in caryopses of dormant and non-dormantlines of wild oat (Avena fatua L.) was determined using NMRmicroimaging. Non-dormant seeds absorbed water more rapidlythan dormant seeds during imbibition on distilled water. Thiseffect was detected first in the embryo-scutellar region (8h) and later in the proximal endosperm (12 h). Cutting the testaand pericarp close to the embryo or scarification with KOH promotedrapid embryo/scutellum hydration and germination. Cutting atthe middle part of the caryopsis did not enhance embryo hydrationnor did it greatly improve germination. The sensitivity of waterdistribution to the phytochrome germination effect was examined.Significant differences in imbibitional water uptake by embryos-scutellumtissue were detected by 18 h following red-light (germinationpromoter) compared with far-red (germination inhibitor) treatment.The results indicated that both the rate and the sequence ofembryo/scutellum hydration were important in initiating germinationin dormant seeds. A refinement of the model that describes waterimbibition in wild oat seeds during the early stages of germinationis discussed. Key words: Water uptake, water distribution, Avena fatua, seed coat modification, light quality, dormant and non-dormant seeds     

Phytochrome-controlled extension growth of Avena sativa L. seedlings

The effects of continuous red and far-red light and of brief light pulses on the growth kinetics of the mesocotyl, coleoptile, and primary leaf of intact oat (Avena sativa L.) seedlings were investigated. Mesocotyl lengthening is strongly inhibited, even by very small amounts of P_fr, the far-red light absorbing form of phytochrome (e.g., by [P_fr]0.1% of total phytochrome, established by a 756-nm light pulse). Coleoptile growth is at first promoted by P_fr, but apparently inhibited later. This inhibition is correlated in time with the rupturing of the coleoptile tip by the primary leaf, the growth of which is also promoted by phytochrome. The growth responses of all three seedling organs are fully reversible by far-red light. The apparent lack of photoreversibility observed by some previous investigators of the mesocotyl inhibition can be explained by an extremely high sensitivity to P_fr. Experiments with different seedling parts failed to demonstrate any further obvious interorgan relationship in the light-mediated growth responses of the mesocotyl and coleoptile. The organspecific growth kinetics, don't appear to be influenced by P_fr destruction. Following an irradiation, the growth responses are quantitatively determined by the level of P_fr established at the onset of darkness rather than by the actual P_fr level present during the growth period.Abbreviation P_fr far-red light absorbing form of phytochrome     

Highly regenerative tissue cultures from axillary tiller buds of sexually mature oat plants (Avena sativa L.)

Summary Competent tissue cultures were initiated from axillary tiller buds and immature leaves of two cultivars ofAvena sativa L. and cultured on agar nutrient medium containing 2 mg/l of 2,4-dichlorophenoxyacetic acid and 1 mg/l benzyladenine. Using a technique of selective excision and subculturing of the shoot-forming tissues and rejecting the root-froming tissues, we regenerated numerous plants either on hormone-free medium or by allowing the subculture with hormone to age under usual culture-room light conditions. This research was supported in part through a grant to A. W. G. from BARD (Binational Agricultural Research and Development Foundation). N. S. S. is grateful to the Ministry of Education and Culture, Government of India, New Delhi, for the award of a National Scholarship for study abroad 1980–81.     

Effects of osmotic stress on polar auxin transport in Avena mesocotyl sections

Segments of mesocotyls of Avena sativa L. transported [1-¹⁴C]indol-3yl-acetic acid (IAA) with strictly basipetal polarity. Treatment of the segments with solutions of sorbitol caused a striking increase in basipetal auxin transport, which was greatest at concentrations around 0.5 M. Similar effects were observed with mannitol or quebrachitol as osmotica, but with glucose or sucrose the increases were smaller. Polar transport was still detectable in segments treated with 1.2 M sorbitol. The effects of osmotic stress on the polar transport of auxin were reversible, but treatment with sorbital solutions more concentrated than 0.5 M reduced the subsequent ability of mesocotyl segments to grow in response to IAA. The increased transport of auxin in the osmotically stressed segments could not be explained in terms of an increased uptake from donor blocks. The velocity of transport declined with higher concentrations of osmoticum. The reasons for the enhancement of auxin transport by osmotic stress are not known.     

Influence of nutrient supply and plant growth regulators on phytotoxicity of imazamethabenz in wild oat (Avena fatua L.)

The influences of nutrient supply and plant growth regulators on the phytotoxicity of imazamethabenz in wild oat (Avena fatua L.) were evaluated in the greenhouse. Wild oat plants supplied with half-strength rather than one-eighth-strength Hoagland solution were more susceptible to imazamethabenz, showing greater growth reduction in main shoot and tillers. The improved herbicide efficacy at higher nutrient levels appeared related to increased herbicide interception by the greater leaf surface available. Leaves developing at either nutrient level did not differ significantly in epicuticular wax, so differential absorption appeared unlikely. Wild oat plants supplemented with nutrient, switching from low to high levels at the time of herbicide application, were as susceptible to imazamethabenz or even more so than plants growing with a constant high level of nutrition. The wild oat pure-line Montana 73, a strongly tillering line, was more susceptible to imazamethabenz than the limited-tillering line, Crop Science 40. Both 2,4-D and GA₃ reduced imazamethabenz-induced tillering. Imazamethabenz efficacy was increased by GA₃ but not by 2,4-D. These results support the hypothesis that lowering apical dominance of wild oat increases imazamethabenz activity in tillers, and that increased tillering following sublethal doses of imazamethabenz treatment is associated with the release of apical dominance.