Induction of vivipary in Avena fatua

An investigation was conducted under controlled conditions to determine whether treatments designed to maximize the availability of water during seed development could induce viviparous germination in wild oats ( Avena fatua L.). Panicles of three genetic lines, which differed in their degree of dormancy, were kept in darkness at ca 100% RH and 20±1°C and were either supplied with water through the cut end of the rachis or left attached to the plant which was exposed to light. In the non-dormant line, germination of both primary and secondary caryopses on excised panicles increased with their stage of development when treated, i.e., 0, 5 and 10 days after anthesis. Germination of primary caryopses varied between 70 and 80% and was similar on both isolated and attached panicles treated at 10 and 5 days after anthesis, respectively. The percentage germination was considerably lower in all treatments of the two dormant lines and was inversely related to the genetically determined difference in their degree of dormancy. In these dormant lines germination was significantly lower on the intact plant than on the detached panicles. Water potential measurements suggested that this difference may be due partly to the transpiration-induced negative ψ_xyin the stem which may contribute to the inhibition of embryo growth and thus to the prevention of viparous germination.     

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     

Metabolism of diclofop-methyl in cell cultures of Avena sativa and Avena fatua

The metabolism of the herbicide, diclofop-methyl (methyl-2-[4-(2', 4'-dichlorophenoxy) phenoxy]propanoate), in cell suspension cultures of Avena sativa L. (cv. Garry) and in callus of Avena fatua L. (transferred to liquid) was determined as a function of time (8 h to about 3 weeks) and was compared to previous metabolism data from intact plants. A. fatua metabolized ¹⁴C-labeled diclofop-methyl more rapidly than A. sativa, but the metabolites formed were similar if not identical. Within 2 days, approximately 50% of the total ¹⁴C recovered was in A. fatua cells whereas less than 15% was in A. sativa cells. In older cultures of A. fatua, the amounts of ¹⁴C in the cells and in the medium were about 45% each; 10 to 12% was in the non-extractable cell residue. The ¹⁴C recovered from A. sativa cells increased to a maximum of about 35% at 7 days and then slowly decreased to about 18% by 21 days, whereas the ¹⁴C in the medium of A. sativa decreased to about 60% at 7 days and then increased to over 75% by 21 days. The nonextractable ¹⁴C residue was 5% or less even after 21 days. Major metabolites in methanolic extracts of cells of both A. sativa and A. fatua were diclofop (2-[4-(2', 4'-dichlorophenoxy)phenoxy] propanoate), diclofop hydroxylated at an undetermined position on the 2,4-dichlorophenyl ring (ring OH-diclofop), and conjugates of diclofop and ring-OH diclofop.     

Developmental Differences Between Germinating After-ripened and Dormant Excised Avena fatua L. Embryos

Based on physiological and molecular differences associatedwith the germination of after-ripened and dormant caryopsesand excised embryos, it has been hypothesized that various methodsof after-ripening are the only treatments that facilitate thetransition of dormant wild oat embryos to a non-dormant state.To further investigate this hypothesis, analytical methods wereused to evaluate physical and temporal changes associated withgermination and subsequent growth of after-ripened and dormantexcised embryos (AR-embryos and D-embryos, respectively) inducedto germinate with fructose (Fru) and/or gibberellic acid (GA).While chemical treatments of Fru, GA, and Fru+GA have littleeffect on the germination and short-term growth of AR-embryos,they do induce germination of D-embryos. Growth following germinationof D-embryos varied according to treatment with the combinationof Fru+GA inducing the greatest growth over the duration ofthe experiment. Even considering differences in the time tocomplete germination, growth of D-embryos was not comparablewith that of AR-embryos. This provides physical evidence thatchemical treatments induce germination without fulfilling therequirements for normal after-ripening-enhanced germination/growth,and indicates that fructose and/or gibberellic acid do not removethe dormancy-block or rate limiting step in the same manneras after-ripening. Avena fatua ; after-ripening; dormancy; fructose; germination; gibberellic acid; wild oats     

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     

Genome structure and evolution in the allohexaploid weed Avena fatua L. (Poaceae).

Allohexaploid wild oat, Avena fatua L. (Poaceae; 2n = 6x = 42), is one of the world's worst weeds, yet unlike some of the other Avena hexaploids, its genomic structure has been relatively little researched. Consequently, in situ hybridisation was carried out on one accession of A. fatua using an 18S-25S ribosomal DNA (rDNA) sequence and genomic DNA from A. strigosa (AA-genome diploid) and A. clauda (CC-genome diploid) as probes. Comparing these results with those for other hexaploids studied previously: (i) confirmed that the genomic composition of A. fatua was similar to the other hexaploid Avena taxa (i.e., AACCDD), (ii) identified major sites of rDNA on three pairs of A/D-genome chromosomes, in common with other Avena hexaploids, and (iii) revealed eight chromosome pairs carrying intergenomic translocations between the A/D- and C-genomes in the accession studied. Based on karyotype structure, the identity of some of these recombinant chromosomes was proposed, and this showed that some of these could be divided into two types, (i) those common to all hexaploid Avena species analysed (3 translocations) and (ii) one translocation in this A. fatua accession not previously observed in reports on other hexaploid Avena species. If this translocation is found to be unique to A. fatua, then this information, combined with more traditional morphological data, will add support to the view that A. fatua is genetically distinct from other hexaploid Avena species and thus should retain its full specific status.     

Action of Respiratory Inhibitors on Seed Germination and Oxygen Uptake in Avena fatua L.

The effects of sodium azide, potassium cyanide (cytochrome oxidaseinhibitors), and salicylhydroxamic acid (SHAM; an alternativerespiration inhibitor) on germination and respiration of Avenafatua L. seeds were studied. Azide and cyanide released seeddormancy at similar concentrations and treatment durations.Cyanide, however, stimulated germination of seeds with littleafter-ripening, whereas azide had no effect under similar conditionsunless the seeds were after-ripened for several months; theduration of after-ripening required for seeds to respond toazide varied with seed batch. There was also a greater lag priorto germination in the case of azide, compared to cyanide treatedseeds. SHAM inhibited the stimulation of germination and respirationby azide, but not by cyanide. Furthermore, respiration induced by azide or cyanide could notbe inhibited by the subsequent application of SHAM. These findingssuggest that the respiration stimulated by azide and cyanideis not alternative (SHAM-sensitive) and, therefore, this respiratorypathway cannot be involved in the stimulation of germinationby cytochrome oxidase inhibitors. While embryos excised fromcontrol, azide or cyanide pretreated seeds had the capacityto perform alternative respiration, the actual contributionof this pathway was negligible. A large proportion of respirationof embryos excised from azide or cyanide pretreated seeds wasresidual, i.e. insensitive to both SHAM and cyanide. Alternative respiration, azide, cyanide, dormancy, salicylhydroxamic acid, wild oats     

A simulation model of Avena fatua L. (wild-oat) growth and development

An eco-physiological simulation model of the growth and development of Avena fatua was parameterised and tested. The model simulates growth ofA. fatua, in kg dry matter ha^-1 day^-1 from sowing to maturity as a function of irradiance, temperature and various species characteristics. Parameter values were derived from the literature and from field experiments, including both autumn and spring sowings of A. fatua over three years at two sites in southern England. With two exceptions, a single set of parameter values was sufficient to accurately simulate the emergence, growth and development of both autumn and spring cohorts over all years and sites. The two exceptions were the result of differences between autumn and spring cohorts of A. fatua in the rate of early leaf area growth and in the relationship between specific leaf area and developmental stage.     

Within-family selection in Avena fatua and A. barbata

Summary Twenty families each of Avena fatua and A. barbata drawn from a natural population were used for measuring the response to within-family selection for the two extremes in heading date and seed size. The estimates of the relative amounts of between- and within-family variation were interpreted in relation to the realized responses to show that A. fatua has greater genetic variability than A. barbata which, on the other hand, has more phenotypic plasticity. These results support our model on the alternative adaptive strategies in the two species discussed earlier.

---

Zusammenfassung Je 20 Familien von Avena fatua und A. barbata, die aus natürlichen Populationen entnommen worden waren, wurden zur Messung der Reaktion auf intrafamiliäre Selektion auf die beiden Extreme des Schoß-Termins und der Samengröße verwendet. Die Schätzungen der relativen Beiträge der Variation zwischen und innerhalb der Familien wurden in Beziehung zu der erzielten Reaktion interpretiert, um zu zeigen, daß A. fatua eine größere genetische Variabilität und A. barbata eine größere phänotypische Plastizität als die jeweils andere Art besitzt. Die vorliegenden Ergebnisse stützen unser früher diskutiertes Modell der alternativen adaptiven Strategien in diesen beiden Spezies.

     

Dormancy Implications of Phosphorus Levels in Developing Caryopses of Wild Oats (Avena fatua L.)

The element phosphorus made up 0.5% of the dry weight of dehulled Avena fatua caryopses 7 days after anthesis (DAA), half of it inorganic (P_i). Caryopses detached and pierced 7 DAA germinated in vitro with a rapid drop in P_i levels. By 15–20 DAA caryopsis dry weight had increased three- to fourfold, but phosphorus made up less than 0.04% of the dry weight of this enlarged caryopsis. Caryopses at this stage germinated readily without piercing if incubated in vitro. A further decrease in P_i accompanied by a marked increase in phytate phosphorus began about 15 DAA and continued during later seed maturation. By 20 DAA, when embryos were relatively mature and endosperm cell division had ceased, a decrease in caryopsis water content (as a percentage of dry weight) began, and seed dormancy became apparent. As starch and phytate reserves accumulated, P_i and water levels of the caryopsis diminished. Higher levels of endogenous P_i coincided with the anabolic events of initial seed formation and, to a lesser extent, with anabolic events of seed germination. Decreasing P_i levels coincided with accumulation of nutrient reserves, lowering of water content, and the initiation of dormancy. The data suggest that (1) enzymes associated with the formation and development of the embryo may be activated by the high P_i levels present during initial seed differentiation; (2) embryo quiescence and dormancy are facilitated by the drop of P_i levels which accompanies the accumulation of starch and phytate reserves; and (3) the increase in P_i which accompanies seed afterripening aids in the termination of dormancy and the resumption of germination. Received August 15, 1996; accepted December 2, 1996     

Dormancy-associated embryonic mRNAs and proteins in imbibing Avena fatua caryopses

The mechanisms controlling seed dormancy maintenance and release are not understood. To characterize the molecular events accompanying dormancy release, two-dimensional gel electrophoresis was used to monitor changes in soluble proteins and in vitro translation products of embryonic mRNA populations during imbibition of dormant and nondormant (after-ripened) Avena fatua L. caryopses. No differences were observed between in vitro translation products of mRNA extracted from dry dormant and nondormant embryos. However, the expression patterns of several imbibition- and germination-associated mRNAs were temporally modulated during the first 24 h of imbibition. Two dormancy-associated mRNAs, represented by polypeptides D₁ and D₂, were differentially overexpressed in dormant embryos after 3 h of imbibition. mRNA levels for D₁ and D₂ were about 8- and 3-fold higher, respectively, in dormant embryos than in nondormant embryos after 3 h of imbibition. Overexpression of D₁ continued through 12 h of imbibition, while expression of both mRNAs fell to low and equivalent amounts in dormant and nondormant embryos after 24 h. Similar dormancy-associated changes in two soluble proteins were observed during imbibition. The results demonstrate that steady-state levels of specific mRNAs and proteins change during early imbibition of dormant and nondormant A. fatua embryos and indicate that these changes may be associated with differential gene expression responsible for the maintenance of dormancy.     

Variation in stomatal aperture in leaves of Avena fatua L. observed by low-temperature scanning electron microscopy

Abstract. A novel technique to record the variability of stomatal aperture over the leaf surface is described. This combines observations of leaf surfaces using low-temperature scanning electron microscopy (LTSEM), with digital image analysis to produce the most accurate aperture measurements obtained to date. Leaf samples are rapidly immobilized by cryo-fixation in liquid nitrogen and stored in a purpose-built cryo-storage system. Specimens can be collected in the field, remote from the cryopreparation system, and stored for up to several weeks before being examined on the LTSEM. The advantages of this method are that the time frame of the measurements is accurately known, and is identical for all stomatal apertures in a sample, and the precision of the measurements is not limited by the resolving power of the microscope. Measurements of stomatal aperture were obtained from leaves of field grown Avena fatua using the above procedure. Leaf surface conductance (g_sur) was determined by porometry immediately before cryo-fixation of the same region of the leaf. Measurements of aperture size showed a high degree of variability within each specimen, with coefficients of variation similar to those found in previous studies. Stomatal conductance (g_s) was calculated from stomatal dimensions using formulae derived elsewhere. A linear regression between the computed values of g_s and porometric estimates of g_sur showed good agreement with the regression line passing through the origin with a slope of 1.0 (R²=0.96). Applications of the experimental system are discussed.     

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     

Gibberellic acid controls specific acid-phosphatase isozymes in aleurone cells and protoplasts of Avena fatua L.

In the presence of gibberellic acid (GA₃) aleurone layers and isolated aleurone protoplasts of Avena fatua accumulate specific isozymes of acid phosphatase (EC 3.1.3.2). Some of these may be involved in mobilizing aleurone-grain phosphate reserves during germination. The hormone also controls secretion of other specific molecular forms of the enzyme that probably assist in endosperm hydrolysis. The accumulation and secretion of putative cell-wall-associated isozymes are stimulated by the action of GA₃ in isolated protoplasts. This effect however, is apparently over-ridden in the intact tissue, possibly by a cell-wall-based feedback mechanism.Abbreviations GA₃ gibberellic acid - pI isoelectric point(s)     

Characterization of cDNA clones for differentially expressed genes in embryos of dormant and nondormant Avena fatua L. caryopses

The molecular regulation of seed dormancy was investigated using differential display to visualize and isolate cDNAs representing differentially expressed genes during early imbibition of dormant and nondormant Avena fatua L. embryos. Of about 3000 cDNA bands examined, 5 cDNAs hybridized with mRNAs exhibiting dormancy-associated expression patterns during the first 48 h of imbibition, while many more nondormancy-associated cDNAs were observed. Dormancy-associated clone AFD1 hybridized with a 1.5 kb mRNA barely detectable in dry dormant and nondormant embryos that became more abundant in dormant embryos after 24 h of imbibition. Clone AFD2 hybridized with two mRNAs, a 1.3 kb message constitutively expressed in dormant and nondormant embryos and a 0.9 kb message present at higher levels in dormant embryos after 3 h of imbibition. Nondormancy-associated clones AFN1, AFN2 and AFN3 hybridized with 1.5 kb, 1.7 kb and 1.1 kb mRNAs, respectively, that were more abundant in nondormant embryos during imbibition. Expression patterns of some mRNAs in dormant embryos induced to germinate by GA₃ treatment were different than water controls, but were not identical to those observed in nondormant embryos. DNA sequence analysis revealed 76% sequence identity between clone AFN3 and a Citrus sinensis glutathione peroxidase-like cDNA, while significant sequence similarities with known genes were not found for other clones. Southern hybridization analyses showed that all clones represent low (1 to 4) copy number genes.     

Necessity of gibberellin for stimulatory effect of KAR1 on germination of dormant Avena fatua L. caryopses

Avena fatua L. florets (caryopses enclosed by lemma and palea) were partially dormant at 10–20 °C and did not germinate at temperatures outside this range. After-ripening florets at 25 °C for 12 weeks completely removed dormancy. Caryopses (florets without lemma and palea) were able to germinate totally at 20 °C. Karrikinolide (KAR₁) and gibberellic acid (GA₃) applied at 10–25 °C partially or markedly induced germination of dormant florets and caryopses, respectively. Both florets and caryopses were more sensitive to KAR₁ than to GA₃. To obtain similar effects, 1,000 to 10,000 times lower concentrations of KAR₁ than GA₃ were required. After-ripening with time gradually increased sensitivity of caryopses to these regulators. Likewise, after-ripened, non-dormant caryopses were sensitive to KAR₁ and GA₃. Inhibitors of gibberellin biosynthesis, ancymidol, paclobutrazol and flurprimidol inhibited the effect of KAR₁. This inhibition was reversed by GA₃. Caryopses pre-incubated in water with ancymidol or paclobutrazol in the presence or absence of KAR₁ germinated completely but with different rates after transfer to GA₃. KAR₁ probably requires gibberellin biosynthesis to stimulate germination of dormant Avena fatua L. caryopses. Both KAR₁ and GA₃ increased α-amylase, β-amylase and dehydrogenases activities during imbibition before visible germination occurred.     

Aluminum resistance mechanisms in oat (Avena sativa L.)

Background and aims

Enhanced aluminum (Al) resistance has been observed in dicots over-expressing enzymes involved in organic acid synthesis; however, this approach for improving Al resistance has not been investigated in monocots. Among the cereals, oat (Avena sativa L.) is considered to be Al resistant, but the basis of resistance is not known.

Methods

A hydroponic assay and hematoxylin staining for Al accumulation in roots were used to evaluate Al resistance in 15 oat cultivars. Malate and citrate release from roots was measured over a 24?h period. A malate dehydrogenase gene, neMDH, from alfalfa (Medicago sativa L.) was used to transform oat.

Results

Oat seedlings were highly resistant to Al, as a concentration of 325?μM AlK(SO₄)₂ was needed to cause a 50% decrease in root growth. Most oat cultivars tested are naturally resistant to high concentrations of Al and effectively excluded Al from roots. Al-dependent release of malate and Al-independent release of citrate was observed. Al resistance was enhanced in a transgenic oat line with the highest accumulation of neMDH protein. However, overall root growth of this line was reduced and expression of neMDH in transgenic oat did not enhance malate secretion.

Conclusions

Release of malate from oat roots was associated with Al resistance, which suggests that malate plays a role in Al resistance of oat. Over-expression of alfalfa neMDH enhanced Al resistance in some lines but was not effective alone for crop improvement.