Key Role for Endogenous Gibberellins in the Control of Seed Germination

The stimulative action of applied gibberellins (GA) on seedgermination has been reported for a large number of speciesin the past three decades. Applied GA often replaces the needfor environmental stimuli like specific temperature pretreatmentor light. Therefore, it has been suggested that endogenous GAsare essential intermediates in the stimulation of germination.Endogenous GAs have been identified in seeds of a limited numberof species, but most of the evidence for a regulatory role isentirely circumstantial. The use of isogenic mutants with lesionsin the GA biosynthesis has presented direct evidence of a keyrole for endogenous GAs in the regulation of germination. GAdeficiency absolutely prevents germination in seeds of Arabidopsisthaliana and tomato. Two different mechanisms of action exist The first one involves hydrolysis of reserve food. In seedsof tomato a factor, probably GA, diffuses prior to germinationfrom embryo to endosperm where it induces hydrolysis of gaJactomannan-richcell walls that are part of the mechanical resistance to theprotrusion of the radicle. A second mechanism of GA action consistsof a direct effect on the growth potential of the embryo. InArabidopsis the stimulation of germination by light dependson the ability of the seeds to synthesize GA, but light alsoenhances the sensitivity of the seeds to GA. Dry storage andpre-incubation at 2°C also increased the responsivenessof Arabidopsis seeds to GA     

Activation of the Cell Cycle in Tomato (Lycopersicon esculentum Mill.) Seeds during Osmoconditioning as Related to Temperature and Oxygen

Using flow cytometric analyses of the nuclear DNA content, westudied the effects of various conditions of osmopriming onthe activation of the cell cycle in embryo root tips of tomato(Lycopersicon esculentum‘Elko’) seeds. In dry untreatedseeds, 90.7% of the nuclei revealed 2C signals. Priming of seedsin polyethylene glycol-8000 (PEG) improved the germination rateof seeds transferred onto water at 15 °C. This was associatedwith an increase in 4C signals when priming was carried outat -1.0 and -1.5 MPa. Priming at -2.0 MPa enhanced subsequentgermination but had no effect on DNA replication. For temperaturesduring priming up to 25 °C, a positive linear correlationexisted between the efficiency of the treatment, evaluated bythe reciprocal of time to obtain 50% germination at 15 °C,and the frequency of 4C nuclei or the 4C/2C values. Such a correlationdid not exist when priming was performed at higher temperatures.At least 5% oxygen in the atmosphere was required during primingfor the induction of DNA replication and for the enhancementof subsequent germination. In the presence of 5x10^-4M and 10^-3MNaN₃during priming, most of the cells were maintained with 2CDNA levels and the treatment had no stimulatory effect on germination.The results show a positive linear relationship between thefrequency of 4C DNA nuclei or the 4C/2C ratio and the improvingeffect of priming. However, in suboptimal conditions of priming(-2.0 MPa or temperatures higher than 25 °C), the improvementof seed germination was not associated with the onset of DNAreplication.Copyright 1999 Annals of Botany Company Cell cycle, germination, osmopriming, oxygen, temperature, Lycopersicon esculentum, tomato.     

Differential response of Trichogramma wasps to extreme sex pheromone types of the noctuid moth Heliothis virescens

1. Chemical espionage in nature may occur when predators or parasitoids home in on animal or plant communication signals. Parasitoid wasps are known to use pheromones emitted by adults hosts to locate host eggs, larvae or pupae. The response of Trichogramma egg parasitoids to a synthetic sex pheromone blend of moths has been shown in a number of studies over the past 40 years. 2. Trichogramma pretiosum (Hymenoptera, Trichogrammatidae) is a tiny parasitic wasp, attacking the eggs of the noctuid moth Heliothis virescens (Lepidoptera, Noctuidae). This study investigated whether T. pretiosum homes in on the sex pheromone of H. virescens at close range. The arrestment response of the wasps to sex pheromone gland extracts of two types of female moths, so‐called high and low females, was also tested, referring to two selected extreme pheromone types of H. virescens. The study also investigated whether the wasps would mount females, possibly to hitchhike with them. 3. The wasps were arrested by the common, ‘low’ pheromone, but not by the rare, ‘high’ pheromone or by extracts from male hairpencils. The wasps did not show a preference for separate sex pheromone compounds, but when pre‐exposed to the major sex pheromone component of H. virescens before the tests together with H. virescens eggs, they did show a preference, indicating learning behaviour. In the mounting experiments, mated females were mounted significantly more than virgin females or males, suggesting that hitchhiking is a strategy used by these wasps to locate moth eggs. 4. This represents the first study to show a differential response of parasitoid wasps to two different sex pheromone types in a single host species. The results warrant further investigations into the potential role of parasitic wasps in the evolution of sexual communication in moths.     

Competition between maize and Echinochloa crus-galli analysed by a hyperbolic regression model

Yield reduction of maize in relation to naturally established populations of Echinochloa crus-galli and Chenopodium album was studied in field experiments over 2 years in which the maize was grown at a wide range of weed densities. Both the crop and the weeds were harvested at intervals during the season. The competitive relations were described accurately by a model based on a hyperbolic relation between yield and plant density. The model can be linearised by considering the reciprocals of the average weight per plant. However, estimating the regression coefficients by linear regression introduced a severe bias due to heterogeneity of variances. Estimation was improved by applying non-linear regression, using a logarithmic transform of the yield equation. Fitted regressions were used to interpolate the yield data to standard weed densities. At a density of 100 Echinochloa plants m-², maize yields were reduced by 8% and 82% in 1982 and 1983, respectively, illustrating the problems in generalising the results of one competition experiment to the other. Three possible fields of application of the competition model are discussed, i.e. adjustment of experimental plot yield for variation in weed population, prediction of expected crop yield losses, and prediction of long-term changes in weed seed populations.     

Artificial night lighting disrupts sex pheromone in a noctuid moth

1. One major, yet poorly studied, change in the environment is the increase in nocturnal light pollution. Although this strongly alters the habitat of nocturnal species, the ecological consequences are poorly known. Moths are well known to be attracted to artificial light sources, but artificial light may affect them in other ways as well. 2. In this study, female Mamestra brassicae moths were subjected to various types of low‐intensity artificial night lighting with contrasting spectral compositions (green‐rich, red‐rich, warm white) or to a dark control treatment and the effects on their sex pheromone production and composition were tested. 3. Artificial night lighting reduced sex pheromone production and altered the chemical composition of the pheromone blend, irrespective of spectral composition. Specifically, amounts of the main pheromone component Z11‐16:Ac were reduced, while the deterring compounds Z9‐14:Ac, Z9‐16:Ac, and Z11‐16:OH were increased relative to Z11‐16:Ac when females were kept under artificial light. These changes may reduce the effectiveness of the sex pheromones, becoming less attractive for males. 4. These results show for the first time that artificial light at night affects processes that are involved in moth reproduction. The potential for mitigation through manipulation of the spectral composition of artificial light appears limited.     

Optimization of novel polymorphic microsatellites in muskox (Ovibos Moschatus) leads to an increased estimate of muskox microsatellite diversity

Initial microsatellite studies suggested that muskoxen display the lowest microsatellite polymorphism in a large ungulate. We optimized 17 di‐nucleotide microsatellites with longer repeats from a muskox DNA library and surveyed 18 animals from across their natural range. Loci with perfect repeats were more variable than imperfect loci: H_perfect = 0.504 ± 0.045 (± SE) vs. H_imperfect = 0.067 ± 0.055. Our H_perfect estimate is higher than previous low estimates of H_E = 0.018 and H_E = 0.059.     

Amounts of Nuclear DNA and Internal Morphology of Gibberellin- and Abscisic Acid-deficient Tomato (Lycopersicon esculentum Mill.) Seeds During Maturation, Imbibition and Germination

Using X-ray photography and flow cytometry, the internal morphologyand DNA replication activity of wild type (wt), GA- (gib-1 )and ABA-deficient (sit^w ) tomato (Lycopersicon esculentum Mill.cv. Moneymaker) mutant seeds were studied. During seed formation,from 30 to 45 d after pollination (DAP) the endosperm becomessolid and the seed starts to gain desiccation tolerance. Atthis time significant changes occur in the amounts of DNA inradicle tip cells. At 30 DAP, radicle tip cells of the threegenotypes manifest about 60% of 2C, 30% of 4C and 10% of 8Camounts of DNA. Upon maturation (45 DAP onwards), most cellsin the seeds of the three genotypes arrest in the G₁phase ofthe cell-cycle with 2C amounts of DNA. However, a relativelyhigh proportion of cells with 4C amounts of DNA was detectedin the radicle tip cells ofsit^w compared with wild type andgib-1. At the well-matured stage (60 DAP), there were about 2% ofseeds with free space in wild type andgib-1 , and about 13%insit^w . At the over-matured stage (75 DAP), even more seedswith free space were found insit^w , whereas no increase in theproportion of the seeds with free space was detected in theother two genotypes. In -1.0 MPa PEG-6000 with or without 10µM GA₄₊₇, no germination occurred in well-matured wildtype andgib-1 seeds, whether or not they were dried after harvest.However,sit^w seeds were able to germinate both in over-maturefruit and in -1.0 MPa PEG-6000. Priming of dried seeds in -1.0MPa PEG induced a large amount of free space in almost all seedsof the three genotypes, and nuclear DNA synthesis in the radicletip cells of wild type andsit^w seeds. However, PEG priming offresh (non-dried) seeds had no effect on the amount of freespace and 2C/4C DNA ratios in wild type orgib-1 seeds, but didinduce free space in about 20–25% ofsit^w seeds and provoked4C signals insit^w seeds. Removal of the endosperm and testaopposite the radicle tip of seeds resulted in root protrusion,the induction of free space and an increase of 4C DNA signalsin the three genotypes. It is concluded that ABA is crucialfor the efficient arrest of tomato embryo radicle tip cellsin G₁phase upon maturation, whereas GAs play an important rolein re-initiating 4C DNA levels upon germination. Dormancy; flow cytometry; free space; Lycopersicon esculentum ; maturation; priming; seed; tomato     

Effects of Carbon Source, Gibberellins and Ethylene on Wall-Bound Invertase Activity in Callus of Convolvulus arvensis

The growth rate of Convolvulus callus on a sucrose-containing medium appeared to be largely independent of the activity of cell wall invertase. The increase of this enzyme activity which occurs upon subculturing was unaffected within the first 24 h by the presence of the substrate sucrose. Neither substitution by glucose or fructose, nor complete deletion of the carbon source had any effect. Gibberellins apparently were not involved in the initiation and/or control of the increase of wall-bound invertase activity occurring upon subculturing. Exogenous ethylene was unable to mimick this effect of subculturing but when applied immediately after subculturing it had a synergistic effect on the increase of invertase. Inhibition occurred, however, when exposure to ethylene was delayed until after 24 h of incubation. These findings suggest a role of ethylene in the control of wall-bound invertase activity.