Background and Aims
The smoke-derived compound karrikinolide (KAR1) shows significant potential as a trigger for the synchronous germination of seeds in a variety of plant-management contexts, from weed seeds in paddocks, to native seeds when restoring degraded lands. Understanding how KAR1 interacts with seed physiology is a necessary precursor to the development of the compound as an efficient and effective management tool. This study tested the ability of KAR1 to stimulate germination of seeds of the global agronomic weed Brassica tournefortii, at different hydration states, to gain insight into how the timing of KAR1 applications in the field should be managed relative to rain events.
Seeds of B. tournefortii were brought to five different hydration states [equilibrated at 15 % relative humidity (RH), 47 % RH, 96 % RH, fully imbibed, or re-dried to 15 % RH following maximum imbibition] then exposed to 1 nm or 1 µm KAR1 for one of five durations (3 min, 1 h, 24 h, 14 d or no exposure).
Dry seeds with no history of imbibition were the most sensitive to KAR1; sensitivity was lower in seeds that were fully imbibed or fully imbibed then re-dried. In addition, reduced sensitivity to KAR1 was associated with an increased sensitivity to exogenously applied abscisic acid (ABA).
Seed water content and history of imbibition were found to significantly influence whether seeds germinate in response to KAR1. To optimize the germination response of seeds, KAR1 should be applied to dry seeds, when sensitivity to ABA is minimized.
Karrikinolide; karrikins; butenolide; smoke; germination stimulant; seed water content; abscisic acid; ABA; gibberellin; weed; Brassica tournefortii
Background and Aims
Tersonia cyathiflora (Gyrostemonaceae) is a fire ephemeral with an obligate requirement for smoke to germinate. Whether it is stimulated to germinate by 3-methyl-2H-furo[2,3-c]pyran-2-one (karrikinolide, KAR1), the butenolide isolated from smoke that stimulates the germination of many other smoke-responsive species, is tested.
Seeds of T. cyathiflora were buried in autumn following collection and were exhumed 1 year later, as this alleviates dormancy and enables seeds to germinate in response to smoke-water. Exhumed seeds were tested with smoke-water and KAR1. Fresh preparations of these solutions were again tested on seeds exhumed 2 months later under a broader range of conditions. They were also tested on Grevillea eriostachya (Proteaceae) and Stylidium affine (Stylidiaceae) to confirm the activity of KAR1.
T. cyathiflora seeds germinated in response to smoke-water but not to KAR1. In contrast, G. eriostachya and S. affine germinated in response to both smoke-water and KAR1.
Although many smoke-responsive seeds germinate in the presence of KAR1, this does not apply universally. This suggests that other chemical(s) in smoke-water may play an important role in stimulating the germination of certain species.
Butenolide; germination; karrikinolide; smoke; 3-methyl-2H-furo[2,3-c]pyran-2-one; Grevillea; Stylidium; Tersonia cyathiflora; Gyrostemonaceae
Background and Aims
Dry fruits remain around the seeds at dispersal in a number of species, especially the Brassicaceae. Explanations for this vary, but usually involve mechanisms of innate dormancy. We speculate that, instead, a persistent fruit may give additional protection through control of dehydration, to species growing in arid or Mediterranean environments where water is sporadic.
X-rays and weight measurements were used to determine the extent to which Raphanus raphanistrum seeds within mature fruits imbibe water, and germination tests determined the roles of the fruit and seed coat in seed dormancy. Rates of water uptake and desiccation, and seedling emergence were compared with and without the fruit. Finally, germinability of seeds extracted from fruits was determined after various periods of moist conditions followed by a range of dry conditions.
Most seeds rapidly take up water within the fruit, but they do not fully imbibe when compared with naked seeds. The seed coat is more important than the dry fruit wall in maintaining seed dormancy. The presence of a dry fruit slows emergence from the soil by up to 6–8 weeks. The fruit slows the rate of desiccation of the seed to a limited extent. The presence of the fruit for a few days during imbibition somehow primes more seeds to germinate than if the fruit is absent; longer moist periods within the pod appear to induce dormancy.
The fruit certainly modifies the seed environment as external conditions change between wet and dry, but not to a great extent. The major role seems to be: (a) the physical restriction of imbibition and germination; and (b) the release and then re-imposition of dormancy within the seed. The ecological significance of the results requires more research under field conditions.
Wild radish; Raphanus raphanistrum; imbibition; desiccation; dry fruit wall; germination; dormancy; X-ray
Background and Aims
Chenopodium album is well-known as a serious weed and is a salt-tolerant species inhabiting semi-arid and light-saline environments in Xinjiang, China. It produces large amounts of heteromorphic (black and brown) seeds. The primary aims of the present study were to compare the germination characteristics of heteromorphic seeds, the diversity of plant growth and seed proliferation pattern of the resulting plants, and the correlation between NaCl stress and variation of seed heteromorphism.
The phenotypic characters of heteromorphic seeds, e.g. seed morphology, seed mass and total seed protein were determined. The effects of dry storage at room temperature on dormancy behaviour, the germination response of seeds to salinity stress, and the effect of salinity on growth and seed proliferation with plants derived from different seed types were investigated.
Black and brown seeds differed in seed morphology, mass, total seed protein, dormancy behaviour and salinity tolerance. Brown seeds were large, non-dormant and more salt tolerant, and could germinate rapidly to a high percentage in a wider range of environments; black seeds were salt-sensitive, and a large proportion of seeds were dormant. These characteristics varied between two populations. There was little difference in growth characteristics and seed output of plants produced from the two seed morphs except when plants were subjected to high salinity stress. Plants that suffered higher salinity stress produced more brown (salt-tolerant) seeds.
The two seed morphs of C. album exhibited distinct diversity in germination characteristics. There was a significant difference in plant development and seed proliferation pattern from the two types of seeds only when the parent plants were treated with high salinity. In addition, seed heteromorphism of C. album varied between the two populations, and such variation may be attributed, at least in part, to the salinity.
Chenopodium album; development of descendants; salinity tolerance; seed germination; variation of seed heteromorphism
A new oxyvinylglycine has been identified as a naturally occurring herbicide that irreversibly arrests germination of the seeds of grassy weeds, such as annual bluegrass (Poa annua), without significantly affecting the growth of established grass seedlings and mature plants or germination of the seeds of broadleaf plant species (dicots). Previously, Pseudomonas fluorescens WH6 and over twenty other rhizosphere bacteria were isolated and selected for their ability to arrest germination of P. annua seeds. The Germination-Arrest Factor (GAF, 1) responsible for this developmentally specific herbicidal action has now been isolated from the culture filtrate of P. fluorescens WH6. Purification of this highly polar, low molecular weight natural product allowed its structure to be assigned as 4-formylaminooxy-l-vinylglycine based on NMR spectroscopic and mass spectrometric data, in combination with d/l-amino acid oxidase reactions to establish the absolute configuration. Assay results for P. annua inhibition by related compounds known to regulate plant growth are presented, and a cellular target for 1 is proposed. Furthermore, using bioassays, TLC, and capillary NMR spectroscopy, it has been shown that GAF (1) is secreted by all other herbicidally-active rhizosphere bacteria in our collection.
Direct seeding is replacing transplanting in rice. Early flooding suppresses weeds but selective action is compromised by the sharing of flood-tolerance traits. Understanding adaptive traits in both species is therefore a prerequisite for developing direct seeding systems that control weeds while leaving rice seedlings relatively unharmed.
Background and aims
Direct seeding of rice is being adopted in rainfed and irrigated lowland ecosystems because it reduces labour costs in addition to other benefits. However, early flooding due to uneven fields or rainfall slows down seed germination and hinders crop establishment. Conversely, early flooding helps suppress weeds and reduces the costs of manual weeding and/or dependence on herbicides; however, numerous weed species are adapted to lowlands and present challenges for the use of flooding to control weeds. Advancing knowledge on the mechanisms of tolerance of flooding during germination and early growth in rice and weeds could facilitate the development of improved rice varieties and effective weed management practices for direct-seeded rice.
Rice genotypes with a greater ability to germinate and establish in flooded soils were identified, providing opportunities to develop varieties suitable for direct seeding in flooded soils. Tolerance of flooding in these genotypes was mostly attributed to traits associated with better ability to mobilize stored carbohydrates and anaerobic metabolism. Limited studies were undertaken in weeds associated with lowland rice systems. Remaining studies compared rice and weeds and related weed species such as Echinochloa crus-galli and E. colona or compared ecotypes of the same species of Cyperus rotundus adapted to either aerobic or flooded soils.
Tolerant weeds and rice genotypes mostly developed similar adaptive traits that allow them to establish in flooded fields, including the ability to germinate and elongate faster under hypoxia, mobilize stored starch reserves and generate energy through fermentation pathways. Remarkably, some weeds developed additional traits such as larger storage tubers that enlarge further in deeper flooded soils (C. rotundus). Unravelling the mechanisms involved in adaptation to flooding will help design management options that will allow tolerant rice genotypes to adequately establish in flooded soils while simultaneously suppressing weeds.
Predicting outcomes of transgene flow from arable crops requires a system perspective that considers ecological and evolutionary processes within a landscape context. In Europe, the arable weed Raphanus raphanistrum is a potential hybridization partner of oilseed rape, and the two species are ecologically linked through the common herbivores Meligethes spp. Observations in Switzerland show that high densities of Meligethes beetles maintained by oilseed rape crops can lead to considerable damage on R. raphanistrum. We asked how increased insect resistance in R. raphanistrum – as might be acquired through introgression from transgenic oilseed rape – would affect seed production under natural herbivore pressure. In simulation experiments, plants protected against Meligethes beetles produced about twice as many seeds as unprotected plants. All stages in the development of reproductive structures from buds to pods were negatively affected by the herbivore, with the transition from buds to flowers being the most vulnerable. We conclude that resistance to Meligethes beetles could confer a considerable selective advantage upon R. raphanistrum in regions where oilseed rape is widely grown.
Apparent competition; crop–wild gene flow; Meligethes beetles; oilseed rape; Raphanus raphanistrum; transgenic plants
The micropylar endosperm is a major regulator of seed germination in endospermic species, to which the close Brassicaceae relatives Arabidopsis thaliana and Lepidium sativum (cress) belong. Cress seeds are about 20 times larger than the seeds of Arabidopsis. This advantage was used to construct a tissue-specific subtractive cDNA library of transcripts that are up-regulated late in the germination process specifically in the micropylar endosperm of cress seeds. The library showed that a number of transcripts known to be up-regulated late during germination are up-regulated in the micropylar endosperm cap. Detailed germination kinetics of SALK lines carrying insertions in genes present in our library showed that the identified transcripts do indeed play roles during germination. Three peroxidases were present in the library. These peroxidases were identified as orthologues of Arabidopsis AtAPX01, AtPrx16, and AtPrxIIE. The corresponding SALK lines displayed significant germination phenotypes. Their transcripts were quantified in specific cress seed tissues during germination in the presence and absence of ABA and they were found to be regulated in a tissue-specific manner. Peroxidase activity, and particularly its regulation by ABA, also differed between radicles and micropylar endosperm caps. Possible implications of this tissue-specificity are discussed.
Arabidopsis thaliana; cress; Lepidium sativum; micropylar endosperm cap; peroxidases; seed germination; subtractive cDNA library
Smoke released from burning vegetation functions as an important environmental signal promoting the germination of many plant species following a fire. It not only promotes the germination of species from fire-prone habitats, but several species from non-fire-prone areas also respond, including some crops. The germination stimulatory activity can largely be attributed to the presence of a highly active butenolide compound, 3-methyl-2H-furo[2,3-c]pyran-2-one (referred to as karrikin 1 or KAR1), that has previously been isolated from plant-derived smoke. Several hypotheses have arisen regarding the molecular background of smoke and KAR1 action.
In this paper we demonstrate that although smoke-water and KAR1 treatment of maize kernels result in a similar physiological response, the gene expression and the protein ubiquitination patterns are quite different. Treatment with smoke-water enhanced the ubiquitination of proteins and activated protein-degradation-related genes. This effect was completely absent from KAR1-treated kernels, in which a specific aquaporin gene was distinctly upregulated.
Our findings indicate that the array of bioactive compounds present in smoke-water form an environmental signal that may act together in germination stimulation. It is highly possible that the smoke/KAR1 'signal' is perceived by a receptor that is shared with the signal transduction system implied in perceiving environmental cues (especially stresses and light), or some kind of specialized receptor exists in fire-prone plant species which diverged from a more general one present in a common ancestor, and also found in non fire-prone plants allowing for a somewhat weaker but still significant response. Besides their obvious use in agricultural practices, smoke and KAR1 can be used in studies to gain further insight into the transcriptional changes during germination.
Strigolactones (SL) and karrikins (KAR) both contain essential butenolide moieties, and both require the F-box protein MAX2 to control seed germination and photomorphogenesis in Arabidopsis thaliana. A new discovery that SL and KAR also require related α/β-hydrolase proteins for such activity suggests that they operate through a similar molecular mechanism. Based on structural similarity, a previously proposed mode of action for SL was also considered for KAR, but recent structure-activity studies suggest that this mechanism may not apply. Here we rationalise these observations into a hypothesis whereby different α/β-hydrolases distinguish SL and KAR by virtue of their non-butenolide moieties and catalyze nucleophilic attack on the butenolide. The products would be different for SL and KAR, and in the case of SL they have no biological activity. The inference is that nucleophilic attack on SL and KAR by α/β-hydrolases is required for their bioactivity, but the hydrolysis products are not.
D14; KAI2; MAX2; butenolide; karrikin; strigolactone; α/β-hydrolase
Background and Aims
It has been hypothesized that soil moisture conditions could affect the dormancy status of buried weed seeds, and, consequently, their sensitivity to light stimuli. In this study, an investigation is made of the effect of different soil moisture conditions during cold-induced dormancy loss on changes in the sensitivity of Polygonum aviculare seeds to light.
Seeds buried in pots were stored under different constant and fluctuating soil moisture environments at dormancy-releasing temperatures. Seeds were exhumed at regular intervals during storage and were exposed to different light treatments. Changes in the germination response of seeds to light treatments during storage under the different moisture environments were compared in order to determine the effect of soil moisture on the sensitivity to light of P. aviculare seeds.
Seed acquisition of low-fluence responses during dormancy release was not affected by either soil moisture fluctuations or different constant soil moisture contents. On the contrary, different soil moisture environments affected seed acquisition of very low fluence responses and the capacity of seeds to germinate in the dark.
The results indicate that under field conditions, the sensitivity to light of buried weed seeds could be affected by the soil moisture environment experienced during the dormancy release season, and this could affect their emergence pattern.
Dark germination; dormancy loss; light; low fluence response (LFR); phytochrome; Polygonum aviculare; soil moisture; soil moisture fluctuations; very low fluence response (VLFR); weed seeds
Twenty-four weeds commonly found in commercial potato fields in Quebec were evaluated for their host suitability to the root-lesion nematode, Pratylenchus penetrans, under greenhouse conditions. Brown mustard (Brassica juncea) and rye (Secale cereale) were included as susceptible controls and forage pearl millet hyb. CFPM 101 (Pennisetum glaucum) as a poor host. Pratylenchus penetrans multiplied well on 22 of the 24 weed species tested (Pf/Pi ≥ rye or brown mustard). Cirsium arvense, Leucanthemum vulgare and Matricaria discoida were classified as very good hosts with a Pf/Pi ranging from 1.60 to 2.54, while Ambrosia artemisiifolia and Cyperus esculentus were classified as poor hosts with a Pf/Pi from 0.01 to 0.15. Amaranthus powellii, A. retrqflexus, Raphanus raphanistrum, Rorippa palustris, Cerastium fontanum, Spergula arvensis, Stellaria media, Chenopodium album, Vicia cracca, Elytrigia repens, Digitaria ischaemum, Echinochloa crusgalli, Panicum capillare, Setaria faberii, S. pumila, S. viridis, Polygonum convolvulus, P. scabrum and P. persicaria were intermediate hosts with Pf/Pi values ranging from 0.33 to 2.01. The plant species and the botanical family had a significant impact on nematode reproduction. The Brassicaceae family resulted in the greatest reproduction of P. penetrans, and the Cyperaceae resulted in the least. The plant life-cycle (annual vs. perennial) had no impact on nematode population.
brown mustard; host range; pearl millet; potato; Pratylenchus penetrans; root-lesion nematode; rye; weed
Background and Aims
The duration of the plant life cycle is an important attribute that determines fitness and coexistence of weeds in arable fields. It depends on the timing of two key life-history traits: time from seed dispersal to germination and time from germination to flowering. These traits are components of the time to reproduction. Dormancy results in reduced and delayed germination, thus increasing time to reproduction. Genotypes in the arable seedbank predominantly have short time to flowering. Synergy between reduced seed dormancy and reduced flowering time would create stronger contrasts between genotypes, offering greater adaptation in-field. Therefore, we studied differences in seed dormancy between in-field flowering time genotypes of shepherd's purse.
Genotypes with early, intermediate or late flowering time were grown in a glasshouse to provide seed stock for germination tests. Secondary dormancy was assessed by comparing germination before and after dark-incubation. Dormancy was characterized separately for seed myxospermy heteromorphs, observed in each genotype. Seed carbon and nitrogen content and seed mass were determined as indicators of seed filling and resource partitioning associated with dormancy.
Although no differences were observed in primary dormancy, secondary dormancy was weaker among the seeds of early-flowering genotypes. On average, myxospermous seeds showed stronger secondary dormancy than non-myxospermous seeds in all genotypes. Seed filling was similar between the genotypes, but nitrogen partitioning was higher in early-flowering genotypes and in non-myxospermous seeds.
In shepherd's purse, early flowering and reduced seed dormancy coincide and appear to be linked. The seed heteromorphism contributes to variation in dormancy. Three functional groups of seed dormancy were identified, varying in dormancy depth and nitrate response. One of these groups (FG-III) was distinct for early-flowering genotypes. The weaker secondary dormancy of early-flowering genotypes confers a selective advantage in arable fields.
Co-adaptation; Capsella bursa-pastoris; shepherd's purse; seedbank; flowering time; seed heteromorphism; light; nitrate; primary dormancy; secondary dormancy; weed; myxospermy; mucilage
Meals produced when oil is extracted from seeds in the Brassicaceae have been shown to suppress weeds and soilborne pathogens. These seed meals are commonly used individually as soil amendments; the goal of this research was to evaluate seed meal mixes of Brassica juncea (Bj) and Sinapis alba (Sa) against Meloidogyne incognita. Seed meals from Bj ‘Pacific Gold’ and Sa ‘IdaGold’ were tested alone and in combinations to determine rates and application times that would suppress M. incognita on pepper (Capsicum annuum) without phytotoxicity. Rates of soil application (% w/w) for the phytotoxicity study were: 0.5 Sa, 0.2 Bj, 0.25 Sa + 0.25 Bj, 0.375 Sa + 0.125 Bj, 0.125 Sa + 0.375 Bj, and 0, applied 0 – 5 weeks before transplant. Overall, 0.2% Bj was the least toxic meal to pepper seedlings. By comparison, 0.5% S. alba seed meal did not reduce lettuce (Lactuca sativa) seed germination at week 0, but all seed meal treatments containing B. juncea prevented or significantly reduced germination at week 0. The seed meals did not affect lettuce seed germination at weeks 1-5, but hypocotyl growth was reduced by all except 0.2% Bj at weeks 1, 4 and 5. Brassica juncea and Sa meals were tested for M. incognita suppression at 0.2, 0.15, 0.1 and 0.05%; mixtures were 0.1% Sa + 0.1% Bj, 0.15% Sa + 0.05% Bj, and 0.05% Sa + 0.15% Bj. All treatments were applied 2 weeks before transplant. The 0.2% Bj and 0.05% Sa + 0.15% Bj treatments overall had the longest shoots and highest fresh weights. Eggs per g root were lowest with 0.1 – 0.2% Bj amendments and the seed meal mixtures. The results indicate that Bj and some Bj + Sa mixtures can be applied close to transplant to suppress M. incognita populations on pepper; consequently, a seed meal mixture could be selected to provide activity against more than one pest or pathogen. For pepper, care should be taken in formulating mixtures so that Sa rates are low compared to Bj.
amendment; biofuel byproducts; Brassica; glucosinolate; management; Meloidogyne incognita; mustard seed meal; root-knot nematode; Sinapis
Background and Aims
Pathogen–seed interactions may involve a race for seed resources, so that seeds that germinate more quickly, mobilizing reserves, will be more likely to escape seed death than slow-germinating seeds. This race-for-survival hypothesis was tested for the North American seed pathogen Pyrenophora semeniperda on seeds of the annual grass Bromus tectorum, an invasive plant in North America. In this species, the seed germination rate varies as a function of dormancy status; dormant seeds germinate slowly if at all, whereas non-dormant seeds germinate quickly.
Three experimental approaches were utilized: (a) artificial inoculations of mature seeds that varied in primary dormancy status and wounding treatment; (b) naturally inoculated undispersed seeds that varied in primary dormancy status; and (c) naturally inoculated seeds from the carry-over seed bank that varied in degree of secondary dormancy, habitat of origin and seed age.
In all three approaches, seeds that germinated slowly were usually killed by the pathogen, whereas seeds that germinated quickly frequently escaped. Pyrenophora semeniperda reduced B. tectorum seed banks. Populations in drier habitats sustained 50 times more seed mortality than a population in a mesic habitat. Older carry-over seeds experienced 30 % more mortality than younger seeds.
Given the dramatic levels of seed death and the ability of this pathogen to reduce seed carry-over, it is intriguing to consider whether P. semeniperda could be used to control B. tectorum through direct reduction of its seed bank.
Biocontrol; biotic resistance; cheatgrass; Drechslera campanulata; invasive species; mycoherbicide; pathogen; seed bank; seed-borne
Ionotropic glutamate receptors principally mediate fast excitatory transmission in the brain. Among the three classes of ionotropic glutamate receptors, kainate receptors (KARs) display a categorical brain distribution, which has been historically defined by 3H-radiolabeled kainate binding. Compared with recombinant KARs expressed in heterologous cells, synaptic KARs exhibit characteristically slow rise-time and decay kinetics. However, the mechanisms responsible for these unique KAR properties remain unclear. Here we found that both the distinct high affinity biding pattern in the mouse brain and the channel properties of native KARs are determined by the KAR auxiliary subunit Neto1. Through modulation of agonist binding affinity and off-kinetics of KARs, but not trafficking of KARs, Neto1 determines both KAR high affinity binding pattern and the distinctively slow kinetics of postsynaptic KARs. By regulating KAR-EPSC kinetics, Neto1 can control synaptic temporal summation, spike generation and fidelity.
Plant defensins are an important component of the innate defence system of plants where they form protective antimicrobial barriers between tissue types of plant organs as well as around seeds. These peptides also have other activities that are important for agricultural applications as well as the medical sector. Amongst the numerous plant peptides isolated from a variety of plant species, a significant number of promising defensins have been isolated from Brassicaceae species. Here we report on the isolation and characterization of four defensins from Heliophila coronopifolia, a native South African Brassicaceae species.
Four defensin genes (Hc-AFP1-4) were isolated with a homology based PCR strategy. Analysis of the deduced amino acid sequences showed that the peptides were 72% similar and grouped closest to defensins isolated from other Brassicaceae species. The Hc-AFP1 and 3 peptides shared high homology (94%) and formed a unique grouping in the Brassicaceae defensins, whereas Hc-AFP2 and 4 formed a second homology grouping with defensins from Arabidopsis and Raphanus. Homology modelling showed that the few amino acids that differed between the four peptides had an effect on the surface properties of the defensins, specifically in the alpha-helix and the loop connecting the second and third beta-strands. These areas are implicated in determining differential activities of defensins. Comparing the activities after recombinant production of the peptides, Hc-AFP2 and 4 had IC50 values of 5-20 μg ml-1 against two test pathogens, whereas Hc-AFP1 and 3 were less active. The activity against Botrytis cinerea was associated with membrane permeabilization, hyper-branching, biomass reduction and even lytic activity. In contrast, only Hc-AFP2 and 4 caused membrane permeabilization and severe hyper-branching against the wilting pathogen Fusarium solani, while Hc-AFP1 and 3 had a mild morphogenetic effect on the fungus, without any indication of membrane activity. The peptides have a tissue-specific expression pattern since differential gene expression was observed in the native host. Hc-AFP1 and 3 expressed in mature leaves, stems and flowers, whereas Hc-AFP2 and 4 exclusively expressed in seedpods and seeds.
Two novel Brassicaceae defensin sequences were isolated amongst a group of four defensin encoding genes from the indigenous South African plant H. coronopifolia. All four peptides were active against two test pathogens, but displayed differential activities and modes of action. The expression patterns of the peptide encoding genes suggest a role in protecting either vegetative or reproductive structures in the native host against pathogen attack, or roles in unknown developmental and physiological processes in these tissues, as was shown with other defensins.
The host suitability to Ditylenchus destructor of seven common weed species in peanut (Arachis hypogaea) fields in South Africa was determined. Based on the number of nematodes per root unit, white goosefoot (Chenopodium album), feathertop chloris (Chloris virgata), purple nutsedge (Cyperus rotundus), jimson weed (Datura stramonium), goose grass (Eleusine indica), khaki weed (Tagetes minuta), and cocklebur (Xanthium strumarium) were poor hosts. Ditylenchus destructor survived on all weed species; population densities increased in peanut hulls and caused severe damage to seeds of peanut grown after weeds. Roots of purple nutsedge left in the soil suppressed populations of D. destructor and root and pod development in peanut grown after the weed. However, nematode populations in peanut hulls and seeds were not suppressed. Some weed species, especially purple nutsedge which is common in peanut fields, can be used to indicate the presence of D. destructor in the absence of peanut.
Arachis hypogaea; Chenopodium album; Chloris virgata; cocklebur; Cyperus rotundus; Datura stramonium; Ditylenchus destructor; Eleusine indica; feathertop chloris; goose grass; host status; jimson weed; khaki weed; peanut; purple nutsedge; South Africa; Tagetes minuta; Xanthium strumarium
Background and Aims
Seed survival in the soil contributes to population persistence and community diversity, creating a need for reliable measures of soil seed bank persistence. Several methods estimate soil seed bank persistence, most of which count seedlings emerging from soil samples. Seasonality, depth distribution and presence (or absence) in vegetation are then used to classify a species' soil seed bank into persistent or transient, often synthesized into a longevity index. This study aims to determine if counts of seedlings from soil samples yield reliable seed bank persistence estimates and if this is correlated to seed production.
Seeds of 38 annual weeds taken from arable fields were buried in the field and their viability tested by germination and tetrazolium tests at 6 month intervals for 2·5 years. This direct measure of soil seed survival was compared with indirect estimates from the literature, which use seedling emergence from soil samples to determine seed bank persistence. Published databases were used to explore the generality of the influence of reproductive capacity on seed bank persistence estimates from seedling emergence data.
There was no relationship between a species' soil seed survival in the burial experiment and its seed bank persistence estimate from published data using seedling emergence from soil samples. The analysis of complementary data from published databases revealed that while seed bank persistence estimates based on seedling emergence from soil samples are generally correlated with seed production, estimates of seed banks from burial experiments are not.
The results can be explained in terms of the seed size–seed number trade-off, which suggests that the higher number of smaller seeds is compensated after germination. Soil seed bank persistence estimates correlated to seed production are therefore not useful for studies on population persistence or community diversity. Confusion of soil seed survival and seed production can be avoided by separate use of soil seed abundance and experimental soil seed survival.
Arable weeds; Bifora testiculata; Carthamus lanatus; Centaurea solstitialis; longevity index; seed bank persistence; soil seed bank
The Brassicaceae family includes the model plant Arabidopsis thaliana as well as a number of agronomically important species such as oilseed crops (in particular Brassica napus, B. juncea and B. rapa) and vegetables (eg. B. rapa and B. oleracea).
Separated by only 10-20 million years, Brassica species and Arabidopsis thaliana are closely related, and it is expected that knowledge obtained relating to Arabidopsis growth and development can be translated into Brassicas for crop improvement. Moreover, certain aspects of plant development are sufficiently different between Brassica and Arabidopsis to warrant studies to be carried out directly in the crop species. However, mutating individual genes in the amphidiploid Brassicas such as B. napus and B. juncea may, on the other hand, not give rise to expected phenotypes as the genomes of these species can contain up to six orthologues per single-copy Arabidopsis gene. In order to elucidate and possibly exploit the function of redundant genes for oilseed rape crop improvement, it may therefore be more efficient to study the effects in one of the diploid Brassica species such as B. rapa. Moreover, the ongoing sequencing of the B. rapa genome makes this species a highly attractive model for Brassica research and genetic resource development.
Seeds from the diploid Brassica A genome species, B. rapa were treated with ethyl methane sulfonate (EMS) to produce a TILLING (Targeting Induced Local Lesions In Genomes) population for reverse genetics studies. We used the B. rapa genotype, R-o-18, which has a similar developmental ontogeny to an oilseed rape crop. Hence this resource is expected to be well suited for studying traits with relevance to yield and quality of oilseed rape. DNA was isolated from a total of 9,216 M2 plants and pooled to form the basis of the TILLING platform. Analysis of six genes revealed a high level of mutations with a density of about one per 60 kb. This analysis also demonstrated that screening a 1 kb amplicon in just one third of the population (3072 M2 plants) will provide an average of 68 mutations and a 97% probability of obtaining a stop-codon mutation resulting in a truncated protein. We furthermore calculated that each plant contains on average ~10,000 mutations and due to the large number of plants, it is predicted that mutations in approximately half of the GC base pairs in the genome exist within this population.
We have developed the first EMS TILLING resource in the diploid Brassica species, B. rapa. The mutation density in this population is ~1 per 60 kb, which makes it the most densely mutated diploid organism for which a TILLING population has been published. This resource is publicly available through the RevGenUK reverse genetics platform http://revgenuk.jic.ac.uk.
• Background and Aims The Brassicaceae family encompasses numerous species of great agronomic importance, belonging to such genera, as Brassica, Raphanus, Sinapis and Armoracia. Many of them are characterized by extensive intraspecific diversity of phenotypes. The present study focuses on the polymorphism of number, appearance and chromosomal localization of ribosomal DNA (rDNA) sites and, when possible, in relation to polyploidy, in 42 accessions of Brassica species and ten accessions of Diplotaxis, Eruca, Raphanus and Sinapis species.
• Methods Chromosomal localization of ribosomal DNA was carried out using dual colour fluorescence in situ hybridization (FISH) with 5S rDNA and 25S rDNA sequences as probes on enzymatically digested root-tip meristematic cells.
• Key Results Loci for 5S and 18S–5.8S–25S rDNA were determined for the first time in six taxa, and previously unreported rDNA constellations were described in an additional 12 accessions. FISH revealed frequent polymorphism in number, appearance and chromosomal localization of both 5S and 25S rDNA sites. This phenomenon was most commonly observed in the A genome of Brassica, where it involves exclusively pericentromeric sites of 5S and 25S rRNA genes. The intraspecific polymorphism was between subspecies/varieties or within a variety or cultivar (i.e. interindividual).
• Conclusions The number of rDNA sites can differ up to 5-fold in species with the same chromosome number. In addition to the eight previously reported chromosomal types with ribosomal genes, three new variant types are described. The extent of polymorphism is genome dependent. Comparing the A, B and C genomes revealed the highest rDNA polymorphism in the A genome. The loci carrying presumably inactive ribosomal RNA genes are particularly prone to polymorphism. It can also be concluded that there is no obvious polyploidization-related tendency to reduce the number of ribosomal DNA loci in the allotetraploid species, when compared with their putative diploid progenitors. The observed differences are rather caused by the prevailing polymorphism within the diploids and allotetraploids. This would make it difficult to predict expected numbers of rDNA loci in natural polyploids.
5S rDNA; 25S rDNA; 18S–5.8S–25S ribosomal RNA genes; Brassicaceae; chromosomes; FISH
In the life cycle of higher plants, seed development is a key process connecting two distinct sporophytic generations. Seed development can be divided into embryo morphogenesis and seed maturation. An essential metabolic function of maturing seeds is the deposition of storage compounds that are mobilised to fuel post-germinative seedling growth. Given the importance of seeds for food and animal feed and considering the tremendous interest in using seed storage products as sustainable industrial feedstocks to replace diminishing fossil reserves, understanding the metabolic and developmental control of seed filling constitutes a major focus of plant research. Arabidopsis thaliana is an oilseed species closely related to the agronomically important Brassica oilseed crops. The main storage compounds accumulated in seeds of A. thaliana consist of oil stored as triacylglycerols (TAGs) and seed storage proteins (SSPs). Extensive tools developed for the molecular dissection of A. thaliana development and metabolism together with analytical and cytological procedures adapted for very small seeds have led to a good description of the biochemical pathways producing storage compounds. In recent years, studies using these tools have shed new light on the intricate regulatory network controlling the seed maturation process. This network involves sugar and hormone signalling together with a set of developmentally regulated transcription factors. Although much remains to be elucidated, the framework of the regulatory system controlling seed filling is coming into focus.
Arabidopsis; seed maturation; triacylglycerols (TAGs); seed storage proteins (SSPs)
Strong seed dormancy has been an obstacle for field production of Echinacea species. Previous research on overcoming Echinacea seed dormancy has been extensive and focused on treatment methods, which involve time and expense, and are incompatible with organic production if synthetic chemicals are used. We have attempted to genetically reduce seed dormancy through selection and breeding in Echinacea, by using E. pallida as a model species. Three accessions were used in this study. Nine parent plants of each accession selected from early, in-dark germinated seeds (in-dark plants) or from late, in-light seeds (in-light plants) were planted and grouped by accession and germination treatment method for seed production through a polycross method. Germination tests indicated that these in-dark plants produced seed (in-dark seed) with significantly reduced seed dormancy when tested under light or dark conditions in comparison to the seed of the in-light plants (in-light seed). Among the three accessions, the in-dark seed germinated at much higher rates than did the in-light seed, more than 2× at 25°C under light and up to an 83× increase in darkness, and up to an 8× increase over the corresponding parental seed lots under comparable germination conditions. In addition to these increases in germination, the in-dark seed showed early and synchronized germination as compared to the in-light seed. Since these results were achieved through only one cycle of selection and breeding, they strongly suggest that we have developed a very effective method for modifying seed dormancy in Echinacea.
seed germination; pale purple coneflower; medicinal crop; breeding method
Germination of endospermic seeds is partly regulated by the micropylar endosperm, which acts as constraint to radicle protrusion. Gibberellin (GA) signalling pathways control coat-dormancy release, endosperm weakening, and organ expansion during seed germination. Three GIBBERELLIN INSENSITIVE DWARF1 (GID1) GA receptors are known in Arabidopsis thaliana: GID1a, GID1b, and GID1c. Molecular phylogenetic analysis of angiosperm GID1s reveals that they cluster into two eudicot (GID1ac, GID1b) groups and one monocot group. Eudicots have at least one gene from each of the two groups, indicating that the different GID1 receptors fulfil distinct roles during plant development. A comparative Brassicaceae approach was used, in which gid1 mutant and whole-seed transcript analyses in Arabidopsis were combined with seed-tissue-specific analyses of its close relative Lepidium sativum (garden cress), for which three GID1 orthologues were cloned. GA signalling via the GID1ac receptors is required for Arabidopsis seed germination, GID1b cannot compensate for the impaired germination of the gid1agid1c mutant. Transcript expression patterns differed temporarily, spatially, and hormonally, with GID1b being distinct from GID1ac in both species. Endosperm weakening is mediated, at least in part, through GA-induced genes encoding cell-wall-modifying proteins. A suppression subtraction hybridization (SSH) cDNA library enriched for sequences that are highly expressed during early germination in the micropylar endosperm contained expansins and xyloglucan endo-transglycosylases/hydrolases (XTHs). Their transcript expression patterns in both species strongly suggest that they are regulated by distinct GID1-mediated GA signalling pathways. The GID1ac and GID1b pathways seem to fulfil distinct regulatory roles during Brassicaceae seed germination and seem to control their downstream targets distinctly.
Arabidopsis thaliana; endosperm weakening; expansin; GIBBERELLIN INSENSITIVE DWARF1; Lepidium sativum; seed germination; xyloglucan endo-transglycosylase/hydrolase
Arbuscular mycorrhizal fungi (AMF) are known for their beneficial effects on plants. However, there is increasing evidence that some ruderal plants, including several agricultural weeds, respond negatively to AMF colonization. Here, we investigated the effect of AMF on the growth of individual weed species and on weed-crop interactions.
First, under controlled glasshouse conditions, we screened growth responses of nine weed species and three crops to a widespread AMF, Glomus intraradices. None of the weeds screened showed a significant positive mycorrhizal growth response and four weed species were significantly reduced by the AMF (growth responses between −22 and −35%). In a subsequent experiment, we selected three of the negatively responding weed species – Echinochloa crus-galli, Setaria viridis and Solanum nigrum – and analyzed their responses to a combination of three AMF (Glomus intraradices, Glomus mosseae and Glomus claroideum). Finally, we tested whether the presence of a crop (maize) enhanced the suppressive effect of AMF on weeds. We found that the growth of the three selected weed species was also reduced by a combination of AMF and that the presence of maize amplified the negative effect of AMF on the growth of E. crus-galli.
Our results show that AMF can negatively influence the growth of some weed species indicating that AMF have the potential to act as determinants of weed community structure. Furthermore, mycorrhizal weed growth reductions can be amplified in the presence of a crop. Previous studies have shown that AMF provide a number of beneficial ecosystem services. Taken together with our current results, the maintenance and promotion of AMF activity may thereby contribute to sustainable management of agroecosystems. However, in order to further the practical and ecological relevance of our findings, additional experiments should be performed under field conditions.