Strong bottom-up and weak top-down effects in soil: nematode-parasitized insects and nematode-trapping fungi

The soils of the Bodega Marine Reserve (BMR, Sonoma County, California) contain many nematode-trapping fungi and many ghost moth larvae parasitized by entomopathogenic nematodes. The current study determined whether these nematode-parasitized moth larvae, which can produce very large numbers of nematodes, enhanced the population densities of nematode-trapping fungi and whether the fungi trapped substantial numbers of nematodes emerging and dispersing from moths. Wax moths were used in place of ghost moths because the former are easier to obtain. When nematode-parasitized moth larvae were added to laboratory microcosms containing BMR field soil, the population densities of four nematode-trapping fungi increased substantially. The greatest increase in population density was by Arthrobotrys oligospora, which uses adhesive networks to capture nematodes. A. oligospora population density increased about 10 times when the added moth larvae were parasitized by the nematode Heterorhabditis marelatus and about 100 times when added moth larvae were parasitized by the nematode Steinernema glaseri. Other trapping fungi endemic to the soil and enhanced by nematode-parasitized moth larvae included Myzocytium glutinosporum, Drechslerella brochopaga, and Gamsylella gephyropaga, which produce adhesive spores, constricting rings, and adhesive branches, respectively. The data suggest that the previously documented abundance and diversity of nematode-trapping fungi in BMR soil can be explained, at least in part, by nematode-parasitized insects, although that inference requires further studies with ghost moths. The strong bottom-up enhancement of nematode-trapping fungi was not matched by a strong top-down suppression of nematodes, i.e. the fungi trapped fewer than 30% of dispersing nematodes.     

Effect of soil acidity on the entomophilic nematode Steinernema kraussei Steiner

Summary The free-living stages of Steinernema kraussei (Rhabditida), an endoparasite of Cephalcia abietis L. (Hymenoptera), are exposed to different soil conditions when searching for host nymphs. Field studies and laboratory experiments showed that soil acidity plays a major role in the nematode's ability to parasitize Cephalcia nymphs. Under field conditions in Picea abies forests positive correlations between soil pH and both nematode density and insects parasitized were found. Acidic soil with pH levels below 4.0 may limit the nematode's host-finding. Controlled experiments in the laboratory under different pH conditions confirmed these correlations.Dedicated to the late Prof. Dr. W. Kühnelt     

中国热带地区不同作物长期轮作对土壤线虫群落结构及多样性的影响

A field experiment was carried out from 2003 to 2013 in the Wanzhong Farm of the Hainan Island, China, to determine the effects of two long-term banana rotations on the abundance and trophic groups of soil nematode communities in the island. The experiment was set out as a randomized complete block design with three replications of three treatments: banana-pineapple rotation(AB), banana-papaya rotation(BB) and banana monoculture(CK) in a conventional tillage system. Soil samples were taken at depths of 0–10, 10–20 and 20–30 cm, and nematodes were extracted by a modified cotton-wool filter method and identified to the genus level. Nematode ecological indices of Shannon-Weaver diversity(H′), dominance index(λ), maturity index(MI), plant parasite index(PPI), structure index(SI), enrichment index(EI), and channel index(CI) were calculated. A total of 28 nematode genera with relative abundance over 0.1% were identified, among which Tylenchus and Paratylenchus in the AB, Thonus in the BB, Tylenchus and Helicotylenchus in the CK were the dominant genera. The rotation soils favored bacterivores, fungivores and omnivores-predators with high colonizer-persister(c-p) values. Soil food web in the rotation systems was highly structured, mature and enriched as indicated by SI, MI and EI values, respectively. Higher abundance of bacterivores and lower values of CI suggested that the soil food web was dominated by a bacterial decomposition pathway in rotation soils. Nematode diversity was much higher after a decade of rotation.Soil depth had significant effects on the abundance of soil nematodes, but only on two nematode ecological indices(λ and MI).     

The nematode-trapping fungus Arthrobotrys oligospora in soilof the Bodega marine reserve: distribution and dependenceon nematode-parasitized moth larvae

Of the 13 nematode-trapping fungi previously detected at the Bodega Marine Reserve (BMR, Sonoma County, CA, USA), Arthrobotrys oligospora is by far the most abundant. Why A. oligospora is so abundant is unclear, but the answer may involve bush lupines (Lupinus arboreus), ghost moth larvae (Hepialus californicus), and insect-parasitic nematodes (Heterorhabditis marelatus). Previous research documented a dramatic increase of A. oligospora in BMR soil with the addition of an H. marelatus-parasitized moth larva. The current study tested two predictions based on the hypothesis that the H. marelatus-parasitized ghost moth larva is a unique and important resource for A. oligospora at BMR. First, because ghost moth larvae are concentrated in soil under bush lupines, we predicted that A. oligospora numbers would be greater under lupines than away from lupines. Second, we predicted that A. oligospora would be enhanced more by moth larvae containing living H. marelatus than by moth larvae containing dead H. marelatus or no H. marelatus or by nematodes alone. The first prediction was supported by data from a field study (A. oligospora population density was greater beneath lupines than in grasses 2 m away), but the difference was small. The second prediction was not supported by data from a laboratory experiment (dead moth larvae caused dramatic increases in A. oligospora numbers whether or not the dead moth larvae contained living nematodes). While H. marelatus are clearly unnecessary for the large increase in A. oligospora numbers, the importance of nematodes in general remains unclear because addition of dead moth larvae always resulted in large increases in bacterivorous nematodes and because addition of nematodes alone enhanced A. oligospora in one trial but not in two others.     

Diversity for abiotic and biotic stress resistance in the wild annual Cicer species

Data on 228 accessions of eight annual wild Cicer species and 20 cultivated chickpea check lines were evaluated for diversity in response to six of the most serious biotic and abiotic stresses which reduce crop yield and production stability of chickpea, i.e., ascochyta blight, fusarium wilt, leaf miner, bruchid, cyst nematode, and cold. Relative frequencies of score reactions to the above six stresses were recorded from all the annual wild Cicer species and the cultivated taxon. Patterns of distribution and amount of variation of the resistance reactions differed between stresses and species. C. bijugum, C. pinnatifidum and C. echinospermum showed accessions with at least one source of resistance (1 to 4 score reactions) to each stress. Overall, C. bijugum showed the highest frequencies of the highest categories of resistance. Next in performance was C. pinnatifidum followed by C. judaicum, C. reticulatum and C. echinospermum. Furthermore, C. bijugum had the highest number of accessions with multiple resistance to the six stresses; two accessions were resistant to five stresses and 16 to four. According to the Shannon-Weaver diversity indices (H), five species showed discrete mean diversity indices which varied from 0.649 in C. pinnatifidum to 0.526 in C. judaicum, whereas C. chorassanicum, C. cuneatum and C. yamashitae showed the lowest Hs, which were respectively 0.119, 0.174 and 0.216. Pair-wise correlations among the six biotic and abiotic stresses showed the possibility of combining these resistances. Interestingly, multiple resistant accessions were predominantly of Turkish origin.     

Suppression of Meloidogyne javanica by Pseudomonas aeruginosa IE-6S in tomato: the influence of NaCl, oxygen and iron levels

Understanding the environmental factors that influence the suppression of disease-suppressive strains of Pseudomonas fluorescens is an essential step toward improving the level and reliability of their biocontrol activity. A 0.8 M NaCl concentration was optimal for in vitro survival and growth of IE-6S⁺ while, nematicidal activity by IE-6S⁺ was maximal when the bacterium was exposed to 0.4 M NaCl. The bacterium was highly sensitive to high (1.6 M) NaCl concentration. Culture filtrate of the bacterium resulting from the medium supplemented with 0.2 or 0.4 M NaCl showed the presence of secondary metabolite, hydrogen cyanide (HCN). Soil amendment with IE-6S⁺ alone or in conjunction with up to 0.8 M NaCl enhanced bacterial efficacy towards Meloidogyne javanica, the root-knot nematode. Soil amendment with NaCl up to 0.8 M also resulted in enhanced bacterial rhizosphere colonization and growth of tomato seedlings. Protein content of the shoot was reduced when soil was amended with 1.6 M NaCl. Inner root establishment of the bacterium was greatly affected in the soils treated with 1.6 M NaCl. Under in vitro conditions, IE-6S⁺ showed enhanced growth when kept at ambient oxygen conditions while the growth of bacterium affected when incubated at low oxygen conditions. Culture filtrate of the bacterium resulting from low oxygen level caused greater mortality of M. javanica juveniles in vitro compared with the filtrates obtained from ambient oxygen conditions. Culture filtrate from low oxygen conditions also showed the presence of hydrogen cyanide while those from ambient oxygen condition did not. Under glasshouse conditions, regardless of bacterial application, nematode penetration rate was greater when the pots were watered from the top; nematode penetration was lowered in bacterized pots compared with non-bacterized controls. IE-6S⁺ applied in the pots either watered from the top or bottom had no significant impact on growth of tomato but protein contents of the leaves increased after treatment with the bacterium. Rhizosphere and inner root colonization of the bacterium increased when the pots were watered from the top. Under in vitro conditions, with an increased iron concentration in the form of FeEDDHA, growth of IE-6S⁺ and its nematicidal activity increased. Culture filtrate of IE-6S⁺ obtained from liquid King's B medium supplemented with 0.5 or 1.0 mM FeEDDHA showed the presence of HCN. Under glasshouse conditions, soil treated with FeEDDHA alone did not reduce nematode penetration rates but did reduce greatly when applied in conjunction with IE-6S⁺. FeEDDHA applied at 0.5 mg/kg of soil in combination with IE-6S⁺ significantly enhanced plant growth and leaf protein contents. FeEDDHA at 1 mg/kg of soil increased bacterial populations both in the rhizosphere and inner root tissues of tomato.     

Effect of a multispecies nematode population on the root, corm, and shoot growth of East African Musa genotypes

A study to determine the impact of a multispecies nematode population on the root, corm, and shoot growth of East African Musa genotypes was carried out. Eight genotypes comprising the plantain “Gonja” (Musa AAB group), the dessert banana “Sukali Ndizi” (AAB), the beer banana “Kayinja” (ABB), and five East African Highland bananas (AAA-EA) were assessed at flower emergence of the plant crop. Root damage and plant growth characteristics were assessed on both infected and noninfected plants. This study showed that Radopholus similis and Helicotylenchus multicinctus were recovered in highest numbers from infected mother plants. Significantly (P<0.05) higher shoot and root damage was observed in the infested plot compared to the noninfested plot. The differences in nematode damage observed among the different Musa genotypes confirm the variability in susceptibility to nematodes. Most of the East African Highland bananas and “Gonja” had a significant reduction in root system size. In addition, toppling and lengthening of the period to flower emergence of the plant crop was much more pronounced in these genotypes. In contrast to previous reports, this study indicated that a multispecies nematode infection significantly (P<0.05) reduced shoot and root growth of “Kayinja”. “Sukali Ndizi”, however, was observed to be tolerant to nematode infection as a small reduction in the root system size was associated with a negligible effect on the corm and shoot growth characteristics. Therefore, the percentage reduction in root and shoot growth due to nematode infection is not constant but depends on the plant genotype.     

Influence of temperature on the motility of Pseudomonas oryzihabitans and control of Globodera rostochiensis

The motility and efficacy of Pseudomonas oryzihabitans as a biocontrol agent against the potato cyst nematode Globodera rostochiensis were studied with respect to temperature. The influence of soil moisture on bacterial movement was also tested. In a closed container trial, P. oryzihabitans significantly reduced invasion of second stage juveniles (J2) of G. rostochiensis in potato roots, its effect being more marked at 25 and 21 °C than at 17 °C. P. oryzihabitans motility in vitro was optimal at 26 °C and inhibited at temperatures below 18 °C. In soil, both temperature and matric potential affected bacterial movement. At 16 °C its movement and survival were suppressed, but they were unaffected at 25 °C. At both temperatures the biocontrol agent moved faster in the wetter (−0.03 MPa) than in the drier soil (−0.1 MPa). These results suggest that temperature is a key factor in determining the potential of P. oryzihabitans as a biocontrol agent.     

一个水稻类感干尖线虫卷叶突变体的遗传分析与基因定位

筛选和鉴定水稻卷叶突变体是研究叶片形态建成机理的前提.本研究利用甲基磺酸乙酯对常规粳稻秀水09进行诱变,获得了一个稳定遗传的类感干尖线虫卷叶突变体(nematode infestation mimic rolling leaf mutant,nir).与野生型相比,nir全生育期叶尖卷曲、枯萎,表型与干尖线虫侵染症状相似.此外,突变体植株矮小,叶片表面较光滑,有效分蘖数少.采用nir与秀水09杂交的F2群体进行遗传分析,结果表明nir的类感干尖线虫卷叶性状受一对隐性核基因控制.利用nir与籼稻93-11杂交获得的F2群体,采用分子标记技术将目的基因最终定位在水稻第二染色体2 ～ 88w和2～82w之间,遗传距离为1.1 cM,物理距离245 kb.研究结果表明,nir很可能是一个尚未报道的新基因,进一步克隆nir基因有可能揭示与现有报道的卷叶基因不同的卷叶调控机理,为筛选理想株型、培育高产水稻品种提供理论基础和供试材料.     

农杆菌介导抗根结线虫Bt cry6A基因转化茄子的研究

本试验以茄子下胚轴和子叶为外植体,通过对抗生素选择压筛选、不同抗生素抑菌效果比较、农杆菌侵染时间、预培养时间、共培养时间等条件优化,建立了适用于茄子的遗传转化体系,并利用农杆菌介导法将抗根结线虫Bt cry6A基因导入茄子中,以期获得抗线虫的茄子材料,探索新的抗线虫育种途径,为利用茄子转基因技术培育和改良茄子品质提供理论依据.结果表明,75mg· L-1的卡那霉素可以有效地抑制愈伤组织的产生;200mg· L-1特美汀能够完全抑制农杆菌的生长;最佳侵染外植体为子叶,在预培养4d,侵染10min,共培养1d时转化率最高;获得的56株Kan抗性植株中有15株具有GUS活性;经过PCR、RT-PCR验证,初步确认Bt cry6A基因已整合至茄子基因组中,并在转录水平上正常表达;根结线虫幼虫接种鉴定表明,转Bt cry6A基因茄子再生植株对南方根结线虫抗性明显提高.