玉米根冠细胞的脱落及其对根与根际关系的影响

以光学与电子显微镜技术观察了玉米幼苗根冠细胞脱落进程与胞间关系的变化。发现随着外沿细胞的脱离,固有的胞间连丝渐次伸展、孔径扩展,进而断裂为半连丝。半连丝内侧端仍与质膜保持固有的连接;外侧端向壁外伸展,直接与根际介质沟通,从而在根冠共质体与根际质外体之间建立了动态性的单向共质联系。以成列囊泡在半连丝中存在与由断口端溢出、半连丝呈现活跃ＡＴＰ酶活性、外源不透膜示踪物向胞内原生质掺入以及原生质的穿壁等实例证明,半连丝仍具生理活性,可作为根冠共质体与根际介质间物质与信息交流的直接通道。     

Rhizosphere Microbial Community Structure in Relation to Root Location and Plant Iron Nutritional Status

Root exudate composition and quantity vary in relation to plant nutritional status, but the impact of the differences on rhizosphere microbial communities is not known. To examine this question, we performed an experiment with barley (Hordeum vulgare) plants under iron-limiting and iron-sufficient growth conditions. Plants were grown in an iron-limiting soil in root box microcosms. One-half of the plants were treated with foliar iron every day to inhibit phytosiderophore production and to alter root exudate composition. After 30 days, the bacterial communities associated with different root zones, including the primary root tips, nonelongating secondary root tips, sites of lateral root emergence, and older roots distal from the tip, were characterized by using 16S ribosomal DNA (rDNA) fingerprints generated by PCR-denaturing gradient gel electrophoresis (DGGE). Our results showed that the microbial communities associated with the different root locations produced many common 16S rDNA bands but that the communities could be distinguished by using correspondence analysis. Approximately 40% of the variation between communities could be attributed to plant iron nutritional status. A sequence analysis of clones generated from a single 16S rDNA band obtained at all of the root locations revealed that there were taxonomically different species in the same band, suggesting that the resolving power of DGGE for characterization of community structure at the species level is limited. Our results suggest that the bacterial communities in the rhizosphere are substantially different in different root zones and that a rhizosphere community may be altered by changes in root exudate composition caused by changes in plant iron nutritional status.     

Tree Species Composition Influences Enzyme Activities and Microbial Biomass in the Rhizosphere: A Rhizobox Approach

Monoculture causes nutrient losses and leads to declines in soil fertility and biomass production over successive cultivation. The rhizosphere, a zone of usually high microbial activities and clearly distinct from bulk soil, is defined as the volume of soil around living roots and influenced by root activities. Here we investigated enzyme activities and microbial biomass in the rhizosphere under different tree compositions. Six treatments with poplar, willow, and alder mono- or mixed seedlings were grown in rhizoboxes. Enzyme activities associated with nitrogen cycling and microbial biomass were measured in all rhizosphere and bulk soils. Both enzyme activities and microbial biomass in the rhizosphere differed significantly tree compositions. Microbial biomass contents were more sensitive to the changes of the rhizosphere environment than enzyme activities. Tree species coexistence did not consistently increase tested enzyme activities and microbial biomass, but varied depending on the complementarities of species traits. In general, impacts of tree species and coexistence were more pronounced on microbial composition than total biomass, evidenced by differences in microbial biomass C/N ratios stratified across the rhizosphere soils. Compared to poplar clone monoculture, other tree species addition obviously increased rhizosphere urease activity, but greatly reduced rhizosphere L-asparaginase activity. Poplar growth was enhanced only when coexisted with alder. Our results suggested that a highly productive or keystone plant species in a community had greater influence over soil functions than the contribution of diversity.     

Effect of Four Organophosphorus Insecticides on Microbial Activities in Soil

Laboratory tests were conducted with four organophosphorus insecticides, Bay 37289 (O-ethyl O-2,4,5-trichlorophenyl ethylphosphonothioate), diazinon [O,O-diethyl O-(2-isopropyl-4-methyl-6-pyrimidinyl) phosphorothioate], Dursban (O,O-diethyl O-3,5,6-trichloro-2-pyridyl phosphorothioate), and Zinophos (O,O-diethyl O-2-pyrazinyl phosphorothioate), applied to a sandy loam at rates of 10 and 100 mug/g to determine whether these materials caused any serious effects on microbial activities related to soil fertility. All insecticides showed an effect on fungi and bacteria for the first and second week of incubation, but, subsequently, the populations returned to levels similar to those obtained in the controls. All insecticide applications increased ammonium production, but, in some instances, there appeared to be a slight depression of nitrification. Sulfur oxidation was equal to or better than that obtained with untreated soil in most cases. There was no significant effect on phosphorus mineralization. Oxygen consumption indicated that microbial respiration increased in proportion to the concentration of insecticides, suggesting the possibilities of microbial degradation of the insecticides or their degradation products and of uncoupling oxidative phosphorylation.     

根腐病对茅苍术根际微生物群落多样性的影响

研究茅苍术根腐病与根际土壤微生物群落之间的关系,为茅苍术根腐病微生物调控防治技术提供依据。比较了未种植茅苍术土壤、茅苍术健康植株及根腐病病株根际土壤的土壤养分和酶活性变化,并结合Illumina MiSeq高通量测序,研究了茅苍术根腐病土壤中细菌和真菌的群落结构变化。茅苍术感染根腐病后,植株根际土壤pH降低,有机质、速效氮和速效磷显著增加;蔗糖酶活性增加,过氧化氢酶活性降低;细菌和真菌的多样性和丰度均显著降低。病原菌如镰刀菌属(Fusarium)和阿太菌属(Athelia)的丰度显著增加,而被孢霉属(Mortierella)、青霉属(Penicillium)和假单胞菌属(Pseudomonas)等病原微生物拮抗菌的丰度降低,这可能是诱发茅苍术根腐病的主要原因。通过分析细菌、真菌多样性及群落构成变化,表明多样性指数显著降低和群落结构失衡的打破可能和土壤pH、养分和酶活性的变化密切相关。     

Rhizosphere Microbial Characterization in Petroleum-Contaminated Soil

Contamination of soil with petroleum compounds is of concern worldwide. Although there are a variety of physical and chemical technologies available to remediate petroleum waste sites, biological methods are often used due to lower cost and public acceptance. Growth and enhanced activity of microbial communities in contaminated soil is a key factor for the success of bioremediation. Establishing vegetation in petroleum-contaminated soil may enhance microbial activity and remediation success even further by providing root exudates to the rhizosphere microorganisms. In this study, microorganisms were characterized in petroleum-contaminated soils and sediments quantitatively and qualitatively based on enumeration and metabolic diversity assessments. Contaminated soils and sediments were obtained from a phytoremediation field demonstration project in California. Microbial numbers in the unvegetated soil, based on plate counts and most probable number of hydrocarbon degraders, were significantly lower than the vegetated soils. Metabolic microbial characterization using BIOLOG was also conducted and based on principle component analysis (PCA), there was a distinct difference between the metabolic diversity of microbial communities in vegetated and unvegetated soils. Results from this research indicate that the presence and type of plants, and level of contamination may greatly influence microbial communities in polluted soils.     

氮沉降及菌根真菌对长白落叶松苗木根系构型及根际酶活性的影响

为探究氮沉降和接种菌根真菌对长白落叶松苗木根系构型和根际土壤酶活性的影响,以1年生长白落叶松(Larix olgensis)的盆栽菌根苗(简称+M,混合接种8种外生菌根真菌)和非菌根苗(简称-M,未接种处理)为研究对象,设置4个氮沉降处理(不施氮(0N,0 kg·N·hm^-1·yr^-1)、低氮(LN,15 kg·N·hm^-1·yr^-1)、中氮(MN,30 kg·N·hm^-1·a^-1)和高氮(HN,60 kg·N·hm^-1·a^-1)),测定直径0~0.5 mm根系的总根长、总表面积、总体积和根尖数等根系形态指标,对比分析氮沉降和接种菌根真菌处理对苗木根际土壤酶(β-1,4葡萄糖苷酶(BG)、亮氨酸氨基肽酶(LAP)、β-1,4-N-乙酰-氨基葡糖氨糖苷酶(NAG)、酸性磷酸酶(ACP)和碱性磷酸酶(ALP))活性的影响。结果表明:①长白落叶松苗木直径0~0.5 mm根系的总根长、总表面积、总体积和根尖数均随氮浓度的递增呈下降的趋势;在0N、LN和MN处理下,-M处理的根系形态指标均高于+M处理。②随氮浓度增加,+M和-M处理苗木根际土壤中BG、LAP、ACP和ALP活性均呈先增加后下降的趋势,而NAG活性呈下降的趋势。③+M和-M处理下,长白落叶松直径0~0.5 mm根系的形态指标与根际土壤BG活性均呈显著负相关关系(P<0.05);除根尖数外,其它根系形态指标与NAG活性相关性均为正相关(P<0.05)。综上所述,苗木菌根化处理削弱了氮沉降对落叶松苗木根系构型的影响;而低氮处理下,+M处理对苗木根际土壤酶的活化程度高于-M处理。     

Tylenchulus semipenetrans Alters the Microbial Community in the Citrus Rhizosphere

Infection of citrus seedlings by Tylenchulus semipenetrans was shown to reduce subsequent infection of roots by Phytophthora nicotianae and to increase plant growth compared to plants infected by only the fungus. Hypothetical mechanisms by which the nematode suppresses fungal development include nutrient competition, direct antibiosis, or alteration of the microbial community in the rhizosphere to favor microorganisms antagonistic to P. nicotianae. A test of the last hypothesis was conducted via surveys of five sites in each of three citrus orchards infested with both organisms. A total of 180 2-cm-long fibrous root segments, half with a female T. semipenetrans egg mass on the root surface and half without, were obtained from each orchard site. The samples were macerated in water, and fungi and bacteria in the suspensions were isolated, quantified, and identified. No differences were detected in the numbers of microorganism species isolated from nematode-infected and uninfected root segments. However, nematode-infected root segments had significantly more propagules of bacteria at all orchard sites. Bacillus megaterium and Burkholderia cepacia were the dominant bacterial species recovered. Bacteria belonging to the genera Arthrobacter and Stenotrophomonas were encountered less frequently. The fungus community was dominated by Fusarium solani, but Trichoderma, Verticillum, Phythophthora, and Penicillium spp. also were recovered. All isolated bacteria equally inhibited the growth of P. nicotianae in vitro. Experiments using selected bacteria, T. semipenetrans, and P. nicotianae, alone or in combination, were conducted in both the laboratory and greenhouse. Root and stem fresh weights of P. nicotianae-infected plants treated with T. semipenetrans, B. cepacia, or B. megaterium were greater than for plants treated only with the fungus. Phytophthora nicotianae protein in roots of fungus-infected plants was reduced by nematodes (P ≤ 0.001), either alone or in combination with either bacterium. However, treatment with bacteria did not affect P. nicotianae development in roots. The results suggest different mechanisms by which T. semipenetrans, B. cepacia, and B. megaterium may mitigate virulence of P. nicotianae.     

嫁接辣椒根系特征及根际土壤酶活性与青枯病抗性的关系

以‘卫士’辣椒为砧木,‘新丰2号’为接穂嫁接,通过人工接种青枯病菌研究嫁接和自根辣椒根系特征、根际土壤微生物及酶活性的变化,探讨嫁接辣椒的抗病机理.结果显示:接种青枯病菌前,嫁接辣椒的根系重量、总长度、总体积、表面积、根尖数和分叉数均显著高于自根苗,根系活力、根际土壤放线菌数量和比例,以及根际土壤酶(多酚氧化酶、过氧化物酶和脱氢酶)活性也明显高于自根苗.接种青枯病菌后,嫁接辣椒的根系受伤程度较自根苗轻,根系重量、总长度、总体积、表面积、根尖数和分叉数的降低幅度均显著小于自根苗,根系活力、根际土壤微生物数量、放线菌比例及土壤酶活性明显大于自根苗.研究表明,嫁接辣椒根系发达,根系活力增强,根际土壤放线菌比例增加及酶活性提高是其青枯病抗性增强的重要原因.     

Functional Potential of Soil Microbial Communities in the Maize Rhizosphere

Microbial communities in the rhizosphere make significant contributions to crop health and nutrient cycling. However, their ability to perform important biogeochemical processes remains uncharacterized. Here, we identified important functional genes that characterize the rhizosphere microbial community to understand metabolic capabilities in the maize rhizosphere using the GeoChip-based functional gene array method. Significant differences in functional gene structure were apparent between rhizosphere and bulk soil microbial communities. Approximately half of the detected gene families were significantly (p<0.05) increased in the rhizosphere. Based on the detected gyrB genes, Gammaproteobacteria, Betaproteobacteria, Firmicutes, Bacteroidetes and Cyanobacteria were most enriched in the rhizosphere compared to those in the bulk soil. The rhizosphere niche also supported greater functional diversity in catabolic pathways. The maize rhizosphere had significantly enriched genes involved in carbon fixation and degradation (especially for hemicelluloses, aromatics and lignin), nitrogen fixation, ammonification, denitrification, polyphosphate biosynthesis and degradation, sulfur reduction and oxidation. This research demonstrates that the maize rhizosphere is a hotspot of genes, mostly originating from dominant soil microbial groups such as Proteobacteria, providing functional capacity for the transformation of labile and recalcitrant organic C, N, P and S compounds.     

Microbial Complexes Occurring on the Apogeotropic Roots and in the Rhizosphere of Cycad Plants

The microbial complexes of soil, the rhizosphere, and the rhizoplane of the apogeotropic (coralloid) roots of cycad plants were comparatively studied. The aseptically prepared homogenates of the surface-sterilized coralloid roots did not contain bacterial microsymbiont, indicating that in the root tissues the symbiosis is a two-component one (plant–cyanobacteria). At the same time, associated bacteria belonging to different taxonomic groups were detected in increasing amounts in the cycad rhizoplane, rhizosphere, and the surrounding soil. The bacterial communities found in the cycad rhizoplane and the surrounding soil were dominated by bacteria from the genus Bacillus. The saprotrophic bacteria and fungi colonizing the cycad rhizosphere and rhizoplane were dominated by microorganisms capable of degrading the plant cell walls. The local degradation of the cell wall was actually observed on the micrographs of the thin sections of cycad roots in the form of channels through which symbiotic cyanobacterial filaments can penetrate into the cortical parenchyma.     

复合微生态菌剂对黄瓜根际土壤微生物数量及酶活的影响

采用盆栽试验的方法,研究黄瓜移苗时施入复合微生态菌剂对黄瓜不同生育时期根际土壤中微生物数量和土壤酶活动态变化的影响,结果显示复合微生态菌剂施入前期可提高土壤中细菌和放线菌的数量,随生育时期的延长效果变弱,于生育后期可有效降低真菌和尖孢镰刀菌的数量;复合微生态菌剂处理可提高土壤脲酶、过氧化氢酶、蔗糖酶和中性磷酸酶的活性,但随生育时期的延长,对中性磷酸酶和脲酶的效果减弱。可见复合土壤微生态菌剂的施入应该于适宜时期补施,才能保证其积极的功效。     

Effect of a Sinorhizobium meliloti Strain with a Modified putA Gene on the Rhizosphere Microbial Community of Alfalfa

The success of a rhizobial inoculant in the soil depends to a large extent on its capacity to compete against indigenous strains. M403, a Sinorhizobium meliloti strain with enhanced competitiveness for nodule occupancy, was recently constructed by introducing a plasmid containing an extra copy of a modified putA (proline dehydrogenase) gene. This strain and M401, a control strain carrying the same plasmid without the modified gene, were used as soil inoculants for alfalfa in a contained field release experiment at León, Spain. In this study, we determined the effects of these two strains on the indigenous microbial community. 16S rRNA genes were obtained from the rhizosphere of alfalfa inoculated with strain M403 or strain M401 or from noninoculated plants by amplification of DNA from soil with bacterial group-specific primers. These genes were analyzed and compared by restriction fragment length polymorphism and temperature gradient gel electrophoresis. The results allowed us to differentiate between alterations in the microbial community apparently caused by inoculation and by the rhizosphere effect and seasonal fluctuations induced by the alfalfa plants and by the environment. Only moderate inoculation-dependent effects could be detected, while the alfalfa plants appeared to have a much stronger influence on the microbial community.     

Effect of Temperature on the Activities of Cytochrome c Oxidase and Respiration in Cassava Root Mitochondria

Cosmid pR4Cl is a derivative of multicopy plasmid pIJ365 which has an insertion of the cos (cohesive end site) region of actinophage R4 [T. Morino, H. Takahashi and H. Saito, Mol. Gen. Genet., 198, 228 (1985)]. When the donor R4 phage was propagated in S. lividans carrying the plasmid, the phage lysate contained transducing particles which encapsulated head-to-tail concatemers of the plasmid DNA. These particles could mediate transfer of the plasmid at a high frequency. We examined conditions that gave a maximum transduction frequency of cosmid pR4Cl. Conditions which depress R4 phage propagation, such as incubation of recipient S. parvulus at a high temperature, improved the frequency. Obviously such conditions minimized the lethal effect of viable phage propogation. The highest transduction frequency obtained so far was around 3 × 10^-3 transductants per infected phage when S. lividans was used as the recipient. This was about 30 per cent of the cosmid transducing particles estimated from the cosmid DNA content in the transducing lysate. The significance of cosmid transduction for gene manipulation in Streptomyces strains is also discussed.     

Molecular Studies on the Role of a Root Surface Agglutinin in Adherence and Colonization by Pseudomonas putida

Pseudomonas putida aggressively colonizes root surfaces and is agglutinated by a root surface glycoprotein. Mutants of P. putida derived chemically or by Tn5 insertion demonstrated enhanced or decreased agglutinability. Two nonagglutinable Tn5 mutants (Agg⁻) and two mutants with enhanced agglutinability (Agg^s) possessed Tn5 in unique restriction sites. Agg⁻ mutants colonized root surfaces of seedlings grown from inoculated seeds, but at levels lower than those observed with the Agg⁺ parent. In short-term binding studies, Agg⁻ cells adhered at levels that were 20- to 30-fold less than those for Agg⁺ parental cells. These data suggest that the agglutination interaction plays a role in the attachment of P. putida to root surfaces.     

Effects of Microbial Community Diversity on the Survival of Pseudomonas aeruginosa in the Wheat Rhizosphere

Ecological theory suggests that microbial communities with greater microbial diversity would be less susceptible to invasion by potential opportunistic pathogens. We investigated whether the survival of the opportunistic pathogen Pseudomonas aeruginosa in the wheat rhizosphere would be affected by the presence of natural and constructed microbial communities of various diversity levels. Three levels of microbial community diversity were derived from wheat roots by a dilution/extinction approach. These wheat rhizosphere inocula, as well as a gnotobiotic microbial community consisting of seven culturable wheat rhizobacterial isolates, were introduced into the nutrient solution of hydroponically grown wheat plants on the day of planting. Phenotypic characterization of the culturable microbial communities on R2A medium, Shannon microbial diversity index, community-level physiological profiles, and terminal restriction fragment length polymorphisms were used to assess the varying microbial diversity levels. At day 7 the roots were invaded with P. aeruginosa and the number of P. aeruginosa colony forming units per root were measured at day 14. The average number of surviving P. aeruginosa cells was 3.52, 4.90, 7.18, 6.65 log₁₀ cfu/root in the high, medium, low, and gnotobiotic microbial community diversity level treatments, respectively. The invasibility of the rhizosphere communities by P. aeruginosa was inversely related to the level of diversity from the dilution extinction gradient. The gnotobiotic community did not confer protection against P. aeruginosa invasion. Although these data indicate that invasibility is inversely related to diversity, further study is needed to both reproduce these findings and define the specific mechanisms of the diversity effect.     

VA菌根真菌Glomus mosseae对棉花根区微生物量和生物量的影响

ＶＡ菌根真菌Ｇｌｏｍｕｓｍｏｓｓｅａｅ对棉花根区微生物量和生物量的影响顾向阳,胡正嘉（华中农业大学土化系,武汉４３００７０）ＥｆｆｅｃｔｏｆＶＡＭＦｕｎｇｕｓＧｌｏｍｕｓｍｏｓｓｅａｅｏｎＭｉｃｒｏｂｉａｌＰｏｐｕｌａｔｉｏｎａｎｄＢｉｏｍａｓｓｉｎ...