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18 páginas.- 6 figuras.-1 tabla.- 67 referencias..- Supplementary Information The online version contains supplementary material available at https://doi.org/10.1007/s11104-023-05981-0 .- Prepint de este artículo en https://www.biorxiv.org/content/10.1101/2023.02.24.529317v1, Aims: Soil-borne pathogens severely damage the yield and quality of crops worldwide. Plant and soil microbiomes (e.g. in the rhizosphere) intimately interact with the plant, the pathogen and influence outcomes of disease infection. Investigation of how these microbiomes respond to disease infection is critical to develop solutions to control diseases. Methods: Here, we conducted a field experiment and collected healthy and crown rot disease infected (caused by Fusarium pseudograminearum, Fp) wheat plants. We investigated their microbiomes in different compartments, plant immune responses and interactions with the pathogen (Fp) aiming at advancing our knowledge on microbiome-mediated regulation of plant responses to pathogens. Results: We found that Fp colonised wheat plants significantly in terms of relative abundances, accounting for 11.3% and 60.7% of the fungal communities in the rhizosphere and roots, respectively. However, Fp presented with a small fraction of the leaf microbiome, up to 1.2%. Furthermore, Fp-infection led to significant changes in the composition of microbiomes in the rhizosphere and root while had little impact on leaves. We further found that wheat defence signalling pathways, microbiomes and the pathogen intimately correlated with each other in structural equation modelling. As such, we also identified ecological clusters explained changes in the wheat defence signalling pathways. Lastly, microbial co-occurrence network complexity was higher in Fp-infected plants relative to healthy plants, suggesting that Fp-infection may have led to changes in the wheat microbial community structure. Conclusions: We provide novel evidence that soil-borne diseases disrupt belowground plant microbiomes influencing the responses of plant immunity to pathogens. © 2023, The Author(s), under exclusive licence to Springer Nature Switzerland AG.

AimsPhyllosphere bacteria play critical roles in plant growth promotion, disease suppression and global nutrient cycling but remain understudied.MethodsIn this project, we examined the bacterial community on the phyllosphere of eight diverse lines ofBrassica napusfor ten weeks in Saskatoon, Saskatchewan Canada.ResultsThe bacterial community was shaped largely by plant growth stage with distinct communities present before and after flowering. Bacterial diversity before flowering had 111 core members with high functional potential, with the peak of diversity being reached during flowering. After flowering, bacterial diversity dropped quickly and sharply to 16 members of the core community, suggesting that the plant did not support the same functional potential anymore.B. napusline had little effect on the larger community, but appeared to have more of an effect on the rare bacteria.ConclusionsOur work suggests that the dominant bacterial community is driven by plant growth stage, whereas differences in plant line seemed to affect rare bacteria. The role of these rare bacteria in plant health remains unresolved.

Made available in DSpace on 2021-06-25T10:53:33Z (GMT). No. of bitstreams: 0 Previous issue date: 2021-01-01 Aims: Xanthomonas campestris pv. campestris (Xcc) survival was evaluated in the phyllosphere and rhizosphere of 20 crop species, as well as in the rhizosphere of cabbage cultivated in six soil types. Methods: The crops aerial parts were inoculated, and the rhizosphere soil was infested with bacterial suspension (107 CFU.mL−1) of 3098C Xcc strain, which is rifampicin-resistant and pathogenic to brassica plants. Xcc recovery from the samples was performed in a semi-selective culture medium. Results: The highest average survival periods of Xcc in the phyllosphere were obtained from cabbage (70 days), tomato (67 days), wheat and pumpkin (63 days), black oat and radish (56 days) and sweet pepper (49 days). Low Xcc survival occurred in the rhizosphere of most crops evaluated when compared to the soil in the absence of plants. The highest average period of survival in the rhizosphere was observed for cabbage (53 days). Survival periods in lettuce, pumpkin, wheat and mustard ranged from 21 to 28 days. In the rhizosphere of cabbage cultivated in six soils, survival periods ranged from 8 to 17 days. Conclusions: Sunflower, carrot, zucchini, spinach, sunn hemp, sorghum, maize and chive are recommended for rotating systems with brassicas because Xcc showed low survival in the phyllosphere and rhizosphere of these crops. Departamento de Proteção Vegetal Faculdade de Ciências Agronômicas (FCA) Universidade Estadual Paulista “Júlio de Mesquita Filho” (UNESP), Avenida Universitária, 3780 Departamento de Produção e Melhoramento Vegetal Faculdade de Ciências Agronômicas (FCA) Universidade Estadual Paulista “Júlio de Mesquita Filho” (UNESP), Avenida Universitária, 3780 Departamento de Proteção Vegetal Faculdade de Ciências Agronômicas (FCA) Universidade Estadual Paulista “Júlio de Mesquita Filho” (UNESP), Avenida Universitária, 3780 Departamento de Produção e Melhoramento Vegetal Faculdade de Ciências Agronômicas (FCA) Universidade Estadual Paulista “Júlio de Mesquita Filho” (UNESP), Avenida Universitária, 3780

The development and dispersal of seeds as well as their transition to seedlings represent perhaps the most critical stages of a plant’s life cycle. The endophytic and epiphytic microbial interactions that take place in, on, and around seeds during these stages of the plant’s life cycle may have profound impacts on plant ecology, health, and productivity. While our understanding of the seed microbiota has lagged far behind that of the rhizosphere and phyllosphere, many advances are now being made. This review explores the microbial associations with seeds through various stages of the plant life cycle, beginning with the earliest stages of seed development on the parent plant and continuing through the development and establishment of seedlings in soil. This review represents a broad synthesis of the ecological and agricultural literature focused on seed-microbe interactions as a means of better understanding how these interactions may ultimately influence plant ecology, health, and productivity in both natural and agricultural systems. Our current understanding of seed-microbe associations will be discussed, with an emphasis on recent findings that specifically highlight the emerging contemporary understanding of how seed-microbe associations may ultimately impact plant health and productivity. The diversity and dynamics of seed microbiomes represent the culmination of complex interactions with microbes throughout the plant life cycle. The richness and dynamics of seed microbiomes is revealing exciting new opportunities for research into plant-microbe interactions. Often neglected in plant microbiome studies, the renaissance of inquiry into seed microbiomes is offering exciting new insights into how the diversity and dynamics of the seed microbiome with plant and soil microbiomes as well as the microbiomes of dispersers and pollinators. It is clear that the interactions taking place in and around seeds indeed have significant impacts on plant health and productivity in both agricultural and natural ecosystems.

To assess the impacts of soil microbes and plant genotype on the composition of maize associated bacterial communities. Two genotypes of Brazilian maize were planted indoors on sterile sand, a deep underground subsoil, and a nutrient-rich topsoil from the Amazon jungle (terra preta). DNA was extracted from rhizospheres, phyllospheres, and surface sterilized roots for 16S rDNA fingerprinting and next generation sequencing. Neither plant genotype nor soil type appeared to influence bacterial diversity in phyllospheres or endospheres. Rhizospheres showed strikingly similar 16S rDNA ordination of both fingerprinting and sequencing data, with soil type driving grouping patterns and genotype having a significant impact only on sterile sand. Rhizospheres grown in non-sterile soils contained greater bacterial diversity than sterile-sand grown ones, however the dominant OTUs (species of Proteobacteria and Bacteroidetes) were found in all rhizospheres suggesting seeds as a common source of inoculum. Rhizospheres of the commercial hybrid appeared to contain less bacterial diversity than the landrace. Maize rhizospheres receive diverse bacteria from soil, are influenced by the genotype or treatment of the seed, and are dominated by species of Proteobacteria, Actinobacteria, Bacteroidetes, and Firmicutes. As many dominant 16S rDNA sequences were observed in rhizospheres grown in both sterile and non-sterile substrate, we conclude that the most common bacterial cells in juvenile maize rhizospheres are seed transmitted.

The objective of this study was to elucidate the composition of bacterial communities from the soil, and endosphere and phyllosphere of upper and lower leaves and clarify the responses to elevated CO2 and/or soil temperature. Using 454 pyrosequencing, the 16S rRNA gene was analyzed from various bacterial communities in a rice paddy that was exposed to different atmospheric CO2 concentrations (ambient, +200 μmol.mol−1) and soil temperatures (ambient, +2 °C). The treatments of elevated temperature and elevated CO2 plus temperature exerted significant influence on the structure of bacterial communities from the lower leaf endosphere. A significant influence of elevated CO2 plus temperature on the community structure was also observed in the upper leaf phyllosphere. The richness and diversity of bacterial communities from the lower leaf phyllosphere, upper leaf endosphere, and upper leaf phyllosphere were significantly affected by elevated CO2 plus temperature. However, we did not observe any significant influence of all climate change treatments (elevated CO2, elevated temperature, and their combination) on the richness, diversity, and structure of soil bacterial communities. We also did not observe any significant effect of the single factor, elevated CO2, on the structure of the leaf endosphere and phyllosphere bacterial communities. Enterobacteriaceae and Xanthomonadaceae were the most shifted phylotypes in response to elevated temperature and elevated CO2 plus temperature. Soil bacterial communities were more resistant to the tested climate change factors compare with foliar bacterial communities. Temperature was a more important factor in shaping the structure of foliar bacterial communities compared with CO2. The response of leaf-associated bacterial communities could be influenced by the leaf location (upper leaf or lower leaf) within the rice plants and by the habitats (leaf endosphere or phyllosphere).

The fungal communities in living and decomposed leaves of European Beech (Fagus sylvatica) were compared to identify the phyllosphere fungi involved in litter decomposition at a site in Bavaria, Germany. New primers were designed to cover a broad range of fungal ribosomal DNA sequence diversity. Following ‘environmental PCR’, clone libraries from each of five samples of living leaves (surface-sterilized and untreated), freshly fallen, initially and highly decomposed leaves, were screened using RFLP fingerprinting. Statistical analysis (ANOSIM) revealed that the fungal communities colonizing living (a) and initially decomposed leaves (c) significantly differed between each other and from freshly fallen (b) and highly decomposed leaves (d). Fungal assemblages of a and d were statistically indistinguishable from each other and from the endophyllous fungal community in living leaves. The results showed that endophyllous fungi play a role throughout the whole decomposition process of beech leaf litter. Therefore, clarification of the life cycle of certain endophytic and/or soil fungi may only be achieved by considering both phyllosphere and soil habitats.

In the present study, attempts were made to identify the potential of bacterial strains for promoting Arachis hypogaea L. growth. Four hundred and thirty three bacteria were isolated from rhizosphere, phyllosphere and plant tissues from peanuts cultivated in the producing area of Cordoba, Argentina. From this collection, 37 epiphytic isolates and 73 endophytic isolates were selected on the basis of tricalcium phosphate solubilizing activity. These isolates were further tested for other plant growth-promoting attributes and some of them evaluated to examine the effect of inoculation on peanut growth. Siderophore production was observed in a high percentage of the isolates, especially in the root nodule endophytes. Antibiosis was evaluated against the phytopathogen fungus Sclerotinia minor and S. Sclerotiorum. Endophytes from nodules showed the highest levels of fungal growth inhibition. A low number of isolates was able to produce auxin like molecules and inoculation of peanut seedlings with these bacteria showed variability on seed germination enhancement. Isolate J49, identified to belong to genus Pantoea, was the most promising bacterium because it increases peanut plant biomass in inoculation experiments. Peanut soils in the province of Cordoba harbor bacteria with major plant growth promotion properties which represent a potential source of new strains that could be used as biological inoculants in agriculture.

This paper reviews strategies for manipulating plants and their root-associated microorganisms to improve plant health and productivity. Some strategies directly target plant processes that impact on growth, while others are based on our knowledge of interactions among the components of the rhizosphere (roots, microorganisms and soil). For instance, plants can be engineered to modify the rhizosphere pH or to release compounds that improve nutrient availability, protect against biotic and abiotic stresses, or encourage the proliferation of beneficial microorganisms. Rhizobacteria that promote plant growth have been engineered to interfere with the synthesis of stress-induced hormones such as ethylene, which retards root growth, and to produce antibiotics and lytic enzymes active against soilborne root pathogens. Rhizosphere engineering also can involve the selection by plants of beneficial microbial populations. For example, some crop species or cultivars select for and support populations of antibiotic-producing strains that play a major role in soils naturally suppressive to soil-borne fungal pathogens. The fitness of root-associated bacterial communities also can be enhanced by soil amendment, a process that has allowed the selection of bacterial consortia that can interfere with bacterial pathogens. Plants also can be engineered specifically to influence their associated bacteria, as exemplified by quorum quenching strategies that suppress the virulence of pathogens of the genus Pectobacterium. New molecular tools and powerful biotechnological advances will continue to provide a more complete knowledge of the complex chemical and biological interactions that occur in the rhizosphere, ensuring that strategies to engineer the rhizosphere are safe, beneficial to productivity, and substantially improve the sustainability of agricultural systems.

Rhizosphere microorganisms have two faces, like Janus the Roman god of gates and doors who symbolizes changes and transitions, from one condition to another. One face looks at the plant root, the other sees the soil. The ears and the nose sense the other gods around and the mouths are wide open, swallowing as much as they can, and as described in Chapter 11, they also are busy talking. These faces may as well represent Hygieia (the Greek god of Health and Hygiene, the prevention of sickness and the continuation of good health) and Morta (the Roman god of death) for rhizosphere microbes can be beneficial, and promote plant growth and well being (Chapter 12) or detrimental, causing plant sickness and death (Chapter 13). It can be argued that many rhizosphere microbes are “neutral”, faceless saprophytes that decompose organic materials, perform mineralization and turnover processes. While most may not directly interact with the plant, their effects on soil biotic and abiotic parameters certainly have an impact on plant growth. Maybe they are Janus’ feet, the unsung heroes of the rhizosphere. This chapter addresses some aspects of the taxonomical and functional microbial diversity of the rhizosphere. Bacteria, Archea, viruses and Fungi will be at the heart of our discussion, while other rootassociated eukaryotes are the subjects of other chapters

Drosera villosa var. villosa A. St.-Hil is a carnivorous plant that grows in Brazilian floo- ded soils very poor on nutrients, including low levels of N. Under these conditions, the plant shows vigorous growth, low root number, low number of captured prey (less than 50%) and a great assemblage of bacteria associated with the roots and leaves that grow in N-free medium. These preliminary results have led us to investigate the number of colony forming units (log CFU) in the roots (rhizosphere and endo- rhizosphere) and leaves (phyllosphere and endo- phyllosphere) of D. villosa var. villosa by the tenfold serial dilution technique in two N-free culture media. The results showed that the phyllosphere had 6.65 log CFU g dry leaf -1 and the rhizosphere 6.47 log CFU g dry soil -1 , with the lowest value detected in the endophyllosphere (4.39 log CFU g dry leaf -1 ). Sixty-three different bacteria morphotypes were isolated from the surface and interior of the roots and leaves and the amplification of the DNA with specific prim- ers detected the nifH gene in 34 of those strains. The DNAs of the 34 strains were compared by the BOX-PCR technique and a great diversity was observed, with the bacteria clustering at a final level of similarity of only 12%. The strains were also submitted to the partial sequencing of the 16S rRNA gene and several genera of N2-fixing bacteria were detected, including Bacillus, Burkholderia, Methylobacterium, Paeni- bacillus, Pseudomonas and Sphingomonas.

The colonizing ability of a transcipient strain of Bacillus megaterium carrying a lepidopteran-specific cryIA (a) gene of Bacillus thuringiensis in the phyllospheres of various economically important plants was studied. Similar experiments were also carried out using the parental B. thuringiensis var. kurstaki strain HD1 for a comparison. While the transcipient remained on the leaves of cotton and okra for more than 28 days, its survival in phyllospheres of mulberry, peanut, chickpea, tomato and rice was rather limited to about 3 – 5 days. The persistence of B. thuringiensis, on the other hand, was extremely short (i.e. less than 4 days) on all the crop plants tested.

Bacteria, yeasts and filamentous fungi colonizing immature, mature and senescing primary leaves of field grown Beta vulgaris (sugar beet) were analysed over a complete growing season. Greatest microbial numbers were detected on senescing primary leaves and these numbers increased over most of the season. The number of colonizers detected on mature leaves was found to be stable over most of the study. Filamentous fungi and yeasts were identified to the genus level and the communities found to have greatest diversity during the summer months. There was no consistent pattern of diversity according to leaf type. Two genera of filamentous fungi, Cladosporium and Alternaria and two yeast genera, Cryptococcus and Sporobolomyces were the most numerous fungal populations isolated. Only 8 filamentous fungi and 3 yeast genera were commonly isolated on PDA (potato dextrose agar). Bacterial strains (1236) were isolated on Tryptic Soy Broth (TSB) agar and identified to species, or in some cases sub-species level, by analysis of their fatty acid methyl ester (FAME) profiles. Isolated bacteria were grouped into 78 named and 37 unnamed species clusters. Greatest number of bacterial species were isolated from young plants and leaves, sampled during the autumn months. Bacterial community diversity was lowest in mid-summer and winter months. Pseudomonas was the most commonly isolated genus and Erwinia herbicola the most common species. P. aureofaciens was the only species isolated from soil that was also isolated from the phyllosphere of B. vulgaris throughout the season.

The results of a three year investigation on phyllospheric nitrogen fixation in Pinus nigra and Pseudotsuga menziesii growing in Central Italy are reported. The highest levels of nitrogen fixation in Pinus nigra and Pseudotsuga menziesii needles were reached during spring in three year old needles (from 6.1 to 7.6 and from 8.1 to 10.2 nmoles of N2 fixed h-1 g-1 needles) collected in the center of the canopy, while the lowest values (less than 1.5 and 3.0 nmoles of N2 fixed h-1 g-1 needles) were detected during summer in young needles collected in the lateral branches (areas more exposed to the light and with low humidity). Microorganisms belonging to the genera Pseudomonas, Bacillus, Achromobacter, Klebsiella and Mycobacterium were isolated from the needles of either Pinus nigra or Pseudotsuga menziesii.

Nitrogen fixation (acetylene reduction) was found in intact tomato (Lycopersicon esculentum Mill ‘Pusa Ruby’) plants in the field, in pots and also in aseptic cultures. The unsterilized as well as sterilized rhizoplane and phylloplane of the plant when assayed separately also responded to the test. From root bits of tomato sterilized upto 20 minutes with 0.1% mercuric chloride, growth of the bacterium from the interior of the root into the medium was observed thereby indicating their presence within the endorhizosphere. Phase contrast and electron microscopic studies of the root system of tomato revealed the presence of bacterial colonies in the epidermis, cortex and vascular bundles. Bacterial numbers in the endosphere, of root and leaf were 30×104 and 12×104, respectively, per gram fresh weight of tissue. The bacteria were predominantly rod-shaped 1.4–4.8×0.9–1.95 μm in 24-h-old cultures, pleomorphic, polar or bipolary flagellated having β-hydroxy butyrate granules. The bacterium has been identified as a new species of Azospirillum.

An investigation was conducted to study the levels of nitrogen fixation on the leaf or sheath surfaces of four cultivars of paddy plants by using acetylene reduction technique. Varying levels of positive nitrogenase activity were observed on all the leaf surfaces. Sheath of IET 1991 cultivar showed a higher rate of fixation than the leaf surface. All the nitrogen-fixing organisms on the leaf or sheath surfaces belonged to the genus Beijerinckia. There was no correlation between the bacterial density and the level of fixation. Scanning electron microscopic data revealed that the upper surface of IET 1991 leaf was highly silicified and the microflora was either scanty or nil while the lower surface appeared quite different and harboured more micro-organisms. Similarly, the inner surface of sheath was devoid of silicification and showed the presence of micro-organisms.

Three herbicides, Benthiocarb, 2,4-D and Fluchloralin were evaluated for their effect on leaf surface microflora of potato. In general, the application of herbicides resulted into a drop in the microbial population. Throughout the study herbicide treated leaves harboured less population compared to the untreated ones.

Analysis of phyllosphere microflora showed that in the resistant cultivar (PHB-14) there was a significantly higher population of fungi, gram positive and gram negative bacteria, compared to susceptible cultivar (NHB-3) under healthy and diseased situations. The cultivars during earhead stage supported maximum phyllosphere fungal and gram negative bacterial populations.

Ninetten aminoacids, twelve sugars, eleven organic acids and ten phenols were detected in the leaf exudates of three cultivars of chilli. The number of aminoacids, sugars, organic acids and phenols increased as the plants grew older. More aminoacids and sugars were detected in the exudate from the susceptible cultivar (Malwa). More organic acids and phenols were detected from the resistant cultivar (Simla). The leaf exudate of the resistant cultivar (Simla) inhibited spore germination of the pathogen (Alternaria solani) while that of susceptible (Malwa) stimulated spore germination. The cultivar ‘Patna’ which is moderately resistant, occupied an intermediate position. Spore germination of the isolated fungi was enhanced in leaf exudate of susceptible cultivar (Malwa), while leaf exudates of the moderately resistant (Patna) and resistant (Simla) inhibited spore germination of the majority of fungi isolated. Most of the antagonistic fungi were not isolated from the susceptible cultivar and the percentage spore germination of these fungi was less in leaf exudate of the susceptible cultivar, while leaf exudates of resistant cultivars enhanced the percentage spore germination of antagonistic fungi,viz Aspergillus flavus, A. fumigatus, A. versicolor, Penicillium citrinum, P. restrictum andTrichoderma viride.

Thein vitro effects of twelve commonly used pesticides, including the fungicides Cuman-L (Ziram) and Hinosan (Ediphenphos), the acaricides Nuvacron (Monocrotophos), Ekatin 25EC (Thiometon) and Ekalux-G5 (Quinalphos) and other insecticides namely Dimecron (Phosphamidon), Anthio 25EC (Formothion), Baytex (Fenthion), Metasystox (Methyl demeton) and Phosalone (Zolone), on the nitrogen fixing bacteria isolated from the phyllosphere of some crop plants were tested. A very wide variation of the effect of these pesticides on the different organisms was noted. At a concentration of 700 μg/ml in the medium, most of the pesticides completely inhibited growth of the nitrogen fixing bacterial isolates. However, with some pesticides, when used at a lower concentration, a degree of growth stimulation was recorded.

The utility of spraying some known N2-fixing microorganisms on rice leaves grown both in N-less sand culture and under field conditions was examined. The effect was compared with that of spraying a phyllosphere N2-fixing isolate of Klebsiella, KUPBR2, and application of nitrogenous fertilizers. All the growth parameters studied including dry weight and N-content were enhanced. Under field conditions number of tillers was increased by 26% withKlebsiella pneumoniae M5al and by 65% with Aphanothece. The dry weight of the plants was enhanced by 61–119%. The yield per 10 m2 was almost doubled with Aphanothece, Beijerinckia 8007,Mycobacterium flavum, K. pneumoniae M5al and KUPBR2. The increases observed withStreptomyces sp. G12 though less spectacular was significant at 1% level with respect to several growth parameters.K. pneumoniae M5al,M. flavum andStreptomyces sp. G12 exhibited nitrogenase activity both in laboratory culture and in association with rice plants.

The beneficial effect of spraying some highly active phyllosphere N2-fixing microorganisms on high and low yielding cultivars of rice plants as compared with that of urea applied at different doses are described. The dry weight, N-content, 1000 grain weight, and yield were remarkably increased in all cases with the application of phyllosphere microorganisms. The performance of two isolates KUP4 and KUPBR2 with IR-8 and IR-26 rice, was better than that of 52 kg urea-N per hectare. IR-579 rice leaves in association with some phyllosphere bacteria reduced acetylene at the rate of 664–816 nmoles/g leaf/h. In IR-26 rice the effect of application of bacterial suspension at three phases of plant growth corresponded very well with that of urea application in three split doses under identical conditions. Recommended fertilizer rates produced the same yield as the half dose plus bacterial spray in the cultivars Pankaj and Rupsail. Fertilizer application in Pankaj and Rupsail rice reduced nitrogenase activity and the beneficial effects of phyllosphere N2-fixation was reduced by 40–55%.

The mulberry leaves were shown to harbour substantial populations of bacteria, streptomycetes, yeasts, and moulds. Azotobacter and Beijerinckia were observed to contribute to nearly 5 to 10 per cent of the bacterial population. When grown in water culture under sterile conditions, Azotobacter inoculation on the leaf or root surface was found to increase plant growth, dry wt, and nitrogen content of the mulberry. The beneficial effect of Azotobacter was largely influenced by the presence of a carbon source in the plant nutrient solution. The root inoculation in comparison to leaf application was found to confer greater benefits to the growing plant. The presence of carbohydrates and amino acids in the leaf leachates of mulberry was shown. The mutual beneficial nature of the association of the plant and Azotobacter has been brought to light.

The rhizosphere is a hot spot of microbial interactions as exudates released by plant roots are a main food source for microorganisms and a driving force of their population density and activities. The rhizosphere harbors many organisms that have a neutral effect on the plant, but also attracts organisms that exert deleterious or beneficial effects on the plant. Microorganisms that adversely affect plant growth and health are the pathogenic fungi, oomycetes, bacteria and nematodes. Most of the soilborne pathogens are adapted to grow and survive in the bulk soil, but the rhizosphere is the playground and infection court where the pathogen establishes a parasitic relationship with the plant. The rhizosphere is also a battlefield where the complex rhizosphere community, both microflora and microfauna, interact with pathogens and influence the outcome of pathogen infection. A wide range of microorganisms are beneficial to the plant and include nitrogen-fixing bacteria, endo- and ectomycorrhizal fungi, and plant growth-promoting bacteria and fungi. This review focuses on the population dynamics and activity of soilborne pathogens and beneficial microorganisms. Specific attention is given to mechanisms involved in the tripartite interactions between beneficial microorganisms, pathogens and the plant. We also discuss how agricultural practices affect pathogen and antagonist populations and how these practices can be adopted to promote plant growth and health.

Nitrogen fixation on leaf surfaces of sugarcane, sorghum, ragi, bamboo and mulberry plants was determined by using acetylene reduction assay. The data revealed varying levels of positive nitrogenase activity on the surfaces. Beijerinckia was observed to be the predominant diazotroph on the leaves. No correlation between fixation rates and $C_3$ or $C_4$ plant species was discerned. The possibility of improving phyllosphere nitrogen fixation has been discussed in light of the above observations.

The removal of fungal spores, urediniospores ofMelampsora medusae and conidia ofPestalozzia sp., from the leaf surfaces ofPopulus xeuramericana (Dode) Guinier cv. I-488 was assessed using three cultural techniques conventionally employed in phylloplane studies. The method of removal and the original density of spore deposition, but not the interaction of these factors, were significant determinants of variability in spore removal. Irrespective of the original density of deposition, the leaf print method was the most, and the leaf washing technique the least, efficient means of spore removal from the leaf surface. Factors which could contribute to this difference in efficiency are discussed.

A culture of a bacterium present in the phyllosphere of water hyacinth (Eichornia crassipes) which was identified earlier to beAzotobacter chroococcum was sprayed on two varieties of paddy and a variety of wheat in microplots under field conditions. There was considerable increase in the yield of the crops. In both the crops, the increase in yield due to spray could easily be compared to that by treatment with fertilizers.

Water hyacinth (Eichhornia crassipes) harbours Azotobacter chroococcum in large numbers on and in its leaves. This may account for its prolific growth of the plants in water containing only traces of combined nitrogen. re]19721201