Evidence is increasing that the saprophytic microflora of the rhizosphere includes both deleterious and beneficial elements that have the potential to influence plant growth and crop yields significantly (13, 16, 25, 27, 57, 72, 77, 78). The deleterious microorganisms affect plant growth negatively, but they do not necessarily parasitize the plant tissue. Their deleterious activities include alterations of the supply of water, ions, and plant growth substances by changing root functions and/or by limiting root growth (1,57, 67, 77). The mechanisms by which beneficial nonsymbiotic microorganisms are considered to affect plant growth positively include promotion of the availability and uptake of mineral nutrients (6, 7, 27), provision of plant-growth substances (27, 56, 57), and, most of all, suppression of deleterious rhizosphere microorganisms (13, 27, 28, 72, 77). Beneficial rhizobacteria that promote plant growth, supposedly by competing for iron with deleterious rhizosphere microorganisms, have recently become known as plant growth-promoting rhizobacteria (PGPR) (13, 41, 67, 77). This paper reviews recent knowledge on deleterious and beneficial rhizosphere bacteria, their interactions, and their effect on yield as related to cropping practices.

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Interactions of Deleterious and Beneficial Rhizosphere Microorganisms and the Effect of Cropping Practices

A Manual of Soil Fungi

Manual of microbiological methods

▪ Abstract The spermosphere represents a short-lived, rapidly changing, and microbiologically dynamic zone of soil surrounding a germinating seed. It is analogous to the rhizosphere, being established largely by the carbon compounds released into the soil once the seed begins to hydrate. These seed exudations drive the microbial activities that take place in the spermosphere, many of which can have long-lasting impacts on plant growth and development as well as on plant health. In this review, I discuss the nature of the spermosphere habitat and the factors that give rise to its character, with emphasis on the types of microbial activities in the spermosphere that have important implications for disease development and biological disease control. This review, which represents the first comprehensive synthesis of the literature on spermosphere biology, is meant to illustrate the unique nature of the spermosphere and how studies of interactions in this habitat may serve as useful experimental models for tes...

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Microbial dynamics and interactions in the spermosphere.

The phytotoxic or promoting effect of bacterial secretions on root growth of young lettuce seedlings (Lactuca sativa) was measured under axenic conditions. It was assumed that the inhibitory or promoting effects of either deleterious rhizobacteria (DRB) or of plant growth promoting rhizobacteria (PGPR) were auxin mediated. To avoid measurements of either parasitism or competition, seedlings were placed adjacent to bacterial colonies, with no contact between the organisms. Auxin excretion rate, evaluated by thin-layer chromatography (TLC) combined with Salkowski's reagent, indicated that all bacteria examined produced and released indole-3-acetic acid (IAA). High levels of IAA (76.6 μM) were excreted by four DRB (Micrococcus luteus, Streptoverticillium sp., Pseudomonas putida, and Gluconobacter sp.) during 84 hr of incubation. High concentrations of IAA released by DRB accounted for the suppression of root growth. Other unidentified fractions in the eluates of DRB also inhibited root elongation, but to a lesser extent. Like DRB, four isolates of PGPR (Agrobacterium sp., Alcaligenes piechaudii, and two different strains of Comamonas acidovorans) secreted IAA, but at lower levels (16.4 μM during a similar period of incubation). PGPR secreted growth promoting substances other than IAA, and these are now being investigated.

Is IAA the Major Root Growth Factor Secreted from Plant-Growth-Mediating Bacteria?

One of the factors giving rise to soil sickness in apple orchards is the rhizosphere microflora. The composition of the microbial coenosis in the rhizosphere changes with increasing age of the apple trees. An increase in the counts of micromycetes and actinomycetes and a decrease in bacterial counts was found in agreement with the decreasing pH of the rhizosphere soil. The presence of fluorescent pseudomonads in the rhizosphere of old apple trees was rare, but the planting of apple seedling into sick soil induced their proliferation. The relative proportion of individual genera of micromycetes changed according to the tree age; fungi of the genus Mucor were more often found in the rhizosphere of younger trees than in that of older ones while fungi of the genus Penicillium had an opposite trend. Biological tests showed that Penicillium fungi form the majority of the phytotoxic microflora. The amount of phytotoxic micromycetes was higher in ‘sick’ soil as compared with control soil in which apple trees had not been grown for at least 15 years. Higher numbers of phytotoxic micromycetes were isolated also from the rhizosphere of apple seedlings grown in ‘sick’ soil as compared with those growing in control soil. An increase in the amount of phytotoxic micromycetes in apple tree rhizosphere could be induced by mere addition of 5% (w/w) ‘sick’ soil to the soil in which apple trees were grown for the first time. By adding sterilized ‘sick’ soil, the amount of phytotoxic micromycetes in the apple seedling rhizosphere was not affected. Increased numbers of phytotoxic micromycetes affected negatively the growth of apple trees and the morphology of apple tree roots. This demonstrated the possibility of transfer of a factor participating in the etiology of soil sickness in apple orchards.

Rhizosphere micro-organisms in relation to the apple replant problem

An investigation was made of the extent to which the rhizosphere effect is reflected in the composition of mycoflora in the wheat rhizosphere during the initial stages of growth and during the vegetation period. It was found that during the initial phases of wheat growth no marked change took place in fungal counts in the rhizosphere and in control soil or on the roots. The fungal count in the course of the vegetation period was markedly lower in the rhizosphere and in fallow soil than on the roots.

Rhizosphere microflora of wheat

Rhizosphere microflora of wheat. I. Composition and properties of bacterial flora during the first stages of wheat growth, II. Composition and properties of bacterial flora during the vegetation period of wheat.

A single-step spraying of wheat during shooting under field conditions with solutions of CCC (chlorocholine chloride), CCC and urea, CCC and Aminex (ammonium salt of 4-chloro-2-methylphenoxyacetic acid), or CCC, urea and Aminex caused changes both in numbers and composition of the rhizosphere mycoflora. The numbers both in the rhizosphere of differently treated plants and in the free soil decreased during vegetation. A more pronounced effect in the number of fungi was demonstrated in plants treated only with CCC. The difference was more considerable during first 10 days after the spray. As far as the relative occurrence of individual genera in the rhizosphere soil is concerned, fungi of the genera Penicillium Link ex Fr., Fusarium Link ex Fr., Verticillium Nees and Trichoderma Pers were most influenced after the treatment with the used agents.

Rhizosphere mycoflora of wheat after foliar application of chlorocholine chloride, urea and 4-chloro-2-methylphenoxyacetic acid.

http://link.springer.com/content/pdf/10.1007/BF02927145.pdf

Rhizosphere microflora of wheat@@@МИКРОФЛОРА РИЗОСФЕРЫ ПШЕНИЦЫ: II. Composition and properties of bacterial flora during the vegetation period of wheat@@@II. Состав и особенности бактериальной ϕлоры в течение периода вегетации пшеницы

Vlasta Čatská

Papers

Rhizosphere micro-organisms in relation to the apple replant problem

Rhizosphere microflora of wheat

Rhizosphere microflora of wheat. I. Composition and properties of bacterial flora during the first stages of wheat growth, II. Composition and properties of bacterial flora during the vegetation period of wheat.

Rhizosphere mycoflora of wheat after foliar application of chlorocholine chloride, urea and 4-chloro-2-methylphenoxyacetic acid.